Re: Robustness to packet reordering

[Christian Huitema <[e-mail]>]

RACK is the default for QUIC. The detection gap is set to 3 by default, 
that is, a packet considered lost if a packet N+3 or later has been 
received. Implementations are free to implement dynamic RACK gaps and 
the ACK Frequency extension can ask the peer to send packets 
accordingly, but AFAIK most implementations go for robustness and keep 
the detection gap fixed.

Note that QUIC does not "reorder" packets -- packets are processed as 
soon as they arrive.

-- Christian Huitema

On 2/5/2025 10:50 PM, Ingemar Johansson S wrote:
> Hi Christian
>
> I was perhaps a bit unclear, sorry.
> I refer to "in the stack".
>
> The RACK function in Linux TCP increases the reordering window when packet reordering is detected and thus avoids fast retransmits. In current Linux TCP the reordering window can increase to accept up to one RTT reordering.
>
> Do any current QUIC stack support this RACK functionality and/or is it planned.
>
> /Ingemar
>
>> -----Original Message-----
>> From: Christian Huitema <[e-mail]>
>> Sent: Thursday, 6 February 2025 02:00
>> To: Ingemar Johansson S
>> <ingemar.s.johansson=[e-mail]>; [e-mail]
>> Cc: Ingemar Johansson S <[e-mail]>
>> Subject: Re: Robustness to packet reordering
>>
>>
>> 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
>>
>>