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Internet Networking Spring 2003

Internet Networking Spring 2003. Tutorial 12 Limited Transmit RFC 3042 Long Thin Networks RFC 2757. Limited Transmit RFC 3042. The problem. cwnd can be smaller than 4 Due to a large number of packet loss, less than 3 dup acks will arrive

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Internet Networking Spring 2003

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  1. Internet NetworkingSpring 2003 Tutorial 12 Limited TransmitRFC 3042 Long Thin NetworksRFC 2757

  2. Limited TransmitRFC 3042

  3. The problem • cwnd can be smaller than 4 • Due to a large number of packet loss, less than 3 dup acks will arrive • This causes a timeout rather than a Fast Retransmit.

  4. TCP modification • The sender will send a new segment for each dup-Ack it receives. • The amount of data is limited to cwnd+2 • cwnd does NOT change due to a dup-ack.

  5. Example 3 cwnd=3 4 5 ACK(3) ACK(3) 6 7 ACK(3) ACK(3) Congestion avoidance ---- Fast retransmit Sstresh = 3/2 = 2 Cwnd = sstresh+3=2+3=5 3 X ACK(8)

  6. Example #2 3 cwnd=3 ------- 4 5 ACK(3) ACK(3) ACK(6) 6 7 cwnd=4 ------- 8 9

  7. ECN and Limited Transmit • ECN can inform of congestion without dropping the packet, thus allowing TCP with small cwnd to be aware of congestion while avoiding timeouts. • ECN requires the co-operation of the network, while Limited Transmit does not. • Can be simultaneously used.

  8. (Wireless) Long Thin NetworksRFC 2757

  9. Definitions • Long: High delay • Longer RTT. • Thin: Low bandwidth • The delay*bw product can be small, what influence does it have on cwnd ? • Example: wireless network. • Counter examples: • Satellite (Long Fat Networks) • Wireless LAN (Short Fat Networks) • More information can be found at: • http://www.cas.mcmaster.ca/~wmfarmer/SE-4C03-02/projects/student_work/mcmahoj.html • RFC 2757.

  10. Wireless network

  11. BER: Bit Error Rate • Higher error rate than a wired link • Possible solutions: • At the link layer: DLC that ensures FIFO and reliability (as learned in the previous course) • No DLC at the link layer: • PEP: Performance Enhancing Proxies (Indirect-TCP) • Lower MTU

  12. DLC and duplicate retransmission TCP RTO Link layer timeout Redundant transmission

  13. PEP: Performance Enhancing Proxies. • Instead of End-to-End TCP: • User -> Intermediate Node (IN) -> Wire-line Internet. IN internet Application Application TCP TCP TCP IP IP IP IP MAC MAC Link Link Link Link

  14. Indirect TCP

  15. PEP: Pros • Shield the wire Internet from the wireless connection and vice-versa. • TCP for the wireless link can be enhanced, due to the knowledge we have on that specific network. • Different cwnd and RTO • Since the RTT of the single wireless link is smaller than the whole route RTT, faster local recovery is achieved. • More freedom for the wireless part; Doesn’t have to be TCP compatible.

  16. PEP: Cons • TCP end-to-end Semantic is broken • IN is usually a single point of failure. • Movement of the client can cause IN change; Since we are referring to W-LTN and not W-LAN, This has much less probability. • Overloading of the IN: much problematic in LFN than LTN. The IN holds 2 stacks of TCP connections. • Handles disconnections poorly • Possible BS buffer overloading. Higher buffer increases hand-off latency when changing cells.

  17. Indirect TCP • Split TCP connection into 2 TCPs • BS acts as a proxy and relays all data • FHsends a packet • BS acknowledges this packet and forwards the packet to MH • Packet is lost on wireless link • BS can notices faster due to lower RTT and retransmit packet wireless TCP regular TCP Internet Mobile Host (MH) Fixed Host (FH) Base Station (BS)

  18. PEP: retransmission TCP RTO ACK TCP2 RTO ACK

  19. TCP connection application application application transport transport transport network network network retrans link link link physical physical physical FH BS MH wireless TCP-Aware Link Layer Protocol • Retains local recovery of Split connection approach and link level retransmission schemes • Snoop protocol, by Berkley: not in the course material

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