Subject: [Simple] MSRP, SDP and CPIM

From: Pekka Pessi <[email-masked]>
Date: Tue, 21 Oct 2003 02:35:04 +0300

Hi,

First, I've a problem with SDP usage by MSRP.

The format parameters on SDP media line should be tokens, and "/" is not
allowed within an SDP token. The token issue was discussed in MMUSIC wg,
and the wg consensus was to keep definition of token as is (and fix the
comment in sdp-new).

So, the top-level media types should be put into an SDP attribute just
like accept-wrapped-types. Example:

   m=message 9999 msrp/tcp *
   a=msrp-accept:message/cpim text/plain, text/html

Alternatively, we could mandate that the MSRP content should always be
Message/CPIM. The message/cpim itself mandates one header, Requires. So,
instead of

   m=message 9999 msrp/tcp message/cpim text/plain text/html

we would have something like this

   m=message 9999 msrp/tcp cpim
   a=msrp-accept: text/plain, text/html

So, what would change at MSRP level? Currently we can have an MSRP message
like this:

    MSRP xx SEND
    TR-ID: 123
    Content-Type: text/plain

    Hi, I'm Alice!

Instead of that, we could have a CPIM message with implied MIME type
message/cpim (so, no Content-Type header in MSRP). Here we have CPIM
message without any CPIM headers:

   MSRP xx SEND
   TR-ID: 123

   Content-Type: text/plain

   Hi, I'm Alice!

So, we pay two extra pairs of CR-LF for CPIM support.

Sender can always include CPIM headers, like From and To,

   MSRP xx SEND
   TR-ID: 123

   From: Alice <sip:[email-masked]>
   To: Bob <sip:[email-masked]>

   Content-Type: text/plain

   Hi, I'm Alice!
 
but the recipient can safely ignore them. The only exception is Requires
header. In order to properly process Requires, we need 420 Not
Supported, too. Then it might be possible to extend MSRP using CPIM
Requires header. 

Perhaps the supported or required extensions could be indicated as SDP
attributes, too.

What about signed CPIM messages? The could be sent inside another one

   MSRP xx SEND
   TR-ID: 123

   My-Own-CPIM-Header: please see secured content

   Content-Type: multipart/signed; boundary=boundary;
                 micalg=sha1;
                 protocol=application/pkcs7-signature
   
   --boundary
   Content-Type: message/cpim
 
   From: Alice <sip:[email-masked]>
   To: Bob <sip:[email-masked]>

   Content-Type: text/plain

   Hi, I'm Alice!

   --boundary
   Content-Type: application/pkcs7-signature

   (lots-of-binary-signature-here)
   --boundary--

The real issue of CPIM is that it is just a wrapper around a MIME
object. This is not a problem with most SIP implementations, as they
already support MIME, but there may be some use of MSRP outside SIP?

we could reuse CPIM header structure (quoting, line ending, etc) in
MSRP, if we choose to always have a message/cpim wrapper. I don't mean
that MSRP headers would be part of CPIM message, but that they reuse the
Header ABNF from CPIM.

--Pekka

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