Q13Mobile Computing
Question
What are the challenges faced by Mobile Transport Layer? Explain Snooping TCP.
Answer
Mobile Transport Layer faces high error rates, frequent disconnections, and varying delays. Snooping TCP buffers packets at the base station to retransmit locally without notifying the sender.
Traditional TCP assumes packet loss is due to network congestion and reduces its transmission window. In mobile networks, packet loss is often due to high bit error rates, fading, and frequent handoffs rather than congestion, causing standard TCP to unnecessarily throttle throughput even when the network is not actually overloaded.
- High bit error rates: Wireless links suffer far more corruption and loss than wired links, which TCP misinterprets as congestion.
- Frequent disconnections and handoffs: Movement between cells causes temporary breaks in connectivity, triggering retransmission timeouts and unnecessary slow-start.
- Variable bandwidth and latency: Wireless bandwidth fluctuates with signal quality, making TCP's congestion window estimates unstable.
- Asymmetric links: Uplink and downlink capacities often differ significantly, complicating ACK-clocked flow control.
- End-to-end semantics: Standard TCP is designed for stable wired paths, and any modification for wireless performance must avoid breaking compatibility with fixed-network TCP peers.
Snooping TCP addresses these issues without breaking end-to-end TCP semantics by introducing a 'snooping' agent at the base station, the point where the wired and wireless segments meet. The agent transparently buffers data packets sent to the mobile host and monitors the sequence numbers of acknowledgments (ACKs) flowing back from it. If it detects a missing ACK, or duplicate ACKs indicating a lost or corrupted packet on the wireless hop, it retransmits the buffered packet locally to the mobile node immediately, rather than waiting for the fixed-network sender to time out. Crucially, it also suppresses those duplicate ACKs so they are not forwarded to the sender, preventing the fixed host from mistakenly interpreting wireless loss as congestion and shrinking its congestion window. Because retransmission happens locally over the short wireless hop, recovery is much faster than an end-to-end retransmission, and because the fixed sender's TCP stack is untouched, no changes are required outside the base station and mobile host.