Network Working Group K. Chan Request for Comments: 5127 J. Babiarz Category: Informational Nortel F. Baker Cisco Systems February 2008 Aggregation of Diffserv Service Classes Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
AbstractIn the core of a high-capacity network, service differentiation may still be needed to support applications' utilization of the network. Applications with similar traffic characteristics and performance requirements are mapped into Diffserv service classes based on end- to-end behavior requirements of the applications. However, some network segments may be configured in such a way that a single forwarding treatment may satisfy the traffic characteristics and performance requirements of two or more service classes. In these cases, it may be desirable to aggregate two or more Diffserv service classes into a single forwarding treatment. This document provides guidelines for the aggregation of Diffserv service classes into forwarding treatments.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview of Service Class Aggregation . . . . . . . . . . . . 5 4. Service Classes to Treatment Aggregate Mapping . . . . . . . . 6 4.1. Mapping Service Classes into Four Treatment Aggregates . . 7 4.1.1. Network Control Treatment Aggregate . . . . . . . . . 9 4.1.2. Real-Time Treatment Aggregate . . . . . . . . . . . . 10 4.1.3. Assured Elastic Treatment Aggregate . . . . . . . . . 10 4.1.4. Elastic Treatment Aggregate . . . . . . . . . . . . . 12 5. Treatment Aggregates and Inter-Provider Relationships . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. Normative References . . . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . . 14 Appendix A. Using MPLS for Treatment Aggregates . . . . . . . . 15 A.1. Network Control Treatment Aggregate with E-LSP . . . . . . 17 A.2. Real-Time Treatment Aggregate with E-LSP . . . . . . . . . 17 A.3. Assured Elastic Treatment Aggregate with E-LSP . . . . . . 17 A.4. Elastic Treatment Aggregate with E-LSP . . . . . . . . . . 17 A.5. Treatment Aggregates and L-LSP . . . . . . . . . . . . . . 18
12] affect real-time applications. Moreover, denial of service (DoS) loads, worms, and network disruptions such as that of 11 September 2001 affect routing . Our objective is to prevent disruption to routing (which in turn affects all services) and to protect real-time jitter-sensitive services, while minimizing loss and delay of sensitive elastic traffic. RFC 4594  defines a set of basic Diffserv classes from the points of view of the application requiring specific end-to-end behaviors from the network. The service classes are differentiated based on the application payload's tolerance to packet loss, delay, and delay variation (jitter). Different degrees of these criteria form the foundation for supporting the needs of real-time and elastic traffic. RFC 4594  also provides recommendations for the treatment method of these service classes. But, at some network segments of the end- to-end path, the number of levels of network treatment differentiation may be less than the number of service classes that the network segment needs to support. In such a situation, that network segment may use the same treatment to support more than one service class. In this document, we provide guidelines on how multiple service classes may be aggregated into a forwarding treatment aggregate. This entails having the IP traffic belonging to service classes, expressed using the DSCP (Differentiated Services Code Point), as described by RFC 4594 . Note that in a given domain, we may recommend that the supported service classes be aggregated into forwarding treatment aggregates; however, this does not mean all service classes need to be supported, and hence not all forwarding treatment aggregates need to be supported. A domain may
support a fewer or greater number of forwarding treatment aggregates than recommended by this document. Which service classes and which forwarding treatment aggregates are supported by a domain is up to the domain administration and may be influenced by business reasons or other reasons (e.g., operational considerations). In this document, we've provided: o definitions for terminology we use in this document, o requirements for performing this aggregation, o an example of performing the aggregation when four treatment aggregates are used, and o an example (in the appendix) of performing this aggregation over MPLS using E-LSP, EXP Inferred PHB Scheduling Class (PSC) Label Switched Path (LSP). The treatment aggregate recommendations are designed to aggregate the service classes  in such a manner as to protect real-time traffic and routing, on the assumption that real-time sessions are protected from each other by admission at the edge. The recommendation given is one possible way of performing the aggregation; there may be other ways of aggregation, for example, into fewer treatment aggregates or more treatment aggregates. In the appendix, an example of aggregation over MPLS networks using E-LSP to realize the treatment aggregates is provided. Note that the MPLS E-LSP is just an example; this document does not exclude the use of other methods. This example only considers aggregation of IP traffic into E-LSP. The use of E-LSP by non-IP traffic is not discussed. RFC 2119 .
