Network Working Group D. Black
Request for Comments: 3140 S. Brim
Obsoletes: 2836 B. Carpenter
Category: Standards Track F. Le FaucheurJune 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 (C) The Internet Society (2001). All Rights Reserved.
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.................................................21.1. Usage Scenarios............................................22. Encoding.....................................................33. Signalling the Class Selector Codepoints.....................44. IANA Considerations..........................................55. Security Considerations......................................5
Changes from RFC 2836...........................................5
Full Copyright Statement........................................8
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
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.
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.
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).
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
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
Any specified use of PHBIDs SHOULD allow the use of the eight Case 1
PHBIDs constructed from the Class Selector Codepoints.
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:
- 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 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.
Useful comments were made by members of the IETF Diffserv working
[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.
[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.
David L. Black
42 South St.
Hopkinton, MA 01748
Scott W. Brim
146 Honness Lane
Ithaca, NY 14850
Brian E. Carpenter
1890 Maple Avenue
Evanston, IL 60201
Francois Le Faucheur
Petra B - Les Lucioles
291, rue Albert Caquot
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