RTP question!!

Aparna Saha apsaha at HSS.HNS.COM
Fri Aug 13 04:57:56 EDT 1999

Hello Deepak,

RTP states that in case a source changes its source transport address, it must
also choose a new SSRC identifier . ( Refer to the last line of the description
of SSRC as a field of the RTP Header. Section A.5.1, H.225 ). Hence, no SSRC
collision will be registered.  After changing the DSP (also resulting in change
of IP address - assuming that it is associated with a different NIC), the source
transport address as well as the SSRC changes as a result of which the receiver
perceives this participant to be a new member in the session. Due to the
soft-state approach of RTP, the information about the old transport address and
the old SSRC would be removed subsequently (as it would no longer register any

If the application requires a correlation between the new SSRC and the older
one, the same can be achieved through the CNAME item. The CNAME item in the RTCP
packets generated would be the same for both the sources. However, as one cannot
wait for an RTCP packet to arrive to correlate, this is not fool-proof.

The above scenario is also the one seen in IPv6 mobility, wherein the IP address
of the sender changes.


Deepak Jaiswal <djai at sasi.com> on 08/13/99 12:15:14 PM

Please respond to Deepak Jaiswal <djai at sasi.com>

To:   rakeshj at wipinfo.soft.net (rakesh jain)
cc:   vittaln at agcs.com, h323implementors at imtc.org (bcc: Aparna Saha/HSS)

Subject:  Re: RTP question!!

>  Deepak,
>    Hi.
>    If after changing DSP you are sending RTP/RTCP packets from new
>    IP address that is IP address of new card, than i think, at
>    receiver end it     will be source conflict, because IP adress is
>    different than the Old one. As a result the packets from the
>    new card will be discarded.
>    Pls let me know, if I am wrong.
>  rakesh

Hello Rakesh,

You need to open a multicast channel where in a receiver can recieve
packets from multiple sources.

With Regards,
Deepak Jaiswal
Multimedia Codecs Group                        djai at sasi.com
Silicon Automation Systems Ltd.                http://www.sasi.com
#5, Corporation Division #67, HAL 3rd Stage,   Tel: +91-80-5281461 x4115
Jeevan Bhima Nagar, Bangalore 560 075, India.  Fax: +91-80-5276125
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