QUIC - RTT handling code, improved packet expiry #3975
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ripatel-fd
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Jan 19, 2025
| fd_quic_range_t range; /* CRYPTO data range; FIXME use pkt_meta var instead */ | ||
| ulong stream_id; /* if this contains stream data, | ||
| the stream id, else zero */ | ||
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| ulong expiry; /* time pkt_meta expires... this is the time the | ||
| ack is expected by */ | ||
| ulong tx_time; /* transmit time */ |
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Does it make sense to have separate tx_time and expiry?
Can we not just have a conn-wide expiry timer that adjusts simultaneously for all outgoing packets?
(Maybe something for a future PR)
And expiry = pkt->tx_time + conn->expiry_duration
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That's basically what the PTO does in the spec. I'll consider adding it in this PR
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The new logic is complex enough that it's worth adding regression tests with simulated inputs, IMO.
(e.g. take a pcap of a real QUIC exchange over a lossy link, export the ACK delay samples to a C table, feed into the algo, assert that the derived RTT and expiry timeouts are in a reasonable range)
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In normal running the client keeps sending us streams, and we send acks. Because acks aren't "mandatory ack" we don't get an RTT from them. So also added code to periodically (once per minute) throw in a ping.
Calculates smoothed_rtt and var_rtt (rttvar) according to spec. Uses these to set expiry on packets.
Follow up could include an implementation of the PTO (Probe Time Out) algorithms from the spec.