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. 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 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. 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]. 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). 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.
[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.
[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.
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