# StatusCodeFYI — Full Content Index > The Universal Status Code Encyclopedia ## Hypertext Transfer Protocol - [100 Continue](https://statuscodefyi.com/http/100-continue/) Markdown: https://statuscodefyi.com/http/100-continue.md - [101 Switching Protocols](https://statuscodefyi.com/http/101-switching-protocols/) Markdown: https://statuscodefyi.com/http/101-switching-protocols.md - [102 Processing](https://statuscodefyi.com/http/102-processing/) Markdown: https://statuscodefyi.com/http/102-processing.md - [103 Early Hints](https://statuscodefyi.com/http/103-early-hints/) Markdown: https://statuscodefyi.com/http/103-early-hints.md - [200 OK](https://statuscodefyi.com/http/200-ok/) Markdown: https://statuscodefyi.com/http/200-ok.md - [201 Created](https://statuscodefyi.com/http/201-created/) Markdown: https://statuscodefyi.com/http/201-created.md - [202 Accepted](https://statuscodefyi.com/http/202-accepted/) Markdown: https://statuscodefyi.com/http/202-accepted.md - [203 Non-Authoritative Information](https://statuscodefyi.com/http/203-non-authoritative-information/) Markdown: https://statuscodefyi.com/http/203-non-authoritative-information.md - [204 No Content](https://statuscodefyi.com/http/204-no-content/) Markdown: https://statuscodefyi.com/http/204-no-content.md - [205 Reset Content](https://statuscodefyi.com/http/205-reset-content/) Markdown: https://statuscodefyi.com/http/205-reset-content.md - [206 Partial Content](https://statuscodefyi.com/http/206-partial-content/) Markdown: https://statuscodefyi.com/http/206-partial-content.md - [207 Multi-Status](https://statuscodefyi.com/http/207-multi-status/) Markdown: https://statuscodefyi.com/http/207-multi-status.md - [208 Already Reported](https://statuscodefyi.com/http/208-already-reported/) Markdown: https://statuscodefyi.com/http/208-already-reported.md - [226 IM Used](https://statuscodefyi.com/http/226-im-used/) Markdown: https://statuscodefyi.com/http/226-im-used.md - [300 Multiple Choices](https://statuscodefyi.com/http/300-multiple-choices/) Markdown: https://statuscodefyi.com/http/300-multiple-choices.md - [301 Moved Permanently](https://statuscodefyi.com/http/301-moved-permanently/) Markdown: https://statuscodefyi.com/http/301-moved-permanently.md - [302 Found](https://statuscodefyi.com/http/302-found/) Markdown: https://statuscodefyi.com/http/302-found.md - [303 See Other](https://statuscodefyi.com/http/303-see-other/) Markdown: https://statuscodefyi.com/http/303-see-other.md - [304 Not Modified](https://statuscodefyi.com/http/304-not-modified/) Markdown: https://statuscodefyi.com/http/304-not-modified.md - [307 Temporary Redirect](https://statuscodefyi.com/http/307-temporary-redirect/) Markdown: https://statuscodefyi.com/http/307-temporary-redirect.md - [308 Permanent Redirect](https://statuscodefyi.com/http/308-permanent-redirect/) Markdown: https://statuscodefyi.com/http/308-permanent-redirect.md - [400 Bad Request](https://statuscodefyi.com/http/400-bad-request/) Markdown: https://statuscodefyi.com/http/400-bad-request.md - [401 Unauthorized](https://statuscodefyi.com/http/401-unauthorized/) Markdown: https://statuscodefyi.com/http/401-unauthorized.md - [402 Payment Required](https://statuscodefyi.com/http/402-payment-required/) Markdown: https://statuscodefyi.com/http/402-payment-required.md - [403 Forbidden](https://statuscodefyi.com/http/403-forbidden/) Markdown: https://statuscodefyi.com/http/403-forbidden.md - [404 Not Found](https://statuscodefyi.com/http/404-not-found/) Markdown: https://statuscodefyi.com/http/404-not-found.md - [405 Method Not Allowed](https://statuscodefyi.com/http/405-method-not-allowed/) Markdown: https://statuscodefyi.com/http/405-method-not-allowed.md - [406 Not Acceptable](https://statuscodefyi.com/http/406-not-acceptable/) Markdown: https://statuscodefyi.com/http/406-not-acceptable.md - [407 Proxy Authentication Required](https://statuscodefyi.com/http/407-proxy-authentication-required/) Markdown: https://statuscodefyi.com/http/407-proxy-authentication-required.md - [408 Request Timeout](https://statuscodefyi.com/http/408-request-timeout/) Markdown: https://statuscodefyi.com/http/408-request-timeout.md - [409 Conflict](https://statuscodefyi.com/http/409-conflict/) Markdown: https://statuscodefyi.com/http/409-conflict.md - [410 Gone](https://statuscodefyi.com/http/410-gone/) Markdown: https://statuscodefyi.com/http/410-gone.md - [411 Length Required](https://statuscodefyi.com/http/411-length-required/) Markdown: https://statuscodefyi.com/http/411-length-required.md - [412 Precondition Failed](https://statuscodefyi.com/http/412-precondition-failed/) Markdown: https://statuscodefyi.com/http/412-precondition-failed.md - [413 Content Too Large](https://statuscodefyi.com/http/413-content-too-large/) Markdown: https://statuscodefyi.com/http/413-content-too-large.md - [414 URI Too Long](https://statuscodefyi.com/http/414-uri-too-long/) Markdown: https://statuscodefyi.com/http/414-uri-too-long.md - [415 Unsupported Media Type](https://statuscodefyi.com/http/415-unsupported-media-type/) Markdown: https://statuscodefyi.com/http/415-unsupported-media-type.md - [416 Range Not Satisfiable](https://statuscodefyi.com/http/416-range-not-satisfiable/) Markdown: https://statuscodefyi.com/http/416-range-not-satisfiable.md - [417 Expectation Failed](https://statuscodefyi.com/http/417-expectation-failed/) Markdown: https://statuscodefyi.com/http/417-expectation-failed.md - [418 I'm a Teapot](https://statuscodefyi.com/http/418-im-a-teapot/) Markdown: https://statuscodefyi.com/http/418-im-a-teapot.md - [421 Misdirected Request](https://statuscodefyi.com/http/421-misdirected-request/) Markdown: https://statuscodefyi.com/http/421-misdirected-request.md - [422 Unprocessable Content](https://statuscodefyi.com/http/422-unprocessable-content/) Markdown: https://statuscodefyi.com/http/422-unprocessable-content.md - [423 Locked](https://statuscodefyi.com/http/423-locked/) Markdown: https://statuscodefyi.com/http/423-locked.md - [424 Failed Dependency](https://statuscodefyi.com/http/424-failed-dependency/) Markdown: https://statuscodefyi.com/http/424-failed-dependency.md - [425 Too Early](https://statuscodefyi.com/http/425-too-early/) Markdown: https://statuscodefyi.com/http/425-too-early.md - [426 Upgrade Required](https://statuscodefyi.com/http/426-upgrade-required/) Markdown: https://statuscodefyi.com/http/426-upgrade-required.md - [428 Precondition Required](https://statuscodefyi.com/http/428-precondition-required/) Markdown: https://statuscodefyi.com/http/428-precondition-required.md - [429 Too Many Requests](https://statuscodefyi.com/http/429-too-many-requests/) Markdown: https://statuscodefyi.com/http/429-too-many-requests.md - [431 Request Header Fields Too Large](https://statuscodefyi.com/http/431-request-header-fields-too-large/) Markdown: https://statuscodefyi.com/http/431-request-header-fields-too-large.md - [451 Unavailable For Legal Reasons](https://statuscodefyi.com/http/451-unavailable-for-legal-reasons/) Markdown: https://statuscodefyi.com/http/451-unavailable-for-legal-reasons.md - [499 Client Closed Request](https://statuscodefyi.com/http/499-client-closed-request/) Markdown: https://statuscodefyi.com/http/499-client-closed-request.md - [500 Internal Server Error](https://statuscodefyi.com/http/500-internal-server-error/) Markdown: https://statuscodefyi.com/http/500-internal-server-error.md - [501 Not Implemented](https://statuscodefyi.com/http/501-not-implemented/) Markdown: https://statuscodefyi.com/http/501-not-implemented.md - [502 Bad Gateway](https://statuscodefyi.com/http/502-bad-gateway/) Markdown: https://statuscodefyi.com/http/502-bad-gateway.md - [503 Service Unavailable](https://statuscodefyi.com/http/503-service-unavailable/) Markdown: https://statuscodefyi.com/http/503-service-unavailable.md - [504 Gateway Timeout](https://statuscodefyi.com/http/504-gateway-timeout/) Markdown: https://statuscodefyi.com/http/504-gateway-timeout.md - [505 HTTP Version Not Supported](https://statuscodefyi.com/http/505-http-version-not-supported/) Markdown: https://statuscodefyi.com/http/505-http-version-not-supported.md - [506 Variant Also Negotiates](https://statuscodefyi.com/http/506-variant-also-negotiates/) Markdown: https://statuscodefyi.com/http/506-variant-also-negotiates.md - [507 Insufficient Storage](https://statuscodefyi.com/http/507-insufficient-storage/) Markdown: https://statuscodefyi.com/http/507-insufficient-storage.md - [508 Loop Detected](https://statuscodefyi.com/http/508-loop-detected/) Markdown: https://statuscodefyi.com/http/508-loop-detected.md - [510 Not Extended](https://statuscodefyi.com/http/510-not-extended/) Markdown: https://statuscodefyi.com/http/510-not-extended.md - [511 Network Authentication Required](https://statuscodefyi.com/http/511-network-authentication-required/) Markdown: https://statuscodefyi.com/http/511-network-authentication-required.md - [520 Web Server Returned an Unknown Error](https://statuscodefyi.com/http/520-web-server-returned-unknown-error/) Markdown: https://statuscodefyi.com/http/520-web-server-returned-unknown-error.md - [521 Web Server Is Down](https://statuscodefyi.com/http/521-web-server-is-down/) Markdown: https://statuscodefyi.com/http/521-web-server-is-down.md - [522 Connection Timed Out](https://statuscodefyi.com/http/522-connection-timed-out/) Markdown: https://statuscodefyi.com/http/522-connection-timed-out.md - [523 Origin Is Unreachable](https://statuscodefyi.com/http/523-origin-is-unreachable/) Markdown: https://statuscodefyi.com/http/523-origin-is-unreachable.md - [524 A Timeout Occurred](https://statuscodefyi.com/http/524-a-timeout-occurred/) Markdown: https://statuscodefyi.com/http/524-a-timeout-occurred.md - [525 SSL Handshake Failed](https://statuscodefyi.com/http/525-ssl-handshake-failed/) Markdown: https://statuscodefyi.com/http/525-ssl-handshake-failed.md - [526 Invalid SSL Certificate](https://statuscodefyi.com/http/526-invalid-ssl-certificate/) Markdown: https://statuscodefyi.com/http/526-invalid-ssl-certificate.md - [530 Site Frozen](https://statuscodefyi.com/http/530-site-frozen/) Markdown: https://statuscodefyi.com/http/530-site-frozen.md ## Simple Mail Transfer Protocol - [211 System Status](https://statuscodefyi.com/smtp/211-system-status/) Markdown: https://statuscodefyi.com/smtp/211-system-status.md - [214 Help Message](https://statuscodefyi.com/smtp/214-help-message/) Markdown: https://statuscodefyi.com/smtp/214-help-message.md - [220 Service Ready](https://statuscodefyi.com/smtp/220-service-ready/) Markdown: https://statuscodefyi.com/smtp/220-service-ready.md - [221 Closing Connection](https://statuscodefyi.com/smtp/221-closing-connection/) Markdown: https://statuscodefyi.com/smtp/221-closing-connection.md - [235 Authentication Successful](https://statuscodefyi.com/smtp/235-auth-successful/) Markdown: https://statuscodefyi.com/smtp/235-auth-successful.md - [250 OK](https://statuscodefyi.com/smtp/250-ok/) Markdown: https://statuscodefyi.com/smtp/250-ok.md - [251 User Not Local](https://statuscodefyi.com/smtp/251-user-not-local/) Markdown: https://statuscodefyi.com/smtp/251-user-not-local.md - [252 Cannot Verify User](https://statuscodefyi.com/smtp/252-cannot-verify/) Markdown: https://statuscodefyi.com/smtp/252-cannot-verify.md - [334 Server Challenge](https://statuscodefyi.com/smtp/334-server-challenge/) Markdown: https://statuscodefyi.com/smtp/334-server-challenge.md - [354 Start Mail Input](https://statuscodefyi.com/smtp/354-start-mail/) Markdown: https://statuscodefyi.com/smtp/354-start-mail.md - [421 Service Not Available](https://statuscodefyi.com/smtp/421-service-unavailable/) Markdown: https://statuscodefyi.com/smtp/421-service-unavailable.md - [450 Mailbox Unavailable](https://statuscodefyi.com/smtp/450-mailbox-unavailable/) Markdown: https://statuscodefyi.com/smtp/450-mailbox-unavailable.md - [451 Local Error](https://statuscodefyi.com/smtp/451-local-error/) Markdown: https://statuscodefyi.com/smtp/451-local-error.md - [452 Insufficient Storage](https://statuscodefyi.com/smtp/452-insufficient-storage/) Markdown: https://statuscodefyi.com/smtp/452-insufficient-storage.md - [455 Server Unable to Accommodate](https://statuscodefyi.com/smtp/455-unable-to-accommodate/) Markdown: https://statuscodefyi.com/smtp/455-unable-to-accommodate.md - [500 Syntax Error](https://statuscodefyi.com/smtp/500-syntax-error/) Markdown: https://statuscodefyi.com/smtp/500-syntax-error.md - [501 Syntax Error in Parameters](https://statuscodefyi.com/smtp/501-syntax-parameters/) Markdown: https://statuscodefyi.com/smtp/501-syntax-parameters.md - [502 Command Not Implemented](https://statuscodefyi.com/smtp/502-not-implemented/) Markdown: https://statuscodefyi.com/smtp/502-not-implemented.md - [503 Bad Sequence](https://statuscodefyi.com/smtp/503-bad-sequence/) Markdown: https://statuscodefyi.com/smtp/503-bad-sequence.md - [504 Parameter Not Implemented](https://statuscodefyi.com/smtp/504-parameter-not-implemented/) Markdown: https://statuscodefyi.com/smtp/504-parameter-not-implemented.md - [521 Host Does Not Accept Mail](https://statuscodefyi.com/smtp/521-no-mail/) Markdown: https://statuscodefyi.com/smtp/521-no-mail.md - [530 Authentication Required](https://statuscodefyi.com/smtp/530-auth-required/) Markdown: https://statuscodefyi.com/smtp/530-auth-required.md - [535 Authentication Failed](https://statuscodefyi.com/smtp/535-auth-failed/) Markdown: https://statuscodefyi.com/smtp/535-auth-failed.md - [550 Mailbox Not Found](https://statuscodefyi.com/smtp/550-mailbox-not-found/) Markdown: https://statuscodefyi.com/smtp/550-mailbox-not-found.md - [551 User Not Local](https://statuscodefyi.com/smtp/551-user-not-local/) Markdown: https://statuscodefyi.com/smtp/551-user-not-local.md - [552 Exceeded Storage](https://statuscodefyi.com/smtp/552-exceeded-storage/) Markdown: https://statuscodefyi.com/smtp/552-exceeded-storage.md - [553 Mailbox Name Invalid](https://statuscodefyi.com/smtp/553-name-invalid/) Markdown: https://statuscodefyi.com/smtp/553-name-invalid.md - [554 Transaction Failed](https://statuscodefyi.com/smtp/554-transaction-failed/) Markdown: https://statuscodefyi.com/smtp/554-transaction-failed.md - [556 Domain Does Not Accept Mail](https://statuscodefyi.com/smtp/556-domain-no-mail/) Markdown: https://statuscodefyi.com/smtp/556-domain-no-mail.md ## gRPC Remote Procedure Call - [0 OK](https://statuscodefyi.com/grpc/0-ok/) Markdown: https://statuscodefyi.com/grpc/0-ok.md - [1 CANCELLED](https://statuscodefyi.com/grpc/1-cancelled/) Markdown: https://statuscodefyi.com/grpc/1-cancelled.md - [2 UNKNOWN](https://statuscodefyi.com/grpc/2-unknown/) Markdown: https://statuscodefyi.com/grpc/2-unknown.md - [3 INVALID_ARGUMENT](https://statuscodefyi.com/grpc/3-invalid-argument/) Markdown: https://statuscodefyi.com/grpc/3-invalid-argument.md - [4 DEADLINE_EXCEEDED](https://statuscodefyi.com/grpc/4-deadline-exceeded/) Markdown: https://statuscodefyi.com/grpc/4-deadline-exceeded.md - [5 NOT_FOUND](https://statuscodefyi.com/grpc/5-not-found/) Markdown: https://statuscodefyi.com/grpc/5-not-found.md - [6 ALREADY_EXISTS](https://statuscodefyi.com/grpc/6-already-exists/) Markdown: https://statuscodefyi.com/grpc/6-already-exists.md - [7 PERMISSION_DENIED](https://statuscodefyi.com/grpc/7-permission-denied/) Markdown: https://statuscodefyi.com/grpc/7-permission-denied.md - [8 RESOURCE_EXHAUSTED](https://statuscodefyi.com/grpc/8-resource-exhausted/) Markdown: https://statuscodefyi.com/grpc/8-resource-exhausted.md - [9 FAILED_PRECONDITION](https://statuscodefyi.com/grpc/9-failed-precondition/) Markdown: https://statuscodefyi.com/grpc/9-failed-precondition.md - [10 ABORTED](https://statuscodefyi.com/grpc/10-aborted/) Markdown: https://statuscodefyi.com/grpc/10-aborted.md - [11 OUT_OF_RANGE](https://statuscodefyi.com/grpc/11-out-of-range/) Markdown: https://statuscodefyi.com/grpc/11-out-of-range.md - [12 UNIMPLEMENTED](https://statuscodefyi.com/grpc/12-unimplemented/) Markdown: https://statuscodefyi.com/grpc/12-unimplemented.md - [13 INTERNAL](https://statuscodefyi.com/grpc/13-internal/) Markdown: https://statuscodefyi.com/grpc/13-internal.md - [14 UNAVAILABLE](https://statuscodefyi.com/grpc/14-unavailable/) Markdown: https://statuscodefyi.com/grpc/14-unavailable.md - [15 DATA_LOSS](https://statuscodefyi.com/grpc/15-data-loss/) Markdown: https://statuscodefyi.com/grpc/15-data-loss.md - [16 UNAUTHENTICATED](https://statuscodefyi.com/grpc/16-unauthenticated/) Markdown: https://statuscodefyi.com/grpc/16-unauthenticated.md ## Domain Name System - [0 NOERROR](https://statuscodefyi.com/dns/0-noerror/) Markdown: https://statuscodefyi.com/dns/0-noerror.md - [1 FORMERR](https://statuscodefyi.com/dns/1-formerr/) Markdown: https://statuscodefyi.com/dns/1-formerr.md - [2 SERVFAIL](https://statuscodefyi.com/dns/2-servfail/) Markdown: https://statuscodefyi.com/dns/2-servfail.md - [3 NXDOMAIN](https://statuscodefyi.com/dns/3-nxdomain/) Markdown: https://statuscodefyi.com/dns/3-nxdomain.md - [4 NOTIMP](https://statuscodefyi.com/dns/4-notimp/) Markdown: https://statuscodefyi.com/dns/4-notimp.md - [5 REFUSED](https://statuscodefyi.com/dns/5-refused/) Markdown: https://statuscodefyi.com/dns/5-refused.md - [6 YXDOMAIN](https://statuscodefyi.com/dns/6-yxdomain/) Markdown: https://statuscodefyi.com/dns/6-yxdomain.md - [7 YXRRSET](https://statuscodefyi.com/dns/7-yxrrset/) Markdown: https://statuscodefyi.com/dns/7-yxrrset.md - [8 NXRRSET](https://statuscodefyi.com/dns/8-nxrrset/) Markdown: https://statuscodefyi.com/dns/8-nxrrset.md - [9 NOTAUTH](https://statuscodefyi.com/dns/9-notauth/) Markdown: https://statuscodefyi.com/dns/9-notauth.md - [10 NOTZONE](https://statuscodefyi.com/dns/10-notzone/) Markdown: https://statuscodefyi.com/dns/10-notzone.md - [11 DSOTYPENI](https://statuscodefyi.com/dns/11-dsotypeni/) Markdown: https://statuscodefyi.com/dns/11-dsotypeni.md - [16 BADVERS/BADSIG](https://statuscodefyi.com/dns/16-badvers/) Markdown: https://statuscodefyi.com/dns/16-badvers.md - [17 BADKEY](https://statuscodefyi.com/dns/17-badkey/) Markdown: https://statuscodefyi.com/dns/17-badkey.md - [18 BADTIME](https://statuscodefyi.com/dns/18-badtime/) Markdown: https://statuscodefyi.com/dns/18-badtime.md - [19 BADMODE](https://statuscodefyi.com/dns/19-badmode/) Markdown: https://statuscodefyi.com/dns/19-badmode.md - [20 BADNAME](https://statuscodefyi.com/dns/20-badname/) Markdown: https://statuscodefyi.com/dns/20-badname.md - [21 BADALG](https://statuscodefyi.com/dns/21-badalg/) Markdown: https://statuscodefyi.com/dns/21-badalg.md - [22 BADTRUNC](https://statuscodefyi.com/dns/22-badtrunc/) Markdown: https://statuscodefyi.com/dns/22-badtrunc.md - [23 BADCOOKIE](https://statuscodefyi.com/dns/23-badcookie/) Markdown: https://statuscodefyi.com/dns/23-badcookie.md ## WebSocket Protocol - [1000 Normal Closure](https://statuscodefyi.com/websocket/1000-normal-closure/) Markdown: https://statuscodefyi.com/websocket/1000-normal-closure.md - [1001 Going Away](https://statuscodefyi.com/websocket/1001-going-away/) Markdown: https://statuscodefyi.com/websocket/1001-going-away.md - [1002 Protocol Error](https://statuscodefyi.com/websocket/1002-protocol-error/) Markdown: https://statuscodefyi.com/websocket/1002-protocol-error.md - [1003 Unsupported Data](https://statuscodefyi.com/websocket/1003-unsupported-data/) Markdown: https://statuscodefyi.com/websocket/1003-unsupported-data.md - [1005 No Status Received](https://statuscodefyi.com/websocket/1005-no-status/) Markdown: https://statuscodefyi.com/websocket/1005-no-status.md - [1006 Abnormal Closure](https://statuscodefyi.com/websocket/1006-abnormal-closure/) Markdown: https://statuscodefyi.com/websocket/1006-abnormal-closure.md - [1007 Invalid Payload Data](https://statuscodefyi.com/websocket/1007-invalid-payload/) Markdown: https://statuscodefyi.com/websocket/1007-invalid-payload.md - [1008 Policy Violation](https://statuscodefyi.com/websocket/1008-policy-violation/) Markdown: https://statuscodefyi.com/websocket/1008-policy-violation.md - [1009 Message Too Big](https://statuscodefyi.com/websocket/1009-message-too-big/) Markdown: https://statuscodefyi.com/websocket/1009-message-too-big.md - [1010 Mandatory Extension](https://statuscodefyi.com/websocket/1010-mandatory-extension/) Markdown: https://statuscodefyi.com/websocket/1010-mandatory-extension.md - [1011 Internal Error](https://statuscodefyi.com/websocket/1011-internal-error/) Markdown: https://statuscodefyi.com/websocket/1011-internal-error.md - [1012 Service Restart](https://statuscodefyi.com/websocket/1012-service-restart/) Markdown: https://statuscodefyi.com/websocket/1012-service-restart.md - [1013 Try Again Later](https://statuscodefyi.com/websocket/1013-try-again-later/) Markdown: https://statuscodefyi.com/websocket/1013-try-again-later.md - [1014 Bad Gateway](https://statuscodefyi.com/websocket/1014-bad-gateway/) Markdown: https://statuscodefyi.com/websocket/1014-bad-gateway.md - [1015 TLS Handshake Failure](https://statuscodefyi.com/websocket/1015-tls-handshake/) Markdown: https://statuscodefyi.com/websocket/1015-tls-handshake.md ## File Transfer Protocol - [110 Restart Marker Reply](https://statuscodefyi.com/ftp/110-restart-marker/) Markdown: https://statuscodefyi.com/ftp/110-restart-marker.md - [120 Service Ready Soon](https://statuscodefyi.com/ftp/120-service-ready-soon/) Markdown: https://statuscodefyi.com/ftp/120-service-ready-soon.md - [125 Data Connection Open](https://statuscodefyi.com/ftp/125-data-connection-open/) Markdown: https://statuscodefyi.com/ftp/125-data-connection-open.md - [150 File Status OK](https://statuscodefyi.com/ftp/150-file-status-ok/) Markdown: https://statuscodefyi.com/ftp/150-file-status-ok.md - [200 Command OK](https://statuscodefyi.com/ftp/200-command-ok/) Markdown: https://statuscodefyi.com/ftp/200-command-ok.md - [202 Command Superfluous](https://statuscodefyi.com/ftp/202-superfluous/) Markdown: https://statuscodefyi.com/ftp/202-superfluous.md - [211 System Status](https://statuscodefyi.com/ftp/211-system-status/) Markdown: https://statuscodefyi.com/ftp/211-system-status.md - [212 Directory Status](https://statuscodefyi.com/ftp/212-directory-status/) Markdown: https://statuscodefyi.com/ftp/212-directory-status.md - [213 File Status](https://statuscodefyi.com/ftp/213-file-status/) Markdown: https://statuscodefyi.com/ftp/213-file-status.md - [214 Help Message](https://statuscodefyi.com/ftp/214-help-message/) Markdown: https://statuscodefyi.com/ftp/214-help-message.md - [215 System Type](https://statuscodefyi.com/ftp/215-system-type/) Markdown: https://statuscodefyi.com/ftp/215-system-type.md - [220 Service Ready](https://statuscodefyi.com/ftp/220-service-ready/) Markdown: https://statuscodefyi.com/ftp/220-service-ready.md - [221 Closing Control Connection](https://statuscodefyi.com/ftp/221-closing/) Markdown: https://statuscodefyi.com/ftp/221-closing.md - [225 Data Connection Open](https://statuscodefyi.com/ftp/225-data-open/) Markdown: https://statuscodefyi.com/ftp/225-data-open.md - [226 Closing Data Connection](https://statuscodefyi.com/ftp/226-closing-data/) Markdown: https://statuscodefyi.com/ftp/226-closing-data.md - [227 Entering Passive Mode](https://statuscodefyi.com/ftp/227-passive-mode/) Markdown: https://statuscodefyi.com/ftp/227-passive-mode.md - [228 Entering Long Passive Mode](https://statuscodefyi.com/ftp/228-long-passive/) Markdown: https://statuscodefyi.com/ftp/228-long-passive.md - [229 Entering Extended Passive Mode](https://statuscodefyi.com/ftp/229-extended-passive/) Markdown: https://statuscodefyi.com/ftp/229-extended-passive.md - [230 User Logged In](https://statuscodefyi.com/ftp/230-logged-in/) Markdown: https://statuscodefyi.com/ftp/230-logged-in.md - [250 File Action OK](https://statuscodefyi.com/ftp/250-file-action-ok/) Markdown: https://statuscodefyi.com/ftp/250-file-action-ok.md - [257 Pathname Created](https://statuscodefyi.com/ftp/257-pathname-created/) Markdown: https://statuscodefyi.com/ftp/257-pathname-created.md - [331 Username OK, Need Password](https://statuscodefyi.com/ftp/331-need-password/) Markdown: https://statuscodefyi.com/ftp/331-need-password.md - [332 Need Account](https://statuscodefyi.com/ftp/332-need-account/) Markdown: https://statuscodefyi.com/ftp/332-need-account.md - [350 File Action Pending](https://statuscodefyi.com/ftp/350-pending/) Markdown: https://statuscodefyi.com/ftp/350-pending.md - [421 Service Not Available](https://statuscodefyi.com/ftp/421-service-unavailable/) Markdown: https://statuscodefyi.com/ftp/421-service-unavailable.md - [425 Can't Open Data Connection](https://statuscodefyi.com/ftp/425-no-data-connection/) Markdown: https://statuscodefyi.com/ftp/425-no-data-connection.md - [426 Connection Closed](https://statuscodefyi.com/ftp/426-connection-closed/) Markdown: https://statuscodefyi.com/ftp/426-connection-closed.md - [450 File Action Not Taken](https://statuscodefyi.com/ftp/450-file-not-taken/) Markdown: https://statuscodefyi.com/ftp/450-file-not-taken.md - [451 Local Error](https://statuscodefyi.com/ftp/451-local-error/) Markdown: https://statuscodefyi.com/ftp/451-local-error.md - [452 Insufficient Storage](https://statuscodefyi.com/ftp/452-insufficient-storage/) Markdown: https://statuscodefyi.com/ftp/452-insufficient-storage.md - [500 Syntax Error](https://statuscodefyi.com/ftp/500-syntax-error/) Markdown: https://statuscodefyi.com/ftp/500-syntax-error.md - [501 Syntax Error in Arguments](https://statuscodefyi.com/ftp/501-syntax-args/) Markdown: https://statuscodefyi.com/ftp/501-syntax-args.md - [502 Command Not Implemented](https://statuscodefyi.com/ftp/502-not-implemented/) Markdown: https://statuscodefyi.com/ftp/502-not-implemented.md - [503 Bad Sequence](https://statuscodefyi.com/ftp/503-bad-sequence/) Markdown: https://statuscodefyi.com/ftp/503-bad-sequence.md - [504 Parameter Not Implemented](https://statuscodefyi.com/ftp/504-parameter-not-implemented/) Markdown: https://statuscodefyi.com/ftp/504-parameter-not-implemented.md - [530 Not Logged In](https://statuscodefyi.com/ftp/530-not-logged-in/) Markdown: https://statuscodefyi.com/ftp/530-not-logged-in.md - [532 Need Account for Storing](https://statuscodefyi.com/ftp/532-need-account/) Markdown: https://statuscodefyi.com/ftp/532-need-account.md - [550 File Unavailable](https://statuscodefyi.com/ftp/550-file-unavailable/) Markdown: https://statuscodefyi.com/ftp/550-file-unavailable.md - [551 Page Type Unknown](https://statuscodefyi.com/ftp/551-page-type-unknown/) Markdown: https://statuscodefyi.com/ftp/551-page-type-unknown.md - [552 Exceeded Storage Allocation](https://statuscodefyi.com/ftp/552-exceeded-storage/) Markdown: https://statuscodefyi.com/ftp/552-exceeded-storage.md - [553 File Name Not Allowed](https://statuscodefyi.com/ftp/553-name-not-allowed/) Markdown: https://statuscodefyi.com/ftp/553-name-not-allowed.md ## Session Initiation Protocol - [100 Trying](https://statuscodefyi.com/sip/100-trying/) Markdown: https://statuscodefyi.com/sip/100-trying.md - [180 Ringing](https://statuscodefyi.com/sip/180-ringing/) Markdown: https://statuscodefyi.com/sip/180-ringing.md - [181 Call Is Being Forwarded](https://statuscodefyi.com/sip/181-call-forwarded/) Markdown: https://statuscodefyi.com/sip/181-call-forwarded.md - [182 Queued](https://statuscodefyi.com/sip/182-queued/) Markdown: https://statuscodefyi.com/sip/182-queued.md - [183 Session Progress](https://statuscodefyi.com/sip/183-session-progress/) Markdown: https://statuscodefyi.com/sip/183-session-progress.md - [199 Early Dialog Terminated](https://statuscodefyi.com/sip/199-early-dialog-terminated/) Markdown: https://statuscodefyi.com/sip/199-early-dialog-terminated.md - [200 OK](https://statuscodefyi.com/sip/200-ok/) Markdown: https://statuscodefyi.com/sip/200-ok.md - [202 Accepted](https://statuscodefyi.com/sip/202-accepted/) Markdown: https://statuscodefyi.com/sip/202-accepted.md - [204 No Notification](https://statuscodefyi.com/sip/204-no-notification/) Markdown: https://statuscodefyi.com/sip/204-no-notification.md - [300 Multiple Choices](https://statuscodefyi.com/sip/300-multiple-choices/) Markdown: https://statuscodefyi.com/sip/300-multiple-choices.md - [301 Moved Permanently](https://statuscodefyi.com/sip/301-moved-permanently/) Markdown: https://statuscodefyi.com/sip/301-moved-permanently.md - [302 Moved Temporarily](https://statuscodefyi.com/sip/302-moved-temporarily/) Markdown: https://statuscodefyi.com/sip/302-moved-temporarily.md - [305 Use Proxy](https://statuscodefyi.com/sip/305-use-proxy/) Markdown: https://statuscodefyi.com/sip/305-use-proxy.md - [380 Alternative Service](https://statuscodefyi.com/sip/380-alternative-service/) Markdown: https://statuscodefyi.com/sip/380-alternative-service.md - [400 Bad Request](https://statuscodefyi.com/sip/400-bad-request/) Markdown: https://statuscodefyi.com/sip/400-bad-request.md - [401 Unauthorized](https://statuscodefyi.com/sip/401-unauthorized/) Markdown: https://statuscodefyi.com/sip/401-unauthorized.md - [403 Forbidden](https://statuscodefyi.com/sip/403-forbidden/) Markdown: https://statuscodefyi.com/sip/403-forbidden.md - [404 Not Found](https://statuscodefyi.com/sip/404-not-found/) Markdown: https://statuscodefyi.com/sip/404-not-found.md - [405 Method Not Allowed](https://statuscodefyi.com/sip/405-method-not-allowed/) Markdown: https://statuscodefyi.com/sip/405-method-not-allowed.md - [406 Not Acceptable](https://statuscodefyi.com/sip/406-not-acceptable/) Markdown: https://statuscodefyi.com/sip/406-not-acceptable.md - [407 Proxy Authentication Required](https://statuscodefyi.com/sip/407-proxy-auth-required/) Markdown: https://statuscodefyi.com/sip/407-proxy-auth-required.md - [408 Request Timeout](https://statuscodefyi.com/sip/408-request-timeout/) Markdown: https://statuscodefyi.com/sip/408-request-timeout.md - [410 Gone](https://statuscodefyi.com/sip/410-gone/) Markdown: https://statuscodefyi.com/sip/410-gone.md - [412 Conditional Request Failed](https://statuscodefyi.com/sip/412-conditional-request-failed/) Markdown: https://statuscodefyi.com/sip/412-conditional-request-failed.md - [413 Request Entity Too Large](https://statuscodefyi.com/sip/413-request-too-large/) Markdown: https://statuscodefyi.com/sip/413-request-too-large.md - [414 Request-URI Too Long](https://statuscodefyi.com/sip/414-uri-too-long/) Markdown: https://statuscodefyi.com/sip/414-uri-too-long.md - [415 Unsupported Media Type](https://statuscodefyi.com/sip/415-unsupported-media-type/) Markdown: https://statuscodefyi.com/sip/415-unsupported-media-type.md - [416 Unsupported URI Scheme](https://statuscodefyi.com/sip/416-unsupported-uri-scheme/) Markdown: https://statuscodefyi.com/sip/416-unsupported-uri-scheme.md - [420 Bad Extension](https://statuscodefyi.com/sip/420-bad-extension/) Markdown: https://statuscodefyi.com/sip/420-bad-extension.md - [421 Extension Required](https://statuscodefyi.com/sip/421-extension-required/) Markdown: https://statuscodefyi.com/sip/421-extension-required.md - [422 Session Interval Too Small](https://statuscodefyi.com/sip/422-session-interval-too-small/) Markdown: https://statuscodefyi.com/sip/422-session-interval-too-small.md - [423 Interval Too Brief](https://statuscodefyi.com/sip/423-interval-too-brief/) Markdown: https://statuscodefyi.com/sip/423-interval-too-brief.md - [428 Use Identity Header](https://statuscodefyi.com/sip/428-use-identity-header/) Markdown: https://statuscodefyi.com/sip/428-use-identity-header.md - [430 Flow Failed](https://statuscodefyi.com/sip/430-flow-failed/) Markdown: https://statuscodefyi.com/sip/430-flow-failed.md - [433 Anonymity Disallowed](https://statuscodefyi.com/sip/433-anonymity-disallowed/) Markdown: https://statuscodefyi.com/sip/433-anonymity-disallowed.md - [436 Bad Identity-Info](https://statuscodefyi.com/sip/436-bad-identity-info/) Markdown: https://statuscodefyi.com/sip/436-bad-identity-info.md - [437 Unsupported Certificate](https://statuscodefyi.com/sip/437-unsupported-certificate/) Markdown: https://statuscodefyi.com/sip/437-unsupported-certificate.md - [438 Invalid Identity Header](https://statuscodefyi.com/sip/438-invalid-identity-header/) Markdown: https://statuscodefyi.com/sip/438-invalid-identity-header.md - [439 First Hop Lacks Outbound Support](https://statuscodefyi.com/sip/439-first-hop-lacks-outbound/) Markdown: https://statuscodefyi.com/sip/439-first-hop-lacks-outbound.md - [470 Consent Needed](https://statuscodefyi.com/sip/470-consent-needed/) Markdown: https://statuscodefyi.com/sip/470-consent-needed.md - [480 Temporarily Unavailable](https://statuscodefyi.com/sip/480-temporarily-unavailable/) Markdown: https://statuscodefyi.com/sip/480-temporarily-unavailable.md - [481 Call/Transaction Does Not Exist](https://statuscodefyi.com/sip/481-call-does-not-exist/) Markdown: https://statuscodefyi.com/sip/481-call-does-not-exist.md - [482 Loop Detected](https://statuscodefyi.com/sip/482-loop-detected/) Markdown: https://statuscodefyi.com/sip/482-loop-detected.md - [483 Too Many Hops](https://statuscodefyi.com/sip/483-too-many-hops/) Markdown: https://statuscodefyi.com/sip/483-too-many-hops.md - [484 Address Incomplete](https://statuscodefyi.com/sip/484-address-incomplete/) Markdown: https://statuscodefyi.com/sip/484-address-incomplete.md - [485 Ambiguous](https://statuscodefyi.com/sip/485-ambiguous/) Markdown: https://statuscodefyi.com/sip/485-ambiguous.md - [486 Busy Here](https://statuscodefyi.com/sip/486-busy-here/) Markdown: https://statuscodefyi.com/sip/486-busy-here.md - [487 Request Terminated](https://statuscodefyi.com/sip/487-request-terminated/) Markdown: https://statuscodefyi.com/sip/487-request-terminated.md - [488 Not Acceptable Here](https://statuscodefyi.com/sip/488-not-acceptable-here/) Markdown: https://statuscodefyi.com/sip/488-not-acceptable-here.md - [489 Bad Event](https://statuscodefyi.com/sip/489-bad-event/) Markdown: https://statuscodefyi.com/sip/489-bad-event.md - [491 Request Pending](https://statuscodefyi.com/sip/491-request-pending/) Markdown: https://statuscodefyi.com/sip/491-request-pending.md - [493 Undecipherable](https://statuscodefyi.com/sip/493-undecipherable/) Markdown: https://statuscodefyi.com/sip/493-undecipherable.md - [500 Server Internal Error](https://statuscodefyi.com/sip/500-server-internal-error/) Markdown: https://statuscodefyi.com/sip/500-server-internal-error.md - [501 Not Implemented](https://statuscodefyi.com/sip/501-not-implemented/) Markdown: https://statuscodefyi.com/sip/501-not-implemented.md - [502 Bad Gateway](https://statuscodefyi.com/sip/502-bad-gateway/) Markdown: https://statuscodefyi.com/sip/502-bad-gateway.md - [503 Service Unavailable](https://statuscodefyi.com/sip/503-service-unavailable/) Markdown: https://statuscodefyi.com/sip/503-service-unavailable.md - [504 Server Time-out](https://statuscodefyi.com/sip/504-server-timeout/) Markdown: https://statuscodefyi.com/sip/504-server-timeout.md - [505 Version Not Supported](https://statuscodefyi.com/sip/505-version-not-supported/) Markdown: https://statuscodefyi.com/sip/505-version-not-supported.md - [513 Message Too Large](https://statuscodefyi.com/sip/513-message-too-large/) Markdown: https://statuscodefyi.com/sip/513-message-too-large.md - [580 Precondition Failure](https://statuscodefyi.com/sip/580-precondition-failure/) Markdown: https://statuscodefyi.com/sip/580-precondition-failure.md - [600 Busy Everywhere](https://statuscodefyi.com/sip/600-busy-everywhere/) Markdown: https://statuscodefyi.com/sip/600-busy-everywhere.md - [603 Decline](https://statuscodefyi.com/sip/603-decline/) Markdown: https://statuscodefyi.com/sip/603-decline.md - [604 Does Not Exist Anywhere](https://statuscodefyi.com/sip/604-does-not-exist-anywhere/) Markdown: https://statuscodefyi.com/sip/604-does-not-exist-anywhere.md - [606 Not Acceptable](https://statuscodefyi.com/sip/606-not-acceptable/) Markdown: https://statuscodefyi.com/sip/606-not-acceptable.md - [607 Unwanted](https://statuscodefyi.com/sip/607-unwanted/) Markdown: https://statuscodefyi.com/sip/607-unwanted.md - [608 Rejected](https://statuscodefyi.com/sip/608-rejected/) Markdown: https://statuscodefyi.com/sip/608-rejected.md ## Guides - [Understanding HTTP Status Codes: A Complete Guide](https://statuscodefyi.com/guides/understanding-http-status-codes/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/understanding-http-status-codes.md - [HTTP Redirects: 301 vs 302 vs 307 vs 308](https://statuscodefyi.com/guides/http-redirect-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-redirect-guide.md - [HTTP Caching: Cache-Control, ETags, and 304 Responses](https://statuscodefyi.com/guides/http-caching-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-caching-guide.md - [Complete Guide to HTTP Methods: GET, POST, PUT, PATCH, DELETE](https://statuscodefyi.com/guides/http-methods-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-methods-guide.md - [Essential HTTP Headers Every Developer Should Know](https://statuscodefyi.com/guides/http-headers-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-headers-guide.md - [HTTP/2 vs HTTP/3: What Changed and Why](https://statuscodefyi.com/guides/http2-vs-http3/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http2-vs-http3.md - [HTTP Content Negotiation: Complete Guide](https://statuscodefyi.com/guides/http-content-negotiation-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-content-negotiation-guide.md - [HTTP Range Requests and Partial Content](https://statuscodefyi.com/guides/http-range-requests-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-range-requests-guide.md - [HTTP Cookies: SameSite, Secure, and HttpOnly Explained](https://statuscodefyi.com/guides/http-cookies-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-cookies-guide.md - [HTTP Request Lifecycle: From URL to Response](https://statuscodefyi.com/guides/http-request-lifecycle-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-request-lifecycle-guide.md - [HTTP Conditional Requests: If-Match, If-None-Match, and Beyond](https://statuscodefyi.com/guides/http-conditional-requests-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-conditional-requests-guide.md - [HTTP Trailers: Sending Metadata After the Body](https://statuscodefyi.com/guides/http-trailers-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-trailers-guide.md - [HTTP Proxy Headers: X-Forwarded-For, Via, and Forwarded](https://statuscodefyi.com/guides/http-proxy-headers-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-proxy-headers-guide.md - [HTTP 103 Early Hints: Preloading Before the Response](https://statuscodefyi.com/guides/http-early-hints-guide/) [HTTP Fundamentals] Markdown: https://statuscodefyi.com/guides/http-early-hints-guide.md - [Choosing the Right HTTP Status Codes for REST APIs](https://statuscodefyi.com/guides/choosing-http-status-codes-for-apis/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/choosing-http-status-codes-for-apis.md - [gRPC Status Codes: A Practical Guide](https://statuscodefyi.com/guides/grpc-status-codes-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/grpc-status-codes-guide.md - [How to Implement and Handle Rate Limiting (429)](https://statuscodefyi.com/guides/rate-limiting-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/rate-limiting-guide.md - [API Versioning Strategies: URL, Header, and Query Param](https://statuscodefyi.com/guides/api-versioning-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-versioning-guide.md - [Designing Consistent API Error Responses](https://statuscodefyi.com/guides/api-error-handling-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-error-handling-guide.md - [API Pagination Patterns: Offset, Cursor, and Keyset](https://statuscodefyi.com/guides/api-pagination-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-pagination-guide.md - [API Versioning Strategies: URI, Header, and Query](https://statuscodefyi.com/guides/api-versioning-strategies/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-versioning-strategies.md - [Implementing Webhooks: A Complete Guide](https://statuscodefyi.com/guides/webhooks-implementation-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/webhooks-implementation-guide.md - [GraphQL vs REST: When to Use Which](https://statuscodefyi.com/guides/graphql-vs-rest-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/graphql-vs-rest-guide.md - [OpenAPI Specification: Design-First API Development](https://statuscodefyi.com/guides/openapi-specification-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/openapi-specification-guide.md - [API Deprecation Strategy: Sunset Headers and Migration Paths](https://statuscodefyi.com/guides/api-deprecation-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-deprecation-guide.md - [Designing Bulk API Endpoints: Batch Requests and Partial Failures](https://statuscodefyi.com/guides/api-bulk-operations-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-bulk-operations-guide.md - [HATEOAS and Hypermedia APIs: Beyond REST](https://statuscodefyi.com/guides/api-hypermedia-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-hypermedia-guide.md - [Designing Idempotent APIs: From Theory to Implementation](https://statuscodefyi.com/guides/api-idempotency-design-guide/) [API Design & Best Practices] Markdown: https://statuscodefyi.com/guides/api-idempotency-design-guide.md - [Debugging 502 Bad Gateway Errors](https://statuscodefyi.com/guides/debugging-502-bad-gateway/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-502-bad-gateway.md - [Debugging CORS Errors: A Developer's Guide](https://statuscodefyi.com/guides/debugging-cors-errors/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-cors-errors.md - [Debugging Timeout Errors: 408 and 504](https://statuscodefyi.com/guides/debugging-timeout-errors/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-timeout-errors.md - [Debugging 503 Service Unavailable Errors in Production](https://statuscodefyi.com/guides/debugging-503-service-unavailable/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-503-service-unavailable.md - [TLS/SSL Certificate Troubleshooting Guide](https://statuscodefyi.com/guides/debugging-ssl-certificate-errors/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-ssl-certificate-errors.md - [Step-by-Step DNS Troubleshooting with dig and nslookup](https://statuscodefyi.com/guides/debugging-dns-issues/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-dns-issues.md - [Debugging 429 Too Many Requests](https://statuscodefyi.com/guides/debugging-429-rate-limiting/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-429-rate-limiting.md - [Debugging Connection Reset Errors](https://statuscodefyi.com/guides/debugging-connection-reset/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-connection-reset.md - [Debugging WebSocket Disconnections](https://statuscodefyi.com/guides/debugging-websocket-disconnects/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-websocket-disconnects.md - [Debugging gRPC Errors and Status Codes](https://statuscodefyi.com/guides/debugging-grpc-errors/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-grpc-errors.md - [Debugging 500 Internal Server Error: A Systematic Approach](https://statuscodefyi.com/guides/debugging-500-internal-server-error/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-500-internal-server-error.md - [Debugging Redirect Loops (ERR_TOO_MANY_REDIRECTS)](https://statuscodefyi.com/guides/debugging-redirect-loops/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-redirect-loops.md - [Debugging Mixed Content Errors in HTTPS Pages](https://statuscodefyi.com/guides/debugging-mixed-content/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-mixed-content.md - [Debugging SMTP Delivery Failures: 4xx Temporary vs 5xx Permanent](https://statuscodefyi.com/guides/debugging-smtp-delivery-failures/) [Debugging & Troubleshooting] Markdown: https://statuscodefyi.com/guides/debugging-smtp-delivery-failures.md - 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[Migrating from TLS 1.2 to TLS 1.3: Configuration and Compatibility](https://statuscodefyi.com/guides/tls-12-to-13-migration/) [Migration & Upgrades] Markdown: https://statuscodefyi.com/guides/tls-12-to-13-migration.md - [Reverse Proxy Configuration: Nginx, Caddy, and HAProxy](https://statuscodefyi.com/guides/infra-reverse-proxy-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-reverse-proxy-guide.md - [Load Balancer Health Checks: HTTP, TCP, and gRPC Probes](https://statuscodefyi.com/guides/infra-load-balancer-health-checks-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-load-balancer-health-checks-guide.md - [Zero-Downtime Deployments: Blue-Green, Canary, and Rolling Updates](https://statuscodefyi.com/guides/infra-zero-downtime-deployment-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-zero-downtime-deployment-guide.md - [CDN Configuration for Dynamic and Static Content](https://statuscodefyi.com/guides/infra-cdn-configuration-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-cdn-configuration-guide.md - [TLS Termination: Where to Terminate HTTPS and Why](https://statuscodefyi.com/guides/infra-tls-termination-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-tls-termination-guide.md - [Rate Limiting at the Edge: WAF, CDN, and API Gateway Strategies](https://statuscodefyi.com/guides/infra-rate-limiting-at-edge-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-rate-limiting-at-edge-guide.md - [Connection Draining: Graceful Shutdown Without Dropping Requests](https://statuscodefyi.com/guides/infra-connection-draining-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-connection-draining-guide.md - [Observability for HTTP Status Codes: Metrics, Logs, and Alerts](https://statuscodefyi.com/guides/infra-observability-status-codes-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-observability-status-codes-guide.md - [Request ID and Distributed Tracing: Correlating Logs Across Services](https://statuscodefyi.com/guides/infra-request-id-tracing-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-request-id-tracing-guide.md - [Custom Error Pages: 404, 500, 502, and Maintenance Pages](https://statuscodefyi.com/guides/infra-error-page-design-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-error-page-design-guide.md - [HTTP Access Log Management: Rotation, Parsing, and Analysis](https://statuscodefyi.com/guides/infra-log-rotation-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-log-rotation-guide.md - [Secret Management for API Keys, Tokens, and Certificates](https://statuscodefyi.com/guides/infra-secret-management-guide/) [Production Infrastructure] Markdown: https://statuscodefyi.com/guides/infra-secret-management-guide.md - [API Gateway Patterns: Architecture and Use Cases](https://statuscodefyi.com/guides/gateway-introduction-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-introduction-guide.md - [API Gateway Rate Limiting Patterns: Token Bucket, Sliding Window, and Quotas](https://statuscodefyi.com/guides/gateway-rate-limiting-patterns-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-rate-limiting-patterns-guide.md - [Circuit Breaker Pattern at the API Gateway](https://statuscodefyi.com/guides/gateway-circuit-breaker-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-circuit-breaker-guide.md - [API Gateway Request Routing: Path, Header, and Content-Based](https://statuscodefyi.com/guides/gateway-request-routing-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-request-routing-guide.md - [Authentication at the API Gateway: JWT Validation, OAuth, and API Keys](https://statuscodefyi.com/guides/gateway-authentication-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-authentication-guide.md - [Request and Response Transformation at the API Gateway](https://statuscodefyi.com/guides/gateway-request-transformation-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-request-transformation-guide.md - [API Gateway Retry and Timeout Policies](https://statuscodefyi.com/guides/gateway-retry-timeout-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-retry-timeout-guide.md - [CORS Handling at the API Gateway](https://statuscodefyi.com/guides/gateway-cors-handling-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-cors-handling-guide.md - [API Gateway Logging and Analytics](https://statuscodefyi.com/guides/gateway-logging-analytics-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-logging-analytics-guide.md - [API Versioning Through the Gateway: URI, Header, and Media Type](https://statuscodefyi.com/guides/gateway-versioning-guide/) [API Gateway Patterns] Markdown: https://statuscodefyi.com/guides/gateway-versioning-guide.md - [API Testing Strategies: Unit, Integration, Contract, and E2E](https://statuscodefyi.com/guides/testing-api-testing-strategies-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-api-testing-strategies-guide.md - [Mock Servers for API Development: WireMock, Prism, and Custom Mocks](https://statuscodefyi.com/guides/testing-mock-server-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-mock-server-guide.md - [Contract Testing with Pact: Consumer-Driven API Contracts](https://statuscodefyi.com/guides/testing-contract-testing-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-contract-testing-guide.md - [Load Testing APIs: k6, Locust, and Artillery](https://statuscodefyi.com/guides/testing-load-testing-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-load-testing-guide.md - [Chaos Engineering for APIs: Injecting Faults and Status Codes](https://statuscodefyi.com/guides/testing-chaos-engineering-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-chaos-engineering-guide.md - [API Schema Validation Testing: OpenAPI, JSON Schema, and Schemathesis](https://statuscodefyi.com/guides/testing-api-schema-validation-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-api-schema-validation-guide.md - [HTTP Recording and Replay: VCR, Polly, and Nock](https://statuscodefyi.com/guides/testing-http-recording-replay-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-http-recording-replay-guide.md - [Testing Error Scenarios: Simulating 4xx and 5xx Responses](https://statuscodefyi.com/guides/testing-error-scenario-testing-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-error-scenario-testing-guide.md - [API Security Testing: OWASP ZAP, Burp Suite, and Automated Scanning](https://statuscodefyi.com/guides/testing-api-security-testing-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-api-security-testing-guide.md - [API Performance Benchmarking: Establishing and Tracking Baselines](https://statuscodefyi.com/guides/testing-performance-benchmarking-guide/) [Testing & Mocking] Markdown: https://statuscodefyi.com/guides/testing-performance-benchmarking-guide.md ## Glossary - [Idempotency](https://statuscodefyi.com/glossary/idempotency/) [HTTP Concepts]: A property of HTTP methods where making the same request multiple times produces the same result as making it once. GET, PUT, DELETE, and HEAD are ide - [Safe Methods](https://statuscodefyi.com/glossary/safe-methods/) [HTTP Concepts]: HTTP methods that do not modify server state. GET, HEAD, OPTIONS, and TRACE are considered safe methods. Browsers, crawlers, and proxies can freely ma - [Content Negotiation](https://statuscodefyi.com/glossary/content-negotiation/) [HTTP Concepts]: The mechanism by which a client and server agree on the best representation of a resource. The client sends Accept, Accept-Language, Accept-Encoding, - [Status Code](https://statuscodefyi.com/glossary/status-code/) [HTTP Concepts]: A three-digit numeric code in an HTTP response that indicates the outcome of a request. Status codes are grouped into five classes: 1xx (Informational - [Reason Phrase](https://statuscodefyi.com/glossary/reason-phrase/) [HTTP Concepts]: A short textual description that accompanies an HTTP status code in the response status line, such as 'Not Found' for 404 or 'OK' for 200. In HTTP/1.1 - [Media Type (MIME Type)](https://statuscodefyi.com/glossary/media-type/) [HTTP Concepts]: A standardized identifier for the format of data transmitted over the internet. Media types follow the pattern type/subtype (e.g., text/html, applicat - [Response Header](https://statuscodefyi.com/glossary/response-header/) [HTTP Concepts]: Metadata fields sent by the server as part of an HTTP response. Response headers provide information about the server, the response body, caching dire - [Request Method (HTTP Verb)](https://statuscodefyi.com/glossary/request-method/) [HTTP Concepts]: The action a client wants to perform on a resource, specified in the request line. Standard HTTP methods include GET (retrieve), POST (create), PUT (r - [Redirect](https://statuscodefyi.com/glossary/redirect/) [HTTP Concepts]: An HTTP mechanism where the server instructs the client to request a different URL. Redirects use 3xx status codes: 301 (Moved Permanently), 302 (Foun - [Cookie](https://statuscodefyi.com/glossary/cookie/) [HTTP Concepts]: A small piece of data stored by the browser and sent back to the server with every subsequent request. Cookies are set via the Set-Cookie response hea - [Session](https://statuscodefyi.com/glossary/session/) [HTTP Concepts]: A server-side mechanism for maintaining state across multiple HTTP requests from the same client. Since HTTP is stateless, sessions use a unique ident - [HTTP/2](https://statuscodefyi.com/glossary/http2/) [HTTP Concepts]: The second major version of the HTTP protocol, standardized in RFC 7540. HTTP/2 introduced binary framing, multiplexing, header compression (HPACK), s - [HTTP/3](https://statuscodefyi.com/glossary/http3/) [HTTP Concepts]: The third major version of HTTP, standardized in RFC 9114, built on QUIC instead of TCP. QUIC provides built-in encryption (TLS 1.3), zero-RTT connect - [REST (Representational State Transfer)](https://statuscodefyi.com/glossary/rest/) [HTTP Concepts]: An architectural style for designing networked applications. RESTful APIs use standard HTTP methods (GET, POST, PUT, DELETE) to perform CRUD operation - [GraphQL](https://statuscodefyi.com/glossary/graphql/) [HTTP Concepts]: A query language and runtime for APIs developed by Facebook. Unlike REST, which exposes multiple endpoints, GraphQL uses a single endpoint where clien - [Keep-Alive](https://statuscodefyi.com/glossary/keep-alive/) [HTTP Concepts]: A mechanism that reuses a single TCP connection for multiple HTTP requests, reducing latency by avoiding repeated TCP handshakes. - [Chunked Transfer Encoding](https://statuscodefyi.com/glossary/chunked-transfer/) [HTTP Concepts]: An HTTP transfer mechanism where data is sent in chunks, allowing streaming responses without knowing the total content length upfront. - [Conditional Request](https://statuscodefyi.com/glossary/conditional-request/) [HTTP Concepts]: An HTTP request that includes conditions (If-Match, If-None-Match, If-Modified-Since) allowing the server to respond with 304 Not Modified if the reso - [Range Request](https://statuscodefyi.com/glossary/range-request/) [HTTP Concepts]: An HTTP mechanism allowing clients to request only a portion of a resource using the Range header, enabling resumable downloads and video streaming. - [Trailer Headers](https://statuscodefyi.com/glossary/trailer-headers/) [HTTP Concepts]: HTTP headers sent after the message body in chunked transfer encoding, useful for checksums or digital signatures computed during transmission. - [Rate Limiting](https://statuscodefyi.com/glossary/rate-limiting/) [API Design]: A technique to control the number of API requests a client can make within a time window, preventing abuse and ensuring fair resource allocation. - [Pagination](https://statuscodefyi.com/glossary/pagination/) [API Design]: A pattern for dividing large datasets into discrete pages, using cursor-based or offset-based approaches to efficiently retrieve subsets of data. - [API Versioning](https://statuscodefyi.com/glossary/api-versioning/) [API Design]: Strategies for maintaining backward compatibility when evolving APIs, including URL path versioning (/v1/), header versioning, and query parameter ver - [Content-Type](https://statuscodefyi.com/glossary/content-type/) [API Design]: An HTTP header indicating the media type of the request or response body (e.g., application/json, text/html), essential for correct parsing by the rec - [Accept Header](https://statuscodefyi.com/glossary/accept-header/) [API Design]: An HTTP request header that specifies which media types the client can process, enabling server-driven content negotiation. - [HATEOAS](https://statuscodefyi.com/glossary/hateoas/) [API Design]: Hypermedia As The Engine Of Application State — a REST constraint where API responses include links to related resources, enabling clients to discover - [Idempotency