IP Addresses Universal address regardless of layer 2 architecture Each address is that of an interface, not necessarily a host A host may have more than one interface and therefore more than one IP address IP address affects the path selected to deliver data to a host

Path Based on Address RAB I2I5 I1I3I4 Host A sends data to host B at address I4 Host A sends data to host B at address I5

Host Router Bridge I7 I3 I2 I1 I5 I4 I8

Datagram Delivery In the Internet Protocol, the basic unit of data is called a datagram Each datagram is individually addressed Decide if destination is on a network to which device (host/router) is attached If on the same network, deliver datagram directly (direct delivery) If on other than a directly attached network, send datagram to a predefined router for delivery (indirect delivery)

Datagram Delivery Given a destination IP address, a datagram eventually needs direct delivery But datagram must be encapsulated into the layer 2 architecture for delivery (frame) How to find the layer 2 (MAC) address that is associated with the destination IP address? Could build a table Could develop a protocol

Address Resolution Protocol (ARP) Ethernet BC IA A IBIC PAPB PC At higher level protocol we wish to use addresses IA, IB, IC At final delivery we need to use PA, PB, PC We must therefore map IX PX

Ethernet BC IA A IBIC PA PBPC O Station O needs to send a datagram to address IX Station O sends a special frame to broadcast address Frame contains address IX Station IX is expected to respond with its Physical address Station O retains this mapping in its memory (cache) Station O also includes its own IP and Physical addresses in the frame F

Address Resolution Protocol This is sort of a call for ‘Who on this network has address IX’ Response from target station contains its Physical address Actual protocol is defined in RFC 826 Frame format also defined This is a general protocol, not unique to IP

ARP Frame CRC SourceT Type = 0806

ARP Frame CRC 1111….111 Source T Type = 0806

ARP Frame Hardware type: 1 for Ethernet Protocol type: 0800 for Internet Protocol Hardware length: 6 for Ethernet Protocol length: 4 for IP Operation: 1 for ARP Request, 2 for Reply Sender hardware address Sender protocol address Target hardware address: 0s on a request Target protocol address

ARP – The Protocol A broadcast is received Check the frame type – 0806 = ARP Check Type and address of sender (Is) Update cache Check Operation – Is it a Request Check Target address (It) – Is it me? Swaps Hardware and Protocol address Inserts own Hardware address Set Operation to Reply (2) Sends response (unicast)

Gratuitous ARP Station send ARP request for its own IP address Let other stations know its mapping and any changes in hardware address Checks to see if any other station has this same IP address

Proxy ARP Sometimes it is desirable for one device (router) to respond on behalf of other hosts Often used to respond for devices on the other end of a serial line connected to the router If a station can respond for another, is this a security concern?

Reverse ARP Extension of ARP protocol to allow a station to obtain its IP address given its hardware addressed Originally used with diskless stations This function has been replaced with more modern protocols BOOTP DHCP

Position of ARP and RARP in TCP/IP Stack

Tools - Utilities Packet Capture – Analyzer Ethereal capture/analyzer Windows version Unix version Ethereal_XTRA on course Web site WinPcap – required for ethereal Analyzer – another capture utility Windump