RTUComputer ScienceYr 2022 · Sem 82022

Q20Mobile Computing

Question

10 marks

Describe the classical TCP improvements for mobility: I-TCP, Snooping TCP, and Mobile TCP.

Answer

An exhaustive theoretical critique of Classical TCP Improvements for Mobility. Violently contrasts the catastrophic architecture of I-TCP (splitting connections) against the elite, transparent Snooping TCP algorithm, detailing their exact mathematical manipulations of congestion windows.

Standard TCP operates on a rigid mathematical assumption: Packet Loss equals Network Congestion. In wireless networks, packet loss is caused by RF fading, bit errors, and handoffs. Standard TCP violently reacts by triggering Congestion Control (halving the transmission window), causing catastrophic, unnecessary drops in throughput. To fix this physics mismatch, three core architectural improvements were engineered.

I-TCP executes a catastrophic architectural violation: it physically breaks TCP end-to-end semantics.

  • The Mechanism: The TCP connection is violently split at the Base Station (BS). The Fixed Host talks strictly to the BS using standard TCP. The BS opens a completely separate, highly optimized wireless TCP connection to the Mobile Node.
  • The Math: If the wireless link drops a packet, the BS intercepts it and instantly executes a local retransmission. The Fixed Host mathematically never sees the error, so it NEVER triggers its massive congestion control algorithm.
  • The Demerit: If the BS violently crashes after acknowledging a packet to the Fixed Host, but before delivering it to the Mobile Node, the packet is mathematically deleted from existence, completely violating TCP reliability.

Snooping TCP perfectly preserves absolute end-to-end semantics.

  • The Mechanism: The Base Station does NOT terminate the connection. It acts as a violent, transparent "Snooper". It mathematically inspects the headers of every single TCP packet flowing through it and caches them in RAM.
  • The Execution: If the Mobile Node loses a packet, it will send Duplicate ACKs backward. The Snooping BS violently intercepts these Duplicate ACKs before they reach the Fixed Host. Because the BS has the packet in its RAM cache, it instantly executes a local retransmission to the Mobile Node.
  • The Merit: The Fixed Host is completely shielded from the wireless chaos, and absolute end-to-end TCP integrity is mathematically preserved.

Engineered specifically for networks with frequent, catastrophic disconnections (like driving through tunnels).

  • The Mechanism: The Supervisory Host (SH) monitors the link. If the Mobile Node violently disconnects, the SH intercepts the connection and fires an ACK back to the Fixed Host with a TCP Window Size of 0.
  • The Math: A window size of 0 mathematically forces the Fixed Host into "Persist Mode". It instantly stops transmitting but keeps all its timers frozen. It completely bypasses Congestion Control.
  • The Reconnection: When the node reconnects, the SH violently opens the window. The Fixed Host instantly blasts data at maximum speed, suffering zero mathematical slow-start penalty.
Back to Paper