Key](https://statuscodefyi.com/glossary/idempotency-key/) [API Design]: A unique identifier sent with API requests to ensure that retried requests don't create duplicate side effects, commonly used in payment APIs. - [Webhook](https://statuscodefyi.com/glossary/webhook/) [API Design]: A user-defined HTTP callback triggered by an event in a source system, pushing real-time notifications to a specified URL instead of requiring polling - [API Gateway](https://statuscodefyi.com/glossary/api-gateway/) [API Design]: A server that acts as a single entry point for API requests, handling routing, rate limiting, authentication, and load balancing before forwarding to - [OpenAPI Specification](https://statuscodefyi.com/glossary/openapi/) [API Design]: A standard format (formerly Swagger) for describing RESTful APIs, including endpoints, request/response schemas, authentication methods, and examples. - [GraphQL Query](https://statuscodefyi.com/glossary/graphql-query/) [API Design]: A read operation in GraphQL that allows clients to request exactly the data fields they need in a single request, avoiding over-fetching and under-fet - [REST Constraints](https://statuscodefyi.com/glossary/rest-constraints/) [API Design]: The six architectural constraints that define REST: client-server, stateless, cacheable, uniform interface, layered system, and code-on-demand (option - [API Throttling](https://statuscodefyi.com/glossary/api-throttling/) [API Design]: A mechanism that limits the processing rate of API requests, queuing or rejecting excess requests to protect backend services from overload. - [Request Body](https://statuscodefyi.com/glossary/request-body/) [API Design]: The data payload sent with HTTP requests (typically POST, PUT, PATCH), containing the resource representation to be created or updated on the server. - [Response Body](https://statuscodefyi.com/glossary/response-body/) [API Design]: The data payload returned by the server in an HTTP response, containing the requested resource, error details, or action results. - [SDK (Software Development Kit)](https://statuscodefyi.com/glossary/sdk/) [API Design]: A collection of libraries, tools, and documentation that simplifies integration with an API or protocol in a specific programming language. SDKs handl - [API Deprecation](https://statuscodefyi.com/glossary/api-deprecation/) [API Design]: The process of marking an API version or endpoint as obsolete, giving consumers a sunset date to migrate before the endpoint is removed. Deprecation i - [Bulk Operations](https://statuscodefyi.com/glossary/bulk-operations/) [API Design]: An API pattern for creating, updating, or deleting multiple resources in a single request to reduce HTTP round trips. A bulk endpoint accepts an array - [Hypermedia](https://statuscodefyi.com/glossary/hypermedia/) [API Design]: The principle of embedding links and controls within API responses so clients discover available actions dynamically rather than hard-coding URLs. Hyp - [JSON:API](https://statuscodefyi.com/glossary/json-api/) [API Design]: A specification for building JSON APIs that standardizes resource structure, relationships, pagination, and error formats to reduce bikeshedding over - [TLS Handshake](https://statuscodefyi.com/glossary/tls-handshake/) [Security]: The process by which a client and server establish a secure, encrypted connection. During the handshake, they negotiate the TLS version, select a ciph - [TLS Certificate (SSL Certificate)](https://statuscodefyi.com/glossary/certificate/) [Security]: A digital document that verifies the identity of a website and enables encrypted connections. Certificates are issued by Certificate Authorities (CAs) - [CORS (Cross-Origin Resource Sharing)](https://statuscodefyi.com/glossary/cors/) [Security]: A browser security mechanism that controls which domains can make requests to a server. By default, browsers block cross-origin requests (same-origin - [Cipher Suite](https://statuscodefyi.com/glossary/cipher-suite/) [Security]: A named combination of cryptographic algorithms used in a TLS connection. A cipher suite specifies the key exchange algorithm, authentication method, - [Authentication](https://statuscodefyi.com/glossary/authentication/) [Security]: The process of verifying the identity of a client or user. HTTP supports multiple authentication schemes: Basic (base64 username:password), Bearer (to - [XSS (Cross-Site Scripting)](https://statuscodefyi.com/glossary/xss/) [Security]: A security vulnerability where attackers inject malicious scripts into web pages viewed by other users. Stored XSS persists in the database, reflected - [CSRF (Cross-Site Request Forgery)](https://statuscodefyi.com/glossary/csrf-attack/) [Security]: An attack that tricks a user's browser into making unintended requests to a website where the user is authenticated. The attacker exploits the browser - [SQL Injection](https://statuscodefyi.com/glossary/sql-injection/) [Security]: A code injection attack that exploits vulnerabilities in database query construction. Attackers insert malicious SQL code through user input fields th - [HSTS (HTTP Strict Transport Security)](https://statuscodefyi.com/glossary/hsts/) [Security]: A security policy mechanism that forces browsers to only connect to a website over HTTPS. The server sends a Strict-Transport-Security response header - [CSP (Content Security Policy)](https://statuscodefyi.com/glossary/csp/) [Security]: An HTTP security header that controls which resources a browser is allowed to load for a given page. CSP mitigates XSS and data injection attacks by s - [OAuth 2.0](https://statuscodefyi.com/glossary/oauth/) [Security]: An authorization framework that enables third-party applications to access a user's resources without exposing their credentials. OAuth 2.0 defines fo - [JWT (JSON Web Token)](https://statuscodefyi.com/glossary/jwt/) [Security]: A compact, URL-safe token format for securely transmitting claims between parties. A JWT consists of three Base64URL-encoded parts: header (algorithm - [API Key](https://statuscodefyi.com/glossary/api-key/) [Security]: A unique identifier used to authenticate API requests, typically passed as a header or query parameter, simpler than OAuth but less secure for user-fa - [Bearer Token](https://statuscodefyi.com/glossary/bearer-token/) [Security]: An opaque access token sent in the Authorization header (Bearer scheme) that grants the holder access to protected resources without additional proof - [Mutual TLS (mTLS)](https://statuscodefyi.com/glossary/mutual-tls/) [Security]: A TLS authentication mode where both client and server present certificates, providing bidirectional identity verification commonly used in microservi - [Rate Limit Bypass](https://statuscodefyi.com/glossary/rate-limit-bypass/) [Security]: Techniques attackers use to circumvent rate limiting, including IP rotation, header manipulation, and distributed requests, and countermeasures to pre - [Same-Origin Policy](https://statuscodefyi.com/glossary/same-origin-policy/) [Security]: A browser security mechanism that restricts scripts from one origin from accessing resources of another origin, forming the foundation that CORS selec - [Clickjacking](https://statuscodefyi.com/glossary/clickjacking/) [Security]: An attack that tricks users into clicking hidden elements by overlaying a transparent iframe over a legitimate page, causing unintended actions such a - [SSRF (Server-Side Request Forgery)](https://statuscodefyi.com/glossary/ssrf/) [Security]: An attack where an attacker manipulates a server into issuing HTTP requests to internal resources — such as cloud metadata endpoints, internal APIs, o - [Security Headers](https://statuscodefyi.com/glossary/security-headers/) [Security]: A collection of HTTP response headers that harden web application security by instructing browsers how to behave when handling content. Key headers in - [TCP/IP](https://statuscodefyi.com/glossary/tcp-ip/) [Networking]: The foundational protocol suite of the internet. TCP (Transmission Control Protocol) provides reliable, ordered delivery of data between applications. - [Three-Way Handshake](https://statuscodefyi.com/glossary/three-way-handshake/) [Networking]: The process used to establish a TCP connection between client and server. The client sends a SYN packet, the server responds with SYN-ACK, and the cli - [Port Number](https://statuscodefyi.com/glossary/port/) [Networking]: A 16-bit number (0-65535) that identifies a specific service or process on a networked host. Well-known ports include 80 (HTTP), 443 (HTTPS), 25 (SMTP - [Latency](https://statuscodefyi.com/glossary/latency/) [Networking]: The time delay between sending a request and receiving a response. Network latency is measured in milliseconds and affected by physical distance, rout - [Timeout](https://statuscodefyi.com/glossary/timeout/) [Networking]: A limit on how long a client or server will wait for a response before giving up. Connection timeouts occur when a TCP connection cannot be establishe - [Load Balancer](https://statuscodefyi.com/glossary/load-balancer/) [Networking]: A device or software that distributes incoming network traffic across multiple backend servers. Load balancers improve availability, reliability, and - [Reverse Proxy](https://statuscodefyi.com/glossary/reverse-proxy/) [Networking]: A server that sits between clients and backend servers, forwarding client requests and returning server responses. Reverse proxies provide SSL termina - [NAT (Network Address Translation)](https://statuscodefyi.com/glossary/nat/) [Networking]: A technique that maps private IP addresses to a public IP address, allowing multiple devices on a local network to share a single public IP. NAT is pe - [Firewall](https://statuscodefyi.com/glossary/firewall/) [Networking]: A network security system that monitors and controls incoming and outgoing traffic based on predetermined rules. Firewalls operate at different layers - [Proxy Server (Forward Proxy)](https://statuscodefyi.com/glossary/proxy-server/) [Networking]: An intermediary server that sits between a client and the internet, forwarding client requests on their behalf. Forward proxies provide anonymity, con - [DNS over HTTPS (DoH)](https://statuscodefyi.com/glossary/dns-over-https/) [Networking]: A protocol for performing DNS resolution via the HTTPS protocol. DoH encrypts DNS queries and responses, preventing eavesdropping and manipulation by - [IPv6](https://statuscodefyi.com/glossary/ipv6/) [Networking]: The latest version of the Internet Protocol, designed to replace IPv4. IPv6 uses 128-bit addresses (e.g., 2001:0db8::1) compared to IPv4's 32-bit addr - [Subnet (Subnetwork)](https://statuscodefyi.com/glossary/subnet/) [Networking]: A logical subdivision of an IP network created by dividing the IP address into network and host portions using a subnet mask. For example, 192.168.1.0 - [Connection Pooling](https://statuscodefyi.com/glossary/connection-pooling/) [Networking]: Reusing existing network connections instead of creating new ones for each request, significantly reducing latency and resource consumption in high-tr - [TCP Reset (RST)](https://statuscodefyi.com/glossary/tcp-reset/) [Networking]: A TCP segment that abruptly terminates a connection without the normal four-way handshake, often caused by firewalls, port scanners, or application er - [MTU (Maximum Transmission Unit)](https://statuscodefyi.com/glossary/mtu/) [Networking]: The largest packet size that can be transmitted over a network link without fragmentation, typically 1500 bytes for Ethernet. - [Socket](https://statuscodefyi.com/glossary/socket/) [Networking]: A software endpoint for bidirectional network communication, identified by an IP address and port number, forming the foundation of all network protoc - [UDP (User Datagram Protocol)](https://statuscodefyi.com/glossary/udp/) [Networking]: A connectionless transport protocol that sends discrete datagrams without establishing a session, guaranteeing neither ordering nor delivery. Because - [Bandwidth](https://statuscodefyi.com/glossary/bandwidth/) [Networking]: The maximum rate of data transfer across a network path, measured in bits per second (bps, Mbps, Gbps). Bandwidth is the capacity of the pipe, while t - [DNS Lookup](https://statuscodefyi.com/glossary/dns-lookup/) [Networking]: The process of querying DNS servers to translate a hostname into an IP address before a TCP connection can be established. A full recursive lookup tra - [Cache-Control](https://statuscodefyi.com/glossary/cache-control/) [Caching & Performance]: An HTTP header that specifies caching directives for requests and responses. Common directives include max-age (seconds until stale), no-cache (must r - [ETag (Entity Tag)](https://statuscodefyi.com/glossary/etag/) [Caching & Performance]: An HTTP response header that provides a unique identifier for a specific version of a resource. Clients can send the ETag back in an If-None-Match hea - [CDN (Content Delivery Network)](https://statuscodefyi.com/glossary/cdn/) [Caching & Performance]: A globally distributed network of servers that caches and delivers content from locations geographically close to users. CDNs reduce latency, improve - [Stale-While-Revalidate](https://statuscodefyi.com/glossary/stale-while-revalidate/) [Caching & Performance]: A Cache-Control extension directive (defined in RFC 5861) that allows a cache to serve a stale response while asynchronously revalidating it in the ba - [Cache Invalidation](https://statuscodefyi.com/glossary/cache-invalidation/) [Caching & Performance]: The process of removing or marking cached data as stale when the underlying content changes. Cache invalidation is notoriously difficult ('There are o - [Edge Caching](https://statuscodefyi.com/glossary/edge-caching/) [Caching & Performance]: The practice of storing cached content at edge servers located geographically close to end users, rather than at the origin server. Edge caches are th - [Last-Modified](https://statuscodefyi.com/glossary/last-modified/) [Caching & Performance]: An HTTP response header indicating when the resource was last changed, used with If-Modified-Since for conditional requests and cache validation. - [Vary Header](https://statuscodefyi.com/glossary/vary-header/) [Caching & Performance]: An HTTP response header that tells caches which request headers affect the response, ensuring that different versions of a resource are cached separat - [Cache Busting](https://statuscodefyi.com/glossary/cache-busting/) [Caching & Performance]: Techniques for forcing browsers and CDNs to fetch fresh content by changing URLs (adding query strings, content hashes, or version numbers to file nam - [HTTP Compression](https://statuscodefyi.com/glossary/http-compression/) [Caching & Performance]: Reducing response size using algorithms like gzip, Brotli, or zstd, configured via Accept-Encoding and Content-Encoding headers. - [Prefetching](https://statuscodefyi.com/glossary/prefetch/) [Caching & Performance]: A browser optimization that proactively fetches resources likely needed for future navigations, using link rel="prefetch" or the Link header. - [Time to First Byte (TTFB)](https://statuscodefyi.com/glossary/ttfb/) [Caching & Performance]: The time between sending an HTTP request and receiving the first byte of the response, a key performance metric indicating server processing speed. - [Connection Reuse](https://statuscodefyi.com/glossary/connection-reuse/) [Caching & Performance]: Sending multiple HTTP requests over a single TCP connection (HTTP/1.1 keep-alive or HTTP/2 multiplexing) to avoid the overhead of repeated connection - [Brotli Compression](https://statuscodefyi.com/glossary/brotli/) [Caching & Performance]: A modern lossless compression algorithm developed by Google that achieves 15–25% better compression ratios than gzip for text-based web resources such - [HTTP Caching Layers](https://statuscodefyi.com/glossary/http-caching-layers/) [Caching & Performance]: The hierarchy of caches a response passes through between the origin server and the end user's browser. Layers include the browser cache, service work - [Cache Hit Ratio](https://statuscodefyi.com/glossary/cache-hit-ratio/) [Caching & Performance]: The percentage of requests served directly from cache without contacting the origin server, expressed as hits / (hits + misses). A high cache hit rati - [Push Cache (HTTP/2)](https://statuscodefyi.com/glossary/push-cache/) [Caching & Performance]: The least-persistent caching layer in HTTP/2, storing server-pushed resources only for the lifetime of a single HTTP/2 connection. Unlike the browser - [Immutable Response](https://statuscodefyi.com/glossary/immutable-response/) [Caching & Performance]: A Cache-Control extension (Cache-Control: immutable) that tells browsers the response body will never change for the duration of its max-age, allowing - [Service Worker Cache](https://statuscodefyi.com/glossary/service-worker-cache/) [Caching & Performance]: A programmable JavaScript cache layer that runs in the browser background and intercepts network requests via fetch event handlers. Service workers en - [103 Early Hints](https://statuscodefyi.com/glossary/103-early-hints/) [Caching & Performance]: An informational HTTP status code that allows the server to send preliminary Link response headers before the final response is ready, enabling browse - [Retry Strategy](https://statuscodefyi.com/glossary/retry-strategy/) [Error Handling]: A pattern for automatically retrying failed requests with techniques like exponential backoff, jitter, and maximum retry limits to handle transient fa - [Circuit Breaker](https://statuscodefyi.com/glossary/circuit-breaker/) [Error Handling]: A resilience pattern that prevents cascading failures by stopping requests to a failing service after a threshold of errors, allowing it time to recov - [Graceful Degradation](https://statuscodefyi.com/glossary/graceful-degradation/) [Error Handling]: A design approach where a system continues operating with reduced functionality when components fail, rather than failing completely. - [Fallback](https://statuscodefyi.com/glossary/fallback/) [Error Handling]: An alternative response or behavior used when the primary service or resource is unavailable, such as serving cached data or a default value. - [Error Boundary](https://statuscodefyi.com/glossary/error-boundary/) [Error Handling]: A mechanism for catching and handling errors at a specific level in a system, preventing unhandled errors from propagating and crashing the entire app - [Dead Letter Queue](https://statuscodefyi.com/glossary/dead-letter-queue/) [Error Handling]: A queue where messages that cannot be processed are stored for later analysis, commonly used in asynchronous systems to handle persistent failures. - [Health Check](https://statuscodefyi.com/glossary/health-check/) [Error Handling]: An endpoint or mechanism