Internetworking

• universal service
• internetworks
• routers
• protocols: TCP/IP
• IP addresses (start)

Universal Service

• any one network technology -- Ethernet, ATM, Frame Relay, SONET, FDDI, token ring, modems -- only interconnects some computers
  
• we want and need to talk to all computers
• we need to connect different networks together
• bridges are limited:
  • different frame formats are not always interconvertible
  • different addresses are not compatible with each other
  • as a network grows sufficiently large, technologies using broadcasting (e.g. Ethernet, FDDI) become too inefficient
• ATM, Frame Relay, and SONET can scale to very large sizes, but are limited in different ways, for example ATM is very inefficient over Ethernet because of packet sizes, and Ethernet cannot offer any guaranteed performance

Internetworks and Routers

• an internetwork is two or more networks connected by routers
• a router can interconnect multiple networks
• the router forwards packets based on its routing table, queueing packets if necessary, dropping packets if there is no route or no room in the queue
• multiple routes allow greater internet reliability
• an internet is a virtual network: it is formed from many smaller networks

Internet Protocols

• IP is the most widespread Internet Protocol
• TCP, the Transport Control Protocol, is the most widespread transport-layer protocol used on top of IP
• all the routers in an internet must speak the same Internet Protocol (IP)
• changing the Internet Protocol is very challenging, because the same protocol must be changed on the whole internet at the same time
• bridging: having a system that can translate between two different networks.
• Example: bridging between an Ethernet and an FDDI network -- all packets are broadcast on both sides, with headers translated appropriately
• tunneling: using one network to carry data for another
• Example: in a VPN (Virtual Private Network), we might use ATM to carry Ethernet packets from one side of the virtual network to the other -- in this case the Ethernet is tunneled over ATM.
• both bridging and tunneling are more limited than internets

Requirements for Internet Protocols

• universal addressing
• hierarchical:
  • addressing
  • routing
• end-to-end delivery
• fragmentation
• resource management: packet lifetime, bandwidth allocation
• open protocols, standard implementations, interoperability testing

IP Addresses

• each Internet host interface is configured with a unique 32-bit address
• this 32-bit number is made up of four bytes, each with value 0..255
• this 32-bit number is usually written as a sequence of 4 byte values (in decimal) separated by dots: 128.171.10.1 is

  10000000 10101011 00001010 00000001
  

  which is 80AB0A01 in hex
• the address is configured, not hardwired
• IP addresses are hierarchical:
  • the first n bits, n < 31, represent the network
  • the remaining 32-n bits represent the host within the network (host numbers are assigned at the discretion of the network administrator)
  • only the network numbers must be globally known, and only the last-hop router must know how to reach the final host
  • n is different for different networks
• how do we know what n is?
  • lookup table
  • encode in the first few bits

IP Address Classes

• if the first bit is 0, n = 7, and the host part has 24 bits (IP addresses 0.0.0.0 through 127.255.255.255) -- class A addresses
• if the first two bits are 10, n = 14, and the host part has 16 bits (IP addresses 128.0.0.0 through 191.255.255.255) -- class B addresses
• if the first three bits are 110, n = 21, and the host part has 8 bits (IP addresses 192.0.0.0 through 223.255.255.255) -- class C addresses
• other addresses are reserved (e.g. 1110 for multicast -- class E, and 1111 -- class E)

Limitations of Address Classes

• the subdivision into address classes is very rigid
• around 1992, could foresee running out of class B addresses
• host needs to know n for this network to be able to tell if an address is on the same or a different network -- why?
• routers needs to know n for each destination network
• solution:
  • manually specify n for each end system
  • automatically distribute n with each network destination address when distributing routing information (link-state or distance-vector)

IP Address Masks

• specify n with a 32-bit number which has n ones followed by 32-n zeros
• examples:
  • class A: 255.0.0.0
  • class B: 255.255.0.0
  • a network with 16 addresses: 255.255.255.240
• if A is an address, and M is a mask, A and M gives the network part of the address, which is also the network address
• if A is an address, and M is a mask, A and (not M) gives the host part of the address