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l10 TCP Friendliness, DCCP, NATs, and STU

# l10 TCP Friendliness, DCCP, NATs, and STU - Lecture 10 TCP...

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Lecture 10: TCP Friendliness, DCCP, NATs, and STUN

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TCP Friendliness
Congestion Control TCP dynamically adapts its rate in response to congestion AIMD causes flows to converge to fair goodput But how do losses (e.g., bit errors) affect goodput? What about UDP?

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Chiu Jain Phase Plots Flow A rate (bps) Flow B rate (bps) Fair A=B Efficient A+B=C overload underload t 1 t 2 t 3 t 4 t 5 t 6
Responding to Loss Set threshold to cwnd 2 On timeout - Set cwnd to 1 - Causes TCP to enter slow start On triple duplicate ACK (Reno) - Set cwnd to cwnd 2 - Retransmit missing segment - Causes TCP to stay in congestion avoidance

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Analyzing TCP Simply Assume all segments are MSS long Assume a packet loss rate p Assume a constant RTT Assume p is small (no timeouts)
Analysis Window size W cuts to W 2 after a loss Grows to W after W 2 RTTs Goodput = 3 4 · W · MTU · 1 RTT

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Window Size p = 1 ( W 2 +( W 2 +1)+ ... + W ) p 1 3 8 W 2 W q 8 3 · p Goodput = 3 4 · q 8 3 · p · MTU · 1 RTT Goodput = 1 . 22 · MTU RTT · p Constant factor changes based on delayed acks, etc.
TCP Friendliness Don’t want other protocols to disrupt TCP UDP happily shuts down TCP flows “TCP friendliness:” obeying TCP congestion control as per prior goodput equation - Does not imply acting like TCP - E.g., does not require abrupt window changes

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ledbat WG “The LEDBAT WG is chartered to standardize a congestion control mechanism that should saturate the bottleneck, maintain low delay, and yield to standard TCP.” TCP-friendliness is insufficient for modern P2P applications - Flow fairness, not application fairness - TCP fills queues Elastic workloads vs. inelastic workloads

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DCCP
DCCP Datagram Congestion Control Protocol (DCCP) provides congestion control for unreliable datagrams (RFC 4340) Connection-oriented protocol - Request-response-ack establishment - Close-reset or CloseReq-Close-reset teardown Counts packets, not bytes

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DCCP Segment
Sequence Numbers Every DCCP packet uses a new sequence number - Data - Acknowledgements - Control traffic Acknowledgements are for last packet received - Not cumulative acknowledgements - Does not succinctly describe connection history - Options can give packet vectors

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Synchronization DCCP uses sequence number windows to protect from attacks Large bursts of losses cause packets to fall past windows Need to resynchronize
Synchronization Exchange

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Synchronization on Reset Problem
Synchronization on Reset Solution

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l10 TCP Friendliness, DCCP, NATs, and STU - Lecture 10 TCP...

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