[R-C] Sender- vs. receiver-based, yet again...

[Michael Welzl]

Hi,

Sorry for bringing this up yet again - but my thoughts just keep returning to the question of sender-vs.-receiver based congestion control, because it's a seemingly simple problem for which we should be able to agree on a clear answer?!

As most of you probably know by now, I'm critical of a receiver-based approach as in RRTCC. (In fact, as I will explain further below, I argue against a *combined* sender+receiver approach). Here is a plus/minus list as I see it:


PLUS no. 1: it helps to reduce the amount of feedback. That's really the ONLY benefit.

MINUS no. 1: it requires the same mechanism on the sender and receiver side, which means that, if RTP becomes a "framework" for congestion control mechanisms, we must signal which mechanism is used to the other side. In contrast, we don't have to do this with TCP's (implemented but not standardized) pluggable congestion control. This is, to me, the biggest problem, as it requires code updates to happen on both sides for a new mechanism to be deployed.

MINUS no. 2: it could make combined congestion control between multiple flows via the FSE more complicated. Anyway it seems we'll need an FSE on the sender AND receiver side, but with receiver-based congestion control, we'll face the question: for two flows between the same two hosts, sharing the same bottleneck, should the receivers exchange information and coordinate their feedback, or should the senders exchange information and coordinate their behavior?  => when congestion control mechanisms are split across sender and receiver, the right answer to this question becomes a per-mechanism decision.


Please amend this list if I'm missing something. Regarding the one PLUS above, here's a critical look at the benefits of reducing feedback **via receiver-side congestion control**:

- less feedback means a greater chance for the feedback to be dropped, which translates into a greater chance for undesirable sender behavior
- we deal with interactive traffic, in which case a lot of the feedback could hopefully be piggybacked onto payload packets
- in some cases there will be other parallel flows (e.g. the data channel in rtcweb) that already give us all the feedback that we need, this is another chance for feedback reduction
- receiver-side-congestion-control-based feedback reduction as in RRTCC means to reduce the feedback whenever feedback isn't urgent for the sender. I'd argue that, in fact, we should reduce feedback only when the channel carrying it (the backwards channel) is congested, and that is a generic function, not bound to the sender-side congestion control behavior.


So how urgent is it to reduce feedback, anyway? Some time ago, Bill Ver Steeg wrote:

***
1- The upstream/return traffic is (generally) assumed to be a precious resource by the RTP community. This is partially due to multicast considerations (which we can mostly ignore here) and partially due to asynchronous networks. Note that the CableTV community has found it necessary to play games with TCP acks in Cable Modems because of upstream bandwidth limitations, even for unicast TCP traffic. This is an ugly hack that I will not defend here, but it is indicative of the problem space. The net-net is that we have to be somewhat conservative in our sending of acks, or we will end up in similar trouble.
***

As he states, multicast is out of scope. Plus, I think we should care more about technical reasons than what the RTP community considers to be a precious resource. As for the CableTV example, one can always find examples where people started doing useless things  :-)   but consider the following: many parallel TCP connections, each sending lots of feedback, work okay for maaaany end users that 1) surf the Internet or (bringing it closer to rtcweb thinking)  2) use P2P applications. And TCP doesn't even do any form of congestion control on ACKs (an RFC exists but noone uses it (so far)).

I'm not saying ACKs never cause problems. I'm saying it's probably rare that they do, and reducing their number WHEN they cause trouble is good enough (and, I would argue, a better decision).


Let's take a step back and summarize the situation:
1) congestion-control-based-feedback-reduction has at least one significant MINUS (no. 1), probably two
2) the benefits of doing it seem minimal
3) not being based on congestion along the backwards path, it misses the point somewhat...


As a result of the above, I would suggest to move all congestion control functionality to the sender side and keep the receiver side dumb, as in TCP, but amended with some generic (i.e. not mechanism-specific) feedback-reduction functions (ACK congestion control; piggybacking; omitting feedback when it is known (e.g. via a signal from the sender) that we get feedback from another flow).

Alternatively, we could still remove the MINUS no. 1 above by agreeing on a generic sender behavior. IIRC, in RRTCC, the sender is rather simple - it follows the rate dictated by the receiver and has a timeout behavior. If all congestion control mechanisms are installed on the receiver side only, with the same agreed generic sender behavior for everyone, we can also have pluggable congestion control without a need for signalling.

It's just the idea of multiple proposals for congestion control involving newly described sender AND receiver behavior that strikes me as bad design.

Thoughts?

Cheers,
Michael