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TCP (Part II)

TCP (Part II). Shivkumar Kalyanaraman Rensselaer Polytechnic Institute shivkuma@ecse.rpi.edu http://www.ecse.rpi.edu/Homepages/shivkuma. TCP interactive data flow TCP bulk data flow TCP congestion control TCP timers TCP futures and performance

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TCP (Part II)

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  1. TCP (Part II) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute shivkuma@ecse.rpi.edu http://www.ecse.rpi.edu/Homepages/shivkuma

  2. TCP interactive data flow • TCP bulk data flow • TCP congestion control • TCP timers • TCP futures and performance Ref: Chap 19-24; RFC 793, 1323, 2001, papers by Jacobson, Karn/Partridge Overview

  3. Reliability models • Reliability fundamentally requires redundancy to recover from uncertain loss or other failure modes. • Two types of redundancy: • Spatial redundancy: independent backup copies • Forward error correction (FEC) codes • Problem: requires huge overhead, since the FEC is also part of the packet(s) it cannot recover from erasure of all packets • Temporal redundancy: retransmit if packets lost/error • Requires trading off response time for reliability • Design of status reports and retransmission optimization (see next slide) important

  4. Temporal Redundancy model

  5. Status report design • Cumulative acks: • Robust to losses on the reverse channel • Can work with go-back-N retransmission • Cannot pinpoint blocks of data which are lost • The first lost packet can be pinpointed because the receiver would generate duplicate acks • Selective acks: • For a byte-stream model like TCP, need to specify ranges of bytes received (requires large overhead) • SACK is a TCP option over-and-above the cumulative acks • Bitmaps are not efficient because a bit is needed for every byte • NAKs have same problems like SACKs and bitmaps, but also are not robust to reverse channel losses

  6. Retransmission optimization • Default retransmission: • Go-back-N: I.e. retransmit the entire window. • Triggered by timeout or persistent loss in TCP • Not efficient if windows are large: high speed n/ws • Selective retransmission: • Retransmit one packet based upon duplicate acks • Recovers quickly from isolated loss, but not from burst loss • SACK allows pinpointing retransmissions to just cover ranges of lost packets • Such retransmitted packets must finally be confirmed by acks since SACK is only an option and not reliable

  7. TCP Interactive Data Flow • Problems: • Overhead: 40 bytes header + 1 byte data • To batch or not to batch: response time important • Batching acks: • Delay-ack timer: piggyback ack on echo • 200 ms timer (fig 19.3) • Batching data: • Nagle’s algo: Don’t send packet until next ack is received. • Developed because of congestion in WANs

  8. TCP Bulk Data Flow • Sliding window: • Send multiple packets while waiting for acks (fig 20.1) upto a limit (W) • Receiver need not ack every packet • Acks are cumulative. • Ack # = Largest consecutive sequence number received + 1 • Two transfers of the data can have different dynamics (eg: fig 20.1 vs fig 20.2) • Receiver window field: • Reduced if TCP receiver short on buffers

  9. TCP Bulk Data Flow (Contd) • End-to-end flow control • Window update acks: receiver ready • Default buffer sizes: 4096 to 16384 bytes. • Ideal: window and receiver buffer = bandwidth-delay product • TCP window terminology: figs 20.4, 20.5, 20.6 • Right edge, Left edge, usable window • “closes” => left edge (snd_una) advances • “opens” => right edge advances (receiver buffer freed => receiver window increases) • “shrinks” => right edge moves to left (rare)

  10. S S S S The Congestion Problem • Problem: demand outstrips available capacity … • Q: Will the “congestion” problem be solved when: • a) Memory becomes cheap (infinite memory)? No buffer Too late • b) Links become cheap (high speed links)? Replace with 1 Mb/s All links 19.2 kb/s S S S S File Transfer time = 5 mins File Transfer Time = 7 hours

  11. c) Processors become cheap (fast routers switches)? Ans: None of the above solves congestion ! • Congestion: Demand > Capacity • It is a dynamic problem => Static solutions are not sufficient • TCP provides a dynamic solution A C S B D Scenario: All links 1 Gb/s. A & B send to C.

