[SIPForum-techwg] Route header in ACK to a negative response

[S S]

I have a question regarding the inclusion of a Route header in the ACK sent
after receiving a negative response.

This is the scenario:

   Alice           Proxy 1          Proxy 2            Bob
     |                |                |                |
     |   INVITE F1    |                |                |
     |--------------->|   INVITE F2    |                |
     |     100  F3    |--------------->|   INVITE F4    |
     |<---------------|     100  F5    |--------------->|
     |                |<---------------|                |
     |                |                |      486 F6    |
     |                |                |<---------------|
     |                |                |     ACK F7     |
     |                |      486 F8    |--------------->|
     |                |<---------------|                |
     |                |      ACK F9    |                |
     |     486 F10    |--------------->|                |
     |<---------------|                |                |
     |     ACK F11    |                |                |
     |--------------->|                |                |
     |                |                |                |


The above call flow is from RFC 3665 Section 3.9. Here, the original Invite
(F1) is sent with a Route header. After the '486 Busy Here' response is
returned to Alice, the corresponding ACK (F11) does not contain the Route
header.

This behavior is in conflict with RFC 3262 Section 17.1.1.3 (Construction of
ACK) which specifies that:

If the INVITE request whose response is being acknowledged had Route
header fields, those header fields MUST appear in the ACK.  This is
to ensure that the ACK can be routed properly through any downstream
stateless proxies.


What should be the expected behavior of the SIP client, Alice, in such a
scenario? Should the ACK contain the Route header or should it be left out.
What is the reasoning behind either course of action?

Thanks,
Sangram
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