Abstract

The Real-time Transport Protocol (RTP) is a common transport choice for interactive multimedia communication applications. The performance of these applications typically depends on a well-functioning congestion control algorithm. To ensure a seamless and robust user experience, a well-designed RTP-based congestion control algorithm should work well across all access network types. This document describes test cases for evaluating performances of candidate congestion control algorithms over cellular and Wi-Fi networks.¶

1. Introduction

Wireless networks (both cellular and Wi-Fi [IEEE802.11]) are an integral and increasingly more significant part of the Internet. Typical application scenarios for interactive multimedia communication over wireless include video conferencing calls in a bus or train as well as live media streaming at home. It is well known that the characteristics and technical challenges for supporting multimedia services over wireless are very different from those of providing the same service over a wired network. Although the basic test cases as defined in [RFC8867] have covered many common effects of network impairments for evaluating RTP-based congestion control schemes, they remain to be tested over characteristics and dynamics unique to a given wireless environment. For example, in cellular networks, the base station maintains individual queues per radio bearer per user hence it leads to a different nature of interactions between traffic flows of different users. This contrasts with a typical wired network setting where traffic flows from all users share the same queue at the bottleneck. Furthermore, user mobility patterns in a cellular network differ from those in a Wi-Fi network. Therefore, it is important to evaluate the performance of proposed candidate RTP-based congestion control solutions over cellular mobile networks and over Wi-Fi networks respectively.¶

[RFC8868] provides guidelines for evaluating candidate algorithms and recognizes the importance of testing over wireless access networks. However, it does not describe any specific test cases for performance evaluation of candidate algorithms. This document describes test cases specifically targeting cellular and Wi-Fi networks.¶

2. Cellular Network Specific Test Cases

A cellular environment is more complicated than its wireline counterpart since it seeks to provide services in the context of variable available bandwidth, location dependencies, and user mobilities at different speeds. In a cellular network, the user may reach the cell edge, which may lead to a significant number of retransmissions to deliver the data from the base station to the destination and vice versa. These radio links will often act as a bottleneck for the rest of the network and will eventually lead to excessive delays or packet drops. An efficient retransmission or link adaptation mechanism can reduce the packet loss probability, but some packet losses and delay variations will remain. Moreover, with increased cell load or handover to a congested cell, congestion in the transport network will become even worse. Besides, there exist certain characteristics that distinguish the cellular network from other wireless access networks such as Wi-Fi. In a cellular network:¶

• The bottleneck is often a shared link with relatively few users.¶

  • The cost per bit over the shared link varies over time and is different for different users.¶
  • Leftover/unused resources can be consumed by other greedy users.¶
• Queues are always per radio bearer, hence each user can have many such queues.¶
• Users can experience both inter- and intra-Radio Access Technology (RAT) handovers (see [HO-def-3GPP] for the definition of "handover").¶
• Handover between cells or change of serving cells (as described in [HO-LTE-3GPP] and [HO-UMTS-3GPP]) might cause user plane interruptions, which can lead to bursts of packet losses, delay, and/or jitter. The exact behavior depends on the type of radio bearer. Typically, the default best-effort bearers do not generate packet loss, instead, packets are queued up and transmitted once the handover is completed.¶
• The network part decides how much the user can transmit.¶

• The cellular network has variable link capacity per user.¶

  • It can vary as fast as a period of milliseconds.¶
  • It depends on many factors (such as distance, speed, interference, different flows).¶
  • It uses complex and smart link adaptation, which makes the link behavior ever more dynamic.¶
  • The scheduling priority depends on the estimated throughput.¶
• Both Quality of Service (QoS) and non-QoS radio bearers can be used.¶

Hence, a real-time communication application operating over a cellular network needs to cope with a shared bottleneck link and variable link capacity, events like handover, non-congestion-related loss, and abrupt changes in bandwidth (both short term and long term) due to handover, network load, and bad radio coverage. Even though 3GPP has defined QoS bearers [QoS-3GPP] to ensure high-quality user experience, it is still preferable for real-time applications to behave in an adaptive manner.¶

Different mobile operators deploy their own cellular networks with their own set of network functionalities and policies. Usually, a mobile operator network includes a range of radio access technologies such as 3G and 4G/LTE. Looking at the specifications of such radio technologies, it is evident that only the more recent radio technologies can support the high bandwidth requirements from real-time interactive video applications. Future real-time interactive applications will impose even greater demand on cellular network performance, which makes 4G (and beyond) radio technologies more suitable for such genre of application.¶

The key factors in defining test cases for cellular networks are:¶

• Shared and varying link capacity¶
• Mobility¶
• Handover¶

However, these factors are typically highly correlated in a cellular network. Therefore, instead of devising separate test cases for individual important events, we have divided the test cases into two categories. It should be noted that the goal of the following test cases is to evaluate the performance of candidate algorithms over the radio interface of the cellular network. Hence, it is assumed that the radio interface is the bottleneck link between the communicating peers and that the core network does not introduce any extra congestion along the path. Consequently, this document has left out of scope the combination of multiple access technologies involving both cellular and Wi-Fi users. In this latter case, the shared bottleneck is likely at the wired backhaul link. These test cases further assume a typical real-time telephony scenario where one real-time session consists of one voice stream and one video stream.¶

