Header fields

Request and Response Header Fields

Alert-Info (Ring-back tone)
Allow-Events (u)
Call-ID (i)
Contact (m)
CSeq
Date (See RFC 1123 for format)
Encryption (Defined in RFC 2543 not included in RFC 3261)
From (f) (Option in RFC 2543, mandatory in RFC 3261)
Organization
Record-Route
Retry-After
Subject (s)
Supported
Timestamp
To (t)
User-Agent
Via (v)

Request Header Fields

Accept
Accept-Contact
Accept-Encoding
Accept-Language (The language tags are registered by IANA. The primary tag is an ISO-639 language abbreviation.)
Authorization
Call-Info
Event (o)
Hide The Hide header field was defined in RFC 2543 but has been deprecated from RFC 3261.
In-Reply-To
Join Join header Internet-Draft
Priority RFC 3323
Privacy
Proxy-Authorization
Proxy-Require
P-OSP-Auth-Token
P-Asserted-Identity
P-Preferred-Identity
Max-Forwards
Reason
Refer-To (r) RFC 3515 Refer Method
Referred-By (b) “The SIP Referred-By Mechanism,” IETF Internet-Draft
Reply-To
Replaces
Reject-Contact (j) Caller Preferences and Callee Capabilities for the Session Initiation Protocol (SIP),” IETF Internet-Draft,
Request-Disposition
Require
Response-Key The Response-Key header field was defined in RFC 2543 but was deprecated in RFC 3261
Route
RAck
Session-Expires
Subscription-State

Response Header Fields

Authentication Info
Error-Info
Min-Expires
Min-SE
Proxy-Authenticate
Server
Unsupported
Warning
WWW-Authenticate
RSeq

Message Body Header Fields

Allow
Content-Encoding
Content-Disposition
Content-Language
Content-Length (l)
Content-Type The Content-Type header field is used to specify the Internet media type in the message body.
Expires
MIME-Version

Hop-By-Hop Header Fields

Alert-Info
Call-Info
Content-Length
Date
Error-Info
Max-Forwards
Organization
Priority
Proxy-Authenticate
Proxy-Authorization
Proxy-Require
Record-Route
Reason
Require
Via
WWW-Authenticate