Internet Draft QoS Signalling Requirements for Wireless November 2001 Internet Engineering Task Force G Karagiannis INTERNET-DRAFT D. Partain Expires October 2001 V. Rexhepi L. Westberg Ericsson November 2001 QoS Signalling Requirements for Wireless Networks draft-karagiannis-nsis-wireless-requirements-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract Real-time applications impose very strict quality of service (QoS) requirements on the underlying transmission network. This level of Karagiannis, et al. Expires May 2002 [Page 1] Internet Draft QoS Signalling Requirements for Wireless November 2001 QoS can only be achieved by means of QoS management on an end-to-end basis (i.e., end user to end user), from application to application, as well as potentially across many domains since the Internet is a concatenation of many domains, usually managed by different QoS administrators (operators). The requirement for end-to-end QoS management does not necessarily mean that a single resource reservation signalling protocol must be applied end-to-end. In fact, it is most likely that the end-to-end QoS management architecture will consist of many interoperable and concatenated QoS management architectures rather than one global end- to-end QoS infrastructure. This is also true in wireless networks, which usually consist of three network parts: a wireless interface part, and a wired part of the wireless network, and the backbone of the wireless network. Each of these parts in the wireless network imposes different requirements on the QoS signalling solution. This memo outlines a set of requirements for signalling QoS needs in the wired part of IP-based wireless networks. These requirements are applicable to any IP-based wireless network that supports a large volume of real-time traffic (mixed with best effort traffic). Karagiannis, et al. Expires May 2002 [Page 2] Internet Draft QoS Signalling Requirements for Wireless November 2001 1. Introduction A wireless network, seen from a QoS domain perspective, usually consists of three parts: a wireless interface part (the "radio interface"), a wired part of the wireless network and the backbone of the wireless network, as shown in the figure below. Note that this figure should not be seen as an architectural overview of wireless networks but rather as showing the conceptual QoS domains in a wireless network. |--| |GW| |--| |--| |MH|--- . |--| / |-------| . /--|base | |--| . |station|-|ER|.... |-------| |--| . |--| back- |--| |---| |----| ...|ER|.......|ER|..|BGW|.."Internet"..|host| -- |-------| |--| . |--| bone |--| |---| |----| |--| \ |base |-|ER|... . |MH| \ |station| |--| . |--|--- |-------| . MH = mobile host |--| ER = edge router <----> |GW| GW = gateway Wireless link |--| BGW = border gateway ... = interior nodes <-------------------> Wired part of wireless network <----------------------------------------> Wireless Network Each of these parts impose different requirements on the QoS signalling solution being used: * Wireless interface: The solution for the air interface link has to ensure flexibility and spectrum efficient transmission of IP packets. However, this link layer QoS is solved in the same way as any other last hop problem by allowing a host to request the proper QoS profile. * Wired part of the wireless network: This is the part of the network that is closest to the base stations. It is an Karagiannis, et al. Expires May 2002 [Page 3] Internet Draft QoS Signalling Requirements for Wireless November 2001 IP network although some parts logically perform tunneling of the end user data. In cellular networks, the wired part of the wireless network is denoted as a radio access network. This part of the wireless network has different characteristics when compared to traditional IP networks: 1. The network supports a high proportion of real-time traffic. The majority of the traffic transported in the wired part of the wireless network is speech, which is very sensitive to delays and delay variation (jitter). 2. The network must support mobility. Many wireless networks are able to provide a combination of soft and hard handover procedures. When handover occurs, reservations need to be established on new paths. The establishment time has to be as short as possible since long establishment times for reservations degrade the performance of the wireless network. Moreover, for maximal utilization of the radio spectrum, frequent handover operations are required. 3. These links are typically rather bandwidth-limited. 4. The wired transmission in such a network contains a relatively high volume of expensive leased lines. Overprovisioning might therefore be prohibitively expensive. 5. The radio base stations are spread over a wide geographical area and are in general situated a large distance from the backbone. * Backbone of the wireless network: the requirements imposed by this network are similar to the requirements imposed by other types of backbone networks. Due to these very different characteristics and requirements, often contradictory, different QoS signalling solutions might be needed in each of the three network parts. This memo outlines a set of requirements for QoS signalling specifically in the wired part of IP- based wireless networks. Karagiannis, et al. Expires May 2002 [Page 4] Internet Draft QoS Signalling Requirements for Wireless November 2001 2. Requirements for the Wired Part of Wireless Networks Based upon the wireless network characteristics as described in the previous section, there are a number of requirements on any QoS signalling mechanism that will be used in the wired part of the network. These requirements are: 1. Modular, Simple, with Minimal Impact on Performance 2. Ability to deal with mobility (handover) 3. On-demand, dynamic signalling for efficient network utilization 4. Ability to deal with severe congestion 5. Unicast transport 6. Scalably Manageable Without adequately addressing all of these requirements, it will not be possible to provide appropriate QoS in this network. In addition, the following are specifically non-requirements: 1. Handling multicast flows 2. Signalling across multiple administrative domains If a QoS signalling protocol can provide these at little cost, that would obviously be useful. However, these requirements are far outweighed by the requirement for simplicity and scalability. In the sections that follow, each of the requirements is explained in more detail. 2.1. Modular, Simple, with Minimal Impact on Performance Although obvious, it bears saying that any QoS signalling mechanism must be as modular, simple, and scalable as possible and that it should have a minimal effect on the overall performance of the network. 2.1.1. Modular While it is clear that a set of mechanisms will not be standardized that is only applicable to the wired part of the wireless network, it must be possible to use only a part of the QoS signalling mechanism that is applicable to that network. That is, a standardized QoS Karagiannis, et al. Expires May 2002 [Page 5] Internet Draft QoS Signalling Requirements for Wireless November 2001 signalling mechanism should be a toolkit from which one can choose the applicable tools in order to manage QoS in a particular networking environment. 2.1.2. Simple It is critical that the reservation mechanism be as simple as possible to implement in hardware in the interior nodes since in most cases there will be more interior routers (<= 10 depending on network structure) in the path between the edges than there are edge nodes (there are typically two edge nodes located in a communication path). As such, the scheme must be optimized for the interior nodes and not for the edge nodes, thus reducing the requirements placed on the functionality of the interior routers. 2.1.3. Minimal Impact on Performance In wireless networks, the interior nodes will have to support a higher number of micro-flows (user connections) compared to the edge nodes. Therefore a resource reservation scheme should be very simple at the interior nodes while it might use more complex mechanisms at the edge nodes. In particular, this means is that the interior nodes must not be required to have per flow responsibilities. The performance of each network node that is used in an end-to-end communication path has an impact on the end-to-end performance. As such, the end-to-end performance of the communication path can be improved by optimizing the performance of interior nodes. One of the factors that can contribute to this optimization is the minimization of the resource reservation signaling protocol processing load on each device. When the dynamic reservation of the resources is on a per micro-flow basis, the resource reservation signaling protocol could easily overload a router, causing severe performance degradation. The QoS signalling mechanism must be designed to minimize the cycles spent on processing the signalling messages. One outcome of this requirement is that it uses a simplified lightweight model in the interior nodes and places complex per-flow handling at the edges. This requires mapping of per-flow traffic parameters at the edges into a necessary set of parameters needed for setting up reservations in interior nodes. The edge routers typically already have to perform per-session management/control, and hence complex per flow-handling is not a significant burden. Karagiannis, et al. Expires May 2002 [Page 6] Internet Draft QoS Signalling Requirements for Wireless November 2001 2.2. Ability to deal with mobility (handover) In order to support mobile users, the QoS signalling mechanism must be highly performant for at least the following reasons: * Handover rates In mobile networks, the admission control process has to cope with far more admission requests than call setups alone would generate. For example, in the GSM (Global System for Mobile communications) case, mobility usually generates an average of one to two handovers per call. For third generation networks (such as UMTS), where it is necessary to keep radio links to several cells simultaneously (macro-diversity), the handover rate is significantly higher (see for example [KeMc01]). * Fast reservations Handover can also cause packet losses. This happens when the processing of an admission request causes a delayed handover to the new base station. In this situation, some packets might be discarded, and the overall speech quality might be degraded significantly. Moreover, a delay in handover may cause degradation for other users. In the worst case scenario, a delay in handover may cause the connection to be dropped if the handover occurred due to bad air link quality. Therefore, it is critical that QoS signalling in connection with handover be carried out very quickly. Furthermore, when the network is overloaded, it is preferable to keep reservations for previously established flows while blocking new requests. Therefore, the resource reservation requests in connection with handover should be given higher priority than new requests for resource reservation. 2.3. On-demand, dynamic signalling for efficient network utilization The resource reservation scheme must facilitate the network utilization to the highest degrees possible. Wireless networks typically use a large number of expensive leased lines, which means that efficient network utilization has a direct economic impact on network operators. Karagiannis, et al. Expires May 2002 [Page 7] Internet Draft QoS Signalling Requirements for Wireless November 2001 Real-time services require that a portion of network resources is available to them. These resources can be reserved on a static or dynamic basis, or potentially based on some kind of measurement of network load. Both measurement-based and static reservations may result in a poorly utilized network, primarily due to the fact that the network resources are typically reserved for peak real-time traffic values. Mobility in the network makes static configuration even less desirable as the resources will be used even less effectively. By using dynamic reservation, this problem will be avoided since the resources are reserved on demand. There is, of course, a tradeoff. Dynamic reservations will mean that the load from resource reservation will be much higher than if static reservation of resources is used. If the dynamic reservation of the resources is done on a micro-flow basis, the resulting network load from resource reservation might be quite high. In networks where resources are very expensive (as is the case for many wireless networks), efficient network utilization is of critical financial importance. As such, static configuration is simply not an option and an appropriately engineered solution allowing dynamic resource reservation is needed. 2.4. Ability to deal with severe congestion In case of a route change or link failure a severe congestion situation may occur in the network. Typically, routing algorithms are able to adapt and change their routing decisions to reflect changes in the topology and traffic volume. In such situations the re-routed traffic will have to follow a new path. Interior nodes located on this new path may become overloaded, since they suddenly might need to support more traffic than they have capacity for. These severe congestion situations will severely affect the overall performance of the traffic passing through such nodes. Therefore, the QoS signalling solution should be able to efficiently solve this situation. 2.5. Unicast transport The vast majority of the traffic in the wireless network is point-to- point unicast transport. As such, the QoS signalling mechanism must deal with unicast effectively. Karagiannis, et al. Expires May 2002 [Page 8] Internet Draft QoS Signalling Requirements for Wireless November 2001 2.6. Scalably Manageable Any strategy for resource management must be done in such a way that it is manageable in a very large network. In networks made up of many thousands of routers, changing of even a single parameter in all routers may be prohibitively difficult. Minimizing the involvement of the operator (or the operator's management tools) is therefore an important requirement. 3. Non-goals for QoS Signalling in Wireless Networks The characteristics of wireless networks mean that a number of issues do not need to be considered in designing a QoS signalling mechanism. Of course, a QoS signalling mechanism that addresses these issues without compromising on the more critical requirements would certainly be acceptable. 3.1. Handling multicast flows Given that the traffic in wireless access networks is unicast, these networks do not require a QoS signalling mechanism which deals with multicast. 