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Wireless and Mobile Systems Design. Lecture 11 Mobile Networks: TCP in Wireless Networks. Lecture Objectives. Describe TCP’s flow control mechanism

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Wireless and mobile systems design l.jpg

Wireless and Mobile Systems Design

Lecture 11Mobile Networks:TCP in Wireless Networks


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Lecture Objectives

  • Describe TCP’s flow control mechanism

  • Describe operation of TCP Reno and TCP Vegas, including congestion avoidance (congestion control), slow start, and fast retransmission and recovery mechanisms

  • Describe performance problems of TCP in wireless networks

  • Summarize proposed schemes to overcome performance limitations of TCP in wireless networks

Mobile Networks: TCP in Wireless Networks 2


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Agenda

  • TCP overview

    • Flow control

    • Congestion avoidance, slow start, and retransmission

    • TCP Reno and TCP Vegas

  • TCP in wireless networks

  • Solutions to TCP performance problems in wireless networks

Mobile Networks: TCP in Wireless Networks 3


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TCP Flow Control

  • TCP inherently supports flow control to prevent buffer overflow at the receiver

    • Useful for fast sender transmitting to slower receiver

  • Receiver advertises a window (wnd) in acknowledgements returned to the sender

  • Sender cannot send more than wnd unacknowledged bytes to the receiver

Src

Dest

Limits amount ofdata that destinationmust buffer

Mobile Networks: TCP in Wireless Networks 4


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TCP Flow Control Example

Sender

Receiver

wnd = 1200

500 bytes

500 bytes

wnd = 200

200 bytes

wnd = 500

500 bytes

Mobile Networks: TCP in Wireless Networks 5


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Flow Control Can Limit Throughput (1)

  • Let rtt be the round-trip time, i.e., the time from sending a segment until an acknowledgement (ACK) is received

  • Let t = wnd/b be the time to transmit a full “window” of data, where b is link bandwidth

Sender

Receiver

t

wndbytes

rtt

Mobile Networks: TCP in Wireless Networks 6


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Flow Control Can Limit Throughput (2)

  • For a link with a high delay-bandwidth product (rttb), the flow control window can limit throughput for the connection

    • In this case, t rtt

    • Throughput is wnd/rtt

Sender

Receiver

t

wndbytes

rtt

Mobile Networks: TCP in Wireless Networks 7


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TCP Congestion Avoidance

  • Congestion avoidance (control) was added to TCP in an attempt to reduce congestion inside the network

  • A much harder problem …

    • Requires the cooperation of multiple senders

    • Must rely on indirect measures of congestion

  • Implemented at sender

Src

Dest

Attempts to reducebuffer overflow insidethe network

Mobile Networks: TCP in Wireless Networks 8


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Recent History of TCP

  • TCP has been improved over the years

    • More robust estimates of round-trip time

    • Faster recovery from packet loss

    • Congestion avoidance and improvements

  • TCP Reno

    • Developed by Van Jacobsen in 1990

    • Improvement to TCP Tahoe (1988)

    • Added fast recovery and fast retransmit

  • TCP Vegas

    • Developed by Brakmo and Peterson in 1995

    • New congestion avoidance algorithm

Mobile Networks: TCP in Wireless Networks 9


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TCP Operation

  • Flow control (already discussed)

  • Congestion avoidance

    • Introduce a congestion window (cwnd), in addition to flow control window (wnd)

    • Need to manage size of congestion window

  • Slow start

    • Aggressively grow congestion window until congestion is detected

    • In Reno, aggressively reduce rate when invoked

  • Loss detection and retransmission

    • Fast retransmission and recovery

    • Less severe adjustment congestion window size

Mobile Networks: TCP in Wireless Networks 10


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Congestion Avoidance: TCP Reno (1)

  • TCP can maintain a congestion window size, cwnd, at the sender

    • Sender can transmit up to minimum of cwndand wnd bytes

  • TCP Reno uses packet loss as an indicator of network congestion

    • Most packet loss occurs due to congestion at intermediate routers since IP has no congestion control mechanism

