tsvwg Q. Xiong Internet-Draft ZTE Corporation Intended status: Informational J. Liu Expires: 12 December 2025 GEE Space 10 June 2025 Technical Considerations for Transport Layer Protocols Optimization for Satellite Networks (T4SAT) draft-xiong-tsvwg-technical-considerations-t4sat-00 Abstract This document analyses the gaps of the existing transport layer technologies and provides technical considerations for Transport Layer Protocols Optimization for Satellite Networks (T4SAT). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 12 December 2025. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Xiong & Liu Expires 12 December 2025 [Page 1] Internet-Draft Technical Considerations for T4SAT June 2025 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 2 3. Technical Considerations for T4SAT . . . . . . . . . . . . . 2 3.1. Optimization of Congestion Control Algorithms . . . . . . 3 3.2. Enhancement of Congestion Notification . . . . . . . . . 3 3.3. Adaptations of Transport Protocols . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Informative References . . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction It demands high-bandwidth and high-throughput data transmission in LEO (Low Earth Orbit) satellite networks as described in [I-D.yang-tsvwg-leo-transport-req]. The performance of transport protocols will be impacted by the unique characteristics such as networks-highly dynamic topologies, long and variable propagation delays and time-varying channel errors. The optimization of transport layer technologies in LEO satellite networks should consider: *the congestion control algorithms should be optimized to adapt the increasing bursty packet loss and significant delay variability. *the congestion control should decouple channel errors from congestion signals while adapting to rapid topology changes. *the ACK mechanism should adapt the transport layer to reduce spurious retransmissions. This document analyses the gaps of the existing transport layer technologies and provides technical considerations for Transport Layer Protocols Optimization for Satellite Networks (T4SAT). 2. Definition of Terms This document uses the terms defined in [I-D.yang-tsvwg-leo-transport-req]: 3. Technical Considerations for T4SAT Xiong & Liu Expires 12 December 2025 [Page 2] Internet-Draft Technical Considerations for T4SAT June 2025 3.1. Optimization of Congestion Control Algorithms As per [I-D.yang-tsvwg-leo-transport-req], frequent handovers and connection switching will increase packet loss ratio (e.g. over 60%). The congestion control algorithms should be selected and optimized to tolerate the high packet loss ratio and predict bandwidth to control the congestion before the queues overflow. Bottleneck Bandwidth and Round-trip propagation time (BBR) is a congestion-based congestion control algorithm for TCP, which actively measures bottleneck bandwidth (BtlBw) and round-trip propagation time (RTprop) based on the model to calculate the bandwidth delay product (BDP) and then to adjust the transmission rate to maximize throughput and minimize latency. In LEO satellite networks, the dynamic topology could add significant delay variability beyond the inherent propagation latency and it also consumes valuable satellite capacity. The congestion control algorithms should be optimized to adapt seamlessly to latency variations and RTT fluctuation based on precise RTT measurements. 3.2. Enhancement of Congestion Notification As per [I-D.yang-tsvwg-leo-transport-req], random packet loss will lead to the failure of congestion control mechanisms. For example, when receiving a congestion notification with packet loss, the loss- based congestion control algorithms, such as Congestion-based Upon Bandwidth-Information (CUBIC), will reduce the congestion window and the throughput will dramatically decrease. In LEO satellite networks, the packet loss will be divided into channel-error loss and congestion-based loss. The time-varying channel-error loss will occur when packets are corrupted or dropped due to physical layer impairments in the communication channel. It needs to retransmit the lost packets without reducing congestion window. The congestion notification should distinguish channel-error loss and congestion-based loss. Explicit Congestion Notification (ECN) can be used to achieve an end-to-end congestion notification and the type of packet loss could be encoding in the ECN markings. 3.3. Adaptations of Transport Protocols As per [I-D.yang-tsvwg-leo-transport-req], long and variable propagation delay is one of unique characteristics in LEO satellite networks. The propagation delays (e.g.20-150ms RTT) are higher exceeding the terrestrial networks. The Retransmission Timeout (RTO) mechanism will frequently misinterpret propagation delay as congestion. The acknowledgments (ACKs) are not arriving on time, leading to unnecessary congestion window reduction and packet Xiong & Liu Expires 12 December 2025 [Page 3] Internet-Draft Technical Considerations for T4SAT June 2025 retransmissions. Moreover, asymmetric uplink and downlink capacities cause ACK congestion and buffer overflows. The ACK mechanism in LEO satellite networks should adapt the transport layer to give ACKs higher priority during handovers and guarantee ACK delivery even during orbital transitions, reducing spurious retransmissions caused by orbital motion. 4. Security Considerations To be discussed in future versions of this document. 5. IANA Considerations Currently this document does not make an IANA requests. 6. Informative References [I-D.yang-tsvwg-leo-transport-req] Yang, F. and T. Tsou, "Transport Layer Protocol Requirement for LEO satellite", Work in Progress, Internet-Draft, draft-yang-tsvwg-leo-transport-req-00, 16 March 2025, . Authors' Addresses Quan Xiong ZTE Corporation Email: xiong.quan@zte.com.cn Jie Liu GEE Space Email: jie.liu33@geely.com Xiong & Liu Expires 12 December 2025 [Page 4]