Use of the SCPS Transport Layer to Improve Application Performance over Satellite and Wireless Systems

Back in the late 1990s there were considerable discussions of the performance issues associated with TCP based applications traversing satellite environments. One popular mechanism to address performance concerns is a called a Performance Enhancing Proxy (PEP), also known as a transport layer proxy or gateway. This technique splits the TCP connection into three separate connections:

1. standard TCP connection between the local application and the local PEP;
2. an advanced enhanced protocol (i.e., SCPS-TP) to transmit data across the satellite or otherwise challenged link;
3. standard TCP connection between the peer PEP and the peer application.

The inter-PEP protocol is tuned and optimized to the characteristics of the satellite link. The advantage of using this technique is the end systems and applications are not modified and any way but still receive the performance enhancing benefits of the PEP

These PEPs do not modify the application protocol, thus applications can operate end-to-end. In other words, these PEP techniques are totally transparent to the applications. Technically, these gateways perform a technique called spoofing in which they intercept the TCP connection in the middle and terminate that connection as if the gateway were the intended destination. A PEP, typically bracketing the satellite network, splits a single TCP connection into three separate connections. These gateways communicate via standard TCP when talking to each of the end-systems and a third connection using an optimized rate based protocol is used when transferring the data across the PEP connection between them.

This technique allows the properties of satellite networks that degrade performance to be isolated and avoids them from manifesting themselves to the end system's TCP. For example, corruption loss on the satellite network does not cause the transmission rate to be cut in half, and congestion loss on the terrestrial network does not cause data to be retransmitted and thus consume consuming satellite bandwidth which may be a scarce resource. This also allows the deployment of protocols that can be tuned to match the characteristics of the satellite link without affecting the end-systems. Therefore the default TCP parameters in the end-systems can be left unchanged, as they are suitable for the terrestrial environment in which they operate.