    • Packet losses due to bit errors are rare

  • TCP Reno is reactive with respect to congestion

    • Responds to loss of packets indicated by timeout or duplicate ACKs

Mobile Networks: TCP in Wireless Networks 11


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Congestion Avoidance: TCP Reno (2)

  • When packet loss occurs, congestion window size is reduced

    • Due to timeout: cwnd = 1 and enter slow start

    • Due to duplicate ACKs: cwnd = cwnd/2 + 3segment_size

  • Congestion window size is increased when data is successfully acknowledged

    • Slow start

      • Slow start active if cwnd ssthresh (threshold)

      • During slow start, congestion window increased by segment_size for every ACK received  opens the window exponentially

    • Congestion avoidance

      • cwnd = cwnd + (1/cwnd) + segment_size/8 for every ACK received  additive growth in window size (about one segment every round trip time)

Mobile Networks: TCP in Wireless Networks 12


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Congestion Window in TCP Reno

G. Xylomenos, G. C. Polyzos, P. Mahonen, and M. Saaranen, “TCP Performance Issues over Wireless Links,” IEEE Communications Magazine, Vol. 39, No. 4, pp. 52-58, April 2001.

Mobile Networks: TCP in Wireless Networks 13


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Congestion Avoidance: TCP Vegas (1)

  • Sets congestion window size based on difference between the expected and actual data rates

    • Goal is to control the number of outstanding bytes in queues in the network (i.e., the backlog in queue)

  • Define…

    • cwnd: Current congestion window size

    • rtt*: Minimum (“congestion-free”) round-trip time

    • rtt: Actual (with congestion) round-trip time

    • diff: Estimated backlog in queue

    • : low threshold for diff (want diff > )

    • : high threshold for diff (want diff < )

  • diff = (cwnd/rtt* –cwnd/rtt) rtt*

Mobile Networks: TCP in Wireless Networks 14


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Congestion Avoidance: TCP Vegas (2)

  • Estimated backlog in queue (repeated here)

    • diff = (cwnd/rtt* –cwnd/rtt) rtt*

  • TCP Vegas attempts to keep at least  bytes, but fewer than  bytes, in queue

    • If diff < , increase cwnd by 1

    • If diff > , decrease cwnd by 1

    • Otherwise (  diff  ), cwnd is not changed

  • TCP Vegas provides a proactive response to congestion

    • Congestion window changed gradually as observed backlog (delay) changes

Mobile Networks: TCP in Wireless Networks 15


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Congestion Avoidance: TCP Vegas (3)

ExpectedThroughput

Throughput

/rtt

/rtt

ActualThroughput

C

LinearlyIncreasing

LinearlyDecreasing

cwnd+

cwnd

Window Size

cwnd+

Mobile Networks: TCP in Wireless Networks 16


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Slow Start Mechanism

  • The goal of the slow start mechanism is to detect and avoid congestion as a connection begins or after a timeout

    • Slow start threshold (sshtresh) set to half of cwnd when congestion is detected

    • Slow start is active if cwnd ssthresh

    • Initially, cwnd = 1 segment

  • TCP Reno doubles the congestion window every round-trip time if no loss occurs

  • TCP Vegas doubles the congestion window every other round-trip time if no loss occurs

Mobile Networks: TCP in Wireless Networks 17


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Loss Detection: TCP Reno

  • Coarse-grain timeout indicates packet loss

    • Sender starts a timer when TCP segment is sent

    • Timeout occurs if ACK not received before timeout

    • Retransmission occurs

    • Slow start is invoked (big reduction in rate!)

  • Three duplicate ACKs indicate packet loss

    • Receiver required to send an ACK if it receives an out of order segment – a segment may be out of order or lost

    • Sender assumes loss when it receives three duplicate ACKs

    • Fast retransmission and recovery mechanism – retransmit the requested segment (which is presumed lost after three duplicate ACKs) without waiting for a timeout

    • Congestion avoidance (smaller reduction in rate)

Mobile Networks: TCP in Wireless Networks 18


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Loss Detection: TCP Vegas

  • Coarse-grain timeout mechanism

    • Same as for TCP Reno

  • Fine-grain timeout mechanism

    • If a duplicate ACK is received and the round-trip time of the first unacknowledged segment exceeds the fine-grain timeout value, then segment loss is assumed and requested segment is retransmitted

