Diffserv Working Group Dan Grossman Internet Draft Motorola, Inc. Expires: April 2000 draft-ietf-diffserv-new-terms-00.txt October, 1999 New Terminology for Diffserv 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. To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract This memo captures Diffserv working group agreements concerning new and improved terminology. It is intended as a living document for use by the Diffserv working group, and especially for use of authors of Diffserv drafts. It is expected that the terminology in this memo will be incorporated into the existing Diffserv RFCs when they are updated. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. 1. Introduction As the Diffserv work has evolved, there have been several cases where terminology has needed to be created or the definitions in [1] and [2] have needed to be refined. This memo was created to capture and test group agreements on terminology, rather than attempting to revise the base RFCs and recycle them at proposed standard. Diffserv authors are encouraged to use the new terminology Grossman [Page 1] draft-ietf-diffserv-new-terms-00.txt October 1999 whereever appropriate. [Author's note: the following represents in part the Author's understanding of the agreements. However, in some cases, the Author found it necessary to elaborate or expand. The Author has also polled the Diffserv chairs and incorporated their recollection into this memo. Every attempt will be made to refine this memo based on comments from the group. No claim is made that the ¸00 version of this memo represents a group consensus.) 2. Terminology related to Service Level Agreements (SLAs) The Diffserv Architecture [2] uses the term "Service Level Agreement" (SLA) to describe the "service contract... that specifies the forwarding service a customer should receive". The SLA may include traffic conditioning rules which (at least in part) constitute a Traffic Conditioning Agreement (TCA). A TCA is "an agreement specifying classifier rules and any corresponding traffic profiles and metering, marking, discarding and/or shaping rules which are to apply...." As work progressed in Diffserv, it came to be believed that the notion of an "agreement" implied considerations that were of a pricing, contractual or other business nature, as well as those that were strictly technical. There also could be other technical considerations in such an agreement (e.g., service availability) which are not addressed by Diffserv. It was therefore agreed that the notions of SLAs and TCAs would be taken to represent the broader context, and that new terminology would be used to describe those elements of service and traffic conditioning that are addressed by Diffserv. - A Service Level Specfication (SLS) is a set of parameters and their values which together define the service offered to a traffic stream by a DS domain. - A Traffic Conditioning Specification (TCS) is a set of parameters and their values which together specify a set of classfier rules and a traffic profile. A TCS is an integral element of an SLS. Note that the definition of "Traffic stream" is unchanged from RFC 2475. A traffic stream can be an individual microflow or a group of microflows (i.e., in a source or destination DS domain) or it can be a BA. Thus, an SLS may apply in the source or destination DS domain to a single microflow or group of microflows, as well as to a BA in any DS domain. 2. PHB Group RFC 2475 deines a PHB group to be: "a set of one or more PHBs that can only be meaningfully specified and implemented simultaneously, due to a common constraint applying to all PHBs in the set such as a queue servicing or queue Grossman [Page 2] draft-ietf-diffserv-new-terms-00.txt October 1999 management policy. A PHB group provides a service building block that allows a set of related forwarding behaviors to be specified together (e.g., four dropping priorities). A single PHB is a special case of a PHB group." RFC 2497 [3] is entitled "Assured Forwarding PHB Group", and uses the term AF PHB group consistently in discussing the set of twelve AF PHBs. However, this usage is not consistent with RFC 2475. There is no common constraint which applies to BAs having different AF classes. Indeed, packets having different AF classes must be forwarded independently. Therefore, each of the four AF classes constitutes a separate PHB group, each having three PHBs corresponding to three drop precedences. A new definition is thus needed to describe a set of related PHB groups. PHB Group Family: a set of two or more PHB groups which are specified together and have similar relationships among their constituent PHBs, but which lack any common constraint. A PHB group family provides a service building block that allows a set of related PHB groups to be specified together (e.g., three classes of PHB groups). 3. Definition of the DS Field Diffserv uses six bits of the IPV4 or IPV6 header to convey the Diffserv Codepoint (DSCP), which selects a PHB. RFC 2474 attempts to rename the TOS octet of the IPV4 header, and Traffic Class octet of the IPV6 header, respectively, to the DS field. The DS Field has a six bit Diffserv Codepoint and two "currently unused bits". Several participants in the Diffserv working group have pointed out that this leads to inconsistencies. In particular, the CU bits of the DS Field have not been assigned to Diffserv (and in fact are being used by RFC 2481 [] for explicit congestion notification). A DSCP is, depending on context, either an encoding which selects a PHB or a sub- field in the DS field which contains that encoding. [Author's note: there was no working group consensus on this subject. This is my attempt at an intellectually satisfying solution, albeit one that will require readers to switch between two sets of terminology until RFC 2474 can be updated] For use in future drafts, including the next update to RFC 2474, the following definitions should apply: - the Differentiated Services Field (DSField) is the six most significant bits of either the IPV4 TOS octet or the IPV6 Traffic Class octet. Grossman [Page 3] draft-ietf-diffserv-new-terms-00.txt October 1999 - the Differentiated Services Codepoint (DSCP) is a value which is encoded in the DS field, and which each DS Node MUST use to select the PHB which is to be experienced by each packet it forwards. The two least significant bits of the IPV4 TOS octet and the IPV6 Traffic Class octet are not presently used by Diffserv. 4. Ordered aggregates and PHB scheduling classes Work on Diffserv support by MPLS LSRs led to the realization that a concept was needed in Diffserv to capture the notion of a set of BAs with a common ordering constraint. This presently applies to AF behavior aggregates, since a DS node may not reorder packets of the same microflow if they belong to the same AF class. This would, for example, prevent an MPLS LSR which was also a DS node from discriminating between packets of an AF BA based on drop precedence and forwarding packets of the same AF class but different drop precedence over different LSPs. The following new terms are defined. PHB Scheduling Class: A PHB group for which a common constraint is that ordering of packets must be preserved Ordered Aggregate (OA): A set of Behavior Aggregates that share an ordering constraint. All of the packets of an OA are members of the same PHB scheduling class. 5. Security Considerations Security considerations are addressed in RFC 2475. Acknowledgements References [1] RFC 2474. [2] Blake, Black, Carlson, Davies, Wang and Weiss "An Architecture for Differentiated Services", RFC 2475, December 1998. [3] Heinanen and Guerin, "Assured Forwarding PHB Group", RFC 2497 Author's Address Grossman [Page 4] draft-ietf-diffserv-new-terms-00.txt October 1999 Dan Grossman Motorola, Inc. 20 Cabot Blvd. Mansfield, MA 02048 Email: dan@dma.isg.mot.com Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. 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