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5. TRANSPORT LAYER. 2. Reminder: TCP Protocol. Slow Start Congestion Avoidance (Control) Fast Retransmit Fast Recovery. TCP: Slow Start. Used when initiating a connection or after a timeout Set cwnd to 1 segment At each ACK , increase cwnd by 1 segment (exponential increase)

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5 transport layer

5. TRANSPORT LAYER

2


Reminder tcp protocol

Reminder:TCP Protocol

  • Slow Start

  • Congestion Avoidance (Control)

  • Fast Retransmit

  • Fast Recovery


Tcp slow start

TCP:Slow Start

  • Used when initiating a connection or after a timeout

    • Set cwnd to 1 segment

    • At each ACK, increase cwnd by 1 segment (exponential increase)

      • Slow start is not slow!

    • Switch to congestion avoidance once cwnd is one half of what it was when congestion occurred

      • After each burst, increase cwnd by 1 segment (linear increase)


Tcp congestion control

TCP:Congestion Control

  • Basic timeout and retransmission

    • If sender receives no ACK for data sent, timeout and retransmit

    • Go To Slow Start (Exponential back-off)

      • cwnd is one half of what it was when congestion occurred

    • Timeout value based on mean and variance of RTT


Tcp congestion avoidance control

TCP:Congestion Avoidance (Control)

  • Uses congestion window (cwnd) for flow control

  • Cwnd set to 1/2 of its value when congestion loss occurred

  • Sender can send up to minimum of advertised window and cwnd

  • Then, additive increase cwnd (at most 1 at each RTT)

    • Careful way to approach limit of the network


Tcp fast retransmit and fast recovery

TCP:Fast Retransmit and Fast Recovery

  • After three duplicate ACKs, assume packet loss, data still flowing

  • Sender resends missing segment

  • Set cwnd to ½ of current cwnd plus 3 segments

  • For each duplicate ACK, increment cwnd by 1

  • When new data ACKed, do regular congestion avoidance


Overview of tcp solutions for wireless networks

Overview of TCPSolutions for Wireless Networks

Focus on eliminating the confusion between congestion loss and all other reasons

Many approaches developed for single-hop wireless systems

Snoop

I-TCP

M-TCP

End to end

SACK

Explicit error notification

Explicit congestion notification (e.g. RED)

Several solutions for multi-hop

A-TCP

Freeze-TCP

Applicability

Clean Layering

Improvement inEfficiency

Layer Violations

Trade-off

8


Tcp problems in wmns

TCPProblems in WMNs

Efficiency – TCP assumes that a missing (or late) ACK is due to network congestion and slows down:

Rate drops quickly if the missing ACK shows up fast enough

Rate drops to zero if it times out

Causes for missing ACKs in WMNs:

Wireless channel errors

Broken routes due to mobility (both users and wireless routers)

Delays due to MAC contention

Interplay between MAC and TCP back-off mechanisms

9


5 transport layer

Wireless Transport Layer Challenges

  • Low bandwidth: typical in wireless networks

     High efficiency requirement

10


5 transport layer

Wireless Transport Layer Challenges

  • Large bandwidth-delay product (BDP):large end-to-end delay

    affects multihop wireless networks (WMNs)

     BDP requires large buffer at sender/receiver

     Large congestion window

    [1]: congestion window = n/4 (n=num of hops) maximizes TCP throughput in

    ideal conditions

    [2]: Alternative solution: split-connection protocols (BDP is small in each part)

[1] Fu Z, Zerfos P, Luo H, Lu S, Zhang L and Gerla M “The impact of multihop wireless channel on TCP throughput and loss,” In Proc. of IEEE INFOCOM, 2003.

[2] Balakrishnan H, Padmanabhan VN and Katz RH, “Network aymmetry: the effects of asymmetry on TCP performance,” ACM MONET Journal, Mobile Networks and Applications, 1999.