3.2. Signalling across multiple administrative domains Real-time applications require a high level of QoS from the underlying transmission network, which can only be achieved by managing the QoS on an end-to-end basis, potentially across many domains. However, this does not mean that the QoS signalling must be applied end-to-end. The end-to-end QoS management architecture might consist of many interoperable edge-to-edge QoS management architectures. Each of these edge-to-edge QoS management architectures might use a different edge-to-edge resource reservation protocol. This will increase the flexibility and the openness of the transmission network since various access networks that are using the same transmission network and different edge-to-edge QoS management architectures will be able to interoperate. It is realistic to assume that end-to-end communication in IP networks as well as the end-to-end QoS management architectures will be managed by more than one operator. Furthermore, it also realistic to assume that an edge-to-edge resource reservation protocol can be Karagiannis, et al. Expires May 2002 [Page 9] Internet Draft QoS Signalling Requirements for Wireless November 2001 managed by one single operator. As such, it is reasonable to allow a QoS signalling scheme to a single operator domain by using applicable parts of a modular architecture. This will ensure that each operator can optimize the QoS management architecture for their needs. Moreover, this limitation (a single operator domain) means that the reservation scheme does not need to handle the issues inherent in a multi-operator domain, thus further simplifying the scheme. 4. Conclusion The need for a QoS signalling mechanism in wireless networks is great. The requirements outlined in this memo must be fulfilled in order to be able to use IP effectively in wireless networks. Without QoS signalling and appropriate resource reservation mechanisms, it will be significantly more difficult to take advantage of using IP as transport in wireless networks. These wireless networks support a high proportion of real-time traffic, must support mobility, the transport links are typically rather bandwidth-limited on relatively expensive leased lines, and the network is spread over a wide geographical area. The QoS signalling mechanism must be designed so that these networks can support users' needs for QoS and be used efficiently. 5. References [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., Weiss, W., "An Architecture for Differentiated Services", IETF RFC 2475, December 1998. [KeMc01] Kempf, J., McCann, P., Roberts, P., "IP Mobility and the CDMA Radio Access Network", IETF Draft, draft-kempf-cdma-appl-02.txt, Work in progress, September 2001. 6. Acknowledgments The editors wish to thank colleagues at Ericsson for value feedback. Karagiannis, et al. Expires May 2002 [Page 10] Internet Draft QoS Signalling Requirements for Wireless November 2001 7. Editors' Addresses Georgios Karagiannis Ericsson EuroLab Netherlands B.V. Institutenweg 25 P.O.Box 645 7500 AP Enschede The Netherlands EMail: Georgios.Karagiannis@eln.ericsson.se David Partain Ericsson Radio Systems AB P.O. Box 1248 SE-581 12 Linkoping Sweden EMail: David.Partain@ericsson.com Vlora Rexhepi Ericsson EuroLab Netherlands B.V. Institutenweg 25 P.O.Box 645 7500 AP Enschede The Netherlands EMail: Vlora.Rexhepi@eln.ericsson.se Lars Westberg Ericsson Research Torshamnsgatan 23 SE-164 80 Stockholm Sweden EMail: Lars.Westberg@era.ericsson.se Table of Contents 1 Introduction .................................................... 3 2 Requirements for the Wired Part of Wireless Networks ............ 5 2.1 Modular, Simple, with Minimal Impact on Performance ........... 5 2.1.1 Modular ..................................................... 5 2.1.2 Simple ...................................................... 6 2.1.3 Minimal Impact on Performance ............................... 6 2.2 Ability to deal with mobility (handover) ...................... 7 2.3 On-demand, dynamic signalling for efficient network utiliza­ Karagiannis, et al. Expires May 2002 [Page 11] Internet Draft QoS Signalling Requirements for Wireless November 2001 tion ......................................................... 7 2.4 Ability to deal with severe congestion ........................ 8 2.5 Unicast transport ............................................. 8 2.6 Scalably Manageable ........................................... 9 3 Non-goals for QoS Signalling in Wireless Networks ............... 9 3.1 Handling multicast flows ...................................... 9 3.2 Signalling across multiple administrative domains ............. 9 4 Conclusion ...................................................... 10 5 References ...................................................... 10 6 Acknowledgments ................................................. 10 7 Editors' Addresses .............................................. 11 Karagiannis, et al. Expires May 2002 [Page 12]