  12. i i   • If information about i , and  is known in a central location where control of i can be effected with zero time delays, the congestion problem is solved. • Problems: • Incomplete information (eg: loss indications) • Distributed solution required • Congestion and control/measurement locations different • Time-varying, heterogeneous time-delays

  13. TCP Congestion Control • Window flow control: avoid receiver overrun • Dynamic window congestion control: avoid/control network overrun • Observation: Not a good idea to start with a large window and dump packets into network • Treat network like a black box and start from a window of 1 segment (“slow start”) • Increase window size exponentially (“exponential increase”) over successive RTTs => quickly grow to claim available capacity. • Technique: Every ack: increase cwnd (new window variable) by 1 segment. • Effective window = Min(cwnd, Wrcvr)

  14. Dynamics • Rate of acks = rate of packets at the bottleneck: “Self-clocking” property. 2nd RTT 3rd RTT 4th RTT 1st RTT 100 Mbps 10 Mbps Router Q

  15. Congestion Detection • Packet loss as an indicator of congestion. • Set slow start threshold (ssthresh) to min(cwnd, Wrcvr)/2 • Retransmit pkt, set cwnd to 1 (reenter slow start) Receiver Window Timeout Congestion Window (cwnd) IdleInterval ssthresh 1 Time (units of RTTs)

  16. Congestion avoidance • Increment cwnd by 1 per ack until ssthresh • Increment by 1/cwnd per ack afterwards (“Congestion avoidance” or “linear increase”) • Idea: ssthresh estimates the bandwidth-delay product for the connection. • Initialization: ssthresh = Receiver window or default 65535 bytes. Larger values thru options. • If source is idle for a long time, cwnd is reset to one MSS.

  17. Implications of using packet loss as congestion indicator • Late congestion detection if the buffer sizes larger • Higher speed links or large buffers => larger windows => higher probability of burst loss • Interactions with retransmission algorithm and timeouts • Implications of ack-clocking • More batching of acks => bursty traffic (harder to manage) • Less batching leads to a large fraction of Internet traffic being just acks (huge overhead) • Additive Increase/Multiplicative Decrease Dynamics: • TCP approximates these dynamics

  18. Timeout and RTT Estimation • Timeout: for robust detection of packet loss • Problem: How long should timeout be ? • Too long => underutilization; too short => wasteful retransmissions • Solution: adaptive timeout: based on RTT • RTT estimation: • Early method: exponential averaging: • R  *R + (1 - )*M { M =measured RTT} • RTO = *R { = delay variance factor} • Suggested values:  = 0.9,  = 2

  19. RTT Estimation • Jacobson [1988]: this method has problems w/ large RTT fluctuations • New method: Use mean & deviation of RTT • A = smoothed average RTT • D = smoothed mean deviation • Err = M - A { M = measured RTT} • A  A + g*Err {g = gain = 0.125} • D  D + h*(|Err| - D) {h = gain = 0.25} • RTO = A + 4D • Integer arithmetic used throughout. Complex initialization process ...

  20. Timer Backoff/Karn’s Algorithm • Timer backoff: If timeout, RTO = 2*RTO {exponential backoff} • Retransmission ambiguity problem: • During retransmission, it is unclear whether an ack refers to a packet or its retransmission. Problem for RTT estimation • Karn/Partridge: don’t update RTT estimators during retransmission. • Restart RTO only after an ack received for a segment that is not retransmitted

  21. Fast Retransmit and Recovery • Goals: • Timeout avoidance: The 500 ms timer granularity can have an adverse performance impact especially for high speed n/ws • Selective retransmission: Especially when packets are dropped due to error or light congestion • Fast Recovery: Converge quickly to a state of congestion avoidance (linear increase) with half-current window -- the assumed ideal window size. • Observation: Receivers are required to send an immediate duplicate acknowledgment when they receives out-of-order data segments.