11


5 transport layer

Wireless Transport Layer Challenges

  • Frequent Blackouts: such as link/route failures easily occur in wireless environment

     Differentiate packet losses due to congestion or unreliable link

    (non-congestion losses)

    [1]: Classical TCPs do not differentiate the losses  throughput quick drop and slow

    recovery

    [2]: Mechanism for packet loss differentiation in TCPWMNs:

    WMNs avoid single-point-of-failure issue but link failures still occur

    due to wireless channels and mesh client mobility

    [3]: Explicit Link Failure Notification (ELFN) differentiates congestion losses and link

    failures

[1] Xylomenos G, Polyzos GC, Mahonen P and Saaranen M, “ TCP performance issues over wireless links,”IEEE Communications Mag., 2001

[2] Chandran K, Raghunathan S and Prakash R, “A feedback-based scheme for improving TCP performance in ad hoc wireless networks,”IEEE Personal Communications, 2001.

[3] Holland G and Vaidya NH, “Analysis of TCP performance over mobile ad hoc networks,”

Proc. ACMMOBICOM, 1999.

12


5 transport layer

Wireless Transport Layer Challenges

  • Dynamic Network Connectivity:

    • Variable link quality, fluctuating traffic load and user mobility cause large variations in the round-trip-time (RTT) calculations.

       RTT is critical for transport layer protocols, especially for TCP

      RTT Value: its measure is heavily affected by the end-to-end packet delay

      TCP Congestion Control: the timeout mechanism is based on the RTT value

13


5 transport layer

Wireless Transport Layer Challenges

  • Network Asymmetry (BW, losses, latency)

    • Different paths for data and ACK packets or the same path in different time instants or different directions

       experience different packet bandwidth, loss rate and

      latency

14


5 transport layer

Wireless Transport Layer Challenges

  • Possible Solutions for Network Asymmetry

    [1]: poor TCP performance in MANETs (e.g., ACK packet loss due to

    poor link in forward direction do not indicate congestion in

    reverse)

    [2]: definition of network asymmetry and proposed solutions such as

    ACK filtering, ACK congestion control, etc.

[1] Xu S and Saadawi T, “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks,”IEEE Communications Mag., 2001

[2] Balakrishnan H, Padmanabhan VN and Katz RH, “Network asymmetry: the effects of asymmetry on TCP performance,”ACM MONET, Mobile Networks and Applications, 1999.

15


5 transport layer

Wireless Transport Layer Challenges

  • Heterogeneity:

    - Wired and wireless networks

    - Network services vary from reliable datatransfer

    to real-time multimedia such as live video streaming.

    - Reliable transport for data; Timely delivery and smooth rate variation.

    • TCP Starvation by UDP traffic

16


5 transport layer

Wireless Transport Layer Challenges

  • Possible Solutions for Heterogeneity

     Split one connection into two or more connections so that each of them experiences homogeneous network characteristics

    [1]: Indirect TCP, where one end-to-end connectionis split into a wired connection and a wireless connection and separate mechanism are applied for the transport protocol

    [2]: Snooping module in the network layer to add timeout or duplicate ACKs

    designed for one-hop wireless network connected to wired networks)

[1] Bakre A and Badrinath BR, “I-TCP: indirect TCP for mobile hosts,” in Proc. 15th IEEEInt. Conf. on Distributed Computing Systems, 1995.

[2] Balakrishnan H, Padmanabhan VN and Katz RH. “Network asymmetry: the effects of asymmetry on TCP performance,”ACM MONET, Mobile Networks and Applications, 1999

17


Wireless transport layer challenges

Wireless Transport Layer Challenges

Unfairness

Due to network layer unfairness

Due to variation in round trip delays

Likely both will be fixed if network layer fairness is ensured

TCP

IP

DLC

Unfair

PHY

18


Wireless transport layer challenges1

Wireless Transport Layer Challenges

Unfair service between short and long flows:

In WMNs, as the number of hops on a path increases, the probability of a link failure and consequent packet losses on the path increase.

This implies that shorter flows enjoy an unfair advantage in throughput compared to longer flows.

19


Wireless transport layer challenges2

Wireless Transport Layer Challenges

  • Effect of Multi-Channel Operations:

    • Interference levels or multi-path channel characteristics can be very different,

      which makes end-to-end rate adaptation and congestion control mechanisms inefficient for WMNs.

20


5 transport layer

Network Congestion and Contention:

End-to-end congestion control mechanisms cannot react

quickly enough to transient congestion conditions because of

unpredictable RTT delay between the sender and receiver.

