Re: [AVT] Port mappings, NAT, and RAMS

["Bill Ver Steeg (versteb)" <versteb at cisco.com>]

I agree that we should make a general solution for mapping unicast
sessions onto unicast sessions. 

We need to solve this problem using data flows that will work across
multiple vendors' solutions, and thus need to do it in a standards
forum. The IETF is the correct place to do this.

Bill VerSteeg 

-----Original Message-----
From: Zeev Vax [mailto:zeevvax at microsoft.com] 
Sent: Tuesday, June 30, 2009 9:07 AM
To: Dave Oran (oran); Roni Even
Cc: Ali C. Begen (abegen); 'VAN CAENEGEM Tom'; Bill Ver Steeg (versteb);
avt at ietf.org
Subject: RE: [AVT] Port mappings, NAT, and RAMS

I agree this should be a separate draft and we should work on a more
generic mechanism.
Two comments I have, first being a general problem it might have been
solved already so I think we should research the area before investing
in another draft. Second given such functionality is typically part of
an application and not staying only in the lower network layers, the
application can solve it as part of the startup sequence or other data
delivery mechanism it has.

Zeev

-----Original Message-----
From: David R Oran [mailto:oran at cisco.com]
Sent: Tuesday, June 30, 2009 6:10 AM
To: Roni Even
Cc: 'Ali C. Begen (abegen)'; 'VAN CAENEGEM Tom'; 'Bill Ver Steeg
(versteb)'; Zeev Vax; avt at ietf.org
Subject: Re: [AVT] Port mappings, NAT, and RAMS


On Jun 30, 2009, at 6:55 AM, Roni Even wrote:

> Hi guys,
> My view is that this is not a general NACK problem but only for the 
> case of SSM and retransmission with similar box architecture to the 
> RAMS solution.
> If you have another case please explain
I believe it is actually a MORE general problem than NACK or RAMS. I
arises from mixing SSM sessions and unicast sessions for the same RTP-
based application. That's because multicast enforces port symmetry among
all the receivers while unicast needs per-receiver port selection.

So far we have two uses for mixing multicast and unicast:
retransmission-based repair, and RAMS. I suspect more uses will arise in
the future. I can think of a few off the top of my head:
- customized per-user overlays (e.g. to be alpha-blended with the
video) or object-level replacement
- network-coded error recovery streams
- extra media info for re-integration when switching back from unicast
to multicast in a fast-forward-to-realtime capability when doing
"pause-live-content" style services
- adaptive switching between multicast and unicast based on number of
simultaneous receivers.

I have a strong intuition that doing a generic mechanism to atomically
signal ports for unicast sessions using the RTCP feedback machinery of
the SSM session would be highly advisable.

So, I think:
a) we need to solve this problem
b) it exists even in the absence of NATs
c) we need a generic mechanism
d) it should be documented in a separate draft, and not part of RAMs or
AVPF.

If we can agree to these points, then we can have a more detailed
discussion of the appropriate machinery. I have opinions on that subject
too.

DaveO.



> Roni
>
>> -----Original Message-----
>> From: avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of

>> Ali C. Begen (abegen)
>> Sent: Tuesday, June 30, 2009 10:19 AM
>> To: VAN CAENEGEM Tom; Bill Ver Steeg (versteb); Zeev Vax; Roni Even; 
>> avt at ietf.org
>> Subject: Re: [AVT] Port mappings, NAT, and RAMS
>>
>> Hi Tom,
>>
>> Let's get the input from the WG on these points. I believe going for 
>> a more general solution that will be applicable to both RAMS and NACK

>> is more desirable.
>>
>> BR,
>> -acbegen
>>
>>
>>
>>> -----Original Message-----
>>> From: VAN CAENEGEM Tom [mailto:Tom.Van_Caenegem at alcatel-lucent.be]
>>> Sent: Monday, June 29, 2009 9:17 AM
>>> To: Ali C. Begen (abegen); Bill Ver Steeg (versteb); Zeev Vax; Roni
>> Even; avt at ietf.org
>>> Subject: RE: Port mappings, NAT, and RAMS
>>>
>>>
>>> Ali,
>>>
>>> See some comments below (TOM).
>>>
>>> -----Original Message-----
>>> From: Ali C. Begen (abegen) [mailto:abegen at cisco.com]
>>> Sent: vrijdag 26 juni 2009 17:41
>>> To: VAN CAENEGEM Tom; Bill Ver Steeg (versteb); Zeev Vax; Roni Even;

>>> avt at ietf.org
>>> Subject: RE: Port mappings, NAT, and RAMS
>>>
>>> Hi Tom,
>>>
>>> [snip]
>>>> The reason why we need pubports is that it indicates "explicitly"
>> the
>>>> port the client wants to use to receive the burst is the port it
>> used
>>>> to send the RAMS-R/pubports message. I believe we need this
>> explicit
>>>> indication, ow, we will be making an implicit assumption. Note that

