1. CS – 647: Advanced Topics in Wireless Networks
Transport Layer for Mobile Ad hoc Networks
CS – 647: Advanced Topics in Wireless Networks
Drs. Baruch Awerbuch and Amitabh Mishra
Computer Science Department
Johns Hopkins

2. Reading
Chapter 7 – Ad Hoc & Sensor Networking, Cordeiro & Agrawal, 2007
One of the suggested text for the course

3. 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

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

5. Network Architecture at a Crossroads
Introduction
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

6. Revisiting the Current Transport Architecture
Introduction
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?

7. 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?

8. 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.

9. 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)

10. 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

11. Overview of TCP Concepts
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
We may need a second slide here to put a figure the typical sawtooth of TCP
Slow-start
Congestion
avoidance

12. TCP Basics
Slow-start
Congestion
avoidance

13. 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

14. What is Different in MANETs?
Overview
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

15. Overview of Best Protocols
TCP-Westwood [Casetti et. al.]
Estimate bandwidth to alleviate the effect of wireless errors.
TCP-Jersey [Xu et. al.]
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.

16. Why Does TCP Fail in MANETs?

17. Specific problems of TCP over MANETs
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.

18. Specific problems of TCP over MANETs
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

19. Specific problems of TCP over MANETs
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

20. Drawback of TCP Exponential Back Off

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

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

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

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

25. Long Term Network Partition
No communication between the partitions

26. 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

27. 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 , 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

28. 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

29. 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

30. Network Layer 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

31. 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 exacerbated 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

32. Path Asymmetry Impact
In Ad hoc networks, there are several asymmetries
Loss Rate Asymmetry: It takes place when the backward path is significantly more error prone than 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 and Data are contending for the same

33. Route Asymmetry
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

34. Overview of Results The best TCP variants: TCP mechanisms:
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.

35. 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