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SIP headers

Following the start line, SIP messages contain SIP headers (also called SIP fields) that convey message attributes and to modify message meaning. SIP headers are similar to HTTP header fields and always have the following format:

<header_name>:<value>

SIP messages can include the SIP headers listed in the following table:

SIP headers
SIP Header Description
Allow Lists the set of SIP methods supported by the UA generating the message. All methods, including ACK and CANCEL, understood by the UA MUST be included in the list of methods in the Allow header field, when present. For example:

Allow: INVITE, ACK, OPTIONS, CANCEL, BYE
Call-ID A globally unique identifier for the call, generated by the combination of a random string and the sender’s host name or IP address. The combination of the To, From, and Call-ID headers completely defines a peer-to-peer SIP relationship between the sender and the receiver. This relationship is called a SIP dialog.

Call-ID: ddeg45e793@10.31.101.30
Contact Included in SIP request messages, the Contact header contains the SIP URI of the sender of the SIP request message. The receiver uses this URI to contact the sender. For example:

Contact: Sender <sip:sender@10.31.100.20>t
Content-Length The number of bytes in the message body (in bytes).

Content-Length: 126
Content-Type In addition to SIP headers, SIP messages include a message body that contains information about the content or communication being managed by the SIP session. The Content-Type header specifies what the content of the SIP message is. For example, if you are using SIP with SDP, the content of the SIP message is SDP code.

Content-Type: application/sdp
CSeq The command sequence header contains a sequence integer that is increased for each new SIP request message (but is not incremented in the response message). This header also incudes the request name found in the request message request-line. For example:

CSeq: 1 INVITE
Expires Gives the relative time after which the message (or content) expires. The actual time and how the header is used depends on the SIP method. For example:

Expires: 5
From Identifies the sender of the message. Responses to a message are sent to the address of the sender. The following example includes the sender’s name (Sender) and the sender’s SIP address (sender@10.31.101.20.):

From: Sender <sip:sender@10.31.101.20>
Max-forwards An integer in the range 0-255 that limits the number of proxies or gateways that can forward the request message to the next downstream server. Also called the number of hops, this value is decreased every time the message is forwarded. This can also be useful when the client is attempting to trace a request chain that appears to be failing or looping in mid-chain.

For example: Max-Forwards: 30
P-Asserted-Identity The P-Asserted-Identity header is used among trusted SIP entities to carry the identity of the user sending a SIP message as it was verified by authentication. See RFC 3325. The header contains a SIP URI and an optional display-name, for example:

P-Asserted-Identity: "Example Person" <sip:10.31.101.50>
RAck Sent in a PRACK request to support reliability of information or provisional response messages. It contains two numbers and a method tag. For example:

RAck: 776656 1 INVITE
Record-Route Inserted into request messages by a SIP proxy to force future requests to be routed through the proxy. In the following example, the host at IP address 10.31.101.50 is a SIP proxy. The lr parameter indicates the URI of a SIP proxy in Record-Route headers.

Record-Route: <sip:10.31.101.50;lr>
Route Forces routing for a request message through one or more SIP proxies. The following example includes two SIP proxies:

Route: <sip:172.20.120.10;lr>, <sip:10.31.101.50;lr>
RSeq The RSeq header is used in information or provisional response messages to support reliability of informational response messages. The header contains a single numeric value. For example:

RSeq: 33456
To Identifies the receiver of the message. The address in this field is used to send the message to the receiver. The following example includes the receiver’s name (Receiver) and the receiver’s SIP address (receiver@10.31.101.30.):

To: Receiver <sip:receiver@10.31.101.30>
Via Indicates the SIP version and protocol to be used for the SIP session and the address to which to send the response to the message that contains the Via field. The following example Via field indicates to use SIP version 2, UDP for media communications, and to send the response to 10.31.101.20 using port 5060.

Via: SIP/2.0/UDP 10.31.101.20:5060