Network asymmetry and ACK bunching problem

Wireless Transport Layer Challenges

21


5 transport layer

Wrap Up:

Transport Layer Challenges

  • Avoid TCP shortcomings while being TCP-compatible

  • Support real-time traffic in wireless mesh networks

  • Provide integration with other layers

  • Infer and react to observations at other layers

  • Consider the impact of mobility

22


5 transport layer

Transport Layer Protocols

for Reliable Data Transport

  • For WMNs same design guidelines and methodologies as in

    Multihop Ad Hoc networks

  • Large number of solutions proposed so far for Multihop Ad

    Hoc networks

  • Classification into two types

    • TCP variants

    • Entirely new transport protocols

23


5 transport layer

TCP Variants

  • Classified intoseveral types:

    1.Differentiation between packet losses and congestions

    2. Window optimization

    3. ACK optimization

    4. Adaptive control of transmission rates

24


5 transport layer

TCP VARIANTS

1.Differentiation between packet losses and congestions

1. Packet Loss vs Congestion:

A packet loss due to congestion or to errors specific of a Multihop Ad Hoc network (e.g., low link quality, route failure, route change due to mobility, etc.)

  • Classical TCP scheme:

    • Packet loss always considered due to congestion  invoke

      Congestion Control Mechanism

    • The frequent usage of the Congestion Control Mechanism

      degrades severely the performance

       A differentiation of the packet losses is needed

25


5 transport layer

TCP VARIANTS:

1.Differentiation between packet losses and congestions

Chandran K, Raghunathan S and Prakash R, “A feedback-based scheme for improving TCP performance in ad hoc wireless networks,” IEEE Personal Communications, 2001

  • Solution 1:Detection of error events and feedback to the TCP sender

     TCP-Feedback (TCP-F)

    * Congestion/route failure handled separately

    * In case of route failure TCP-F explicitly informs the source

    * TCP-F detects the route failure in the networking layer as part of

    a routing protocol

26


5 transport layer

TCP-FMechanism:

1.Differentiation between packet losses and congestions

Chandran K, Raghunathan S and Prakash R,

“A feedback-based scheme for improving TCP performance in ad hoc

wireless networks,” IEEE Personal Communications, 2001

* A node detects a failure  sends a route failure notification to the next-hop

* When the source node receives the notification  it stops sending packets and it enters a snooze state:

1. It freezes all timers and windows

2. It starts a route failure timer(according to the worst case of route re-

establishment)

* When the source node receives a route re-establishment message 

It stops the snooze state:

1. It flushes out unacknowledged packets (no wait for ACKs)

2.It falls back to standard TCP

27


5 transport layer

TCP VARIANTS:

1.Differentiation between packet losses and congestions

Holland G and Vaidya NH, “Analysis of TCP performance over

mobile ad hoc networks,”in Proc. ACM MOBICOM, 1999.

 Explicit Link Failure Notification (ELFN)

* Differentiates packet loss for congestion/link failure

* In case of route failure ELFN notifies the sender (through ICMP or routing messages) similarly to TCP-F:

  • The sender disables the transmission timer and it enters the stand-by mode(a probe packet is sent periodically)

    1. It sends a probepacket (to check if a new route has been established)

    2. If the answer is positive it goes back to normal TCP state

28


5 transport layer

TCP VARIANTS:

1.Differentiation between packet losses and congestions

Dyer T and Boppana R, “A comparison of TCP performance over three routing protocols for mobile ad hoc networks,”

Proc. of ACM MOBIHOC 2001.

