Chapter 5 - The Sample QoS Network
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In this section we'll present a sample network which we will use as a basis for subsequent discussion. The sample network is intended to reflect a realistic network incorporating multiple subnets of varying types. It is illustrated below:
The two large routed networks in the center of the diagram represent large network providers. The peripheral networks represent customer networks. There are three corporate or campus customer networks illustrated and two individual home customer networks. The network providers can be considered transit networks, as they contain no hosts or end-stations. The various customer networks contain hosts. The bold dashed ovals separate the larger network into sub-networks. For the sake of simplicity, we assume that these also correspond to administrative domains (ADs)1.
All the corporate or campus networks are illustrated as a combination of smaller routed networks, ATM networks and 802 LANs. Private customer networks are illustrated as single hosts connected via dial-up lines. Interconnections between networks are not clearly identified (other than the dial-in connections to the private customer networks). Interconnections could range from SONET rings to high speed leased lines, to xDSL connections, cable connections, low-speed modems, and so on. Interconnections may be represented as networks in their own right. Generally, some pair of interconnection devices is implied.
Assumptions Regarding the Sample Network
We assume that the network:
Includes an arbitrary number of concatenated subnetworks of arbitrary media.
Is required to provide a combination of high quality and low quality guarantees on an end-to-end basis.
Must meet certain objectives in terms of efficiency of resource usage.
Must meet certain objectives in terms of overhead of QoS mechanisms.
Must be manageable.
Subnet Local QoS Mechanisms
Each subnetwork provides local QoS mechanisms. These include:
Various traffic handling mechanisms in devices, as appropriate for the scale and media of the subnet.
Policy servers (PDPs) and policy data-stores which provide QoS top-down provisioning capabilities, as well as interaction with end-to-end QoS signaling (as described previously).
Agents in various network devices that are able to participate in end-to-end QoS related signaling.
Global QoS Mechanisms
Other QoS mechanisms are global in the sense that they span multiple sub-networks. These include:
Per-conversation, end-to-end RSVP signaling, which is generated by certain hosts for certain application traffic.
Inter-domain or intra-domain signaling in the form of aggregated RSVP, MultiProtocol Label Switching (MPLS) signaling, bandwidth broker interactions, and so forth2.
High level cross-network provisioning and configuration applications.
The Role of RSVP in Providing High Quality End-to-End QoS
As discussed previously, guarantees must be valid end-to-end, across multiple subnets. Lower quality guarantees can be provided without requiring tight coupling between the QoS mechanisms in different subnets. However, high quality guarantees require tight coupling between these mechanisms.
As an example, it is possible to independently configure devices in each subnet (in a top-down manner) to prioritize some set of traffic (as identified by IP port) above best-effort traffic (BBE service). This will indeed improve the quality of service perceived by the prioritized application, in all parts of the network. However, this is a low quality guarantee, as it makes no specific commitments regarding available bandwidth or latency.
On the other hand, consider the quality of guarantee required to support a videoconference. A videoconferencing application requires that all subnets between the videoconferencing peers be able to provide a significant amount of bandwidth at a low latency. To do so efficiently requires that all devices along the data path commit the required amount of low latency bandwidth, for the duration of the videoconference. As we have seen, high quality guarantees such as these generally require signaling across network devices in order to make efficient use of network resources. In our sample network, multiple subnets, based on multiple media (and varying traffic handling mechanisms) must be coordinated via this signaling. RSVP with intserv is particularly suitable for this purpose because it expresses QoS requirements in high-level, abstract terms. Agents in each subnet are able to translate the media independent, abstract requests into parameters that are meaningful to the specific subnet media. The ISSLL (Integrated Services Over Specific Link Layers) working group of the IETF has focused on the definitions of mappings from integrated services (intserv) to numerous media, including 802 networks, ATM, slow links (e.g. traditional modems) and, recently, diffserv.
In our model, hosts generate RSVP signaling when it is necessary to obtain high quality guarantees. The network listens to this signaling at strategic points in the network. We will refer to devices that participate in RSVP signaling as RSVP agents or alternatively as signaling or admission control agents. As we have shown, appointing such agents at varying densities can provide varying quality/efficiency products. At a minimum we assume one or more admission control agents in each subnet. Each agent uses the mappings defined in ISSLL to translate high level end-to-end RSVP requests into parameters that are meaningful to the media for which the agent is responsible. The admission control agent then determines, based on resource availability and/or policy decisions, (with the cooperation of PDPs) whether an RSVP request is admissible or not. Any admission control agent along the route from sender to receiver may veto an RSVP request for resources. Requests that are not vetoed by any device are considered admitted and result in the return of an RSVP RESV message to the requesting transmitting host.
An important component of the end-to-end service model described above is the mapping from intserv services to the corresponding traffic handling mechanisms in each of the subnets on the end-to-end path. As mentioned previously, the definition of such mappings is the responsibility of the ISSLL working group of the IETF. A mapping includes definition of the underlying media-specific service suitable to provide the intserv service. It also includes admission control guidelines. These are used to determine the marginal impact that will result from admission of additional traffic to an underlying traffic handling mechanism. Based on this impact, additional traffic may be admitted or may be refused admission.
Host QoS Mechanisms
Generation of RSVP signaling for conversations requiring high quality guarantees, including identification of both the user and the application requesting resources
Hosts generate RSVP signaling for conversations requiring high quality guarantees. These include conversations generating both quantifiable and non-quantifiable traffic, so long as they are persistent. Hosts then proceed to mark and schedule traffic based on the results of the signaling requests. If a signaling request for resources at a specific intserv service level is admitted, the host will mark traffic on the corresponding conversation with the appropriate DSCP and 802.1p marks based on the ISSLL mapping from intserv to diffserv and 802, respectively. (Note that the network may override default mappings). If a signaling request specifies quantifiable parameters, the host schedules traffic in accordance with the requested parameters.
Although the role of the host is most pronounced in the context of signaled QoS, it may also participate in supporting top-down provisioned QoS. It does so by enabling policy agents to provision classification, scheduling and marking information in transmitting hosts, to control traffic that is non-persistent (for which signaling messages are not generated).
|1||All devices within a single AD are managed by a single administrator with consistent economic objectives. The notion of ADs is generally recursive, in the sense that there may be multiple ADs within a larger AD, just as there may be local governments subject to a federal government.|
|2||The terms MPLS and Bandwidth Broker are defined later in this document.|