(as individual)
On Sep 2, 2010, at 8:37 AM, Christer Holmberg wrote:
Hi Cullen,
Do these changes allow an SBC on the signaling path to change the contents
of the MSRP messages
without the end points being able to detect that? I'm sure it will be easier
to answer this once we have
a new draft.
Sessmatch does not make it any easier for an SBC in the signalling path to
change the content of the MSRP messages.
For an SBC to do MSRP message modification it will have to implement MSRP
B2BUA functionality - no matter if sessmatch is supported by the endpionts or
not.
I'm going to have to push on this one a little bit:
I think it _does_ make it easier for an SBC to change MSRP messages because it
makes it easier for the SBC to put itself into the media path in the first
place. This is no different than for any other sort of media.
The need to implement MSRP b2bUA functionality exists if the SBC changes the
To-Path and From-Path headers. It may or may not be required if it wants to
change _bodies_, depending on whether that change requires chunk-reassembly or
changes the length of the body. And assuming, of course, there is no end-to-end
protection on the MSRP messages in the first place--and we all know that in
practice there probably won't be.
I think what Christer is talking about is, if sessmatch did not exist, then if
an SBC were to insert itself into the MSRP media path, it would be _forced_
then to rewrite the To-Path and From-Path in each MSRP request to match the
change it made in the SDP. If it did not do so, the endpoints would detect the
change and treat it as an error. That is, sessmatch does not enable the
insertion in the first place--it just makes things cheaper from a processing
perspective.
Note that any end-to-end integrity protection on the SDP, such as RFC4474 or
s/mime, will prevent SBC insertion, with or without sessmatch. Just like for
any other media.
Regards,
Christer
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