Enhancing TCP Performance Over Satellite Links: Strategies and Protocol Recommendations

1. IETF Working Group CSCI 344 Spring 1998 Presentation Beth Johnson TCP over Satellite (tcpsat) IETF WG Presentation

2. TCP over Satellite ( tcpsat)

3. General Description • GOAL - To create an informational RFC describing the issues affecting TCP throughput over satellite links. • ISSUES: • - domains for each issue • - network topology • - satellite orbit • LEO- ( Low Earth Orbit ) • MEO-( Medium Earth Orbit ) • GSO-( Geostationary Orbit ) • - link rates • - fixing protocol • - fixing implementation IETF WG Presentation

4. Scope of working group • Transport layer issues affecting TCP over Satellite • Existing TCP options • Compliant implementation with known improved performance over satellite links • Recommendations of well understood protocol changes • Identification of protocol changes that are potentially promising IETF WG Presentation

5. URL and Mailing List URLs: http://www.ieft.org/html.charters/tcpsat-charter.html http://tcpsat.lerc.nasa.gov/tcpsat/ Mailing List: General Discussion:tcp-over-satellite@listserv.trw.com To Subscribe: majordomo@listserve.trw.com In Body: subscribe tcp-over-satellite IETF WG Presentation

6. Enhancing TCP over Satellite Channels using Standard Mechanisms • Satellite channel characteristics have an effect on the way transport protocols (ex. TCP) behave. • When protocols such as TCP perform poorly, channel utilization is low. • Improving TCP in the satellite environment IETF WG Presentation

7. Satellite Characteristics • Delay in the delivery of a message over a satellite link due to finite speed of light and the altitude of communications satellites • Many communications satellites are located at GSO at an altitude of approximately 36,000 km. • -orbit period is the same as Earth’s • -ground station can always “see” • -one round-trip (RTT) would be about 558 ms • Other orbits of communications satellites at LEO and MEO • -use constellations of satellites for constant coverage • -propagation delay from several milliseconds to 80 ms IETF WG Presentation

8. Fundamental Characteristics • Noise: The strength of a radio signal falls in proportion to the square of the distance traveled. • Bandwidth: The radio spectrum is a limited natural resource. • - bandwidth available to satellite systems is limited IETF WG Presentation

9. Advantages of Satellite • Natural broadcast capability • Multicast applications • Reach geographically remote areas • Reach mobile users IETF WG Presentation

10. Disadvantages of Satellites • Long feedback loop • Large delay * bandwidth product (DBP) • - defines the amount of data a protocol should have “in flight” • Transmission errors • Asymmetric use • Variable round trip times • Intermittent connectivity ( in non-GSO) IETF WG Presentation

11. Two non-TCP Mechanisms • Path MTU Discovery: • - used to determine the maximum packet size a connection can use on a given network path without being subjected to IP packet fragmentation • - disadvantage is that it may cause a long pause before TCP is able to start sending data • Forward Error Correction (FEC): • - should be used to bring the performance of the link to at least fiber quality IETF WG Presentation

12. Standard TCP Mechanisms • Congestion Control algorithms: • - slow start - initializing the congestion window to one segment waits for corresponding acknowledgement (ACK) • - congestion avoidance • - fast retransmit • - fast recovery IETF WG Presentation

13. Recommendations for TCP over Satellite • Path-MTU Discovery • FEC • Fast Retransmit • Fast Recovery • TCP Large Window: • - window scaling • - PAWS - RTTM • TCP SACKS IETF WG Presentation

14. Requirements for TCP over Satellite • Slow Start • Congestion Avoidance IETF WG Presentation

15. Ongoing TCP Research Related to Satellites • Outlines mechanisms that may help the TCP better utilize the bandwidth provided by long-delay satellite environments IETF WG Presentation

16. Satellite Architectures • Asymmetric satellite networks • Satellite link as last hop • Hybrid satellite networks • Point-to-point satellite networks • Point-to-multipoint satellite networks • Multiple satellite hops IETF WG Presentation

17. Connection Setup • TCP uses a three-way handshake to setup a connection between two hosts • T/TCP bypasses the three-way handshake sends data and connection setup information • T/TCP requires changes in the TCP stacks of both the sender and receiver IETF WG Presentation

18. Slow Start • used to gradually increase the size of TCP’s sliding window • Larger Initial Window • - initial window be more than a single segment • - triggers more ACKs opening window more rapidly • - requires changes to the sender TCP stack • Byte Counting • - window increases based on the number of previously unacknowledged bytes ACKed, rather than on the number of ACKs received • - leads to slightly larger line-rate bursts of segments IETF WG Presentation

19. Slow Start cont. • -requires changes to the senders TCP stack • Terminating Slow Start • - use the packet-pai algorithm to determine a more appropriate value for sstresh • - requires changes to the senders TCP stack IETF WG Presentation

20. Spoofing • break a TCP connection between a client and a server into two parts: • - client and its gateway router over satellite/wireless link • - gateway router and server over Internet/wired link • gateway breaks incoming TCP connections in two by acting on clients behalf • allows the server to complete the transfer without delays of the satellite • allows the gateway to use more appropriate version of TCP over the satellite hop • requires modifications to the gateway routers IETF WG Presentation

21. Multiple Data Connections • Start transmission uses an effective window of N segments rather than a single segment • Transfer increases the window by Nsegments per RTT rather than one segment • Larger over all window size • Overall window decrease in the face of dropped segments is reduced when using N parallel connections • The use of multiple parallel connections in a shared network, such as the Internet may lead to congestive collapse IETF WG Presentation

22. ACK Spacing • Bursts can be spread over time by making a gateway separate ACKs by at least two segments between ACKs • Allow the sender to transmit at the correct rate and thus avoid dropped segments • Implemented at the router IETF WG Presentation

23. TCP Header Compression • Replaces the data in the TCP and IP headers that remains constant, changes slowly, or changes in a predictable manner • The sender first sends a full TCP header including in it a connection number that the sender will use to reference the connection • Receiver stores the full header and uses it as a template, filling in some fields from the limited information contained in later compressed headers • Requires changes at both the sender and receiving ends IETF WG Presentation

24. 42nd IETF Meeting Los Angles, CA March 30 - April 3 TCP over Satellite meets: Tuesday, March 31 9:00 - 11:15 IETF WG Presentation