| Network Working Group | Z. Sarker |
| Internet-Draft | I. Johansson |
| Intended status: Informational | Ericsson AB |
| Expires: January 6, 2020 | X. Zhu |
| J. Fu | |
| W. Tan | |
| M. Ramalho | |
| Cisco Systems | |
| July 5, 2019 |
Evaluation Test Cases for Interactive Real-Time Media over Wireless Networks
draft-ietf-rmcat-wireless-tests-08
The Real-time Transport Protocol (RTP) is a common transport choice for interactive multimedia communication applications. The performance of such applications typically depends on a well-functioning congestion control algorithm. To ensure 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 such congestion control algorithms over LTE and Wi-Fi networks.
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 January 6, 2020.
Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Wireless networks (both cellular and Wi-Fi [IEEE802.11]) are an integral part of the Internet. Mobile devices connected to the wireless networks account for an increasingly more significant portion of the media traffic over the Internet. Application scenarios range from video conferencing calls in a bus or train to media consumption by someone on a living room couch. 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. Even though basic test cases for evaluating RTP-based congestion control schemes as defined in [I-D.ietf-rmcat-eval-test] have covered many effects of the impairments common to both wired and wireless networks, there remain characteristics and dynamics unique to a given wireless environment. For example, in LTE 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 wired networks, where traffic flows from all users share the same queue. 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.
RMCAT evaluation criteria document [I-D.ietf-rmcat-eval-criteria] provides the guideline 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 networks such as LTE networks and Wi-Fi networks.
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.
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 amount of retransmissions to deliver the data from the base station to the destination and vice versa. These network links or 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 there will still be some packet losses and delay variations. Moreover, with increased cell load or handover to a congested cell, congestion in the transport network will become even worse. Besides, there are certain characteristics which make the cellular network different from and more challenging than other types of access networks such as Wi-Fi and wired network. In a cellular network --
Hence, a real-time communication application operating in such a cellular network needs to cope with a shared bottleneck link and variable link capacity, events like handover, non-congestion related loss, abrupt changes in bandwidth (both short term and long term) due to handover, network load and bad radio coverage. Even though 3GPP define QoS bearers [QoS-3GPP] to ensure high-quality user experience, adaptive real-time applications are desired.
Different mobile operators deploy their own cellular network with their own set of network functionalities and policies. Usually, a mobile operator network includes 2G, EDGE, 3G and 4G radio access technologies. Looking at the specifications of such radio technologies it is evident that only 3G and 4G radio technologies can support the high bandwidth requirements from real-time interactive video applications. The future real-time interactive application will impose even greater demand on cellular network performance which makes 4G (and beyond radio technologies) more suitable access technology for such genre of application.
The key factors to define test cases for cellular networks are
However, for cellular networks, it is very hard to separate such events from one another as these events are heavily related. Hence instead of devising separate test cases for all those important events, we have divided the test case 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 add any extra congestion in the path. Also, the combination of multiple access technologies such as one user has LTE connection and another has Wi-Fi connection is kept out of the scope of this document. However, later those additional scenarios can also be added in this list of test cases. While defining the test cases we assumed a typical real-time telephony scenario over cellular networks 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 LTE (and 5G) networks, and actually such tests are already available today, these tests cannot in the general case be carried out in a deterministic fashion or to ensure repeatability. The main reason is that these networks are in the control of cellular operators and there exist 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 an LTE network simulator is used for the test cases defined in this document, for example --- NS-3 LTE simulator [LTE-simulator].
The goal of this test is to evaluate the performance of the candidate congestion control algorithm under varying network load. The network load variation is created by adding and removing network users a.k.a. User Equipments (UEs) during the simulation. In this test case, each of the user/UE in the media session is an RMCAT compliant endpoint. The arrival of users follows a Poisson distribution proportional to the length of the call so as to keep the number of users per cell fairly constant during the evaluation period. At the beginning of the simulation, there should be enough time to warm-up the network. This is to avoid running the evaluation in an empty network where network nodes are having empty buffers, low interference at the beginning of the simulation. This network initialization period is therefore excluded from the evaluation period.
