Mobile Computing & Software Engineering Lab Tunneling at The Transport and Application Levels Prof. Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN NTUT, TAIWAN 1
Mobile Computing & Software Engineering Lab Introduction Exploring the techniques system managers and programmers use to exploit alternative topologies NTUT, TAIWAN 2
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 3
Mobile Computing & Software Engineering Lab Example Two Ethernet sites (LAN) and one WAN using X.25 interconnects hosts at the two sites Using TCP/IP Using TCP/IP Ethernet 1 Ethernet 2 WAN host host using using using X.25 x.25 x.25 NTUT, TAIWAN 4
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 5
Mobile Computing & Software Engineering Lab Example Using TCP/IP Using TCP/IP Internet Ethernet 1 Ethernet 2 router router Virtual circuit WAN host host using using using X.25 x.25 x.25 NTUT, TAIWAN 6
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 7
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 8
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 9
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 10
Mobile Computing & Software Engineering Lab 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. NTUT, TAIWAN 11
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 12
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 13
Mobile Computing & Software Engineering Lab Client Server application application Socket Socket simulation simulation library library OS OS with with X.25 X.25 WAN using X.25 NTUT, TAIWAN 14
Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 15
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