Helper Protocols

Protocols that either make it easier for IP to do its job, or extend the capabilities of the network layer.

Address Resolution Protocol• Network interfaces have physical addresses.• IP packets are addressed to logical addresses.• How can we make sure the packet gets to the right

physical machine in a dynamic addressing environment?

• ARP is the primary address mapping protocol used.• ARP request is broadcast with IP address of receiver.• The receiver, and only the receiver, replies with its physical

address.• Replies are cached for some length of time so that new ARP

requests don’t need to be sent if a transmission takes multiple packets.

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Assigning logical addresses• RARP: Finds the logical address for a machine when

only the physical address is known.• BOOTP:

• One machine functions as a BOOTP server. This machine has a table matching NIC addresses with IP addresses.

• Requests for logical addresses are broadcast and the BOOTP server responds.

• DHCP:• Blends BOOTP capability (static address allocation) with dynamic

address allocation

• Along with static table, keeps a second table of available IP addresses.

Internet Control Message Protocol• IP is unreliable and connectionless, for the

benefit of efficiency.

• ICMP allow for other types of messages to be exchanged between hosts:

• Error-reporting: When an error occurs in the transmission of a packet, an error report is sent to the sending host

• Query: Are you alive?

Internet Group Management Protocol

• IGMP allows for the creation of groups and group addresses for the purpose of multicasting.

• Using IGMP messages, hosts can:– Enter groups– Leave groups

• Routers can:– Check membership