This test case also includes user mobility and some competing traffic. The latter includes both same kind of flows (with same adaptation algorithms) and different kind of flows (with different services and congestion control schemes). The investigated congestion control algorithms should show maximum possible network utilization and stability in terms of rate variations, lowest possible end to end frame latency, network latency and Packet Loss Rate (PLR) at different cell load level.
Each mobile user is connected to a fixed user. The connection between the mobile user and fixed user consists of an LTE radio access, an Evolved Packet Core (EPC) and an Internet connection. The mobile user is connected to the EPC using LTE radio access technology which is further connected to the Internet. The fixed user is connected to the Internet via wired connection with sufficiently high bandwidth, for instance, 10 Gbps, so that the system is resource-limited on the wireless interface. The wired connection to the Internet in this setup does not introduce any network impairments to the test; it only adds 10 ms of one-way propagation delay.
The path from the fixed user to the mobile users is defined as "Downlink" and the path from the mobile users to the fixed user is defined as "Uplink". We assume that only uplink or downlink is congested for mobile users. Hence, we recommend that the uplink and downlink simulations are run separately.
uplink
++))) +-------------------------->
++-+ ((o))
| | / \ +-------+ +------+ +---+
+--+ / \----+ +-----+ +----+ |
/ \ +-------+ +------+ +---+
UE BS EPC Internet fixed
<--------------------------+
downlink
Figure 1: Simulation Topology
The values enclosed within "[ ]" for the following simulation attributes follow the same notion as in [I-D.ietf-rmcat-eval-test]. The desired simulation setup is as follows --
The goal of this test is to evaluate the performance of candidate congestion control algorithm when users visit part of the network with bad radio coverage. The scenario is created by using a larger cell radius than that in the previous test case. In this test case, each of the user/UE in the media session is an RMCAT compliant endpoint. The arrival of users follows a Poisson distribution proportional to the length of the call, so as to keep the number of users per cell fairly constant during the evaluation period. At the beginning of the simulation, there should be enough amount of time to warm-up the network. This is to avoid running the evaluation in an empty network where network nodes are having empty buffers, low interference at the beginning of the simulation. This network initialization period is therefore excluded from the evaluation period.
This test case also includes user mobility and some competing traffic. The latter includes the same kind of flows (with same adaptation algorithms). The investigated congestion control algorithms should result in maximum possible network utilization and stability in terms of rate variations, lowest possible end to end frame latency, network latency and Packet Loss Rate (PLR) at different cell load levels.
Same as defined in Section 3.1.1
The desired simulation setup is the same as the Varying Network Load test case defined in Section 3.1 except the following changes:
RMCAT evaluation criteria document [I-D.ietf-rmcat-eval-criteria] defines the metrics to be used to evaluate candidate algorithms. However, looking at the nature and distinction of cellular networks we recommend that at least the following metrics be used to evaluate the performance of the candidate algorithms for the test cases defined in this document.
The desired metrics are --
Given the prevalence of Internet access links over Wi-Fi, it is important to evaluate candidate RMCAT congestion control solutions over test cases that include Wi-Fi access lines. Such evaluations should also highlight the inherently different characteristics of Wi-Fi networks in contrast to wired networks:
In summary, the presence of Wi-Fi access links in different network topologies can exert different impact on the network performance in terms of application-layer effective throughput, packet loss rate, and packet delivery delay. These, in turn, influence the behavior of end-to-end real-time multimedia congestion control.
Unless otherwise mentioned, test cases in this section are described using the underlying 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. As Wi-Fi standards evolve over time -- for instance, with the introduction of the emerging Wi-Fi 6 (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 could be the bottleneck. In the following sections, we describe basic test cases for both scenarios separately. The same set of performance metrics as in [I-D.ietf-rmcat-eval-test]) should be collected for each test case.
All test cases described below can be carried out using simulations, e.g. based on [ns-2] or [ns-3]. When feasible, it is also 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, to verify the viability of the candidate schemes.
The test scenarios below are intended to mimic the setup of video conferencing over Wi-Fi connections from the home. Typically, the Wi-Fi home network is not congested and the bottleneck is present over the wired home access link. Although it is expected that test evaluation results from this section are similar to those from test cases defined for wired networks (see [I-D.ietf-rmcat-eval-test]), it is still worthwhile to run through these tests as sanity checks.
