Re: [tcpm] [tsvwg] Cross-area alignment on "L4S and RACK"

"Scharf, Michael" <> Fri, 09 November 2018 06:21 UTC

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From: "Scharf, Michael" <>
To: Bob Briscoe <>, "" <>
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Thread-Topic: [tsvwg] Cross-area alignment on "L4S and RACK"
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Subject: Re: [tcpm] [tsvwg] Cross-area alignment on "L4S and RACK"
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I don’t see the fundamental difference between a link technology that guarantees in-order delivery within a flow (say by flow hashing, see ECMP) vs. some link scheme that guarantees in-order delivery, say, only for some fraction of the traffic characterized by some other means (say, other header bits).

Clearly, reordering is a problem that needs to be discussed in the IETF as a whole. And we could discuss whether a revision of RFC 3366 is needed.

But I am not convinced that this topic has to be tied into an experiment such as L4S.


From: Bob Briscoe []
Sent: Friday, November 9, 2018 6:42 AM
To: Scharf, Michael;
Subject: Re: [tsvwg] Cross-area alignment on "L4S and RACK"


That is the point I thought you were making, and my point /is/ about that.

If an application needs all the packets (i.e. a reliable protocol), it's not useful to reduce the latency of some packets but not all. That's where the in-order requirement comes from.

However, if a stream-based transport is used (i.e. TCP) the /application/ still gets in-order delivery from TCP. It just doesn't have to require links to provide in order delivery as well.

Note that there can be multiple flows over one link. So 'in-order delivery' for a link does not mean in order of TCP sequence number. It means 'delivered in the order that packets arrived at the link' (the link ingress adds its own sequence numbers, and the link egress buffers them until it can send out in the same order, or until a time-out or max no. of link retransmissions).

So, if the link loses a packet from flow A (the link isn't aware of microflows, I'm just saying the packet happens to be from flow A), link-layer in-order delivery will hold back all packets sent after that packet (from all flows, not just flow A) until the link repairs the gap or gives up.

So hop-by-hop in-order delivery gives every flow the same delay as the worst delayed flow.

Of course, someone building a network for DETNET would avoid link technologies like WiFi or LTE that frequently lose and retransmit packets. But wherever there is variable delay in a link, guaranteed in-order delivery per-hop means every flow is guaranteed to always get the worst delay from every link, which would have been experienced by only one flow without per-hop in-order delivery.

And many of the applications of DETNET involve dumb industrial machinery that just expects everything to arrive in order and to a clocked schedule. But it's always as effective to deploy a resequencing buffer as the penultimate hop, screwed onto the receiving machine if necessary, rather than require per-hop resequencing in the network.

So, why do Internet links often ensure in-order delivery even though TCP puts everything in order for the application?

Short answer: the 3 DupACK rule (which is to do with loss-recovery, a different issue from the discussion above).

Long answer: As flow rates have scaled up, the typical time between 3 packet arrivals has become so small that TCP's 3-DupACK rule makes links have to provide delivery with only a very small re-ordering degree - so small that it's easier for a link to just deliver everything in the order it received it; not because TCP doesn't put packets back in order, but just so as not to trigger TCP into generating spurious retransmissions.

On 08/11/2018 16:35, Scharf, Michael wrote:
Inline [ms]

From: Bob Briscoe []
Sent: Thursday, November 8, 2018 12:56 PM
To: Scharf, Michael;<>
Subject: Re: [tsvwg] Cross-area alignment on "L4S and RACK"


This is merely a symptom of a difference in opinion on where the resequencing function should be located.

  *   L4S takes the end-to-end approach saying that it is sufficient to do resequencing in one place (the receiving host) if it is needed. Then any resequencing delay only affects that flow.
  *   The DETNET approach is saying the resequencing must be done hop-by-hop. This means there appears to be no resequencing needed on the receiver (except if there's re-ordering on the final hop).

  *   In order to guarantee no resequencing in the network (DETNET approach), only the the worst case latency can be guaranteed, and every packet has to have that same worst-case latency.
  *   When you leave resequencing to the receiver (end-to-end approach), most of the packets arrive earlier than they would with DETNET, but out of order ones might not. Then the application chooses whether it wants to wait.
[ms] My point is different: There seem to be other working groups in the IETF that do talk about “low latency” as well but apparently they also do need in-order delivery of packets, too. So the assumption that e.g. a link layer technology can simply disable in-order delivery for all “low latency” applications seems not correct. The reality may be a bit more complex.
The 3 DupACK rule in TCP led the Internet not to follow the end-to-end principle on re-sequencing. Now we're removing the 3 DupACK rule, we can take advantage of the e2e principle.
[ms] For TCP, three DupACKS are a SHOULD in RFC 5681 and RFC 5681 is standards track. So the bar for “removing” that rule for TCP is not low…
[ms] Also, the end-to-end principle comes with tradeoffs. For instance, relying on the end-to-end principle for recovery of bit errors is not efficient. For in-order delivery there are tradeoffs as well, see e.g. Section 4 in draft-ietf-tcpm-rack-04. There may be no free lunch.
Of course, an application might choose to use TCP and L4S, rather than UDP and L4S (e.g QUIC). Then there could still be HoL blocking in the receiving TCP stack. But at least there's no HoL blocking in the network, and the app can choose between stream (TCP) or datagram (UDP).
[ms] If LRO/GRO in the receiving TCP stack gets messed up, I fail to see the benefit (see draft-ietf-tcpm-rack-04).

On 06/11/2018 19:20, Scharf, Michael wrote:

A comment on the TCPM presentation "L4S and RACK":

As far as I understand, the DETNET WG in RTG area has quite some uses cases for ultra-low latency transport - in particular also with bounded jitter. And some of these applications (e.g. using UDP) apparently may not be able to tolerate *any* out-of-order packet delivery.

So perhaps some cross-area alignment on ulta-low-latency application requirements would be useful?


(who recently had to review draft-ietf-detnet-architecture)



Bob Briscoe                     



Bob Briscoe