12/6/2015CST Computer Networks1 Reverse Address Resolution CST 415

12/6/2015CST Computer Networks2 Topics Definitions The Problem RARP bootp DHCP

12/6/2015CST Computer Networks3 Definitions RARP Reverse Address Resolution Protocol – The protocol defined to resolve the physical MAC address with the IP address of a machine that does not know it’s own IP address.

12/6/2015CST Computer Networks4 The Problem - Defined When a host first boots onto a network: –It will already know it’s physical address since this address is embedded in the NIC controller. –It may not know it’s IP address »A diskless workstation. –At best, this host can send out an broadcast Packet since the broadcast physical address is pre-defined

12/6/2015CST Computer Networks5 The Problem - Defined How can this poor, lonely, host get the information it needs to talk to it’s peers on the network? Suggestions? Hmmm…. Maybe we should put together an RFC and define a Protocol.

12/6/2015CST Computer Networks6 The Problem - Defined The host computer in question will broadcast out an RARP request pleading for somebody to send back it’s associated IP address. “Help! Who am I?” If luck prevails, there is a RARP server on the Ethernet segment who will answer. “Here you are, you diskless wonder. Now don’t forget so quickly this time.”

12/6/2015CST Computer Networks7 RARP Deficiencies: –Each physical LAN segment must have a RARP server. »Broadcast MAC addresses may not cross bridges and do not cross routers. –RARP servers need to know “a-priori” the binding of all MAC address to IP addresses of the network segment on which they reside. –RARP tables can get relatively large which impacts the speed of address look-up.

12/6/2015CST Computer Networks8 bootp TCP/IP Bootstrap protocol Designed to do the same job as RARP Also allows an initial bootstrap of a device to take place. Built on top of UDP as a transport mechanism The utilization of UDP allows the IP address server to reside on a different physical network segment.

12/6/2015CST Computer Networks9 bootp TCP/IP Bootstrap protocol Bootstrapping is a 2-phase process –Obtaining an IP address –Support of a higher level protocol for loading of an operating system, then starting the system. bootp is based on a client-server architecture.

12/6/2015CST Computer Networks10 bootp See RFC 951 for message format.

12/6/2015CST Computer Networks11 bootp TCP/IP Bootstrap protocol When Internet started to take off, dynamic address binding became important. bootp does not handle dynamic address binding. DHCP was introduced to handle dynamic binding.

12/6/2015CST Computer Networks12 DHCP Dynamic Host Configuration Protocol Based on bootp as an underlying protocol Dynamically assign addresses to clients. Centrally manage assigned IP addresses at the DHCP server. The DHCP server has a pool of IP addresses it can hand out to hardware devices dynamically roaming into and out of the network.

12/6/2015CST Computer Networks13 DHCP DHCP allocates IP address in three modes: 1.Manual Allocation – Pre configured for IP to MAC address allocation. 2.Dynamic Allocation – The server gives out an IP address from a pool. The address expires after a “lease” time and must be renewed. 3.Automatic Allocation – A client gets an assigned IP address from a pool, then always gets reassigned that address.

12/6/2015CST Computer Networks14 DHCP See RFC 2131 for Message Structure