TCP in Wireless Networks

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1 Outline Lecture 10 TCP Performance and QoS in Wireless s TCP Performance in wireless networks TCP performance in asymmetric networks WAP Kurose-Ross: Chapter 3, 6.8 On-line: TCP over Wireless Systems Problems and Enhancements Sid 2 David Gundlegård, ITN TCP TCP in Wireless s Connection oriented protocol Error control, flow control, sequence control, congestion control Sid 4 David Gundlegård, ITN

2 TCP TCP Performance Sliding window The window size determines the number of packets (segments/bytes) that can be sent without an ACK The window size is controlled by the receiver (flow control) and by the internal congestion control (congwin) The smallest of the two is chosen Flow control in order to avoid buffer overflow at the receiver Congestion control in order to avoid buffer overflow in the network TCP performance sensitive to delay (and delay variation) From e.g. lecture 8: R max =W/RTT Throughput dependent on RTT Typically larger delay in wireless networks -> decreased throughput Another more severe problem Congestion control Sid 5 David Gundlegård, ITN Sid 6 David Gundlegård, ITN Congestion Control How does TCP detect congestion? How does TCP know when a packet is lost? Congestion Control Avoids network congestion by Slow start Additive increase Multiplicative decrease TCP slows down due to congestion Decreases congestion window Sid 7 David Gundlegård, ITN Sid 8 David Gundlegård, ITN

3 Problems Solution Categories Works well i wired networks Packet loss often caused by congestion A problem in wireless networks Packet loss typically not caused by congestion Lost packets due to bad radio environment Furthermore long delays due to FEC, interleaving, layer 2 retransmission, handover (timeout) Bursty nature of frame errors Link layer solutions Co-operation between data link and transport layer in order to maximize efficiency? E.g. Snooping, TULIP, Delayed ACKs, shared channel scheduling Split solutions Divide the TCP connection into two separate connections E.g. Indirect-TCP, Mobile-TCP, METP, WAP 1.x End-to-end solutions TCP modifications ECN, ELN, Selective ACK, TCP Vegas Sid 9 David Gundlegård, ITN Sid 10 David Gundlegård, ITN Improvement #0 Fast retransmit and Fast recovery (standard TCP) N duplicate ACKs triggers (fast) retransmission Doesn t wait for timeout N=3 (why?) Duplicate ACKs does not trigger slow start (why?) Cwnd and ssthresh cut in half Improvements #1 Split TCP Lower delay and jitter on both wireless and wired side Retransmissions does not propagate to wired network Possible wireless TCP optimisations Violates end-to-end semantics Double TCP processing Wireless gateway trusted host? (e.g. IPsec) access point (foreign agent) wired Internet wireless TCP standard TCP Sid 11 David Gundlegård, ITN Sid 12 David Gundlegård, ITN

4 Improvements #2 Improvements #3 mobile host Snooping Isolate retransmissions to wireless part Still keep end-to-end semantics Snoop the TCP segments and cache segments until acknowledged Suppress duplicate acknowledgements Most profitable towards wireless host local retransmission correspondent foreign host agent snooping of ACKs Sid 13 David Gundlegård, ITN buffering of data end-to-end TCP connection wired Internet Explicit Loss Notification (ELN or ECN) Performance degradation due to congestion/corruption mixup Modify the protocol by inserting a ELN field, which can be set by wireless gateways and propagated by end hosts Selective acknowledgements (SACK) Standard TCP can only handle one packet loss per RTT Duplicate ACKs only sent for one packet With SACK it is possible to ACK up to three later blocks of data Less retransmissions and timeouts TCP Vegas Sid 14 David Gundlegård, ITN Other Improvements Increased maximum window size (more than standard 64 kb) High speed networks and long delay networks Increased initial CongWin (slow start) Narrowband networks Increased MTU (MTU discovery) Efficiency Timestamps Better RTT estimation TCP in Asymmetric s Sid 15 David Gundlegård, ITN

