【计算机网络课程教学课件】ch5-2

(a) An internetwork. (b) A graph of the internetwork.
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5.5.7 Fragmentation
Problem: Different networks may impose different maximum packet sizes with various causes. This means that we may have to split a packet into smaller ones when forwarding it through an intermediate network: fragmentation.
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5.5.5 Tunneling
Solution: You don’t solve it, but instead
consider, for example, IP as a universal network protocol. Lots of universal network protocols are available
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Fragments may pass through unreliable networks, and may be lost, we need to decide what to do about lost fragments. Solution: Discard the entire packet (what do you think will happen then?).
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Transparent and non-transparent fragmentation Computer Networks - 5 - 2
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Fragmentation--Reassembly
Problem: When we create fragments, how do we paste them together again:
Main problem: Addressing – different networks use completely different addresses, so how do I address a host in an IP network when I’ve only got something else?
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Connecting Networks
A collection of interconnected networks.
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Computer Neic idea
We can solve a lot of the internetworking
problems when we can assume that the source and destination are on the same type of network. In that case, we need only to tunnel packets through intermediate networks.
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5.5.1 How Networks Differ
Don’t worry: it’s impossible to resolve all differences. The solution is to just take a simple approach (like the Internet). We now only consider the starred (*) issues.
5.5 Internetworking
Problem: Great! We have all these networks, with all different protocol stacks, and now we just to let them talk to each other. Non-Solution: Enforce all networks to run the same protocol stack. That’s asking for a lot of trouble, and effectively saying that we are not allowed to make any progress. Solution: Construct all kinds of gateways that connect to different kinds of networks
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5.6 The Network Layer in the Internet
The IP Protocol IP Addresses Internet Control Protocols OSPF – The Interior Gateway Routing Protocol BGP – The Exterior Gateway Routing Protocol Internet Multicasting Mobile IP IPv6
a fragment may be fragmented again by successive intermediate
networks when a fragment arrives at the destination, we have to know exactly where it fits into the original packet.
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5.5.4 Connectionless Internetworking
Basic idea: Have the network layer offer only datagram services: unreliable, unordered packet flow. Often, connection-oriented services are not even supported (Internet).
One way of numbering the fragments is to use a tree A better numbering system is for the internetwork protocol to define an elementary fragment size small enough that the elementary fragment can pass through every network. When a packet is fragmented, all the pieces are equal to the elementary fragment size except the last one, which may be shorter. The internet header must provide the original packet number and the number of the elementary fragment contained in the packet.
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5.5.6 Internetwork Routing
Routing through an internet is similar to routing within a single subnet, but with some added complications. Making a graph model of this situation is complicated by the fact that every router can directly access every other router connected to any network to which it is connected Once the graph has been constructed, routing algorithm can be applied: within each network an interior gateway protocol is used; between networks, an exterior gateway protocol is used. Autonomous System (AS): an independent network
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The relay systems for internetworking
repeaters at the physical layer for boosting signals. bridges to make the interconnection at the data link layer. multiprotocol routers for forwarding, and possibly splitting up packets (bridges can’t do the latter). transport gateways for coupling byte streams in different networks. application gateways, e.g., for handling electronic mail between OSI and TCP/IP networks.