For new terms introduced by this document, we provide the definition here: o Treatment Aggregate. This term is defined as the aggregate of Diffserv service classes . A treatment aggregate is concerned only with the forwarding treatment of the aggregated traffic, which may be marked with multiple DSCPs. A treatment aggregate differs from Behavior Aggregate  and Traffic Aggregate , each of which indicate the aggregated traffic having a single Diffserv codepoint and utilizing a single Per Hop Behavior (PHB). For terms from existing RFCs, we provide the reference to the appropriate section of the relevant RFC that contain the definition: o Real-Time and Elastic Applications and their traffic. Section 3.1 of RFC 1633 . o Diffserv Service Class. Section 1.3 of RFC 4594 . o MPLS E-LSP, EXP Inferred PHB Scheduling Class (PSC) Label Switched Path (LSP). Section 1.2 of RFC 3270 . o MPLS L-LSP, Label Only Inferred PHB Scheduling Class (PSC) Label Switched Path (LSP). Section 1.3 of RFC 3270 . 3] may be required. These aggregations have the following requirements: 1. The end-to-end network performance characteristic required by the application MUST be supported. This performance characteristic is represented by the use of Diffserv service classes . 2. The treatment aggregate MUST meet the strictest requirements of its member service classes. 3. The treatment aggregate SHOULD only contain member service classes with similar traffic characteristic and performance requirements. 4. The notion of the individual end-to-end service classes MUST NOT be destroyed when aggregation is performed. Each domain along the end-to-end path may perform aggregation differently, based on the original end-to-end service classes. We recommend an easy
way to accomplish this by not altering the DSCP used to indicate the end-to-end service class. But some administrative domains may require the use of their own marking; when this is needed, the original end-to-end service class indication must be restored upon exiting such administrative domains. One possible way of achieving this is with the use of tunnels to encapsulate the end- to-end traffic. 5. Each treatment aggregate has limited resources; hence, traffic conditioning and/or admission control SHOULD be performed for each service class aggregated into the treatment aggregate. Additional admission control and policing may be used on the sum of all traffic aggregated into the treatment aggregate. In addition to the above requirements, we have the following suggestions: 1. The treatment aggregate and assigned resources may consider historical traffic patterns and the variability of these patterns. For example, a point-point service (e.g., pseudowire) may have a very predictable pattern, while a multipoint service (e.g., VPLS, Virtual Private LAN Service) may have a much less predictable pattern. 2. In addition to Diffserv, other controls are available to influence the traffic level offered to a particular traffic aggregate. These include adjustment of routing metrics, and usage of MPLS-based traffic engineering techniques. This document only describes the aggregation of IP traffic based on the use of Diffserv service classes . RFC 4594  has been defined to allow, in many instances, mapping of two or possibly more service classes into a single forwarding treatment aggregate. Notice that there is a relationship/trade-off between link speed, queue depth, delay, and jitter. The degree of aggregation and hence the number of treatment aggregates will depend on the aggregation's impacts on loss, delay, and jitter. This depends on whether the speed of the links and scheduler behavior, being used to implement the aggregation, can minimize the effects of mixing traffic with different packet sizes and transmit rates on queue depth. A general rule-of-thumb is that higher link speeds allow for more aggregation/ smaller number of treatment aggregates, assuming link utilization is within the engineered level.
RFC 4594  into four treatment aggregates. The use of four treatment aggregates assumes that the resources allocated to each treatment aggregate are sufficient to honor the required behavior of each service class . We use the performance requirement (tolerance to loss, delay, and jitter) from the application/end-user as a guide on how to map the service classes into treatment aggregates. We have also used section 3.1 of RFC 1633  to provide us with guidance on the definition of Real-Time and Elastic applications. An overview of the mapping between service classes and the four treatment aggregates is provided by Figure 1, with the mapping being based on performance requirements. In Figure 1, the right side columns of "Service Class" and "Tolerance to Loss/ Delay/Jitter" are from Figure 2 of RFC 4594 . It is recommended that certain service classes be mapped into specific treatment aggregates. But this does not mean that all the service classes recommended for that treatment aggregate need to be supported. Hence, for a given domain, a treatment aggregate may contain only a subset of the service classes recommended in this document, i.e., the service classes supported by that domain. A domain's treatment of non-supported service classes should be based on the domain's local policy. This local policy may be influenced by its agreement with its customers. Such treatment may use the Elastic Treatment Aggregate, dropping the packets, or some other arrangements. Our example of four treatment aggregates is based on the basic differences in performance requirement from the application/end-user perspective. A domain may choose to support more or fewer treatment aggregates than the four recommended. For example, a domain may support only three treatment aggregates and map any network control traffic into the Assured Elastic treatment aggregate. This is a choice the administrative domain has. Hence, this example of four treatment aggregates does not represent a minimum required set of treatment aggregates one must implement; nor does it represent the maximum set of treatment aggregates one can implement.
