TCP Issues in Wired Networks

TCP is a connection-oriented transport control protocol that offers the essential congestion control required to ensure reliable packet delivery. TCP was specifically designed to operate in wired network. The error rate in wired network is relatively low. TCP assumes that packet loss occurs only due to the network congestion and hence, makes corresponding transmission rate adjustment to its congestion window. TCP congestion window size has a great impact on network performance. There is an optimal value of the TCP congestion window size for a given network topology and traffic pattern at which the channel can be used at the maximum rate. Generally, TCP does not work around this optimal value and hence, the selection of optimal point is a difficult task. This results in decreased throughput and increased packet loss. Larger the congestion window size, the number of packets in the channel will also be larger and hence, the contention at the channel increases. Small congestion window comparatively offer the best performance.

Drawback of TCP

 Congestion control mechanism enforced by the TCP fails to predict the network condition at the earlier stage leads to the large queue build-up. Reaction to the congestion is happened only after the packet drop retransmission of which leads to increased delay. This kind of TCP mechanism affects the QoS of network that handles the multimedia traffic.

Explicit Congestion Control (XCC) protocols

Explicit Congestion Control (XCC) protocols overcome the drawback of TCP. In XCP, routers update status of the network capacity to source as a feedback. Source adjusts its sending rate according to the information in feedback. Network is enriched with stable throughput, and full utilization using XCP.

NS2 Solution:

  • Network traffic is varied to create congestion and to analyze the consequences.

  • Inbuilt variable for Congestion window size can be assigned for the TCP object of the sender node in tcl script and that can be accessed periodically to check its size variation according to the network condition.

  • Congestion control mechanism of the TCP is evaluated for the different traffic rate.

  • XCP object is associated with source and the throughput is analyzed for the varying traffic conditions.

  • XCP performs better than TCP during congestion for the multimedia traffic.

Related Titles:

  • Filipe Abrantes, Joa˜o Taveira Arau´ jo and Manuel Ricardo, “Explicit Congestion Control Algorithms for Time Varying Capacity Media” IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 10, NO. 1, JANUARY 2011.