Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Mobility Support (Mobile IP)

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages

Agent Advertisement and Agent Solicitation Messages

Provision already exists for exchanges of data between a router and a node, in the form of ICMP messages that are used for the regular IP router discovery process. Two messages are used for this purpose: Router Advertisement messages that let routers tell local nodes that they exist and describe their capabilities, and Router Solicitation messages that let a node prompt a router to send an advertisement. These are described in their own topic in the ICMP section.

Given the similarity to normal router discovery, it made sense to implement agent discovery as a modification to the existing process rather than set up a whole new system. The messages used in the agent discovery process are:

• Agent Advertisement: This is a message transmitted regularly by a router acting as a Mobile IP agent. It consists of a regular Router Advertisement message that has one or more extensions added that contain Mobile-IP-specific information for mobile nodes.
  
  
• Agent Solicitation: This message can be sent by a mobile IP device to nudge a local agent to send an Agent Advertisement.

The use of these messages is described in the Mobile IP standard in detail, and unsurprisingly, is very similar to how regular Router Advertisement and Router Solicitation messages are employed. Agents are normally configured to send out Agent Advertisements on a regular basis, with the rate set to ensure reasonably fast contact with mobile nodes without consuming excessive network bandwidth. They are required to respond to any Agent Solicitation messages they receive by sending an Advertisement. It is possible that some agents may be configured to send Advertisements only upon receipt of a Solicitation.

Mobile nodes are required to accept and process Agent Advertisements. They distinguish these from regular Router Advertisements by looking at the size of the message. They then parse the extension(s) to learn the capabilities of the local agent. They determine whether they are on their home network or a foreign network, and in the case of a foreign agent, how the agent should be used. Mobile nodes are required to use Agent Advertisements to detect when they have moved, using one of two algorithms defined in the standard, and also to detect when they have returned back to their home network after they have been traveling. They are also required to be able to send Agent Solicitation messages if they don't receive an Agent Advertisement after a certain period of time. They are restricted to sending these only infrequently, however, again to keep traffic manageable.

Now let's look at the formats of the two message types.

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages