lecture 11 mobile networks: tcp in wireless networks wireless and mobile systems design

Lecture 11Mobile Networks:

TCP in Wireless Networks

Wireless and Mobile Systems Design

Mobile Networks: TCP in Wireless Networks 2

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


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


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


TCP Flow Control Example

Sender Receiverwnd = 1200

500 bytes

500 bytes

wnd = 200

200 bytes

wnd = 500

500 bytes


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

rttwndbytes


Flow Control Can Limit Throughput (2)

● For a link with a high delay-bandwidth product (rttb), the flow control window can limit throughput for the connection■ In this case, t rtt■ Throughput is wnd/rtt

Sender Receiver

t

rttwndbytes


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


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


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


Congestion Avoidance: TCP Reno (1)

● TCP can maintain a congestion window size, cwnd, at the sender■ Sender can transmit up to minimum of cwnd and 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


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 + 3segment_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)


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.


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*



● 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


cwnd+

LinearlyIncreasing

cwnd+cwnd

C

ExpectedThroughput


ActualThroughput

LinearlyDecreasing

/rtt/rtt

Throughput

Window Size


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


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)


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


cwnd cwnd/2 + 3

TCP Reno Behavior

time

cwnd

SS CA SS CA CA

Duplicate ACK CA

SS: Slow startCA: Collision avoidance

cwnd 1

Timeout SS


SS

TCP Vegas Behavior

● Converges more smoothly … assuming sufficiently large buffers

time

cwnd

CA


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


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


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


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


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


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


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


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


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


General Solution Approaches

● Link-layer approaches● Split-connection approaches● End-to-end approaches


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


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


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”


Split-Connection Protocols (2)

TCP TCP*

Logical TCP Connection

SplitConnection

HostHost

AP


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


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


Indirect TCP: Overview

WiredNetwork

FixedHost

MobileHost

TCPProxy

Standard TCP

StandardTCP

“Wireless” TCP*

Indirect TCP

(* Normal TCP or modified transport protocol)


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


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


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


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


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

WiredNetwork

FixedHost

MobileHost

Standard TCP

SnoopAgent


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


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


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


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


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


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

lecture 11 mobile networks: tcp in wireless networks wireless and mobile systems design

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