Lecture 4

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Unformatted text preview: o big to send –  Classic method, dated •  Discovery (next) –  Find the largest packet that fits on the network path and use it –  IP uses today instead of fragmenta8on 62 31 10/23/13 IPv4 Fragmenta8on •  Routers fragment packets that are too large to forward •  Receiving host reassembles to reduce load on routers Fragment! Reassemble! Fits on first link 63 IPv4 Fragmenta8on Fields •  Header fields used to handle packet size differences –  Iden8fica8on, Fragment offset, MF/DF control bits Payload (e.g., TCP segment) 64 32 10/23/13 IPv4 Fragmenta8on Procedure •  Routers split a packet that is too large: –  –  –  –  –  Typically break into large pieces Copy IP header to pieces Adjust length on pieces Set offset to indicate posi8on Set MF (More Fragments) on all pieces except last •  Receiving hosts reassembles the pieces: –  Iden8fica8on field links pieces together, MF tells receiver when it has all pieces 65 IPv4 Fragmenta8on (2) Before MTU = 2300 ID = 0x12ef Data Len = 2300 Offset = 0 MF = 0 Aler MTU = 1500 ID = Data Len = Offset = MF = (Ignore length of headers) ID = Data Len = Offset = MF = 66 33 10/23/13 IPv4 Fragmenta8on (3) Before MTU = 2300 ID = 0x12ef Data Len = 2300 Offset = 0 MF = 0 Aler MTU = 1500 ID = 0x12ef Data Len = 1500 Offset = 0 MF = 1 ID = 0x12ef Data Len = 800 Offset = 1500 MF = 0 67 IPv4 Fragmenta8on (4) •  It works! –  Allows repeated fragmenta8on •  But fragmenta8on is undesirable –  More work for routers, hosts –  Tends to magnify loss rate –  Security vulnerabili8es too 68 34 10/23/13 Path MTU Discovery •  Discover the MTU that will fit –  So we can avoid fragmenta8on –  The method in use today •  Host tests path with large...
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This document was uploaded on 04/04/2014.

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