Subject: [AVT] Size of RTCP packets using IPv6

From: Belling Thomas <[email-masked]>
Date: Fri, 13 Feb 2004 17:44:02 +0100

Hi all,

within the 3GPP there were some discussions about realistic sizes of RTCP compound packets in an IPv6 Network.

Apart from the IPv6/UDP header each packet contains at least a SR (52 bytes) or RR (32 bytes), plus SDES package including a CNAME is also mandated in each compound RTCP package. This includes a header of fixed size (6 bytes), plus a "host" or "user@host" part of variable size..

The last part is tricky. According to RFC 1132 (which was written for IPv4, not taking into account the larger IPv6 address space), "Host software MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters." Expressing a full IPv6 address in ASCI notation also requires up to 39 characters. I assume one should expect this part to be at least around 100 bytes. Other opinions?

There were also some ideas to restrict the recommended size of compound RTCP packets to around 90 bytes (exluding IP/UDP), and optimise transmission for such a size.
I doubt this is a good idea:
*	My guess of a "minimum" size already exceeds this value.
*	translators and mixers should combine RTCP packets
*	A host should send only one RTCP compound packet by report interval, because this is assumed in the bandwidth/frequency algorithms for calculating the RTCP sending times. This implies the host should combine any information it needs to send in this period within one compoung packet.
*	Host may therefore need to combine several sender and receiver reports (e.g. if multiple senders/receivers are involved, or multiple media streams per host), or may need to add mandatory RTCP extension packets defined in RTP profiles.
Do you agree?

Acoording to your experience, what are realsitic sizes of RTCP packets with IPv6?

Thank you for your help.

Thomas Belling

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From: Magnus Westerlund <[email-masked]>
Date: Tue, 17 Feb 2004 17:58:42 +0100

Hi Thomas,

[Taking chair hat off]

I would like to point out to you that using explicit IPv6 addresses is 
probably not the smartest move for having a short and compact CNAME. I 
think that a CNAME of around 25 bytes is realistic if one makes the 
effort to find short and good CNAMEs. For a mobile phone the 
[email-masked] would result in such a size as long as one tries to 
use short domain names. It would also fullfill the requirements on a 
CNAME. I do not expect that a mobile phone will be capable of handling 
two simultanous non-related media sessions. Thus this would provide 
global unique names.

See more comments inline:

Belling Thomas wrote:

> Hi all,
> 
> 
> within the 3GPP there were some discussions about realistic sizes of 
> RTCP compound packets in an IPv6 Network.
> 
> Apart from the IPv6/UDP header each packet contains at least a SR (52 
> bytes) or RR (32 bytes), plus SDES package including a CNAME is also 
> mandated in each compound RTCP package. This includes a header of fixed 
> size (6 bytes), plus a "host" or "user@host" part of variable size..
> 
> The last part is tricky. According to RFC 1132 (which was written for 
> IPv4, not taking into account the larger IPv6 address space), "Host 
> software MUST handle host names of up to 63 characters and SHOULD handle 
> host names of up to 255 characters." Expressing a full IPv6 address in 
> ASCI notation also requires up to 39 characters. I assume one should 
> expect this part to be at least around 100 bytes. Other opinions?

I think this depends very much on the scenario and where the user is 
connected. A lot of people can have a rather short ones. I think Colin 
at his previous work could have has one like this:

[email-masked]

This is a 20 characters, and me taking the liberty to invent a host name 
of 8 characters. Then on the other side the maximum length is 255 
characters.

I think that rather many will suffice with a maximum of 40 chars. A few 
extreme cases could come up to the 100 you mention. However this takes 
either a surprisingly long domain name or explicit IPv6 address combined 
with the user name from hell.

> 
> There were also some ideas to restrict the recommended size of compound 
> RTCP packets to around 90 bytes (exluding IP/UDP), and optimise 
> transmission for such a size.
> 
>     * A host should send only one RTCP compound packet by report
>       interval, because this is assumed in the bandwidth/frequency
>       algorithms for calculating the RTCP sending times. This implies
>       the host should combine any information it needs to send in this
>       period within one compoung packet.
>     * Host may therefore need to combine several sender and receiver
>       reports (e.g. if multiple senders/receivers are involved, or
>       multiple media streams per host), or may need to add mandatory
>       RTCP extension packets defined in RTP profiles.
> 
> Do you agree?
> 

I think a size of RTCP packets of around 100 bytes without lower layers 
are possible, under the following restrictions from the scenario

- VoIP one to one communication, i.e. no conferences with mixers, one 
SSRC per end-point.
- One selects CNAME in a smart way.
- One does not have any RTCP extensions.
- No need for further SDES items.

However if one needs to support a wider application scenario than the 
one the calculations was done for, then of course the size will be bigger.

I would also like to point out the greater need for RTCP in a conference 
application. In a conference application you have greater need to use 
the SDES items to identify who is actually speaking. This is even more 
important if some kind of mixer, or traffic reflector is used. Also the 
need for SSRC collision detection, network feedback becomes more 
important as one has more participants in a session.

> Acoording to your experience, what are realsitic sizes of RTCP packets 

I think this is a very hard question as it depends on the scenario. From 
60 bytes of RTCP when on is alone in the session, to a whole MTU 
reporting on a multiple of sources, plus using extended reporting, for 
example RFC 3611.

I am still hoping that this 3GPP debate will be resolved in such a way 
that RTCP will not be turned off, and that RTCP can received a 
reasonable QoS in regards to the media flow.

Best Regards

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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