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![Page 1: 1 TCP for Wireless and Mobile Hosts Nitin H. Vaidya University of Illinois at Urbana-Champaign nhv@uiuc.edu nhv © 2001 Nitin.](https://reader035.fdocuments.in/reader035/viewer/2022070307/551b1bd055034607418b5843/html5/thumbnails/1.jpg)
1
TCP for Wireless and Mobile Hosts
Nitin H. Vaidya
University of Illinois at Urbana-Champaign
http://www.crhc.uiuc.edu/~nhv
© 2001 Nitin Vaidya
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2
Notes
Names in brackets, as in [Vaidya99], refer to a document in the list of references
Many charts included in these slides are based on similar results presented in graphs in published literatures. Since, in many cases, exact numbers are not provided in the papers, the charts in these slides are based on “guess-timates” obtained from published graphs. Please refer original sources for accurate data.
This handout may not be as readable as the original slides, since the slides contain colored text and figures.
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3
Notes
PowerPoint source for tutorial slides and reference list for the tutorial are presently available at
http://www.cs.tamu.edu/faculty/vaidya/
(follow the link to Seminars)
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Internet Engineering Task Force (IETF)Activities
IETF pilc (Performance Implications of Link Characteristics) working group http://www.ietf.org/html.charters/pilc-charter.html http://pilc.grc.nasa.gov Refer [Dawkins99] and [Montenegro99] for an overview of
related work
IETF tcpsat (TCP Over Satellite) working group http://www.ietf.org/html.charters/tcpsat-charter.html http://tcpsat.grc.nasa.gov/tcpsat/ Refer [Allman98] for overview of satellite related work
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5
Internet Engineering Task Force (IETF)Activities
IETF manet (Mobile Ad-hoc Networks) working group http://www.ietf.org/html.charters/manet-charter.html
IETF mobileip (IP Routing for Wireless/Mobile Hosts) working group http://www.ietf.org/html.charters/mobileip-charter.html
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Tutorial Outline
Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance
Classification Discussion of selected approaches
TCP over satellite
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Tutorial Outline
Impact of mobility on TCP performance Approaches to improve TCP performance in
presence of mobility Issues in multi-hop wireless networks Issues needing further work References
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Notable Omissions
Wireless ATM
WAP (Wireless Application Protocol)
http://www.wapforum.com
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Wireless Technologies
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Wireless Technologies
Wireless local area networks Cellular wireless Satellites Multi-hop wireless Wireless local loop
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Wireless Local Area Networks
Local area connectivity using wireless communication IEEE 802.11 WLAN Standard Example: WaveLan, Aironet Wireless LAN may be used for
last hop to a wireless host wireless connectivity between hosts on the LAN
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Cellular Wireless
Space divided into cells A base station is responsible to communicate with
hosts in its cell Mobile hosts can change cells while communicating Hand-off occurs when a mobile host starts
communicating via a new base station
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Multi-Hop Wireless
May need to traverse multiple links to reach a destination
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Multi-Hop Wireless - MobilityMobile Ad Hoc Networks (MANET)
Mobility causes route changes
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Multi-Hop Wireless Metricom’s Ricochet Network
Around 28.8 Kbps (128 Kbps to come)
Poletopradio
Wireless hosts
internet
modem
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Satellites
Geostationary Earth Orbit (GEO) Satellites example: Inmarsat
SAT
ground stations
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Satellites
Low-Earth Orbit (LEO) Satellites example: Iridium (66 satellites) (2.4 Kbps data)
SAT
ground stations
SAT
SAT
constellation
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Satellites
GEO long delay - 250-300 ms propagation delay
LEO relatively low delay - 40 - 200 ms large variations in delay - multiple hops/route changes,
relative motion of satellites, queueing
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Wireless Connectivity - Characteristics Transmission errors
Wireless LANs - 802.11, Hyperlan Cellular wireless Multi-hop wireless Satellites
Low bandwidth Cellular wireless Packet radio (e.g., Metricom)
Long or variable latency GEO, LEO satellites Packet radio - high variability
Asymmetry in bandwidth, error characteristics Satellites (example: DirectPC)
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Transmission Control Protocol / Internet Protocol
TCP/IP
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Internet Protocol (IP)
Packets may be delivered out-of-order
Packets may be lost
Packets may be duplicated
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Transmission Control Protocol (TCP)
Reliable ordered delivery
Implements congestion avoidance and control
Reliability achieved by means of retransmissions if necessary
End-to-end semantics Acknowledgements sent to TCP sender confirm delivery of
data received by TCP receiver Ack for data sent only after data has reached receiver
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TCP Basics
Cumulative acknowledgements
An acknowledgement ack’s all contiguously received data
TCP assigns byte sequence numbers For simplicity, we will assign packet sequence
numbers Also, we use slightly different syntax for acks than
normal TCP syntax In our notation, ack i acknowledges receipt of packets
through packet i
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40 39 3738
3533
Cumulative Acknowledgements
A new cumulative acknowledgement is generated only on receipt of a new in-sequence packet
41 40 3839
35 37
3634
3634
i data acki
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Delayed Acknowledgements
An ack is delayed until another packet is received, or delayed ack timer expires (200 ms typical)
Reduces ack traffic
40 39 3738
3533
41 40 3839
35 37
New ack not producedon receipt of packet 36,
but on receipt of 37
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Duplicate Acknowledgements
A dupack is generated whenever an
out-of-order segment arrives at the receiver
40 39 3738
3634
42 41 3940
36 36
Dupack
(Above example assumes delayed acks)On receipt of 38
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Duplicate Acknowledgements Duplicate acks are not delayed Duplicate acks may be generated when
a packet is lost, or a packet is delivered out-of-order (OOO)
40 39 3837
3634
41 40 3739
36 36
DupackOn receipt of 38
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Number of dupacks depends on how much OOO a packet is
40 39 3837
3634
41 40 3739
36 36
Dupack
42 41 3940
36 36 38
New Ack
New AckNew Ack
New Ack
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New Ack
DupackNew Ack
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Window Based Flow Control
Sliding window protocol Window size minimum of
receiver’s advertised window - determined by available buffer space at the receiver
congestion window - determined by the sender, based on feedback from the network
2 3 4 5 6 7 8 9 10 11 131 12
Sender’s window
Acks received Not transmitted
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Window Based Flow Control
2 3 4 5 6 7 8 9 10 11 131 12
Sender’s window
2 3 4 5 6 7 8 9 10 11 131 12
Sender’s window
Ack 5
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Ack Clock
TCP window flow control is “self-clocking”
New data sent when old data is ack’d
Helps maintain “equilibrium”
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Window Based Flow Control
Congestion window size bounds the amount of data that can be sent per round-trip time
Throughput <= W / RTT
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Ideal Window Size
Ideal size = delay * bandwidth delay-bandwidth product
What if window size < delay*bw ? Inefficiency (wasted bandwidth)
What if > delay*bw ? Queuing at intermediate routers
• increased RTT due to queuing delays Potentially, packet loss
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How does TCP detect a packet loss?
Retransmission timeout (RTO)
Duplicate acknowledgements
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Detecting Packet Loss Using Retransmission Timeout (RTO)
At any time, TCP sender sets retransmission timer for only one packet
If acknowledgement for the timed packet is not received before timer goes off, the packet is assumed to be lost
RTO dynamically calculated
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Retransmission Timeout (RTO) calculation
RTO = mean + 4 mean deviation Standard deviation average of (sample – mean) Mean deviation average of |sample – mean| Mean deviation easier to calculate than standard deviation Mean deviation is more conservative
Large variations in the RTT increase the deviation, leading to larger RTO
2 2
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Timeout Granularity
RTT is measured as a discrete variable, in multiples of a “tick”
1 tick = 500 ms in many implementations
smaller tick sizes in more recent implementations (e.g., Solaris)
RTO is at least 2 clock ticks
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Exponential Backoff
Double RTO on each timeout
Packettransmitted
Time-out occursbefore ack received,packet retransmitted
Timeout interval doubled
T1 T2 = 2 * T1
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Fast Retransmission
Timeouts can take too long how to initiate retransmission sooner?
