TCP- and UDP-based applications

Problem: Evaluate TCP

Why does TCP perform poorly in ad-hoc/vehicular networks?

• Developed for wire-line networks.

• Assumes all losses are due to congestion.

Many TCP variants have been proposed.

• How good are they?

• Are they sufficient?

Are there any other alternatives?

• Are non-TCP protocols the solution?

Overview of TCP concepts

Conventional TCP: Tahoe, Reno, New-Reno.

Sending rate is controlled by:

• Congestion window (cwnd): limits the number of packets in flight.

• Slow-start threshold (ssthresh): when congestion avoidance starts.

Loss detection.

• 3 duplicate ACKs (faster, more efficient).

• The retransmission timer expires (slower, less, efficient).

Overview of congestion control mechanisms:

• Slow-start phase: cwnd starts from 1 and increases exponentially.

• Congestion avoidance (CA): cwnd increases linearly.

• Fast retransmit and fast recovery: Triggered by 3 duplicate ACKs.

Wireless networks and ad-hoc

1. Mobility

• Route stability and availability.

2. High bit error rate

• Packets can be lost due to “noise”.

3. Unpredictability/Variability

• Difficult to estimate time-out, RTT, bandwidth.

4. Contention: packets compete for airtime

• Intra-flow and inter-flow contentions.

5. Long connections have poor performance

• More than 4 hops throughput drops dramatically.

Why does TCP fail wireless and in ad-hoc networks?

TCP misinterprets route failures as congestion

• Effects: Reduce sending rate.

• Buffered packets (Data and ACKs) at intermediate nodes are dropped.

• Sender encounters timeouts.

  • Under prolonged disconnection, a series of timeouts may be encountered.

TCP misinterprets wireless errors as congestion

• Effects: Incorrect execution of congestion control -> performance drops.

• The wireless channel is error-prone compared to the wireline.

  • Fading, interference, noise.

A delay spike causes TCP to invoke unnecessary retransmissions

• Effects: Performance drops and many unnecessary retransmissions exist.

• Variability: Spikes are not uncommon here.

• Spikes throw off parameter estimation and tuning.

  • RTO, window size, slow-start threshold.

Inefficiency due to the loss of retransmitted packet

• Effects: performance drops significantly under a high-loss environment.

• Losing a retransmitted packet hurts.

  • TCP can recover from one loss (fast retransmission).

• Wired networks: packet loss rate is low.

• Here, high packet loss makes the problem significant.