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Header Fields

Every SIP header field in the IANA registry, its compact form, and the RFC that defines it.

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Every header field in the IANA registry, its compact form where one exists, and the RFC that defines it.

Source. https://www.iana.org/assignments/sip-parameters/sip-parameters-2.csv — the IANA Header Fields registry, retrieved 2026-07-30, 134 fields. The same arrangement as SIP request methods and SIP response codes: RFC 3261 defines 47 and later RFCs register the rest.

Compact forms

Nineteen fields have a single-letter alias. RFC 3261 section 7.3.3 makes them exactly equivalent to the long form, so a parser that misses one misreads the message — and an attacker who knows which ones a tool ignores can hide a header in plain sight. COMPACT_HEADERS in src/sip/parser.rs carries all nineteen, and compact_headers_match_the_iana_registry holds it to this table.

CompactHeader fieldDefined in
aAccept-ContactRFC 3841
bReferred-ByRFC 3892
cContent-TypeRFC 3261
dRequest-DispositionRFC 3841
eContent-EncodingRFC 3261
fFromRFC 3261
iCall-IDRFC 3261
jReject-ContactRFC 3841
kSupportedRFC 3261
lContent-LengthRFC 3261
mContactRFC 3261
oEventRFC 6665, RFC 6446
rRefer-ToRFC 3515
sSubjectRFC 3261
tToRFC 3261
uAllow-EventsRFC 6665
vViaRFC 3261, RFC 7118
xSession-ExpiresRFC 4028
yIdentityRFC 8224

All header fields

The Description column quotes RFC 3261 section 20 for the fields it defines. A field registered by a later RFC carries its reference without a description, because no single section defines them all — follow the link.

