Mobile and Ad hoc Networks

1. Mobile and Ad hoc Networks Background of Ad hoc Wireless Networks Wireless Communication Technology and Research Ad hoc Routing and Mobile IP and Mobility Wireless Sensor and Mesh Networks Student Presentations Transport Layer for Adhoc Networks http://web.uettaxila.edu.pk/CMS/SP2012/teAWNms/

2. Outline • Overview of TCP • The problems of TCP over MANETs • Overview of best transport protocols • In depth • Specific problems of TCP over MANETs • Details of major TCP variants • Discussion - other efforts • Conclusion

3. Data stream in Wired Network ACKs stream Data stream in a MANET TCP Source TCP Sink N 1 2 3 4 ACKs stream TCP in Wired Network and MANET

4. Network Architecture at a Crossroads • Wireline-centric network design is “obsolete” • New network environments have emerged • Ad hoc, sensors, consumer-owned, delay-tolerant • New networking technologies have emerged • UWB, Cooperative approaches, MIMO, Directed antennas • The R&D community recognizes the need for change

5. Revisiting the Current Transport Architecture • The vision: • Wireless as an integral part of the network • Multiple wireless hops: not just the last mile (Cellular) • Pockets of wireless ad hoc connectivity • A new protocol stack is required • Is TCP/IP capable of delivering?

6. Problem Statement • Why does TCP perform poorly in MANETs? • Developed for Wireline networks • Assumes all losses congestion related • Many TCP variants have been proposed • How good are they? • Are they sufficient? • Are there any other alternatives? • Are non-tcp protocols the solution?

7. Our Goal • Identify the problems of TCP in MANETs. • Evaluate various major TCP variants. • 12 TCP variants, 7 improvement techniques • Observations: • Most TCP variants are NOT sufficient. • A new transport layer protocol may be/is needed.

8. TCP Basics • Byte Stream Delivery • Connection-Oriented: Two communicating TCP entities (the sender and the receiver) must first agree upon the willingness to communicate • Full-Duplex: TCP almost always operates in full-duplex mode, • TCP exhibit asymmetric behavior only during connection start and close sequences (i.e., data transfer in the forward direction but not in the reverse, or vice versa)

9. Reliable TCP Guarantees • A number of mechanisms help provide the guarantees: • Checksums: To detect errors with either the TCP header or data • Duplicate data detection: Discard duplicate copies of data that has already been received • Retransmissions: • For lost and damaged data • Due to lack of positive acknowledgements • Timeout period calls for a retransmission • Sequencing:To deliver the byte stream data to an application in order • Timers:Various static and dynamic timers used for deciding when to retransmit • Window: For flow control in the form of a data transmission window size

10. Overview of TCP Concepts http://en.wikipedia.org/wiki/TCP_congestion_avoidance_algorithm • Conventional TCP: Tahoe, Reno, New-Reno • Sending rate is controlled by • Congestion window (cwnd): limits the # of packets in flight • Slow-start threshold (ssthresh): when CA start • Loss detection • 3 duplicate ACKs (faster, more efficient) • Retransmission timer expires (slower, less efficient) • Overview of congestion control mechanisms • Slow-start phase: cwnd start from 1 and increase exponentially • Congestion avoidance (CA): increase linearly • Fast retransmit and fast recovery: Trigger by 3 duplicate ACKs Slow-start Congestion avoidance

11. Slow-start Congestion avoidance TCP Basics

12. Congestion Control • Slow Start (SS): A mechanism to control the transmission rate) • When TCP connection starts (Initial Value): CWND =1, • congestion window increases by one segment for each acknowledgement returned • Congestion Avoidance (CA): Used to reduce the transmission rate • When Slow Start drops one or more packets due to congestion • Fast Retransmit: Sender receiving triple duplicate ACKs • Immediate transmission of missing packet without waiting for the Retransmission Timeout to expire • Fast Recovery: In SS or CA when sender receiving triple duplicate ACKs  Sender only enters Congestion Avoidance mode

13. What is Different in MANETs? • Mobility • Route stability and availability • High bit error rate • Packets can be lost due to “noise” • Unpredictability/Variability • Difficult to estimate time-out, RTT, bandwidth • Contention: packets compete for airtime • Intra-flow and inter-flow contentions • Long connections have poor performance • More than 4 hops thruput drops dramatically

14. Overview of Best Protocols • TCP-Westwood[Casetti et. al.] • Estimate bandwidth to alleviate the effect of wireless errors. • TCP-Jersey [Xu et. al.] • Estimate bandwidth to alleviate the effect of wireless errors. • Congestion warning assists the determination of packet loss due to wireless error from congestion. • ATP[Sundaresan et. al.] • Rate based transmission, periodic rate feedback, no timeout concept, reliability provided by SACK. • Split-TCP [Kopparty et. al.] • Separating congestion control from reliability. • Dropped packets are recovered from the most recent proxy instead of the source.

15. Why Does TCP Fail in MANETs? RTO: default maximum retransmission timeout value

16. TCP in MANET • TCP misinterprets route failures as congestion • Effects: Reduce sending rate • Buffered packets (Data and ACKs) at intermediate nodes are dropped. • Sender encounters timeout. • Under prolonged disconnection, a series of timeouts may be encountered.

