TCP for Wireless and Mobile Hosts

TCP for Wireless and Mobile Hosts
Nitin H. Vaidya University of Illinois at Urbana-Champaign © 2001 Nitin Vaidya

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.

Notes PowerPoint source for tutorial slides and reference list for the tutorial are presently available at (follow the link to Seminars)

Internet Engineering Task Force (IETF) Activities
IETF pilc (Performance Implications of Link Characteristics) working group Refer [Dawkins99] and [Montenegro99] for an overview of related work IETF tcpsat (TCP Over Satellite) working group Refer [Allman98] for overview of satellite related work

Internet Engineering Task Force (IETF) Activities
IETF manet (Mobile Ad-hoc Networks) working group IETF mobileip (IP Routing for Wireless/Mobile Hosts) working group

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

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

Notable Omissions Wireless ATM WAP (Wireless Application Protocol)

Wireless Technologies

Wireless Technologies
Wireless local area networks Cellular wireless Satellites Multi-hop wireless Wireless local loop

Wireless Local Area Networks
Local area connectivity using wireless communication IEEE WLAN Standard Example: WaveLan, Aironet Wireless LAN may be used for last hop to a wireless host wireless connectivity between hosts on the LAN

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

Multi-Hop Wireless May need to traverse multiple links to reach a destination

Multi-Hop Wireless - Mobility Mobile Ad Hoc Networks (MANET)
Mobility causes route changes

Multi-Hop Wireless Metricom’s Ricochet Network
Around 28.8 Kbps (128 Kbps to come) Wireless hosts modem Poletop radio internet

Satellites Geostationary Earth Orbit (GEO) Satellites
example: Inmarsat SAT ground stations

Satellites Low-Earth Orbit (LEO) Satellites
example: Iridium (66 satellites) (2.4 Kbps data) SAT constellation SAT SAT ground stations

Satellites GEO LEO long delay - 250-300 ms propagation delay
relatively low delay ms large variations in delay - multiple hops/route changes, relative motion of satellites, queueing

Wireless Connectivity - Characteristics
Transmission errors Wireless LANs , Hyperlan Cellular wireless Multi-hop wireless Satellites Low bandwidth Packet radio (e.g., Metricom) Long or variable latency GEO, LEO satellites Packet radio - high variability Asymmetry in bandwidth, error characteristics Satellites (example: DirectPC)

Transmission Control Protocol / Internet Protocol TCP/IP

Internet Protocol (IP)
Packets may be delivered out-of-order Packets may be lost Packets may be duplicated

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

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

Cumulative Acknowledgements
A new cumulative acknowledgement is generated only on receipt of a new in-sequence packet 40 39 38 37 33 34 35 36 41 40 39 38 34 35 36 37 i data i ack

Delayed Acknowledgements
An ack is delayed until another packet is received, or delayed ack timer expires (200 ms typical) Reduces ack traffic New ack not produced on receipt of packet 36, but on receipt of 37 40 39 38 37 33 35 41 40 39 38 35 37

Duplicate Acknowledgements
A dupack is generated whenever an out-of-order segment arrives at the receiver 40 39 37 38 36 34 42 41 Dupack (Above example assumes delayed acks) On receipt of 38

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 37 38 34 36 41 40 39 37 36 36 Dupack On receipt of 38

Number of dupacks depends on how much OOO a packet is
40 39 37 38 34 36 New Ack New Ack 41 40 39 37 34 36 36 New Ack New Ack Dupack 42 41 40 39 36 36 38 New Ack Dupack New Ack

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 Sender’s window 1 2 3 4 5 6 7 8 9 10 11 12 13 Acks received Not transmitted

Sender’s window 1 2 3 4 5 6 7 8 9 10 11 12 13 Ack 5 2 3 4 5 6 7 8 9 10 11 13 1 12 Sender’s window

Ack Clock TCP window flow control is “self-clocking”
New data sent when old data is ack’d Helps maintain “equilibrium”

Congestion window size bounds the amount of data that can be sent per round-trip time Throughput <= W / RTT

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

How does TCP detect a packet loss?
Retransmission timeout (RTO) Duplicate acknowledgements

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

Retransmission Timeout (RTO) calculation
RTO = mean + 4 mean deviation Standard deviation s : s = average of (sample – mean) Mean deviation d = average of |sample – mean| Mean deviation easier to calculate than standard deviation Mean deviation is more conservative: d >= s Large variations in the RTT increase the deviation, leading to larger RTO 2 2

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

Exponential Backoff Double RTO on each timeout T1 T2 = 2 * T1
Timeout interval doubled Packet transmitted Time-out occurs before ack received, packet retransmitted