Fast retransmit
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Detecting Packet Loss Using DupacksFast Retransmit Mechanism
Dupacks may be generated due to packet loss, or out-of-order packet delivery
TCP sender assumes that a packet loss has occurred if it receives three dupacks consecutively
12 8 7910113 dupacks are also generated if a packetis delivered at least 3 places beyond itsin-sequence location
Fast retransmit useful only if lower layers deliver packets“almost ordered” ---- otherwise, unnecessary fast retransmit
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Congestion Avoidance and Control
Slow Start initially, congestion window size cwnd = 1 MSS
(maximum segment size) increment window size by 1 MSS on each new ack slow start phase ends when window size reaches the
slow-start threshold
cwnd grows exponentially with time during slow start factor of 1.5 per RTT if every other packet ack’d factor of 2 per RTT if every packet ack’d Could be less if sender does not always have data to send
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Congestion Avoidance
On each new ack, increase cwnd by 1/cwnd packets
cwnd increases linearly with time during congestion avoidance 1/2 MSS per RTT if every other packet ack’d 1 MSS per RTT if every packet ack’d
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0
2
4
6
8
10
12
14
0 1 2 3 4 5 6 7 8
Time (round trips)
Con
gest
ion
Win
dow
size
(s
egm
ents
)
Slow start
Congestionavoidance
Slow start threshold
Example assumes that acks are not delayed
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Congestion Control
On detecting a packet loss, TCP sender assumes that network congestion has occurred
On detecting packet loss, TCP sender drastically reduces the congestion window
Reducing congestion window reduces amount of data that can be sent per RTT throughput may decrease
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Congestion Control -- Timeout
On a timeout, the congestion window is reduced to the initial value of 1 MSS
The slow start threshold is set to half the window size before packet loss more precisely,
ssthresh = maximum of min(cwnd,receiver’s advertised window)/2 and 2 MSS
Slow start is initiated
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0
5
10
15
20
25
0 3 6 9 12 15 20 22 25
Time (round trips)
Con
gest
ion
win
dow
(se
gmen
ts)
ssthresh = 8 ssthresh = 10
cwnd = 20
After timeout
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Congestion Control - Fast retransmit
Fast retransmit occurs when multiple (>= 3) dupacks come back
Fast recovery follows fast retransmit
Different from timeout : slow start follows timeout timeout occurs when no more packets are getting across fast retransmit occurs when a packet is lost, but latter
packets get through ack clock is still there when fast retransmit occurs no need to slow start
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Fast Recovery
ssthresh =
min(cwnd, receiver’s advertised window)/2 (at least 2 MSS)
retransmit the missing segment (fast retransmit) cwnd = ssthresh + number of dupacks when a new ack comes: cwnd = ssthreh
enter congestion avoidance
Congestion window cut into half
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0
2
4
6
8
10
Time (round trips)
Win
dow
size
(seg
men
ts)
After fast retransmit and fast recovery window size isreduced in half.
Receiver’s advertized window
After fast recovery
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TCP Reno
Slow-start Congestion avoidance Fast retransmit Fast recovery
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Fast Recovery
Fast recovery can result in a timeout with multiple losses per RTT
. TCP New-Reno [Hoe96]
stay in fast recovery until all packet losses in window are recovered
can recover 1 packet loss per RTT without causing a timeout
Selective Acknowledgements (SACK) [mathis96rfc2018] provides information about out-of-order packets received by
receiver
can recover multiple packet losses per RTT
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Impact of transmission errorson TCP performance
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53
Tutorial Outline
Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance
Classification Discussion of selected approaches
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Random Errors
If number of errors is small, they may be corrected by an error correcting code
Excessive bit errors result in a packet being discarded, possibly before it reaches the transport layer
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Random Errors May Cause Fast Retransmit
40 39 3738
3634
Example assumes delayed ack - every other packet ack’d
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Random Errors May Cause Fast Retransmit
41 40 3839
3634
Example assumes delayed ack - every other packet ack’d
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Random Errors May Cause Fast Retransmit
42 41 3940
36
Duplicate acks are not delayed
36
dupack
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Random Errors May Cause Fast Retransmit
40
363636
Duplicate acks
4143 42
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Random Errors May Cause Fast Retransmit
41
3636
3 duplicate acks triggerfast retransmit at sender
4244 43
36
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Random Errors May Cause Fast Retransmit
Fast retransmit results in retransmission of lost packet reduction in congestion window
Reducing congestion window in response to errors is unnecessary
Reduction in congestion window reduces the throughput
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Sometimes Congestion Response May be Appropriate in Response to Errors
On a CDMA channel, errors occur due to interference from other user, and due to noise [Karn99pilc] Interference due to other users is an indication of
congestion. If such interference causes transmission errors, it is appropriate to reduce congestion window
If noise causes errors, it is not appropriate to reduce window
When a channel is in a bad state for a long duration, it might be better to let TCP backoff, so that it does not unnecessarily attempt retransmissions while the channel remains in the bad state [Padmanabhan99pilc]
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This Tutorial
We consider errors for which reducing congestion window is an inappropriate response
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Impact of Random Errors [Vaidya99]
0
400000
800000
1200000
1600000
16384 32768 65536 131072
1/error rate (in bytes)
bits/sec
Exponential error model2 Mbps wireless full duplex linkNo congestion losses
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Note
Since results from different papers are not necessarily obtained using same system model, comparison of absolute numbers in different graphs may not be valid
Observe trends, rather than absolute numbers
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Burst Errors May Cause Timeouts
If wireless link remains unavailable for extended duration, a window worth of data may be lost driving through a tunnel passing a truck
Timeout results in slow start Slow start reduces congestion window to 1 MSS,
reducing throughput Reduction in window in response to errors
unnecessary
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Random Errors May Also Cause Timeout
Multiple packet losses in a window can result in timeout when using TCP-Reno (and to a lesser extent
when using SACK)
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Impact of Transmission Errors
TCP cannot distinguish between packet losses due to congestion and transmission errors
Unnecessarily reduces congestion window Throughput suffers
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68
Tutorial Outline
Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance
Classification Discussion of selected approaches
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Classification of Schemes to Improve Performance of TCP in Presence of Transmission Errors
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Techniques to Improve TCP Performancein Presence of Errors
Classification 1
Classification based on nature of actions taken to
improve performance
Hide error losses from the sender if sender is unaware of the packet losses due to errors, it will
not reduce congestion window
Let sender know, or determine, cause of packet loss if sender knows that a packet loss is due to errors, it will not
reduce congestion window
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71
Techniques to Improve TCP Performancein Presence of Errors
Classification 2
Classification based on where modifications are needed
At the sender node only
At the receiver node only
At intermediate node(s) only
Combinations of the above
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Ideal Behavior
Ideal TCP behavior: Ideally, the TCP sender should simply retransmit a packet lost due to transmission errors, without taking any congestion control actions Such a TCP referred to as Ideal TCP Ideal TCP typically not realizable
Ideal network behavior: Transmission errors should be hidden from the sender -- the errors should be recovered transparently and efficiently
Proposed schemes attempt to approximate one of the above two ideals
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73
Tutorial Outline
Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance
Classification Discussion of selected approaches
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74
Selected Schemes to Improve Performance of TCP in Presence of Transmission Errors
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Caveat
When describing various schemes, only the major features are presented
Often, some additional features are present in these schemes, to optimize their performance
We will not cover all the details, only the most relevant ones
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Various Schemes
Link level mechanisms Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
For a brief overview, see [Dawkins99,Montenegro99]
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77
Link Level Mechanisms
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Link Layer MechanismsForward Error Correction
Forward Error Correction (FEC) [Lin83] can be use to correct small number of errors
Correctable errors hidden from the TCP sender
FEC incurs overhead even when errors do not occur Adaptive FEC schemes [Eckhardt98] can reduce the
overhead by choosing appropriate FEC dynamically
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Link Layer MechanismsLink Level Retransmissions
Link level retransmission schemes retransmit a packet at the link layer, if errors are detected
Retransmission overhead incurred only if errors occur unlike FEC overhead
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Link Layer Mechanisms
In general
Use FEC to correct a small number of errors
Use link level retransmission when FEC capability is exceeded
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Link Level Retransmissions
wireless
physical
link
network
transport
application
physical
link
network
transport
application
physical
link
network
transport
application
rxmt
TCP connection
Link layer state
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Link Level RetransmissionsIssues
How many times to retransmit at the link level before giving up? Finite bound -- semi-reliable link layer No bound -- reliable link layer
What triggers link level retransmissions? Link layer timeout mechanism Link level acks (negative acks, dupacks, …) Other mechanisms (e.g., Snoop, as discussed later)
How much time is required for a link layer retransmission? Small fraction of end-to-end TCP RTT Large fraction/multiple of end-to-end TCP RTT
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Link Level RetransmissionsIssues
Should the link layer deliver packets as they arrive, or deliver them in-order? Link layer may need to buffer packets and reorder if
necessary so as to deliver packets in-order
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Link Level RetransmissionsIssues
Retransmissions can cause head-of-the-line blocking
Although link to receiver 1 may be in a bad state, the link to receiver 2 may be in a good state
Retransmissions to receiver 1 are lost, and also block a packet from being sent to receiver 2
Base station
Receiver 1
Receiver 2
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Link Level RetransmissionsIssues
Retransmissions can cause congestion losses
Attempting to retransmit a packet at the front of the queue, effectively reduces the available bandwidth, potentially making the queue at base station longer
If the queue gets full, packets may be lost, indicating congestion to the sender
Is this desirable or not ?
