6 ICS132-congestion-control

6 ICS132-congestion-control - 5 Congestion Control Too many...

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5. Congestion Control Too many packets in part of the subnet Performance Degrades: Congestion Network Layer provides congestion control to ensure timely delivery of packets from souce to destination
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Congestion Control z Causes of congestion z Types of congestion control schemes z Solving congestion
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5.1 Causes of Congestion 1. Exhaustion of buffer space 2. Deadlock
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5.1.1 Exhaustion of Buffer Space z Routers maintain packet queues (or buffers) z Buffers fill up if: ± Routers are too slow, OR ± Combined input traffic rate exceeds the outgoing traffic rate z Insufficient buffer space leads to congestion
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Exhaustion of Buffer Space (cont’d) 50 Mbps 50 Mbps 50 Mbps 50 Mbps Router Buffer
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5.1.2 Deadlock z The first router cannot proceed until the second router does something, and the second router cannot proceed until the first router does something z Both routers come to a completely halt and stay that way forever
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Types of Deadlock 1. Store and Forward Lockup ± Direct Store and Forward Lockup ± Indirect Store and Forward Lockup 2. Reassembly Lockup
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Direct Store and Forward Lockup z Simplest lockup between two routers z Example: ± Suppose router A has five buffers, all of which are queued for output to router B ± Similarly, router B has five buffers, all of which are queued for output to router A ± If there is flow control on the link between routers A and B, then neither router can accept any incoming packets from the other. They are both stuck.
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Direct Store and Forward Lockup (cont’d) Router A Router B To/from other routers To/from other routers To/from other routers To/from other routers
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Indirect Store and Forward Lockup The same thing can happen on a larger scale
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Reassembly Lockup z In some network layer implementations, the sending router must split messages into multiple network layer packets z Receiving routers reassemble split up packets into a single packet z If the receiving router’s buffer fills up with incomplete packets, it cannot reassemble any more packets
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Reassembly Lockup (cont’d) s=source, p=part s=0, p=3 s=0, p=4 s=0, p=5 s=1, p=1 s=1, p=2 s=1, p=4 s=1, p=5 s=2, p=1 s=2, p=2 s=2, p=3 s=2, p=5 s=3, p=1 s=3, p=3 s=3, p=4 s=3, p=5 Input 0 Input 1 Input 2 Input 3 s=0, p=2 Buffer size = 16 packet segments
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5.2 Types of Congestion Control
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This note was uploaded on 03/24/2011 for the course CS 132 taught by Professor Suda,t during the Fall '08 term at UC Irvine.

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6 ICS132-congestion-control - 5 Congestion Control Too many...

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