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Network Working Group S. Brim Request for Comments: 2836 B. Carpenter Category: Standards Track F. Le Faucheur

                                                              May 2000
               Per Hop Behavior Identification Codes

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Table of Contents:

 1. Introduction................................................. 1
 1.1. Usage Scenarios............................................ 2
 2. Encoding..................................................... 3
 3. IANA Considerations.......................................... 4
 4. Security considerations...................................... 4
 References...................................................... 4
 Authors' Addresses.............................................. 5
 Intellectual Property........................................... 6
 Full Copyright Statement........................................ 7

1. Introduction

 Differentiated Services [RFC 2474, RFC 2475] introduces the notion of
 Per Hop Behaviors (PHBs) that define how traffic belonging to a
 particular behavior aggregate is treated at an individual network
 node. In IP packet headers, PHBs are not indicated as such; instead
 Differentiated Services Codepoint (DSCP) values are used. There are
 only 64 possible DSCP values, but there is no such limit on the
 number of PHBs. In a given network domain, there is a locally defined
 mapping between DSCP values and PHBs. Standardized PHBs recommend a
 DSCP mapping, but network operators may choose alternative mappings.

Brim, et al. Standards Track [Page 1] RFC 2836 Per Hop Behavior Identification Codes May 2000

 In some cases it is necessary or desirable to identify a particular
 PHB in a protocol message, such as a message negotiating bandwidth
 management or path selection, especially when such messages pass
 between management domains. Examples where work is in progress
 include communication between bandwidth brokers, and MPLS support of
 In certain cases, what needs to be identified is not an individual
 PHB, but a set of PHBs. One example is a set of PHBs that must follow
 the same physical path to prevent re-ordering.  An instance of this
 is the set of three PHBs belonging to a single Assured Forwarding
 class, such as the PHBs AF11, AF12 and AF13 [RFC 2597].
 This document defines a binary encoding to uniquely identify PHBs
 and/or sets of PHBs in protocol messages. This encoding MUST be used
 when such identification is required.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [RFC2119].

1.1. Usage Scenarios

 Diffserv services are expected to be supported over various
 underlying technologies which we broadly refer to as "link layers"
 for the purpose of this discussion. For the transport of IP packets,
 some of these link layers make use of connections or logical
 connections where the forwarding behavior supported by each link
 layer device is a property of the connection. In particular, within
 the link layer domain, each link layer node will schedule traffic
 depending on which connection the traffic is transported in. Examples
 of such "link layers" include ATM and MPLS.
 For efficient support of diffserv over these link layers, one model
 is for different Behavior Aggregates (BAs) (or sets of Behavior
 Aggregates) to be transported over different connections so that they
 are granted different (and appropriate) forwarding behaviors inside
 the link layer cloud. When those connections are dynamically
 established for the transport of diffserv traffic, it is very useful
 to communicate at connection establishment time what forwarding
 behavior(s) is(are) to be granted to each connection by the link
 layer device so that the BAs transported experience consistent
 forwarding behavior inside the link layer cloud. This can be achieved
 by including in the connection establishment signaling messages the
 encoding of the corresponding PHB, or set of PHBs, as defined in this
 document.  Details on proposed usage of PHB encodings by some MPLS
 label distribution protocols (RSVP and LDP) for support of Diff-Serv
 over MPLS, can be found in [MPLS-DS].

Brim, et al. Standards Track [Page 2] RFC 2836 Per Hop Behavior Identification Codes May 2000

 In another approach, the ATM Forum has a requirement to indicate
 desired IP QOS treatments in ATM signaling, so that ATM switches can
 be just as supportive of the desired service as are IP forwarders.
 To do so the Forum is defining a new VC call setup information
 element is which will carry PHB identification codes (although will
 be generalized to do more if needed).

2. Encoding

 PHBs and sets of PHBs are encoded in an unsigned 16 bit binary field.
 The 16 bit field is arranged as follows:
 Case 1: PHBs defined by standards action, as per [RFC 2474].
 The encoding for a single PHB is the recommended DSCP value for that
 PHB, left-justified in the 16 bit field, with bits 6 through 15 set
 to zero.  Note that the recommended DSCP value MUST be used, even if
 the network in question has chosen a different mapping.
 The encoding for a set of PHBs is the numerically smallest of the set
 of encodings for the various PHBs in the set, with bit 14 set to 1.
 (Thus for the AF1x PHBs, the encoding is that of the AF11 PHB, with
 bit 14 set to 1.)
     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   |         DSCP          | 0   0   0   0   0   0   0   0   X   0 |
 Case 2: PHBs not defined by standards action, i.e. experimental or
 local use PHBs as allowed by [RFC 2474]. In this case an arbitrary 12
 bit PHB identification code, assigned by the IANA, is placed left-
 justified in the 16 bit field. Bit 15 is set to 1, and bit 14 is zero
 for a single PHB or 1 for a set of PHBs.  Bits 12 and 13 are zero.
     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   |                      PHB id code              | 0   0   X   1 |
 Bits 12 and 13 are reserved either for expansion of the PHB
 identification code, or for other use, at some point in the future.

Brim, et al. Standards Track [Page 3] RFC 2836 Per Hop Behavior Identification Codes May 2000

3. IANA Considerations

 IANA is requested to create a new assignment registry for "Per-Hop
 Behavior Identification Codes", initially allowing values in the
 range 0 to 4095 decimal.
 Assignment of values in this field require:
  1. the identity of the assignee
  2. a brief description of the new PHB, with enough detail to

distinguish it from existing standardized and non-standardized

    PHBs. In the case of a set of PHBs, this description should cover
    all PHBs in the set.
   -a reference to a stable document describing the PHB in detail.
 During the first year of existence of this registry, IANA is
 requested to refer all requests to the IETF diffserv WG for review.
 Subsequently, requests should be reviewed by the IETF Transport Area
 Directors or by an expert that they designate.
 If the number of assignments begins to approach 4096, the Transport
 Area Directors should be alerted.

4. Security Considerations

 This encoding in itself raises no security issues. However, users of
 this encoding should consider that modifying a PHB identification
 code may constitute theft or denial of service, so protocols using
 this encoding must be adequately protected.


 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC 2474] 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
 [RFC 2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
            and W. Weiss, "An Architecture for Differentiated
            Services", RFC 2475, December 1998.
 [RFC 2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
            "Assured Forwarding PHB Group", RFC 2597, June 1999.

Brim, et al. Standards Track [Page 4] RFC 2836 Per Hop Behavior Identification Codes May 2000

 [MPLS-DS]  MPLS Support of Differentiated Services, Francois Le
            Faucheur, Liwen Wu, Bruce Davie, Shahram Davari, Pasi
            Vaananen, Ram Krishnan, Pierrick Cheval, Juha Heinanen,
            Work in Progress.

Authors' Addresses

 Scott W. Brim
 146 Honness Lane
 Ithaca, NY 14850
 Brian E. Carpenter
 c/o iCAIR
 Suite 150
 1890 Maple Avenue
 Evanston, IL 60201
 Francois Le Faucheur
 Cisco Systems
 Petra B - Les Lucioles
 291, rue Albert Caquot
 06560 Valbonne

Brim, et al. Standards Track [Page 5] RFC 2836 Per Hop Behavior Identification Codes May 2000

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive

Brim, et al. Standards Track [Page 6] RFC 2836 Per Hop Behavior Identification Codes May 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
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 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Brim, et al. Standards Track [Page 7]

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