    • If a non-duplicate ACK is received after a retransmission and the round-trip time of the segment exceeds the fine-grain timeout value, then segment loss is assumed and retransmission occurs

Mobile Networks: TCP in Wireless Networks 19


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TCP Reno Behavior

cwnd

Duplicate ACK

 CA

Timeout

 SS

cwndcwnd/2 + 3

cwnd 1

time

SS

CA

SS

CA

CA

SS: Slow start

CA: Collision avoidance

Mobile Networks: TCP in Wireless Networks 20


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TCP Vegas Behavior

  • Converges more smoothly … assuming sufficiently large buffers

cwnd

time

SS

CA

Mobile Networks: TCP in Wireless Networks 21


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TCP Reno Pros and Cons (1)

  • TCP Reno benefits

    • Simple bandwidth estimation scheme

    • Aggressive congestion avoidance mechanism ensures bandwidth when connected to TCP Vegas connections

    • More widely deployed, probably due to its maturity and aggressiveness

  • TCP Reno problems

    • Constantly updates window size

      • Can lead to periodic oscillation in window size

      • Can lead to oscillation in round trip times, causing delay jitter and inefficient bandwidth utilization

      • Can have many retransmissions of the same packets after a packet is dropped

Mobile Networks: TCP in Wireless Networks 22


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TCP Reno Pros and Cons (2)

  • TCP Reno problems (continued)

    • Connections with shorter round trip times can update congestion window sizes more quickly

      • Such connections can received an unfair share of network capacity

      • TCP Reno is biased against connections with longer delays

Mobile Networks: TCP in Wireless Networks 23


Tcp vegas pros and cons l.jpg
TCP Vegas Pros and Cons

  • TCP Vegas benefits

    • Fair bandwidth estimation scheme

      • Window update rate does not depend only on round-trip time as in TCP Reno

    • Smooth sending rate and efficient link utilization when queue sizes are large (window stabilizes between  and )

    • TCP Vegas detects losses faster than TCP Reno and can recover from multiple drops more efficiently

  • TCP Vegas problems

    • Cannot compete with more aggressive TCP Reno connections

    • Vegas may not stabilize if buffers are small, leading to behavior that is similar to that of TCP Reno

Mobile Networks: TCP in Wireless Networks 24


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TCP Reno versus TCP Vegas

  • TCP Vegas generally outperforms TCP Reno in a homogeneous environment

    • TCP Vegas achieves between 40% and 70% better throughput

    • TCP Vegas has 20% to 50% of the losses compared to the TCP Reno

  • Factors

    • Slow-start and congestion avoidance have the greatest influence on throughput

    • Congestion detection mechanism during congestion avoidance has only minor or negative effect on throughput

    • Congestion detection mechanism may exhibit problems related to fairness among competing connections

Mobile Networks: TCP in Wireless Networks 25


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Agenda

  • TCP overview

    • Flow control

    • Congestion avoidance, slow start, and retransmission

    • TCP Reno and TCP Vegas

  • TCP in wireless networks

  • Solutions to TCP performance problems in wireless networks

Mobile Networks: TCP in Wireless Networks 26


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TCP Problems with Wireless

  • Packet loss in wireless networks typically due to…

    • Bit errors due to wireless channel impairments

    • Handoffs due to mobility

    • Possibly congestion, but not often

  • As we’ve seen, TCP assumes packet loss is due to…

    • Congestion in the network

    • Packet reordering, but not often

  • In a wireless network, TCP congestion avoidance can be triggered by packet loss

    • TCP’s mechanisms do not respond well to packet loss due to bit errors or handoffs

    • Performance of TCP-based applications can suffer

Mobile Networks: TCP in Wireless Networks 27


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More TCP Problems with Wireless

  • Bursts of errors may occur due to low signal strength or longer period of noise

    • More than one packet lost in TCP

    • More likely to be detected as a timeout  enter slow start!

