TCP for Wireless and Mobile Hosts Nitin H. Vaidya University of Illinois at Urbana-Champaign [email protected] http://www.crhc.uiuc.edu/~nhv © 2001 Nitin Vaidya 1 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. 2 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) 3 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 4 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 5 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 6 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 7 Notable Omissions Wireless ATM WAP (Wireless Application Protocol) http://www.wapforum.com 8 Wireless Technologies 9 Wireless Technologies Wireless local area networks Cellular wireless Satellites Multi-hop wireless Wireless local loop 10 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 11 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 12 Multi-Hop Wireless May need to traverse multiple links to reach a destination 13 Multi-Hop Wireless - Mobility Mobile Ad Hoc Networks (MANET) Mobility causes route changes 14 Multi-Hop Wireless Metricom’s Ricochet Network Around 28.8 Kbps (128 Kbps to come) Wireless hosts modem Poletop radio internet 15 Satellites Geostationary Earth Orbit (GEO) Satellites example: Inmarsat SAT ground stations 16 Satellites Low-Earth Orbit (LEO) Satellites example: Iridium (66 satellites) (2.4 Kbps data) constellation SAT SAT SAT ground stations 17 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 18 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) 19 Transmission Control Protocol / Internet Protocol TCP/IP 20 Internet Protocol (IP) Packets may be delivered out-of-order Packets may be lost Packets may be duplicated 21 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 22 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 23 Cumulative Acknowledgements A new cumulative acknowledgement is generated only on receipt of a new in-sequence packet 40 39 33 38 34 41 35 40 34 39 35 i 37 data 38 36 i 36 ack 37 24 Delayed Acknowledgements An ack is delayed until another packet is received, or delayed ack timer expires (200 ms typical) New ack not produced Reduces ack traffic on receipt of packet 36, but on receipt of 37 40 39 38 33 41 37 35 40 39 35 38 37 25 Duplicate Acknowledgements A dupack is generated whenever an out-of-order segment arrives at the receiver 40 39 38 37 34 42 41 36 40 36 (Above example assumes delayed acks) 39 36 Dupack On receipt of 38 26 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 41 40 36 39 37 36 36 Dupack On receipt of 38 27 Number of dupacks depends on how much OOO a packet is 40 39 37 34 41 36 New Ack 40 39 34 New Ack 41 40 36 New Ack New Ack 37 36 New Ack 42 38 36 Dupack 39 36 Dupack 38 New Ack 28 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 29 Window Based Flow Control Sender’s window 1 2 3 4 5 6 7 8 9 10 11 12 13 Ack 5 1 2 3 4 5 6 7 8 9 10 11 12 13 Sender’s window 30 Ack Clock TCP window flow control is “self-clocking” New data sent when old data is ack’d Helps maintain “equilibrium” 31 Window Based Flow Control Congestion window size bounds the amount of data that can be sent per round-trip time Throughput <= W / RTT 32 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 33 How does TCP detect a packet loss? Retransmission timeout (RTO) Duplicate acknowledgements 34 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 35 Retransmission Timeout (RTO) calculation RTO = mean + 4 mean deviation Standard deviation s : s 2 = average of (sample – mean)2 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 36 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 37 Exponential Backoff Double RTO on each timeout T1 T2 = 2 * T1 Timeout interval doubled Packet transmitted Time-out occurs before ack received, packet retransmitted 38 Fast Retransmission Timeouts can take too long how to initiate retransmission sooner? Fast retransmit 39 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 12 8 11 10 9 7 3 dupacks are also generated if a packet is delivered at least 3 places beyond its in-sequence location Fast retransmit useful only if lower layers deliver packets “almost ordered” ---- otherwise, unnecessary fast retransmit 40 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 41 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 42 Congestion Window size (segments) 14 Congestion avoidance 12 10 8 Slow start threshold 6Slow start 4 2 0 0 1 2 3 4 5 6 7 8 Time (round trips) Example assumes that acks are not delayed 43 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 44 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 45 25 cwnd = 20 20 15 10 ssthresh = 10 ssthresh = 8 5 25 22 20 15 12 9 6 3 0 0 Congestion window (segments) After timeout Time (round trips) 46 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 47 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 48 Window size (segments) After fast recovery 10 Receiver’s advertized window 8 6 4 2 0 0 2 4 6 8 10 12 14 Time (round trips) After fast retransmit and fast recovery window size is reduced in half. 49 TCP Reno Slow-start Congestion avoidance Fast retransmit Fast recovery 50 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 51 Impact of transmission errors on TCP performance 52 Tutorial Outline Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance Classification Discussion of selected approaches 53 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 54 Random Errors May Cause Fast Retransmit 40 39 38 37 34 36 Example assumes delayed ack - every other packet ack’d 55 Random Errors May Cause Fast Retransmit 41 40 34 39 38 