Transport Protocols for Wireless Networks CMPE 293 - Spring 2001 Marcelo M. de Carvalho.
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Transcript of Transport Protocols for Wireless Networks CMPE 293 - Spring 2001 Marcelo M. de Carvalho.
Outline
• Overview: – Transport Protocols & TCP– Limitations & Problems in Wireless
• TCP for Single-Hop Networks– Improving the Performance for TCP:
classes of protocols
• TCP for Multi-Hop Networks– TCP for MANETs
Traditional Transport Protocols
• Reliable transport protocols have been tuned for networks composed of wired links and stationary hosts.
• They adapt to prevailing end-to-end delay conditions throughout the life of a connection;
• Main Assumption: Increases in delay are interpreted as packet losses caused by congestion.
Sources of Errors in Wireless Links
• Pauses due to handoff between cells;
• Packet losses due to futile transmissions: mobile host out of reach of other transceivers (little or no overlap between cells);
• Packet losses due to transmission errors in wireless links.
How does TCP work?
• TCP continually measure how long acknowledgments take to return;
• If }{2}{ delayVardelayEdelay – Retransmit packet;– Initiate congestion control procedure:
• Drop transmission window size;• Activate slow-start algorithm;• Reset retransmission timer to a backoff interval that
doubles with each consecutive time-out.
Smooth Handoff
• Cellular networks should strive to provide smooth handoffs in order to eliminate packet losses during cell crossings.
• No overlaps are also good!!!– High aggregate bandwidth: adjacent cells can
use the same portion of the spectrum;– Support for low-powered mobile receivers;– Accurate location information
Retransmission Timers• Long pauses are partly due to inaccurate
retransmission timers.
• TCP implementations have coarse timers (300- to 500-millisecond resolution);
• Small timeout:– multiple reductions of the slow-start threshold;– multiple backoffs of the retransmission timer;– multiple retransmissions before the routes
become consistent.
Fast Retransmissions
• IDEA: Resume communication immediately after handoffs complete, without waiting for a retransmission timeout.
• Modern TCPs: activated when a transmitter receives triplicate acknowledgments from a receiver;
• Once a greeting arrives at the MH, TCP invokes the fast retransmission procedure.
Comparision of Mechanisms
• End-to-end protocols
• Split-connection protocols
• Link-layer protocols
• Hybrid protocols
End-to-end Protocols
• Sender is aware of the existence of wireless hops.
• Selective Acknowledgments (SACKs): sender can recover from multiple packet losses without resorting to a coarse timeout.
• Explicit Loss Notification (ELFN): the sender can distinguish between congestion and other forms of losses.
Split-connection Protocols
• Goal: to hide any non-congestion-related losses from the TCP sender.
• TCP connection is split between a sender and receiver into two separate connections at the base station:– TCP connection over wired link;– Specialized protocol over wireless link.
I-TCP: Indirect TCP
MH
MSR
FH
• MH = Mobile Host• MSR = Mobile Support Router• FH = Fixed Host
I-TCP TCP
TCP/IP in Mobile Environment
• Main reason for throughput degradation:– Loss of TCP segments during cell crossovers,
especially with non-overlapped cells.
• Effects:– Lost segments trigger exponential back off and
congestion control at the transmitting host. – Congestion recovery phase may last for
several seconds.
Indirect Protocol
• Different flow control and congestion control for wireless and wired links;
• Separate transport protocol supports disconnections, moves and other wireless related features;
• MSR manages much of the overhead;
• Faster reaction to mobility due to proximity between MSR and MH.
I-TCP Basics
move
MSR-2
FH
MH
MH socket
MH
MH socket
MSR-1 MSR-1
MSR1mhsocket
MSR1fhsocket
MSR2fhsocket
MSR2mhsocket
FH socket
I-TCP Handoff
Regular TCP
Wireless TCP
Link-layer Protocols
• Two main classes:– Error correction using techniques such as
Forward Error Correction;– Retransmission of lost packets in response to
automatic repeat request (ARQ) messages.
• Tuned to the characteristics of the wireless link.
Hybrid Protocols: The Snoop Prootocol
• An agent monitors every packet and maintains a cache of TCP segments that have not yet been acknowledged.
• Packet loss is detected by the arrival of a small number of duplicate acks or by a local timeout.
• The agent retransmits the lost packet and suppresses the duplicate acks.
Observations
• TCP-aware link-layer protocol with selective acknowledgments performs the best;
• Split-connection approaches is not a requirement for good performance.
• Selective acknowledgment is very useful in lossy links, especially for burst losses.
• Explicit Loss Notification is worth to try.
TCP Performance over MANETs
• Goals: I– nvestigate the impact of link failures due to
mobility on TCP performance;– Define expected throughput;– Enhance throughput with Explicity Link Failure
Notification (ELFN).
Simulation Environment
• NS Network Simulator;• TCP-Reno over IP on an 802.11 wireless
network;• Dynamic Source Routing (DSR) Protocol;• BSD ARP protocol (to resolve IP addresses to
MAC addresses);• 30 nodes in a 1500 X 300 meter area moving
according to the random waypoint mobility model.
Expected Throughput
• ti = duration of time for which the shortest path from the sender to receiver contains i hops.
• Ti = throughput obtained over a linear chain using i hops.
ThroughputExpected
1
1
ii
iii
t
Tt
TCP with ELFN
• Implementation:– Use ICMP message as a notice to the TCP
sender;– If the routing protocol sends a route failure
message to the sender, then the notice can be piggy-backed on it.
• TCP’s response: disable congestion control mechanism until route has been restored.
Observations
• Routing protocol has a significant impact on TCP performance (cache and propagation of stale routes);
• More aggressive cache management protocols are needed.