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rfc:rfc3140

Network Working Group D. Black Request for Comments: 3140 S. Brim Obsoletes: 2836 B. Carpenter Category: Standards Track F. Le Faucheur

                                                             June 2001
               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 (2001).  All Rights Reserved.

Abstract

 This document defines a 16 bit encoding mechanism for the
 identification of differentiated services Per Hop Behaviors in
 protocol messages.  It replaces RFC 2836.

Table of Contents

 1. Introduction.................................................2
 1.1. Usage Scenarios............................................2
 2. Encoding.....................................................3
 3. Signalling the Class Selector Codepoints.....................4
 4. IANA Considerations..........................................5
 5. Security Considerations......................................5
 Changes from RFC 2836...........................................5
 Acknowledgements................................................6
 References......................................................6
 Authors' Addresses..............................................6
 Intellectual Property...........................................7
 Full Copyright Statement........................................8

Black, et al. Standards Track [Page 1] RFC 3140 Per Hop Behavior Identification Codes June 2001

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.
 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
 diffserv.
 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.
 This document replaces RFC 2836, which omitted considerations for the
 Class Selector codepoints.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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.

Black, et al. Standards Track [Page 2] RFC 3140 Per Hop Behavior Identification Codes June 2001

 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].
 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 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

Black, et al. Standards Track [Page 3] RFC 3140 Per Hop Behavior Identification Codes June 2001

 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.
 In both cases, when a single PHBID is used to identify a set of PHBs
 (i.e., bit 14 is set to 1), that set of PHBs MUST constitute a PHB
 Scheduling Class (i.e., use of PHBs from the set MUST NOT cause
 intra-microflow traffic reordering when different PHBs from the set
 are applied to traffic in the same microflow).  The set of AF1x PHBs
 [RFC 2597] is an example of a PHB Scheduling Class.  Sets of PHBs
 that do not constitute a PHB Scheduling Class can be identified by
 using more than one PHBID.

3. Signalling the Class Selector Codepoints

 [RFC 2474] defines the eight DS codepoint values of the form 'xxx000'
 (where x may be '0' or '1') as the Class Selector Codepoints.
 Codepoint 000000 is the recommended DSCP value for the Default PHB,
 and hence the Case 1 PHBID constructed from that codepoint is used to
 signal the Default PHB (see Section 2 above).
 For convenience and consistent operation with networks that employ IP
 Precedence [RFC 1812], the Case 1 format PHBIDs constructed from the
 other seven Class Selector Codepoints may also be used to signal
 PHBs.  In each case, the PHB signaled by such a PHBID is the PHB to
 which the embedded class selector codepoint (or IP Precedence value
 that corresponds to it in non-diffserv domains) is mapped in the
 recipient's network.  Note that different networks will employ
 different mappings; see Section 4 of [RFC 2474] for further
 discussion.
 Any specified use of PHBIDs SHOULD allow the use of the eight Case 1
 PHBIDs constructed from the Class Selector Codepoints.

Black, et al. Standards Track [Page 4] RFC 3140 Per Hop Behavior Identification Codes June 2001

4. 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.

5. 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.
 Just signalling a PHBID SHOULD NOT be sufficient to grant the sender
 access to a PHB that it would otherwise not be able to use.  In cases
 where this is an issue, receivers SHOULD treat received PHBIDs as
 requests for service, and use local policy to determine whether to
 grant or deny such requests.

Changes from RFC 2836

 [RFC 2836] did not consider the Class Selector code points, which are
 covered by section 3 of the present document. A clarification has
 been added at the end of section 2 for the case of PHB Scheduling
 Classes.  The second paragraph of section 5 has been added.

Black, et al. Standards Track [Page 5] RFC 3140 Per Hop Behavior Identification Codes June 2001

Acknowledgements

 Useful comments were made by members of the IETF Diffserv working
 group.

References

 [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
             1998.
 [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.
 [RFC 2836]  Brim, S., Carpenter, B. and F. Le Faucheur, "Per Hop
             Behavior Identification Codes", RFC 2836, May 2000.
 [MPLS-DS]   Le Faucheur, F., et al., "MPLS Support of Differentiated
             Services", Work in Progress.

Authors' Addresses

 David L. Black
 EMC Corporation
 42 South St.
 Hopkinton, MA   01748
 EMail: black_david@emc.com
 Scott W. Brim
 146 Honness Lane
 Ithaca, NY 14850
 USA
 EMail: sbrim@cisco.com

Black, et al. Standards Track [Page 6] RFC 3140 Per Hop Behavior Identification Codes June 2001

 Brian E. Carpenter
 IBM
 c/o iCAIR
 Suite 150
 1890 Maple Avenue
 Evanston, IL 60201
 USA
 EMail: brian@icair.org
 Francois Le Faucheur
 Cisco Systems
 Petra B - Les Lucioles
 291, rue Albert Caquot
 06560 Valbonne
 France
 EMail: flefauch@cisco.com

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
 Director.

Black, et al. Standards Track [Page 7] RFC 3140 Per Hop Behavior Identification Codes June 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Black, et al. Standards Track [Page 8]

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