| Internet-Draft | Max. Ack Delay & QUIC Loss Recovery | September 2024 |
| Eggert | Expires 21 March 2025 | [Page] |
This document updates RFC9002 by specifying how the max_ack_delay is
incorporated into the computation of the loss_delay used for time-based loss
recovery.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://larseggert.github.io/quic-9002-mad/draft-eggert-quic-9002-mad.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-eggert-quic-9002-mad/.¶
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The maximum acknowledgement delay (max_ack_delay) is the maximum amount of
time by which the receiver intends to delay acknowledgments for packets in the
Application Data packet number space, as defined by the eponymous transport
parameter (Section 18.2 of [RFC9000]).¶
This document updates [RFC9002] by specifying how the max_ack_delay is
incorporated into the computation of the loss_delay used for time-based loss
recovery.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Appendix A.10 of [RFC9002] contains pseudo code for detecting lost packets in
a packet number space. That code includes a computation of loss_delay, i.e., a
time in the past before which all sent packets are deemed lost.¶
loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
¶
This computation of loss_delay does not take max_ack_delay into account;
kTimeThreshold (which is 9/8) is the only stretch factor that adds some
affordance for delayed ACK delivery.¶
When the peer ACK delay approaches or (especially) when it exceeds 1/8 * RTT,
this can cause the sender to declare sent packets as lost, causing unneeded
early retransmissions.¶
This document updates this calculation of loss_delay to take max_ack_delay
into account:¶
loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt) + max_ack_delay
¶
As in Section 6.2.1 of [RFC9002], when loss_delay is calculated for Initial
or Handshake packet number spaces, the max_ack_delay in the loss_delay
computation is set to 0, since the peer is expected to not delay ACKs in these
packet number spaces intentionally; see Section 13.2.1 of [RFC9000].¶
No security considerations are introduced by this document.¶
This document has no IANA actions.¶
These individuals suggested improvements to this document:¶