1 Chapter 4: Internetworking (Introduction) Dr. Rocky K. C. Chang 16 March 2004

2 1. The internetworking problem Problem: How to interconnect heterogeneous networks effectively? Three problems with interconnection at the data-link layer: –Do not scale to the number of data-link technologies. –Do not scale to the number of hosts (or networks). –Do not have a common addressing space.

3 1. The internetworking problem S2 S1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7S3H8

4 1.1 Scaling to data-link technologies Conversion between frame structures. Scalability problem as the number of data-link technologies supported increases, e.g., Ethernet FDDI PPP Token ring Frame conversion

5 1.2 Scaling to the network size A switched LAN is a “flat” network---A single broadcast frame reaches every LAN. –VLAN can relieve this problem at the expense of managing VLAN membership. Spanning tree protocol does not scale well to the network size. –Take a longer time for the protocol to converge. –Take a longer time to respond to network state changes.

6 1.3 Uncommon MAC address space The number of bits used in a MAC address may differ. –48-bit IEEE MAC addresses –IBM recommends another locally administered MAC addresses (overriding the burned-in MAC addresses). Each address in a data-link technology must be universally unique, but its uniqueness is not guaranteed when several networks are bridged.

7 2. A layer-three internetworking solution Use IP, XNS, IPX, etc on top of the networks. Replace LAN switches with layer-three switches, more commonly known as routers. Add IP software to each end host (with the whole protocol suite software). Assign an IP address to each network interface.

8 2. A layer-three internetworking solution R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7R3H8

IP: Scaling to data-link technologies Ethernet FDDI PPP Token ring IP Encapsulation and demultiplexing

IP: Scaling to the network size IP network uses hierarchy to achieve scalability. There are at least three levels: –A single IP host (csultra6.comp.polyu.edu.hk) –A IP subnet (four subnets in comp.polyu.edu.hk) –An autonomous system (polyu.edu.hk)

IP: Uncommon MAC Address space Create a logical (unicast) address space to identify network interfaces. Classes A-C for unicast and a class D for multicast: NetworkHost (a) NetworkHost (b) NetworkHost (c) (d) 1

IP software at end hosts The IP software mainly consists of modules for –Application layer, such as DNS –Transport layer: TCP, UDP –Routing layer: IP, ICMP, and others. –Data-link layer: MAC-IP-addresses binding IP addresses MAC addresessHost names DNS ARP RARP

An example A HTTP client is running in m1.sun.com to connect to a HTTP server at The DNS client at the m1.sun.com first obtains the IP address of The application data (HTTP+TCP) will then be encapsulated by an IP datagram with m1.sun.com

An example –IP source address = –IP destination address = Now m1.sun.com needs to run ARP to obtain the MAC address of network interface to the LAN. The IP datagram is then encapsulated in an Ethernet frame with –MAC source address = that of m1.sun.com –MAC destination address = that of

IP software at routers The software at routers is mainly used for routing and datagram forwarding. Each router is running at least a “routing protocol” to construct a routing (or forwarding) table. –Each entry in a routing table consists of IP destination address and the next-hop’s IP address. Upon receiving a datagram, a router forwards it based on a set of forwarding rules and the routing table.

Encapsulation and address binding To transmit IP datagrams over any data-link network, two requirements are needed: –A standard way to encapsulate IP datagrams –Address resolution between IP addresses and MAC addresses Standard RFCs for specifying datagram encap- sulations and possibly address resolutions, e.g., Ethernet (RFC 894), IEEE 802 (RFC 1042), etc. A shared medium uses an Address Resolution Protocol (ARP) for address binding.

Data encapsulation Send out to the network interface You have seen –IP over DIX Ethernet (slide 19 in Chapter 2, part I) –IP over IEEE (slide 22 in Chapter 2, part II) –IP over PPP (slide 22 in Chapter 2, part I) –IP over ATM via AAL 5 (slide 15 in Chapter 3, part III)

Address resolution protocol An ARP request message is data-link broadcasted on the LAN with the target IP address. Every IP host picks up a copy of the message and examine the target IP address. –If matching its IP address, send an ARP reply message back to the sender with its MAC address. –Else, drop the message. To reduce broadcast traffic, each host uses an ARP cache to remember the recent binding.

Address resolution protocol TargetHardwareAddr (bytes 2–5) TargetProtocolAddr (bytes 0–3) SourceProtocolAddr (bytes 2–3) Hardware type = 1ProtocolType = 0x0800 SourceHardwareAddr (bytes 4–5) TargetHardwareAddr (bytes 0–1) SourceProtocolAddr (bytes 0–1) HLen = 48PLen = 32Operation SourceHardwareAddr (bytes 0–3)

An internetworking example On each “hop or link,” both data encapsulation and address resolution occur. R1 ETH FDDI IP ETH TCP R2 FDDI PPP IP R3 PPP ETH IP H1 IP ETH TCP H8