36 Example assumes delayed ack - every other packet ack’d 56 Random Errors May Cause Fast Retransmit 42 41 40 36 39 36 dupack Duplicate acks are not delayed 57 Random Errors May Cause Fast Retransmit 43 42 36 41 40 36 36 Duplicate acks 58 Random Errors May Cause Fast Retransmit 44 43 42 36 41 36 36 3 duplicate acks trigger fast retransmit at sender 59 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 60 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] 61 This Tutorial We consider errors for which reducing congestion window is an inappropriate response 62 Impact of Random Errors [Vaidya99] 1600000 1200000 800000 bits/sec 400000 0 16384 32768 65536 131072 1/error rate (in bytes) Exponential error model 2 Mbps wireless full duplex link No congestion losses 63 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 64 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 65 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) 66 Impact of Transmission Errors TCP cannot distinguish between packet losses due to congestion and transmission errors Unnecessarily reduces congestion window Throughput suffers 67 Tutorial Outline Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance Classification Discussion of selected approaches 68 Classification of Schemes to Improve Performance of TCP in Presence of Transmission Errors 69 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 70 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 71 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 72 Tutorial Outline Wireless technologies TCP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance Classification Discussion of selected approaches 73 Selected Schemes to Improve Performance of TCP in Presence of Transmission Errors 74 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 75 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] 76 Link Level Mechanisms 77 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 78 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 79 Link Layer Mechanisms In general Use FEC to correct a small number of errors Use link level retransmission when FEC capability is exceeded 80 Link Level Retransmissions Link layer state TCP connection application application application transport transport transport network network link link link physical physical physical rxmt wireless network 81 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 82 Link Level Retransmissions Issues 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 83 Link Level Retransmissions Issues Retransmissions can cause head-of-the-line blocking Receiver 1 Base station Receiver 2 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 84 Link Level Retransmissions Issues Retransmissions can cause congestion losses Receiver 1 Base station Receiver 2 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 ? 85 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 86 RTO Variations Wireless Packet loss RTT sample RTO 87 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] 88 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 89 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 90 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 91 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 Lost packet Retransmitted packet 4 3 2 1 92 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 93 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 perpacket 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) 94 Adaptive Link Layer Strategies [Lettieri98,Eckhardt98,Zorzi97] Adaptive protocols attempt to dynamically choose: FEC code retransmission limit frame size 95 Link Layer Retransmissions [Vaidya99] 2000000 1600000 base TCP 1200000 Link layer retransmission 800000 400000 0 1E+05 65536 32768 16384 1/error rate (in bytes) 2 Mbps wireless duplex link with 1 ms delay Exponential error model No congestion losses 20 ms 1 ms 10 Mbps 2 Mbps 96 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 97 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 98 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 99 Split Connection Approach 100 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 101 Split Connection Approach Connection between wireless host MH and fixed host FH goes through base station BS FH-MH = FH-BS FH Fixed Host + BS Base Station BS-MH MH Mobile Host 102 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 Fixed Host BS Base Station MH Mobile Host 103 Split Connection Approach Per-TCP connection state TCP connection TCP connection application application transport transport transport network network network link link link physical physical physical rxmt wireless application 104 Split Connection Approach Indirect TCP [Bakre95,Bakre97] FH - BS connection : Standard TCP BS - MH connection : Standard TCP 105 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 106 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 107 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] 108 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 109 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 110 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 39 40 38 FH 37 BS 40 MH 36 111 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 39 40 38 FH 37 BS 40 MH 36 112 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 40 39 40 BS 38 37 36 39 40 New base station MH MH Hand-off 113 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 114 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 data FH MH ack 115 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 116 