Flow Attribute Notification Protocol is a protocol between neighbor modes which manages cut-through packet forwarding functionalities. In cut-through packet forwarding, a router doesn't perform conventional IP packet processing for received packets. FANP indicates mapping information between a datalink connection and a packet flow to the neighbor node. It helps a pair of nodes manage mapping information. By using FANP, routers such as the CSR (Cell Switch Router) can forward incoming packets based on their datalink-level connection identifiers, bypassing usual IP packet processing. FANP generally runs on ATM networks.

FANP has the following characteristics:

• Soft-state, cut-through path (Dedicated-VC) management
• Protocol between neighbor nodes instead of end-to-end
• Applicable to any connection-oriented, datalink platform.

There are 7 FANP control messages. They are encapsulated into IP packets, apart from the PROPOSE message which uses an extended ATM ARP message format. The destination IP address in the IP packet header signifies the neighbor node's IP address. The source IP address is the sender's IP address. The IP protocol ID is 110.
The following message format exists for: Offer, Ready and Error messages. Propose Ack, Remove and Remove Ack messages do not have the flow ID field.

Protocol Structure

• Version - The Version number. This version is version 1.
• OpCode - C Operation code, the following OpCode values exist: 1 Propose Ack; 2 Offer; 3 Ready; 4 Error; 5 Remove; 6 Remove ACK.
• Checksum - A 16 bit checksum for the whole message.
• VCID type - The type of VCID. The current value is 1. The VCID uniquely identifies the datalink connection between neighbor nodes.
• Flow ID type - If the Flow ID is 0, then the flow ID field is null. If the Flow ID is 1, then the Flow ID field described below is present.
• Reserved - In Offer messages the Refresh Timer field appears here. In error messages, the Error code field appears here.
• Refresh timer - The interval of the Refresh timer, in seconds. (Only appears in Offer messages.) The recommended value is 120.
• Error code - Only appears in Error Messages.
• VCID - Virtual Connection Identification.
• Flow ID - The Flow ID field does not appear in propose ACK, Remove and Remove Ack messages. When there is a flow ID type value of 1, this field contains the source and destination IP addresses of the flow.