TCP OVER AD HOC NETWORK

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1 TCP OVER AD HOC NETWORK Special course on data communications and networks Zahed Iqbal

2 Agenda Introduction Versions of TCP TCP in wireless network TCP in Ad Hoc network Conclusion References Homework Zahed Iqbal 2

3 Introduction TCP implements Flow control Congestion Avoidance & Control Reliable transmission through packet retransmission End-to-End semantics ACK send to sender to confirm delivery Zahed Iqbal 3

4 Flow Control Traffic regulation according to congestion state and connection quality TCP sliding window allows end-to-end flow control Zahed Iqbal 4

5 Congestion Control - I Source Based (depends on RTT, packet loss, or throughput) Gateway Based (based on indication of congestion through early detection and notification) Zahed Iqbal 5

6 Congestion Control - II Consists of Slow Start (SS) Congestion Avoidance (CA) Fast Retransmission and Fast Recovery AIMD (cwnd increased by +1 and decreased by ½) Zahed Iqbal 6

7 Congestion Control - III Zahed Iqbal 7

8 Issues with TCP Fairness Scalability (ability to share bottleneck fairly and decreases with the number of flows) No recovery possible from a single packet loss if the window size drops below Zahed Iqbal 8

9 Versions of TCP - I TCP Reno TCP Tahoe TCP Vegas TCP SACK Zahed Iqbal 9

10 Versions of TCP - II TCP Reno Induce packet losses to estimate available network bandwidth Window size increased if no packet loss (additive increase) window size halved if there is packet loss (multiplicative decrease) Lead to a period oscillation in cwnd, RT delay and queue length Zahed Iqbal 10

11 Versions of TCP - III Reno start fast retransmission and fast recovery if - timeout occurs - Or, Duplication ACK occurs(3 ACK) TCP Tahoe utilizes SS, CA and fast retransmission algorithm sets cwnd size 1/2 of the current window size at timeout window size is increased by 1/cwnd at each ACK Zahed Iqbal 11

12 Versions of TCP - IV Cwnd and receiver advertised window size is sent fast retransmission work same way as RENO except without to wait for timeout to occurs. TCP Vegas Modified SS Improved CA new retransmission mechanisms reduce time to detect lost packets Zahed Iqbal 12

13 Versions of TCP - V TCP Vegas Vegas estimates congestion level and updates the window size according to the difference between expected and actual flow rate no oscillation in window size Zahed Iqbal 13

14 Versions of TCP - VI TCP SACK ACK replies can be delayed and resulting duplicates introduces a selective retransmission when multiple packets are lost SACK Zahed Iqbal 14

15 TCP in wireless Last-Hop - I Problems delays (affects flow and congestion control) Link delay varies with time bcoz of multiple devices share the same wireless link packet loss due to multi-path fading and signal jumping, handoff, high BER Solutions Indirect TCP (I-TCP) resolves disconnection issues in tcp transfers connection state between MSRs Zahed Iqbal 15

16 TCP in wireless Last-Hop - II I-TCP: Since end-to-end retransmissions are slow, TCP connections may be split using as pivot points routers connected to both wireless and wired links. End-to-end connections are split into separate TCP sessions for the wired and wireless parts of the path. I-TCP partitions the TCP connection into 2 segments between the fixed and MSR (regular) between MSR and mobile host (dynamic) I-TCP uses socket migration techniques Zahed Iqbal 16

17 TCP in wireless Last-Hop - IV Courtesy:Bakre, Badrinath Zahed Iqbal 17

18 TCP in wireless Last-Hop - III I-TCP: Split schemes violate end-to-end TCP semantics, since acks may reach the sender before data packets reach their destination. To preserve TCP semantics, acks must be delayed (this reduces throughput). Pivot points face significant overhead, since packets undergo TCP processing twice and considerable per-connection state is maintained there Zahed Iqbal 18

19 TCP in wireless Last-Hop - V TCP Snoop Design Goals Improved Performance No change to TCP at fixed hosts No violation of end-to-end TCP semantics No recompiling/relinking of existing applications snoop agent at base station detects and retransmits locally avoids unnecessary congestion control procedure by the sender ELN is used to send NACK to the mobile host Zahed Iqbal 19

20 TCP in wireless Last-Hop - VI Zahed Iqbal Courtesy:mit.edu 20

21 TCP in Ad Hoc network - I Problems Effect of node mobility Effect of route reconstruction/repair Effect of network partitioning/merging, multipath routing sender mistakes the delay of ACK arrival and increases RTT as a sign of congestion sender begins to reduce window size Effect of High BER throughput performance is reduced Zahed Iqbal 21

22 TCP in Ad Hoc network - II Solutions TCP Feedback (TCP-F) RFN message received TCP state machine SNOOZE ESTABLISHED RFN message received Zahed Iqbal 22

23 TCP in Ad Hoc network - III TCP Feedback (TCP-F) Source stops all data transmission Source freezes all times, cwnd size, values of all state variables source initiates a route failure times (=equal to repair time on worst-case) resumes everything at receiving route repair complete message Zahed Iqbal 23

24 TCP in Ad Hoc network - IV TCP BuS (5 enhancements) 1) Explicit Notification control message ERDN and ERSN are sent from pivoting node for route maintenance. 2) Extended timeout values Timeout value of buffered packets at the source and IN are doubled (is this an optimal value?) 3) Selective retransmission Zahed Iqbal 24

25 TCP in Ad Hoc network - V 4) Avoiding unnecessary request for fast retransmission 5) Reliable transmission of control messages Zahed Iqbal 25

26 Conclusion TCP is successful building a reliable transport mechanism on the top of unreliable datagram delivery service provided by IP with the exception of wireless networks Delays and packet losses can significantly impact the proper operation of TCP Zahed Iqbal 26

27 Conclusion Fair bandwidth allocation among multiple TCP connection is difficult - a potential research issue Problem with TCP in Ad hoc network is more severe than wireless last hop network Zahed Iqbal 27

28 References Chapter 11 of C-K Toh Book TCP-BuS: Improving TCP Performance inwireless Ad Hoc Networks, Dongkyun Kim, C.-K. Toh, and Yanghee Choi SzdocszSzKim.pf/kim01tcpbus.pdf Analysis and Comparison of TCP Reno and Vegas, Jeonghoon Mo, Richard J. La, Venkat Anantharam, and Jean Walrand Department of Electrical Engineering and Computer Sciences University of California at Berkeley PAPERSzSzinfo99.pdf/mo99analysis.pdf RFC 2001 (TCP Tahoe), RFC 2018 (TCP SACK) Zahed Iqbal 28

29 References I-TCP: Indirect TCP for Mobile Hosts, Ajay Bakre, B. R Badrinath Zahed Iqbal 29

30 Homework Q1: What are the similarities and dissimilarities between TCP-Snoop and TCP Reno? Can you describe the situation if TCP Reno is used in Wireless last hop network? Q2. In TCP-BuS, the timeout value for buffered packet at the source and IN are doubled? Do you think that this is an optimal timeout value? Justify your answer Zahed Iqbal 30

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