fn app_limited_congestion_avoidance()

in neqo-transport/src/cc/classic_cc.rs [1139:1251]

• 95 lines of code
• 6 McCabe index (conditional complexity)

    fn app_limited_congestion_avoidance() {
        const CWND_PKTS_CA: usize = CWND_INITIAL_PKTS / 2;
        const BELOW_APP_LIMIT_PKTS: usize = CWND_PKTS_CA - 2;
        const ABOVE_APP_LIMIT_PKTS: usize = BELOW_APP_LIMIT_PKTS + 1;

        let mut cc = ClassicCongestionControl::new(NewReno::default(), Pmtud::new(IP_ADDR, MTU));
        let mut now = now();

        // Change state to congestion avoidance by introducing loss.

        let p_lost = SentPacket::new(
            PacketType::Short,
            1,
            now,
            true,
            Vec::new(),
            cc.max_datagram_size(),
        );
        cc.on_packet_sent(&p_lost, now);
        cwnd_is_default(&cc);
        now += PTO;
        cc.on_packets_lost(Some(now), None, PTO, &[p_lost], now);
        cwnd_is_halved(&cc);
        let p_not_lost = SentPacket::new(
            PacketType::Short,
            2,
            now,
            true,
            Vec::new(),
            cc.max_datagram_size(),
        );
        cc.on_packet_sent(&p_not_lost, now);
        now += RTT;
        cc.on_packets_acked(&[p_not_lost], &RttEstimate::default(), now);
        cwnd_is_halved(&cc);
        // cc is app limited therefore cwnd in not increased.
        assert_eq!(cc.acked_bytes, 0);

        // Now we are in the congestion avoidance state.
        assert_eq!(cc.state, State::CongestionAvoidance);
        // simulate packet bursts below app_limit
        let mut next_pn = 3;
        for packet_burst_size in 1..=BELOW_APP_LIMIT_PKTS {
            // always stay below app_limit during sent.
            let mut pkts = Vec::new();
            for _ in 0..packet_burst_size {
                let p = SentPacket::new(
                    PacketType::Short,
                    next_pn,
                    now,
                    true,
                    Vec::new(),
                    cc.max_datagram_size(),
                );
                next_pn += 1;
                cc.on_packet_sent(&p, now);
                pkts.push(p);
            }
            assert_eq!(
                cc.bytes_in_flight(),
                packet_burst_size * cc.max_datagram_size()
            );
            now += RTT;
            for (i, pkt) in pkts.into_iter().enumerate() {
                cc.on_packets_acked(&[pkt], &RttEstimate::default(), now);

                assert_eq!(
                    cc.bytes_in_flight(),
                    (packet_burst_size - i - 1) * cc.max_datagram_size()
                );
                cwnd_is_halved(&cc); // CWND doesn't grow because we're app limited
                assert_eq!(cc.acked_bytes, 0);
            }
        }

        // Fully utilize the congestion window by sending enough packets to
        // have `bytes_in_flight` above the `app_limited` threshold.
        let mut pkts = Vec::new();
        for _ in 0..ABOVE_APP_LIMIT_PKTS {
            let p = SentPacket::new(
                PacketType::Short,
                next_pn,
                now,
                true,
                Vec::new(),
                cc.max_datagram_size(),
            );
            next_pn += 1;
            cc.on_packet_sent(&p, now);
            pkts.push(p);
        }
        assert_eq!(
            cc.bytes_in_flight(),
            ABOVE_APP_LIMIT_PKTS * cc.max_datagram_size()
        );
        now += RTT;
        let mut last_acked_bytes = 0;
        // Check if congestion window gets increased for all packets currently in flight
        for (i, pkt) in pkts.into_iter().enumerate() {
            cc.on_packets_acked(&[pkt], &RttEstimate::default(), now);

            assert_eq!(
                cc.bytes_in_flight(),
                (ABOVE_APP_LIMIT_PKTS - i - 1) * cc.max_datagram_size()
            );
            // The cwnd doesn't increase, but the acked_bytes do, which will eventually lead to an
            // increase, once the number of bytes reaches the necessary level
            cwnd_is_halved(&cc);
            // increase acked_bytes with each packet
            assert_ne!(cc.acked_bytes, last_acked_bytes);
            last_acked_bytes = cc.acked_bytes;
        }
    }