Re: [net-next,v9,09/15] quic: add congestion control

On Thu, Feb 5, 2026 at 6:55 AM Simon Horman <[email protected]> wrote:
>
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>
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> ---
> quic: add congestion control
>
> This patch introduces quic_cong for RTT measurement and congestion control,
> implementing a state machine with slow start, congestion avoidance, and
> recovery phases using the New Reno algorithm.
>
> > diff --git a/net/quic/cong.c b/net/quic/cong.c
> > --- /dev/null
> > +++ b/net/quic/cong.c
>
> [ ... ]
>
> > +static int quic_cong_check_persistent_congestion(struct quic_cong *cong, u64 time)
> > +{
> > +     u32 ssthresh;
> > +
> > +     /* rfc9002#section-7.6.1:
> > +      *   (smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay) *
> > +      *      kPersistentCongestionThreshold
> > +      */
> > +     ssthresh = cong->smoothed_rtt + max(4 * cong->rttvar, QUIC_KGRANULARITY);
> > +     ssthresh = (ssthresh + cong->max_ack_delay) * QUIC_KPERSISTENT_CONGESTION_THRESHOLD;
> > +     if (cong->time - time <= ssthresh)
> > +             return 0;
>
> [ ... ]
>
> > +/* rfc9002#section-5: Estimating the Round-Trip Time */
> > +void quic_cong_rtt_update(struct quic_cong *cong, u64 time, u32 ack_delay)
> > +{
> > +     u32 adjusted_rtt, rttvar_sample;
> > +
> > +     /* Ignore RTT sample if ACK delay is suspiciously large. */
> > +     if (ack_delay > cong->max_ack_delay * 2)
> > +             return;
> > +
> > +     /* rfc9002#section-5.1: latest_rtt = ack_time - send_time_of_largest_acked */
> > +     cong->latest_rtt = cong->time - time;
>
> The field cong->time is described as "Cached current timestamp" in struct
> quic_cong, but where is it written? The socket struct is zero-initialized,
> so cong->time will always be 0.
>
> When computing RTT with a positive packet send timestamp in the time
> parameter, does this cause unsigned integer underflow? For example, if the
> packet was sent at time 12345, computing 0 - 12345 wraps to a very large
> value.
>
> The same issue appears in quic_cong_check_persistent_congestion() where
> cong->time - time is compared against ssthresh, and in quic_reno_on_packet_lost()
> and quic_reno_on_process_ecn() where recovery_time is set to cong->time.
This is not a problem, cong->time will always be set before processing
the packet on the TX path in the next patch series.

Thanks.