Agent Advertisement

With mobile nodes moving from net to net, where mobility agents may or may not be present some protocol, to let the MNs determine, whether a mobility agent^2.12 is present, is needed. The designers of Mobile IP chose to use the existing ICMP protocol for this purpose. This is done by adding extensions to the ICMP router discovery protocol called Agent Advertisement and Agent Solicitation.

An agent advertisement is an ICMP router advertisement (see appendix D) that has been extended to carry a mobility agent advertisement extension. A mobility agent transmits agent advertisements to advertise services to visiting mobile nodes. The agent advertisement can also carry other extensions as specified by the ICMP protocol. Compared to the ''pure'' ICMP protocol the following restrictions has been imposed on the link-, IP- and ICMP header fields[23].

• The link-layer Destination address of an unicast agent advertisement (the response to an agent solicitation) must be the source link-layer address of the solicitation that prompted the advertisement.
• The TTL (Time To Live) of the IP header must be set to 1. This is to prevent solicitations and advertisements from reaching Mobile Nodes or mobility agents not on the same link.
• The Destination address of the IP header must be either the all-systems-on-this-link multicast address (224.0.0.1) or the limited broadcast address (255.255.255.255). Other means of addressing cannot be used because the mobile node cannot be expected to know the network prefix of the foreign network.
• The Type field of the ICMP header must be 9.
• The Code field of the ICMP header must be 0 if the advertising agent also acts as a router for IP traffic. Otherwise it must be 16 in which case the advertising agent forwards datagrams from the MN to an appropriate router.
• The Lifetime field of the ICMP header must contain the period of time, this advertisement is to be considered valid in the absence of further advertisements.
• The Router address of the ICMP header may appear in the Agent Advertisement.
• the Num addrs of the ICMP header contains the number of router addresses advertised in this message.