Figure 2 shows the network topology of Wi-Fi test cases. The test contains multiple mobile nodes (MNs) connected to a common Wi-Fi access point (AP) and their corresponding wired clients on fixed nodes (FNs). Each connection carries either a RMCAT or a TCP traffic flow. Directions of the flows can be uplink, downlink, or bi-directional.
Uplink
+----------------->+
+------+ +------+
| MN_1 |)))) /=====| FN_1 |
+------+ )) // +------+
. )) // .
. )) // .
. )) // .
+------+ +----+ +-----+ +------+
| MN_N | ))))))) | | | |========| FN_N |
+------+ | | | | +------+
| AP |=========| FN0 |
+----------+ | | | | +----------+
| MN_tcp_1 | )))) | | | |======| MN_tcp_1 |
+----------+ +----+ +-----+ +----------+
. )) \\ .
. )) \\ .
. )) \\ .
+----------+ )) \\ +----------+
| MN_tcp_M |))) \=====| MN_tcp_M |
+----------+ +----------+
+<-----------------+
Downlink
Figure 2: Network topology for Wi-Fi test cases
These test cases assume that the wired portion along the media path is well-provisioned whereas the bottleneck exists over the Wi-Fi access network. This is to mimic the application scenarios typically encountered by users in an enterprise environment or at a coffee house.
Same as defined in Section 4.1.1
This section describes a few test scenarios that are deemed as important for understanding the behavior of a candidate RMCAT solution over a Wi-Fi network.
EDCA/WMM is prioritized QoS with four traffic classes (or Access Categories) with differing priorities. RMCAT flows should achieve better performance (i.e., lower delay, fewer packet losses) with EDCA/WMM enabled when competing against non-interactive background traffic (e.g., file transfers). When most of the traffic over Wi-Fi is dominated by media, however, turning on WMM may actually degrade performance since all media flows now attempt to access the wireless transmission medium more aggressively, thereby causing more frequent collisions and collision-induced losses. This is a topic worthy of further investigation.
When there exist 802.11b devices connected to a modern 802.11 network, they may affect the performance of the whole network. Additional test cases can be added to evaluate the impacts of legacy devices on the performance of the candidate congestion control algorithm.
This document defines a collection of test cases that are considered important for cellular and Wi-Fi networks. Moreover, this document also provides a framework for defining additional test cases over wireless cellular/Wi-Fi networks.
This memo includes no request to IANA.
The security considerations in [I-D.ietf-rmcat-eval-criteria] and the relevant congestion control algorithms apply. The principles for congestion control are described in [RFC2914], and in particular, any new method MUST implement safeguards to avoid congestion collapse of the Internet.
The evaluations of the test cases are intended to carry out in a controlled lab environment. Hence, the applications, simulators and network nodes ought to be well-behaved and should not impact the desired results. It is important to take appropriate caution to avoid leaking non-responsive traffic from unproven congestion avoidance techniques onto the open Internet.
The authors would like to thank Tomas Frankkila, Magnus Westerlund, Kristofer Sandlund, and Sergio Mena de la Cruz for their valuable input and review comments regarding this draft.
| [Heusse2003] | Heusse, M., Rousseau, F., Berger-Sabbatel, G. and A. Duda, "Performance anomaly of 802.11b", in Proc. 23th Annual Joint Conference of the IEEE Computer and Communications Societies, (INFOCOM'03), March 2003. |
| [I-D.ietf-rmcat-cc-requirements] | Jesup, R. and Z. Sarker, "Congestion Control Requirements for Interactive Real-Time Media", Internet-Draft draft-ietf-rmcat-cc-requirements-09, December 2014. |
| [I-D.ietf-rmcat-eval-test] | Sarker, Z., Singh, V., Zhu, X. and M. Ramalho, "Test Cases for Evaluating RMCAT Proposals", Internet-Draft draft-ietf-rmcat-eval-test-10, May 2019. |
| [IEEE802.11] | IEEE, "Standard for Information technology--Telecommunications and information exchange between systems Local and metropolitan area networks--Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", 2012. |
| [LTE-simulator] | "NS-3, A discrete-Event Network Simulator" |
| [ns-2] | "The Network Simulator - ns-2" |
| [ns-3] | "The Network Simulator - ns-3" |