Base station
Receiver 1
Receiver 2
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Link Level RetransmissionsAn Early Study [DeSimone93]
The sender’s Retransmission Timeout (RTO) is a function of measured RTT (round-trip times) Link level retransmits increase RTT, therefore, RTO
If errors not frequent, RTO will not account for RTT variations due to link level retransmissions When errors occur, the sender may timeout & retransmit
before link level retransmission is successful Sender and link layer both retransmit Duplicate retransmissions (interference) waste wireless
bandwidth Timeouts also result in reduced congestion window
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RTO Variations
Packet loss
RTT sample
RTO
Wireless
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A More Accurate Picture
Analysis in [DeSimone93] does not accurately model real TCP stacks
With large RTO granularity, interference is unlikely, if time required for link-level retransmission is small compared to TCP RTO [Balakrishnan96Sigcomm] Standard TCP RTO granularity is often large Minimum RTO (2*granularity) is large enough to allow a
small number of link level retransmissions, if link level RTT is relatively small
Interference due to timeout not a significant issue when wireless RTT small, and RTO granularity large [Eckhardt98]
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Link Level RetransmissionsA More Accurate Picture
Frequent errors increase RTO significantly on slow wireless links RTT on slow links large, retransmissions result in large
variance, pushing RTO up Likelihood of interference between link layer and TCP
retransmissions smaller But congestion response will be delayed due to larger RTO When wireless losses do cause timeout, much time wasted
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Link-Layer RetransmissionsA More Accurate Picture [Ludwig98]
Timeout interval may actually be larger than RTO Retransmission timer reset on an ack If the ack’d packet and next packet were transmitted in a
burst, next packet gets an additional RTT before the timer will go off
1 2
data ack
Timeout = RTO
Effectively, Timeout = RTT of packet 1 + RTO
Reset, Timeout = RTO
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Large TCP Retransmission Timeout Intervals
Good for reducing interference with link level retransmits
Bad for recovery from congestion losses
Need a timeout mechanism that responds appropriately for both types of losses Open problem
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Link Level Retransmissions
Selective repeat protocols can deliver packets out of order
Significantly out-of-order delivery can trigger TCP fast retransmit Redundant retransmission from TCP sender Reduction in congestion window
Example: Receipt of packets
3,4,5 triggers dupacks
6 2 5 234 1
Lost packet
Retransmitted packet
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Link Level RetransmissionsIn-order delivery
To avoid unnecessary fast retransmit, link layer using retransmission should attempt to deliver packets “almost in-order”
6 5 4 223
6 5 2 234
1
1
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Link Level RetransmissionsIn-order delivery
Not all connections benefit from retransmissions or ordered delivery audio
Need to be able to specify requirements on a per-packet basis [Ludwig99] Should the packet be retransmitted? How many times? Enforce in-order delivery?
Need a standard mechanism to specify the requirements open issue (IETF PILC working group)
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Adaptive Link Layer Strategies[Lettieri98,Eckhardt98,Zorzi97]
Adaptive protocols attempt to dynamically choose:
FEC code
retransmission limit
frame size
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Link Layer Retransmissions [Vaidya99]
0
400000
800000
1200000
1600000
2000000
16384
32768
65536
1E+
05
1/error rate (in bytes)
base TCP
Link layerretransmission
2 Mbps wireless duplex link with 1 ms delayExponential error modelNo congestion losses
20 ms 1 ms
10 Mbps 2 Mbps
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Link Layer Schemes: Summary
When is a reliable link layer beneficial to TCP performance?
if it provides almost in-order delivery
and
TCP retransmission timeout large enough to tolerate additional delays due to link level retransmits
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Link Layer Schemes: Classification
Hide wireless losses from TCP sender
Link layer modifications needed at both ends of wireless link TCP need not be modified
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Various Schemes
Link level mechanisms Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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Split Connection Approach
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Split Connection Approach
End-to-end TCP connection is broken into one connection on the wired part of route and one over wireless part of the route
A single TCP connection split into two TCP connections if wireless link is not last on route, then more than two TCP
connections may be needed
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Split Connection Approach
Connection between wireless host MH and fixed host FH goes through base station BS
FH-MH = FH-BS + BS-MH
FH MHBS
Base Station Mobile HostFixed Host
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Split Connection Approach
Split connection results in independent flow control for the two parts
Flow/error control protocols, packet size, time-outs, may be different for each part
FH MHBS
Base Station Mobile HostFixed Host
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Split Connection Approach
wireless
physical
link
network
transport
application
physical
link
network
transport
application
physical
link
network
transport
application rxmt
Per-TCP connection state
TCP connection TCP connection
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Split Connection ApproachIndirect TCP [Bakre95,Bakre97]
FH - BS connection : Standard TCP BS - MH connection : Standard TCP
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Split Connection ApproachSelective Repeat Protocol (SRP) [Yavatkar94]
FH - BS connection : standard TCP BS - FH connection : selective repeat protocol on top
of UDP
Performance better than Indirect-TCP (I-TCP), because wireless portion of the connection can be tuned to wireless behavior
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Split Connection Approach : Other Variations
Asymmetric transport protocol (Mobile-TCP) [Haas97icc]
Low overhead protocol at wireless hosts, and higher overhead protocol at wired hosts smaller headers used on wireless hop (header compression) simpler flow control - on/off for MH to BS transfer MH only does error detection, BS does error correction too No congestion control over wireless hop
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Split Connection Approach : Other Variations
Mobile-End Transport Protocol [Wang98infocom] Terminate the TCP connection at BS
TCP connection runs only between BS and FH
BS pretends to be MH (MH’s IP functionality moved to BS)
BS guarantees reliable ordered delivery of packets to MH
BS-MH link can use any arbitrary protocol optimized for wireless link
Idea similar to [Yavatkar94]
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Split Connection Approach : Classification
Hides transmission errors from sender Primary responsibility at base station If specialized transport protocol used on wireless,
then wireless host also needs modification
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Split Connection Approach : Advantages
BS-MH connection can be optimized independent of FH-BS connection Different flow / error control on the two connections
Local recovery of errors Faster recovery due to relatively shorter RTT on wireless link
Good performance achievable using appropriate BS-MH protocol Standard TCP on BS-MH performs poorly when multiple packet
losses occur per window (timeouts can occur on the BS-MH connection, stalling during the timeout interval)
Selective acks improve performance for such cases
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Split Connection Approach : Disadvantages
End-to-end semantics violated ack may be delivered to sender, before data delivered to the
