Q19Mobile Computing

<h3>Challenges of TCP over Wireless</h3><p>Traditional TCP is optimized for wired networks where packet loss is almost always due to congestion. In wireless networks, packet loss is frequently caused by high bit-error rates, interference, or handoffs. When TCP detects packet loss, it erroneously assumes congestion, sharply reducing its transmission window, leading to severe performance degradation.</p>

<h3>Indirect TCP (I-TCP)</h3><ul><li><strong>Mechanism:</strong> Splits the TCP connection at the base station into two connections: wired (sender to BS) and wireless (BS to mobile host).</li><li><strong>Advantages:</strong> Shields the wired network from wireless errors. Standard TCP runs on the wired side, while an optimized protocol can run on the wireless side.</li><li><strong>Disadvantages:</strong> Violates end-to-end TCP semantics; if the BS crashes, the sender thinks data arrived because the BS acknowledged it. Handoff is complex because the state must be transferred to the new BS.</li></ul>

<h3>Snooping TCP</h3><ul><li><strong>Mechanism:</strong> The base station 'snoops' on passing packets and buffers them. If it detects a lost packet (via duplicate ACKs or timeout), it retransmits it locally to the mobile node and drops duplicate ACKs heading to the sender.</li><li><strong>Advantages:</strong> Maintains end-to-end TCP semantics. No changes required at the sender.</li><li><strong>Disadvantages:</strong> Cannot be used if end-to-end encryption (like IPsec) is employed because the BS cannot read the TCP headers to snoop.</li></ul>

<h3>Mobile TCP (M-TCP)</h3><ul><li><strong>Mechanism:</strong> Splits the connection similarly to I-TCP but maintains end-to-end semantics. It uses a Supervisory Host (SH) which does not send ACKs to the sender until the mobile host acknowledges receipt. If the mobile node disconnects, the SH sets the sender's window size to zero, putting the sender into persist mode.</li><li><strong>Advantages:</strong> Preserves end-to-end semantics and handles frequent, lengthy disconnections well.</li><li><strong>Disadvantages:</strong> Requires modifications to the network infrastructure (introducing Supervisory Hosts) and slightly more complex implementation.</li></ul>