>>>> the rams-r/pubports is in the primary session and burst/ret packets

>>>> are in the retransmission session, which is different than the
>>> primary.
>>>>
>>>> As far as I can see, not everybody agrees on this point, but we
>> need
>>>> to make a decision here before we mandate anything.
>>>>
>>>> TOM: I agree with your last line, but I do not completely agree
>> when
>>>> you mention "implicit" assumption, especially for option 3, where 
>>>> RAMS-R (or
>>>> NACK) and the Pub-port message are in the same compound message. If
>> we
>>>
>>>> "define" that if no pub-report message is sent by the RR, the 
>>>> information towards the Retransmission server is exactly the same
>> as
>>>> when an RTP receiver sends an RTCP compound message made up of an
>> RTCP
>>>
>>>> NACK/RAMS-R together with a pub-port message where the ports are
>>> "zero"
>>>
>>> We "may" be able to say it from RAMS-R but not for NACK since we are
>> not
>>> defining it, we will be overloading its meaning, IMO. And as you/ 
>>> Bill pointed out in other emails, I also believe that this is more 
>>> general problem than just RAMS. We need something like pubports for 
>>> NACKs as well.
>>>
>>>
>>> TOM: indeed, that is why IMO we should define this problem and
>> solution
>>> in a separate draft which applies to any system architecture where a

>>> unicast session from a FT is coupled to a ssm session with that same
>> FT.
>>>
>>>
>>>> -as currently proposed in the RAMS draft now.  I agree that this is

>>>> not true for option 4, where pub-port and RAMS-R or NACK are
>> separate
>>>> messages, but I do not favour the solution of option 4 for the same

>>>> reason you mention in your reply (a new failure mode). So, if
>> option 4
>>>
>>>> is -for that reason- no longer considered, I do not see a problem
>> with
>>>
>>>> defining this rule/clause as expressed above. In other words, there
>> is
>>>
>>>> no need for a pub-report message in this scenario and this results
>> NOT
>>>
>>>> in an "assumption" what-so-ever.
>>>>
>>>>
>>>>> supports RTP/RTCP muxing itself, and if it knows the RS supports 
>>>>> RTP/RTCP muxing too (as indicated in the declarative sdp received
>> by
>>>
>>>>> the
>>>>> RR)
>>>>>
>>>>> -Don't we also allow that for option 3, the port P2 actually
>> equals
>>>>> port P4? This is a kind of hybrid of session and SSRC
>> multiplexing
>>>>> of the primary multicast RTP stream and the 
>>>>> retransmission/acceleration session
>>>>> :  on the unidirectional RTP layer, the primary MC packets and
>> the
>>>>> unicast retransmission packets are clearly session muxed, but the 
>>>>> RTCP
>>>>
>>>>> "return" channel for both streams have exactly the same transport 
>>>>> addresses, but are distinguished by means of a different SSRC.
>> In
>>>>> that
>>>>
>>>> How? Since SSM and retransmission sessions are session muxed, they 
>>>> have the same SSRC. When a client is sending a receiver report, how
>> do
>>>
>>>> we identify which session it belongs to? So, I believe P2 should be

>>>> different than P4.
>>>>
>>>> TOM: you are right. But notice this: RFC 4588 mandates that when 
>>>> session muxing is used for retransmission, than the SSRC should be
>> the
>>> same.
>>>> However the RFC 4588 does not give arguments why it needs to be
>> like
>>>> that. So I ask you: what would or can happen if there is a
>> different
>>>> SSRC used?  If there are no convincing arguments why we must stick
>> to
>>>
>>> Good question. If we don't use the same SSRC in session-muxed RTP 
>>> sessions, it will introduce correlation problems. Receiver/XR 
>>> reports coming from a larger number of receivers will be difficult 
>>> to
>> identify
>>> and match at the server side. There will be SSRC collisions, which
>> will
>>> complicate things further.
>>>
>>> TOM: the big difference with when the RFC 4588 was being discussed
>> and
>>> developed, is that now we are capable of advertising in SDP the SSRC
>> of
>>> the media server, that is in this case the retransmission server 
>>> SSRC used in the retransmission session. In fact for RAMS, we need 
>>> to
>> define
>>> the SSRC of the ssm RTP session, so we can as well do it for the 
>>> retransmission session. Perhaps we should consider this different
>> SSRC
>>> possibility (hence violating RFC 4588) as part of a new draft, the
>> one I
>>> mentioned above.
>>>
>>>
>>>> the same SSRC, then, if we have a good use case to depart from that

>>>> "rule" -and the use case we are discussing here is IMO indeed a
>> valid
>>>> use case- I do not see why we should be limited by such a "rule".
>> The
>>>> "valid" use case is here that RAMS and Retransmission for SSM
>> sessions
>>>
>>>> operates fine through any NAPT and without any dedicated RTCP
>> message.
>>>
>>> -acbegen
>> _______________________________________________
>> Audio/Video Transport Working Group
>> avt at ietf.org
>> https://www.ietf.org/mailman/listinfo/avt
>
> _______________________________________________
> Audio/Video Transport Working Group
> avt at ietf.org
> https://www.ietf.org/mailman/listinfo/avt