that reports the operational status of a service, used by load balancers and orchestrators to route traffic away from unhealt - [Timeout Budget](https://statuscodefyi.com/glossary/timeout-budget/) [Error Handling]: A pattern where the total time allowed for a request is divided among sub-operations, ensuring that no single step can exhaust the entire time budget. - [Backpressure](https://statuscodefyi.com/glossary/backpressure/) [Error Handling]: A flow control mechanism where a consumer signals to a producer to slow down when it cannot keep up with the rate of incoming data. - [Problem Details (RFC 9457)](https://statuscodefyi.com/glossary/problem-details/) [Error Handling]: A standardized JSON format for representing HTTP API errors, including type, title, status, detail, and instance fields for consistent error communica - [Exponential Backoff](https://statuscodefyi.com/glossary/exponential-backoff/) [Error Handling]: A retry timing strategy where the wait interval doubles after each consecutive failure — for example, 1 s, 2 s, 4 s, 8 s — capping at a configured max - [Jitter](https://statuscodefyi.com/glossary/jitter/) [Error Handling]: Random variation added to retry delays to prevent multiple clients from retrying simultaneously after a shared failure event. Without jitter, all clie - [Thundering Herd](https://statuscodefyi.com/glossary/thundering-herd/) [Error Handling]: A problem where a large number of clients simultaneously retry or reconnect after a shared failure, overwhelming the server that has just begun recove - [Bulkhead Pattern](https://statuscodefyi.com/glossary/bulkhead-pattern/) [Error Handling]: A resilience pattern borrowed from ship design that partitions system resources — thread pools, connection pools, semaphores — into isolated compartme - [Error Code Mapping](https://statuscodefyi.com/glossary/error-code-mapping/) [Error Handling]: The practice of translating internal application error codes or exception types to the appropriate protocol-level status codes — for example, mapping - [Idempotent Retry](https://statuscodefyi.com/glossary/idempotent-retry/) [Error Handling]: The practice of safely retrying a failed request because the underlying operation is idempotent — producing the same result regardless of how many tim - [Error Rate](https://statuscodefyi.com/glossary/error-rate/) [Error Handling]: The percentage of requests that result in error responses (HTTP 4xx or 5xx, gRPC non-OK status) over a given time window, expressed as error_count / t - [Poison Message](https://statuscodefyi.com/glossary/poison-message/) [Error Handling]: A message in a queue that repeatedly causes processing failures, blocking forward progress and consuming retry budget. Poison messages arise from malf - [Fail Open / Fail Closed](https://statuscodefyi.com/glossary/fail-open-fail-closed/) [Error Handling]: Two opposing failure modes for a system check or gate. Fail open allows requests through when the check itself is unavailable, prioritizing availabili - [Retry-After Header](https://statuscodefyi.com/glossary/retry-after-header/) [Error Handling]: An HTTP response header that tells the client how long to wait before issuing another request. It is sent with 429 Too Many Requests to indicate rate - [Request-Response Model](https://statuscodefyi.com/glossary/request-response/) [Protocol Design]: A communication pattern where a client sends a request and waits for a server to send back a response. HTTP, SMTP, FTP, SIP, and DNS all follow this m - [RFC (Request for Comments)](https://statuscodefyi.com/glossary/rfc/) [Protocol Design]: A formal document published by the IETF (Internet Engineering Task Force) that defines internet standards, protocols, and best practices. Key RFCs inc - [IANA (Internet Assigned Numbers Authority)](https://statuscodefyi.com/glossary/iana/) [Protocol Design]: The organization responsible for maintaining global registries of internet protocol parameters, including HTTP status codes, media types, port numbers - [Protocol Upgrade](https://statuscodefyi.com/glossary/protocol-upgrade/) [Protocol Design]: The process of switching from one protocol to another over an existing connection. HTTP/1.1's Upgrade header enables switching to WebSocket (101 Switc - [Multiplexing](https://statuscodefyi.com/glossary/multiplexing/) [Protocol Design]: The ability to send multiple requests and responses simultaneously over a single connection. HTTP/2 introduced multiplexing via streams, eliminating h - [Backward Compatibility](https://statuscodefyi.com/glossary/backward-compatibility/) [Protocol Design]: The ability of a newer version of a protocol or system to work with older versions without breaking existing functionality. HTTP/2 maintains backward - [Protocol Buffers (Protobuf)](https://statuscodefyi.com/glossary/protocol-buffer/) [Protocol Design]: A language-neutral, platform-neutral binary serialization format developed by Google. Protocol Buffers define message schemas in .proto files, which a - [Serialization](https://statuscodefyi.com/glossary/serialization/) [Protocol Design]: The process of converting structured data into a format suitable for transmission or storage. Common serialization formats include JSON (text-based, h - [Protocol Negotiation](https://statuscodefyi.com/glossary/protocol-negotiation/) [Protocol Design]: The process by which client and server agree on which protocol version or features to use for communication, such as ALPN for HTTP/2 or TLS version ne - [Message Framing](https://statuscodefyi.com/glossary/message-framing/) [Protocol Design]: The method of delimiting individual messages within a stream of bytes, using techniques like length-prefixed frames (gRPC), delimiters (SMTP CRLF.CRLF - [Wire Format](https://statuscodefyi.com/glossary/wire-format/) [Protocol Design]: The exact byte-level representation of data as transmitted over the network, including encoding, byte ordering, and field layout specified by protocol - [Extension Mechanism](https://statuscodefyi.com/glossary/extension-mechanism/) [Protocol Design]: A protocol feature that allows adding new capabilities without breaking existing implementations, such as HTTP headers, gRPC metadata, or DNS EDNS(0). - [Connection-Oriented](https://statuscodefyi.com/glossary/connection-oriented/) [Protocol Design]: A communication model that requires establishing a persistent logical connection between sender and receiver before any application data is exchanged. - [Connectionless](https://statuscodefyi.com/glossary/connectionless/) [Protocol Design]: A communication model that sends self-contained datagrams without establishing a prior connection or maintaining shared state between sender and recei - [Half-Duplex](https://statuscodefyi.com/glossary/half-duplex/) [Protocol Design]: A communication mode where data can flow in both directions between two endpoints, but only in one direction at a time. Classic HTTP/1.1 request-respo - [Full-Duplex](https://statuscodefyi.com/glossary/full-duplex/) [Protocol Design]: A communication mode where data flows simultaneously in both directions between two endpoints without taking turns. WebSocket connections are full-dup - [ALPN (Application-Layer Protocol Negotiation)](https://statuscodefyi.com/glossary/alpn/) [Protocol Design]: A TLS extension (RFC 7301) that allows the client and server to negotiate which application protocol they will use during the TLS handshake, before an - [Text vs Binary Protocol](https://statuscodefyi.com/glossary/text-vs-binary-protocol/) [Protocol Design]: The distinction between protocols that encode messages as human-readable ASCII or UTF-8 text versus those that encode messages as compact binary data. - [Head-of-Line Blocking](https://statuscodefyi.com/glossary/head-of-line-blocking/) [Protocol Design]: A performance problem where a slow or lost packet blocks the processing of all subsequent packets in the same ordered stream, even if those later pack - [QUIC](https://statuscodefyi.com/glossary/quic/) [Protocol Design]: A UDP-based transport protocol developed by Google and standardized in RFC 9000 that combines TLS 1.3 encryption with multiplexed streams, forming the - [DNS Resolution](https://statuscodefyi.com/glossary/dns-resolution/) [DNS]: The process of translating a human-readable domain name (like example.com) into an IP address. DNS resolution involves recursive queries through a hie - [DNS Record Types](https://statuscodefyi.com/glossary/dns-record-types/) [DNS]: Different categories of data stored in the Domain Name System. Common types include A (IPv4 address), AAAA (IPv6 address), CNAME (canonical name alias - [TTL (Time to Live)](https://statuscodefyi.com/glossary/ttl/) [DNS]: In DNS, the duration in seconds that a DNS record can be cached by resolvers before they must re-query the authoritative nameserver. Lower TTLs mean f - [DNSSEC (DNS Security Extensions)](https://statuscodefyi.com/glossary/dnssec/) [DNS]: A suite of extensions to DNS that adds cryptographic authentication to DNS responses. DNSSEC uses digital signatures to verify that DNS records have n - [DNS over TLS (DoT)](https://statuscodefyi.com/glossary/dns-over-tls/) [DNS]: A protocol for encrypting DNS queries using TLS on a dedicated port (853). Unlike DNS over HTTPS, which uses port 443 and blends with web traffic, DoT - [Anycast](https://statuscodefyi.com/glossary/anycast/) [DNS]: A network addressing method where the same IP address is announced from multiple locations worldwide. Routers direct traffic to the nearest (topologic - [Recursive Resolver](https://statuscodefyi.com/glossary/recursive-resolver/) [DNS]: A DNS server that performs the full resolution process on behalf of the client, querying root, TLD, and authoritative name servers to find the final a - [Authoritative Name Server](https://statuscodefyi.com/glossary/authoritative-nameserver/) [DNS]: A DNS server that holds the actual DNS records for a domain and provides definitive answers, as opposed to recursive resolvers that cache and forward - [Glue Record](https://statuscodefyi.com/glossary/glue-record/) [DNS]: An A or AAAA record provided by a parent zone to break circular dependencies when a name server's hostname is within the domain it serves. - [EDNS(0)](https://statuscodefyi.com/glossary/edns/) [DNS]: Extension Mechanisms for DNS — a backward-compatible extension that allows larger UDP messages, additional flags, and option codes beyond the original - [DNS Zone](https://statuscodefyi.com/glossary/dns-zone/) [DNS]: A portion of the DNS namespace managed by a specific organization, defined by a zone file containing all records for that delegated domain. A zone has - [DNS Propagation](https://statuscodefyi.com/glossary/dns-propagation/) [DNS]: The time it takes for DNS record changes to spread across all recursive resolvers worldwide, typically governed by TTL values on the previous records. - [CNAME Record](https://statuscodefyi.com/glossary/cname-record/) [DNS]: A DNS record that maps an alias domain name to a canonical (true) domain name, creating a redirect at the DNS level. Resolvers follow the chain until - [MX Record](https://statuscodefyi.com/glossary/mx-record/) [DNS]: A DNS record specifying the mail server responsible for receiving email for a domain, with priority values for failover ordering. Lower priority numbe - [SRV Record](https://statuscodefyi.com/glossary/srv-record/) [DNS]: A DNS record that specifies the hostname, port, priority, and weight for a specific service, used by SIP, XMPP, and service discovery systems. The for - [PTR Record](https://statuscodefyi.com/glossary/ptr-record/) [DNS]: A DNS record that maps an IP address to a domain name (reverse DNS), stored under the in-addr.arpa or ip6.arpa zones. PTR records are used for email a - [DNS Cache Poisoning](https://statuscodefyi.com/glossary/dns-cache-poisoning/) [DNS]: An attack that injects forged DNS records into a resolver's cache, redirecting users to malicious servers without their knowledge. Attackers exploit t - [DNS Root Server](https://statuscodefyi.com/glossary/root-server/) [DNS]: One of 13 root server clusters (A through M) that form the top of the DNS hierarchy and direct queries to TLD nameservers such as .com or .org. Despit - [Split-Horizon DNS](https://statuscodefyi.com/glossary/split-horizon-dns/) [DNS]: A DNS configuration that returns different answers based on the source of the query, typically used to serve internal IP addresses to corporate networ - [Wildcard DNS Record](https://statuscodefyi.com/glossary/wildcard-dns/) [DNS]: A DNS record using an asterisk (*) that matches any subdomain not explicitly defined, commonly used for catch-all routing in multi-tenant SaaS platfor - [SMTP Relay](https://statuscodefyi.com/glossary/smtp-relay/) [Email & SMTP]: The process of forwarding email from one SMTP server to another. When you send an email, your mail client submits it to an outgoing SMTP server (port - [SPF, DKIM, and DMARC](https://statuscodefyi.com/glossary/spf-dkim-dmarc/) [Email & SMTP]: Three email authentication standards that prevent spoofing and phishing. SPF (Sender Policy Framework) specifies which servers can send email for a do - [Bounce Rate (Email)](https://statuscodefyi.com/glossary/bounce-rate/) [Email & SMTP]: The percentage of sent emails that are returned undelivered. Hard bounces indicate permanent delivery failures (e.g., SMTP 550 — mailbox not found, in - [Email Deliverability](https://statuscodefyi.com/glossary/email-deliverability/) [Email & SMTP]: The ability of an email to reach the recipient's inbox rather than being blocked, bounced, or filtered to spam. Deliverability depends on sender reput - [MTA (Mail Transfer Agent)](https://statuscodefyi.com/glossary/mta/) [Email & SMTP]: Software responsible for transferring email messages between servers using SMTP. An MTA receives email from a mail client (MUA) or another MTA, perfor - [SMTP Pipelining](https://statuscodefyi.com/glossary/smtp-pipeline/) [Email & SMTP]: An SMTP extension (ESMTP PIPELINING) that allows sending multiple commands without waiting for each response, improving throughput for bulk email deli - [Greylisting](https://statuscodefyi.com/glossary/greylisting/) [Email & SMTP]: An anti-spam technique where the mail server temporarily rejects email from unknown senders with a 4xx code, expecting legitimate servers to retry whi - [Backscatter](https://statuscodefyi.com/glossary/backscatter/) [Email & SMTP]: Misdirected bounce messages sent to forged sender addresses, a form of spam caused by accepting then bouncing messages with spoofed From headers. - [Envelope vs Header](https://statuscodefyi.com/glossary/envelope-vs-header/) [Email & SMTP]: The distinction between SMTP envelope addresses (MAIL FROM/RCPT TO) used for routing, and email headers (From/To) displayed to users — they can differ - [STARTTLS](https://statuscodefyi.com/glossary/starttls/) [Email & SMTP]: An SMTP extension that upgrades an existing plaintext connection to encrypted TLS, different from implicit TLS which starts encrypted from the beginni - [SMTP Authentication (SMTP AUTH)](https://statuscodefyi.com/glossary/smtp-auth/) [Email & SMTP]: An SMTP extension (RFC 4954) requiring clients to authenticate with credentials before the server will relay their messages, preventing open relay abu - [Open Relay](https://statuscodefyi.com/glossary/open-relay/) [Email & SMTP]: An SMTP server that forwards email from any sender to any recipient without authentication, commonly exploited by spammers to send bulk mail while hid - [Email Queue](https://statuscodefyi.com/glossary/email-queue/) [Email & SMTP]: A temporary storage mechanism where the MTA holds outbound messages for delivery, retrying on temporary failures indicated by 4xx SMTP reply codes. Me - [MIME Multipart](https://statuscodefyi.com/glossary/mime-multipart/) [Email & SMTP]: The format for email messages containing multiple content types (text, HTML, attachments) using boundary-delimited parts defined by MIME (RFC 2045). T - [ARC (Authenticated Received Chain)](https://statuscodefyi.com/glossary/arc/) [Email & SMTP]: An email authentication protocol (RFC 8617) that preserves SPF, DKIM, and DMARC validation results across forwarding hops, solving the forwarding auth - [BIMI (Brand Indicators for Message Identification)](https://statuscodefyi.com/glossary/bimi/) [Email & SMTP]: An email standard that displays a brand's verified logo next to authenticated messages in the recipient's inbox, requiring DMARC enforcement at the p= - [DSN (Delivery Status Notification)](https://statuscodefyi.com/glossary/dsn/) [Email & SMTP]: An SMTP extension (RFC 3461) that provides structured delivery reports — success, failure, or delay — back to the original sender. The MAIL FROM comma - [Email Header Injection](https://statuscodefyi.com/glossary/email-header-injection/) [Email & SMTP]: An attack that exploits improperly sanitized user input to inject additional email headers (BCC, CC, Subject, To) into messages generated by a web app - [SMTP Enhanced Status Codes](https://statuscodefyi.com/glossary/smtp-enhanced-status/) [Email & SMTP]: Three-digit dotted codes (e.g., 5.1.1) defined in RFC 3463 that provide more specific diagnostic information than basic three-digit SMTP reply codes. - [MDN (Message Disposition Notification)](https://statuscodefyi.com/glossary/mdn/) [Email & SMTP]: An email mechanism (RFC 8098) where the recipient's mail client sends a notification back to the sender confirming the message was displayed, processe - [WebSocket Frame](https://statuscodefyi.com/glossary/websocket-frame/) [Real-Time & Streaming]: The basic unit of communication in the WebSocket protocol. After the initial HTTP upgrade handshake, data is exchanged as frames rather than HTTP mess - [gRPC Streaming](https://statuscodefyi.com/glossary/grpc-streaming/) [Real-Time & Streaming]: gRPC supports four communication patterns: unary (single request, single response), server streaming (single request, multiple responses), client stre - [SIP Dialog](https://statuscodefyi.com/glossary/sip-dialog/) [Real-Time & Streaming]: A peer-to-peer SIP relationship between two user agents that persists for the duration of a call or session. A dialog is established by a successful I - [WebRTC (Web Real-Time Communication)](https://statuscodefyi.com/glossary/webrtc/) [Real-Time & Streaming]: An open framework that enables real-time audio, video, and data communication directly between browsers without plugins. WebRTC uses ICE (Interactive - [Signaling](https://statuscodefyi.com/glossary/signaling/) [Real-Time & Streaming]: The process of coordinating communication between peers before establishing a direct connection. In WebRTC, signaling involves exchanging session desc - [ICE Candidate](https://statuscodefyi.com/glossary/ice-candidate/) [Real-Time & Streaming]: A network address and port combination that a peer can use for communication in the ICE (Interactive Connectivity Establishment) protocol. ICE gathers - [Server-Sent Events (SSE)](https://statuscodefyi.com/glossary/server-sent-events/) [Real-Time & Streaming]: A unidirectional HTTP-based push technology where the server sends events to the client over a single long-lived connection, simpler than WebSocket fo - [Long Polling](https://statuscodefyi.com/glossary/long-polling/) [Real-Time & Streaming]: A technique where the client makes an HTTP request and the server holds it open until new data is available, simulating real-time push over standard H - [WebSocket Subprotocol](https://statuscodefyi.com/glossary/websocket-subprotocol/) [Real-Time & Streaming]: An application-level protocol negotiated during the WebSocket handshake (Sec-WebSocket-Protocol header) that defines the message format and semantics - [gRPC Metadata](https://statuscodefyi.com/glossary/grpc-metadata/) [Real-Time & Streaming]: Key-value pairs sent as headers/trailers in gRPC calls, used for authentication tokens, tracing IDs, and custom application data separate from the pro - [SIP Registrar](https://statuscodefyi.com/glossary/sip-registrar/) [Real-Time & Streaming]: A SIP server that accepts REGISTER requests and updates the location service database, enabling the SIP network to route incoming calls to the correct - [WebSocket Ping/Pong](https://statuscodefyi.com/glossary/websocket-ping-pong/) [Real-Time & Streaming]: A WebSocket heartbeat mechanism where Ping control frames are sent to check connection liveness, and the recipient must respond with a Pong frame. - [Event Stream](https://statuscodefyi.com/glossary/event-stream/) [Real-Time & Streaming]: The MIME type text/event-stream used by Server-Sent Events, defining a line-based text format with named fields: event (event type), data (payload), i - [WebSocket Close Code](https://statuscodefyi.com/glossary/websocket-close-code/) [Real-Time & Streaming]: A numeric code sent in a WebSocket Close frame indicating the reason for disconnection, defined in RFC 6455. Common codes include 1000 (normal closure - [STUN (Session Traversal Utilities for NAT)](https://statuscodefyi.com/glossary/stun/) [Real-Time & Streaming]: A protocol (RFC 8489) that helps clients behind NAT discover their public IP address and port by querying a STUN server on the public internet. The se - [TURN (Traversal Using Relays around NAT)](https://statuscodefyi.com/glossary/turn/) [Real-Time & Streaming]: A relay protocol (RFC 8656) used when direct peer-to-peer connections fail due to symmetric NAT or restrictive firewalls, routing all media through a - [SDP (Session Description Protocol)](https://statuscodefyi.com/glossary/sdp/) [Real-Time & Streaming]: A text format (RFC 8866) for describing multimedia session parameters — codecs, IP addresses, ports, bandwidth, and encryption keys — exchanged during - [MQTT (Message Queuing Telemetry Transport)](https://statuscodefyi.com/glossary/mqtt/) [Real-Time & Streaming]: A lightweight publish/subscribe messaging protocol (ISO/IEC 20922) designed for IoT and constrained networks, running over TCP with minimal overhead. - [STOMP (Simple Text Oriented Messaging Protocol)](https://statuscodefyi.com/glossary/stomp/) [Real-Time & Streaming]: A text-based messaging protocol commonly used over WebSocket for connecting browsers to message brokers like RabbitMQ and ActiveMQ. STOMP defines a sm - [gRPC-Web](https://statuscodefyi.com/glossary/grpc-web/) [Real-Time & Streaming]: A protocol adaptation that enables browser JavaScript clients to communicate with gRPC backends via an HTTP/1.1 or HTTP/2 proxy translation layer, sin - [Service Mesh](https://statuscodefyi.com/glossary/service-mesh/) [Infrastructure]: A dedicated infrastructure layer for managing service-to-service communication in microservices, handling load balancing, encryption, observability, a - [Sidecar Proxy](https://statuscodefyi.com/glossary/sidecar-proxy/) [Infrastructure]: A proxy deployed alongside each microservice instance to intercept and manage network traffic, implementing service mesh functionality transparently. - [Ingress Controller](https://statuscodefyi.com/glossary/ingress-controller/) [Infrastructure]: A reverse proxy that manages external access to services in a Kubernetes cluster, handling TLS termination, path-based routing, and virtual hosting. - [Service Discovery](https://statuscodefyi.com/glossary/service-discovery/) [Infrastructure]: A mechanism for automatically detecting and locating services in a distributed system, using DNS-based (SRV records) or registry-based (Consul, etcd) - [Load Balancing Algorithm](https://statuscodefyi.com/glossary/load-balancing-algorithm/) [Infrastructure]: Strategies for distributing traffic across backend servers: round-robin, least-connections, weighted, IP-hash, and consistent hashing, each with diffe - [Blue-Green Deployment](https://statuscodefyi.com/glossary/blue-green-deployment/) [Infrastructure]: A deployment strategy using two identical production environments (blue and green) where traffic is switched from the old version to the new version a - [Canary Deployment](https://statuscodefyi.com/glossary/canary-deployment/) [Infrastructure]: A deployment strategy where a new version is gradually rolled out to a small percentage of traffic before full deployment, allowing early detection of - [Health Check Endpoint](https://statuscodefyi.com/glossary/health-check-endpoint/) [Infrastructure]: A dedicated API endpoint (typically /health or /healthz) that returns the operational status of a service, used by load balancers and container orches - [TLS Termination](https://statuscodefyi.com/glossary/tls-termination/) [Infrastructure]: The practice of decrypting TLS traffic at a load balancer or reverse proxy, forwarding unencrypted HTTP to backend services, reducing their computatio - [Connection Draining](https://statuscodefyi.com/glossary/connection-draining/) [Infrastructure]: A graceful shutdown technique where a server stops accepting new connections but continues processing existing requests before terminating. - [Distributed Tracing](https://statuscodefyi.com/glossary/distributed-tracing/) [Infrastructure]: A method of tracking requests as they propagate through multiple microservices, using trace IDs (W3C Trace Context, OpenTelemetry) to correlate logs a - [Observability](https://statuscodefyi.com/glossary/observability/) [Infrastructure]: The ability to understand a system's internal state from its external outputs (logs, metrics, traces), essential for diagnosing issues in distributed - [Correlation ID](https://statuscodefyi.com/glossary/correlation-id/) [Infrastructure]: A unique identifier (commonly X-Request-ID or X-Correlation-ID) propagated through all services handling a single request, enabling end-to-end tracing - [Rate Limiter Algorithm](https://statuscodefyi.com/glossary/rate-limiter-algorithm/) [Infrastructure]: Backend algorithms for enforcing API rate limits, each with different trade-offs in burst tolerance and fairness. Token bucket accumulates tokens at a - [Container Orchestration](https://statuscodefyi.com/glossary/container-orchestration/) [Infrastructure]: Automated management of containerized application deployment, scaling, networking, and health — primarily through Kubernetes, with alternatives like D - [Rolling Update](https://statuscodefyi.com/glossary/rolling-update/) [Infrastructure]: A deployment strategy that replaces application instances one at a time (or in small batches), maintaining availability throughout by ensuring a minim - [Readiness Probe](https://statuscodefyi.com/glossary/readiness-probe/) [Infrastructure]: A Kubernetes health check that determines whether a pod is ready to receive traffic, distinct from liveness probes that check if the pod is alive. Whi - [SLA / SLO / SLI](https://statuscodefyi.com/glossary/sla-slo-sli/) [Infrastructure]: The three layers of reliability engineering measurement. SLI (Service Level Indicator) is the measured metric, such as request success rate or latency - [Configuration Management](https://statuscodefyi.com/glossary/configuration-management/) [Infrastructure]: The practice of managing application settings across environments using environment variables, config servers, or feature flag systems to avoid hardco - [Zero-Downtime Deployment](https://statuscodefyi.com/glossary/zero-downtime-deployment/) [Infrastructure]: Deployment strategies that ensure the application remains fully available throughout the update process, combining health checks, connection draining, - [HTTP Semantics (RFC 9110)](https://statuscodefyi.com/glossary/http-semantics/) [Web Standards]: The core RFC defining HTTP's architecture, including methods, status codes, headers, content negotiation, and conditional requests, independent of the - [URI (Uniform Resource Identifier)](https://statuscodefyi.com/glossary/uri/) [Web Standards]: A string that identifies a resource by location (URL) or name (URN), forming the addressing foundation of HTTP and the web. - [MIME Type](https://statuscodefyi.com/glossary/mime-type/) [Web Standards]: A two-part identifier (type/subtype) that indicates the nature and format of a document, file, or byte stream, standardized by IANA. - [CORS Preflight](https://statuscodefyi.com/glossary/cors-preflight/) [Web Standards]: An automatic OPTIONS request sent by browsers before cross-origin requests with non-simple methods or headers, checking if the server allows the actua - [Content-Disposition](https://statuscodefyi.com/glossary/content-disposition/) [Web Standards]: An HTTP header indicating whether content should be displayed inline or downloaded as an attachment, and suggesting a filename for downloads. - [Transfer-Encoding](https://statuscodefyi.com/glossary/transfer-encoding/) [Web Standards]: An HTTP header specifying the encoding applied to the message body for transfer (chunked, compress, deflate, gzip), distinct from Content-Encoding. - [Link Header](https://statuscodefyi.com/glossary/link-header/) [Web Standards]: An HTTP header (RFC 8288) for conveying typed relationships between resources, used for pagination (rel=next), preloading (rel=preload), and API disco - [Origin](https://statuscodefyi.com/glossary/origin/) [Web Standards]: A combination of scheme, host, and port that defines a security boundary in web browsers, central to the Same-Origin Policy and CORS. - [Referrer-Policy](https://statuscodefyi.com/glossary/referrer-policy/) [Web Standards]: An HTTP header controlling how much referrer information is sent with requests, balancing analytics needs against user privacy. - [Permissions Policy](https://statuscodefyi.com/glossary/feature-policy/) [Web Standards]: An HTTP header (formerly Feature-Policy) that controls which browser features (camera, geolocation, autoplay) a page can use, enhancing security and p - [HSTS Preload](https://statuscodefyi.com/glossary/strict-transport-security/) [Web Standards]: An extension of HSTS where domains are hardcoded into browsers' built-in HTTPS-only lists, ensuring HTTPS from the very first connection. - [Structured Headers (RFC 8941)](https://statuscodefyi.com/glossary/structured-headers/) [Web Standards]: A standardized format for HTTP header values using defined data types (integers, strings, tokens, lists, dictionaries) for consistent parsing across i - [Early Hints (103)](https://statuscodefyi.com/glossary/early-hints/) [Web Standards]: An informational HTTP response that allows servers to send Link headers before the final response, enabling browsers to start preloading resources ear - [Certificate Transparency](https://statuscodefyi.com/glossary/expect-ct/) [Web Standards]: A web security standard that requires TLS certificates to be logged in public, append-only logs, enabling domain owners to detect misissued certificat - [Subresource Integrity (SRI)](https://statuscodefyi.com/glossary/subresource-integrity/) [Web Standards]: A security feature that ensures resources fetched from CDNs haven't been tampered with, by verifying their cryptographic hash against an expected valu - [Web Linking (RFC 8288)](https://statuscodefyi.com/glossary/web-linking/) [Web Standards]: A framework for expressing typed relationships between resources using Link headers or HTML link elements, enabling machine-readable discovery of rela - [Proxy Authentication](https://statuscodefyi.com/glossary/proxy-authentication/) [Web Standards]: An HTTP mechanism where a proxy server requires clients to authenticate before forwarding requests, using the 407 status code and Proxy-Authorization - [Conditional Headers](https://statuscodefyi.com/glossary/conditional-header/) [Web Standards]: HTTP headers (If-Match, If-None-Match, If-Modified-Since, If-Unmodified-Since, If-Range) that make requests conditional on resource state, enabling ef - [X-Forwarded-For](https://statuscodefyi.com/glossary/x-forwarded-for/) [Web Standards]: A de facto HTTP header used by proxies and load balancers to identify the originating client IP address when traffic passes through intermediaries. Ea - [Content-Encoding](https://statuscodefyi.com/glossary/content-encoding/) [Web Standards]: An HTTP response header specifying the compression algorithm applied to the response body before transmission, with common values of gzip, br (Brotli) - [gRPC Channel](https://statuscodefyi.com/glossary/grpc-channel/) [gRPC & RPC]: A logical connection to a gRPC endpoint that handles connection management, load balancing, and name resolution under the hood. Channels maintain a po - [gRPC Interceptor](https://statuscodefyi.com/glossary/grpc-interceptor/) [gRPC & RPC]: Middleware that intercepts gRPC calls for cross-cutting concerns — logging, authentication, metrics, and tracing — similar to HTTP middleware. Interce - [gRPC Deadline](https://statuscodefyi.com/glossary/grpc-deadline/) [gRPC & RPC]: A client-specified absolute time by which a gRPC call must complete, propagated across service hops and enforced by DEADLINE_EXCEEDED (status code 4). - [gRPC Status Code](https://statuscodefyi.com/glossary/grpc-status-code/) [gRPC & RPC]: One of 17 standard numeric codes (0–16) returned by gRPC services, including OK (0), CANCELLED (1), UNKNOWN (2), INVALID_ARGUMENT (3), NOT_FOUND (5), - [gRPC Service Definition](https://statuscodefyi.com/glossary/grpc-service-definition/) [gRPC & RPC]: A .proto file declaration that defines the RPC methods a service exposes, their request and response message types, and streaming modes. The protoc co - [Unary RPC](https://statuscodefyi.com/glossary/unary-rpc/) [gRPC & RPC]: The simplest gRPC pattern — a single request from the client receives a single response from the server, analogous to a standard HTTP request-response - [Server Streaming RPC](https://statuscodefyi.com/glossary/server-streaming-rpc/) [gRPC & RPC]: A gRPC pattern where the client sends one request and the server returns a stream of multiple responses, useful for real-time feeds, large result sets - [Client Streaming RPC](https://statuscodefyi.com/glossary/client-streaming-rpc/) [gRPC & RPC]: A gRPC pattern where the client sends a stream of messages and the server responds with a single message after receiving all client data. This is usef - [Bidirectional Streaming RPC](https://statuscodefyi.com/glossary/bidirectional-streaming/) [gRPC & RPC]: A gRPC pattern where both client and server send streams of messages simultaneously over a single connection. The streams are independent — either sid - [Protobuf Message](https://statuscodefyi.com/glossary/protobuf-message/) [gRPC & RPC]: A structured data type defined in a .proto file with typed fields, used as the default serialization format for gRPC request and response payloads. Pr - [gRPC Reflection](https://statuscodefyi.com/glossary/grpc-reflection/) [gRPC & RPC]: A gRPC service extension that allows clients to discover available services and methods at runtime without prior knowledge of the .proto definitions. - [gRPC Health Checking Protocol](https://statuscodefyi.com/glossary/grpc-health-checking/) [gRPC & RPC]: A standard gRPC service (grpc.health.v1.Health) that load balancers and orchestrators use to determine if a gRPC server is ready for traffic. The prot - [gRPC Load Balancing](https://statuscodefyi.com/glossary/grpc-load-balancing/) [gRPC & RPC]: Client-side or proxy-based distribution of gRPC calls — options include pick-first, round-robin, and xDS-based policies for service mesh integration. - [gRPC Retry Policy](https://statuscodefyi.com/glossary/grpc-retry-policy/) [gRPC & RPC]: A declarative configuration in gRPC service config specifying which status codes are retryable, max attempts, backoff parameters, and retry throttling - [gRPC Error Details](https://statuscodefyi.com/glossary/grpc-error-details/) [gRPC & RPC]: Rich error information attached to gRPC status using google.rpc.Status — includes structured error info, retry info, debug info, field violations, quo - [gRPC-Gateway](https://statuscodefyi.com/glossary/grpc-gateway/) [gRPC & RPC]: A reverse proxy that translates RESTful JSON API requests into gRPC calls, enabling HTTP/JSON clients to consume gRPC services without native gRPC sup - [gRPC Keepalive](https://statuscodefyi.com/glossary/grpc-keepalive/) [gRPC & RPC]: Periodic HTTP/2 PING frames sent by gRPC to detect dead connections and keep NAT mappings alive, configurable via keepalive time, timeout, and permit - [RPC (Remote Procedure Call)](https://statuscodefyi.com/glossary/rpc/) [gRPC & RPC]: A protocol paradigm where a client invokes a function on a remote server as if it were local, abstracting away network communication details. RPC fram - [Protobuf Oneof](https://statuscodefyi.com/glossary/protobuf-oneof/) [gRPC & RPC]: A protobuf feature where only one field in a group can be set at a time, useful for polymorphic message types and variant responses. Setting any field - [gRPC Compression](https://statuscodefyi.com/glossary/grpc-compression/) [gRPC & RPC]: Message-level compression in gRPC (gzip, snappy, zstd) negotiated via grpc-accept-encoding and grpc-encoding headers, reducing bandwidth for large pay - [SIP INVITE](https://statuscodefyi.com/glossary/sip-invite/) [SIP & VoIP]: The SIP method used to initiate a session (phone call, video conference), triggering the offer/answer exchange of session parameters via SDP. The call - [SIP Proxy Server](https://statuscodefyi.com/glossary/sip-proxy/) [SIP & VoIP]: An intermediary server that routes SIP requests on behalf of user agents, performing lookup, authentication, and policy enforcement. Stateless proxies - [SIP Redirect Server](https://statuscodefyi.com/glossary/sip-redirect/) [SIP & VoIP]: A SIP server that responds with a 3xx status code containing alternative contact URIs where the target can be reached, without forwarding the request - [SIP Transaction](https://statuscodefyi.com/glossary/sip-transaction/) [SIP & VoIP]: A single request-response