  • Solution 2:Heuristic to distinguish route errors/congestion

     Fixed Retransmission TimeOut (Fixed RTO)

    * It does not rely on feedback from the lower layers

    * When RTOs occur consecutively without receiving ACKs:

    • The sender assumes that a route error has occurred

      1. It transmits again the unacknowledged packet(without doubling the RTO)

      2. RTO remains fixed untilthe route is re-established and the

      retransmitted packet is acknowledged

29


5 transport layer

TCP VARIANTS:

1.Differentiation between packet losses and congestions

Wang F and Zhang Y, “Improving TCP Performance over Mobile Ad

Hoc Networks with Out-of-order Detection and Response,”

in Proc. ACM MOBIHOC, 2002

  • Solution 3:End-to-End Approach

     TCP Detection of Out of Order and Response (TCP DOOR)

    * Detection and response to out-of-order (OOO) delivery events

    * The detection of OOO events is performed at both ends (sender detects OOO ACK packets and receiver detects OOO data packets)

    * OOO events can be detected only after a route has been recovered from failures

    TCP DOOR is less accurate and responsivethan afeedback-based approach

30


5 transport layer

TCP VARIANTS:

2. Window Optimization

Fu Z, Zerfos P, Luo H, Lu S, Zhang L and Gerla M“The impact of multihop wireless channel on TCP throughput and loss,”

Proc. of IEEE INFOCOM, 2003

TCP congestion control window is designed for wired networks 

no optimal for multihop wireless networks

  • Classical TCP scheme:

    • If 802.11 MAC layer in multihop wireless network  rare buffer

      overflow, packet loss mainly by link-layer drops

    • TCP window usually adapted to the buffer-overflow, much

      different from link-layer drop behavior

       An adaptation of the TCP window to the link layer drop behavior is needed

31


5 transport layer

TCP VARIANTS:

2. Window Optimization

  • Classical TCP window vs link-layer drop behavior:

    • When the TCP window is larger than W* the link-layer drop

      probability gradually increases

    • When the TCP window reaches a W > W*  the link-layer drop

      probability saturates

    • When the TCP window is optimized at W* best TCP throughput

    • Classical window adaptation  keeps increasing until W >> W*

       Causes much larger link-layer drop probability  reduces TCP throughput

32


5 transport layer

TCP VARIANTS:

2. Window Optimization

Problems in computing an optimal TCP window

1) The window is related to the MAC behavior

(multihop wireless network)

2) The optimized window of one flow may be impacted by other

flows in the same network

3) The optimal window size is also related to the number of

hops, routing paths, and other factors

33


5 transport layer

TCP VARIANTS:

2. Window Optimization

Fu Z, Meng X and Lu S, “A transport protocol for supporting multimedia

streaming in mobile ad hoc networks,” IEEE Journal on Selected Areas in

Communications, 2003

  • Solution:Do not explicitly derive optimal window 

    use indirect solution

     Link-Layer Random Early Detection (RED) and adaptive pacing

    • RED records MAC layer average retries

    • RED marks packets (dropped/marked) in the buffer accordingly

    • When average retries are > threshold  adaptive pacing increases the backoff timer by the last transmission time of prev. packet

    • Better coordination among nodes

    • TCP window is as close to W* as possible

34


5 transport layer

TCP VARIANTS:

3. ACK Optimization

Tung L-P, Shih W-K, Cho T-C, Sun, YS and Chen MC, “TCP throughput enhancement over wireless mesh networks,”

IEEE Communications Magazine, 2007

  • In a wireless multihop network,

    - TCP ACKs interfer TCP data packets

    - too many TCP ACKs increase collisions (70% of collisions in TCP)

  • Classical TCP scheme:

    • If 802.11 MAC layer in multihop wireless network  each node

      on the routing path tries to send one frame at a time

    • Spurious retransmissions from sender, more ACKs  higher

      packet losses for collisions with ACK’s

       Limit the number of transmitted ACK’s

35


5 transport layer

TCP VARIANTS:

3. ACK Optimization

  • Solution:Use cumulative ACKs or use a delayed-ACK scheme

    Scheme 1: Dynamic adaptive acknowledgement

    * Accumulates many delayed ACKs

    Scheme 2: Uses a dynamic reaction scheme

    * The timeout interval is adaptively adjusted by considering packet inter-arrival

    times

    • The receiver delays just enough time for ACKs

    • Avoid spurious retransmissions even with high delay variability (multihop wireless networks)

36


5 transport layer

TCP VARIANTS

4. Adaptive control of transmission rates

  • When cumulative ACKs are used  bursty transmissions

  • Classical TCP scheme:

