19-ControlProtoLink

19-ControlProtoLink - ICMP/ARP and General Link-Layer EE122...

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1 ICMP/ARP and General Link-Layer EE122 Fall 2011 Scott Shenker http://inst.eecs.berkeley.edu/~ee122/ Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley
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Announcements • Regrades done: pick up after class • Demo on Wednesday • Today’s lecture is in two parts…. 2
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Questions to be answered in 2 nd half • What does the link-layer do? • How does it do it? • Random access will be in Wednesday’s lecture 3
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Questions to be answered in 1 st half • What must a host know before it can operate? – Local information – Remote information • How can it find this information out? 4
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5 Obtaining information involves… Local : Bootstrapping an end host – Learning its own configuration parameters ( DHCP ) – Learning the link-layer addresses of other nodes ( ARP ) Global : Network control messages – Internet Control Message Protocol ( ICMP ) – Exploiting ICMP for discovering Internet path properties Different mechanisms, for different environments
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Internet versus LAN • Addressing: IP addresses vs MAC addresses (later) • Scale: Huge vs Limited • Management: Ad Hoc vs Managed • Delivery Model: No broadcast vs broadcast 6
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Fitting mechanisms to context…. Local mechanisms : broadcast to find things – “Bootstrapping” Remote mechanisms : investigate path – Can’t broadcast to entire Internet – Can only ask how to use what routing has already found – “Network Control Messages” 7
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Network Control Messages (and how to use them for discovery) 8
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What Errors Might A Router See? • Dead-end: No route to destination • Sign of a loop: TTL expires • Can’t physically forward: packet too big – And has DF flag set • Can’t keep up with traffic: buffer overflowing • Header corruption or ill-formed packets • …. 9
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Which should network react to? • No route to destination? – Host can’t detect or fix routing failure. • TTL expires? – Host can’t detect or fix routing loop. • Packet too big (with DF set)? – Host can adjust packet size, but can’t tell difference between congestion drops and MTU drops • Buffer overflowing? – Transport congestion control can detect/deal with this • Header corruption or ill-formed packets? – Some format problems, but probably not corruption 10
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Router Response to Problems? • Router doesn’t really need to respond – Best effort means never having to say you’re sorry – So, IP could conceivably just silently drop packets • Network is already trying its best – Routing is already trying to avoid loops/dead-ends – Network can’t reduce packet size (in DF packets) – Network can’t reduce load, nor fix format problems • What more can/should it do?
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This note was uploaded on 01/09/2012 for the course ELECTRICAL 122 taught by Professor Shenker during the Spring '11 term at Berkeley.

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19-ControlProtoLink - ICMP/ARP and General Link-Layer EE122...

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