TCP PERFORMANCE OVER AD HOC NETWORKS

1. TCP PERFORMANCE OVER AD HOC NETWORKS Presented by Vishwanee Raghoonundun Assisted by Maheshwarnath Behary MSc Computer Networks Middlesex University

2. Introduction • Definition of Ad hoc networks • TCP/IP in Ad hoc networks • Causes of performance degradation • TCP enhancements in Ad hoc networks • Conclusion • Reference

3. Definition of Ad hoc network • Ad hoc network is a collection of two or more devices equipped with wireless communication and networking capability. An Ad hoc network is infrastructureless as it does not rely on any fixed network entities, i.e, there is no need for any fixed radio base stations, wires or fixed routers. • A mobile ad hoc network (MANET) is a multi-hop connection and is suitable for application areas such as: • battlefield communications • disaster recovery • inimical environment monitoring (where fixed wired infrastructure is not available)

4. TCP/IP in Ad hoc networks • TCP is an end-to-end protocol • Designed to provide flow and congestion control in a network • Connection-oriented protocol • Reliable for retransmitting lost packets in wired networks • Uses Congestion Control Mechanism • TCP/IP cannot distinguish between the nature of the problems on the network and assume network congestion in all cases. • Inefficient due to the wireless, unpredictable nature of ad hoc networks. • Result in poor performance in ad hoc networks

5. Causes of performance degradation • Channel Errors Bit errors corrupt packets in transmission leading to packet (ACK) losses. Sender reduces congestion window to one packet and reacts with a slow start. • Medium Contention and Collision Hidden and exposed terminals situations prevent one node from reaching the other, thus triggering a route failure. Route has to be recomputed and that causes delay resulting in TCP invoking congestion control algorithms if timeout occurs • Mobility Handovers cause link breakage and packet loss. TCP again assumes congestion and reacts accordingly. Furthermore routes have to be recalculated causing delay and throughput is severely degraded. • Congestion Congestion may give rise to buffer overflow and increased link contention, which degrades TCP performance.

6. TCP Enhancements in Ad hoc networks Feedback-based techniques • TCP-F Upon route failure, a Route Failure Notification (RFN) is sent to sender which enters in a ‘snooze’ state, i.e, it stops transmission until it receives a Route Reestablishment Notification (RRN). • TCP-ELFN (Explicit Link Failure Notification) • Upon receiving ELFN, sender disables congestion control mechanism and enters into a “stand-by” mode until a new route is established. • While on stand-by, sender sends a packet to probe network to see if a route has been established. • ATCP (Ad hoc TCP) • If route breakage, sender enters “persist state” • If network congestion, sender enters “congestion control” state • If high bit error rate, sender enters “retransmit” state

7. TCP Enhancements in Ad hoc networks (cntd) TCP without feedback • Uses Adaptive Congestion Window Limit (CWL) Setting, which is dynamically adjusted according to current round-trip hop-count (RTHC) of the path (CWL should be < than RTHC) • It reduces delay at any hop along the path Indirect TCP • Segments a TCP connection into a fixed and a wireless part • Access Point acts as a proxy and relays data in both direction. Neither mobile node nor fixed node knows what takes place at the other end. • In case of packet loss, AP forwards the packet without the transmitter being aware of the problem. • Transmission errors on wireless link cannot propagate to wired link • Loss of end-to-end semantics

8. TCP Enhancements in Ad hoc networks (cntd) Snoop TCP • Foreign agent buffers all packet with destination mobile host and snoops acknowledgements in both directions • To remain transparent foreign agent must not acknowledge data to the correspondent host • Transmission from fixed node to mobile node, sensing no ACK, foreign agent will retransmit loss packet • Transmission from mobile node to fixed node , FA snoops packet streams and on detecting missing packets, will send a NACK to mobile node to activate retransmission • Preserve end-to-end semantics • Useless if encryption scheme is applied to data and require more buffer

9. TCP Enhancements in Ad hoc networks (cntd) Mobile TCP • Suitable for lengthy or frequent disconnections • The Base Station is known as Supervisory Host (SH) • No proxy/does not have a buffering system, instead require sender to retransmit lost packet • If SH does not receive ACK, it assumes disconnection and changes the window size of the sender to 0 - Choking • Maintain TCP end-to-end semantics • Packet loss due to error bits is propagated to sender Freeze TCP • True end-to-end scheme and does not require the involvement of any intermediaries (such as base stations) for flow control • Changes in TCP code are restricted to the mobile client side, making it possible to fully inter-operate with the existing infrastructure

10. Conclusion • Traditional TCP is reliable in wired networks but it needed further enhancements in order to operate efficiently on wireless networks. TCP enhancements considered earlier on, have proved to reduce throughput degradation considerably. However they are not fool-proof as they carry some trade-offs. Further enhancements are required until a fool-proof version is specifically designed for wireless and ad hoc networks.

11. References [1] Jochen Schiller, “Mobile Communications”, Second Edition, 2003 [2] Xiang Chen, Hongqiang Zhai, Jianfeng Wang and Yuguang Fang, “TCP Performance over Mobile Ad Hoc Networks”, http://www.ecel.ufl.edu/jwang/publications_files/tcp_cjece.pdf [3] A. Lasebae, “Mobile and Wireless Networks”.