receiver May not be a problem for applications that do not rely on
TCP for the end-to-end semantics
FH MHBS
40
39
3738
3640
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Split Connection Approach : Disadvantages
BS retains hard state
BS failure can result in loss of data (unreliability) If BS fails, packet 40 will be lost Because it is ack’d to sender, the sender does not buffer 40
FH MHBS
40
39
3738
3640
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Split Connection Approach : Disadvantages
BS retains hard state
Hand-off latency increases due to state transfer Data that has been ack’d to sender, must be moved to new
base station
FH MHBS
40
39
3738
3640
MH
New base station
Hand-off
40
39
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Split Connection Approach : Disadvantages
Buffer space needed at BS for each TCP connection BS buffers tend to get full, when wireless link slower (one
window worth of data on wired connection could be stored at the base station, for each split connection)
Window on BS-MH connection reduced in response to errors may not be an issue for wireless links with small delay-bw
product
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Split Connection Approach : Disadvantages
Extra copying of data at BS copying from FH-BS socket buffer to BS-MH socket buffer increases end-to-end latency
May not be useful if data and acks traverse different paths (both do not go through the base station) Example: data on a satellite wireless hop, acks on a dial-up
channel
FH MH
data
ack
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Various Schemes
Link layer mechanisms Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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TCP-Aware Link Layer
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Snoop Protocol [Balakrishnan95acm]
Retains local recovery of Split Connection approach and link level retransmission schemes
Improves on split connection end-to-end semantics retained soft state at base station, instead of hard state
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Snoop Protocol
FH MHBSwireless
physical
link
network
transport
application
physical
link
network
transport
application
physical
link
network
transport
application
rxmt
Per TCP-connection state
TCP connection
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Snoop Protocol
Buffers data packets at the base station BS to allow link layer retransmission
When dupacks received by BS from MH, retransmit on wireless link, if packet present in buffer
Prevents fast retransmit at TCP sender FH by dropping the dupacks at BS
FH MHBS
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Snoop : Example
FH MHBS
40 39 3738
3634
Example assumes delayed ack - every other packet ack’d
36
37
38
35 TCP statemaintained at
link layer
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Snoop : Example
41 40 3839
3634
36
37
38
35 39
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Snoop : Example
42 41 3940
36
Duplicate acks are not delayed
36
dupack
37
38
39
40
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Snoop : Example
40
363636
Duplicate acks
4143 42
37
38
39
40
41
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Snoop : Example
FH MHBS
41
3636
3744 43
36
37
38
39
40
41
42
Discarddupack
Dupack triggers retransmissionof packet 37 from base station
BS needs to be TCP-aware to
be able to interpret TCP headers
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Snoop : Example
37
36
36
4245 44
36
37
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40
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36
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Snoop : Example
42
36
36
4346 45
36
37
38
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40
41
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41
36
44
TCP sender does notfast retransmit
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Snoop : Example
43
3636
4447 46
36
37
38
39
40
41
42
43
41
36
44
TCP sender does notfast retransmit
45
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Snoop : Example
FH MHBS
44
3636
4548 47
36
42
43
41
36
44
45
43
46
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Snoop [Balakrishnan95acm]
0
400000
800000
1200000
1600000
2000000
16
K
32
K
64
K
12
8K
25
6K
no
erro
r
1/error rate (in bytes)
bit
s/s
ec
base TCP
Snoop
2 Mbps Wireless link
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Snoop ProtocolWhen Beneficial?
Snoop prevents fast retransmit from sender despite transmission errors, and out-of-order delivery on the wireless link
OOO delivery causes fast retransmit only if it results in at least 3 dupacks
If wireless link level delay-bandwidth product is less than 4 packets, a simple (TCP-unaware) link level retransmission scheme can suffice Since delay-bandwidth product is small, the retransmission
scheme can deliver the lost packet without resulting in 3 dupacks from the TCP receiver
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Snoop Protocol : Classification
Hides wireless losses from the sender
Requires modification to only BS (network-centric approach)
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Snoop Protocol : Advantages
High throughput can be achieved performance further improved using selective acks
Local recovery from wireless losses
Fast retransmit not triggered at sender despite out-of-order link layer delivery
End-to-end semantics retained
Soft state at base station loss of the soft state affects performance, but not correctness
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Snoop Protocol : Disadvantages
Link layer at base station needs to be TCP-aware
Not useful if TCP headers are encrypted (IPsec)
Cannot be used if TCP data and TCP acks traverse different paths (both do not go through the base station)
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WTCP Protocol [Ratnam98]
Snoop hides wireless losses from the sender But sender’s RTT estimates may be larger in
presence of errors Larger RTO results in slower response for congestion
losses
FH MHBS
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WTCP Protocol
WTCP performs local recovery, similar to Snoop
In addition, WTCP uses the timestamp option to estimate RTT
The base station adds base station residence time to the timestamp when processing an ack received from the wireless host
Sender’s RTT estimate not affected by retransmissions on wireless link
FH MHBS
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WTCP Example
FH BS MH3 3
34
Numbers in this figure are timestamps
Base station residence time is 1 unit
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WTCP : Disadvantages
Requires use of the timestamp option May be useful only if retransmission times are large
link stays in bad state for a long time link frequently enters a bad state link delay large
WTCP does not account for congestion on wireless hop assumes that all delay at base station is due to queuing and
retransmissions will not work for shared wireless LAN, where delays also
incurred due to contention with other transmitters
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Various Schemes
Link layer mechanisms Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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TCP-Unaware Approximation of TCP-Aware Link Layer
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Delayed Dupacks Protocol [Mehta98,Vaidya99]
Attempts to imitate Snoop, without making the base station TCP-aware
Snoop implements two features at the base station link layer retransmission reducing interference between TCP and link layer
retransmissions (by dropping dupacks)
Delayed Dupacks implements the same two features at BS : link layer retransmission at MH : reducing interference between TCP and link layer
retransmissions (by delaying dupacks)
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Delayed Dupacks Protocol
wireless
physical
link
network
transport
application
physical
link
network
transport
application
physical
link
network
transport
application
rxmt
TCP connection
Link layer state
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Delayed Dupacks Protocol
Link layer retransmission scheme at the base station
Link layer delivers packets out-of-order when transmission errors occur
Why may a link layer deliver packets out-of-order?