Header fieldCompactDefined inDescription
AcceptRFC 3261 section 20.1The Accept header field follows the syntax defined in [H14.1].
Accept-ContactaRFC 3841
Accept-EncodingRFC 3261 section 20.2The Accept-Encoding header field is similar to Accept, but restricts the content-codings [H3.5] that are acceptable in the response.
Accept-LanguageRFC 3261 section 20.3The Accept-Language header field is used in requests to indicate the preferred languages for reason phrases, session descriptions, or status responses carried as message bodies in the response.
Accept-Resource-PriorityRFC 4412
Additional-Identity3GPP TS 24.229 v16.7.0
Alert-InfoRFC 3261 section 20.4When present in an INVITE request, the Alert-Info header field specifies an alternative ring tone to the UAS.
AlertMsg-ErrorRFC 8876
AllowRFC 3261 section 20.5The Allow header field lists the set of methods supported by the UA generating the message.
Allow-EventsuRFC 6665
Answer-ModeRFC 5373
Attestation-Info3GPP TS 24.229 v15.11.0
Authentication-InfoRFC 3261 section 20.6The Authentication-Info header field provides for mutual authentication with HTTP Digest.
AuthorizationRFC 3261 section 20.7The Authorization header field contains authentication credentials of a UA.
Call-IDiRFC 3261 section 20.8The Call-ID header field uniquely identifies a particular invitation or all registrations of a particular client.
Call-InfoRFC 3261 section 20.9The Call-Info header field provides additional information about the caller or callee, depending on whether it is found in a request or response.
Cellular-Network-Info3GPP TS 24.229 v13.9.0
ContactmRFC 3261 section 20.10A Contact header field value provides a URI whose meaning depends on the type of request or response it is in.
Content-DispositionRFC 3261 section 20.11The Content-Disposition header field describes how the message body or, for multipart messages, a message body part is to be interpreted by the UAC or UAS.
Content-EncodingeRFC 3261 section 20.12The Content-Encoding header field is used as a modifier to the “media-type”.
Content-IDRFC 8262
Content-LanguageRFC 3261 section 20.13See [H14.12].
Content-LengthlRFC 3261 section 20.14The Content-Length header field indicates the size of the message- body, in decimal number of octets, sent to the recipient.
Content-TypecRFC 3261 section 20.15The Content-Type header field indicates the media type of the message-body sent to the recipient.
CSeqRFC 3261 section 20.16A CSeq header field in a request contains a single decimal sequence number and the request method.
DateRFC 3261 section 20.17The Date header field contains the date and time.
DC-Info3GPP TS 24.229 v19.4.1
Encryption (Deprecated)RFC 3261
Error-InfoRFC 3261 section 20.18The Error-Info header field provides a pointer to additional information about the error status response.
EventoRFC 6665, RFC 6446
ExpiresRFC 3261 section 20.19The Expires header field gives the relative time after which the message (or content) expires.
Feature-CapsRFC 6809
Flow-TimerRFC 5626
FromfRFC 3261 section 20.20The From header field indicates the initiator of the request.
GeolocationRFC 6442
Geolocation-ErrorRFC 6442
Geolocation-RoutingRFC 6442
Hide (Deprecated)RFC 3261
History-InfoRFC 7044
IdentityyRFC 8224
Identity-Info (deprecated by [RFC8224])RFC 8224
In-Reply-ToRFC 3261 section 20.21The In-Reply-To header field enumerates the Call-IDs that this call references or returns.
Info-PackageRFC 6086
JoinRFC 3911
Max-BreadthRFC 5393
Max-ForwardsRFC 3261 section 20.22The Max-Forwards header field must be used with any SIP method to limit the number of proxies or gateways that can forward the request to the next downstream server.
MIME-VersionRFC 3261 section 20.24See [H19.4.1].
Min-ExpiresRFC 3261 section 20.23The Min-Expires header field conveys the minimum refresh interval supported for soft-state elements managed by that server.
Min-SERFC 4028
OrganizationRFC 3261 section 20.25The Organization header field conveys the name of the organization to which the SIP element issuing the request or response belongs.
Origination-Id3GPP TS 24.229 v15.11.0
P-Access-Network-InfoRFC 7315
P-Answer-StateRFC 4964
P-Asserted-IdentityRFC 3325
P-Asserted-ServiceRFC 6050
P-Associated-URIRFC 7315
P-Called-Party-IDRFC 7315
P-Charge-InfoRFC 8496
P-Charging-Function-AddressesRFC 7315
P-Charging-VectorRFC 7315
P-DCS-Billing-InfoRFC 5503
P-DCS-LAESRFC 5503
P-DCS-OSPSRFC 5503
P-DCS-RedirectRFC 5503
P-DCS-Trace-Party-IDRFC 5503
P-Early-MediaRFC 5009
P-Media-AuthorizationRFC 3313
P-Preferred-IdentityRFC 3325
P-Preferred-ServiceRFC 6050
P-Private-Network-IndicationRFC 7316
P-Profile-KeyRFC 5002
P-Refused-URI-ListRFC 5318
P-Served-UserRFC 5502, RFC 8498
P-User-DatabaseRFC 4457
P-Visited-Network-IDRFC 7315
PathRFC 3327
Permission-MissingRFC 5360
Policy-ContactRFC 6794
Policy-IDRFC 6794
PriorityRFC 3261 section 20.26The Priority header field indicates the urgency of the request as perceived by the client.
Priority-Share3GPP TS 24.229 v13.16.0
Priority-Verstat3GPP TS 24.229
Priv-Answer-ModeRFC 5373
PrivacyRFC 3323
Proxy-AuthenticateRFC 3261 section 20.27A Proxy-Authenticate header field value contains an authentication challenge.
Proxy-AuthorizationRFC 3261 section 20.28The Proxy-Authorization header field allows the client to identify itself (or its user) to a proxy that requires authentication.
Proxy-RequireRFC 3261 section 20.29The Proxy-Require header field is used to indicate proxy-sensitive features that must be supported by the proxy.
RAckRFC 3262
ReasonRFC 3326
Reason-PhraseRFC 6873
Record-RouteRFC 3261 section 20.30The Record-Route header field is inserted by proxies in a request to force future requests in the dialog to be routed through the proxy.
Recv-InfoRFC 6086
Refer-Events-AtRFC 7614
Refer-SubRFC 4488
Refer-TorRFC 3515
Referred-BybRFC 3892
Reject-ContactjRFC 3841
Relayed-Charge3GPP TS 24.229 v12.14.0
ReplacesRFC 3891
Reply-ToRFC 3261 section 20.31The Reply-To header field contains a logical return URI that may be different from the From header field.
Request-DispositiondRFC 3841
RequireRFC 3261 section 20.32The Require header field is used by UACs to tell UASs about options that the UAC expects the UAS to support in order to process the request.
Resource-PriorityRFC 4412
Resource-Share3GPP TS 24.229 v13.7.0
Response-Key (Deprecated)RFC 3261
Response-Source3GPP TS 24.229 v15.11.0
Restoration-Info3GPP TS 24.229 v12.14.0
Retry-AfterRFC 3261 section 20.33The Retry-After header field can be used with a 500 (Server Internal Error) or 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client and with
RouteRFC 3261 section 20.34The Route header field is used to force routing for a request through the listed set of proxies.
RSeqRFC 3262
Security-ClientRFC 3329
Security-ServerRFC 3329
Security-VerifyRFC 3329
ServerRFC 3261 section 20.35The Server header field contains information about the software used by the UAS to handle the request.
Service-Interact-Info3GPP TS 24.229 v13.18.0
Service-RouteRFC 3608
Session-ExpiresxRFC 4028
Session-IDRFC 7989
SIP-ETagRFC 3903
SIP-If-MatchRFC 3903
SubjectsRFC 3261 section 20.36The Subject header field provides a summary or indicates the nature of the call, allowing call filtering without having to parse the session description.
Subscription-StateRFC 6665
SupportedkRFC 3261 section 20.37The Supported header field enumerates all the extensions supported by the UAC or UAS.
Suppress-If-MatchRFC 5839
Target-DialogRFC 4538
TimestampRFC 3261 section 20.38The Timestamp header field describes when the UAC sent the request to the UAS.
TotRFC 3261 section 20.39The To header field specifies the logical recipient of the request.
Trigger-ConsentRFC 5360
UnsupportedRFC 3261 section 20.40The Unsupported header field lists the features not supported by the UAS.
User-AgentRFC 3261 section 20.41The User-Agent header field contains information about the UAC originating the request.
User-to-UserRFC 7433
ViavRFC 3261 section 20.42The Via header field indicates the path taken by the request so far and indicates the path that should be followed in routing responses.
WarningRFC 3261 section 20.43The Warning header field is used to carry additional information about the status of a response.
WWW-AuthenticateRFC 3261 section 20.44A WWW-Authenticate header field value contains an authentication challenge.