1. Prof. Younghee Lee 1 Computer networks u Lecture 10: TCP: wireless network Prof. Younghee Lee

2. 2 What is the problem? u TCP assumes that packet loss is due to congestion. u Wireless Network: –Higher transmission error rate –Frequent handoff –Burst error => time out => ss

3. Prof. Younghee Lee 3 What is the problem? u Reducing cwnd make sense? –On a CDMA channel, errors occur due to interference from other user, and due to noise »Interference due to other users is an indication of congestion. If such interference causes transmission errors, it is appropriate to reduce cwnd »If noise causes errors, it is not appropriate to reduce cwnd. –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 u Solution: –“wireless-aware TCP” (I-TCP, ProxyTCP, Snoop-TCP, split connections...) –New TCP version?: Reactive, Proactive

4. Prof. Younghee Lee 4 Various solution IEEE database has 599 papers on Wireless TCP as of Fall 2004…

5. Prof. Younghee Lee 5 Various Solution u Link level mechanisms u Split connection approach u TCP-Aware link layer: snoop u TCP-Unaware approximation of TCP-aware link layer: Delayed Dupacks u Explicit notification: no cwnd control on packet loss u Receiver-based discrimination u Sender-based discrimination

6. Prof. Younghee Lee 6 Link level retransmission? u Advantages –No changes at TCP u Disadvantages –Possible retransmission same packet at both link layer and TCP –Reordering –Highly variable delay u Retx on ARQ »CDMA, TDMA use ARQ »AIRMAIL - ARQ and FEC »Why? u FEC? –Correctable errors hidden from the TCP sender –Incurs overhead => adaptive FEC [Eckhardt98]; choosing appropriate FEC dynamically u Possible choice –Low error rate => FEC –High / bursty error => link level retransmission

7. Prof. Younghee Lee 7 Link level retransmission? u Retransmissions can cause head-of-the-line blocking –One BS to multiple receivers –BS uses FIFO –A receiver in a bad state => multiple retransmission => block forwarding the messages to receivers in good state u Retransmission => increase RTT therefore RTO

8. Prof. Younghee Lee 8 Link level retransmission? u Large TCP Retransmission Timeout Intervals –Good for reducing interference with link level retransmits »How many times to retransmit at the link layer before giving up? u No bound: reliable but large RTT => TCP level retransmission? –Bad for recovery from congestion losses –Need a timeout mechanism that responds appropriately for both types of losses: congestion loss? Link error? u When is that beneficial to TCP performance? –in-order delivery is beneficial to TCP: selective retransmission, Go back N –Large TCP retransmission timeout

9. Prof. Younghee Lee 9 Split connection approach: Indirect TCP(I-TCP) u 2 TCP connections –SACK for BS MH u Advantages –Optimize performance to wireless TCP –No changes to TCP for FH u Disadvantages –Violate end to end semantic: Ack.. –High overhead due to dual stack at BS, high delay due to queueing delay at BS, extra copying of data at BS –Overhead is heavy when handoff is frequent –BS retains hard state: BS fails? => hand-off latency increases due to state transfer… FH BS MH cell2 cell1 TCP TCP Tailored for wireless

10. Prof. Younghee Lee 10 Indirect TCP(I-TCP)(2) u Implementation: proxy I-TCP proxy User space Kernel Socket layer TCP layer IP layer To FHTo MH FH socket MH socket

11. Prof. Younghee Lee 11 Indirect TCP(I-TCP)(3) Wireless link: 2Mbps Ethernet link: 10Mbps Local area throughput Wide area throughput TCP I-TCP I-TCP(M->F) TCP(F->M) I-TCP(F->M) TCP(M->F)

12. Prof. Younghee Lee 12 Split connection approach: Variations 1. Selective Repeat Protocol(SRP) over UDP for tuning to wireless behavior –Mobile-End Transport Protocol »Terminate the TCP connection at BS u TCP connection runs only between BS and FH »BS pretends to be MH (MH’s IP functionality moved to BS): (metaphor: NI Host) »BS guarantees reliable ordered delivery of packets to MH u BS-MH link can use any arbitrary protocol optimized for wireless link –Performance better than I-TCP »Why? 2. Asymmetric transport protocol ( Mobile-TCP) –Lower overhead protocol at MH, higher overhead protocol at BS & FH »Smaller header (header compression), simpler on/off flow control, only error detection, no congestion control at wireless link ; BS does error correction 3. Mobile End Transport Protocol: BS pretends to be MH(MH’s IP function also) FH BS MH cell2 cell1 TCP SRP over UDP

13. Prof. Younghee Lee 13 TCP aware link layer : snoop u UC Berkeley for wireless last-hop network: Improves on split connection –end-to-end semantics retained –soft state at base station, instead of hard state u Snoop agent present at BS: FH to MH –Lost segments are detected and retransmitted locally –Last-hop round times are estimated –Suppression of duplicated ACKs corresponding to wireless losses avoids unnecessary invocation of fast retransmission. »retransmit on wireless link, if packet present in buffer –For MH to FH: Explicit Loss Notification(ELN): decoupling of retransmission from congestion control –NACK implementation similar to TCP SACK –Requires modification to only BS –Not useful if TCP headers are encrypted (IPsec) FH BS MH cell2 cell1 TCP Packet retransmission

14. Prof. Younghee Lee 14 WTCP u Motivation –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 u WTCP protocol –WTCP performs local recovery, similar to Snoop –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

15. Prof. Younghee Lee 15 Implementation comparison Performance: no enough information to quantify which one has the best performance. I-TCP is reported by different group to have decent improvement. Snoop degrades when wireless transmission becomes the bottleneck.

