Indirect TCP (I-TCP) is a mobility-enhanced version of the traditional Transmission Control Protocol (TCP) designed specifically to address the challenges introduced by wireless and mobile environments. I-TCP achieves this by breaking the end-to-end TCP connection into two separate connections: one over the wired part of the network and another over the wireless segment. This architecture is particularly useful in addressing the high error rates and variable latency commonly found in wireless networks.

Key Characteristics of I-TCP:

Split Connection: The main feature of I-TCP is the splitting of the TCP connection into two segments:

  • Wired Segment: Between the source and a mobile support router (MSR) or mobility agent.
  • Wireless Segment: Between the MSR and the mobile device.

Isolation of Errors: By splitting the connection, errors on the wireless link are isolated from the wired network. This way, standard TCP congestion control mechanisms aren’t mistakenly triggered by wireless errors.

Local Retransmissions: If there are packet losses on the wireless link, retransmissions are handled locally by the MSR, which means faster recovery and less bandwidth consumption on the wired network.

End-to-End Semantics: Although the TCP connection is split, I-TCP tries to maintain end-to-end semantics. The MSR acts as a proxy, terminating the TCP connection from the wired side and initiating a new one towards the mobile device.

State Management: The MSR maintains the state of the mobile device’s connections, which allows for buffering and handling handoffs.

Advantages:

  • Improved Performance: By handling wireless errors locally, the performance on the wired network is not affected.
  • Compatibility: I-TCP allows the mobile host to interact with legacy systems without any modifications to them.

Challenges:

  • Breaking End-to-End Semantics: Even though I-TCP tries to maintain end-to-end semantics, introducing an intermediate entity (the MSR) that terminates and starts TCP connections can introduce challenges, especially with applications that expect a true end-to-end connection.
  • Single Point of Failure: The MSR can become a bottleneck or single point of failure in the network.
  • Added Complexity: Introducing an MSR and maintaining connection states add complexity to the network design.

Conclusion:

I-TCP is an approach to enhance TCP performance in mobile and wireless environments by isolating the characteristics of wireless links from the wired network. While it addresses many challenges of mobile networks, it introduces new complexities that need to be managed.