TSVWG F. Yang
Internet Draft China Mobile
Intended status: Informational T. Tsou
Expires: September 16, 2025 Tiktok
March 16, 2025
Transport Layer Protocol Requirement for LEO satellite
draft-yang-tsvwg-leo-transport-req-00.txt
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Abstract
In recent years, high-bandwidth LEO (Low Earth Orbit) satellite
networks, such as Starlink and OneWeb, have seen tremendous
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development and are gradually becoming an important part of the
global Internet. However, due to the unique characteristics of
satellite networks, using TCP for data transmission faces challenges
in multiple aspects, such as high latency caused by long-distance
propagation and high error rates due to signal attenuation. This
proposal analyzes the typical characteristics of LEO satellite
transmission and their impact on transport layer protocols, and
finally summarizes the basic requirements that need to be considered
for designing transport layer protocols tailored to LEO satellites.
Table of Contents
1. Introduction .................................................. 2
2. Conventions used in this document ............................. 3
3. Problem Statement ............................................. 3
4. Requirement for transport layer protocol ...................... 5
5. Security Considerations ....................................... 7
6. References .................................................... 7
6.1. Normative References ..................................... 7
6.2. Informative References ................................... 7
7. Acknowledgments ............................................... 8
1. Introduction
In recent years, high-bandwidth LEO (Low Earth Orbit) satellite
networks, such as Starlink and OneWeb, have achieved significant
development and are gradually becoming an important part of the
global Internet. However, the LEO satellite networks have unique
characteristics, which impacts TCP for data transmission.
o Frequent Handovers and Connection Switching: LEO satellites orbit
the Earth at high speeds (approximately 7.8 km/s), leading to
frequent handovers between ground terminals and satellites, as
well as between satellites themselves (every few to tens of
minutes). This can cause routing fluctuations, packet loss or
reordering, and failure of congestion control mechanisms.
o Signal Quality and Random Packet Loss: Signal quality is affected
by the distance between satellites and the ground, inter-
satellite links, Doppler shifts, and weather conditions. These
factors increase the error rate, leading to random packet loss
and triggering congestion window reduction, which in turn reduces
throughput.
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o Solar interference and Packet Loss: Periodic sun interference can
interrupt signals between satellites, causing packet loss and
triggering TCP timeout retransmissions. This further leads to
congestion window reduction and impacts throughput.
Currently, there are many standardization efforts targeting
satellite networks, mostly focusing on routing. For example, TVR
addresses dynamic routing, and TIPTOP focuses on QUIC deployment for
deep-space satellites. However, there is still a lack of
standardization work targeting the transport layer for LEO satellite
networks.
2. Conventions used in this document
LEO: Low Earth Orbit
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Problem Statement
The unique characteristics of LEO networks-highly dynamic
topologies, long and variable propagation delays, time-varying
channel errors pose fundamental challenges to conventional transport
layer protocols like TCP. These protocols, designed for terrestrial
networks, fail to adapt to the space environment, leading to
degraded throughput, increased latency, and unreliable connectivity.
Study in [2] by Netflix analyzed on-demand video streaming over LEO
with one million LEO households across 85 countries for over two
years. Starlink users experience overall perceptual video quality
similar to non-Starlink networks, they are more likely to experience
bitrate switches and network rebuffers due to Starlink's network
conditions. About half of sessions with a bitrate switch experience
a resolution of less than 1080p. Video rebuffers are significantly
more likely to occur over Starlink than a Top 10 ISP, and 40% more
likely to occur relative to any non-Starlink network. It also finds
that over 95% of Starlink's throughput is lower than alternative
networks, which is nearly always 50% of what a top 10 ISP offers and
falls below 20 Mb/s. This eventually leads to bitrate switching and
rebuffer, which contributes negatively to overall quality of
experience over Starlink.
Study in [3] investigated the performance of TCP and UDP in the
Starlink network. TCP vs. UDP downlink test results consistently
reveal that UDP outperforms TCP in satellite networks, with the mean
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throughput being 128 Mbps and 29 Mbps, respectively. And the root
cause is there is a much higher occurrence of packet loss in both
the uplink and downlink directions in Starlink network. Based on
this finding, the author verified that TCP parallelism can optimize
bandwidth utilization by distributing data across multiple
connections, thereby mitigating the impact of TCP congestion control.
Study in [4] reveals that the endless and bursty packet losses over
unstable LEO satellite links impose significant challenges on
guaranteeing the quality of experience (QoE) of Web applications.
They found that in the Starlink network environment, both page load
time and speed index are much higher than those in the wired network.
Specifically, the 60th/80th/90th percentile page load time in
Starlink are about 245.0%/202.4%/136.3% higher than those in Optimum.
Similarly, the 60th/80th/90th percentile speed index in Starlink is
about 185.3%/241.6%/219.3% higher.
Various optimizations at the transport protocol level have been
proposed, such as end-to-end optimizations like SCPS-TP and MP-TCP,
redundancy coding like FEC, cross-layer optimizations like ECN, and
congestion control algorithm optimizations like TCP Westwood and TCP
Eifel, as well as AI-enhanced congestion control. This gives us a
more indication on what problem we should focused on.
o Bursty Packet Losses: bursty packet losses are a common
phenomenon in LEO satellite networks due to various factors such
as signal blockage from obstacles, rapid movement of satellites,
and interference from other signals. These bursty losses can
severely disrupt data transmission, leading to retransmission
delays and increased latency. Traditional error correction and
recovery mechanisms may not be effective in handling such bursty
losses, requiring the development of more robust and adaptive
techniques.
o Variable Round Trip Times: the dynamic nature of LEO satellite
networks results in highly variable Round Trip Times (RTTs). As
satellites move rapidly across the sky, the distance between the
sender and receiver changes constantly, causing fluctuations in
signal propagation delay. Additionally, network congestion and
routing changes can further exacerbate RTT variations. These
unpredictable RTTs can lead to inefficient resource utilization
and performance degradation in transmission protocols that rely
on accurate timing and synchronization.
