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Re: [sipcore] Comments on draft-mohali-sipcore-reason-call-forwarding-00

"Worley, Dale R (Dale)" <dworley@avaya.com> Mon, 18 October 2010 21:11 UTC

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From: "Worley, Dale R (Dale)" <dworley@avaya.com>
To: "sipcore@ietf.org" <sipcore@ietf.org>
Date: Mon, 18 Oct 2010 17:12:17 -0400
Thread-Topic: [sipcore] Comments on draft-mohali-sipcore-reason-call-forwarding-00
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Subject: Re: [sipcore] Comments on draft-mohali-sipcore-reason-call-forwarding-00
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Here are some more comments on this draft:

The term "call diversion" sounds more inclusive to the naive ear than
"call forwarding".  If the two terms are used identically in the
draft, it seems better to use only the former, especially because the
abbreviation "CDIV" is being used.

Section 3 does not give any BNF for CDIV, though it quotes all of the
established BNF for the Reason header. Clearly, what is intended (but
not stated) is:

    protocol  =/  "CDIV"

In regard to listing the CDIV cause values, it seems to me that there
should be an existing encoding of call diversion reasons somewhere
within the 3GPP specification, and it would be more effective for the
draft to reference that table (which would be maintained by 3GPP)
rather than defining a new IANA registry (which would be used almost
exclusively by 3GPP).

In the example, it appears that the 302 response F3 is informing the
proxy that the call should be sent to Voicemail.  In that case, the
302 would also carry the Reason header explaing why the diversion is
being done, which the proxy would subsequently transcribe into the
History-Info header of the new INVITE F5:

    F3: 302 Bob -> proxy.example.com

    SIP/2.0 302 Moved Temporarily
    Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik80k7g-1
    Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK-klj79f
    From: Alice <sip:+15551001@example.com;user=phone>; tag=1234567
    To: sip:+15551002@example.com;user=phone;tag=765432
    Call-ID: c3x842276298220188511
    Contact: <sip:voicemail@example.com>
    Reason: CDIV;cause=6;text="Deflection immediate response"
    CSeq: 1 INVITE
    Content-Length: 0

In the example, F5 has two "Reason" headers attached to the URI at
index 1.  If you have more than one "header" attached to a URI, only
the first is introduced with "?", the remainder are introduced with
"&":

    History-Info: <sip:+15551002@example.com;user=phone?Reason=SIP
              %3Bcause=302%3Btext="Moved Temporarily"&Reason=CDIV
              %3Bcause=6%3Btext="Deflection immediate response">;index=1,
              <sip:voicemail@example.com>;index=1.1;mp=1

There are also some difficulties regarding the History-Info headers in
the examle; they don't appear to be consistently applied to all the
INVITEs.

Dale

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