    • Cumulative ACKs are desired in multihop wireless network

    • The congestion control at the sender can invoke bursty ACKs

      transmissions  further contentions in the link layer

       A solution is needed to control the transmission rate

  • 37


    5 transport layer

    TCP VARIANTS

    4. Adaptive Control of Transmission Rates

    ElRakabawy SM, Klemm A and Lindemann C,

    “TCP with adaptive pacing for multihop wireless networks”,

    Proc. ACM MOBIHOC, 2005

    • Solution:Adjusts the transmission rate by considering several metrics

       TCP Adaptive Pacing (TCP-AP) uses three metrics:

      1. Congestion window

      2. Contention on the end-to-end path based on a co-efficient of RTT

      variations (cov RTT)

      3. Spatial-reuse constraint (FHD) based on the 4-hop propagation

      delay (FHD) starting from the sender

    38


    5 transport layer

    TCP VARIANTS

    4. Adaptive Control of Transmission Rates

    • TCP-AP (Adaptive Pacing) computes the inter-packet Delay (Dip)

      Dip = FHD(1 + 2 cov RTT )

    • Dip becomes the timeout value of the pacing timer  when timeout occurs:

      • Congestion window is checked for transmitting a new packet

      • A new packet is sent if possible, otherwise the TCP stay idle

    39


    5 transport layer

    TCP VARIANTS

    4. Adaptive Control of Transmission Rates

    • TCP-AP:

      Hybrid scheme  sender rate control & congestion control

      Advantages:

    • No impact on TCP end-to-end semantics

    • No changes required on lower layer protocols (routing, MAC)

    40


    5 transport layer

    ALTERNATIVE SOLUTION:

    Separate Sublayer for New TCP Functions

    Liu J and Singh S, “ATCP: TCP for mobile ad hoc networks,” IEEE Journals on Selected Areas in Communications, 2001

    • TCP variants  they may need undesired modifications to the standard TCP/IP protocol suite

       An intermediate protocol between TCP and the network layer

       Adaptive TCP (ATCP) protocol

      * Studied for mobile ad hoc network

      * Protocol between TCP and IP layers

    41


    5 transport layer

    ATCP utilizes network layer feedback (from the intermediate nodes) to take appropriate actionsNetwork feedback is:ICMP: The Destination Unreachable ICMP message indicates route disruption ECN: Indicates network congestionWith ECN enabled, time out and 3 dup ACKs are assumed to no longer be due to congestion

    ATCP Approach

    42


    5 transport layer

    RTO or 3rd dup ACK:Retransmits unACKed segmentsACK with ECN flag: Invokes congestion controlDestination Unreachable ICMP message: Stops transmission; Enter Persistent Mode Wait until a new route is found, resume transmissionATCP monitors TCP state and spoofs TCP in such a way to achieve the above behaviors

    TCP/ATCP Behavior

    43


    5 transport layer

    ATCP

    Liu J and Singh S, “ATCP: TCP for mobile ad hoc networks,” IEEE Journals on Selected Areas in Communications, 2001

    * Goal:

    Avoid standard TCP being impacted by

    • Re-routing time out

    • Packet loss due to error

    • Out-of-order packet due to multipath routing

    • Network partitions

    44


    5 transport layer

    ATCP Functions

    1. If network partition  persistent state of TCP sender (no timeout start)

    2. If packet loss due to error instead of congestion  TCP sender retransmits packets (no congestion control)

    3. If network congestion  regular TCP congestion control

    * ATCP is supposed to know when a packet is lost for congestion or network errors and when there is network partition or route failure

    45


    5 transport layer

    - ATCP improves TCP performance- Maintains high throughput since TCP’s unnecessary congestion control is avoided- Saves network resources by reducing number of unnecessary re-transmissions- End-to-End TCP semantics are maintained ATCP is transparent Nodes with and without ATCP can set up TCP connections normally

    Advantages of ATCP

    46


    5 transport layer

    ALTERNATIVE SOLUTION:

    Ad hoc Transport Protocol (ATP)

    Sundaresan K, Anantharaman V, Hsieh H-Y and Sivakumar R,

    ATP: a reliable transport protocol for ad-hoc networks.

