Subject: [AVT] Couple of clarifications on RFC2833

From: "Hashmi, Mohamed Hadi" <[email-masked]>
Date: Mon, 31 Mar 2003 15:15:16 -0700

Question-1

the RFC 2833, section 3.6 states the following

"If an event continues for more than one period, the source generating the events should send a new event packet with the RTP timestamp value corresponding to the beginning of the event and the duration of the event increased correspondingly."

What this "corresponding" mean?  Does it mean that all the RTP packets related to that event should use the SAME timestamp as that of its first packet or does it mean that timestamp for next packet= timestamp of the first packet + duration of event?

I have seen some implementations where all the RTP packets for one specific DTMF event use the timestamp as that of first packet. But this creates an interesting situation where audio RTP packets will have latest time stamp and then we have DTMF RTP packet carrying a previous timestamp. Since sequence number are incremented sequentially for both audio as well dtmf RTP packets, the dtmf  packets with higher sequence number will have smaller timestamp value compared to an audio RTP packet with  smaller sequence number.

Question-2

While using inband DTMF detection the sequence numbers are incremented by "one" for every packet sent by the media gateway for that call whether they are dtmf or audio. That means in an RTP stream for example ,  sequence numbers 5,8,9,10,11 could belong to a DTMF event RTP packets while 1,2,3,4,6,7,12,13 to audio RTP packets. This is perfectly alright with in-band DTMF detection.

How should the  sequence numbers be assigned  for out-of-band DTMF detection?  As in this case there would be two RTP stream going to different IP address /port, one for audio and another one for DTMF. If sequence numbers are assigned as above then both audio receivers as well as DTMF receiver are going to get gaps in sequence numbers ? Or should there be one set of sequence numbers for audio stream and another for DTMF stream?

Will appreciate any clarification on these 

Thanks

Hadi

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Colin Perkins <[email-masked]>
Date: Mon, 31 Mar 2003 18:06:51 -0500

--> "Hashmi, Mohamed Hadi" writes:
>"If an event continues for more than one period, the source generating the
>events should send a new event packet with the RTP timestamp value
>corresponding to the beginning of the event and the duration of the event
>increased correspondingly."
>
>What this "corresponding" mean?  Does it mean that all the RTP packets
>related to that event should use the SAME timestamp as that of its first
>packet or does it mean that timestamp for next packet timestamp of the
>first packet + duration of event?

My understanding is that all the packets comprising a single event have the
same timestamp. The second (and subsequent) packets have the same timestamp
as the first packet, but increased duration fields. 

>I have seen some implementations where all the RTP packets for one specific
>DTMF event use the timestamp as that of first packet. But this creates an
>interesting situation where audio RTP packets will have latest time stamp
>and then we have DTMF RTP packet carrying a previous timestamp. Since
>sequence number are incremented sequentially for both audio as well dtmf
>RTP packets, the dtmf  packets with higher sequence number will have
>smaller timestamp value compared to an audio RTP packet with  smaller
>sequence number.

So? RTP does not mandate that packets are sent in order of increasing
timestamp.

>Question-2
>
>While using inband DTMF detection the sequence numbers are incremented by
>"one" for every packet sent by the media gateway for that call whether they
>are dtmf or audio. That means in an RTP stream for example ,  sequence
>numbers 5,8,9,10,11 could belong to a DTMF event RTP packets while 1,2,3,
>4,6,7,12,13 to audio RTP packets. This is perfectly alright with in-band
>DTMF detection.
>
>How should the  sequence numbers be assigned  for out-of-band DTMF
>detection?  As in this case the re would be two RTP stream going to
>different IP address /port, one for audio and another one for DTMF. If
>sequence numbers are assigned as above then both audio receivers as well as
>DTMF receiver are going to get gaps in sequence numbers ? Or should there
>be one set of sequence numbers for audio stream and another for DTMF
>stream?  

If there are two separate RTP streams, they each have their own sequence
number space. Accordingly, you'd use one set of sequence numbers for audio
stream and another for DTMF stream.

Colin

---

From: "Bino George" <[email-masked]>
Date: Sun, 30 Mar 2003 09:19:23 +0530

Hello Hashmi:
This issue is discussed a lot in this mailing list. Please scan through the
mailing list or google for "RFC2833 + Confusions"
Ex:
http://www1.ietf.org/mail-archive/working-groups/avt/current/msg01693.html

Finally, Please read my comments  also.
----- Original Message -----
From: Hashmi, Mohamed Hadi <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, April 01, 2003 3:45 AM
Subject: [AVT] Couple of clarifications on RFC2833

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----

Question-1

What this "corresponding" mean?  Does it mean that all the RTP packets
related to that event should use the SAME timestamp as that of its first
packet or does it mean that timestamp for next packet= timestamp of the
first packet + duration of event?

Bino: Please visualize the DTMF events starting from a RTP TS and after that
the DTMF event packet  will represent its duration. If you really want a
notion of Current RTP TS you can add up  Events Starting RTP TS + Duration.

I have seen some implementations where all the RTP packets for one specific
DTMF event use the timestamp as that of first packet. But this creates an
interesting situation where audio RTP packets will have latest time stamp
and then we have DTMF RTP packet carrying a previous timestamp. Since
sequence number are incremented sequentially for both audio as well dtmf RTP
packets, the dtmf  packets with higher sequence number will have smaller
timestamp value compared to an audio RTP packet with  smaller sequence
number.

Bino: Logic is very simple. Read  session 3.2 ( Simultaneous Generation of
Audio and Events) and read RFC 2198 (RTP Payload for Redundant Audio Data).
Otherwise go ahead with a simple solution, support only outofband DTMF or
inband DTMF. This will dramatically improve the interoperability of your
device.

Question-2

While using inband DTMF detection the sequence numbers are incremented by
"one" for every packet sent by the media gateway for that call whether they
are dtmf or audio. That means in an RTP stream for example ,  sequence
numbers 5,8,9,10,11 could belong to a DTMF event RTP packets while
1,2,3,4,6,7,12,13 to audio RTP packets. This is perfectly alright with
in-band DTMF detection.

How should the  sequence numbers be assigned  for out-of-band DTMF
detection?  As in this case there would be two RTP stream going to different
IP address /port, one for audio and another one for DTMF. If sequence
numbers are assigned as above then both audio receivers as well as DTMF
receiver are going to get gaps in sequence numbers ? Or should there be one
set of sequence numbers for audio stream and another for DTMF stream?

Bino: Please  read RFC 2198 (RTP Payload for Redundant Audio Data).

Thanks

Hadi