CS4470 Computer Networking Protocols 6/10/2018 CS4470 Computer Networking Protocols 6. Internetworking Huiping Guo Department of Computer Science California State University, Los Angeles

Outline Network technologies Motivation of Internetworking Internet and routers IP addressing ARP 6. Internetworking CS470

Network technologies Ethernet FDDI Run over different cables 6/10/2018 Network technologies Ethernet Run over different cables Twisted-pair Ethernet: 10Base-T Fast Ethernet: 100Base-TX, 100Base-FX Gigabit Ethernet: 1000baseX, 1000baseT Physical layers are different FDDI Ring based token network Run on fiber FDDI has since been effectively obsolesced by fast Ethernet 6. Internetworking CS470

Network technologies PPP (point-to-point protocol) A data link protocol commonly used in establishing a direct connection between two networking nodes For both router-router leased line connection and the dial-up host-router connection. Common leased line services DS1(T1), DS3(T3) Over SONET (Synchronized Optical Network): OC1, OC12, OC24, OC36, OC48 6. Internetworking CS470

Motivation of internetworking Why internetworking There are different network technologies Ethernet, Token ring, ATM, Wireless LAN, … Connect multiple physical networks into a large, uniform communication system Challenges Heterogeneity Scalability 6. Internetworking CS470

Challenges of internetworking Heterogeneity Users on one type of network should be able to communicate with users on other types of networks Establishing connectivity between hosts on two different networks may require traversing several other different networks Each network has its own addressing scheme, media access control protocols, service model,… 6. Internetworking CS470

Challenges of internetworking Scalability The Internet has roughly doubled in size each year for 20 years Routing How to find an efficient path through a network with millions/billions of nodes Addressing How to provide suitable identifiers for all nodes 6. Internetworking CS470

Bridges and switches Can bridges and switches be used to interconnect different networks? Bridges and switches only know frames Different networks use incompatible frame format A frame created for one network technology cannot be transmitted on a network that uses a different technology 6. Internetworking CS470

Interconnect heterogeneous networks Hardware Routers: interconnect physical networks, also called gateways Software IP protocol Provides universal service Universal service A user on any computer in any networks can talk to any other user 6. Internetworking CS470

What is an internetwork Directly connected or switched network Such a network uses one technology, such as Ethernet, ATM, Token ring Also called physical network Internetwork A logical network built on a collection of physical networks Routers are the nodes that connect the physical networks 6. Internetworking CS470

A simple internetwork router host FDDI Token Ring H5 H4 H6 H3 H2 H1 H8 Ethernet Point-to-Point Link (e.g., ISDN) router host 6. Internetworking CS470

Software Achieve universal service Complexity of the software Software is needed on computers and routers Complexity of the software Deal with different frame format Routing how to find an efficient (loop free) path through a network with millions/billions of nodes Addressing how to provide suitable identifiers for all the nodes Does the MAC address work? 6. Internetworking CS470

MAC addresses don’t work Flat structure Not efficient to forward data based on MAC addresses Take space to store Different in different networks Hierarchical addresses preferred An address is divided into two parts Part 1: identify the network Part2: identify the computer Make forwarding efficient 6. Internetworking CS470

IP addresses Specified by Internet Protocol When we address a datagram Sending host puts destination internet address in datagram Destination address can be interpreted by any intermediate router Routers examine address and forward datagram on to the destination All of these addresses are virtual they are defined in software, not hardware 6. Internetworking CS470

IP Addresses Each host is assigned a 32-bit number Unique across entire Internet Dotted decimal notation: 123.144.2.23 Each IP address is divided into a prefix and a suffix Prefix identifies network to which computer is attached No two networks can be assigned the same network number Suffix identifies computer within that network No two computers on the same network can have the same suffix, but computers on different networks can have the same suffix Address format makes routing efficient 6. Internetworking CS470

IP addresses and MAC Addresses 32-bit IP address: network-layer address used to get datagram to destination network MAC (or LAN or physical or Ethernet) address: used to get frame from one interface to another physically-connected interface (same network) 6. Internetworking CS470

Resolving Addresses Hardware only recognizes MAC addresses IP only uses IP addresses Consequence: software needed to perform translation Known as address resolution 6. Internetworking CS470

Address Resolution Protocol Layer 2 protocol Given A locally-connected network, N IP address C of computer on N Find Hardware address for C Technique Address Resolution Protocol 6. Internetworking CS470

ARP is local to a network One computer can resolve the address of another computer only if both computers attach to the same network A computer never resolves the address of a computer on a remote network! 6. Internetworking CS470

ARP is local to a network (cont.) A sends data to B A uses B’s IP address A resolves B’s IP address to B’s hardware address A sends the frame to B directly 6. Internetworking CS470

ARP is local to a network (cont.) A sends data to F A uses F’s IP address A determines that the data must travel through R1 A resolves R1’s hardware address A sends the frame to R1 directly It’s the responsibility of R1 to send the frame to F 6. Internetworking CS470

Address Resolution Protocol (ARP) Keep mappings in table Table entry contains pair of addresses for one computer IP address Hardware address Build table automatically as needed 6. Internetworking CS470

ARP Table Lookup Use a simple list containing IP address and hardware address for each host on net Search on IP address and extract corresponding hardware address Note that all IP addresses have same prefix; can save space by dropping prefix 6. Internetworking CS470

ARP Table Lookup Sequential search may be prohibitively expensive (O(n)) Can use indexing or hashing for O(1) lookup Indexing - use hostid part of IP address as list (array) index Hashing - use hashing function on hostid to generate list index 6. Internetworking CS470

ARP on your computer Open a command window StartRuncmd Display the arp table on your compuer Under MS-DOS, type arp –a Clear the arp table Under MS-DOS, type arp –d * 6. Internetworking CS470

ARP Table How to get the table in the first place? Address resolution with message exchange Each computer on the network participates in address resolution by agreeing to answer the resolution requests for its address A computer that needs to resolve an address sends a broadcast request on the network. All machines receive the request and examine the requested address If the request matches a computer’s address, the computer responds 6. Internetworking CS470

ARP Message Exchange Example W sends a broadcast message asking for Y’s MAC address Everyone hears the message Only Y responds with a Unicast message (still broadcast on bus) 6. Internetworking CS470

ARP protocol example 1 (in a LAN) A wants to send datagram to B B’s MAC address not in A’s ARP table. A 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 237.196.7.23 237.196.7.78 237.196.7.14 237.196.7.88 B 6. Internetworking CS470

ARP protocol example 1 (in a LAN) A broadcasts ARP query datagram, containing B's IP address Dest MAC address = FF-FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP datagram, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator 6. Internetworking CS470

ARP protocol example 2 (to another LAN) A sends a datagram from A to B via R, assume A knows B’s IP address A R B 6. Internetworking CS470

ARP protocol example 2 (to another LAN) Two types of nodes Host: one adapter, IP address and one ARP table Router: one adapter, one IP Address and one ARP table for each interface Router has Two interfaces Two IP addresses Two adapters Two ARP tables 6. Internetworking CS470

ARP protocol example 2 (to another LAN) A creates datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s adapter sends frame R’s adapter receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B 6. Internetworking CS470