exchange in SIP identified by the branch parameter in the Via header, with built-in retransmission timers for unreliable tra - [SIP User Agent](https://statuscodefyi.com/glossary/sip-user-agent/) [SIP & VoIP]: A SIP endpoint that initiates (UAC — User Agent Client) or receives (UAS — User Agent Server) SIP requests, typically a VoIP phone, softphone, or WebR - [SIP Trunk](https://statuscodefyi.com/glossary/sip-trunk/) [SIP & VoIP]: A virtual connection between a PBX and a service provider that replaces traditional ISDN or analog phone lines, carrying voice calls as SIP signaling - [RTP (Real-time Transport Protocol)](https://statuscodefyi.com/glossary/rtp/) [SIP & VoIP]: A protocol for delivering audio and video over IP networks with timestamps and sequence numbers for synchronization, jitter compensation, and packet l - [SRTP (Secure Real-time Transport Protocol)](https://statuscodefyi.com/glossary/srtp/) [SIP & VoIP]: An encrypted extension of RTP that provides confidentiality, authentication, and replay protection for voice and video media streams. SRTP uses AES en - [SIP ACK](https://statuscodefyi.com/glossary/sip-ack/) [SIP & VoIP]: The SIP method sent by the caller to confirm receipt of a final response to an INVITE, completing the three-way handshake that establishes a dialog. A - [SIP BYE](https://statuscodefyi.com/glossary/sip-bye/) [SIP & VoIP]: The SIP method used to terminate an active session (end a call), sent by either party in the dialog and triggering media stream teardown. BYE can only - [SIP CANCEL](https://statuscodefyi.com/glossary/sip-cancel/) [SIP & VoIP]: A SIP method that aborts a pending INVITE transaction before a final response is received, used when the caller hangs up during ringing. CANCEL must m - [SIP OPTIONS](https://statuscodefyi.com/glossary/sip-options/) [SIP & VoIP]: A SIP method used to query a server or user agent about its capabilities — supported methods, codecs, extensions, and Max-Forwards limit — without est - [SIP SUBSCRIBE/NOTIFY](https://statuscodefyi.com/glossary/sip-subscribe-notify/) [SIP & VoIP]: SIP methods for event notification — SUBSCRIBE requests ongoing updates on a resource (presence status, call state, message waiting), and NOTIFY deliv - [B2BUA (Back-to-Back User Agent)](https://statuscodefyi.com/glossary/b2bua/) [SIP & VoIP]: A SIP entity that acts as both a UAC and UAS simultaneously, terminating one SIP leg and originating another independent dialog — used by session bord - [SBC (Session Border Controller)](https://statuscodefyi.com/glossary/session-border-controller/) [SIP & VoIP]: A network device at the edge of a VoIP network that provides security, NAT traversal, codec transcoding, and SIP normalization for voice traffic. SBCs - [Codec Negotiation](https://statuscodefyi.com/glossary/codec-negotiation/) [SIP & VoIP]: The process of agreeing on audio and video codecs (G.711, G.722, Opus, H.264, VP8) between SIP or WebRTC endpoints during SDP offer/answer exchange. T - [SIP Response Classes](https://statuscodefyi.com/glossary/sip-response-classes/) [SIP & VoIP]: SIP response codes grouped like HTTP — 1xx (Provisional: processing or ringing), 2xx (Successful: request accepted), 3xx (Redirection: try alternate a - [VoIP (Voice over IP)](https://statuscodefyi.com/glossary/voip/) [SIP & VoIP]: Technology for delivering voice communication and multimedia sessions over IP networks instead of traditional circuit-switched telephone networks. VoI - [Presence (SIP)](https://statuscodefyi.com/glossary/presence/) [SIP & VoIP]: A SIP event package that conveys the availability status (online, busy, away, do not disturb) of a user agent, used by instant messaging and unified c - [PBX (Private Branch Exchange)](https://statuscodefyi.com/glossary/pbx/) [SIP & VoIP]: A private telephone switching system that connects internal extensions and routes calls to the public telephone network, increasingly IP-based (IP-PBX - [FTP Active Mode](https://statuscodefyi.com/glossary/ftp-active-mode/) [FTP & File Transfer]: The original FTP data transfer mode where the server initiates a connection back to the client on a client-specified port. The client sends a PORT com - [FTP Passive Mode](https://statuscodefyi.com/glossary/ftp-passive-mode/) [FTP & File Transfer]: An FTP data transfer mode where the client initiates both the control and data connections to the server, solving firewall and NAT traversal problems - [FTP Control Channel](https://statuscodefyi.com/glossary/ftp-control-channel/) [FTP & File Transfer]: The persistent TCP connection on port 21 used for sending FTP commands and receiving reply codes throughout an FTP session. The control channel remain - [FTP Data Channel](https://statuscodefyi.com/glossary/ftp-data-channel/) [FTP & File Transfer]: A separate TCP connection used exclusively for transferring file contents and directory listings in FTP. Unlike the persistent control channel, a new - [SFTP (SSH File Transfer Protocol)](https://statuscodefyi.com/glossary/sftp/) [FTP & File Transfer]: A secure file transfer protocol that runs over SSH on port 22, providing encryption, authentication, and file management operations in a single connec - [FTPS (FTP Secure)](https://statuscodefyi.com/glossary/ftps/) [FTP & File Transfer]: FTP extended with TLS encryption to secure both the control and data channels. Implicit FTPS (port 990) requires TLS from the very start of the connec - [FTP Reply Code](https://statuscodefyi.com/glossary/ftp-reply-code/) [FTP & File Transfer]: A three-digit status code returned by the FTP server after every client command, structured similarly to HTTP status codes. The first digit indicates - [FTP Anonymous Access](https://statuscodefyi.com/glossary/ftp-anonymous/) [FTP & File Transfer]: A mode where users connect to an FTP server with the reserved username 'anonymous' and typically an email address as the password, without requiring r - [FTP Transfer Mode (ASCII vs Binary)](https://statuscodefyi.com/glossary/ftp-ascii-vs-binary/) [FTP & File Transfer]: FTP supports two data transfer modes that determine how file bytes are treated during transmission. ASCII mode converts line endings between the local - [FTP Resume (REST Command)](https://statuscodefyi.com/glossary/ftp-resume/) [FTP & File Transfer]: The FTP REST (Restart) command sets a byte offset restart point before a subsequent RETR or STOR command, enabling interrupted downloads or uploads to - [SCP (Secure Copy Protocol)](https://statuscodefyi.com/glossary/scp/) [FTP & File Transfer]: A file transfer protocol that uses SSH for both authentication and encryption, allowing files to be copied between a local host and a remote host or b - [FTP Chroot Jail](https://statuscodefyi.com/glossary/ftp-chroot/) [FTP & File Transfer]: A security technique that restricts FTP users to their home directory by changing the apparent root of the filesystem to that directory upon login. Us - [WebDAV (Web Distributed Authoring and Versioning)](https://statuscodefyi.com/glossary/webdav/) [FTP & File Transfer]: An extension to HTTP (RFC 4918) that adds methods for collaborative file management on remote web servers, including PROPFIND, MKCOL, COPY, MOVE, LOCK - [rsync](https://statuscodefyi.com/glossary/rsync/) [FTP & File Transfer]: A utility for efficient incremental file synchronization that computes the differences between source and destination files and transmits only the cha - [FTP MLSD (Machine-Readable Listing)](https://statuscodefyi.com/glossary/ftp-mlsd/) [FTP & File Transfer]: A modern FTP command defined in RFC 3659 that returns structured directory listings with standardized fact formats (size, type, modify, perm), replaci - [FTP NAT Traversal](https://statuscodefyi.com/glossary/ftp-nat-traversal/) [FTP & File Transfer]: Techniques for establishing FTP data connections through NAT devices and firewalls, which is complex because FTP embeds IP addresses in protocol messa - [Transfer Integrity](https://statuscodefyi.com/glossary/transfer-integrity/) [FTP & File Transfer]: Mechanisms to verify that files arrive at their destination completely unchanged after a transfer. Common approaches include pre- and post-transfer ch - [FTP over TLS (RFC 4217)](https://statuscodefyi.com/glossary/ftp-over-tls/) [FTP & File Transfer]: The formal specification for securing FTP connections with TLS, covering both the control and data channels. RFC 4217 defines explicit security negoti - [File Locking](https://statuscodefyi.com/glossary/file-locking/) [FTP & File Transfer]: A mechanism to prevent concurrent modifications to the same file by multiple clients or processes simultaneously. Advisory locks (such as POSIX flock) - [Bandwidth Throttling](https://statuscodefyi.com/glossary/bandwidth-throttling/) [FTP & File Transfer]: The intentional limiting of data transfer rates to prevent a single client or process from consuming all available network bandwidth. FTP servers, web - [OAuth Authorization Code Flow](https://statuscodefyi.com/glossary/oauth-authorization-code/) [Authentication & OAuth]: The most secure OAuth 2.0 grant type for server-side applications, where the authorization server issues a short-lived authorization code to the clien - [OAuth Client Credentials Flow](https://statuscodefyi.com/glossary/oauth-client-credentials/) [Authentication & OAuth]: An OAuth 2.0 grant type for machine-to-machine authentication where the client authenticates directly with the authorization server using its own clie - [OAuth Implicit Flow (Legacy)](https://statuscodefyi.com/glossary/oauth-implicit-flow/) [Authentication & OAuth]: A deprecated OAuth 2.0 grant type that returned access tokens directly in the URL fragment after the authorization redirect, without an intermediate c - [PKCE (Proof Key for Code Exchange)](https://statuscodefyi.com/glossary/pkce/) [Authentication & OAuth]: An OAuth 2.0 security extension (RFC 7636) that prevents authorization code interception attacks in public clients. Before the authorization request, - [OIDC (OpenID Connect)](https://statuscodefyi.com/glossary/oidc/) [Authentication & OAuth]: An identity layer built on top of OAuth 2.0 that adds standardized user authentication to the authorization framework. In addition to access tokens, O - [Refresh Token](https://statuscodefyi.com/glossary/refresh-token/) [Authentication & OAuth]: A long-lived credential issued alongside an access token that allows the client to obtain new access tokens without requiring the user to re-authentic - [Access Token](https://statuscodefyi.com/glossary/access-token/) [Authentication & OAuth]: A short-lived credential that grants access to protected resources on behalf of a user or client. Access tokens are typically presented in the HTTP Au - [Token Introspection (RFC 7662)](https://statuscodefyi.com/glossary/token-introspection/) [Authentication & OAuth]: An OAuth 2.0 endpoint defined in RFC 7662 where resource servers can send an access or refresh token to the authorization server and receive structure - [Token Revocation (RFC 7009)](https://statuscodefyi.com/glossary/token-revocation/) [Authentication & OAuth]: An OAuth 2.0 endpoint defined in RFC 7009 that allows clients to explicitly notify the authorization server that a token is no longer needed, triggeri - [OAuth Scopes](https://statuscodefyi.com/glossary/scopes/) [Authentication & OAuth]: A mechanism in OAuth 2.0 that limits what an access token is permitted to do by attaching a set of named permissions to it. Clients declare the scopes - [JWT Claims](https://statuscodefyi.com/glossary/jwt-claims/) [Authentication & OAuth]: The key-value pairs in the payload section of a JSON Web Token that convey information about the subject and the token itself. Registered claims are s - [JWK (JSON Web Key)](https://statuscodefyi.com/glossary/jwk/) [Authentication & OAuth]: A JSON data structure defined in RFC 7517 for representing a cryptographic key — RSA, EC, or symmetric — including its key type, algorithm, and public - [SAML (Security Assertion Markup Language)](https://statuscodefyi.com/glossary/saml/) [Authentication & OAuth]: An XML-based open standard for exchanging authentication and authorization data between an Identity Provider (IdP) and a Service Provider (SP). The Id - [SSO (Single Sign-On)](https://statuscodefyi.com/glossary/sso/) [Authentication & OAuth]: An authentication scheme that allows a user to log in once and gain access to multiple related but independent applications without re-authenticating - [MFA (Multi-Factor Authentication)](https://statuscodefyi.com/glossary/mfa/) [Authentication & OAuth]: A security mechanism that requires users to provide two or more independent verification factors from different categories to prove their identity: so - [TOTP (Time-Based One-Time Password)](https://statuscodefyi.com/glossary/totp/) [Authentication & OAuth]: An algorithm defined in RFC 6238 that generates short-lived numeric codes from a shared secret key and the current Unix time, typically producing a 6- - [HTTP Basic Authentication](https://statuscodefyi.com/glossary/basic-auth/) [Authentication & OAuth]: The simplest HTTP authentication scheme, defined in RFC 7617, where the client encodes the username and password as base64(username:password) and send - [HTTP Digest Authentication](https://statuscodefyi.com/glossary/digest-auth/) [Authentication & OAuth]: An HTTP authentication scheme defined in RFC 7616 that challenges the client with a server-generated nonce, then expects an MD5 hash of the username, - [Session Token](https://statuscodefyi.com/glossary/session-token/) [Authentication & OAuth]: A server-generated opaque identifier, typically stored in a Set-Cookie header, that maps to server-side session data such as the authenticated user ID - [Passkey (WebAuthn)](https://statuscodefyi.com/glossary/passkey/) [Authentication & OAuth]: A phishing-resistant passwordless authentication credential based on public-key cryptography, standardized by the FIDO2 and WebAuthn (W3C) specificati - [TLS Versions](https://statuscodefyi.com/glossary/tls-versions/) [TLS & Encryption]: The evolution of the TLS/SSL protocol — SSL 3.0 (deprecated), TLS 1.0 and 1.1 (deprecated since 2021), TLS 1.2 (current baseline supported everywhere) - [TLS 1.3](https://statuscodefyi.com/glossary/tls-1-3/) [TLS & Encryption]: The latest version of the Transport Layer Security protocol (RFC 8446), released in 2018. TLS 1.3 reduces the handshake to one round trip (1-RTT) and - [Certificate Authority (CA)](https://statuscodefyi.com/glossary/certificate-authority/) [TLS & Encryption]: A trusted third party that issues digital certificates after verifying that the requester controls the domain (DV), belongs to a real organisation (OV - [Certificate Chain](https://statuscodefyi.com/glossary/certificate-chain/) [TLS & Encryption]: The ordered sequence of certificates that links a server's leaf certificate back to a trusted root CA. A typical chain is: leaf certificate → one or m - [Certificate Pinning](https://statuscodefyi.com/glossary/certificate-pinning/) [TLS & Encryption]: A security technique where the client hardcodes the expected certificate fingerprint or public key hash and rejects any TLS connection that presents a - [Certificate Transparency (CT)](https://statuscodefyi.com/glossary/certificate-transparency/) [TLS & Encryption]: A public logging framework (RFC 6962) that requires all publicly trusted TLS certificates to be recorded in one or more append-only CT logs before bro - [Forward Secrecy (PFS)](https://statuscodefyi.com/glossary/forward-secrecy/) [TLS & Encryption]: A property of a key exchange mechanism where compromising the server's long-term private key does not allow decryption of previously recorded encrypte - [SNI (Server Name Indication)](https://statuscodefyi.com/glossary/sni/) [TLS & Encryption]: A TLS extension (RFC 6066) where the client includes the target hostname in the ClientHello message before the TLS handshake completes. SNI allows a s - [OCSP (Online Certificate Status Protocol)](https://statuscodefyi.com/glossary/ocsp/) [TLS & Encryption]: A protocol (RFC 6960) for checking in real time whether a TLS certificate has been revoked by its issuing CA, as a more efficient alternative to downl - [OCSP Stapling](https://statuscodefyi.com/glossary/ocsp-stapling/) [TLS & Encryption]: A TLS extension (RFC 6066) where the server periodically fetches a signed time-stamped OCSP response from the CA and attaches ('staples') it to the TL - [Let's Encrypt](https://statuscodefyi.com/glossary/lets-encrypt/) [TLS & Encryption]: A free, automated, and open Certificate Authority operated by the Internet Security Research Group (ISRG) that issues domain-validated (DV) TLS certif - [ACME Protocol (RFC 8555)](https://statuscodefyi.com/glossary/acme-protocol/) [TLS & Encryption]: Automatic Certificate Management Environment — an IETF standard protocol (RFC 8555) that enables fully automated certificate issuance, renewal, and re - [TLS Session Resumption](https://statuscodefyi.com/glossary/tls-resumption/) [TLS & Encryption]: Mechanisms that allow a client to skip the full TLS handshake when reconnecting to a server it has previously established a session with. TLS 1.2 supp - [0-RTT (Zero Round Trip Time)](https://statuscodefyi.com/glossary/zero-rtt/) [TLS & Encryption]: A TLS 1.3 feature (RFC 8446 Section 2.3) that allows a client to send application data in the very first message of a resumed session, before receivin - [Key Exchange](https://statuscodefyi.com/glossary/key-exchange/) [TLS & Encryption]: The cryptographic process by which two parties agree on a shared secret key over an insecure channel without transmitting the secret itself. The origi - [PKI (Public Key Infrastructure)](https://statuscodefyi.com/glossary/public-key-infrastructure/) [TLS & Encryption]: The framework of policies, hardware, software, people, and procedures for creating, managing, distributing, using, storing, and revoking digital certi - [SSL Stripping](https://statuscodefyi.com/glossary/ssl-stripping/) [TLS & Encryption]: A man-in-the-middle attack in which an attacker positioned between a client and server intercepts an HTTP request and forwards it to the server over H - [CT Log (Certificate Transparency Log)](https://statuscodefyi.com/glossary/ct-log/) [TLS & Encryption]: An append-only, cryptographically verifiable log of all TLS certificates submitted to the Certificate Transparency ecosystem. CT logs use a Merkle tre - [TLS Fingerprinting (JA3/JA4)](https://statuscodefyi.com/glossary/tls-fingerprinting/) [TLS & Encryption]: A passive network analysis technique that creates a hash fingerprint of a TLS ClientHello message by extracting fields such as TLS version, cipher sui - [Post-Quantum TLS](https://statuscodefyi.com/glossary/post-quantum-tls/) [TLS & Encryption]: The integration of quantum-resistant key exchange and signature algorithms into TLS to protect against future