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cs / ee 143 Communication Networks Chapter 7 Transport

cs / ee 143 Communication Networks Chapter 7 Transport. Text: Walrand & Parakh , 2010 Steven Low CMS, EE, Caltech. Transport services. UDP Datagram service No congestion control No error/loss recovery Lightweight. TCP Connection oriented service Congestion control

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cs / ee 143 Communication Networks Chapter 7 Transport

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  1. cs/ee 143 Communication NetworksChapter 7 Transport Text: Walrand & Parakh, 2010 Steven Low CMS, EE, Caltech

  2. Transport services • UDP • Datagram service • No congestion control • No error/loss recovery • Lightweight • TCP • Connection oriented service • Congestion control • Error/loss recovery • Heavyweight

  3. UDP 1 ~ 65535 (216-1) UDP header ≤ 65535 Bytes – 8 Bytes (UDP header) – 20 Bytes (IP header) Usually smaller to avoid IP fragmentation (e.g., Ethernet MTU 1500 Bytes)

  4. TCP TCP header

  5. Example TCP states 3-way handshake 4-way handshake Possible issue: SYN flood attack Result in large numbers of half-open connections and no new connections can be made.

  6. RTT 1 2 W 1 2 W data ACKs 1 2 W 1 2 W Window Flow Control • ~ W packets per RTT • Lost packet detected by missing ACK Source time Destination time

  7. ARQ (Automatic Repeat Request) Go-back-N • TCP • Sender & receiver negotiate whether or • not to use Selective Repeat (SACK) • Can ack up to 4 blocks of contiguous • bytes that receiver got correctly • e.g. [3; 10, 14; 16, 20; 25, 33] Selective repeat

  8. Window control • Limit the number of packets in the network to window W • Source rate = bps • If W too small then rate « capacity If W too big then rate > capacity => congestion • Adapt W to network (and conditions)

  9. TCP window control • Receiver flow control • Avoid overloading receiver • Set by receiver • awnd: receiver (advertised) window • Network congestion control • Avoid overloading network • Set by sender • Infer available network capacity • cwnd: congestion window • Set W = min (cwnd, awnd)

  10. TCP congestion control • Source calculates cwnd from indication of network congestion • Congestion indications • Losses • Delay • Marks • Algorithms to calculate cwnd • Tahoe, Reno, Vegas, …

  11. TCP Congestion Controls • Tahoe (Jacobson 1988) • Slow Start • Congestion Avoidance • Fast Retransmit • Reno (Jacobson 1990) • Fast Recovery • Vegas (Brakmo & Peterson 1994) • New Congestion Avoidance

  12. TCP Tahoe (Jacobson 1988) window time CA SS : Slow Start : Congestion Avoidance : Threshold

  13. Slow Start • Start with cwnd = 1 (slow start) • On each successful ACK increment cwnd cwndcnwd + 1 • Exponential growth of cwnd each RTT: cwnd 2 x cwnd • Enter CA when cwnd >= ssthresh

  14. Congestion Avoidance • Starts when cwndssthresh • On each successful ACK: cwndcwnd + 1/cwnd • Linear growth of cwnd each RTT: cwndcwnd + 1

  15. Packets 1 7 2 3 4 5 6 Acknowledgements 1 2 3 3 3 3 Packet Loss • Assumption: loss indicates congestion • Packet loss detected by • Retransmission TimeOuts (RTO timer) • Duplicate ACKs (at least 3) (Fast Retransmit)

  16. Fast Retransmit • Wait for a timeout is quite long • Immediately retransmits after 3 dupACKs without waiting for timeout • Adjusts ssthresh flightsize = min(awnd, cwnd) ssthresh max(flightsize/2, 2) • Enter Slow Start (cwnd = 1)

  17. Summary: Tahoe • Basic ideas • Gently probe network for spare capacity • Drastically reduce rate on congestion • Windowing: self-clocking for every ACK { if (W < ssthresh) then W++ (SS) else W += 1/W (CA) } for every loss { ssthresh = W/2 W = 1 } Seems a little too conservative?

  18. TCP Reno (Jacobson 1990) SS CA for every ACK { W += 1/W(AI) } for every loss { W = W/2 (MD) } How to halve W without emptying the pipe? Fast Recovery

  19. Fast recovery • Idea: each dupACK represents a packet having left the pipe (successfully received) • Enter FR/FR after 3 dupACKs • Setssthresh max(flightsize/2, 2) • Retransmit lost packet • Set cwndssthresh +ndup (window inflation) • Wait till W=min(awnd, cwnd) is large enough; transmit new packet(s) • On non-dup ACK, set cwndssthresh(window deflation) • Enter CA

  20. 1 2 3 4 5 6 7 8 1 9 10 11 0 0 0 0 0 0 0 Exit FR/FR 11 7 9 4 4 4 4 4 Example: FR/FR • Fast retransmit • Retransmit on 3 dupACKs • Fast recovery • Inflate window while repairing loss to fill pipe S time time R 8 cwnd 8 ssthresh

  21. Summary: Reno • Basic ideas • dupACKs: halve W and avoid slow start • dupACKs: fast retransmit + fast recovery • Timeout: slow start dupACKs congestion avoidance FR/FR timeout retransmit slow start

  22. FR/FR 1 3 0 0 0 0 2 0 timeout 8 unack’d pkts 5 8 9 Multiple loss in Reno? • On 3 dupACKs, receiver has packets 2, 4, 6, 8, cwnd=8, retransmits pkt 1, enter FR/FR • Next dupACK increment cwnd to 9 • After a RTT, ACK arrives for pkts 1 & 2, exit FR/FR, cwnd=5, 8 unack’edpkts • No more ACK, sender must wait for timeout 2 3 4 5 6 7 8 9 0 1 S time D time

  23. New RenoFall & Floyd ‘96, (RFC 2583) • Motivation: multiple losses within a window • Partial ACK takes Reno out of FR, deflates window • Sender may have to wait for timeout before proceeding • Idea: partial ACK indicates lost packets • Stays in FR/FR and retransmits immediately • Retransmits 1 lost packet per RTT until all lost packets from that window are retransmitted • Eliminates timeout

  24. Delay-based TCP: Vegas (Brakmo & Peterson 1994) • Reno with a new congestion avoidance algorithm • Converges (provided buffer is large) ! window time CA SS

  25. Congestion avoidance • Each source estimates number of its own packets in pipe from RTT • Adjusts window to maintain estimate # of packets in queues between aand b for every RTT { • if W/RTTmin – W/RTT< a / RTTmin then W++ • if W/RTTmin – W/RTT> b / RTTminthen W-- } for every loss W := W/2

  26. Implications • Congestion measure = end-to-endqueueing delay • At equilibrium • Zero loss • Stable window at full utilization • Nonzero queue, larger for more sources • Convergence to equilibrium • Converges if sufficient network buffer • Oscillates like Reno otherwise

  27. Theory-guided design: FAST We will study them further in TCP modeling in the following weeks A simple model of AIMD (Reno) for example…

  28. Summary • UDP header/TCP header • TCP 3-way/4-way handshake • ARQ: Go-back-N/selective repeat • Tahoe/Reno/New Reno/Vegas/FAST -- useful details for your project • Simply model of AIMD

  29. 33 Why both TCP and UDP? Most applications use TCP, as this avoids re-inventing error recovery in every application But some applications do not need TCP For example: Voice applicationsSome packet loss is fine. Packet retransmission introduces too much delay. For example: an application that sends just one message, like DNS/SNMP/RIP. TCP sends several packets before the useful one. We may add reliability at application layer instead.

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