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NARP: NBMA Address Resolution Protocol

The NBMA Address Resolution Protocol (NARP) allows a source terminal (a host or router), wishing to communicate over a Non-Broadcast, Multi-Access (NBMA) link layer network, to find out the NBMA addresses of a destination terminal if the destination terminal is connected to the same NBMA network as the source.

A conventional address resolution protocol, such as ARP for IP, may not be sufficient to resolve the NBMA address of the destination terminal, since it only applies to terminals belonging to the same IP subnetwork, whereas an NBMA network can consist of multiple logically independent IP subnets.

Once the NBMA address of the destination terminal is resolved, the source may either start sending IP packets to the destination (in a connectionless NBMA network such as SMDS) or may first establish a connection to the destination with the desired bandwidth and QOS characteristics (in a connection oriented NBMA network such as ATM).

An NBMA network can be non-broadcast either because it technically doesn't support broadcasting (e.g., an X.25 network) or because broadcasting is not feasible for one reason or another (e.g., an SMDS broadcast group or an extended Ethernet would be too large).

Protocol Structure - NARP: NBMA Address Resolution Protocol

8		16	32bit
Version		Hop Count	Checksum
Type		Code	Unused
Destination IP address
Source IP address
NBMA Len.	NBMA address (variable length)

• Version - NARP version number. Currently this value is 1.
• Hop Count - Indicates the maximum number of NASs that a request or reply is allowed to traverse before being discarded.
• Checksum - Standard IP checksum over the entire NARP packet (starting with the fixed header).
• Type - NARP packet type. The NARP Request has a type code 1, NARP Reply has a type code 2.
• Code - A response to an NARP request may contain cached information. If an authoritative answer is desired, then code 2.
• Source and Destination IP Address - Respectively, these are the IP addresses of the NARP requestor and the target terminal for which the NBMA address is destined.
• NBMA Length and NBMA Address - The NBMA length field is the length of the NBMA address of the source terminal in bits.

Related Protocols
ARP

Sponsor Source

NARP is defined by IETF (www.ietf.org ) in RFC 1735

Reference