space transport protocols

VSAT系统对TCP的改进

https://www.vsat-systems.com/broadband-satellite-internet/index.html

TCP/IP over satellite, some background

TCP/IP is the "language" of the Internet. TCP functions by sending packets of data, and then waits for acknowledgments of receipt. These acknowledgments signal the sender to transmit more data. When acknowledgments return slowly, TCP slows the speed at which data is sent. This is done to prevent network overloading as TCP assumes it is already congested.

TCP works by starting a TCP/IP session slowly. Speed builds as the networks' capacity to carry traffic is verified by the rate at which acknowledgements are received. In Internet terminology, this effect is known as "slow-start".

TCP was designed for terrestrial networks where packets of data travel shorter distances and not all the way to the geostationary orbit. Larger distances mean greater latency (600 ms or more for satellite links). Thus causes TCP to expect an acknowledgement before the round trip to the remote site is completed. As TCP was originally designed for low-latency terrestrial networks, it does not understand that a satellite is involved. The resulting network operates as if the satellite latency was caused by congestion. If uncorrected, this effect causes all packets over a satellite network to be sent at the slow-start rate.

TCP/IP Spoofing，TCP欺骗，适用于VSAT这种不需要卫星作为端节点的场景；若卫星需要作为端节点，则不适用。

VSAT Systems employs TCP/IP acceleration also known as TCP Spoofing. This technique compensates for the space-link transit time using state-of-the-art routers, switches and protocol processors which are fine tuned for satellite applications. This equipment appears to TCP as if it were the remote location, while acting as a relay or forwarder for data packets going to and from the remote satellite location. When the spoofing equipment receives Internet traffic destined for a remote satellite location, it acknowledges receipt of the packet immediately on behalf of the remote site. This mitigates the slow-start effects and data packets begin to follow immediately.

In this manner, the latency is "hidden" because the acknowledgments are returned rapidly. As a result, TCP moves out of slow-start mode quickly and builds speed to reach maximum levels.

The VSAT Systems acceleration equipment also watches for real acknowledgements coming back from the remote site and suppresses them. If the acknowledgement is not received from the remote site, the system automatically re-sends the packet from its buffer. Thus, our satellite-connected sites communicate seamlessly with servers on the terrestrial Internet.