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Does the SIP un-register tag value (in the From: field) should match the register tag value? i.e. if:

REGISTER sip:registrar.biloxi.com SIP/2.0
Via: SIP/2.0/UDP bobspc.biloxi.com:5060;branch=z9hG4bKnashds7
Max-Forwards: 70
To: Bob <sip:[email protected]>
From: Bob <sip:[email protected]>;tag=456248

Does the un-register MUST use the same tag=456248?

2 Answers 2

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The question you need to ask is, does the REGISTER packet require that tag value?

If it does, the answer is Yes, because an UNREGISTER is just a REGISTER with an Expires Header set to 0 (or some other really small number).

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  • Yes, but this is not a dialog, these are just 2 register commands: one with expiry > 0 and the other with expiry=0, so why should the tag value be the same? Oct 30, 2013 at 16:15
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    That is true, REGISTER events do seem to be out of dialog, and the tags are there more for INVITE entities, to keep legs. RE-INVITES, etc separate.. Which seems to answer my question, I don't think the REGISTER must have a tag, it should use the Call-ID field as an identifier.
    – NickW
    Oct 30, 2013 at 16:50
  • I will accept your answer, though it doesn't explain why the SBC rejects my un-register which differs from the register request only in the tag value... Oct 30, 2013 at 16:55
  • It doesn't, you might want to ask the Devs of that software what RFC they used to decide that, because I can find no MUST for tags in the REIGISTER section: 10.2 RFC 3261, in fact they say " A REGISTER request does not establish a dialog."
    – NickW
    Oct 30, 2013 at 17:10
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Consider the case where a user registers two identical devices using the same SIP account from the same location (IP). A SIP server/proxy may use that information to distinguish the two devices.

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  • Shouldn't the registrar update the original registered record in this case? Oct 30, 2013 at 16:48
  • No, as those are two different registrations. What the server/proxy needs to know is the available locations to send INVITEs. If the second device unregisters, the server must destroy any references to it, so not to send INVITEs to it. I am not sure there are many devices that implement this, usually SIP servers/proxies consider a registration as dead if the device hasn't re-registered after a certain amount of time. To my experience, this is not a blocker nor something to waste time on
    – manjiki
    Oct 30, 2013 at 17:01
  • Wouldn't the CALL-ID be the identifier in this case?
    – NickW
    Oct 30, 2013 at 17:11
  • RFC's section 19.3 says: "When a UA sends a request outside of a dialog, it contains a From tag only, providing "half" of the dialog ID. The dialog is completed from the response(s), each of which contributes the second half in the To header field" but I am not sure it applies in this case.
    – manjiki
    Oct 31, 2013 at 11:23

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