5 Asymmetry Bandwidth asymmetry Typically factor of in bandwidth between downlink and uplink E.g. satellite and ADSL networks Media access asymmetry Often present in base station / access point networks Typically no random access downlink Loss rate asymmetry s with different downlink/uplink technology Different frequency, transmitter, antenna etc. Problem in both directions Bandwidth Asymmetry Unidirectional transfer Normalized bandwidth ratio k B = bandwidth uplink/downlink P = packet size uplink/downlink More than one ACK every k datapackets will saturate uplink before downlink Bidirectional transfer A single uplink data packet can increase queuing delay dramatically k = B P d d B P u u Sid 17 David Gundlegård, ITN Sid 18 David Gundlegård, ITN Media Access Asymmetry Improvements With central base station MAC easier downlink than uplink Uplink throughput and delay variation can cause bad utilisation of the downlink Uplink bandwidth management TCP header compression ACK filtering ACK congestion control ECN for ACKs ACK priority Handling infrequent ACKS Sender adaption (take into account cumulative ACKs) ACK reconstruction node inserts ACK to the ACK stream No changes in end systems Sid 19 David Gundlegård, ITN Sid 20 David Gundlegård, ITN

6 QoS Context Bearer Service QoS in Wireless s Access Core Public Internet Access Access Core Sid 22 David Gundlegård, ITN QoS Characteristics Wired VS Wireless Circuit Switched VS Packet Switched? Random Access, Polling or Dedicated resources? End-to-end VS per-hop Support for QoS negotiation? Admission control? Flow isolation, policing and priority? Hub Core Switch R R - Random access - Changing conditions R Switch Sid 23 David Gundlegård, ITN Sid 24 David Gundlegård, ITN

7 QoS in Cellular s GSM, GPRS, UMTS Strong support for QoS Circuit Switched parts of GSM/UMTS no problem (within the operators network) QoS Negotiation possible during PDP Context setup Heading towards all-ip network DiffServ + RSVP + PDP context? Spreads to other networks? What about multiple radio access networks? UMTS signalling in WLAN or Internet QoS in UMTS QoS in WLAN Sid 25 David Gundlegård, ITN Legacy MAC Distributed Coordination Function (DCF) DCF CSMA/CA CSMA/CA (Short Interframe Spacing) CSMA/CA + RTS/CTS PIFS (PCF IFS) Point Coordination Function (PCF) DIFS (DCF IFS) Access Point (AP) polls stations when they are allowed to transmit Random backoff timer Contention window (CW) PCF/DCF usage broadcast by AP Superframe DIFS medium busy DIFS PIFS contention next frame Beacon Contention Free Period (CFP) = PCF Contention Period (CP) = DCF direct access if medium is free DIFS + post backoff expired t Sid 27 David Gundlegård, ITN Sid 28 David Gundlegård, ITN

8 DCF + RTS/CTS Large frames or hidden terminal problem Request to send (RTS) PCF Designed for QoS support! Contention free period advertised by AP Clear to send (CTS) Allocation Vector (NAV) set by stations AP polls stations after PIFS sender receiver other stations DIFS RTS CTS data NAV (RTS) NAV (CTS) defer access ACK DIFS contention data t point coordinator wireless stations stations NAV t 0 medium busy t 1 PIFS D 1 U 1 SuperFrame NAV D 2 U Sid 29 David Gundlegård, ITN Sid 30 David Gundlegård, ITN QoS Issues in Legacy MAC e Solutions DCF No guarantee/priority at all PCF Beacon delay due to transmissions at the end of the superframe Unknown transmission time of polled stations Up to 2312 bytes per frame Different modulation -> different transmission time New MAC functions Enhanced DCF and Hybrid Coordination Function (HCF) 8 different Traffic Categories (TC) New IFS: Arbitration IFS (AIFS) >= DIFS TC priority parameters in EDCF AIFS CWmin/CWmax CW change due to collision Transmission Opportunities (TXOP) advertised in Beacon frame (EDCF-TXOP) or Polling Frame (HCF-TXOP) Sid 31 David Gundlegård, ITN Sid 32 David Gundlegård, ITN

9 802.11e EDCF e HCF Polling messages include duration Polling possible both during CFP and CP PIFS priority Additionally support for reservation period allowing stations to communicate QoS needs Example: AIFS high = 2, AIFS low = 10 CWmin = CWmax = 7 ->? Sid 33 David Gundlegård, ITN Sid 34 David Gundlegård, ITN Next time QoS Wireless s Heterogenous s Sid 35 David Gundlegård, ITN

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