Re: [Ecn-sane] [tsvwg] Compatibility with singlw queue RFC3168 AQMs

[Mikael Abrahamsson <swmike@swm.pp.se>]

On Wed, 7 Aug 2019, Sebastian Moeller wrote:

> 	I guess I am overly naive, but as far as I can tell only TCP has 
> this strong shuffling-sensitivity, UDP itself does not care 
> (applications still might dislike packet shuffling). Could the

I know applications that require packets (using UDP) to be arriving in 
order, because an out of order packet is considered a lost packet. These 
applications are designed with the presumtion that the network deliver the 
packets in order (at least within the 5 tuple).

I guess basic TCP (without SACK) can be argued as the same property?

> intermediate hops not simply block TCP and just pass on UDP? This should 
> at least avoid the medium idling while waiting for the straggling 
> packets. I guess that is tricky in that a medium's ARQ might not look 
> past its own headers, "sequence identifiers" and checksums, clearly that 
> is not enough to get to the protocol ID (add to this IPv6 extension 
> headers and the required deep dive).

I think the argument can be made that TCP actually is less sensitive to 
packet reordering (with SACK) than UDP. With TCP it's fairly well 
understood what happens, with UDP we have no idea, because we don't know 
what application is running.

> The fact that is not implemented yet, indicates to me that the ECT(1) 
> thing is also not likely to make more inroads, what am I missing?

The whole concept of "let's try to figure out what traffic can be 
delivered out of order and what needs to be in order" is a fairly new 
concept. I am not surprised this hasn't been implemented yet. There is 
absolutely no current consensus on what traffic can be re-ordered or not. 
L4S suggests an implicit mark on "new" transports that are allowed to be 
re-ordered. I have not seen any such proposal before. Typically the L2/ARQ 
designers are sitting there designing their thing without knowing anything 
about L3 or L4 basically. Having this kind of cross-layer approach is kind 
of new from what I can see.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se