Subject: one SSRC for each participants??

From: Sylvie Brunelle <[email-masked]>
Date: Thu, 18 May 2000 11:32:48 +0100

Hi everybody,

Does someone know if all participants of a RTP session do have an SSRC
or if it is only for sources.

Indeed when I read the definition of the SSRC, I understand that only
sources do have an SSRC.
"Synchronization source (SSRC): The source of a stream of RTP
             packets, identified by a 32-bit numeric SSRC identifier
             carried in the RTP header so as not to be dependent upon
             the network address. All packets from a synchronization
             source form part of the same timing and sequence number
             space, so a receiver groups packets by synchronization
             source for playback. Examples of synchronization sources
             include the sender of a stream of packets derived from a
             signal source such as a microphone or a camera, or an RTP
             mixer (see below)..."

But if I look at the format of the RTCP packets, I see that there is
always the SSRC of the packet sender specified in the header of the RTCP
packet and that participants that are only receivers do send RTCP
packets (RR...). Thus that would involve that ALL participants of an RTP
session (even receivers) do have an SSRC.

Is that right?
Cheers
Sylvie

---

From: Colin Perkins <[email-masked]>
Date: Thu, 18 May 2000 13:13:38 +0100

Sylvie,

>Indeed when I read the definition of the SSRC, I understand that only
...
>session (even receivers) do have an SSRC.

Yes. ALL participants in an RTP session have an SSRC.

Colin

---

From: "Nam Kyu Kim" <[email-masked]>
Date: Fri, 19 May 2000 10:30:44 +0900

Dear Colin and all

I have one question for clear understanding..

For example, in case there are two participant-one is a sender another is 
a only receiver, ssrc value of RTCP control packets(RR, SDES, BYE) in 
receiver is the same as sender's ssrc?

SR of sender
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|    RC   |   PT=SR=200   |             length            | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         SSRC of sender                        |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|              NTP timestamp, most significant word             | sender
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
:

RR of receiver
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|    RC   |   PT=RR=201   |             length            | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     SSRC of packet sender                     |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                 SSRC_1 (SSRC of first source)                 | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| fraction lost |       cumulative number of packets lost       |   1
:

SDES of receiver
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|    SC   |  PT=SDES=202  |             length            | header
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                          SSRC/CSRC_1                          | chunk
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   1
|                           SDES items                          |
:

Underlined SSRCs of above packet formats are all same?
Or not same?
Or participant only receiving can not send SDES, BYE and APP?

-----------------------------------------------------
Nam Kyu Kim 
R & D Team 9
Mobile Telecommunication Terminal Division
HYUNDAI ELECTRONICS INDUSTRIES Co., Ltd.

TEL : (82-2)580-5511       FAX : (82-2)580-5487
Home : (82-2)2282-3077   C.P. : 017-211-3077
E-mail : [email-masked]
-----------------------------------------------------

---

From: Colin Perkins <[email-masked]>
Date: Fri, 19 May 2000 15:20:45 +0100

--> Nam Kyu Kim writes:
>For example, in case there are two participant-one is a sender another 
>is a only receiver, ssrc value of RTCP control packets(RR, SDES, BYE) 
>in receiver is the same as sender's ssrc?

The SSRC in SR and SDES packets is that of the participant generating
those SR/SDES packets. The first SSRC in an RR packet is that of the
participant generating that RR packet, the other SSRC(s) are those of
the participants being reported upon.

>SR of sender

Not the same.

>Or participant only receiving can not send SDES, BYE and APP?

A receiver is allowed to send SDES, BYE and APP packets.

Colin

---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 19 May 2000 10:32:48 -0400

> Nam Kyu Kim wrote:
> is

Let A be the sender, and B be the receiver.

> SR of sender

This is SSRC of A.

> block

The first SSRC (of packet sender) is SSRC of B.
The second SSRC (SSRC_1, of first source) is the SSRC of A.

> |                          SSRC/CSRC_1                          |
> chunk

SSRC/CSRC_1 is the SSRC of B, since you have indicated that this is the
SDES from the receiver, B.

> Or participant only receiving can not send SDES, BYE and APP?

The receiving-only participant can send SDES, BYE or APP. In fact, it
SHOULD send SDES. It can even send BYE (but only if it has previously
send an SDES).

-Jonathan R.
-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (732) 741-4778
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com