TCP-Aware Link Layer 117 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 118 Snoop Protocol Per TCP-connection state TCP connection application application application transport transport transport network network link link link physical physical physical FH BS rxmt wireless network MH 119 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 120 Snoop : Example 35 36 TCP state maintained at link layer 37 38 40 39 38 FH 37 BS 34 MH 36 Example assumes delayed ack - every other packet ack’d 121 Snoop : Example 35 39 36 37 38 41 40 34 39 38 36 122 Snoop : Example 37 40 38 39 42 41 40 36 39 36 dupack Duplicate acks are not delayed 123 Snoop : Example 37 40 38 41 39 43 42 36 41 40 36 36 Duplicate acks 124 Snoop : Example 44 37 40 38 41 39 42 43 FH 37 41 BS Discard dupack MH 36 36 Dupack triggers retransmission 36 of packet 37 from base station BS needs to be TCP-aware to be able to interpret TCP headers 125 Snoop : Example 45 37 40 38 41 39 42 44 43 42 37 36 36 36 36 126 Snoop : Example 46 37 40 43 38 41 44 39 42 45 43 42 36 TCP sender does not fast retransmit 41 36 36 36 127 Snoop : Example 47 37 40 43 38 41 44 39 42 45 46 44 43 41 TCP sender does not fast retransmit 36 36 36 36 128 Snoop : Example 42 45 43 46 44 48 47 45 FH 44 BS 41 MH 43 36 36 36 36 129 Snoop [Balakrishnan95acm] bits/sec 2000000 1600000 1200000 base TCP Snoop 800000 400000 0 no error 256K 128K 64K 32K 16K 1/error rate (in bytes) 2 Mbps Wireless link 130 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 131 Snoop Protocol : Classification Hides wireless losses from the sender Requires modification to only BS (network-centric approach) 132 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-oforder link layer delivery End-to-end semantics retained Soft state at base station loss of the soft state affects performance, but not correctness 133 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) 134 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 135 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 136 WTCP Example 3 3 FH BS 4 MH 3 Numbers in this figure are timestamps Base station residence time is 1 unit 137 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 138 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 139 TCP-Unaware Approximation of TCP-Aware Link Layer 140 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) 141 Delayed Dupacks Protocol Link layer state TCP connection application application application transport transport transport network network link link link physical physical physical rxmt wireless network 142 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-andgo 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 143 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 144 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 145 Delayed Dupacks : Example 35 36 Link layer state 37 38 40 39 38 37 34 36 Example assumes delayed ack - every other packet ack’d Link layer acks are not shown 146 Delayed Dupacks : Example 36 37 38 39 41 40 39 38 BS 34 35 36 Removed from BS link layer buffer on receipt of a link layer ack (LL acks not shown in figure) 147 Delayed Dupacks : Example 37 40 38 39 42 41 40 36 39 36 dupack Duplicate acks are not delayed 148 Delayed Dupacks : Example 37 40 38 41 39 43 42 36 Original ack 41 40 36 36 Duplicate acks 149 Delayed Dupacks : Example 37 44 39 41 40 42 43 37 36 dupack Base station forwards dupacks 41 36 dupacks 36 Delayed dupack 150 Delayed Dupacks : Example 37 42 40 43 41 45 44 36 dupacks 42 36 37 36 36 Delayed dupacks 151 Delayed Dupacks : Example 37 43 41 44 42 46 45 43 42 36 TCP sender does not fast retransmit 41 Delayed dupacks are discarded if lost packet received before delay D expires 152 Delayed Dupacks [Vaidya99] 2000000 1600000 base TCP 1200000 800000 dupack delay 80ms + LL Retransmit 400000 0 1E+05 65536 32768 16384 1/error rate Only LL retransmit 20 ms (in bytes) 20 ms 10 Mbps 2 Mbps 2 Mbps wireless duplex link with 20 ms delay No congestion losses 153 Delayed Dupacks [Vaidya99] 160000 140000 120000 100000 80000 60000 40000 20000 0 base TCP dupack delay 80ms + LL Retransmit 1E+05 65536 32768 16384 1/error rate Only LL retransmit (in bytes) 20 ms 20 ms 10 Mbps 2 Mbps 5% packet loss due to congestion 154 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 155 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 156 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 157 Explicit Notification 158 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 159 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 160 Explicit Notification Space Communication Protocol StandardsTransport Protocol (SCPS-TP) Satellite wireless Ground station TCP destinations 161 Space Communication Protocol StandardsTransport 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 corruptionexperienced bit TCP sender, after receiving an ack with corruptionexperienced bit, does not back off until it receives an ack without that bit (even if timeout or fast retransmit 162 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 MH 4 3 wireless hole at 2 2 1 BS 1 1 4 3 1 1 1 Dupack with ELN set FH 163 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 164 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 165 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 37 Partial ack 36 Cumulative ack 166 Variations Base station may or may not locally buffer and retransmit lost packets Partial ack for all packets or a subset ? 