The TCP/IP Guide - Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages

This is the main extension used to convey Mobile IP capabilities of the agent to mobile nodes on the local network. It contains numerous fields used as part of the various processes described in this overall section on Mobile IP.

The structure of the Mobility Agent Advertisement Extension is described in Table 81 and shown graphically in Figure 130.

**Table 81: Mobile IP *Mobility Agent Advertisement Extension* Format**
Field Name	Size (bytes)	Description
*Type*	1	*Extension Type:* Identifies the Agent Advertisement extension type. For the Mobility Agent Advertisement Extension, it is set to 16.
*Length*	1	*Length:* Length of the extension in bytes, excluding the Type and Length fields. Thus, it is equal to 6 plus 4 for each care-of address in the message.
*Sequence Number*	2	*Sequence Number:* A sequential counter set to zero when the router initializes and then incremented for each advertisement sent out.
*Registration Lifetime*	2	*Registration Lifetime:* The maximum length of time, in seconds, the agent is willing to accept for registration requests. A value of 65,535 (all ones) means “infinite”. Note that this field is for registration only and has no relation to the regular Lifetime field in the regular Router Advertisement part of the message.
*Flags*	1
*Reserved*	1	*Reserved:* Sent as zero and ignored by recipient.
*Care-Of Addresses*	Variable (4 per address)	*Care-Of Addresses:* Zero or more addresses provided by a foreign agent for a mobile node to use as a foreign agent care-of address. A foreign agent must always provide at least one address in its advertisement; a router that cannot act as a foreign agent will typically omit this field.