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Network Working Group                                           D. BlackRequest for Comments: 3140                                       S. BrimObsoletes:2836                                             B. CarpenterCategory: Standards Track                                 F. Le Faucheur                                                               June 2001Per Hop Behavior Identification CodesStatus 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 replacesRFC 2836.Table of Contents1. Introduction.................................................21.1. Usage Scenarios............................................22. Encoding.....................................................33. Signalling the Class Selector Codepoints.....................44. IANA Considerations..........................................55. Security Considerations......................................5   Changes fromRFC 2836...........................................5   Acknowledgements................................................6   References......................................................6   Authors' Addresses..............................................6   Intellectual Property...........................................7   Full Copyright Statement........................................8Black, et al.               Standards Track                     [Page 1]

RFC 3140         Per Hop Behavior Identification Codes         June 20011. 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 replacesRFC 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 (seeSection 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; seeSection 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 20014. 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:      -  the identity of the assignee      -  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 fromRFC 2836   [RFC 2836] did not consider the Class Selector code points, which are   covered bysection 3 of the present document. A clarification has   been added at the end ofsection 2 for the case of PHB Scheduling   Classes.  The second paragraph ofsection 5 has been added.Black, et al.               Standards Track                     [Page 5]

RFC 3140         Per Hop Behavior Identification Codes         June 2001Acknowledgements   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.comBlack, 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.comIntellectual 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 inBCP-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 2001Full 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]