  • Delay is often very high

    • Round-trip time can be very long and variable

    • TCP’s timeout mechanisms may not work well

    • Problem exacerbated by link-level retransmission

  • Links may be asymmetric

    • Delayed ACKs in the slow direction can limit throughput in the fast direction

Mobile Networks: TCP in Wireless Networks 28


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Week 13 In-Class Laboratory

  • Experiments to consider…

    • Influence of bit errors in the wireless channel on TCP performance

    • TCP Reno versus TCP Vegas in this environment

  • Interactions are relatively complex

    • Typical studies use simulation, which provides a very controlled environment

    • We’re being a bit bold in trying to do experimental measurements

  • There is no at-home exercise for this week

    • You will be responsible for findings and observations on the final exam

Mobile Networks: TCP in Wireless Networks 29


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Agenda

  • TCP overview

    • Flow control

    • Congestion avoidance, slow start, and retransmission

    • TCP Reno and TCP Vegas

  • TCP in wireless networks

  • Solutions to TCP performance problems in wireless networks

Mobile Networks: TCP in Wireless Networks 30


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General Solution Approaches

  • Link-layer approaches

  • Split-connection approaches

  • End-to-end approaches

Mobile Networks: TCP in Wireless Networks 31


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Link-Layer Protocols (1)

  • Hide losses not due to congestion from the sender by making link appear to be more reliable

    • Link-level automatic retransmission request (ARQ)

    • Forward error correction (FEC) codes

    • Hybrid ARQ and FEC

  • Advantages

    • Requires no change to existing sender behavior

    • Matches layered protocol model

  • Problem

    • Interactions with TCP, e.g., fast retransmission by TCP can be triggered by delays due to link-level timeout and retransmission

Mobile Networks: TCP in Wireless Networks 32


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Link-Layer Protocols (2)

  • Negative interactions with TCP can be reduced by making the link-level protocol TCP-aware

    • Example: Snoop TCP

    • Advantages

      • Attempts to retransmit locally and suppress duplicate acknowledgements

      • State is soft, so handoff is simplified

    • Disadvantage

      • May not completely shield TCP from the effects of mobility and the wireless link

Mobile Networks: TCP in Wireless Networks 33


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Split-Connection Protocols (1)

  • Hide the wireless link entirely by terminating the TCP connection prior to the wireless link

    • At the base station or access point

  • Use a special protocol or regular TCP over the wireless link

  • Example: Indirect TCP

  • Problems

    • Extra protocol overhead

    • Violates end-to-end semantics of TCP

    • Complicates handoff due to state information at the access point or base station where the protocol is “split”

Mobile Networks: TCP in Wireless Networks 34


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Split-Connection Protocols (2)

Host

Host

Logical TCP Connection

AP

TCP

TCP*

Split

Connection

Mobile Networks: TCP in Wireless Networks 35


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End-to-End Protocols (1)

  • Make TCP sender aware that some losses are not due to congestion and, thus, avoid congestion control when not needed

  • Use selective acknowledgement (SACKs) for “fine-grained” error recovery

    • SACK RFC

    • SMART

  • Use explicit loss notification (ELN) to distinguish between congestion and other losses

Mobile Networks: TCP in Wireless Networks 36


End to end protocols 2 l.jpg
End-to-End Protocols (2)

  • Advantages

    • Maintains end-to-end semantics of TCP

    • Introduces no extra overhead at base stations for protocol processing or handoff

  • Disadvantages

    • Requires modified TCP

    • May not operate efficiently, e.g., for packet reordering versus packet loss

Mobile Networks: TCP in Wireless Networks 37


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Indirect TCP: Overview

Standard TCP

WiredNetwork

TCPProxy

FixedHost

MobileHost

StandardTCP

“Wireless” TCP*

Indirect TCP

(* Normal TCP or modified transport protocol)

Mobile Networks: TCP in Wireless Networks 38


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Indirect TCP: Handoff

  • An access point or router can act as a Mobile IP foreign agent and as the TCP proxy for Indirect TCP (I-TCP)

  • If the mobile host moves to a different foreign agent, a handoff is needed for Mobile IP

  • If the mobile host moves to a different proxy, a handoff of the full TCP state is needed for I-TCP

    • Buffered data

    • Sequence numbers

    • Port

Mobile Networks: TCP in Wireless Networks 39


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Indirect TCP: Advantages

  • Does not require changes to TCP at the hosts in the fixed network

  • Errors from the wireless link are corrected at the TCP proxy and, thus, do not propagate through the fixed network

  • New protocol affects only a limited part of the Internet

  • Optimizations possible over wireless link

    • Variance in delay between proxy and mobile host may be small, permitting optimized TCP

    • Opportunity for header compression, etc.