• Only an issue when the link layer does not use stop-and-go protocol
With OOO link layer delivery, loss of a packet from one flow does not block delivery of packets from another flow
If in-order delivery is enforced, when retransmission for a packet is being performed, packets from other other flows may also be blocked from being delivered to the upper layer
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Delayed Dupacks Protocol
TCP receiver delays dupacks (third and subsequent) for interval D, when out-of-order packets received
Dupack delay intended to give link level retransmit time to succeed
Benefit: Delayed dupacks can result in recovery from a transmission loss without triggering a response from the TCP sender
Disadvantage: Recovery from congestion losses delayed
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Delayed Dupacks Protocol
Delayed dupacks released after interval D, if missing packet not received by then
Link layer maintains state to allow retransmission Link layer state is not TCP-specific
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Delayed Dupacks : Example
40 39 3738
3634
Example assumes delayed ack - every other packet ack’d
Link layer acks are not shown
36
37
38
35
Link layer state
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Delayed Dupacks : Example
BS
41 40 3839
3634
36
37
38
39
35 Removed from BS link layer buffer on receipt of alink layer ack (LL acks not shown in figure)
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Delayed Dupacks : Example
42 41 3940
36
Duplicate acks are not delayed
36
dupack
37
38
39
40
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Delayed Dupacks : Example
40
363636
Duplicate acks
4143 42
37
38
39
40
41
Original ack
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Delayed Dupacks : Example
41
3636
3744 43
36
37
39
40
41
42
Base station forwards dupacks
dupack dupacksDelayeddupack
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Delayed Dupacks : Example
37
3636
4245 44
36
37
40
41
42
36dupacks
Delayed dupacks
43
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Delayed Dupacks : Example
424346 45
36
37
41
42
43
41
TCP sender does notfast retransmit
44
Delayed dupacks arediscarded if lost
packet received beforedelay D expires
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Delayed Dupacks [Vaidya99]
0
400000
800000
1200000
1600000
2000000
16384
32768
65536
1E+
05
1/error rate (in bytes)
base TCP
dupack delay80ms + LLRetransmitOnly LLretransmit
2 Mbps wireless duplex link with 20 ms delayNo congestion losses
20 ms 20 ms
10 Mbps 2 Mbps
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Delayed Dupacks [Vaidya99]
020000400006000080000
100000120000140000160000
16
38
4
32
76
8
65
53
6
1E
+0
5
1/error rate (in bytes)
base TCP
dupack delay80ms + LLRetransmitOnly LLretransmit
5% packet loss due to congestion
20 ms 20 ms
10 Mbps 2 Mbps
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Delayed Dupacks Scheme : Advantages
Link layer need not be TCP-aware
Can be used even if TCP headers are encrypted
Works well for relatively small wireless RTT (compared to end-to-end RTT)
relatively small delay D sufficient in such cases
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Delayed Dupacks Scheme : Disadvantages
Right value of dupack delay D dependent on the wireless link properties
Mechanisms to automatically choose D needed
Delays dupacks for congestion losses too, delaying congestion loss recovery
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Various Schemes
Link-layer retransmissions Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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Explicit Notification
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Explicit Notification SchemesGeneral Philosophy
Approximate Ideal TCP behavior: Ideally, the TCP sender should simply retransmit a packet lost due to transmission errors, without taking any congestion control actions
A wireless node somehow determines that packets are lost due to errors and informs the sender using an explicit notification
Sender, on receiving the notification, does not reduce congestion window, but retransmits lost packet
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Explicit Notification Schemes
Motivated by the Explicit Congestion Notification (ECN) proposals [Floyd94]
Variations proposed in literature differ in
who sends explicit notification how they know to send the explicit notification what the sender does on receiving the notification
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Explicit NotificationSpace Communication Protocol Standards-
Transport Protocol (SCPS-TP)
Satellite
Ground station
wireless
TCP destinations
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Space Communication Protocol Standards-Transport Protocol (SCPS-TP)
The receiving ground station keeps track of how many packets with errors are received (their checksums failed)
When the error rate exceeds a threshold, the ground station sends corruption experienced messages to destinations of recent error-free TCP packets destinations are cached
The TCP destinations tag acks with corruption-experienced bit
TCP sender, after receiving an ack with corruption-experienced bit, does not back off until it receives an ack without that bit (even if timeout or fast retransmit occurs)
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Explicit Loss Notification [Balakrishnan98]when MH is the TCP sender
Wireless link first on the path from sender to receiver The base station keeps track of holes in the packet
sequence received from the sender When a dupack is received from the receiver, the
base station compares the dupack sequence number with the recorded holes if there is a match, an ELN bit is set in the dupack
When sender receives dupack with ELN set, it retransmits packet, but does not reduce congestion window
MH FHBS4 3 2 1 134
wireless
Recordhole at 2
111 1
Dupack with ELN set
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Explicit Bad State Notification [Bakshi97]when MH is TCP receiver
Base station attempts to deliver packets to the MH using a link layer retransmission scheme
If packet cannot be delivered using a small number of retransmissions, BS sends a Explicit Bad State Notification (EBSN) message to TCP sender
When TCP sender receives EBSN, it resets its timer timeout delayed, when wireless channel in bad state
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Partial Ack Protocols [Cobb95][Biaz97]
Send two types of acknowledgements A partial acknowledgement informs the sender that a
packet was received by an intermediate host (typically, base station)
Normal TCP cumulative ack needed by the sender for reliability purposes
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Partial Ack Protocols
When a packet for which a partial ack is received is detected to be lost, the sender does not reduce its congestion window loss assumed to be due to wireless errors
37
36
Partial ack
37
Cumulative ack
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Variations
Base station may or may not locally buffer and retransmit lost packets
Partial ack for all packets or a subset ?
37
36
Partial ack
37
Cumulative ack
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Explicit Loss Notification [Biaz99thesis]when MH is TCP receiver
Attempts to approximate hypothetical ELN proposed in [Balakrishnan96] for the case when MH is receiver
Caches TCP sequence numbers at base station, similar to Snoop. But does not cache data packets, unlike Snoop.
Duplicate acks are tagged with ELN bit before being forwarded to sender if sequence number for the lost packet is cached at the base station
Sender takes appropriate action on receiving ELN
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Explicit Loss Notification [Biaz99thesis]when MH is TCP receiver
37
36
37
3839
39
38
Sequence numberscached at base station
37 37
Dupack with ELN
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Various Schemes
Link-layer retransmissions Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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Receiver-Based Discrimination Scheme
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Receiver-Based Scheme [Biaz98Asset]
MH is TCP receiver Receiver uses a heuristic to guess cause of packet
loss When receiver believes that packet loss is due to
errors, it sends a notification to the TCP sender TCP sender, on receiving the notification, retransmits
the lost packet, but does not reduce congestion window
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Receiver-Based Scheme
Packet loss due to congestion
FH MHBS
1012 11
FH MHBS
11
1012
T
Congestion loss
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Receiver-Based Scheme
Packet loss due to transmission error
FH MHBS
1012 11
FH MHBS
101112Error loss
2 T
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Receiver-Based Scheme
Receiver uses the inter-arrival time between consecutively received packets to guess the cause of a packet loss
On determining a packet loss as being due to errors, the receiver may tag corresponding dupacks with an ELN bit, or send an explicit notification to sender
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Receiver-Based SchemeDiagnostic Accuracy [Biaz99Asset]
Congestion losses Error losses
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Receiver-Based Scheme : Disadvantages
Limited applicability
The slowest link on the path must be the last wireless hop to ensure some queuing will occur at the base station
The queueing delays for all packets (at the base station) should be somewhat uniform multiple connections on the link will make inter-packet
delays variable
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Receiver-Based Scheme : Advantages
Can be implemented without modifying the base station (an “end-to-end” scheme)
May be used despite encryption, or if data & acks traverse different paths
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Various Schemes
Link-layer retransmissions Split connection approach TCP-Aware link layer TCP-Unaware approximation of TCP-aware link layer Explicit notification Receiver-based discrimination Sender-based discrimination
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Sender-Based Discrimination Scheme
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Sender-Based Discrimination Scheme [Biaz98ic3n,Biaz99techrep]
Sender can attempt to determine cause of a packet loss
If packet loss determined to be due to errors, do not reduce congestion window
Sender can only use statistics based on round-trip times, window sizes, and loss pattern unless network provides more information (example: explicit
loss notification)
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Heuristics for Congestion Avoidance
loadload
RTT
throughput
knee
cliff
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Heuristics for Congestion Avoidance
Define condition C as a function of congestion window size and observed RTTs
Condition C evaluated when a new RTT is calculated condition C typically evaluates to 2 or 3 possible values for now assume 2 values: TRUE or FALSE
If (C == True) reduce congestion window
Several proposals for condition C
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Heuristics for Congestion AvoidanceSome proposals
Normalized Delay Gradient [jain89]
r = [RTT(i)-RTT(i-1)] / [RTT(i)+RTT(i-1)]
w = [W(i)-W(i-1)] / [W(i)+W(i-1)]
Condition C = (r/w > 0)
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Heuristics for Congestion AvoidanceSome proposals
Normalized Throughput Gradient [Wang91]
Throughput gradient
TG(i) = [T(i) - T(i-1) ] / [ W(i)-W(i-1)]
Normalized Throughout Gradient
NTG = TG(i) / TG(1)
Condition C = (NTG < 0.5)
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Heuristics for Congestion AvoidanceSome proposals
TCP Vegas [Brakmo94]
expected throughput ET = W(i) / RTTmin
actual throughput AT = W(i) / RTT(i)
Condition C = ( ET-AT > beta)
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Sender-Based Heuristics
Record latest value evaluated for condition C
When a packet loss is detected
if last evaluation of C is TRUE, assume packet loss is due to congestion
else assume that packet loss is due to transmission errors
If packet loss determined to be due to errors, do not reduce congestion window
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Sender-Based SchemesDiagnostic Accuracy [Biaz99ic3n]
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Sender-Based SchemesDiagnostic Accuracy [Biaz99ic3n]
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Sender-Based Heuristics : Disadvantage
Does not work quite well enough as yet !!
