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TCP Automatic IW

TCP Automatic IW. draft-touch-tcpm-automatic-iw-01 IETF 82 Joe Touch, USC/ISI. Overview. TCP IW constant requires per-decade updates Originally defined as 1 MSS [Ja88] Originally implemented as 2 MSS Redefined as min (3 MSS, 4KB) [RFC2414, 1998]

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TCP Automatic IW

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  1. TCP Automatic IW draft-touch-tcpm-automatic-iw-01 IETF 82 Joe Touch, USC/ISI

  2. Overview • TCP IW constant requires per-decade updates • Originally defined as 1 MSS [Ja88] • Originally implemented as 2 MSS • Redefined as min(3 MSS, 4KB) [RFC2414, 1998] • Proposed as 10 MSS [draft-ietf-tcpm-initcwnd] • TCP IW can auto-adjust like TCP • AIMD • Over long timescales • React to loss during IW (as best possible) only • Goal - experimental

  3. Proposed algorithm • At first boot: • IW = maxIW • Increment loss counter when: • SYN-ACK includes ECN • Retransmit is within [ISN, ISN + IW] • Monthly or conn_count > max_conn: • AIMD iff loss/conn_count > loss_thresh

  4. Algorithm properties • Self-adjusting • No need to revisit every decade • Conservative • AIMD • TCP-friendly operation • Operates over very long timescales • Affects only IW (start, restart) • Low-effort • Act only during ECN or retransmission

  5. Proposed constants • AI <= 2 MSS • MD <= 0.5 • Max_conn <= 1000 • IW is multiple of 2 (helps compr. ACKs) • Loss_thresh >= 95% • AI limited to 2 MSS/year (??)

  6. Issues • Algorithm design • Basic function • Triggers (SYN-ACK ECN, retransmission) • MSS vs. byte • Seqno wrap • Constants • (change based on discussion) • False positives • Due to reordering • Granularity • Per machine, interface, subnet • Additional state • Per-conn – ISN, seqno wrap • Across reboots (what if not available?)

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