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Congestion Control

Congestion Control. 1. Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem!.

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Congestion Control

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  1. Congestion Control 1

  2. Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem! Principles of Congestion Control Transport Layer

  3. two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput lout lin : original data unlimited shared output link buffers Host A Host B Causes/costs of congestion: scenario 1 Transport Layer

  4. one router, finite buffers sender retransmission of lost packet Causes/costs of congestion: scenario 2 Host A lout lin : original data l'in : original data, plus retransmitted data Host B finite shared output link buffers Transport Layer

  5. always: (goodput) “perfect” retransmission only when loss: retransmission of delayed (not lost) packet makes larger (than perfect case) for same l l l > = l l l R/2 in in in R/2 R/2 out out out R/3 lout lout lout R/4 R/2 R/2 R/2 lin lin lin a. b. c. Causes/costs of congestion: scenario 2 “costs” of congestion: • more work (retrans) for given “goodput” • unneeded retransmissions: link carries multiple copies of pkt Transport Layer

  6. four senders multihop paths timeout/retransmit l l in in Host A Host B Causes/costs of congestion: scenario 3 Q:what happens as and increase ? lout lin : original data l'in : original data, plus retransmitted data finite shared output link buffers Transport Layer

  7. Host A Host B Causes/costs of congestion: scenario 3 lout another “cost” of congestion: • when packet dropped, any “upstream transmission capacity used for that packet was wasted! Transport Layer

  8. end-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at Approaches towards congestion control two broad approaches towards congestion control: Transport Layer

  9. Network assisted congestion control • Active Queue Management • monitor queue, do not just drop upon overflow more intelligent decisions • maintain low average queue length, alleviate phase effects, enforce fairness • Explicit Congestion Notification (ECN) • Instead of dropping, set a bit; • reduced loss - major benefit!

  10. Explicit Congestion Control (ECN) • Receiver informs sender about bit; sender behaves as if a packet was dropped • actual communication between end nodes and the network • Typical incentives: • sender = server; efficiently use connection, fairly distribute bandwidth • use ECN as it was designed • receiver = client; goal = high throughput, does not care about others • ignore ECN flag, do not inform sender about it • Need to make it impossible for receiver to lie about ECN flag when it was set • Solution: nonce = random number from sender, deleted by router when setting ECN • Sender believes “no congestion” if correct nonce is sent back

  11. ECN Bits in IP Header Value Name 00 Not-ECT (Not ECN Capable Transport) 10 ECT(0) (ECN Capable Transport (0) ) 01 ECT(1) (ECN Capable Transport(1) ) 11 CE (Congestion Experienced) 2 bits => 4 ECN Codepoints

  12. ECN Bits in TCP Header ECE flag - ECN-Echo flag CWR flag - Congestion Window Reduced flag

  13. ECN in action • Nonce provided by bit combination: • ECT(0): ECT=1, CE=0 • ECT(1): ECT=0, CE=1 • Nonce usage specification still experimental

  14. ABR: available bit rate: “elastic service” if sender’s path “underloaded”: sender should use available bandwidth if sender’s path congested: sender throttled to minimum guaranteed rate RM (resource management) cells: sent by sender, interspersed with data cells bits in RM cell set by switches (“network-assisted”) NI bit: no increase in rate (mild congestion) CI bit: congestion indication RM cells returned to sender by receiver, with bits intact Case study: ATM ABR congestion control Transport Layer

  15. two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell sender’ send rate thus maximum supportable rate on path EFCI bit in data cells: set to 1 in congested switch if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell Case study: ATM ABR congestion control Transport Layer

  16. Questions?

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