TCP over Wireless Links ECE 256 Spring 2009
Today’s Discussions • Setting up the context (recap) • MAC (radios, HTP, antennas, rate/power/CS control), routing • Motivating a Transport Layer • The e2e argument • The basics of flow control and congestion control • From wireline to wireless • Peek into the large gamut of research in wireless TCP • Snoop, ELN, Vegas, Reno, Paris, Hollywood (just kidding !!) • Performance over wireless links • What’s hot now ?
Recall 1: PHY and MAC MAC MAC PHY PHY • Spread Spectrum radios (DS and FH) • RTS/CTS and Carrier Sensing for Hidden Terminals • Directional antennas to reduce interference • Rate control to extract max capacity from available SINR • Power control for spatial reuse & energy savings – topology control • TDMA scheduling, multi-channel use, encryption security • … and many more
Routing Routing Routing Routing Routing Recall 2: Network Layer • The first view of the network • Coping up with (uncontrolled) user mobility • Flooding the network reactively, or proactive updation • Mobile IP, coping with handoffs, etc. • Ad hoc routing – discovery, optimal metric, maintenance, caching • Secure routing – Routes bypassing malicious nodes
TCP TCP We have a route, now what ? Data NETWORK Data Data • Transport packets quickly and reliably over this network • Network properties often unknown (or difficult to track) • Where is the congestion ? How much cross traffic ? • What is the bottleneck bandwidth ? • How much buffers at intermediate nodes ? • Motivation for end to end TCP
The End to End Argument [Reed,Clark] “The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the end points of the communication system. Therefore, providing that questioned function as a feature of thecommunication system itself is not possible.” In other words, as a middle man, don’t attempt to do it, if you cannot do it completely. Let the end point handle it completely !! Caveat: Middle-man involvment OK for performance optimization Question is: Who is the end point ? TCP ? Application layer ? Human user ? … the debate goes on
Stop & Wait Pipelined Go Back N Selective Repeat Some transmission methods
Stop-and-wait operation sender receiver first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R
Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-Back-N, selective repeat Pipelined protocols
Pipelining: increased utilization sender receiver first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK ACK arrives, send next packet, t = RTT + L / R Increase utilization by a factor of 3!
Sender: k-bit seq # in pkt header “window” of up to N, consecutive unack’ed pkts allowed Go-Back-N • ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” • may receive duplicate ACKs • timer for each in-flight pkt • timeout(n): retransmit pkt n and all higher seq # pkts in window
receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unACKed pkt sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts Selective Repeat
Makes sense to transmit only the lost packets But this is true under what assumption ? Can you say a case in which Go-BACK-N might be better Selective Request
data from above : if next available seq # in window, send pkt timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+N]: mark pkt n as received if n smallest unACKed pkt, advance window base to next unACKed seq # receiver sender Selective repeat pkt n in [rcvbase, rcvbase+N-1] • send ACK(n) • out-of-order: buffer • in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-N,rcvbase-1] • ACK(n) otherwise: • ignore
Example: seq #’s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size? Selective repeat: dilemma What if pkts go out of order
TCP Congestion Control • Problem Definition • How much data should I pump into the network to ensure • Intermediate router queues not filling up • Fairness achieved among multiple TCP flows • Why is this problem difficult? • TCP cannot have information about the network • Only TCP receiver can give some feedbacks
The TCP Intuition Pour water Collect water
The Control Problem • Two main components in TCP • Flow Control and Congestion Control • Flow Control • If receiver’s bucket filling up, pour less water • Congestion Control • Don’t pour too much if there are leaks in intermediate pipes • Regulate your flow based on how much is leaking out • Aggressive pouring calls for retransmission of lost packets • Conservative pouring lower e2e capacity • Challenge: At what rate(t) should you pour ? Why ?