--------------------------------------------------------------------- |Treatment | Tolerance to ||Service Class | Tolerance to | |Aggregate | Loss |Delay |Jitter|| | Loss |Delay |Jitter| |==========+======+======+======++===============+======+======+======| | Network | Low | Low | Yes || Network | Low | Low | Yes | | Control | | | || Control | | | | |==========+======+======+======++===============+======+======+======| | Real- | Very | Very | Very || Telephony | VLow | VLow | VLow | | Time | Low | Low | Low ||---------------+------+------+------| | | | | || Signaling | Low | Low | Yes | | | | | ||---------------+------+------+------| | | | | || Multimedia |Low - | Very | Low | | | | | || Conferencing |Medium| Low | | | | | | ||---------------+------+------+------| | | | | || Real-time | Low | Very | Low | | | | | || Interactive | | Low | | | | | | ||---------------+------+------+------| | | | | || Broadcast | Very |Medium| Low | | | | | || Video | Low | | | |==========+======+======+======++===============+======+======+======| | Assured | Low |Low - | Yes || Multimedia |Low - |Medium| Yes | | Elastic | |Medium| || Streaming |Medium| | | | | | | ||---------------+------+------+------| | | | | || Low-Latency | Low |Low - | Yes | | | | | || Data | |Medium| | | | | | ||---------------+------+------+------| | | | | || OAM | Low |Medium| Yes | | | | | ||---------------+------+------+------| | | | | ||High-Throughput| Low |Medium| Yes | | | | | || Data | |- High| | |==========+======+======+======++===============+======+======+======| | Elastic | Not Specified || Standard | Not Specified | | | | | ||---------------+------+------+------| | | | | || Low-Priority | High | High | Yes | | | | | || Data | | | | --------------------------------------------------------------------- Figure 1: Treatment Aggregate and Service Class Performance Requirements As we are recommending to preserve the notion of the individual end- to-end service classes, we also recommend that the original DSCP field marking not be changed when treatment aggregates are used. Instead, classifiers that select packets based on the contents of the DSCP field should be used to direct packets from the member Diffserv service classes into the queue that handles each of the treatment aggregates, without remarking the DSCP field of the packets. This is
summarized in Figure 2, which shows the behavior each treatment aggregate should have, and the DSCP field marking of the packets that should be classified into each of the treatment aggregates. ------------------------------------------------------------ |Treatment |Treatment || DSCP | |Aggregate |Aggregate || | | |Behavior || | |==========+==========++=====================================| | Network | CS || CS6 | | Control |(RFC 2474)|| | |==========+==========++=====================================| | Real- | EF || EF, CS5, AF41, AF42, AF43, CS4, CS3 | | Time |(RFC 3246)|| | |==========+==========++=====================================| | Assured | AF || CS2, AF31, AF21, AF11 | | Elastic |(RFC 2597)||-------------------------------------| | | || AF32, AF22, AF12 | | | ||-------------------------------------| | | || AF33, AF23, AF13 | |==========+==========++=====================================| | Elastic | Default || Default, (CS0) | | |(RFC 2474)||-------------------------------------| | | || CS1 | ------------------------------------------------------------ Figure 2: Treatment Aggregate Behavior Notes for Figure 2: For Assured Elastic and Elastic Treatment Aggregates, please see sections 4.1.3 and 4.1.4, respectively, for details on additional priority within the treatment aggregate. RFC 4594  characterizes as being included in the Network Control service class. Traffic in the Network Control Treatment Aggregate should be carried in a common queue or class with a PHB as described in RFC 2474 , section 126.96.36.199 for Class Selector (CS). This treatment aggregate should have a lower probability of packet loss and bear a relatively deep target mean queue depth (min-threshold if RED (Random Early Detection) is being used).
