Tunneling at The Transport and Application Levels Prof. Chuan-Ming - - PowerPoint PPT Presentation

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Tunneling at The Transport and Application Levels

• Prof. Chuan-Ming Liu

Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN

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Introduction

Exploring the techniques system managers and programmers use to exploit alternative topologies

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Multitprotocol Environments

Not all machines provide complete TCP/IP support Not all organizations use TCP/IP to interconnect computers Actually, most large organizations have several groups of machines and each group has its own protocol suite Disadvantages

Duplication of effort Limitations on interoperability

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Example

Two Ethernet sites (LAN) and one WAN using X.25 interconnects hosts at the two sites Ethernet 1

Using TCP/IP

Ethernet 2

Using TCP/IP

WAN using X.25

host using x.25 host using x.25

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Mixing Network

Network may run an alternative protocol Transport-level service delivers packets from one point to another as easily as packet-switching hardware It is possible to substitute any transport-level switching service for a single physical link in a packet switching system Many internets have been built that use transport-level switching services instead of physical networks

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Example

Ethernet 1

Using TCP/IP

Ethernet 2

Using TCP/IP

WAN using X.25

host using x.25 host using x.25 router router Internet Virtual circuit

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Example

When the routers boots, they use X.25 to form a conventional, transport-level virtual circuit to one another across the X.25 WAN The X.25 service does not know that the data being sent across the virtual circuit consists of IP datagram

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Example

With the two routers, a user on any host can invoke standard TCP/IP client software that contact a server on any other host Neither the user nor the client-server applications need to know that datagrams pass across an X.25 network when they travel from the Ethernet at one site to the Ethernet at the other

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Limit on Circuit Allocation

From the example, it seems ok to add additional sites (routers) to extend the topology X.25 network limits the number of circuits that a single computer can allocate simultaneously – typically 16 or 32 virtual circuit An organization with N sites needs (N*(N- 1))/2 circuits to interconnect all of them

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Dynamic Circuit Allocation

How to overcome the limitation for circuit allocation? Allocate circuit on demand and close circuit that are not being used Next-hub address – the address of the next router to which the datagram should be sent The router consults the table of active X.25 virtual circuits to decide

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Problem for Dynamic Allocation

When the router decide to add a virtual circuit and the table of active circuits is full?

Select the circuit which is the least recently used to close

After sending the datagram, the circuit remains open to minimize the delay and cost.

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Encapsulation v.s. Tunneling

Encapsulation: the process of placing an IP datagram inside a network packet or frame so that it can be sent across an underlying network Tunneling: the use of a high-level transport network service to sent datagrams from one point to another The difference lies in if IP transmits datagrams in hardware packets or uses a high-level transport service to deliver them

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Application-level Tunneling

Provides a communication path between a client and a server Not all OS support TCP/IP protocol and transport-level tunneling To implement application-level tunneling

Building socket simulation library

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WAN using X.25

Client application Socket simulation library OS with X.25 Server application Socket simulation library OS with X.25

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Dialup Phone Lines

Modems allows two computers to communication across the dialup telephone system SLIP: Serial Line IP PPP: Point-to-Point Protocol The use of dialup is analogous to the notion of tunneling The dialup telephone system should be viewed as a connection-oriented physical network. SLIP and PPP define a form of encapsulation