COMPUTER NETWORKS Chapter 05 The Network Layer Part 2

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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Connecting Networks A collection of interconnected 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.

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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.

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5.5.2 How Networks Can Be Connected

(a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.

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5.5.3 Concatenated Virtual Circuits Basic idea: Assume the constituent networks support virtual circuits. In that case, the internet can use virtual circuit technology through concatenation.

concatenation.

Note: If one of the constituent networks does not support virtual circuits, or, for example, provides only unreliable data transport, simple concatenation will be hard.

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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).

• 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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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

Tunneling Basic idea Ø 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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