    Proc. ACM MOBIHOC, 2003

    • An entirely new transport protocol  compatibility issue for WMNs

      • E.g., ATP  stand-alone wireless network assumption

         not valid for WMNs

      • WMNs integration with the Internet and many other wireless networks

    • Transport protocols for WMNs must be compatible with

      TCPs in other networks

    47


    5 transport layer

    ALTERNATIVE SOLUTION:

    Ad hoc Transport Protocol (ATP)

    Sundaresan K, Anantharaman V, Hsieh H-Y and Sivakumar R, ATP: a reliable transport protocol for ad-hoc networks.

    Proc. ACM MOBIHOC, 2003

    • Many fundamental problems exist in TCP  develop entirely new

      transport protocols for ad hoc networks

      Ad hoc Transport Protocol (ATP)

      • Rate-based transmission

        1) Quick start  initial rate estimation

        2) Delay-based approach for congestion detection (no ambiguity on congestion/non-congestion losses)

        3) No retransmission timeout

        4) Decoupled congestion control and reliability

      • Better performance (e.g., delay, throughput, and fairness) than the TCP variants

    48


    5 transport layer

    Layer coordinationUses feedback from network nodes for congestion detection, avoidance, and controlRate based transmissionsAvoids impact of bursty trafficDecoupling of congestion control and reliability

    Ad Hoc Transport Protocol (ATP)

    49


    5 transport layer

    Congestion control uses feedback from the network; - Reliability is ensured through receiver feedback and selective ACK- Assisted congestion control- Adapts sending rate based on feedback from intermediate nodes- TCP friendliness and fairness achieved through feedback from intermediate nodes

    Ad Hoc Transport Protocol (ATP)

    50


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    • End-to-end delivery of real-time traffic

       UDP transport

    • UDP alone

      * cannot guarantee real-time:

      may starve TCP connections in the same network

    • Additional protocols over UDP: Real-Time Protocol (RTP) and

      Real-Time transport protocol (RTCP)

    • Rate Control Protocol (RCP) is also needed over RTP/RTCP for

      congestion control

    51


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    • RCP for wired networks:

      • Additive-increase multiplicative-decrease (AIMD)-based

      • Equation-based

      • Not applicable to wireless networks with packet errors and link failures

    • RCP for wireless networks:

       RCP should differentiate between congestion losses or wireless channels losses

      • Loss differentiation algorithms (LDAs) with congestion control

        have been developed (not applicable to WMNs though)

    52


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    Cen S, Cosman PC and Voelker GM, “End-to-end differentiation of congestion and wireless losses,” IEEE/ACM Trans. Networking, 2003

    • Loss Differentiation Algorithms (LDAs):

      * The hybrid LDA is the most effective

      * Only one wireless link is considered on the path between sender and receiver

      • not applicable to WMNs scenarios

    53


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    Fu Z, Meng X and Lu S, “A transport protocol for supporting multimedia streaming in mobile ad hoc networks,” IEEE Journal on Selected Areas in Communications, 2003

    • RCP for mobile ad hoc networks:

      • Adaptive detection rate control (ADTFRC):

        * End-to-end multi-metric joint detection approach

        for TCP-friendly rate control schemes

    54


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    Fu Z, Meng X and Lu S, “A transport protocol for supporting multimedia streaming in mobile ad hoc networks,” IEEE Journal on Selected Areas in Communications, 2003

    Problems of ADTFRC:

    • Not sufficient detection accuracy for multimedia traffic

    • Do not distinguish different non-congestion packet losses

    55


    5 transport layer

    Transport Layer Protocols

    for Real-Time Delivery

    • RCP for WMNs:

      • No new RCP has been proposed so far

      • No effective RCP for ad hoc networks can be adopted

        and tailored for WMNs

      • RCP for WMNs is a new research area

    56


    Transport layer protocols for wmns

    Transport Layer Protocols for WMNs

    There exist only a few transport layer protocols for WMNs.

    • More attention has been given to routing and MAC protocols:

      • If these can provide enough reliability and quality, the standard TCP and UDP protocols can be used.

        • More convenient for users and network administrators but…

      • Standard transport protocols cannot always meet the needs of WMNs’ applications.

