27-12-2012, 01:52 PM
MOBILE IP AND TCP
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Motivation for Mobile IP
• Routing
– based on IP destination address, network prefix (e.g. 129.132.13)
determines physical subnet
– change of physical subnet implies change of IP address to have a
topological correct address (standard IP) or needs special entries in the
routing tables
• Changing the IP-address?
– adjust the host IP address depending on the current location
– almost impossible to find a mobile system, DNS updates are too slow
– TCP connections break
– security problems
• Change/Add routing table entries for mobile hosts?
– worldwide!
– does not scale with the number of mobile hosts and frequent changes in
their location
Requirements to Mobile IP (RFC 2002)
• Compatibility
– support of the same layer 2 protocols as IP
– no changes to current end-systems and routers required
– mobile end-systems can communicate with fixed systems
• Transparency
– mobile end-systems keep their IP address
– continuation of communication after interruption of link possible
– point of connection to the fixed network can be changed
• Efficiency and scalability
– only little additional messages to the mobile system required
(connection typically via a low bandwidth radio link)
– world-wide support of a large number of mobile systems
• Security
– authentication of all registration messages
Terminology
• Mobile Node (MN)
– system (node) that can change the point of connection
to the network without changing its IP address
• Home Agent (HA)
– system in the home network of the MN, typically a router
– registers the location of the MN, tunnels IP datagrams to the COA
• Foreign Agent (FA)
– system in the current foreign network of the MN, typically a router
– typically the default router for the MN
• Care-of Address (COA)
– address of the current tunnel end-point for the MN (at FA or MN)
– actual location of the MN from an IP point of view
– can be chosen, e.g., via DHCP
• Correspondent Node (CN)
Network integration
• Agent Advertisement
– HA and FA periodically send advertisement messages into their
physical subnets
– MN listens to these messages and detects, if it is in the home or a
foreign network (standard case for home network)
– MN reads a COA from the FA advertisement messages
• Registration (always limited lifetime!)
– MN signals COA to the HA via the FA, HA acknowledges via FA to MN
– these actions have to be secured by authentication
• Advertisement
– HA advertises the IP address of the MN (as for fixed systems), i.e.
standard routing information
– routers adjust their entries, these are stable for a longer time (HA
responsible for a MN over a longer period of time)
– packets to the MN are sent to the HA,
– independent of changes in COA/FA
Mobile IP and IPv6
• Mobile IP was developed for IPv4, but IPv6 simplifies the protocols
– security is integrated and not an add-on, authentication of registration is
included
– COA can be assigned via auto-configuration (DHCPv6 is one
candidate), every node has address auto-configuration
– no need for a separate FA, all routers perform router advertisement
which can be used instead of the special agent advertisement
– MN can signal a sender directly the COA, sending via HA not needed in
this case (automatic path optimization)
– „soft“ hand-over, i.e. without packet loss, between two subnets is
supported
• MN sends the new COA to its old router
• the old router encapsulates all incoming packets for the MN and forwards
them to the new COA
• authentication is always granted
DHCP: Dynamic Host Configuration Protocol
• Application
– simplification of installation and maintenance of networked computers
– supplies systems with all necessary information, such as IP address,
DNS server address, domain name, subnet mask, default router etc.
– enables automatic integration of systems into an Intranet or the Internet,
can be used to acquire a COA for Mobile IP
• Client/Server-Model
– the client sends via a MAC broadcast a request to the DHCP server
(might be via a DHCP relay)
TCP slow-start
• sender calculates a congestion window for a receiver
• start with a congestion window size equal to one segment
• exponential increase* of the congestion window up to the
congestion threshold, then linear increase
• missing acknowledgement causes the reduction of the congestion
threshold to one half of the current congestion window
• congestion window starts again with one segment