Fast Retransmission Timeouts can take too long Fast retransmit
how to initiate retransmission sooner? Fast retransmit

Detecting Packet Loss Using Dupacks Fast 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 3 dupacks are also generated if a packet is delivered at least 3 places beyond its in-sequence location 12 8 7 9 10 11 Fast retransmit useful only if lower layers deliver packets “almost ordered” ---- otherwise, unnecessary fast retransmit

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

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

Example assumes that acks are not delayed
Congestion avoidance Slow start threshold Slow start Example assumes that acks are not delayed

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

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

After timeout cwnd = 20 ssthresh = 10 ssthresh = 8

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

Congestion window cut into half
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

Receiver’s advertized window
After fast recovery Receiver’s advertized window After fast retransmit and fast recovery window size is reduced in half.

TCP Reno Slow-start Congestion avoidance Fast retransmit Fast recovery

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

Impact of transmission errors on TCP performance

Impact of transmission errors on TCP performance Approaches to improve TCP performance Classification Discussion of selected approaches

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

Random Errors May Cause Fast Retransmit
40 39 37 38 36 34 Example assumes delayed ack - every other packet ack’d

41 40 39 38 34 36 Example assumes delayed ack - every other packet ack’d

42 41 40 39 36 36 dupack Duplicate acks are not delayed

43 42 41 40 36 36 36 Duplicate acks

44 43 42 41 36 36 36 3 duplicate acks trigger fast retransmit at sender

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

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]

This Tutorial We consider errors for which reducing congestion window is an inappropriate response

Impact of Random Errors [Vaidya99]
Exponential error model 2 Mbps wireless full duplex link No congestion losses

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

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

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)

Impact of Transmission Errors
TCP cannot distinguish between packet losses due to congestion and transmission errors Unnecessarily reduces congestion window Throughput suffers

Classification of Schemes to Improve Performance of TCP in Presence of Transmission Errors

Techniques to Improve TCP Performance in 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

Techniques to Improve TCP Performance in 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

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

Selected Schemes to Improve Performance of TCP in Presence of Transmission Errors

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

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]

Link Level Mechanisms

Link Layer Mechanisms Forward 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

Link Layer Mechanisms Link 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

Link Layer Mechanisms In general
Use FEC to correct a small number of errors Use link level retransmission when FEC capability is exceeded

Link Level Retransmissions
Link layer state TCP connection wireless physical link network transport application rxmt

Link Level Retransmissions Issues
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

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

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 Receiver 1 Receiver 2 Base station

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 ? Receiver 1 Receiver 2 Base station

Link Level Retransmissions An 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

RTO Variations Wireless Packet loss RTT sample RTO

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]

Link Level Retransmissions A 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

Link-Layer Retransmissions A 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 data ack 1 2 Timeout = RTO Reset, Timeout = RTO Effectively, Timeout = RTT of packet 1 + RTO

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

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 Lost packet Retransmitted packet 6 2 5 4 3 2 1

Link Level Retransmissions In-order delivery
To avoid unnecessary fast retransmit, link layer using retransmission should attempt to deliver packets “almost in-order” 6 5 4 3 2 2 1 6 5 2 4 3 2 1

Link Level Retransmissions In-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)

Adaptive Link Layer Strategies [Lettieri98,Eckhardt98,Zorzi97]
Adaptive protocols attempt to dynamically choose: FEC code retransmission limit frame size

Link Layer Retransmissions [Vaidya99]
20 ms 1 ms 10 Mbps 2 Mbps 2 Mbps wireless duplex link with 1 ms delay Exponential error model No congestion losses

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

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

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

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

Connection between wireless host MH and fixed host FH goes through base station BS FH-MH = FH-BS BS-MH FH BS MH Fixed Host Base Station Mobile Host

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 BS MH Fixed Host Base Station Mobile Host

Per-TCP connection state TCP connection TCP connection wireless physical link network transport application rxmt

Split Connection Approach Indirect TCP [Bakre95,Bakre97]
FH - BS connection : Standard TCP BS - MH connection : Standard TCP

Split Connection Approach Selective 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

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

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]

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

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

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 MH BS 40 39 37 38 36

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 MH BS 40 39 37 38 36

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 MH BS 40 39 37 38 36 New base station Hand-off

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

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 data FH MH ack

Various Schemes Link layer mechanisms Split connection approach

TCP-Aware Link Layer

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

Snoop Protocol Per TCP-connection state TCP connection physical link
network transport application physical link network transport application physical link network transport application rxmt FH BS MH wireless

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 BS MH

Snoop : Example 35 TCP state maintained at link layer 36 37 38 40 39
FH BS MH 34 36 Example assumes delayed ack - every other packet ack’d