    57


    Transport layer protocols for wmns1

    Transport Layer Protocols for WMNs

    58

    Transport protocols proposed for mobile ad hoc networks or other multihop networks can be used but…

    • These protocols may not be a good choice for WMNs:

      • Many TCP enhancements for ad hoc networks focus on route

        failure due to mobility  Rare in WMNs.

      • Most existing transport protocols for ad hoc networks only consider an isolated multihop wireless network  WMNs are connected to Internet.


    Transport layer protocols for wmns2

    Transport Layer Protocols for WMNs

    WMNs’ architectures are different from

    other wireless networks:

    • WMNs are typically connected to the Internet backbone using gateways:

      • The end points may be inside the Internet backbone  They cannot be easily modified or changed.

    59


    Transport layer protocols for wmns3

    Transport Layer Protocols for WMNs

    60

    Large amount of traffic come from or go to the Internet backbone instead of flowing within WMNs:

    • Interactions between MAC and transport layers are significantly different from a general multihop wireless network

    • Existing solutions may not be able to deliver good performance to WMNs


    Transport layer protocols for wmns4

    Transport Layer Protocols for WMNs

    • A mesh router must collect the packets from all the connected devices.

      • This results in a significant buffer usage.

      • TCP Vegas and its descendants use buffer occupancy as a congestion metric.

        • Congestion is detected earlier when compared to loss based TCP variants but…

        • Delay-based algorithms are not adequate when single gateways may be part of different TCP flows, potentially with different properties.

          Novel transport protocols considering specific features of

          WMNs should be developed!

    61


    Transport protocols based on hop by hop control

    Transport Protocols Based on Hop-by-Hop Control

    Motivation

    • In WMNs, the most frequent cause of packet loss is due to bit errors in packets:

      • It is a wise strategy to consider link layer performance enhancement as a functional block of a transport protocol.

    62


    Transport protocols based on hop by hop control1

    Transport Protocols Based on Hop-by-Hop Control

    63

    Considering end-to-end transmission at the transport layer, if a packet is lost at an intermediate node due to bit-error:

    • End-to-end retransmission wastes the successful transmissions before this node

    • Needs the packet to traverse the same path again  can lead to higher delay and waste of resources

    • Large number of ACKs can be generated  consume a large percentage of bandwidth


    Transport protocols based on hop by hop control2

    Transport Protocols Based on Hop-by-Hop Control

    Raniwala et al., “Evaluation of a stateful transport protocol for multi-channel

    wireless mesh networks,” in Proc. of IEEE Int. Workshop on Quality of Service, 2007.

    Stateful Transport Protocol

    • Uses hop-by-hop retransmission instead of end-to-end transmission.

    • Requires the routers inside the network to maintain states for transport layer functions (thus the name)

      • The congestion control is performed through a rate control scheme at the sender of a connection.

      • It is not compatible with standard TCP…

    64


    5 transport layer

    Stateful Transport Protocol

    • In WMNs, not all nodes can implement proprietary protocols:

      • Clients to be connected to WMNs may have to use standard TCP

      • Nodes residing in the wired network usually use standard TCP too

    65


    5 transport layer

    Stateful Transport Protocol

    66

    To support connections to these nodes, a proxy protocol is needed at the ingress/egress nodes to split the end-to-end connections into three subconnections:

    • A subconnection from mobile clients to ingress node of WMNs.

    • A subconnection from the ingress to the egress node of WMNs.

      • Here is where the stateful transport protocol enters the game.

      • (Link Aware Reliable Transport Protocol)

    • A subconnection from the egress node to the end node in the wired network.


    5 transport layer

    Stateful Transport Protocol:Link-aware ReliableTransport Protocol (LRTP)

    • The core function of the stateful transport protocol

    • Three key components:

      • Rate Control

      • Link-layer Retransmissions

      • NACK and End-to-End Retransmission

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    5 transport layer

    Stateful Transport Protocol: Transmission Rate Control

    Performed by the sender of a flow as follows:

    • The link capacity is measured based on the service time

      of link layer packets.

      • Service time: the time interval from a packet is scheduled for

        transmission until a MAC ACK is received.

        2. The measured link capacity is allocated to all flows on

        the same link using the max-min algorithm.

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    5 transport layer

    Stateful Transport Protocol

    69

    3. The calculated transmission rate of a flow is embedded in its

    packets as a stamp.