cryptographically relevant quantum compu - [Layer 4 Load Balancing](https://statuscodefyi.com/glossary/layer-4-load-balancing/) [Load Balancing & Proxying]: Load balancing performed at the OSI transport layer (Layer 4) using TCP/UDP IP addresses and port numbers to route traffic without inspecting applicat - [Layer 7 Load Balancing](https://statuscodefyi.com/glossary/layer-7-load-balancing/) [Load Balancing & Proxying]: Load balancing at the OSI application layer (Layer 7) that inspects HTTP/2/gRPC application data — headers, URLs, cookies, query strings, and gRPC met - [Sticky Sessions (Session Affinity)](https://statuscodefyi.com/glossary/sticky-sessions/) [Load Balancing & Proxying]: A load balancing configuration that ensures all requests from the same client are consistently routed to the same backend server for the duration of a - [Consistent Hashing](https://statuscodefyi.com/glossary/consistent-hashing/) [Load Balancing & Proxying]: A hashing strategy that maps both keys and servers onto a circular hash ring, so that adding or removing a server causes only a small fraction of keys - [Weighted Round Robin](https://statuscodefyi.com/glossary/weighted-round-robin/) [Load Balancing & Proxying]: A load balancing algorithm that extends basic round-robin by assigning a numeric weight to each backend server. Requests are distributed in proportion - [Least Connections](https://statuscodefyi.com/glossary/least-connections/) [Load Balancing & Proxying]: A dynamic load balancing algorithm that routes each new request to the backend server currently handling the fewest active connections. Unlike round-r - [Forward Proxy](https://statuscodefyi.com/glossary/forward-proxy/) [Load Balancing & Proxying]: A proxy server that sits between clients and the broader internet, forwarding outbound requests on behalf of those clients. The destination server see - [Transparent Proxy](https://statuscodefyi.com/glossary/transparent-proxy/) [Load Balancing & Proxying]: A proxy that intercepts and redirects network traffic without requiring any client configuration — the client is unaware that a proxy is present. Tran - [PROXY Protocol](https://statuscodefyi.com/glossary/proxy-protocol/) [Load Balancing & Proxying]: A protocol created by HAProxy that prepends a small header to a proxied TCP connection containing the original client's IP address and port. When a lo - [HAProxy](https://statuscodefyi.com/glossary/haproxy/) [Load Balancing & Proxying]: High Availability Proxy — a free, open-source, high-performance TCP and HTTP load balancer and proxy server widely used in production infrastructure. - [Nginx (as Reverse Proxy)](https://statuscodefyi.com/glossary/nginx-proxy/) [Load Balancing & Proxying]: Nginx is a high-performance web server that is widely deployed as a reverse proxy and software load balancer in front of application servers. In proxy - [Envoy Proxy](https://statuscodefyi.com/glossary/envoy-proxy/) [Load Balancing & Proxying]: An open-source, high-performance L7 proxy developed by Lyft and donated to the CNCF, designed as the data plane of service mesh architectures. Envoy s - [SSL Offloading](https://statuscodefyi.com/glossary/ssl-offloading/) [Load Balancing & Proxying]: The practice of decrypting incoming TLS traffic at a dedicated load balancer, ADC (Application Delivery Controller), or hardware appliance so that bac - [GSLB (Global Server Load Balancing)](https://statuscodefyi.com/glossary/global-server-load-balancing/) [Load Balancing & Proxying]: A technique for distributing traffic across multiple geographically dispersed data centres or cloud regions, directing each user to the nearest or hea - [Backend Pool](https://statuscodefyi.com/glossary/backend-pool/) [Load Balancing & Proxying]: The set of origin server instances registered with a load balancer that are eligible to receive forwarded requests. A backend pool is defined with one - [Failover](https://statuscodefyi.com/glossary/failover/) [Load Balancing & Proxying]: The automatic process of switching incoming traffic from a failed primary server, data centre, or service instance to a healthy standby or backup. Fai - [Request Routing](https://statuscodefyi.com/glossary/request-routing/) [Load Balancing & Proxying]: The set of rules and logic that a load balancer or API gateway uses to determine which backend service or server should handle each incoming request. - [Active-Passive HA](https://statuscodefyi.com/glossary/active-passive-ha/) [Load Balancing & Proxying]: A high-availability topology in which a primary (active) node handles all production traffic while an identical standby (passive) node remains idle, c - [Connection Multiplexing (Proxy)](https://statuscodefyi.com/glossary/connection-multiplexing/) [Load Balancing & Proxying]: A proxy optimisation technique where many client-side connections are tunnelled through a smaller number of persistent backend connections, reducing t - [xDS (Discovery Service Protocol)](https://statuscodefyi.com/glossary/xds/) [Load Balancing & Proxying]: A family of gRPC-based data plane APIs used by Envoy Proxy and compatible proxies to receive dynamic configuration from a control plane without restar ## Debugging Scenarios - [502 Bad Gateway in Nginx Reverse Proxy](https://statuscodefyi.com/scenarios/http/502-bad-gateway-nginx/) [HTTP] - [502 Bad Gateway Behind Cloudflare](https://statuscodefyi.com/scenarios/http/502-bad-gateway-cloudflare/) [HTTP] - [503 Service Unavailable During Traffic Spikes](https://statuscodefyi.com/scenarios/http/503-service-unavailable-overload/) [HTTP] - [500 Internal Server Error in Django](https://statuscodefyi.com/scenarios/http/500-internal-server-error-django/) [HTTP] - [500 Internal Server Error in PHP/Apache](https://statuscodefyi.com/scenarios/http/500-internal-server-error-php/) [HTTP] - [404 Not Found in Single Page Applications](https://statuscodefyi.com/scenarios/http/404-not-found-spa/) [HTTP] - [Getting 401 Instead of 403 (or Vice Versa)](https://statuscodefyi.com/scenarios/http/401-vs-403-api/) [HTTP] - [403 Forbidden Due to CORS Misconfiguration](https://statuscodefyi.com/scenarios/http/cors-error-403/) [HTTP] - [429 Too Many Requests — Rate Limiting Hits](https://statuscodefyi.com/scenarios/http/429-too-many-requests/) [HTTP] - [408 Request Timeout on Slow API Calls](https://statuscodefyi.com/scenarios/http/408-request-timeout-slow-api/) [HTTP] - [301 Redirect Loop (ERR_TOO_MANY_REDIRECTS)](https://statuscodefyi.com/scenarios/http/301-redirect-loop/) [HTTP] - [413 Content Too Large — File Upload Rejected](https://statuscodefyi.com/scenarios/http/413-payload-too-large/) [HTTP] - [504 Gateway Timeout — Upstream Not Responding](https://statuscodefyi.com/scenarios/http/504-gateway-timeout/) [HTTP] - [Mixed Content Errors After HTTPS Migration](https://statuscodefyi.com/scenarios/http/mixed-content-upgrade/) [HTTP] - [422 Unprocessable Content — API Validation Failures](https://statuscodefyi.com/scenarios/http/422-validation-error-api/) [HTTP] - [403 Forbidden Due to CSRF Token Mismatch](https://statuscodefyi.com/scenarios/http/403-csrf-token-mismatch/) [HTTP] - [400 Bad Request — Malformed JSON Body](https://statuscodefyi.com/scenarios/http/400-bad-request-malformed-json/) [HTTP] - [405 Method Not Allowed — Wrong HTTP Verb](https://statuscodefyi.com/scenarios/http/405-method-not-allowed-wrong-verb/) [HTTP] - [409 Conflict — Optimistic Locking Failure](https://statuscodefyi.com/scenarios/http/409-conflict-optimistic-locking/) [HTTP] - [511 Network Authentication Required — Captive Portal](https://statuscodefyi.com/scenarios/http/511-network-authentication-required-captive-portal/) [HTTP] - [304 Not Modified — Cache Serving Stale Content](https://statuscodefyi.com/scenarios/http/304-not-modified-stale-cache/) [HTTP] - [431 Request Header Fields Too Large — Cookie Overflow](https://statuscodefyi.com/scenarios/http/431-request-header-fields-too-large-cookies/) [HTTP] - [502 Bad Gateway — WebSocket Upgrade Failure Through Proxy](https://statuscodefyi.com/scenarios/http/502-websocket-upgrade-proxy-failure/) [HTTP] - [206 Partial Content — Video Streaming Buffering Issues](https://statuscodefyi.com/scenarios/http/206-partial-content-video-streaming/) [HTTP] - [426 Upgrade Required — HTTP/1.1 to HTTP/2 Migration](https://statuscodefyi.com/scenarios/http/426-upgrade-required-http2-migration/) [HTTP] - [550 Mailbox Not Found — Email Bouncing](https://statuscodefyi.com/scenarios/smtp/550-mailbox-not-found/) [SMTP] - [421 Service Unavailable — SMTP Connection Refused](https://statuscodefyi.com/scenarios/smtp/421-smtp-connection-refused/) [SMTP] - [Email Rejected Due to SPF/DKIM/DMARC Failures](https://statuscodefyi.com/scenarios/smtp/spf-dkim-failures/) [SMTP] - [550 Relay Access Denied — Cannot Send Through SMTP Server](https://statuscodefyi.com/scenarios/smtp/smtp-relay-denied/) [SMTP] - [554 Transaction Failed — Blacklisted IP](https://statuscodefyi.com/scenarios/smtp/554-blacklisted-ip/) [SMTP] - [535 Authentication Failed — App Password Required](https://statuscodefyi.com/scenarios/smtp/535-auth-failed-app-password/) [SMTP] - [452 Insufficient Storage — Mailbox Full](https://statuscodefyi.com/scenarios/smtp/452-mailbox-full/) [SMTP] - [503 Bad Sequence of Commands — AUTH Before STARTTLS](https://statuscodefyi.com/scenarios/smtp/503-bad-sequence-auth-starttls/) [SMTP] - [421 Service Unavailable — STARTTLS Required](https://statuscodefyi.com/scenarios/smtp/421-starttls-required/) [SMTP] - [550 Domain Has No Mail — MX Record Missing](https://statuscodefyi.com/scenarios/smtp/550-mx-record-missing/) [SMTP] - [gRPC UNAVAILABLE — Connection Failures](https://statuscodefyi.com/scenarios/grpc/grpc-unavailable-connection/) [gRPC] - [gRPC DEADLINE_EXCEEDED — Timeout Issues](https://statuscodefyi.com/scenarios/grpc/grpc-deadline-exceeded/) [gRPC] - [gRPC PERMISSION_DENIED — Auth and Authorization Issues](https://statuscodefyi.com/scenarios/grpc/grpc-permission-denied/) [gRPC] - [RESOURCE_EXHAUSTED — gRPC Message Size Limit Exceeded](https://statuscodefyi.com/scenarios/grpc/grpc-resource-exhausted-message-size/) [gRPC] - [UNAUTHENTICATED — Missing or Invalid gRPC Metadata](https://statuscodefyi.com/scenarios/grpc/grpc-unauthenticated-metadata/) [gRPC] - [INTERNAL — Protobuf Serialization Mismatch](https://statuscodefyi.com/scenarios/grpc/grpc-internal-protobuf-mismatch/) [gRPC] - [FAILED_PRECONDITION — Stale State in Distributed System](https://statuscodefyi.com/scenarios/grpc/grpc-failed-precondition-stale-state/) [gRPC] - [CANCELLED — Client Abandoning Slow gRPC Requests](https://statuscodefyi.com/scenarios/grpc/grpc-cancelled-client-abandoning/) [gRPC] - [NXDOMAIN — Domain Does Not Exist](https://statuscodefyi.com/scenarios/dns/dns-nxdomain/) [DNS] - [SERVFAIL — DNS Server Failure](https://statuscodefyi.com/scenarios/dns/dns-servfail/) [DNS] - [REFUSED — Recursive Query to Authoritative-Only Server](https://statuscodefyi.com/scenarios/dns/dns-refused-authoritative-only/) [DNS] - [NXDOMAIN — Subdomain Typo or Missing DNS Record](https://statuscodefyi.com/scenarios/dns/nxdomain-subdomain-missing-record/) [DNS] - [SERVFAIL — Lame Delegation](https://statuscodefyi.com/scenarios/dns/dns-servfail-lame-delegation/) [DNS] - [FORMERR — Malformed DNS Query](https://statuscodefyi.com/scenarios/dns/dns-formerr-malformed-query/) [DNS] - [High TTL Causing Delayed DNS Failover](https://statuscodefyi.com/scenarios/dns/dns-high-ttl-delayed-failover/) [DNS] - [NOTAUTH — Unauthorized DNS Dynamic Update](https://statuscodefyi.com/scenarios/dns/dns-notauth-unauthorized-update/) [DNS] - [WebSocket 1006 Abnormal Closure](https://statuscodefyi.com/scenarios/websocket/websocket-1006-abnormal/) [WebSocket] - [1008 Policy Violation — Authentication Timeout](https://statuscodefyi.com/scenarios/websocket/websocket-1008-policy-violation-auth/) [WebSocket] - [1009 Message Too Big — Payload Size Limit](https://statuscodefyi.com/scenarios/websocket/websocket-1009-message-too-big/) [WebSocket] - [1015 TLS Handshake Failure — Certificate Mismatch](https://statuscodefyi.com/scenarios/websocket/websocket-1015-tls-handshake-failure/) [WebSocket] - [1011 Internal Error — Server Crash During Stream](https://statuscodefyi.com/scenarios/websocket/websocket-1011-internal-error-crash/) [WebSocket] - [1002 Protocol Error — Invalid Frame from Proxy](https://statuscodefyi.com/scenarios/websocket/websocket-1002-protocol-error-proxy/) [WebSocket] - [1013 Try Again Later — Server Overloaded](https://statuscodefyi.com/scenarios/websocket/websocket-1013-try-again-later-overloaded/) [WebSocket] - [530 Login Authentication Failed](https://statuscodefyi.com/scenarios/ftp/530-login-failed-passive-mode/) [FTP] - [425 Can't Open Data Connection — Passive Mode Firewall](https://statuscodefyi.com/scenarios/ftp/425-passive-mode-firewall/) [FTP] - [550 File Unavailable — Permission Denied](https://statuscodefyi.com/scenarios/ftp/550-file-unavailable-permission-denied/) [FTP] - [421 Service Unavailable — Connection Limit Reached](https://statuscodefyi.com/scenarios/ftp/421-service-unavailable-connection-limit/) [FTP] - [426 Connection Closed — Transfer Aborted Mid-File](https://statuscodefyi.com/scenarios/ftp/426-connection-closed-transfer-aborted/) [FTP] - [227 Entering Passive Mode — Wrong IP Behind NAT](https://statuscodefyi.com/scenarios/ftp/227-passive-mode-wrong-ip-nat/) [FTP] - [553 File Name Not Allowed — Invalid Characters or Path](https://statuscodefyi.com/scenarios/ftp/553-filename-not-allowed/) [FTP] - [408 Request Timeout in NAT Traversal](https://statuscodefyi.com/scenarios/sip/408-request-timeout-nat-traversal/) [SIP] - [503 Service Unavailable — SIP Trunk Overloaded](https://statuscodefyi.com/scenarios/sip/503-service-unavailable-sip-trunk/) [SIP] - [401 Unauthorized — SIP Registration Failure](https://statuscodefyi.com/scenarios/sip/401-unauthorized-sip-registration/) [SIP] - [486 Busy Here — Call Routing Loop Misdiagnosis](https://statuscodefyi.com/scenarios/sip/486-busy-here-routing-loop/) [SIP] - [488 Not Acceptable Here — Codec Mismatch](https://statuscodefyi.com/scenarios/sip/488-not-acceptable-here-codec-mismatch/) [SIP] ## Specifications - [RFC 9110: HTTP Semantics](https://statuscodefyi.com/specs/rfc-9110/) - [RFC 9111: HTTP Caching](https://statuscodefyi.com/specs/rfc-9111/) - [RFC 9112: HTTP/1.1](https://statuscodefyi.com/specs/rfc-9112/) - [RFC 9113: HTTP/2](https://statuscodefyi.com/specs/rfc-9113/) - [RFC 9114: HTTP/3](https://statuscodefyi.com/specs/rfc-9114/) - [RFC 6585: Additional HTTP Status Codes](https://statuscodefyi.com/specs/rfc-6585/) - [RFC 7807: Problem Details for HTTP APIs](https://statuscodefyi.com/specs/rfc-7807/) - [RFC 9457: Problem Details for HTTP APIs](https://statuscodefyi.com/specs/rfc-9457/) - [RFC 6749: The OAuth 2.0 Authorization Framework](https://statuscodefyi.com/specs/rfc-6749/) - [RFC 6750: The OAuth 2.0 Authorization Framework: Bearer Token Usage](https://statuscodefyi.com/specs/rfc-6750/) - [RFC 7519: JSON Web Token (JWT)](https://statuscodefyi.com/specs/rfc-7519/) - [RFC 6266: Use of the Content-Disposition Header Field in the HTTP Protocol](https://statuscodefyi.com/specs/rfc-6266/) - [RFC 7239: Forwarded HTTP Extension](https://statuscodefyi.com/specs/rfc-7239/) - [RFC 6455: The WebSocket Protocol](https://statuscodefyi.com/specs/rfc-6455/) - [RFC 8288: Web Linking](https://statuscodefyi.com/specs/rfc-8288/) - [RFC 7235: Hypertext Transfer Protocol (HTTP/1.1): Authentication](https://statuscodefyi.com/specs/rfc-7235/) - [RFC 7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content](https://statuscodefyi.com/specs/rfc-7231/) - [RFC 2616: Hypertext Transfer Protocol — HTTP/1.1](https://statuscodefyi.com/specs/rfc-2616/) - [RFC 5321: Simple Mail Transfer Protocol](https://statuscodefyi.com/specs/rfc-5321/) - [RFC 5322: Internet Message Format](https://statuscodefyi.com/specs/rfc-5322/) - [RFC 7208: Sender Policy Framework (SPF) for Authorizing Use of Domains in Email](https://statuscodefyi.com/specs/rfc-7208/) - [RFC 6376: DomainKeys Identified Mail (DKIM) Signatures](https://statuscodefyi.com/specs/rfc-6376/) - [RFC 7489: Domain-based Message Authentication, Reporting, and Conformance (DMARC)](https://statuscodefyi.com/specs/rfc-7489/) - [RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security](https://statuscodefyi.com/specs/rfc-3207/) - [RFC 2920: SMTP Service Extension for Command Pipelining](https://statuscodefyi.com/specs/rfc-2920/) - [RFC 6531: SMTP Extension for Internationalized Email](https://statuscodefyi.com/specs/rfc-6531/) - [gRPC Core: gRPC Status Codes](https://statuscodefyi.com/specs/grpc-status-codes/) - [gRPC Core: Protocol Buffers (protobuf)](https://statuscodefyi.com/specs/grpc-protocol-buffers/) - [RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2)](https://statuscodefyi.com/specs/rfc-7540/) - [gRPC Core: gRPC Streaming](https://statuscodefyi.com/specs/grpc-streaming/) - [RFC 1035: Domain Names - Implementation and Specification](https://statuscodefyi.com/specs/rfc-1035/) - [RFC 1034: Domain Names - Concepts and Facilities](https://statuscodefyi.com/specs/rfc-1034/) - [RFC 4033: DNS Security Introduction and Requirements](https://statuscodefyi.com/specs/rfc-4033/) - [RFC 4034: Resource Records for the DNS Security Extensions](https://statuscodefyi.com/specs/rfc-4034/) - [RFC 4035: Protocol Modifications for the DNS Security Extensions](https://statuscodefyi.com/specs/rfc-4035/) - [RFC 8484: DNS Queries over HTTPS (DoH)](https://statuscodefyi.com/specs/rfc-8484/) - [RFC 7858: Specification for DNS over Transport Layer Security (TLS)](https://statuscodefyi.com/specs/rfc-7858/) - [RFC 6891: Extension Mechanisms for DNS (EDNS(0))](https://statuscodefyi.com/specs/rfc-6891/) - [RFC 7692: WebSocket Per-Message Compression](https://statuscodefyi.com/specs/rfc-7692/) - [RFC 8441: Bootstrapping WebSockets with HTTP/2](https://statuscodefyi.com/specs/rfc-8441/) - [RFC 8307: Well-Known URIs for the WebSocket Protocol](https://statuscodefyi.com/specs/rfc-8307/) - [RFC 959: File Transfer Protocol (FTP)](https://statuscodefyi.com/specs/rfc-959/) - [RFC 2228: FTP Security Extensions](https://statuscodefyi.com/specs/rfc-2228/) - [RFC 2428: FTP Extensions for IPv6 and NATs](https://statuscodefyi.com/specs/rfc-2428/) - [RFC 4217: Securing FTP with TLS](https://statuscodefyi.com/specs/rfc-4217/) - [RFC 3659: Extensions to FTP](https://statuscodefyi.com/specs/rfc-3659/) - [RFC 3261: SIP: Session Initiation Protocol](https://statuscodefyi.com/specs/rfc-3261/) - [RFC 3262: Reliability of Provisional Responses in SIP](https://statuscodefyi.com/specs/rfc-3262/) - [RFC 3263: Session Initiation Protocol (SIP): Locating SIP Servers](https://statuscodefyi.com/specs/rfc-3263/) - [RFC 3265: Session Initiation Protocol (SIP)-Specific Event Notification](https://statuscodefyi.com/specs/rfc-3265/) - [RFC 6665: SIP-Specific Event Notification](https://statuscodefyi.com/specs/rfc-6665/)