  22. Fast Retransmit and Recovery 0 • 3 duplicate acks => assume loss • More duplicate acks => other packets have reached destination safely. • Wait for about 1/2*RTT, and resume transmitting new segments for every subsequent duplicate ack received. Stop this process once the ack for the missing segment received 500 Ack 500 Ack 500 Ack 500 FRR Ack 500 Ack 500

  23. Fast Retransmit and Recovery • Fast Retransmit: Received third duplicate ack: • Set ssthresh to 1/2 of current cwnd • Retransmit the missing segment • Set cwnd to ssthresh+3 • Fast Recovery: For each duplicate ack hence: • Increment cwnd by 1 MSS • New packets are transmitted once cwnd grows large enough. • [If old cwnd was a pipe of length 1*RTT, the network gets a relief period of 1/2*RTT]

  24. FRR (contd) • Upon receiving the next (non-duplicate) Ack: • Set cwnd to ssthresh& enter linear growth phase New packets sent during this phase CWND CWND/2 TIME

  25. FRR problems • Burst loss of 3 pkts => Timeout + window shutdown to cwnd/8 ! CWND W CWND/2 CWND/8 CWND/4 Time 1st Fast Retransmit Timeout 2nd Fast Retransmit

  26. TCP Performance Optimization • SACK: selective acknowledgments: specifies blocks of packets received at destination. • Random early drop (RED) scheme spreads the dropping of packets more uniformly and reduces average queue length and packet loss rate. • Scheduling mechanisms protect well-behaved flows from rogue flows. • Explicit Congestion Notification (ECN): routers use a explicit bit-indication for congestion instead of loss indications.

  27. Congestion control summary • Sliding window limited by receiver window. • Dynamic windows: slow start (exponential rise), congestion avoidance (linear rise), multiplicative decrease. • Adaptive timeout: need mean RTT & deviation • Timer back off and Karn’s algo during retransmission • Go-back-N or Selective retransmission • Cumulative and Selective acknowledgements • Timeout avoidance: FRR • Drop policies, scheduling and ECN

  28. TCP Persist Timer • Receiver flow control can set window to zero • Receiver later sends “window update acks” • But TCP does not transmit acks reliably => update acks may be lost and source may be stuck at a zero window value • TCP uses persist timer to query the receiver periodically to find if the window has been increased. • Persist timer always bounded between 5s and 60s. It does exponential backoff like other timers too.

  29. Silly Window Syndrome • A) The system operates at a small window (sends segments which are not MSS-sized) even if the receiver grants a large window. • B) Receiver advertises small windows. • Solution: batching • Receiver must not advertise small windows • Sender waits until segment full before sending (extension of Nagle’s algo), • It can transmit everything if it is not waiting for any ACK (or if Nagle’s algo has been disabled)

  30. TCP Keepalive timer • Optional timer. • Not part of TCP spec, but found in most implementations. • Not necessary, because “connection” defined by endpoints. • Connection can be “up”as long as source/destination “up”. • Typical use: to detect idle clients or half-open connections and de-allocate server resources tied up to them. Eg: telnet, ftp.

  31. Gigabit Networks • “Higher Bandwidth Networks” • Propagation latency unchanged. • Increasing bandwidth from 1.5Mb/s to 45 Mb/s (factor of 29) decreases file transfer time of 1MB by a factor of 25. • But, increasing from 1 Gb/s to 2 Gb/s gives an improvement of only 10% ! • Transfer time = propagation time + transmission time + queueing/processing. • Design networks to minimize delay (queueing, processing, reduce retransmission latency)

  32. Window Scaling Option • Long Fat Pipe Networks (LFN): Satellite links • Need very large window sizes. • Normally, Max window = 216 = 64 KBytes • Window scale: Window = W × 2Scale Kind = 3 Length = 3 Scale • Max window = 216 × 2255 • Option sent only in SYN and SYN + Ack segments. • RFC 1323

  33. Timestamp option • For LFNs, need accurate and more frequent RTT estimates. • Timestamp option: • Place a timestamp value in any segment. • Receiver echoes timestamp value in ack • If acks are delayed, the timestamp value returned corresponds to the earliest segment being acked. • Segments lost/retransmitted => RTT overestimated

  34. PAWS: Protection against wrapped sequence numbers • Largest receiver window = 2^30 = 1 GB • “Lost” segment may reappear before MSL, and the sequence numbers may have wrapped around • The receiver considers the timestamp as an extension of the sequence number => discard out-of-sequence segment based on both seq # and timestamp. • Reqt: timestamp values need to be monotonically increasing, and need to increase by at least one per window

  35. Summary • Interactive and bulk TCP flow • TCP congestion control • Informal exercises: Perform some of the experiments described in chaps 19-21 to see various facets of TCP in action

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