    • Opportunity for a different transport protocol

Mobile Networks: TCP in Wireless Networks 40


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Indirect TCP: Disadvantages

  • Loss of TCP’s end-to-end semantics

    • What happens if the proxy or the mobile host fails?

  • Handoff overhead can be significant

  • Overhead at the proxy for per packet processing (up to TCP and back down)

    • Can be reduced by good design

  • TCP proxy must be trusted

    • Obvious opportunities for snooping and denial of service

    • End-to-end IP-level privacy and authentication (e.g., using IPSec) must terminate at the proxy

Mobile Networks: TCP in Wireless Networks 41


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Indirect TCP: Wireless Transport

  • I-TCP as originally proposed uses TCP as the wireless transport protocol

    • Timeouts at the wireless sender may stall the original sender on the fixed network

  • Selective acknowledgement protocols have been shown to provide better performance

    • Better suited to wireless link with higher error rate

Mobile Networks: TCP in Wireless Networks 42


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Snoop TCP: Overview

  • Provide reliable link layer that is TCP aware

    • Snoop agent at the access point or foreign agent

    • Buffers data at the ends of the links for retransmissions (instead of going back to TCP end points)

    • “Snoops” on acknowledgements and filters duplicate acknowledgements

Standard TCP

WiredNetwork

SnoopAgent

FixedHost

MobileHost

Mobile Networks: TCP in Wireless Networks 43


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Snoop TCP: Operation (1)

  • Snoop agent monitors and buffers data sent from fixed network to mobile host

  • Snoop agent monitors ACKs from the mobile host

    • Can discard buffer data when acknowledged

    • Can retransmit data when …

      • Delayed ACK, or

      • Duplicate ACK

    • Timeout can be relatively short leading to a fast retransmission

  • Snoop Agent discards duplicate ACKs from mobile host

Mobile Networks: TCP in Wireless Networks 44


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Snoop TCP: Operation (2)

  • Snoop agent discards duplicate data that has already been sent by the agent and acknowledged

  • Snoop agent cannot generate ACKs that are sent back to the fixed host

    • Unlike split-connection schemes, Snoop TCP preserves end-to-end TCP semantics

Mobile Networks: TCP in Wireless Networks 45


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Snoop TCP: Reverse Direction

  • Snoop monitors traffic from mobile host back to fixed host and detects missing segments

  • A negative ACK (NACK) is sent immediately to the mobile host

  • Mobile host can retransmit missing segment, hopefully in time to avoid a TCP timeout at the fixed host

Mobile Networks: TCP in Wireless Networks 46


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Snoop TCP: Advantages

  • Preserves end-to-end TCP semantics

  • Requires no changes in TCP for fixed hosts

  • No changes in TCP are possible for the mobile hosts, but reverse direction traffic can benefit from changes at mobile host

  • There is no need for handoff

  • Automatic fallback to standard TCP

    • No need to ensure that all foreign networks provide a Snoop agent

Mobile Networks: TCP in Wireless Networks 47


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Snoop TCP: Disadvantages

  • Does not fully isolate wireless link errors from the fixed network

  • Mobile host must be modified to handle NACKs for reverse (mobile to fixed) traffic

  • Cannot snoop encrypted datagrams

    • Cannot use with privacy

  • Retransmission of data from agent not authenticated due to protection from replay attacks

    • Cannot use with authentication

Mobile Networks: TCP in Wireless Networks 48


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Summary

  • TCP is a complex protocol

    • Minimal support from underlying protocols

    • Indirect observation of network environment

    • Large number of competing flows from different hosts

    • Congestion avoidance is still a research issue

  • TCP does not perform well in a wireless environment where packets are usually lost due to bit errors, not congestion

  • Schemes have been proposed to address TCP performance problems

    • Link-level recovery

    • Split protocols

    • End-to-end protocols

Mobile Networks: TCP in Wireless Networks 49


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