Reason
Statistics collected by the sender garbled by other traffic on the network
Not much correlation between observed short-term statistics, and onset of congestion
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Sender-Based Heuristics : Advantages
Only sender needs to be modified
Needs further investigation to develop better heuristics investigate longer-term heuristics
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Why do Statistical Technique Perform Poorly? The techniques we evaluated use simple statistics on
RTT and window size W to draw conclusions about state of the network
Unfortunately, correlation between RTT and W is often weak
Fra
ctio
n o
f T
CP
c
on
ne
cti
on
s
Coefficient of correlation (RTT,W)
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Statistical TechniquesFuture Work
Other statistical measures ?
Mechanisms that achieve good TCP throughput despite not-too-good diagnostic accuracy
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TCP in Presence of Transmission ErrorsSummary
Many techniques have been proposed, and several approaches perform well in many environments
Recommendation: Prefer end-to-end techniques End-to-end techniques are those which
do not require TCP-Specific help from lower layers Lower layers may help improve TCP performance without
taking TCP-specific actions. Examples:
• Semi-reliable link level retransmission schemes
• Explicit notification
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Tutorial Outline
Schemes to improves TCP performance in presence of transmission errors
TCP over Satellite Impact of mobility on TCP performance Approaches to improve TCP performance in
presence of mobility Issues in multi-hop wireless networks Issues needing further work References
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TCP Over Satellite
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TCP over Satellite
Geostationary Earth Orbit (GEO) Satellite long latency transmission errors or channel unavailability
Low Earth Orbit (LEO) Satellite relatively smaller delays delays more variable
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Problems Addressed by Various Schemes
Long delay Large delay-bandwidth products Transmission errors
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Improving TCP-over-Satellite [Allman98sept][IETF-TCPSAT]
Larger congestion window (window scale option) maximum window size up to 2^30
Acknowledge every packet (do not delay acks) Selective acks
fast recovery can only recover one packet loss per RTT SACKS allow multiple packet recovery per RTT
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Larger Initial Window[Allman98september] [Allman98august]
Allows initial window size of cwnd to be up to
approximately 4 Kbyte
Larger initial window results in faster window growth during slow start avoids wait for delayed ack timers (which will occur with
cwnd = 1 MSS) larger initial window requires fewer RTTs to reach ssthresh
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Byte Counting [Allman98august]
Increase window by number of new bytes ack’d in an acknowledgement, instead of 1 MSS per ack
Speeds up window growth despite delayed or lost acks
Need to reduce bursts from sender limiting size of window growth per ack rate control
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Space Communications Protocol Standard-Transport Protocol (SCPS-TP) [Durst96]
Sender makes default assumption about source of packet loss default assumption can be set by network manager on a
per-route basis default assumption can be changed due to explicit feedback
from the network
Congestion control algorithm derived from TCP-Vegas, to bound window growth, to reduce congestion-induced losses
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Space Communications Protocol Standard-Transport Protocol (SCPS-TP)
During link outage, TCP sender freezes itself, and resumes when link is restored outage assumed to occur in both directions simultaneously ground station can detect outage of incoming link (for
instance, by low signal levels), and infers outage of outgoing link
ground stations provide link outage information to any sender that attempts to send packets on the outgoing link
sender does not unnecessarily timeout or retransmit until it is informed that link has recovered
Selective acknowledgement protocol to recover losses quickly
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Satellite Transport Protocol (STP) [Henderson98]
Uses split connection approach Protocol on satellite channel different from TCP
selective negative acks when receiver detects losses no retransmission timer transmitter periodically requests receiver to ack received
data reduces reverse channel bandwidth usage when losses are
rare
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Early Acks
Spoofing Ground station acks packets Should take responsibility for delivering packets Early acks from ground station result in faster congestion
window growth
ACKprime approach [Scott98] Acks from ground station only used to grow congestion
window Reliable delivery assumed only on reception of an ack from
the receiver
• this is similar to the partial ack approach [Biaz97]
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Tutorial Outline
TCP over Satellite Impact of mobility on TCP performance Approaches to improve TCP performance in
presence of mobility Issues in multi-hop wireless networks Issues needing further work References
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Impact of Mobility on TCP Performance
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Impact of Mobility
Hand-offs occur when a mobile host starts communicating with a new base station (in cellular wireless systems)
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Impact of Mobility
If link layer performs hand-offs and guarantees reliability despite handoff, then TCP will not be aware of the handoff except for potential delays during handoff
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Impact of Mobility
If hand-off visible to IP Need Mobile IP [Johnson96] packets may be lost while a new route is being established
reliability despite handoff
We consider this case
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Mobile IP [Johnson96]
Router1
Router3
Router2
S MH
Home agent
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Mobile IP [Johnson96]
Router1
Router3
Router2
S MH
Home agent
Foreign agent
move
Packets are tunneledusing IP in IP
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Example Hand-Off Procedure
1. Each base station periodically transmits beacon
2. Mobile host, on hearing stronger beacon from a new BS, sends it a greeting changes routing tables to make new BS its default gateway sends new BS identity of the old BS
OldBS
NewBS
MH
2
1
3
4
5,6
7
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Hand-Off Procedure
3. New BS acknowledges the greeting, and begins to route the MH’s packets
4. New BS informs old BS
5. Old BS changes routing table, to forward any packets for the MH to the new BS
6. Old BS sends an ack to new BS
7. New BS sends handoff-completion message to MH
OldBS
NewBS
MH
2
1
3
4
5,6
7
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Mobile IP
Mobile IP would need to modify the previous hand-off procedure to inform the home agent the identity of the new foreign agent
Triangular optimization can reduce the routing delay Route directly to foreign agent, instead of via home agent
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Hand-off
Hand-offs may result in temporary loss of route to MH with non-overlapping cells, it may be a while before the
mobile host receives a beacon from the new BS
While routes are being reestablished during handoff, MH and old BS may attempt to send packets to each other, resulting in loss of packets
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Impact of Handoffs on Schemes to Improves Performance in Presence of Errors
Split connection approach hard state at base station must be moved to new base station
Snoop protocol soft state need not be moved while the new base station builds new state, packet losses may
not be recovered locally
Frequent handoffs a problem for schemes that rely on significant amount of hard/soft state at base stations hard state should not be lost soft state needs to be recreated to benefit performance
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Techniques to Improve TCP Performance
in Presence of Mobility
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Classification
Hide mobility from the TCP sender
Make TCP adaptive to mobility
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Using Fast Retransmits to Recover from Timeouts during Handoff [Caceres95]
During the long delay for a handoff to complete, a whole window worth of data may be lost
After handoff is complete, acks are not received by the TCP sender
Sender eventually times out, and retransmits If handoff still not complete, another timeout will
occur Performance penalty
Time wasted until timeout occurs Window shrunk after timeout
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0-second Rendezvous Delay : Beacon arrives as soon as cell boundary crossed
Lasttimedtransmit
Cell crossing+ beaconarrives
Handoff completeRoutes updated
Retransmissiontimeout
0 0.15 0.8 sec
1.0
Packet loss Idle sender
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1-second Rendezvous Delay : Beacon arrives 1 second after cell boundary crossed
Lasttimedtransmit
0 0.8
2.0
Timeout 1
Cell crossing
Packet loss
Retransmissiontimeout 2
Handoffcomplete
Beacon arrives
1.0
1.0 1.15
Idle sender
2.8 sec
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Performance [Caceres95]
Four environments
1. No moves
2. Moves (once per 8 sec) between overlapping cells
3. Moves between non-overlapping cells, 0 sec delay
4. Moves between non-overlapping cells, 1 sec delay
Experiments using 2 Mbps WaveLan
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TCP Performance
1600 1510 14001100
0200400600800
10001200140016001800
No move
s
overla
pping
cells
non-o
verla
p/0 d
elay
non-o
verla
p/1 s
ec.