Snoop : Example 35 39 36 37 38 41 40 39 38 34 36

Snoop : Example 37 40 38 39 42 41 40 39 36 36 dupack Duplicate acks are not delayed

Snoop : Example 37 40 38 41 39 43 42 41 40 36 36 36 Duplicate acks

Snoop : Example 37 40 38 41 39 42 44 43 37 41 FH BS MH Discard dupack
36 36 Discard dupack Dupack triggers retransmission of packet 37 from base station BS needs to be TCP-aware to be able to interpret TCP headers 36

Snoop : Example 37 40 43 38 41 39 42 45 44 42 37 36 36 36 36

Snoop : Example 37 40 43 38 41 44 39 42 46 45 43 42 36 41 TCP sender does not fast retransmit 36 36 36

Snoop : Example 37 40 43 38 41 44 39 42 45 47 46 44 43 41 TCP sender does not fast retransmit 36 36 36 36

Snoop : Example 42 45 43 46 44 48 47 45 44 FH BS MH 41 43 36 36 36 36

Snoop [Balakrishnan95acm]
2 Mbps Wireless link

Snoop Protocol When 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

Snoop Protocol : Classification
Hides wireless losses from the sender Requires modification to only BS (network-centric approach)

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

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)

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 BS MH

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 BS MH

WTCP Example 3 3 FH BS MH 4 3 Numbers in this figure are timestamps
Base station residence time is 1 unit

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

Various Schemes Link layer mechanisms Split connection approach

TCP-Unaware Approximation of TCP-Aware Link Layer

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)

Delayed Dupacks Protocol
Link layer state TCP connection wireless physical link network transport application rxmt

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

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

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

Delayed Dupacks : Example
35 Link layer state 36 37 38 40 39 38 37 34 36 Example assumes delayed ack - every other packet ack’d Link layer acks are not shown

36 37 38 39 41 40 39 38 BS 34 36 Removed from BS link layer buffer on receipt of a link layer ack (LL acks not shown in figure) 35

37 40 38 39 42 41 40 39 36 36 dupack Duplicate acks are not delayed

37 40 38 41 39 43 42 41 40 36 36 36 Duplicate acks Original ack

37 39 41 40 42 44 43 37 41 36 36 36 dupack dupacks Delayed dupack Base station forwards dupacks

37 42 40 43 41 45 44 42 37 36 36 36 dupacks 36 Delayed dupacks

37 43 41 44 42 46 45 43 42 36 41 Delayed dupacks are discarded if lost packet received before delay D expires TCP sender does not fast retransmit

Delayed Dupacks [Vaidya99]
20 ms 10 Mbps 2 Mbps 2 Mbps wireless duplex link with 20 ms delay No congestion losses

Delayed Dupacks [Vaidya99]
20 ms 10 Mbps 2 Mbps 5% packet loss due to congestion

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

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

Various Schemes Link-layer retransmissions Split connection approach

Explicit Notification

Explicit Notification Schemes General 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

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

Explicit Notification Space Communication Protocol Standards-Transport Protocol (SCPS-TP)
Satellite wireless TCP destinations Ground station

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)

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 Record hole at 2 4 3 2 1 4 3 1 MH BS FH wireless 1 1 1 1 Dupack with ELN set

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

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

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 37 Partial ack Cumulative ack

Variations Base station may or may not locally buffer and retransmit lost packets Partial ack for all packets or a subset ? 37 36 37 Partial ack Cumulative ack

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

Explicit Loss Notification [Biaz99thesis] when MH is TCP receiver
Sequence numbers cached at base station 39 38 37 39 38 37 36 37 37 Dupack with ELN

Receiver-Based Discrimination Scheme

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

Receiver-Based Scheme
Packet loss due to congestion 12 11 10 FH BS MH T 12 10 FH BS MH 11 Congestion loss

Packet loss due to transmission error 12 11 10 FH BS MH 2 T 12 11 10 Error loss FH BS MH

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

Receiver-Based Scheme Diagnostic Accuracy [Biaz99Asset]
Congestion losses Error losses

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

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

Sender-Based Discrimination Scheme

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)

Heuristics for Congestion Avoidance
throughput cliff knee RTT load load

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

Heuristics for Congestion Avoidance Some 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)

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)

TCP Vegas [Brakmo94] expected throughput ET = W(i) / RTTmin actual throughput AT = W(i) / RTT(i) Condition C = ( ET-AT > beta)

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

Sender-Based Schemes Diagnostic Accuracy [Biaz99ic3n]

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

Sender-Based Heuristics : Advantages
Only sender needs to be modified Needs further investigation to develop better heuristics investigate longer-term heuristics

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 Fraction of TCP connections Coefficient of correlation (RTT,W)

