TCP Options for Low Latency: Maximum ACK Delay and ... - IETF
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TCP Options for Low Latency: Maximum ACK Delay and ... - IETF
How? Negotiate use in an option in TCP handshake... Handles the general/cloud/SaaS case. (Could also use per-route confi
TCP Options for Low Latency: Maximum ACK Delay and Microsecond Timestamps Neal Cardwell Yuchung Cheng Eric Dumazet
IETF 97: Seoul, Nov 2016
Motivation: lower latency, higher throughput Datacenters with commodity 10Gbps Ethernet: RTT 5ms) Google uses 5ms internally since 2013 ([3] mentions 5ms as well) 3
Maximum ACK Delay (MAD) But if RTO is fast even when ACKs are delayed: -> spurious retransmits and congestion control back-off How to know long ACKs may be delayed? Negotiate Max ACK Delay (MAD) in an option in TCP handshake... Small MAD negotiated => enables small min RTO => improves performance
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Microsecond TCP Timestamps: Motivation TCP Timestamps [RFC1323][RFC7323]: finest allowed granularity is 1 ms But RTTs are < 100 us and soon 10 us in the datacenter Benefits of 1 us TCP timestamps: Can undo cwnd reductions in datacenter In datacenter, original and fast retransmit have same 1ms timestamp Can't use TCP timestamps to undo cwnd reduction [RFC3522/RFC4015] Can do fine-grained measurement and diagnostics One-way delay variation for data (e.g. incast queues), ACKs
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Microsecond TCP Timestamps: Implementation When using usec TS, need to adjust a constant in PAWS logic When to expect 32-bit wrap-around in idle connections [RFC7323 sec 5.5]: 1 ms => wraps in ~24 days 1 us => wraps in ~34 minutes 1 ns => wraps in ~2 secs
How? Negotiate use in an option in TCP handshake... Handles the general/cloud/SaaS case (Could also use per-route config if this is intradomain traffic) 6
Options and the TCP Handshake SYN
SYN + ACK
Remote MAD = 10ms usTS = on
ACK Remote MAD = 5ms usTS = on
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Max ACK Delay and Min RTO interaction RTO = f1(RTT) + f2(RTTVAR) + MAD
usTS: use microsecond timestamps? (0 = no usec Timestamps; 1 = usec Timestamps) MAD unit: time units for MAD value (0 = no MAD negotiated; 1=msecs, 2=usecs, 3=nsecs) MAD value: Maximum ACK Delay value (1 ... 999) Total space: 6 bytes - using 2-byte RFC 6994 ExperimentID 4 bytes - if promoted to standard with its own option Kind (no ExperimentID) 9
Status Code for these 2 features has matured at Google (used for all internal TCP traffic) ● ●
Maximum ACK Delay: since Jul 2005 (yes, 11 years ago!) Microsecond timestamps: since Feb 2015 (1.75 years ago)
Verbal interest from at least one other major TCP implementor for MAD Next steps: 1. 2. 3.
Internet Draft Change code to support RFC6994 experimental option format Send code upstream to Linux by Q1 2017 10