37 37 Partial ack 36 Cumulative ack 167 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 168 Explicit Loss Notification [Biaz99thesis] when MH is TCP receiver Sequence numbers cached at base station 39 38 37 39 36 37 38 37 37 Dupack with ELN 169 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 170 Receiver-Based Discrimination Scheme 171 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 172 Receiver-Based Scheme Packet loss due to congestion 12 FH 11 10 BS MH T FH BS 12 11 Congestion loss 10 MH 173 Receiver-Based Scheme Packet loss due to transmission error 12 FH 11 10 BS MH 2T 12 FH BS 11 Error loss 10 MH 174 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 175 Receiver-Based Scheme Diagnostic Accuracy [Biaz99Asset] Congestion losses Error losses 176 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 177 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 178 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 179 Sender-Based Discrimination Scheme 180 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) 181 Heuristics for Congestion Avoidance throughput cliff knee RTT load load 182 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 183 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) 184 Heuristics for Congestion Avoidance Some 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) 185 Heuristics for Congestion Avoidance Some proposals TCP Vegas [Brakmo94] expected throughput ET = W(i) / RTTmin actual throughput AT = W(i) / RTT(i) Condition C = ( ET-AT > beta) 186 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 187 Sender-Based Schemes Diagnostic Accuracy [Biaz99ic3n] 188 Sender-Based Schemes Diagnostic Accuracy [Biaz99ic3n] 189 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 190 Sender-Based Heuristics : Advantages Only sender needs to be modified Needs further investigation to develop better heuristics investigate longer-term heuristics 191 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) 192 Statistical Techniques Future Work Other statistical measures ? Mechanisms that achieve good TCP throughput despite not-too-good diagnostic accuracy 193 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 • • 194 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 195 TCP Over Satellite 196 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 197 Problems Addressed by Various Schemes Long delay Large delay-bandwidth products Transmission errors 198 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 199 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 200 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 201 Space Communications Protocol StandardTransport 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 TCPVegas, to bound window growth, to reduce congestion-induced losses 202 Space Communications Protocol StandardTransport 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 203 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 204 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] • 205 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 206 Impact of Mobility on TCP Performance 207 Impact of Mobility Hand-offs occur when a mobile host starts communicating with a new base station (in cellular wireless systems) 208 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 209 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 210 Mobile IP [Johnson96] S MH Router 3 Home agent Router 1 Router 2 211 Mobile IP [Johnson96] move Router 3 S MH Foreign agent Home agent Router 1 Router 2 Packets are tunneled using IP in IP 212 Example Hand-Off Procedure 1. 2. 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 4 5,6 New BS 1 2 3 MH 7 213 Hand-Off Procedure 3. 4. 5. 6. 7. 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 4 5,6 New BS 1 2 3 MH 7 214 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 215 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 216 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 217 Techniques to Improve TCP Performance in Presence of Mobility 218 Classification Hide mobility from the TCP sender Make TCP adaptive to mobility 219 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 220 0-second Rendezvous Delay : Beacon arrives as soon as cell boundary crossed Cell crossing + beacon arrives Handoff complete Routes updated Last timed transmit Retransmission timeout 1.0 0 0.15 Packet loss 0.8 sec Idle sender 221 1-second Rendezvous Delay : Beacon arrives 1 second after cell boundary crossed Cell crossing Beacon arrives Timeout 1 Last timed transmit Handoff complete 2.0 1.0 0 Retransmission timeout 2 0.8 1.0 Packet loss 1.15 2.8 sec Idle sender 222 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 223 TCP Performance 1800 1600 1400 1200 1000 800 600 400 200 0 o N 1600 1510 1400 1100 Kbit/sec s e ov m la r e ov ng i pp ls l ce no n e ov /0 p rla ay l de n o n ve o - /1 p rla . c se 224 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 225 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 226 0-second Rendezvous Delay Improvement using Fast Retransmit Cell crossing + beacon arrives Handoff complete Routes updated Retransmission timeout does not occur Fast retransmit Last timed transmit 1.0 0 0.15 Packet loss 0.8 Idle sender 227 TCP Performance Improvement 1800 1600 1600 1400 1200 1000 800 600 400 200 0 1 1510 1490 1400 1380 1100 Kbit/sec With fast rxmit 2 3 4 228 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 229 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 230 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 231 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 232 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 FH Ack 1000 BS MH 233 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 234 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) 235 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. 