Jörg Deutschmann

The TCP performance over satellite communications has become a well-known problem, following significant experimentation with Internet services over satellite since the '90s. Several tailored TCP optimisations have been introduced (mainly implementing changes at the sender side, but also at the receiver side in some proposals). In parallel, given the challenge of installing tailored TCP versions directly in the end user system, a set of architectural extensions have been introduced culminating in the concept of a Performance Enhancing Proxy (PEP, RFC 3135), whereby a native end-to-end TCP connection is now commonly split into a series of multiple connection (a split TCP concept). This allows a tailored TCP to be deployed on the satellite link (i.e., between the satellite terminals and gateways to be optimised). Though largely used since the early 2000's, PEPs have always been unable to enhance non-TCP protocols or VPN connections traversing the satellite network segment. Application-layer compression and acceleration was also provided in some PEPs.

Since 2000, there has been a continued effort to evolve the protocol stack for Internet web services, with several updates to the protocols for HTTP-based services. A design of HTTP by Google, known as SDPY, was standardised as HTTP/2. This provided significant improvements in download speed of satellite, but at the same time deployed application-layer encryption and compression – making application-layer acceleration dependent on using an authenticated proxy and impossible within a PEP.

A more recent Google proposal (known as gQUIC) sought a transport other than TCP that uses a UDP substrate with transport encryption. This effort evolved in standardisation by the Internet Engineering Task Force (IETF) and was finally published as IETF QUIC (RFC 9000) in 2021. QUIC is specified for use with HTTP/3, a replacement for HTTP2/TCP. The main leap from classical HTTP services over TCP is in that QUIC uses encrypted datagram connections, with congestion control, flow control, NAT-rebinding and migration algorithms directly implemented within the QUIC protocol. Following standardisation, QUIC and HTTP/3 have been implemented and have been rapidly deployed to the Internet.

Hence, the design rationale of QUIC intrinsically prevents using a classical PEP solution for the optimisation of performance over a satellite system.  Whilst the application-layer performance of HTTP/3 resembles or improves on that of HTTP/2, and the transport design has been shown to operate correctly over satellite with respect to initialisation, protocol timers, and other core functions, experiments have shown that performance of QUIC operated end-to-end over paths comprising a satellite network segment can be lower than offered by TCP using a PEP. This has motivated the scientific community and the satellite industry to think of alternative solutions for QUIC congestion control (CC) to accelerate with the QUIC performance degradation, which is still now at the early stages. QUIC has also been suggested for other applications.

The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt), University of Aberdeen, and Friedrich-Alexander-Universität Erlangen-Nürnberg have built a consortium that is committed to thoroughly analyse the existing approaches and options to improve the performance of TCP over satellite network segment and apply the most appropriate concepts to QUIC congestion control mechanisms as well as understanding the implications of deploying the new approaches as a part of a secure end-to-end architecture. As a result, a novel algorithm will be defined and then verified against the relevant technical requirements. Finally, the resulting new QUIC specifications will be validated using real satellite trials in exemplar scenarios.

In the QUICSAT project, the cooperation between the Friedrich-Alexander University (FAU) Erlangen-Nürnberg and ND SatCom GmbH has the common goal of improving Internet protocols and applications for geostationary satellite connections.

The potential of new technologies (AQM, ECN, BBR and especially QUIC) will be examined. The ultimate goal is that Internet via satellite should perform as good as terrestrial Internet connections.

The high latency of geostationary satellites, the current architecture of Internet protocols and the constantly increasing complexity of Internet applications (especially websites) are the reason why the performance of Internet via satellite is sometimes worse than the performance of terrestrial Internet connections, even if the data rates are comparable. Newer Quality of Service (QoS) mechanisms are currently not used in satellite communication. With QUIC there is also the risk that the performance of satellite internet will decrease due to the non-applicability of Performance Enhancing Proxies.

The project makes a contribution to protocol research, standardization and reference implementations.

This work evaluates the performance of different applications over different Internet access technologies, with focus on Internet access via satellite.

The following Internet access technologies have been selected:

• Geostationary satellites (Konnect/Eutelsat, skyDSL/Eutelsat, Bigblu/Eutelsat, Novostream/Astra Connect)

• Satellite megaconstellations in low Earth orbit (Starlink)

• Terrestrial systems as reference (o2 DSL, Congstar LTE)

Satellite communication is a way to provide broadband internet access all over the world. However, with geostationary satellites the propagation delay leads to very high delays in the magnitude of several hundred milliseconds. In order to improve the interactivity and responsiveness of communication systems, utilizing a second communication link can be highly beneficial.

The Transparent Multichannel IPv6 (TMC-IPv6) Project aims to combine the advantages of multiple heterogeneous communication links. An illustrative example is the combination of a rural DSL connection with low data rate/low latency and a satellite connection with high data rate but high latency, which results in a user’s internet access with high data rate and low latency providing a better Quality of Experience (QoE).

Satellite-based internet access from different operators is provided by our project partners in order to experience realistic satellite communication environment and test potential solutions. The outdoor unit (parabolic antenna) is mounted on the roof of the Wolfgang-Händler-Hochhaus.