Improving TCP Performance over Ad-hoc Network
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Transcript of Improving TCP Performance over Ad-hoc Network
Improving TCP Performanceover Ad-hoc Network
11/28/2000
Xuanming Dong, Duke Lee, and Jin Wang
Course Project for EE228A --- Fall 2000(Professor Jean Walrand)
Outline
TCP basics and congestion control Ad-hoc network basics Impact of ad-hoc network on TCP
performance Explicit Route Notification Simulation and Results Summary
Basic function of TCP
1. Basic Data Transfer: use IP services;2. Reliability: sequence number; positive
ACK; retransmission and time out; checksum;
3. Flow Control: 4. Congestion Control: 5. Multiplexing: addresses or ports within
each host;6. Connections: 7. Precedence and Security:
TCP: Congestion Control
Variables: Congestion Window (W) & Slow Start
threshold size (ssthresh) Indication of congestion:
Timeout and DupACK Congestion Control :
Slow Start + Congestion Avoidance Fast Recovery (Fast retransmission +
Congestion Avoidance)
Ad-hoc Networks
Autonomous mobile nodes are free to move around arbitrarily
network topology may change randomly and rapidly at unpredictable times
Without (necessarily) using a pre-existing infrastructure
Routes between nodes may potentially contain multiple hops
Infrastructure-oriented Wireless Network Have fixed and wired gateways The bridges for these networks are
known as base stations A mobile node within these networks
connects to the nearest base station As the mobile node travels out of
range of one base station and into range of another, a “handoff” occurs
Usually only the last link is wireless
How does mobility affect TCP
High bit error rate cause packets(TCP data segment or ACKs)
corrupted and then congestion control invoked
Route recomputation Discovering a new route may take
significantly longer than RTO and then cause retransmission and invoke congestion control
Network may be temporarily partitioned Cause packets dropped and invoke
congestion control
Route Re-computation
Figure 1: Route Re-Computation
Temporary Network Partition
Figure 2: Temporary Network Partition
mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
TCP sendertimes out.Backs off timer.
Route isrepaired
TCP sendertimes out.Resumessending
Larger route repair delaysespecially harmful
No throughput
No throughput
despite route repair
Why Does Throughput Degrade?
Explicit Route Notification(ERN)
Router-based feedback Need to modify TCP Source & Router
to receive/send feedback Need mechanisms for information
exchange between layers Inform TCP of route failure by
explicit message Let TCP know when route is
repaired By Explicit notification Reduces repeated TCP timeouts
and backoff
TCP FSM
Normal Frozen
ERFN
Frozen Timeout
ERRN
Figure 4: Extended TCP Finite State Machine
Explicit Route Notification(ERN)
Router-based feedback Need to modify TCP Source & Router
to receive/send feedback Need mechanisms for information
exchange between layers Inform TCP of route failure by
explicit message Let TCP know when route is
repaired By Explicit notification Reduces repeated TCP timeouts
and backoff
Support from Router
1. Maintain a packet cache for every routing entry in the routing table. Once the router finds that the routing entry for a packet becomes invalid recently, it caches the packet and creates an ERFN (Explicit Route Failure Notification) packet.
2. Associate a timer with every packet in the cache. If the timer expires, then drop the packet from the cache.
3. If the invalid routing entry is removed from the routing table, drop all packets in its cache and cancel the associated timers.
4. If the invalid routing entry becomes valid again, forward all the cached packets and create an ERRN (Explicit Route Recovery Notification) packet for each cached packet.
Support from Router
Destination Next Hop Metric SeqNo A A 0 3 B B 3 2 C 2 D 6
Cache - C
Cache - D
Figure 3: Routing table in DSDV Routers
Support from TCP
Context Switch The Frozen State
1. Stop sending further packets. 2. Stop invoking congestion control. 3. All TCP timers are frozen, except the new frozen
timeout timer.4. But TCP source may accept ACK packets and advance
the acknowledgment window.
Support from TCP
Normal Frozen
ERFN
Frozen Timeout
ERRN
Figure 4: Extended TCP Finite State Machine
Simulation Results
Source Packet Trace Source Congestion Window trace TCP Throughput
Simulation Results
Figure 6: TCP Source Packet Seq No vs Time (Reno)
-400
100
600
1100
1600
24 26 28 30 32 34 36
Time(Second)
Se
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ce N
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Simulation Results
Figure 7: TCP Source Congestion Window Size vs Time (Reno)
0
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24 26 28 30 32 34 36
Time(Second)
Co
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Win
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Simulation Results
Figure 8: TCP Source Packet Seq No vs Time (ERN)
0
200
400
600
800
1000
1200
1400
1600
24 26 28 30 32 34 36
Time(Second)
Seq
uen
ce N
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Simulation Results
Figure 9: TCP Source Congestion Window Size vs Time (ERN)
0
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10
15
24 26 28 30 32 34 36
Time(Second)
Con
gest
ion
Win
dow
S
ize
Summary
By Xuanming Dong