Qian Zhang Department of Computer Science HKUST Advanced Topics in Next- Generation Wireless Networks Transport Protocols in Ad hoc Networks

Transmission Control Protocol (TCP) Reliable ordered delivery Implements congestion avoidance and control Reliability achieved by means of retransmissions if necessary End-to-end semantics –Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver –Ack for data sent only after data has reached receiver

Overview of TCP concepts Conventional TCP Sending rate is controlled by –Congestion window (cwnd): limits the –Slow-start threshold (ssthresh): when 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 Overview Slow-start Congestion avoidance – Tahoe, Reno, New-Reno, SACK # of packets in flight CA start

Challenges Throughput unfairness –Unfairness at MAC layer –Transport layer should take this into account Resource constraints –Power and bandwidth constraints Separation of congestion control and reliability control Completely decoupled transport layer –Wired network: transport protocol completely separated from underlying layer –Ad hoc network: interaction with network and MAC layer is expected for adaptivity

Challenges (Cont.) Misinterpretation of congestion –Traditional mechanism: packet loss, timeout –Ad hoc loss/delay due to High bit error rate due to varying link condition Packet collisions due to contention and hidden terminal Path breaks due to node mobility Dynamically changing topology –Frequent path breaks –Partitioning and merging of networks –High delay in reestablishment of path

Performance of TCP Several factors affect TCP performance in MANET –Wireless transmission errors –Multi-hop routes on shared wireless medium For instance, adjacent hops typically cannot transmit simultaneously –Route failures due to mobility

Random Errors If number of errors is small, they may be corrected by an error correcting code Excessive bit errors result in a packet being discarded, possibly before it reaches the transport layer Problems –Fast retransmission of lost packet –Reduction in congestion window  reduces the throughput –Burst errors may cause timeouts

Throughput over Multi-Hop Wireless Paths Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop TCP Throughput using 2 Mbps MAC

Throughput Degradations with Increasing Number of Hops Packet transmission can occur on at most one hop among three consecutive hops –Increasing the number of hops from 1 to 2, 3 results in increased delay, and decreased throughput Increasing number of hops beyond 3 allows simultaneous transmissions on more than one link –However, degradation continues due to contention between TCP Data and Acks traveling in opposite directions When number of hops is large enough, the throughput stabilizes due to effective pipelining

Performance Metric: Expected Throughput Compute the expected throughput as where –t i : the duration for which the shortest path from the sender to the receiver contains i hops –T i : the throughput obtained in the fixed, linear network with i hops

Impact of Mobility measured TCP Reno throughput

Throughput Degrades with Increasing Speed …

But NOT Always …

mobility causes link breakage, resulting in route failure TCP data and acks en route discarded Why Does Throughput Degrade? TCP sender times out. Starts sending packets again Route is repaired No throughput despite route repair

mobility causes link breakage, resulting in route failure TCP data and acks en route discarded TCP sender times out. Backs off timer. Route is repaired TCP sender times out. Resumes sending Larger route repair delays especially harmful No throughput despite route repair Why Does Throughput Degrade?

C B D A C B D A C B D A 1.5 second route failure Route from A to D is broken for ~1.5 second When TCP sender times out after 1 second, route still broken TCP times out after another 2 seconds, and only then resumes Low Speed Scenario

C B D A C B D A C B D A 0.75 second route failure Route from A to D is broken for ~ 0.75 second When TCP sender times out after 1 second, route is repaired Higher (double) Speed Scenario

Impact of Caching Route caching has been suggested as a mechanism to reduce route discovery overhead [Broch98] Each node may cache one or more routes to a given destination When a route from S to D is detected as broken, node S may: –Use another cached route from local cache, or –Obtain a new route using cached route at another node

Impact to TCP Average speed (m/s) Actual throughput (as fraction of expected throughput)

Why Performance Degrades With Caching When a route is broken, route discovery returns a cached route from local cache or from a nearby node After a time-out, TCP sender transmits a packet on the new route However, the cached route has also broken after it was cached Another route discovery, and TCP time-out interval Process repeats until a good route is found timeout due to route failure timeout, cached route is broken timeout, second cached route also broken

To Cache or Not to Cache Caching can result in faster route “repair” Faster does not necessarily mean correct If incorrect repairs occur often enough, caching performs poorly Need mechanisms for determining when cached routes are stale

Caching and TCP performance Caching can reduce overhead of route discovery even if cache accuracy is not very high But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs

How to Improve Throughput (Bring Closer to Ideal) Network feedback Inform TCP of route failure by explicit message Let TCP know when route is repaired –Probing –Explicit notification Reduces repeated TCP timeouts and backoff

TCP with ELFN Explicit Link Failure Notification –Not totally new, e.g., ECN bits in TCP –To provide the TCP sender with information about link and route failures, so that it can avoid responding to the failures as if congestion has occurred How does it work? –When a TCP sender receives an ELFN: disables its retransmission timers and enters a “stand-by” mode –While on standby: A packet is sent at periodic intervals to probe the network to see if a route has been established –If an acknowledgment is received: leaves stand-by mode and restores the retransmission timers

Measured TCP Reno w/ELFN throughput

Performance with Explicit Notification

Issues: Network Feedback Network knows best (why packets are lost) Network feedback beneficial  Need to modify transport & network layer to receive/send feedback  Need mechanisms for information exchange between layers [Holland99] discusses alternatives for providing feedback (when routes break and repair) –[Chandran98] also presents a feedback scheme