236 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 237 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 238 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 239 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 240 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 241 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 242 TCP in Mobile Ad Hoc Networks 243 Mobile Ad Hoc Networks (MANET) May need to traverse multiple links to reach a destination 244 Mobile Ad Hoc Networks [IETF-MANET] Mobility causes route changes 245 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 246 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 247 Impact of Multi-Hop Wireless Paths [Holland99] 1600 1400 1200 1000 800 600 400 200 0 TCP Throughtput (Kbps) 1 2 3 4 5 6 7 8 9 10 Number of hops TCP Throughput using 2 Mbps 802.11 MAC 248 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) 249 Impact of Mobility TCP Throughput 2 m/s 10 m/s Ideal throughput (Kbps) 250 Impact of Mobility 20 m/s 30 m/s Ideal throughput 251 Throughput generally degrades with increasing speed … Ideal Average Throughput Over 50 runs Actual Speed (m/s) 252 But not always … 30 m/s 20 m/s Actual throughput Mobility pattern # 253 Why Does Throughput Degrade? mobility causes link breakage, resulting in route failure Route is repaired TCP sender times out. Starts sending packets again No throughput No throughput despite route repair TCP data and acks en route discarded 254 Why Does Throughput Degrade? mobility causes link breakage, resulting in route failure TCP sender times out. Backs off timer. Route is repaired TCP sender times out. Resumes sending No throughput No throughput despite route repair Larger route repair delays especially harmful TCP data and acks en route discarded 255 Why Does Throughput Improve? Low Speed Scenario C B D C D B A C D B 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. 256 Why Does Throughput Improve? Higher (double) Speed Scenario C B D C D B A C D B A 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. 257 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 258 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 259 Performance Improvement With feedback Actual throughput Without network feedback Ideal throughput 2 m/s speed 260 Performance Improvement With feedback Actual throughput Without network feedback Ideal throughput 30 m/s speed 261 throughput as a fraction of ideal Performance with Explicit Notification [Holland99] 1 0.8 Base TCP 0.6 With explicit notification 0.4 0.2 0 2 10 20 30 mean speed (m/s) 262 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 263 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 264 To Cache or Not to Cache Average speed (m/s) 265 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 timeout due to route failure timeout, cached timeout, second cached route is broken route also broken Another route discovery, and TCP time-out interval Process repeats until a good route is found 266 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 267 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 268 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 269 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) Proposal: new RTO = function of old RTO, old route length, and new route length May be too large Will result in slow response to future losses Example: new RTO = old RTO * new route length / old route length Not evaluated yet 270 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 271 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 272 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 • • 273 Other Topics 274 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 275 Twice Algorithm : Example delta 2 delta 276 Channel State Dependent Packet Scheduling [Bhagwat96] Head-of-the Line blocking can occur with FIFO (firstin-first-out) scheduling, if sender attempts to retransmit packets on a channel in a bad state M1 M1 M2 M2 M3 M1 Wireless card M2 M3 277 Channel State Dependent Packet Scheduling Separate queue for each destination Channel state monitor somehow determines if a channel is in burst error state M1 M1 M1 M2 M2 scheduler Wireless card M2 Per M3 destination queues Channel status monitor M3 278 Channel State Dependent Packet Scheduling Packets transmitted on bad channels, only if packets for no other channels present in queues M1 M1 M1 M2 M2 scheduler Wireless card M2 Per M3 destination queues Channel status monitor M3 279 Channel State Dependent Packet Scheduling Needs a reasonably good Channel State Monitor M1 M1 M1 M2 M2 scheduler Wireless card M2 Per M3 destination queues Channel status monitor M3 280 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 281 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 282 Issues for Further Investigation 283 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 284 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 285 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 286 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 287 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 288 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 289 References Please see attached listing for the references cited in the tutorial 290 Thank you !! For more information, send e-mail to Nitin Vaidya at [email protected] © 2001 Nitin Vaidya 291
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