Kbit/sec
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TCP Performance
Degradation in case 2 (overlapping cells) is due to encapsulation and forwarding delay during handoff
Additional degradation in cases 3 and 4 due to packet loss and idle time at sender
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Mitigation Using Fast Retransmit
When MH is the TCP receiver: after handoff is complete, it sends 3 dupacks to the sender this triggers fast retransmit at the sender instead of dupacks, a special notification could also be sent
When MH is the TCP sender: invoke fast retransmit after completion of handoff
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0-second Rendezvous DelayImprovement using Fast Retransmit
Lasttimedtransmit
Cell crossing+ beaconarrives
Handoff completeRoutes updated
Retransmissiontimeoutdoes not occur
0 0.15 0.8
1.0
Packet loss
Fast retransmit
Idle sender
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TCP Performance Improvement
16001510 1490
13801400
1100
0
200
400
600
800
1000
1200
1400
1600
1800
1 2 3 4
Kbit/sec
With fast rxmit
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TCP Performance Improvement
No change in cases 1 and 2, as expected
Improvement for non-overlapping cells
Some degradation remains in case 3 and 4 fast retransmit reduces congestion window
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Improving Performance by Smooth Handoffs [Caceres95]
Provide sufficient overlap between cells to avoid packet loss
or
Buffer packets at BS Discard the packets after a short interval If handoff occurs before the interval expires, forward the
packets to the new base station Prevents packet loss on handoff
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M-TCP [Brown97]
In the fast retransmit scheme [Caceres95] sender starts transmitting soon after handoff BUT congestion window shrinks
M-TCP attempts to avoid shrinkage in the congestion window
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M-TCP UsesTCP Persist Mode
When a new ack is received with receiver’s advertised window = 0, the sender enters persist mode
Sender does not send any data in persist mode except when persist timer goes off
When a positive window advertisement is received, sender exits persist mode
On exiting persist mode, RTO and cwnd are same as before the persist mode
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M-TCP
Similar to the split connection approach, M-TCP splits one TCP connection into two logical parts the two parts have independent flow control as in I-TCP
The BS does not send an ack to MH, unless BS has received an ack from MH maintains end-to-end semantics
BS withholds ack for the last byte ack’d by MH
FH MHBS
Ack 1000Ack 999
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M-TCP
Withheld ack sent with window advertisement = 0, if MH moves away (handoff in progress)
Sender FH put into persist mode during handoff Sender exits persist mode after handoff, and starts
sending packets using same cwnd as before handoff
FH MHBS
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M-TCP
The last ack is not withheld, if BS does not expect any other ack from the MH this happens when the BS has no other unack’d data
buffered locally this is required to prevent a sender timeout at the end of a
transfer (or end of a burst of data)
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M-TCP
Avoids reduction of congestion window due to handoff, unlike the fast retransmit scheme simulation-based performance results look good
Important Question unanswered : Is not reducing the window a good idea?
When host moves, route changes, and new route may be more congested than before.
It is not obvious that starting full speed after handoff is right.
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FreezeTCP [Goff99]
M-TCP needs help from base station Base station withholds ack for one byte The base station uses this ack to send a zero window
advertisement when a mobile host moves to another cell
FreezeTCP requires the receiver to send zero window advertisement (ZWA)
FH MHBS
MobileTCP receiver
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FreezeTCP [Goff99]
TCP receiver determines if a handoff is about to happen determination may be based on signal strength
Ideally, receiver should attempt to send ZWA
1 RTT before handoff Receiver sends 3 dupacks when route is
reestablished No help needed from the base station
an end-to-end enhancement
FH MHBS
MobileTCP receiver
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Using Multicast to Improve Handoffs [Ghai94,Seshan96]
Define a group of base stations including current cell of a mobile host cells that the mobile host is likely to visit next
Address packets destined to the mobile host to the group
Only one base station transmits the packets to the mobile host if rest of them buffer the packets, then packet loss minimized
on handoff
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Using Multicast to Improve Handoffs
Static group definition [Ghai94] groups can be defined taking physical topology into account static definition may not take individual user mobility pattern
into account
Dynamic group definition [Seshan96] implemented using IP multicast groups each user’s group can be different overhead of updating the multicast groups is a concern with
a large scale deployment
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Using Multicast to Improve Handoffs
Buffering at multiple base stations incurs memory overhead
Trade-off between buffering overhead and packet loss
Buffer requirement can be reduced by starting buffering only when a mobile host is likely to leave current cell soon
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Tutorial Outline
TCP over Satellite Impact of mobility on TCP performance Approaches to improve TCP performance in
presence of mobility Issues in multi-hop wireless networks Issues needing further work References
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TCP in Mobile Ad Hoc Networks
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Mobile Ad Hoc Networks (MANET)
May need to traverse multiple links to reach a destination
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Mobile Ad Hoc Networks[IETF-MANET]
Mobility causes route changes
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TCP in Mobile Ad Hoc NetworksIssues
Route changes due to mobility Wireless transmission errors
problem compounded with multiple hops
Out-of-order packet delivery frequent route changes may cause out-of-order delivery TCP does not perform well if packets are significantly OOO
Multiple access protocol choice of MAC protocol can impact TCP performance
significantly
Half-duplex radios cannot send and receive packets simultaneously changing mode (send or receive) incurs overhead
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Throughput over Multi-Hop Wireless Paths [Gerla99]
When contention-based MAC protocol is used, connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop extent of packet delay or drop increases with number of
hops
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Impact of Multi-Hop Wireless Paths [Holland99]
0
200
400
600
800
1000
1200
1400
1600
1 2 3 4 5 6 7 8 9 10
Number of hops
TCPThroughtput(Kbps)
TCP Throughput using 2 Mbps 802.11 MAC
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Ideal Throughput
f(i) = fraction of time for which shortest path length between sender and destination is I
T(i) = Throughput when path length is I From previous figure
Ideal throughput = f(i) * T(i)
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Impact of MobilityTCP Throughput
Ideal throughput (Kbps)
Act
ual t
hrou
ghpu
t
2 m/s 10 m/s
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Impact of Mobility
Ideal throughput
Act
ual t
hrou
ghpu
t
20 m/s 30 m/s
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Throughput generally degrades with increasing
speed …
Speed (m/s)
AverageThroughputOver 50 runs
Ideal
Actual
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But not always …
Mobility pattern #
Actualthroughput
20 m/s
30 m/s
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mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Degrade?
TCP sender times out.Starts sending packets again
Route isrepaired
No throughput
No throughputdespite route repair
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mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Degrade?
TCP sendertimes out.Backs off timer.
Route isrepaired
TCP sendertimes out.Resumessending
Larger route repair delaysespecially harmful
No throughput
No throughput
despite route repair
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Why Does Throughput Improve?Low Speed Scenario
C
B
D
A
C
B
D
A
C
B
D
A
1.5 second route failure
Route from A to D is broken for ~1.5 second.
When TCP sender times after 1 second, route still broken.
TCP times out after another 2 seconds, and only then resumes.