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o Large and Variable Delay-Bandwidth Products: the combination of
high latency and variable bandwidth in LEO satellite networks
creates large and variable delay-bandwidth products. This makes
it challenging to maintain efficient data flow and congestion
control. Transmission protocols need to adapt to these varying
conditions to optimize throughput and avoid network congestion
while ensuring fair resource allocation among multiple users and
applications.
o High Bit Error Rates: the wireless communication environment of
LEO satellites is prone to high bit error rates (BER) due to
factors like signal attenuation, interference, and noise. These
errors can corrupt data packets, leading to retransmissions and
reduced overall network efficiency. Effective error detection and
correction mechanisms are essential to mitigate the impact of
high BER in LEO satellite transmission protocols.
o TCP Fairness: there will be competing connections with different
RTTs. The connections with high RTTs cannot allocate enough
bandwidth. Traditional TCP protocols may not perform well in such
environments due to their inherent assumptions about network
conditions. Developing fair and efficient resource allocation
mechanisms that consider the unique characteristics of LEO
satellite networks is necessary to prevent certain users or
applications from dominating network resources.
o Security: security is a concern in LEO satellite networks, as
they are vulnerable to various attacks such as jamming,
eavesdropping. Transmission protocols must incorporate robust
security measures to protect data integrity, confidentiality, and
availability. This includes encryption, authentication, and
intrusion detection mechanisms tailored to the specific
challenges of LEO satellite environments.
4. Requirement for transport layer protocol
Given the unique characteristics of LEO satellite networks, the
requirements for transport layer protocols should focus on improving
the following aspects:
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o Fast Connection Establishment. In satellite networks, fast
connection establishment is crucial for improving communication
efficiency. For short-lived communication needs, such as bursty
data transmission from mobile users, a slow connection setup
process can waste significant time. TCP needs to optimize its
connection establishment process by adopting fast handshake
algorithms and other techniques to reduce the time required for
connection setup and enhance the responsiveness of satellite
networks.
o High-Bandwidth Support. High-performance transport protocols aim
to support the rapid transmission of large volumes of data, such
as high-definition video streaming and big data file transfers.
When sufficient bandwidth capacity is available in the satellite
link, the protocol should provide high-bandwidth transmission
capabilities to meet the demands of these applications.
o Adaptation to Latency Variations. The latency in satellite
networks may fluctuate due to factors such as satellite handovers
and signal interference, causing RTT (Round-Trip Time) jitter.
Transport protocols need to quickly adapt to this latency jitter
to avoid performance degradation resulting from congestion window
changes triggered by the jitter.
o Error Resistance. Data packet loss in satellite channels can be
caused by obstructions, atmospheric interference, and spatial
distance. More efficient packet loss detection and recovery
mechanisms are needed. For example, redundancy-based encoding
methods can be introduced, where the sender encodes data with
redundancy, and the receiver can use this redundant information
to recover lost packets, reducing the need for retransmissions
and enhancing the reliability of data transmission.
o Resilience to Network Interruptions. Short-term interruptions in
satellite links can easily occur due to sun outages or satellite
coverage limitations. During such interruptions, the transport
protocol should reduce the sending rate to minimize unnecessary
packet transmission. Once the interruption is resolved, the
protocol should quickly restore the bandwidth to ensure efficient
data transfer.
o Reduced Retransmission Rate. Retransmissions increase
transmission delay and consume bandwidth, which are particularly
valuable resources in satellite networks. Transport protocols
need to minimize unnecessary retransmissions through accurate
packet loss detection and effective error recovery mechanisms,
thereby optimizing the overall efficiency of data transmission.
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o Improved Out-of-Order Handling. The complex transmission
environment in satellite networks can cause packets to arrive out
of order at the receiver due to different routing paths and
varying transmission delays. Transport protocols must be capable
of accurately reordering out-of-order packets to ensure the
correct delivery of data.
5. Security Considerations
None.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[2] Liz Izhikevich, Reese Enghardt, Te-Yuan Huang, and Renata
Teixeira. 2024. A Global Perspective on the Past, Present, and
Future of Video Streaming over Starlink. Proc. ACM Meas. Anal.
Comput. Syst. 8, 3, Article 30 (December 2024), 22 pages.
https://doi.org/10.1145/3700412
[3] Bin Hu, Xumiao Zhang, Qixin Zhang, Nitin Varyani, Z. Morley
Mao, Feng Qian, and Zhi-Li Zhang. 2023. LEO Satellite vs.
Cellular Networks: Exploring the Potential for Synergistic
Integration. In Companion of the 19th International Conference
on emerging Networking EXperiments and Technologies (CoNEXT
2023). Association for Computing Machinery, New York, NY, USA,
45-51. https://doi.org/10.1145/3624354.3630588
[4] Jihao Li, Hewu Li, Zeqi Lai, Qian Wu, Yijie Liu, Qi Zhang,
Yuanjie Li, and Jun Liu. 2024. SatGuard: Concealing Endless
and Bursty Packet Losses in LEO Satellite Networks for Delay-
Sensitive Web Applications. In Proceedings of the ACM Web
Conference 2024 (WWW '24). Association for Computing Machinery,
New York, NY, USA, 3053-3063.
https://doi.org/10.1145/3589334.3645639
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7. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Feng Yang
China Mobile
Email: yangfeng@chinamobile.com
Tina Tsou
Tiktok
Email: tina.tsou@tiktok.com
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