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EE450-Discussion12-Fall10

# EE450-Discussion12-Fall10 - Discussion#12 EE450 IP...

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1 Discussion #12 EE450, 11/10/2010 -IP Fragmentation - Addressing and  Subnetting

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2 Problem#1: Description Suppose that a TCP message that contains 2048 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks of the Internet (i.e. from the source host to a router to the destination host ). The first network uses 14-byte headers and has an MTU of 1024 bytes, the second uses 8-byte headers with an MTU of 512 bytes. Each network’s MTU gives the total packet size that may be sent including the network header.
3 Problem#1: Question Give the sizes of the fragments delivered to the network layer at the destination host, assuming all IP headers are 20 bytes. S R D MTU=1024 (Header=14) MTU=512 (Header=8) TCP Data=2048 TCP Header=20 MAC IP TCP IP Header=20 MTU

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4 Problem#1: Solution Across the first network: Packets have room for 1024-14-20=990 bytes of IP- level data. Since 990 is not multiple of 8, each fragment can contain at most 8 x 990/8 = 984 bytes. We need to transfer 2048 +20 bytes of such data. This would be fragmented into fragments of size 984,984 and 100 bytes. For Packet sizes across the first network , add 14+20 bytes to each fragment.
5 Problem#1: Solution Across the second network: Packets have room for 512-8-20=484 bytes of IP-level data. So the 100-byte packet will not be fragmented but the other two 984-byte packets will be fragmented. Since 484 is not multiple of 8, each fragment can contain at most 8 x 484/8 = 480 bytes. Each 984-byte fragment would be fragmented into fragments of size 480,480 and 24 bytes. For Packet sizes across the second network, add 8+20 bytes to each fragment.

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6 Problem#2: Description We have an IP datagram of size 1420 bytes (including the IP header) traversing the sequence of physical networks with different MTUs as follow: Ethernet : MTU = 1500 bytes FDDI : MTU = 4500 bytes PPP : MTU = 532 bytes R1 R2 Ethernet FDDI R3 A B Ethernet PPP
7 Problem#2: Question Show the unfragmented datagram as well as the fragmented datagrams and clearly identify the Header fields used in IP fragmentation. MAC IP IP Header=20, Payload=1400 bytes MTUs: 1500, 4500, 532

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8 Problem#2: Solution The datagram will not be fragmented across the first two networks. Since the MTU of the PPP network is less than the size of the datagram (without the IP header), it will be fragmented to 3 datagrams.
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