Statistical Techniques Future Work
Other statistical measures ? Mechanisms that achieve good TCP throughput despite not-too-good diagnostic accuracy

TCP in Presence of Transmission Errors Summary
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

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

TCP Over Satellite

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

Problems Addressed by Various Schemes
Long delay Large delay-bandwidth products Transmission errors

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

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

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

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

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

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

Early Acks Spoofing ACKprime approach [Scott98]
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]

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

Impact of Mobility on TCP Performance

Impact of Mobility Hand-offs occur when a mobile host starts communicating with a new base station (in cellular wireless systems)

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

Impact of Mobility If hand-off visible to IP We consider this case
Need Mobile IP [Johnson96] packets may be lost while a new route is being established reliability despite handoff We consider this case

Mobile IP [Johnson96] Router 3 MH S Home agent Router 1 Router 2

Mobile IP [Johnson96] move Router 3 S MH Foreign agent Home agent
1 Router 2 Packets are tunneled using IP in IP

Example Hand-Off Procedure
Each base station periodically transmits beacon 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 Old BS New MH 2 1 3 4 5,6 7

Hand-Off Procedure New BS acknowledges the greeting, and begins to route the MH’s packets New BS informs old BS Old BS changes routing table, to forward any packets for the MH to the new BS Old BS sends an ack to new BS New BS sends handoff-completion message to MH Old BS New MH 2 1 3 4 5,6 7

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

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

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

Techniques to Improve TCP Performance in Presence of Mobility

Classification Hide mobility from the TCP sender
Make TCP adaptive to mobility

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

0-second Rendezvous Delay : Beacon arrives as soon as cell boundary crossed
Cell crossing + beacon arrives Retransmission timeout Handoff complete Routes updated Last timed transmit 1.0 0.15 0.8 sec Packet loss Idle sender

1-second Rendezvous Delay : Beacon arrives 1 second after cell boundary crossed
Cell crossing Retransmission timeout 2 Timeout 1 Handoff complete Last timed transmit 1.0 2.0 2.8 sec 0.8 1.0 1.15 Packet loss Idle sender

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

TCP Performance

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

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

0-second Rendezvous Delay Improvement using Fast Retransmit
Cell crossing + beacon arrives Retransmission timeout does not occur Handoff complete Routes updated Fast retransmit Last timed transmit 1.0 0.15 0.8 Packet loss Idle sender

TCP Performance Improvement

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

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

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

M-TCP Uses TCP 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

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 Ack 999 Ack 1000 FH BS MH

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 BS MH

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)

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.

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) Mobile TCP receiver FH BS MH

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 Mobile TCP receiver FH BS MH

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

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

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

TCP in Mobile Ad Hoc Networks

Mobile Ad Hoc Networks (MANET)
May need to traverse multiple links to reach a destination

Mobile Ad Hoc Networks [IETF-MANET]
Mobility causes route changes

TCP in Mobile Ad Hoc Networks Issues
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

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

Impact of Multi-Hop Wireless Paths [Holland99]
TCP Throughput using 2 Mbps MAC

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 = S f(i) * T(i)

Impact of Mobility TCP Throughput
2 m/s 10 m/s Actual throughput Ideal throughput (Kbps)

Impact of Mobility 20 m/s 30 m/s Actual throughput Ideal throughput

Throughput generally degrades with increasing speed …
Ideal Average Throughput Over 50 runs Actual Speed (m/s)

But not always … 30 m/s 20 m/s Actual throughput Mobility pattern #

Why Does Throughput Degrade?
mobility causes link breakage, resulting in route failure TCP data and acks en route discarded Route is repaired TCP sender times out. Starts sending packets again No throughput despite route repair

Why Does Throughput Degrade?
TCP sender times out. Resumes sending Larger route repair delays especially harmful mobility causes link breakage, resulting in route failure TCP data and acks en route discarded TCP sender times out. Backs off timer. Route is repaired No throughput despite route repair

Why Does Throughput Improve? Low Speed Scenario
B B B A A 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.

Why Does Throughput Improve? Higher (double) Speed Scenario
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.

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

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

Performance Improvement
Without network feedback With feedback Actual throughput Ideal throughput 2 m/s speed

Performance Improvement
Without network feedback With feedback Actual throughput Ideal throughput 30 m/s speed

Performance with Explicit Notification [Holland99]

Issues Network 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

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

To Cache or Not to Cache Average speed (m/s)
Actual throughput (as fraction of expected throughput) Average speed (m/s)

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 due to route failure timeout, cached route is broken timeout, second cached route also broken

Issues To 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

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

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

Issues RTO 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

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

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

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

TCP for Wireless and Mobile Hosts

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TCP for Wireless and Mobile Hosts

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