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Why Does Throughput Improve?Higher (double) Speed Scenario
C
B
D
A
C
B
D
A
C
B
D
A
0.75 second route failure
Route from A to D is broken for ~ 0.75 second.
When TCP sender times after 1 second, route is repaired.
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Why Does Throughput Improve?General Principle
TCP timeout interval somewhat (not entirely) independent of speed
Network state at higher speed, when timeout occurs, may be more favorable than at lower speed
Network state Link/route status Route caches Congestion
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How to Improve Throughput(Bring Closer to Ideal)
Network feedback
Inform TCP of route failure by explicit message
Let TCP know when route is repaired Probing Explicit notification
Reduces repeated TCP timeouts and backoff
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Performance Improvement
Without networkfeedback
Ideal throughput 2 m/s speed
With feedback
Act
ua
l thr
oug
hpu
t
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Performance Improvement
Without networkfeedback
With feedback
Ideal throughput 30 m/s speed
Act
ua
l thr
oug
hpu
t
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Performance with Explicit Notification[Holland99]
0
0.2
0.4
0.6
0.8
1
2 10 20 30
mean speed (m/s)
thro
ug
hp
ut
as a
fra
ctio
n o
f id
eal
Base TCP
With explicitnotification
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IssuesNetwork Feedback
Network knows best (why packets are lost)
+ Network feedback beneficial- Need to modify transport & network layer to
receive/send feedback
Need mechanisms for information exchange between layers
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Impact of Caching
Route caching has been suggested as a mechanism to reduce route discovery overhead [Broch98]
Each node may cache one or more routes to a given destination
When a route from S to D is detected as broken, node S may: Use another cached route from local cache, or Obtain a new route using cached route at another node
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To Cache or Not to Cache
Average speed (m/s)Ac t
ual t
hrou
ghpu
t (a s
fra
ctio
n of
exp
ecte
d th
roug
hput
)
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Why Performance Degrades With Caching
When a route is broken, route discovery returns a cached route from local cache or from a nearby node
After a time-out, TCP sender transmits a packet on the new route.However, the cached route has also broken after it was cached
Another route discovery, and TCP time-out interval Process repeats until a good route is found
timeout dueto route failure
timeout, cachedroute is broken
timeout, second cachedroute also broken
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IssuesTo Cache or Not to Cache
Caching can result in faster route “repair”
Faster does not necessarily mean correct
If incorrect repairs occur often enough, caching performs poorly
Need mechanisms for determining when cached routes are stale
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Caching and TCP performance
Caching can reduce overhead of route discovery even if cache accuracy is not very high
But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs
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Issues Window Size After Route Repair
Same as before route break: may be too optimistic
Same as startup: may be too conservative
Better be conservative than overly optimistic Reset window to small value after route repair Impact low on paths with small delay-bw product
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IssuesRTO After Route Repair
Same as before route break If new route long, this RTO may be too small, leading to timeouts
• Except when RTT small compared to clock granularity
Same as TCP start-up (6 second) May be too large Will result in slow response to future losses
Proposal: new RTO = function of old RTO, old route length, and new route length Example: new RTO = old RTO * new route length / old route length Not evaluated yet
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Impact of MAC - Delay Variability
As wireless medium is shared between multiple sources, the round-trip delay is variable
Also, on slow wireless networks, delay is large made larger by send-receive turnaround time
Large and variable delays result in larger RTO On packet loss, timeout takes much longer to occur Idle source (waiting for timeout to occur) lowers TCP
throughput
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Impact of MAC - Delay Variability [Balakrishnan97]
Several techniques may be used to mitigate problem,based on minimizing ack transmissions
to reduce frequency of send-receive turnaround and contention between acks and data
Piggybacking link layer acks with data Sending fewer TCP acks - ack every d-th packet (d
may be chosen dynamically)• but need to use rate control at sender to reduce
burstiness (for large d) Ack filtering - Gateway may drop an older ack in the
queue, if a new ack arrives reduces number of acks that need to be delivered to the
sender
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Out-of-Order Packet Delivery
Route changes may result in out-of-order (OOO) delivery
Significantly OOO delivery confuses TCP, triggering fast retransmit
Potential solutions: Avoid OOO delivery by ordering packets before delivering to
IP layer
• can result in variable delay turn off fast retransmit
• can result in poor performance in presence of congestion
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Other Topics
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Header Compression for Wireless Networks [Degermark96]
In TCP packet stream, most header bits are identical Van Jacobson’s scheme exploits this observation to
compress headers, by only sending the “delta” between the previous and current header
Packet losses result in inefficiency, as headers cannot be reconstructed due to lost information
Packet losses likely on wireless links [Degermark96] proposes a technique that works well despite
single packet loss “twice” algorithm if current packet fails TCP checksum, assume that a single packet is
lost apply delta for the previous packet twice to the current header, and
test checksum again
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Twice Algorithm : Example
delta 2 delta
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Channel State Dependent Packet Scheduling[Bhagwat96]
Head-of-the Line blocking can occur with FIFO (first-in-first-out) scheduling, if sender attempts to retransmit packets on a channel in a bad state
M1 M2 M3M2 M1Wireless
card
M1
M2
M3
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Channel State Dependent Packet Scheduling
Separate queue for each destination Channel state monitor somehow determines if a
channel is in burst error state
M1
M2
M3
M2
M1Wireless
card
M1
M2
M3
scheduler
Channel statusmonitor
Perdestinationqueues
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Channel State Dependent Packet Scheduling
Packets transmitted on bad channels, only if packets for no other channels present in queues
M1
M2
M3
M2
M1Wireless
card
M1
M2
M3
scheduler
Channel statusmonitor
Perdestinationqueues
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Channel State Dependent Packet Scheduling
Needs a reasonably good Channel State Monitor
M1
M2
M3
M2
M1Wireless
card
M1
M2
M3
scheduler
Channel statusmonitor
Perdestinationqueues
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Automatic TCP Buffer Tuning [Semke98]
Using too small buffers can yield poor performance
Using too large buffers can limit number of open connections
Automatic mechanisms to choose buffer size dynamically would be useful
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Tutorial Outline
TCP over Satellite Impact of mobility on TCP performance Approaches to improve TCP performance in
presence of mobility Issues in multi-hop wireless networks Issues needing further work References
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Issues for Further Investigation
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Link Layer Protocols
“Pure” link layer designs that support higher transport performance some recent work in this area as noted earlier
Identifying scenarios where link layer solutions are inadequate
If TCP-awareness is absolutely needed, provide an interface that can be used by other transport protocols too
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End-to-End Techniques
Existing techniques typically require cooperation from intermediate nodes.
Such techniques often not applicable encrypted TCP headers TCP data and acks do not go through same base station
End-to-end techniques would rely on information available only at end nodes Harder to design due to lack of complete information about
errors Explicit Notifications may make that easier
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Impact of Congestion Losses
Past work typically evaluates performance in absence of congestion
Relative performance improvement may change substantially when congestion occurs performance improvement may reduce if congestion is
dominant, or if RTO becomes larger due to wireless errors
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Multiple TCP Transfers
Past work typically measures a single TCP connection over wireless TCP throughput is the metric of choice
When multiple connections share a wireless link, other performance metrics may make sense fairness aggregate throughput
Relative performance improvements of various schemes may change when multiple connections are considered
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TCP Window & RTO Settings After a Move
Congestion window & RTO size at connection open are chosen to be conservative
When a route change occurs due to mobility, which values to use? Same as before route change ---- may be too aggressive Same as at connection open ---- may be too conservative
Answer unclear some proposals attempt to use same values as before route
change, but not clear if that is the best alternative
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TCP for Mobile Ad Hoc Networks
Much work on routing in ad hoc networks Some recent work on TCP for ad hoc networks Need to investigate many issues
MAC-TCP interaction routing-TCP interaction impact of route changes on window size, RTO choice after
move
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References
Please see attached listing for the references cited in the tutorial
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Thank you !!
For more information, send e-mail toNitin Vaidya [email protected]
© 2001 Nitin Vaidya