On 2/6/2025 12:55 PM, Martin Thomson wrote:
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.
Applications that are sensitive to delays will break their traffic into
multiple QUIC streams. In case of packet loss, only one of those streams
will be blocked, the others will be delivered without "head of queue
blocking". Implementing L2 correction will make the response worse, not
better, because all the streams will be delayed for the duration of the
L2 correction instead of just one.
-- Christian Huitema
