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

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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

reading
Reading
  • Chapter 7 – Ad Hoc & Sensor Networking, Cordeiro & Agrawal, 2007
    • One of the suggested text for the course
outline
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
tcp in wired network and manet

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
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
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?
problem statement
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?
our goal
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.
tcp basics
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)
reliable tcp guarantees
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
overview of tcp concepts

Overview

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

Slow-start

Congestion

avoidance

tcp basics1

Slow-start

Congestion

avoidance

TCP Basics
congestion control
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
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
overview of best protocols

Overview

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.
specific problems of tcp over manets
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.
specific problems of tcp over manets1
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
specific problems of tcp over manets2
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

slide21

Impact ofPartitionon Throughput

A

X

Y

P

Z

S

B

C

D

Link Failure

Data transfer continues in spite of failure

No communication between the partitions

slide22

Effects of Partitions on TCP

Node 5 moves away from node 3 (short-term partition)

slide23

5

2

8

1

7

3

9

6

4

The routing protocol reestablishes the path through node 6

Reestablishing Path

long term partition
Long Term Partition

Node 5 moves away from node 3 (long-term partition)

slide25

Long Term Network Partition

No communication between the partitions

tcp throughput
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
impact of lower layers on tcp mac
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
impact of lower layers on tcp mac1
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
capture conditions
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
network layer impact
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
slide31
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
path asymmetry impact
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 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
route asymmetry
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
overview of results
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.
slide35
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