On 2/5/2025 6:21 AM, Ingemar Johansson S wrote:
Hi
A very short question, with possibly many answers:
Do current QUIC stacks support out of order delivery up to one RTT, as is the
case with TCP Linux with RACK enabled.
Define support. Do you mean "at the API" or "in the stack"?
The programming APi for QUIC exposes a set of "streams", as well as
possibly a "datagram" capability. Data sent on one stream is delivered
in sequence on that stream. Multiple streams can be processed in
parallel. There is no guarantee of ordering between stream. If a packet
containing data for stream number N is lost, delivery on that stream
will stop until the loss recovery -- but delivery will continue for
other streams.
Data sent as datagram is delivered whenever the datagram arrives. There
is no guarantee of ordering between datagrams, or between datagrams and
streams. There is also no expectation of loss recovery: if a packet
containing a datagram is lost, the datagram is lost.
In the stack, QUIC stacks are expected to check whether a packet was
"already received", and not process duplicates. Note that if packets are
lost, the frames contained in the packet may be processed differently --
some may not be resent, either because they are now obsolete, or because
the frame has already been received -- for example, in case of spurious
retransmission. The packets carrying the repeated data will have their
own packet number, different from the initial packet.
In order to not process duplicate, implementations have to maintain
knowledge of already received packets. That knowledge typically has some
kind of horizon, such as "the last N packets". Packets that are older
than that will be ignored, because there is no way to tell whether they
are duplicate. The value of that "horizon" is implementation dependent.
About 1 RTT worth of packet makes sense, but implementations could use
something else, like for example 3*PTO.
-- Christian Huitema
