RE: [tsvwg] Robustness to packet reordering

[Ingemar Johansson S <[e-mail]>]

Hi

I think a draft would be good. I can definitely help and add the 5G specifics.
There is a cost aspect with large reordering buffers in UEs (terminals) and  network nodes for 5G as well and given that link throughput increases so will also memory. But I think the main driver behind my interest is to limit head of line blocking.
Historically, reordering buffers in 5G (and before) has been to improve TCP performance. Now that TCP and other protocols have potential to be more robust against packet reordering then I think that it is time to reconsider how things are done.

And with address to Sebasians and Joes discussion, yes, these are relevant arguments and there is probably no “one shoe fits all”. I am for instance not that keen to allow packets to go completely out of sequence from 5G access as retransmissions on the MAC layer are quite common, but that is perhaps more of concern for a number of transport protocol stacks that do not work well when subject out of sequence delivery.

There is concern about performance for RoHC, RoHC is to day mainly used for VoLTE and VoNR which has its own data radio bearer. I see little utility with RoHC otherwise.


One question, would this be a relevant discussion point at the next IETF ?, and if so, which WG ?.

/Ingemar

From: Greg White <g.white=[e-mail]>
Sent: Wednesday, 12 February 2025 00:38
To: [e-mail]
Cc: Ryan Hamilton <[e-mail]>; Martin Thomson <[e-mail]>; Greg White <[e-mail]>; Ingemar Johansson S <[e-mail]>; [e-mail]; [e-mail]
Subject: Re: [tsvwg] Robustness to packet reordering

I really think that  https://www.rfc-editor.org/rfc/rfc3819#section-15 is lacking.  It doesn’t seem to me that it is a case of the advice there not being heeded.  The advice appears to be that you’d better provide guaranteed in-order delivery unless you want to have terrible TCP performance and broken header compression.  It didn’t omit mentioning that it is the header compression receiver’s responsibility to handle reordering, it outright says that it is the subnetwork’s responsibility.  For a BCP, I think we need to be more clear.

The fact that all 5G, Wi-Fi and DOCSIS gear (perhaps other links too) on the planet have specialized protocol functions (frame sequence numbering, resequencing buffers, multiple simultaneous resequencing contexts, logic to handle sequence loss, etc.) to do this feature that we mostly agree is objectively bad, is also evidence that we should be more clear on this point.  The DOCSIS spec for example has 52 pages of text that discuss resequencing requirements, and every cable modem (these are cost sensitive) is required to support 16 simultaneous resequencing contexts (to handle multiple simultaneous service offerings) each of which is required to support up to 13ms of data.  I’ve also been told that there was a recent proposal to eliminate resequencing requirements in 802.11 for Wi-Fi 8, but it was abandoned.

I don’t know about other sections in the document, but I do think we should provide new clarity on the topic of packet reordering.  I’ll put together a strawman draft, but would welcome help.

-Greg

From: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Date: Tuesday, February 11, 2025 at 8:47 AM
To: Greg White <[e-mail]<mailto:[e-mail]>>
Cc: Ryan Hamilton <rch=[e-mail]<mailto:rch=[e-mail]>>, Martin Thomson <[e-mail]<mailto:[e-mail]>>, Greg White <g.white=[e-mail]<mailto:g.white=[e-mail]>>, Ingemar Johansson S <ingemar.s.johansson=[e-mail]<mailto:ingemar.s.johansson=[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: Re: [tsvwg] Robustness to packet reordering

Hi, Greg,

On Feb 10, 2025, at 2:35 PM, Greg White <[e-mail]<mailto:[e-mail]>> wrote:

Joe,

Thanks for pointing to that reference.  I assume that is the most definitive guidance that the IETF has given to L2 networks on the topic, and any future changes to that guidance could take the form of updates to RFC3819.

I agree with you that the sentence you quoted seems reasonable, but in the context of the rest of the text in that section in RFC3819, it seems to me that the warnings about TCP performance and header compression might undercut the recommendation.

The warnings are about compression - and still apply. If the compression algorithm includes dependencies between sequences of packets, then those sequences have to be restored for the compressor to work. Perhaps what isn’t said is that if the compressor has such dependencies, then IT should perform the needed reordering, rather than expecting the network to do so for it. The prior section addresses the impact of loss on compression, but overlooked the impact of reordering.

 I think many L2 designers consider TCP performance to be important (even if they don’t know the details of current implementations), and they also might not be willing to take the risk that their link would break someone’s header compression scheme (users do lots of different things!).

Yes, but again this argues for the compressor to reorder, not L2.

Is there value in updating that section?

Certainly the entire doc could include more recent references and could be subbed for omissions such as above, perhaps providing more comprehensive advice up front, e.g.,:
- whatever you expect L2 to do, do at the ends of L3 in or in front of protocols that depend on those features
- but do NOT engineer the entire L2 for any of those features

But in a sense that’s just reinforcing the advice in the E2E paper, which doesn’t need (IMO) to be revised simply to refer to more recent examples.

At a minimum we could point to RACK, L4S and the QUIC packet reordering threshold along with whatever consensus we can develop around the idea that transports that are interested in performance already (or at least can) implement reordering tolerance, and that the benefits of minimizing delay outweigh any slight benefits provided to older transport implementations.  That said, this thread has seen several opinions (not all in agreement) so it might be challenging to get consensus.

And that’s part of the issue as well; to the extent that our docs lack clear, direct advice, it can be the result of the consensus process.

I don’t particularly think an update is warranted - IMO, what’s needed is for the advice that’s there to be heeded.

Joe


-Greg

From: Joe Touch <[e-mail]<mailto:[e-mail]>>
Date: Friday, February 7, 2025 at 3:18 PM
To: Ryan Hamilton <rch=[e-mail]<mailto:rch=[e-mail]>>
Cc: Martin Thomson <[e-mail]<mailto:[e-mail]>>, Greg White <g.white=[e-mail]<mailto:g.white=[e-mail]>>, Ingemar Johansson S <ingemar.s.johansson=[e-mail]<mailto:ingemar.s.johansson=[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: [tsvwg] Re: Robustness to packet reordering


On Feb 7, 2025, at 2:12 PM, Ryan Hamilton <rch=[e-mail]<mailto:rch=[e-mail]>> wrote:
….
Let's not hobble the performance of modern protocols in order to *potentially* provide minimal improvements to the performance of obsolete implementations.

Agreed. As I noted, RFC3819 still has imo the best advice:


   This suggests that subnetwork implementers should try to avoid packet

   reordering whenever possible, but not if doing so compromises

   efficiency, impairs reliability, or increases average packet delay.