Download ECE/CS 4984: Lecture 11

Wireless and Mobile Systems Design
Lecture 11
Mobile Networks:
TCP in Wireless Networks
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
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
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 of
data that destination
must buffer
Mobile Networks: TCP in Wireless Networks
4
TCP Flow Control Example
Sender
wnd = 1200
Receiver
500 bytes
500 bytes
wnd = 200
200 bytes
wnd = 500
500 bytes
Mobile Networks: TCP in Wireless Networks
5
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
t
rtt
Receiver
wnd
bytes
Mobile Networks: TCP in Wireless Networks
6
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
t
rtt
Receiver
wnd
bytes
Mobile Networks: TCP in Wireless Networks
7
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 reduce
buffer overflow inside
the network
Mobile Networks: TCP in Wireless Networks
8
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
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
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
Mobile Networks: TCP in Wireless Networks
11
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
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
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:
■ rtt*:
■ rtt:
■ diff:
■ :
■ :
Current congestion window size
Minimum (“congestion-free”) round-trip time
Actual (with congestion) round-trip time
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
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
Congestion Avoidance: TCP Vegas (3)
Expected
Throughput
Throughput
/rtt
/rtt
Actual
Throughput
C
Linearly
Increasing
cwnd+
cwnd
cwnd+
Mobile Networks: TCP in Wireless Networks
Linearly
Decreasing
Window Size
16
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
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
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
TCP Reno Behavior
cwnd
Timeout
 SS
cwnd  cwnd/2 + 3
cwnd  1
SS
CA
Duplicate ACK
 CA
SS
CA
CA
time
SS: Slow start
CA: Collision avoidance
Mobile Networks: TCP in Wireless Networks
20
TCP Vegas Behavior
cwnd
SS
CA
time
● Converges more smoothly … assuming sufficiently
large buffers
Mobile Networks: TCP in Wireless Networks
21
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
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
● 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
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
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
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
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
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
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
General Solution Approaches
● Link-layer approaches
● Split-connection approaches
● End-to-end approaches
Mobile Networks: TCP in Wireless Networks
31
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
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
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
Split-Connection Protocols (2)
Logical TCP Connection
Host
Host
TCP
AP
TCP*
Split
Connection
Mobile Networks: TCP in Wireless Networks
35
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 “finegrained” 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)
● 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
Indirect TCP: Overview
Standard TCP
Fixed
Host
Wired
Network
TCP
Proxy
Standard
TCP
Mobile
Host
“Wireless”
TCP*
Indirect TCP
(* Normal TCP or modified transport protocol)
Mobile Networks: TCP in Wireless Networks
38
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
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
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
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
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
Fixed
Host
Wired
Network
Mobile Networks: TCP in Wireless Networks
Snoop
Agent
Mobile
Host
43
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
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
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
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
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
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