Hi Bob,
Thanks for the response. I have a further query on
this section. How does preserving the original tag in
a forwarded 3-6xx response to a request that did not
have a To-Tag, help in debugging? If the proxy, for
some reason, had chosen to modify the tag while
sending the response upstream, then it will have
stored the modified tag in its UAS transaction object
so that it can map the ACK from the upstream element
to this UAS transaction. So what kind of debugging
does the RFC talk about here?
regards
Revathi
==================================================
inline.
----- Original Message -----
From: "Revathi K" <revsuram at hotmail.com>
> Hi,
> I have some basic doubts about the To-tag processing
by proxy.
> When will a proxy ever need to insert a To-tag in a
response?
>
> Section 16.7 of the RFC says that:
>
> 3-6xx responses are delivered hop-by-hop. When
issuing a 3-6xx
> response, the element is effectively acting as a
UAS, issuing
> its own response, usually based on the responses
received from
> downstream elements. An element SHOULD preserve
the To tag
> when simply forwarding a 3-6xx response to a
request that did
> not contain a To tag.
> A proxy MUST NOT modify the To tag in any
forwarded response to
> a request that contains a To tag.
> While it makes no difference to the upstream
elements if the
> proxy replaced the To tag in a forwarded 3-6xx
response,
> preserving the original tag may assist with
debugging.
>
> 1) The above section seems to be contradictory. On
the one hand,
> it says that 3xx-6xx responses are hop-by-hop and
the proxy
> would act as a UAS hen delivering these. This would
mean that
> the proxy can, if it wants, insert/replace the
To-tag in the
> response. However, the section also says that a
proxy must not
> modify the To tag in any forwarded response. So
which of the above
> interpretations is correct?
The key is whether the request had a to-tag. If it
did, it is an in-dialog
request (e.g. re-INVITE, BYE, etc). In that case the
to-tag in the response
MUST always match the to-tag in the request no matter
whether the response
is generated by a UA or a proxy.
For the case of an out-of-dialog request (i.e. no
to-tag in the request), if
the proxy is forwarding one of the responses it
received from a downstream
UAS, it SHOULD retain the to-tag from that response,
unless there is a
really good reason not to.
>
> 2) Also, this above section seems to talk about tha
case where
> the proxy forward a 3xx-6xx respone from a
downstream element.
> What should be the behaviour when the proxy is
generating a
> 3xx-6xx response on its own (i.e. without forwarding
the
> request out)? Should the proxy insert a To-tag on it
own
> in this case?
When the proxy is responding to the request, it is
truly the UAS for the
request, and UAS rules apply (see 8.2.6.2).
If the request had a to-tag, the response MUST have
the same to-tag.
If the request did not have a to-tag, a to-tag MUST be
added to the
response.
>
>
> Any help in understanding the above section of the
RFC correctly
> will be appreciated.
>
> thanks in advance
> Revathi
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