Subject: [Megaco] AVD-3878 H.248.SDPMapper: "m="-line with multiple transport elements

From: "Schwarz Albrecht" <[email-masked]>
Date: Thu, 4 Mar 2010 15:21:36 +0100

Hello Christian, Tommy,
 
got a question for clarification:
the highlighted "m="-line below, with multiple transport elements, is
syntactically not possible with RFC 4566 (2327), but feasible in H.248
due to the alternative option in 7.1.8.1.1.1/H.248.1.
Right?
Thanks,
Albrecht
 
____H.248.1 _____________________________________________________

7.1.8.1.1   Specific syntax for H.248 text encoding

7.1.8.1.1.1          Rules

When text encoding the protocol, the descriptors consist of session
descriptions as defined in SDP ([IETF RFC 2327]). In session
descriptions sent from the MG to the MGC, the SDP must comply with [IETF
RFC 2327].  In session descriptions sent from the MGC to the MG, the
following exceptions to the syntax of [IETF RFC 2327] are allowed:

*            the "s = ", "t = " and "o = " lines are optional;

*            the use of CHOOSE is allowed in place of a single parameter
value; and

*            the use of alternatives is allowed in place of a single
parameter value.

...
 
____ AVD-3878 _____________________________________________________
http://wftp3.itu.int/av-arch/avc-site/2009-2012/1003_Sha/AVD-3878.zip

6.1.2     Relationship with the H.248 Reserve Value concept

SDPCapNeg also allows the ability to specify alternatives (via the use
of "|") in configuration lists. The MGC can also provide this
functionality via the use of over specified property values (or SDP
parameters as per ITU-T H.248.39). Over specification of properties is
equivalent to providing alternatives which effectively is a "or"
function in the same way as "|".

For example, given the following Revised Offer/Answer SDP:

      v=0 

      o=- 25678 753849 IN IP4 192.0.2.1 
      s=  
      c=IN IP4 192.0.2.1 
      t=0 0 
      m=audio 53456 RTP/AVPF 0 18  
      a=tcap:1 RTP/AVPF RTP/AVP RTP/SAVP RTP/SAVPF 
      a=pcfg:1 t=2|1 a=1 

The MGC would construct the following SDP for the first H.248 Group:

      v=0 

      o=- 25678 753849 IN IP4 192.0.2.1 
      s=  
      c=IN IP4 192.0.2.1 
      t=0 0 
      m=audio 53456 RTP/AVP RTP/AVPF 0 18

---

From: Christian Groves <[email-masked]>
Date: Fri, 05 Mar 2010 14:50:58 +1100

Hello Albrecht,

Yes that is correct.

Regards, Christian

Schwarz Albrecht wrote:
> descriptions sent from the MG to the MGC, the SDP must comply with 
> [IETF RFC 2327]. In session descriptions sent from the MGC to the MG, 
> the following exceptions to the syntax of [IETF RFC 2327] are allowed:
>
> • the "s = ", "t = " and "o = " lines are optional;
>
> • the use of CHOOSE is allowed in place of a single parameter value; and
>
> • the use of alternatives is allowed in place of a single parameter value.
>
> ...
> of “|”) in configuration lists. The MGC can also provide this 
> equivalent to providing alternatives which effectively is a “or” 
> function in the same way as “|”.
>