3. PEP TCP¶
In the two first sections of this part, we have seen the problems of TCP over a satellite link and some improvements involving only the end hosts. Now we will see that we can use TCP enhancements that envisage intermediate agents on the path, usually called Performance Enhancing Proxies (PEPs).
Among the PEP proposals, the most promising solution is represented by the splitting approach. The concept of this solution is to separate the satellite link from the rest of the network.
To this end, two alternative architectures can be adopted, namely integrated and distributed PEP. In the first case, the TCP connection established among the end hosts is split in two separated connections, with a single PEP agent in the middle. The first connection makes use of the TCP standard and is terminated on the PEP. The second connection, between PEP and the final user, can exploit an enhanced TCP version compatible with a standard TCP receiver. By contrast, the distributed architecture splits the connection in three sections, by isolating the satellite link between two PEP agents. In this case, it is possible to adopt a proper transport protocol on the satellite link while the end hosts continue to adopt the TCP standard protocol.
Integrated PEPs based on TCP splitting offer higher performance because they are able to confine the satellite impairments (long RTTs and random losses) on the second component of the split connection, where they can be counteracted by specific optimized TCPs.
Despite the good performance provided, there are some important issues related to the splitting approach. First, splitting violates the basic end to end TCP concept : the sender receives acknowledgments from an intermediate agent instead of the final receiver. Second, the intermediate agent needs to access the TCP header of the packets in order to send ACKs back to the sender and perform all its optimization procedures. This prevents the use of IPSEC technique which encrypts the IP payload making the TCP header not available. Thus, an IPSEC flow can not be managed by splitting PEP.