17. TCP in MANET • TCP misinterprets wireless errors as congestion • Effects: Incorrect execution of congestion control  Performance drops. • Wireless channel is error-prone compared to wireline • Fading, interference, noise

18. TCP in MANET • Intra-flow and inter-flow contention • Effects: Increased delay, unpredictability, and unfairness. • Inter-flow contention: contention of nearby flows. • Intra-flow contention: between packets of the same flow (e.g. forward data and reverse ACKs). • Wireline: only packet on same link “compete” • Wireless: all close by devices compete for the channel Two nearby flows Data stream ACKs stream

19. Impact of Partition on Throughput A X Y P Z S B C D Link Failure Data transfer continues in spite of failure No communication between the partitions

20. Effects of Partitions on TCP Node 5 moves away from node 3 (short-term partition)

21. 5 2 8 1 7 3 9 6 4 The routing protocol reestablishes the path through node 6 Reestablishing Path

22. Long Term Partition Node 5 moves away from node 3 (long-term partition)

23. Long Term Network Partition No communication between the partitions

24. TCP Throughput • Larger the number of nodes a TCP connection needs to span, lower is the end-to-end throughput, as there will be more medium contention taking place in several regions of the network • TCP throughput is inversely proportional to the number of hops

25. Impact of Lower Layers on TCP -MAC • It is intended for providing an efficient shared broadcast channel through which the involved mobile nodes can communicate • In IEEE 802.11, RTS/CTS handshake is only employed when the DATA packet size exceeds some predefined threshold • Each of these frames carries the remaining duration of time for the transmission completion, so that other nodes in the vicinity can hear it and postpone their transmissions • The nodes must await an IFS interval and then contend for the medium again • The contention is carried out by means of a binary exponential backoff mechanism which imposes a further random interval • At every unsuccessful attempt, this random interval tends to become higher

26. Impact of Lower Layers on TCP -MAC • Consider a linear topology in which each node can only communicate with its adjacent neighbors • In addition, consider that in Figures (a) and (b) there exist a single TCP connection running between nodes 1 and 5

27. Capture Conditions • In Figure (c) where there are two independent connections,(connection 2-3) (connection 4-5) • Assuming that connection 2-3 experiences collision due to the hidden node problem caused by the active connection 4-5 , node 2 will back off and retransmit the lost frame • At every retransmission, the binary exponential backoff mechanism imposes an increasingly backoff interval, and implicitly, this is actually decreasing the possibility of success for the connection 2-3 to send a packet as connection 4-5 will “dominate” the medium access once it has lower backoff value • In consequence, the connection 2-3 will hardly obtain access to the medium while connection 4-5 will capture it

28. NetworkLayer Impact • Routing strategies play a key role on TCP performance • There have been a lot of proposed routing schemes and, typically, each of them have different effects on the TCP performance

29. DSR • DSR protocol operates on an on-demand basis in which a node wishing to find a new route broadcasts a RREQ packet • The problem with this approach concerns the high probability of stale routes in environments where high mobility as well as medium constraints may be normally present • The problem is aggravated by the fact that other nodes can overhear the invalid route reply and populate their buffers with stale route information • It can be mitigated by either manipulating TCP to tolerate such a delay or by making the delay shorter so that the TCP can deal with them smoothly

30. TORA • TORA has been designed to be highly dynamic by establishing routes quickly and concentrating control messages within a small set of nodes close to the place where the topological change has occurred • TORA makes use of directed acyclic graphs, where every node has a path to a given destination and established initially • This protocol can also suffer from stale route problem similar to the DSR protocol • The problem occurs mainly because TORA does not prioritize shorter paths, which can yield considerable amount of out-of-sequence packets for the TCP receiver, triggering retransmission of packets

31. Path Asymmetry Impact • In Ad hoc networks, there are severalAsymmetries • Loss Rate Asymmetry:It takes place when the backward path is significantly more error pronethan the forward path • Bandwidth Asymmetry:Arises when forward and backward data follow distinct paths with different speeds • Can happen in ad hoc networks when all nodes not have the same interface speed • Media Access Asymmetry:Arises when TCP ACKs andData are contending for the same

32. RouteAsymmetry • Route asymmetry implies having different paths in both directions • Route asymmetry is associated with the possibility of different transmission ranges for the nodes • The inconvenience with different transmission ranges is that it can lead to conditions in which the forward data follow a considerably shorter path than the backward data (TCP ACK) or vice versa --> affecting hop counts and delays (RTT) • Multi-hop paths are prone to have lower throughput and TCP ACKs may face considerable disruptions

33. Overview of Results • The best TCP variants: • TCP-Westwood and TCP-Jersey seem the best. • Both protocols estimate bandwidth more accurately. • TCP mechanisms: • Feedback from intermediate nodes leads to big gains. • The best non-TCP approaches: • Ad-hoc Transport Protocol (ATP)seems to address most issues • Non-window based: estimates achievable rate periodically • Split-TCP:promising new way of looking at transport layer • Dynamically buffer packets mid-path • Key: Separation of congestion control from reliability.

34. Assignment #8 • Write note on the TCP and non-TCP approaches for Transport Layer of Ad hoc Networks highlighted in Yellow.

35. Q&A • ?