16. Prof. Younghee Lee 16 TCP-Unaware Approximation of TCP-Aware Link Layer u 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)

17. Prof. Younghee Lee 17 TCP-Unaware Approximation of TCP-Aware Link Layer u 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 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 »Can be used even if TCP headers are encrypted –Disadvantage: Recovery from congestion losses delayed

18. Prof. Younghee Lee 18 TCP variants u TCP SACK –Selective acknowledgement –The receiver sends back ACKs to inform the sender that the data has been successfully received so that the sender can then retransmit only the missing packet u TCP Westwood

19. Prof. Younghee Lee 19 Explicit Notification u Approximate Ideal TCP behavior: Ideally, the TCP sender should simply retransmit a packet lost due to transmission errors, without taking any congestion control actions u A wireless node somehow determines that packets are lost due to errors and informs the sender using an explicit notification u Sender, on receiving the notification, does not reduce congestion window, but retransmits lost packet u Motivated by the Explicit Congestion Notification (ECN) proposals [Floyd94]

20. Prof. Younghee Lee 20 Explicit Notification u 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 »Why? »timeout delayed, when wireless channel in bad state u There are various Explicit Loss Notification Schemes

21. Prof. Younghee Lee 21 Receiver-Based Scheme u TCP Receiver(MH) 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 uses the inter-arrival time between consecutively received packets to guess the cause of a packet loss: from error? Or congestion? u 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 u Advantages –Can be implemented without modifying the base station –May be used despite encryption, or if data & acks traverse different paths u 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

22. Prof. Younghee Lee 22 Sender based Discrimination Scheme u Sender can attempt to determine cause of a packet loss –For example; »TCP Vegas [Brakmo94] u expected throughput ET = W(i) / RTTmin u actual throughput AT = W(i) / RTT(i) u Condition C = ( ET-AT > beta) »When a packet loss is detected u if last evaluation of C is TRUE, assume packet loss is due to congestion u else assume that packet loss is due to transmission errors u If packet loss determined to be due to errors, do not reduce congestion window u Sender can only use statistics based on round-trip times, window sizes, and loss pattern –unless network provides more information (example: explicit loss notification) u Does not work quite well enough as yet !! u Only sender needs to be modified

23. Prof. Younghee Lee 23 TCPs over Satellite u Long latency u Larger congestion window or larger initial window u SACK: allow multiple packet recovery per RTT u Space Communication Protocol Standard Transport Protocol (SCPS-TP) u Satellite Transport Protocol (STP): split connection approach u Early Acks: –Spoofing: Early acks from ground station result in faster congestion window growth u TCP-Peach [1] –Applications such as HTTP are based on the transfer of small files »the entire transfer occurs within the slow start phase and the connection is not able to fully utilize the available network bandwidth. –sudden start and rapid recovery with the traditional TCP’s congestion avoidance and fast retransmit algorithms »Sudden start introduces the use of dummy packets that are copies of the last data packet the sender has sent. »The sender sends multiple dummy packets between two consecutive data packets »the reception of ACKs for the dummy packets indicates the availability of the unused network resource, and the sender is triggered to increase its sending window quickly. [1] I. F. Akyildiz, G. Morabito, and S. Palazzo, “TCP-Peach: A New Congestion Control Scheme for Satellite IP Networks,” IEEE/ACM Trans. Net., vol. 9, no. 3, June 2001, pp. 307–21.

24. Prof. Younghee Lee 24 Yet another TCP variants u Impact of mobility on TCP Performance –Hand-off, Mobile IP –Rendezvous Delay ; 1 sec after cell boundary crossed –M-TCP, –Freeze TCP »It imposes no restrictions on routers and only requires code modifications at the mobile unit or receiver side. »Mobile handoff or temporary blockage of radio signals by obstacles. »The mobile unit predicts the impending disconnections ; signal strength »The freeze-TCP receiver on the mobile unit proactively sets the advertised window size to zero in the ACK packets in the presence of impending disconnection. u force the sender into persist mode, where it ceases sending more packets while keeping its sending window unchanged. »To implement, cross-layer information must be exchanged, u TCP exposed to handoff algorithms implemented by network interface card(NIC) vendors on the interface devices

25. Prof. Younghee Lee 25 Yet another TCP variants u Issues in multihop wireless networks –TCP for MANET –ATCP »a thin layer inserted between the standard TCP and IP layers »explicit congestion notification (ECN) to detect congestion and distinguish congestion loss from error loss »ICMP Destination Unreachable message to detect a change of route or temporary partition »According to these feedbacks, the ATCP layer puts the TCP sender into either the persist, congestion control, or retransmit state accordingly »The scheme does not modify the TCP code »TCP semantic is maintained.

26. Prof. Younghee Lee 26 Wireless TCP u Reactive approach –TCP new Reno, TCP SACK u Proactive approach –TCP Vegas, TCP Westwood –TCP Veno »same methodology as Vegas »It further suggests a way to differentiate the cause of packet loss u If the number of backlogged packets is below a threshold –Not congested => the loss is considered to be random. –It increases the congestion window in a conservative manner u Otherwise congestive => Reno scheme

27. Prof. Younghee Lee 27 Wireless TCP u Proactive approach –TCP Jersey »the available bandwidth estimation (ABE) algorithm u TCP sender side addition u continuously estimates the bandwidth available to the connection u adjust its transmission rate when the network becomes congested »the congestion warning (CW) router configuration u Routers alert end stations by marking all packets when there is a sign of incipient congestion. u sender of the TCP connection to differentiate packet losses »calculates the optimum congestion window size using ABE, CW »ECN at the routers to implement CW