The TCP Protocol (in a nutshell) • T transmits few packets, waits for ACK • Called slow start • R acknowledges all packet till seq #i by ACK i (optimizations possible) • ACK sent out only on receiving a packet • Can be Duplicate ACK if expected packet not received • ACK reaches T indicator of more capacity • T transmits larger burst of packets (self clocking) … so on • Burst size increased until packet drops (i.e., DupACK) • When T gets DupACK or waits for longer than RTO • Assumes congestion reduces burst size (congestion window)
time TCP Timeline Host A Host B Think of a blind person trying to stand up in a low ceiling room Objective: Don’t bang your head, but stand up quickly one segment RTT two segments four segments
When waited for > RTO After RTO timeout cwnd = 20 ssthresh = 10 ssthresh = 8
The TCP Protocol (in a nutshell) • DupACK not necessarily indicator of congestion • Can happen due to out of order (OOO) delivery of packets • If 3 OOO pkts, then CW need not be cut drastically • The DupACK packet retransmitted • Continue with same pace of transmission as before (fast recovery) • R advertizes its receiver window in ACKs • If filling up, T reduces congestion window Why ?
Fast Recovery on 3 OOO DupACKs After fast recovery Receiver’s advertized window
TCP Round Trip Time and Timeout EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT • Exponential weighted moving average • influence of past sample decreases exponentially fast • typical value: = 0.125
Setting the timeout EstimtedRTT plus “safety margin” large variation in EstimatedRTT -> larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT: TCP Round Trip Time and Timeout DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT| (typically, = 0.25) Then set timeout interval: TimeoutInterval = EstimatedRTT + 4*DevRTT
Several flavors of TCP: combines options / optimizations Reno, Vegas, Eifel, Westwood … Overall TCP has worked well – proven on the internet Then why study it again for wireless networks ?
The TCP Intuition Pour water Collect water
Renewed Challenge • Key assumption in TCP • A packet loss is indicative of network congestion • Source needs to regulate flow by reducing CW • Assumption closely true for wired networks • BER ~ 10 -6 • With wireless, errors due to fading, fluctuations • Need not reduce CW in response … • But, TCP is e2e CANNOT see the network • Thus, TCP cannot classify the cause of loss CHALLENGE
application application application transport transport transport network network network rxmt link link link physical physical physical wireless The Problem Model TCP connection Wireline
Impact of Misclassification Best possible TCP with no errors (1.30 Mbps) TCP Reno (280 Kbps) Sequence number (bytes) Time (s) 2 MB wide-area TCP transfer over 2 Mbps WaveLAN
The Solution Space • Much research on TCP over wireless • Difficult to cover complete ground • We peek into some of the key ideas • 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
Link Layer Mechanisms • Forward error corrections • Add redundancy in the packets to correct bit-errors • TCP retransmissions can be alleviated • Link layer retransmissions • MAC layer ACKnowledgments • Overhead only when errors occur (unlike FEC) Such mechanisms require no change in TCP Is that breaking e2e argument ??
Issues with Link Layer Mechanisms • Link layer cannot guarantee reliability • Have to drop packets after some finite limit • What is the retransmission limit (??) • Retransmission can take quite long • Can be significant fraction of RTT • TCP can timeout and retransmit the same packet again • Increasing RTO can avoid this • But that impacts TCP’s recovery from congestion • Head of the line blocking • Link layer has to keep retransmitting even if bad channel • Blocks other streams Why?
Findings • Link layer retransmission good • When channel errors infrequent • When retransmit time << RTO • When modifying TCP is not an acceptable solution
application application application rxmt transport transport transport network network network link link link physical physical physical wireless 1 TCP = ½ TCP + ½ (TCP or XXX) Per-TCP connection state TCP connection TCP connection Base Station
Splitting Approaches • Indirect TCP [Baker97] • Fixed host (FH) to base station (BS) uses TCP • BS to mobile host (MH) uses another TCP connection • Selective Repeat [Yavatkar94] • Over FH to BS: Use TCP • Over BS to MH: Use selective repeat on top of UDP • No congestion control over wireless [Haas97] • Also use less headers over wireless • Header compression
Issues with Splitting • E2E totally broken • 2 separate connections • BS maintains hard state for each connection • What if MH disconnected from BS ? • Huge buffer requirements at BS • What if BS fails ? • Handoff between BS requires state transfer • What if Data and ACK travel on different routes ? • BS will not see the ACK at all – splitting not feasible Whats the Problem ?
Snoop • Link layer at BS buffers un-acknowledged packets • Now, BS peeks into every returning TCP ACK from MH • If DupACK • Retransmits the necessary packet • Drops the DupACK • DupACK does not reach sender • Prevents fast retransmit
Snoop : Example 35 TCP state maintained at link layer 36 37 38 40 39 38 37 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