    4. When the packet sees a lower value on the next link, the stamp is

    overwritten by the new value.

    5. The receiver calculates an average value and then sends it back to

    the sender via a periodic mechanism.

    6. The sender adjusts its transmission rate accordingly.


    Stateful transport protocol link layer retransmission

    Stateful Transport Protocol:Link-Layer Retransmission

    • If a sent packet gets lost, it is retransmitted at the

      current hop instead of waiting for the receiver to detect it.

    • To avoid head-of-line blocking, a packet to be retransmitted

      is not sent again immediately, but scheduled using a queue for each link and a round-robin scheme among flows.

    • This works on top of the MAC-layer retransmission .

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    Stateful transport protocol nack and end to end retransmission

    Stateful Transport Protocol:NACK and End-to-End Retransmission

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    Buffer overflow or congestion can cause packet loss  In

    this case, the LRTP retransmission is not helpful!

    When the receiver detects this event, it sends a negative

    ACK (NACK) back to the sender.


    Critical problems of lrtp

    Critical Problems of LRTP

    • Inaccuracy of the link capacity estimation scheme:

      • It only captures contention:

        • Related to all the links in the same interference range, not just the one being considered.

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    Critical problems of lrtp1

    Critical Problems of LRTP

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    2. Convergency time of the transmission rate:

    • BW requirement of a flow is determined using the input rate of a flow:

      • Incoming packets in a flow do not really reflect BW

        requirement of a flow before an appropriate rate is

        determined for the sender.

    • LTRP also uses the current BW share for the BW requirement, but …

      • the current BW share is not an accurate value either, as

        the BW requirement of the current flow is not known yet.


    Problems of stateful transport protocol

    Problems of Stateful Transport Protocol

    3. Dependency on the Functionality of the MAC protocol:

    • This scheme may not be always applicable, e.g., IEEE 802.11n: packets are not individually sent and acknowledged, but aggregated with other packets potentially from different flows.

      4. Unfeasibility of the inter-node coordination for bandwidth allocation:

    • The entire network should be coordinated in a multi-hop scenario in order to achieve this.

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    Open research issues

    Open Research Issues

    • For reliable transport protocols, TCP-based protocols have the advantages of simplicity and provide easy compatibility

      • TCP variants developed for mobile ad hoc networks tend to be too complicated for WMNs.

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    Open research issues1

    Open Research Issues

    76

    More efficient schemes for TCP enhancement are expected for WMNs in terms of:

    • Loss differentiation schemes.

    • Congestion detection schemes.

    • Retransmission mechanisms.

    • Congestion control algorithms.

    • Cross-layer optimization is the major challenge:

    • because all problems of TCP performance degradation are actually related to protocols in the lower layers


    Open research issues2

    Open Research Issues

    • For non-TCP based reliable transport protocols, cross-layer solutions are adopted:

      • In both LRTP and AR-TP, retransmission and rate control are mainly done in the link layer

        • Improving the performance of these schemes is still

          an open problem.

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    Open research issues3

    Open Research Issues

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    • How to design a better link/transport cross-layer protocol to achieve higher performance and better compatibility to TCP is an interesting topic

      • WMNs are usually connected to clients and nodes using

        standard TCP, it is critical to have a solution to

        support compatibility with standard TCP


    Open research issues4

    Open Research Issues

    • For real-time delivery, no existing solution from ad hoc networks can be adopted:

      • If UDP is used, brand-new RCPs need to be developed considering the features of WMNs.

      • If DCCP is adopted, it is necessary to improve its performance so that it meets the needs of multimedia applications and is friendly to TCP flows

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    Open research issues5

    Open Research Issues

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    Multicasting in WMNs, e.g., for IP-TVapplications demands new transport protocols

    • Cross-layer design is important for these protocols as multicasting is closely related to both routing and transport protocols


    Open research issues6

    Open Research Issues

    • Interconnection of WMNs with various wireless networks such as IEEE 802.11, 802.16, 802.15, etc. requires novel solutions for transport protocols:

      • Different network technologies show different properties (capacity, errors)

      • Same transport protocol may be not effective for all networks

      • Using different transport protocols in these networks is complicated and costly

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