Published on: 6 June, 2023

What are the functions of home agent and foreign agent in mobile IP? How does I-TCP differ from traditional TCP? Describe with shome agent & foreign agent and I-TCP vs. traditional TCPuitable diagram. 


Home agent is a type of mobility agent. In Mobile Internet Protocol (Mobile IP), a home agent is a router on a mobile node's home network that maintains information about the device's current location, as identified in its care-of address. The home agent uses tunnelling mechanisms to forward Internet traffic so that the device's IP address doesn't have to be changed each time it connects from a different location. A home agent may work in conjunction with a foreign agent, which is a router on the visited network.


A foreign agent which is also a type of mobility agent which works in conjunction with another type of mobility agent known as a home agent to support Internet traffic forwarding for a device connecting to the Internet from any location other than its home network. The home agent tunnels datagrams (packets) intended for the mobile node to a care-of address, and the foreign agent detunnels packets and delivers them to the mobile node.


The Slow start mechanism when the segments are lost of traditional TCP in fixed networks does not work well for wireless networks. To remove this problem indirect TCP(ITCP) was developed.

In I-TCP, the TCP connection between the mobile host (MH) and the correspondent host (CH) is split at the mobility support router (MSR). The connection between the MSR and MH has independent, optimized flow and congestion control from the MSR to CH link.

  • MSR state: Mobile-IP is used to deal with the mobility of the MH. The MSRs transfer the split connection state when a handoff occurs to a new MSR.
  • Changes to TCP semantics: Because the connection is split, separate acknowledgements are generated for the MH to MSR and MSR to CH links - the acknowledgements are not end-to-end. The link characteristics are also different - the MSR to MH link is very fragile. Because they use Mobile-IP, all other TCP semantics are preserved.
  • Performance: I-TCP is strictly better than regular TCP. If there are no hand-offs, then the performance gained by the optimized MSR to MH link outweighs the overhead of copying packets across the links. If there are hand-offs across cells, the fact that congestion control and slow-start for the MSR to CH were avoided resulted in a big win. (Although the window size for the MSR to CH link did decrease, because incoming packets were being buffered and not delivered during the handoff.) Performance in WANs is even better than performance in LANs, because retransmissions and recovery from congestion control on the wireless link is decoupled from the WAN wired link.
  • Implementation details: The I-TCP layer is a user-level process, meaning extra overhead in going from kernel space to user space. This overhead was claimed to be far less than the gain I-TCP provided. Another detail is that forwarding pointer route optimizations between MSRs were used.

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