Re: Robustness to packet reordering

[Martin Thomson <[e-mail]>]

On Fri, Feb 7, 2025, at 04:59, Greg White wrote:
> This is an important topic relating to the expectations and 
> requirements that transport protocols place on layer 2 protocols.  In 
> layer 2 standards bodies that I've been involved in, it has been 
> understood that "the upper layers" expect in-order delivery, 

As far as QUIC goes, it is sensitive to reordering in the network.  Some reordering will be interpreted as damage (Christian cited the relevant parts) and performance suffers in a few minor way when things arrive out of order (ACKs are less efficient, data needs to be held, memory accesses are less likely to be contiguous, etc...).

However, the idea that the network might seek to "fix" these problems, when doing so necessarily involves extra work and delays, is not a good trade.  Stuff that is delayed to "fix" a reordering that happened might delay signals that the QUIC stack could use, even if some data needs to be held at the endpoint.  QUIC packets contain many things, some of which don't need to be strictly ordered to be useful.

That's QUIC though.  As you note, L2 isn't necessarily aware of what the upper layers are getting up to.  But I understand that TCP is getting better too.