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62 computer science engineering 47 102313 topic

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Unformatted text preview: packet –  Routers provide feedback if too large; they tell host what size would have fit 69 Path MTU Discovery (2) MTU=1400 Try 1200 MTU=1200 bytes MTU=900 Try 900 70 35 10/23/13 Path MTU Discovery (3) Test #1 Test #2 MTU=1400 MTU=1200 bytes Try 1200 Test #3 MTU=900 Try 900 71 Path MTU Discovery (4) •  Process may seem involved –  But usually quick to find right size •  Path MTU depends on the path and so can change over 8me –  Search is ongoing •  Implemented with ICMP (next) –  Set DF (Don’t Fragment) bit in IP header to get feedback messages 72 36 10/23/13 Introduc8on to Computer Networks Error Handling with ICMP (§5.6.4) Computer Science & Engineering Topic •  What happens when something goes wrong during forwarding? –  Need to be able to find the problem What happened? XXXXXXX Yikes! 74 37 10/23/13 Internet Control Message Protocol •  ICMP is a companion protocol to IP –  They are implemented together –  Sits on top of IP (IP Protocol=1) •  Provides error report and tes8ng –  Error is at router while forwarding –  Also tes8ng that hosts can use 75 ICMP Errors •  When router encounters an error while forwarding: –  It sends an ICMP error report back to the IP source address –  It discards the problema8c packet; host needs to rec8fy Report then toss it! Oh, now I see … XXXXXXX ICMP report 76 38 10/23/13 ICMP Message Format •  Each ICMP message has a Type, Code, and Checksum •  Olen carry the start of the offending packet as payload...
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