What is the use of a tag in a SIP request?

Question

I have a trivial doubt about SIP. I tried searching on the Internet and referenced many books, but still I cannot find a good reason to add the tag to the SIP request.

SIP Request Example (snapshot from rfc-3261)

INVITE sip:[e-mail] SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: Bob <sip:[e-mail]>
From: Alice <sip:[e-mail]>;tag=1928301774
Call-ID: [e-mail]
CSeq: 314159 INVITE
Contact: <sip:[e-mail]>
Content-Type: application//sdp
Content-Length: 142

According to rfc-3261:

A dialog can be identified using tags, tags, and call IDs.

I know the exact reason for adding to-tag and call-id. What I do not know,

• Why do we need a tag?
• What will be the effect if from-tag was absent (assuming rfc-3261 does not require this)?

Answer 1

[EDIT] "tag" From, : http://andrewjprokop.wordpress.com/2013/09/23/lets-play-sip-tag/

.

, :

Call-ID . , SIP . , , , , - , . Call-ID , , . .

, . - Call-ID, , . , , . ( ), UAC. (To tag), UAS (User Agent Server). UAC From, UAS To. , UAC, From To . UAS SIP (, 180 ), . , . , SIP- From, , , .

Answer 2

@Ani, , , "From-tag" . , / , To From, From To-Tag.

:

, A INVITE B, From A ( ), To Header B ( ). , , B A, , From Header B sip uri B, TO A sip uri A. B , A .

, , From-tag INVITE, To-Tag Re-INVITE , .

Answer 3

- UAC , Call-Id. .

• / - , FROM, Dialog.

• INVITE PSTN, IAM SAM INVITE FROM. . RFC 3578 . , FROM GW , INVITE, .

Answer 4

"" .

PDF http://www.sipknowledge.com/rfc3261_explained_light.zip

8.1.1.3 ""

, 8.3.1 , From , SIP.

Answer 5

, . , CALL-ID . & SIP. , SIP-.

3: ( )

1. , , .

https://www.ietf.org/rfc/rfc2543.txt

2. : JOIN FROM & . , 2 , .

RFC3911- "" (SIP) https://www.rfc-editor.org/rfc/rfc3911.txt

3. : -

   11.5

   When the request was subsequently received, the following checks were made:

   1.   If the Call-ID is new, the request is for a new call,
        regardless of the values of the To and From header fields.
   
   2.   If the Call-ID exists, the request is for an existing call.
        If the To, From, Call-ID, and CSeq values exactly match
        (including tags) those of any requests received previously,
        the request is a retransmission.
   
   3.   If there was no match to the previous step, the To and From
        fields are compared against existing call leg local and
        remote addresses. If there is a match, and the CSeq in the
        request is higher than the last CSeq received on that leg,
        the request is a new transaction for an existing call leg.