Please notice this Network Control Treatment Aggregate is meant to be used for the customer's network control traffic. The provider may choose to treat its own network control traffic differently, perhaps in its own service class that is not aggregated with the customer's network control traffic. 3], in addition to other locally defined classes: Telephony, Signaling, Multimedia Conferencing, Real- time Interactive, and Broadcast Video. Traffic in each service class that is going to be aggregated into the treatment aggregate should be conditioned prior to aggregation. It is recommended that per-service-class admission control procedures be used, followed by per-service-class policing so that any individual service class does not generate more than what it is allowed. Furthermore, additional admission control and policing may be used on the sum of all traffic aggregated into this treatment aggregate. Traffic in the Real-Time Treatment Aggregate should be carried in a common queue or class with a PHB (Per Hop Behavior) as described in RFC 3246  and RFC 3247 . RFC 2597 . The premise of such a service is that an SLA that is negotiated includes a "committed rate" and the ability to exceed that rate (and perhaps a second "excess rate") in exchange for a higher probability of loss using Active Queue Management (AQM)  or Explicit Congestion Notification (ECN) marking  for the portion of traffic deemed to be in excess.
This treatment aggregate may include the following service classes from the Diffserv service classes , in addition to other locally defined classes: Multimedia Streaming, Low Latency Data, OAM, and High-Throughput Data. The DSCP values belonging to the Assured Forwarding (AF) PHB group and class selector of the original service classes remain an important consideration and should be preserved during aggregation. This treatment aggregate should maintain the AF PHB group marking of the original packet. For example, AF3x marked packets should remain AF3x marked within this treatment aggregate. In addition, the class selector DSCP value should not be changed. Traffic bearing these DSCPs is carried in a common queue or class with a PHB as described in RFC 2597 . In effect, appropriate target rate thresholds have been applied at the edge, dividing traffic into AFn1 (committed, for any value of n), AFn2, and AFn3 (excess). The service should be engineered so that AFn1 and CS2 marked packet flows have sufficient bandwidth in the network to provide high assurance of delivery. Since the traffic is elastic and responds dynamically to packet loss, Active Queue Management  should be used primarily to reduce the forwarding rate to the minimum assured rate at congestion points. The probability of loss of AFn1 and CS2 traffic must not exceed the probability of loss of AFn2 traffic, which in turn must not exceed the probability of loss of AFn3 traffic. If RED  is used as an AQM algorithm, the min-threshold specifies a target queue depth for each of AFn1+CS2, AFn2, and AFn3, and the max- threshold specifies the queue depth above which all traffic with such a DSCP is dropped or ECN marked. Thus, in this treatment aggregate, the following inequalities SHOULD hold in queue configurations: o min-threshold AFn3 < max-threshold AFn3 o max-threshold AFn3 <= min-threshold AFn2 o min-threshold AFn2 < max-threshold AFn2 o max-threshold AFn2 <= min-threshold AFn1+CS2 o min-threshold AFn1+CS2 < max-threshold AFn1+CS2 o max-threshold AFn1+CS2 <= memory assigned to the queue Note: This configuration tends to drop AFn3 traffic before AFn2, and AFn2 before AFn1 and CS2. Many other AQM algorithms exist and are used; they should be configured to achieve a similar result.
7] or ECN flagging  is appropriate for such traffic. This treatment aggregate may include the following service classes from the Diffserv service classes , in addition to other locally defined classes: Standard and Low-Priority Data. Treatment aggregates should be well specified, each indicating the service classes it will handle. But in cases where unspecified or unknown service classes are encountered, they may be dropped or be treated using the Elastic Treatment Aggregate. The choice of how to treat unspecified service classes should be well defined, based on some agreements. Traffic in the Elastic Treatment Aggregate should be carried in a common queue or class with a PHB as described in RFC 2474 , section 4.1, "A Default PHB". The AQM thresholds for Elastic traffic MAY be separately set, so that Low Priority Data traffic is dropped before Standard traffic, but this is not a requirement. 3]. This allows the admission control into each treatment aggregate of a provider domain to be based on the admission control of traffic into the supported service classes, as indicated by the discussion in section 4 of this document. If the inter-provider relationship needs to be based on treatment aggregates specified by this document, then the exact treatment aggregate content and representation must be agreed to by the peering providers. Some additional work on inter-provider relationships is provided by inter-provider QoS , where details on supporting real-time services between service providers are discussed. Some related work in ITU-T provided by Appendix VI of Y.1541  may also help with inter-provider relationships, especially with international providers.
RFC 4594 , the Security Consideration discussion of no new security issues indicated by RFC 4594  also applies to treatment aggregates of this document.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.  Babiarz, J., Chan, K., and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, August 2006.  Braden, B., Clark, D., and S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, June 1994.  Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000.  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-Protocol Label Switching (MPLS) Support of Differentiated Services", RFC 3270, May 2002.
 Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J., and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998.  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, June 1999.  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.  Charny, A., Bennet, J., Benson, K., Boudec, J., Chiu, A., Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., and K. Ramakrishnan, "Supplemental Information for the New Definition of the EF PHB (Expedited Forwarding Per-Hop Behavior)", RFC 3247, March 2002.  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.  Choi, B., Moon, S., Zhang, Z., Papagiannaki, K., and C. Diot, "Analysis of Point-To-Point Packet Delay in an Operational Network", INFOCOMM 2004, March 2004, <http://www.ieee-infocom.org/2004/Papers/37_4.PDF>.  Ogielski, A. and J. Cowie, "Internet Routing Behavior on 9/11", March 2002, <http://www.renesys.com/tech/presentations/pdf/ renesys-030502-NRC-911.pdf>.  Nichols, K. and B. Carpenter, "Definition of Differentiated Services Per Domain Behaviors and Rules for their Specification", RFC 3086, April 2001.  MIT Communications Futures Program, "Inter-provider Quality of Service", November 2006, < http://cfp.mit.edu/resources/papers/Interprovider QoS MIT_CFP_WP_9_14_06.pdf>.  International Telecommunications Union, "Network Performance Objectives for IP-Based Services", Recommendation Y.1541, February 2006.
RFC 2983 on Diffserv and Tunnels  and RFC 3270 on MPLS Support of Diffserv  provide a very good background on this topic. This document provides an example of using the E-LSP, EXP Inferred PHB Scheduled Class (PSC) Label Switched Path (LSP), defined by MPLS Support of Diffserv  for realizing the Treatment Aggregates. When treatment aggregates are represented in MPLS using EXP Inferred PSC LSP, we recommend the following usage of the MPLS EXP field for treatment aggregates.
------------------------------------------- |Treatment || MPLS || DSCP | DSCP | |Aggregate || EXP || name | value | |==========++======++=========|=============| | Network || 110 || CS6 | 110000 | | Control || || | | |==========++======++=========|=============| | Real- || 100 || EF | 101110 | | Time || ||---------|-------------| | || || CS5 | 101000 | | || ||---------|-------------| | || ||AF41,AF42|100010,100100| | || || AF43 | 100110 | | || ||---------|-------------| | || || CS4 | 100000 | | || ||---------|-------------| | || || CS3 | 011000 | |==========++======++=========|=============| | Assured || 010* || CS2 | 010000 | | Elastic || || AF31 | 011010 | | || || AF21 | 010010 | | || || AF11 | 001010 | | ||------||---------|-------------| | || 011* || AF32 | 011100 | | || || AF22 | 010100 | | || || AF12 | 001100 | | || || AF33 | 011110 | | || || AF23 | 010110 | | || || AF13 | 001110 | |==========++======++=========|=============| | Elastic || 000* || Default | 000000 | | || || (CS0) | | | ||------||---------|-------------| | || 001* || CS1 | 001000 | ------------------------------------------- Figure 3: Treatment Aggregate and MPLS EXP Field Usage * Note: For Assured Elastic (and Elastic) Treatment Aggregate, the usage of 010 or 011 (000 or 001) as EXP field value depends on the drop probability. Packets in the LSP with EXP field of 011 (001) have a higher probability of being dropped than packets with an EXP field of 010 (000).
The above table indicates the recommended usage of EXP fields for treatment aggregates. Because many deployments of MPLS are on a per- domain basis, each domain has total control of its EXP usage and each domain may use a different EXP field allocation for the domain's supported treatment aggregates. section 4.1.1 of this document and section 3.2 of RFC 4594 . Reinforcing these recommendations, there should be no drop precedence associated with the MPLS PSC used for Network Control Treatment Aggregate because dropping of Network Control Treatment Aggregate traffic should be prevented. section 4.1.2 of this document and in member service classes' sections of RFC 4594 , we want to indicate that Real-Time Treatment Aggregate traffic should not be dropped, as some of the applications whose traffic is carried in the Real-Time Treatment Aggregate do not react well to dropped packets. As indicated in section 4.1.2 of this document, admission control should be performed on each service class contributing to the Real-Time Treatment Aggregate to prevent packet loss due to insufficient resources allocated to Real-Time Treatment Aggregate. Further, admission control and policing may also be applied on the sum of all traffic aggregated into this treatment aggregate.
that with this mapping, during congestion, CS1-marked traffic may be starved. If the domain chooses to support only one drop precedence for this treatment aggregate, we recommend the use of 000 for EXP field marking. section 4 of this document) for treatment aggregate to L-LSP mapping, leaving this to each individual MPLS domain administration.
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