TSVWG T. Meng Internet-Draft H. Shi Intended status: Informational Huawei Technologies Expires: 25 April 2024 23 October 2023 Necessity of Application-Network Collaboration in Wireless Access Scenarios draft-meng-tsvwg-wireless-collaboration-00 Abstract Emerging applications (e.g., extended reality, cloud gaming, and teleoperation) impose stringent bandwidth, latency, reliability requirements on network transport, so as to deliver immersive and interactive user experience. That drives recent discussion on application-network collaboration, especially in wireless access networks. To motivate participation from content and network providers, this memo elaborates the necessity of such collaboration while focusing on wireless access scenarios. 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 25 April 2024. Copyright Notice Copyright (c) 2023 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 Meng & Shi Expires 25 April 2024 [Page 1] Internet-Draft App-Net Collaboration Necessity October 2023 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Importance of Wireless Access . . . . . . . . . . . . . . . . 3 4. Tradeoffs in Wireless Access Networks . . . . . . . . . . . . 3 4.1. Knobs to Compensate Wireless Losses . . . . . . . . . . . 3 4.2. Tradeoffs in Wireless QoS . . . . . . . . . . . . . . . . 4 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Collaboration May Not be a Zero-Sum Game . . . . . . . . 5 5.2. Why not Expose Wireless Losses to Transport Layer . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 8. Reference . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 8.2. Informative Reference . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Thanks to performant congestion control and over-the-top optimizations (e.g., jitter buffer), today's access network can support rich Internet applications mostly using a single pervasive QoS. Nevertheless, as emerging applications (e.g., extended reality, cloud gaming, and teleoperation) keep pursuing immersive and interactive user experience, the single pervasive network QoS starts to be the bottleneck to fulfill their stringent requirements on bandwidth, latency, and reliability at the same time [I-D.ietf-mops-ar-use-case], especially in wireless access networks. That drives discussion on application-network collaboration [RFC9419]. Many recent proposals contribute to use cases and solutions on how to accomplish collaborative signaling between application endpoint and in-network element [I-D.joras-sadcdn] [I-D.herbert-fast] [I-D.wing-cidfi] [I-D.kaippallimalil-tsvwg-media-hdr-wireless] [I-D.shi-quic-structured-connection-id]. To motivate participation from content and network providers, this memo elaborates the necessity of such collaboration while focusing on wireless access scenarios. Meng & Shi Expires 25 April 2024 [Page 2] Internet-Draft App-Net Collaboration Necessity October 2023 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Importance of Wireless Access Stringent latency requirements of emerging applications require edge deployment, making highly fluctuating wireless access links one of the main bottlenecks in user-facing last-mile transport. [Details will be added later.] 4. Tradeoffs in Wireless Access Networks 4.1. Knobs to Compensate Wireless Losses The most important characteristic that distinguishes wireless access networks from wired networks is the inherently unreliable communication media. L2 packet losses on a wireless access link must be much more common than on a wired link. To compensate wireless losses and achieve a practical low loss rate (e.g., below 1%) at transport layer and above, wireless networks can manipulate several knobs, as exemplified below. However, they inevitably come with tradeoffs. * Modulation and Coding Scheme (MCS): Adopting a lower-order modulation (e.g., 16QAM instead of 64QAM) and adding more redundancy (e.g., a lower FEC code rate) can increase wireless transmission reliability and contribute to more consistent latency. However, that impact wireless spectral efficiency, and degrade the bandwidth upper limit (i.e., a lower-order modulation transmits less bits per symbol). * L2 retransmission: This is the most popular way to cover up WiFi and cellular L2 losses. To some extent, it is the outcome of early-day TCP's inability to resist non-congestive losses. Although L2 retransmissions are critical for many congestion control algorithms to fully utilize available wireless bandwidth (some are even still quite popular nowadays, such as CUBIC and Prague), yet they cause high tail latency. Meng & Shi Expires 25 April 2024 [Page 3] Internet-Draft App-Net Collaboration Necessity October 2023 * L2 reordering: Wireless networks such as LTE and 5G also conduct packet reordering together with L2 retransmissions, for purpose of in-order delivery to transport layer. However, upon L2 packet loss, that also blocks subsequent received transport blocks in a low-layer reordering buffer, further deteriorating tail latency. 4.2. Tradeoffs in Wireless QoS Without collaborative signaling between application and network, network is expected to mostly provide a single pervasive transport service to heterogeneous data from possibly many different applications. Such a coarse-granularity QoS should be determined by the highest requirements on each performance indicators. For example, ultra-high bandwidth is needed to accommodate ultra-high definition virtual reality media, ultra-low latency is needed to realize close to real-time motion-to-photon gaming latency, and ultra-high reliability is needed to deliver remote teleoperation instructions. Nevertheless, a individual wireless QoS is bottlenecked by the unreliable communication media. Table 1 shows the corresponding configuration of the above knobs to guarantee an individual QoS indicator. +============================+===================================+ | QoS Objective | Knob Configurations | +============================+===================================+ | High bandwidth | High-order MCS | | | | | (high spectral efficiency) | | +----------------------------+-----------------------------------+ | Consistently low latency | Less or no L2 retransmission, no | | | reordering | +----------------------------+-----------------------------------+ | Low loss, high reliability | Low-order MCS, or | | | | | | L2 retransmission with reordering | +----------------------------+-----------------------------------+ Table 1: Configurations Corresponding to Individual QoS Indicator According to the table, there is Figure 1 showing the tradeoff relations between three QoS indicators. It is quite challenging, if not impossible, for wireless access networks to efficiently provide a pervasive QoS that fulfills ultra high bandwidth, ultra-low latency, and ultra-high reliability at the same time. Meng & Shi Expires 25 April 2024 [Page 4] Internet-Draft App-Net Collaboration Necessity October 2023 +----------------+ +-----| High Bandwidth |-------+ | +----------------+ | High-Order MCS | | High-Order MCS No L2 Retrans. | | L2 Retrans. w/ No Reordering | | L2 Reordering | | +------+------+ +-----+-----+ | Low Latency |-----------------| Low Loss | +-------------+ Low-Order MCS +-----------+ No L2 Retrans. No Reordering Figure 1: Tradeoffs between QoS Indicators The currently off-the-shelf LTE and 5G adopt high-order MCS for high spectral efficiency, and enable both L2 retransmission and reordering to guarantee very low transport-layer packet loss rate. Although contemporary real-time communication (RTC) applications such as video conferencing managed to scale with performant congestion control and over-the-top optimizations (e.g., jitter buffer), emerging immersive applications requiring ultra-low latency (e.g., below 50 ms) will be impeded by the inherent tail latency (e.g., could exceed 100 ms [TailLatency]). 5. Discussion 5.1. Collaboration May Not be a Zero-Sum Game Packet prioritization or stream/flow QoS multiplexing is necessary to handle the tradeoffs resulted from unreliable wireless communication media. That involves collaborative signaling between application and network. This memo notes that application-network collaboration is not necessarily a zero-sum game. That is the case when the wireless access network serves different flows only by tailored configurations of above knobs, without prioritized resource allocation. For example, a low latency flow without L2 retransmissions may not sacrifice the available bandwidth of another classic flow enabling both L2 retransmission and reordering, as long as they run on wireless bearers with fair time/frequency domain resources. 5.2. Why not Expose Wireless Losses to Transport Layer One may argue that recent transport protocols such as QUIC [RFC9000] can handle lost and out-of-order packets well, so that L2 retransmission and reordering can be disabled for all traffic to avoid explicit application-network collaboration. This memo validates those knobs as follows. Meng & Shi Expires 25 April 2024 [Page 5] Internet-Draft App-Net Collaboration Necessity October 2023 * Compared with end-to-end retransmission, local L2 retransmission is more efficient. The former adds at least one RTT that is tens of milliseconds, while the later only needs several milliseconds or even lower depending on radio technologies. Such a difference is crucial for ultra-low latency. * Exposing L2 wireless losses to endpoint confuses recognition of congestion. Out of spectral efficiency, cellular networks usually set the target L2 block error rate to 10% by default. An equivalent loss rate at transport layer, along with frequent out- of-order arrivals, can complicate congestion control, especially when considering the stringent application requirements. 6. Security Considerations Tba. 7. IANA Considerations This memo has no IANA actions. 8. Reference 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, . [RFC9419] Arkko, J., Hardie, T., Pauly, T., and M. Kühlewind, "Considerations on Application - Network Collaboration Using Path Signals", RFC 9419, DOI 10.17487/RFC9419, July 2023, . Meng & Shi Expires 25 April 2024 [Page 6] Internet-Draft App-Net Collaboration Necessity October 2023 [I-D.herbert-fast] Herbert, T., "Firewall and Service Tickets", Work in Progress, Internet-Draft, draft-herbert-fast-07, 7 October 2023, . [I-D.wing-cidfi] Wing, D., Reddy.K, T., and M. Boucadair, "CID Flow Indicator (CIDFI)", Work in Progress, Internet-Draft, draft-wing-cidfi-02, 12 September 2023, . [I-D.kaippallimalil-tsvwg-media-hdr-wireless] Kaippallimalil, J., Gundavelli, S., and S. Dawkins, "Media Header Extensions for Wireless Networks", Work in Progress, Internet-Draft, draft-kaippallimalil-tsvwg- media-hdr-wireless-03, 16 October 2023, . [I-D.shi-quic-structured-connection-id] Shi, H., "Structured Connection ID Carrying Metadata", Work in Progress, Internet-Draft, draft-shi-quic- structured-connection-id-01, 12 September 2023, . 8.2. Informative Reference [I-D.ietf-mops-ar-use-case] Krishna, R. and A. Rahman, "Media Operations Use Case for an Extended Reality Application on Edge Computing Infrastructure", Work in Progress, Internet-Draft, draft- ietf-mops-ar-use-case-13, 22 October 2023, . [I-D.joras-sadcdn] Joras, M., "Securing Ancillary Data for Communicating with Devices in the Network", Work in Progress, Internet-Draft, draft-joras-sadcdn-01, 10 July 2023, . [TailLatency] Meng, Z., Guo, Y., Sun, C., Wang, B., Sherry, J., Liu, H., and M. Xu, "Achieving Consistent Low Latency for Wireless Meng & Shi Expires 25 April 2024 [Page 7] Internet-Draft App-Net Collaboration Necessity October 2023 Real-Time Communications with the Shortest Control Loop", SIGCOMM 2022, DOI 10.1145/3544216.3544225, 2022, . Authors' Addresses Tong Meng Huawei Technologies China Email: mengtong1@huawei.com Hang Shi Huawei Technologies China Email: shihang9@huawei.com Meng & Shi Expires 25 April 2024 [Page 8]