lec25 - ECE 333 Introduction to Communication Networks Fall...

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Unformatted text preview: ECE 333: Introduction to Communication Networks Fall 2002 Lecture 25: & More on Internetworking and IP Last time we introduced the topic of internetworking and discussed routing and addressing in IP. In this lecture we consider some other aspects of both internetworking in general and IP in particular. Fragmentation and Reassembly A basic problem when internetworking networks that are based on different protocols is that each network may have its own maximum packet size. For example in Ethernet can accept packets up to 15000 bytes, while FDDI packets may be up to 4500 bytes long. Two possible approaches for dealing with this are 1. Use a small enough packet so that it can be carried over any network. 2. Allow packets that are too large for a network to be fragmented at routers before crossing over the network, i.e., a large packet is broken into several smaller packets; these packets must then be re-assembled into the original packet. In IPv4, fragmentation is used. Two different approaches to fragmentation - transparent and nontransparent Transparent Fragmentation - the router doing the fragmentation breaks up oversized packets and addresses them all to the same exit router. The exit router re-assembles the fragments and forwards original packet. This is called transparent fragmentation because it is invisible to the other networks in the path and to the hosts. Problems: & All packets must exit at the same router; this is not guaranteed in a connectionless network. & Overhead and delay involved with repeated fragmentation and reassembly. Nontransparent Fragmentation - once a packet is fragmented, it is not reassembled until it reaches the destination. Problems: & Overhead is increased since the ratio of data to header is worse. & Every host must be able to reassemble. & Multiple layers of fragmentation are possible. (Some networks may need to fragment the fragments.) In IPv4, non-transparent fragmentation is used. 5 General Fragmentation Problems Fragments must be numbered so they can be reassembled in the same order. What information is required? Must number all packets and indicate how many packets are in original....
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lec25 - ECE 333 Introduction to Communication Networks Fall...

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