Even though it is possible to carry out tests over operational cellular networks (e.g., LTE/5G), and actually such tests are already available today, these tests cannot in general be carried out in a deterministic fashion to ensure repeatability. The main reason is that these networks are controlled by cellular operators, and there exists various amounts of competing traffic in the same cell(s). In practice, it is only in underground mines that one can carry out near deterministic testing. Even there, it is not guaranteed either as workers in the mines may carry with them their personal mobile phones. Furthermore, the underground mining setting may not reflect typical usage patterns in an urban setting. We, therefore, recommend that a cellular network simulator be used for the test cases defined in this document, for example -- the LTE simulator in [NS-3].¶

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++)))        +-------------------------->
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                      downlink

3. Wi-Fi Networks Specific Test Cases

Given the prevalence of Internet access links over Wi-Fi, it is important to evaluate candidate RTP-based congestion control solutions over test cases that include Wi-Fi access links. Such evaluations should highlight the inherently different characteristics of Wi-Fi networks in contrast to their wired counterparts:¶

• The wireless radio channel is subject to interference from nearby transmitters, multipath fading, and shadowing. These effects lead to fluctuations in the link throughput and sometimes an error-prone communication environment.¶
• Available network bandwidth is not only shared over the air between concurrent users but also between uplink and downlink traffic due to the half-duplex nature of the wireless transmission medium.¶
• Packet transmissions over Wi-Fi are susceptible to contentions and collisions over the air. Consequently, traffic load beyond a certain utilization level over a Wi-Fi network can introduce frequent collisions over the air and significant network overhead, as well as packet drops due to buffer overflow at the transmitters. This, in turn, leads to excessive delay, retransmissions, packet losses, and lower effective bandwidth for applications. Note further that the collision-induced delay and loss patterns are qualitatively different from those caused by congestion over a wired connection.¶
• The IEEE 802.11 standard (i.e., Wi-Fi) supports multi-rate transmission capabilities by dynamically choosing the most appropriate modulation and coding scheme (MCS) for the given received signal strength. A different choice in the MCS Index leads to different physical-layer (PHY-layer) link rates and consequently different application-layer throughput.¶
• The presence of legacy devices (e.g., ones operating only in IEEE 802.11b) at a much lower PHY-layer link rate can significantly slow down the rest of a modern Wi-Fi network. As discussed in [Heusse2003], the main reason for such anomaly is that it takes much longer to transmit the same packet over a slower link than over a faster link, thereby consuming a substantial portion of air time.¶
• Handover from one Wi-Fi Access Point (AP) to another may lead to excessive packet delays and losses during the process.¶
• IEEE 802.11e has introduced the Enhanced Distributed Channel Access (EDCA) mechanism to allow different traffic categories to contend for channel access using different random back-off parameters. This mechanism is a mandatory requirement for the Wi-Fi Multimedia (WMM) certification in Wi-Fi Alliance. It allows for prioritization of real-time application traffic such as voice and video over non-urgent data transmissions (e.g., file transfer).¶

In summary, the presence of Wi-Fi access links in different network topologies can exert different impacts on the network performance in terms of application-layer effective throughput, packet loss rate, and packet delivery delay. These, in turn, will influence the behavior of end-to-end real-time multimedia congestion control.¶

Unless otherwise mentioned, the test cases in this section choose the PHY- and MAC-layer parameters based on the IEEE 802.11n standard. Statistics collected from enterprise Wi-Fi networks show that the two dominant physical modes are 802.11n and 802.11ac, accounting for 41% and 58% of connected devices, respectively. As Wi-Fi standards evolve over time -- for instance, with the introduction of the emerging Wi-Fi 6 (based on IEEE 802.11ax) products -- the PHY- and MAC-layer test case specifications need to be updated accordingly to reflect such changes.¶

Typically, a Wi-Fi access network connects to a wired infrastructure. Either the wired or the Wi-Fi segment of the network can be the bottleneck. The following sections describe basic test cases for both scenarios separately. The same set of performance metrics as in [RFC8867]) should be collected for each test case.¶

We recommend carrying out the test cases as defined in this document using a simulator, such as [NS-2] or [NS-3]. When feasible, it is encouraged to perform testbed-based evaluations using Wi-Fi access points and endpoints running up-to-date IEEE 802.11 protocols, such as 802.11ac and the emerging Wi-Fi 6, so as to verify the viability of the candidate schemes.¶

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                              Downlink

6. References

Contributors

The following individuals contributed to the design, implementation, and verification of the proposed test cases during earlier stages of this work. They have helped to validate and substantially improve this specification.¶

Ingemar Johansson <ingemar.s.johansson@ericsson.com> of Ericsson AB contributed to the description and validation of cellular test cases during the earlier stage of this document.¶

Wei-Tian Tan <dtan2@cisco.com> of Cisco Systems designed and set up a Wi-Fi testbed for evaluating parallel video conferencing streams, based upon which proposed Wi-Fi test cases are described. He also recommended additional test cases to consider, such as the impact of EDCA/WMM usage.¶

Michael A. Ramalho <mar42@cornell.edu> of AcousticComms Consulting (previously at Cisco Systems) applied lessons from Cisco's internal experimentation to the draft versions of the document. He also worked on validating the proposed test cases in a virtual-machine-based lab setting.¶