PHB ID codes: Per Hop Behavior Identification Codes
Compact 16-bit codes to identify how a router should treat traffic.
Brim, S., Carpenter, B., and F. Le Faucheur, "Per Hop Behavior Identification Codes", RFC 2836, DOI 10.17487/RFC2836, May 2000, <https://www.rfc-editor.org/info/rfc2836>.
@misc{rfc2836,
series = {Request for Comments},
number = 2836,
howpublished = {RFC 2836},
publisher = {RFC Editor},
doi = {10.17487/RFC2836},
url = {https://www.rfc-editor.org/info/rfc2836},
author = {S. Brim and B. Carpenter and F. Le Faucheur},
title = {{Per Hop Behavior Identification Codes}},
pagetotal = 7,
year = 2000,
month = may,
} RFC text
Contents
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.
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].
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).
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.
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:
-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.
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.
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.
[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
USA
EMail: sbrim@cisco.com
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
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Full Copyright Statement
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AI ELI5
On this page
§1Identifying Traffic Treatment Behaviors
RFC 2836 defines a compact way to identify Per Hop Behaviors (PHBs), the rules that determine how a router treats traffic at that hop. PHBs are the foundation of Differentiated Services (DiffServ), which allows networks to offer different service levels for different traffic.
In IP packet headers, PHBs are indicated by the Differentiated Services Codepoint (DSCP), a 6-bit field. But there are only 64 possible DSCP values. A network operator can map each DSCP to a PHB, and can define additional PHBs that do not have a standardized DSCP mapping. When routers negotiate bandwidth management or establish MPLS label-switched paths, they must be able to name the PHB or set of PHBs that a connection should receive—and the 64 DSCP values alone are not always enough.
Routers can treat traffic in different ways (fast, reliable, cheap, etc.). Each way is called a PHB. When routers talk to each other about which treatment to use, they need short names for them.
§2PHBs and PHB Sets
A PHB is a forwarding behavior that a router applies to traffic: for example, "low latency for video" or "best effort." In some cases, multiple related PHBs must be handled together—for example, the Assured Forwarding class defines three PHBs (AF11, AF12, AF13) that must follow the same physical path to prevent packet reordering. This group of PHBs is called a PHB set.
The RFC encoding must handle both single PHBs and PHB sets. It also distinguishes between standardized PHBs, which have a recommended DSCP value defined in an RFC, and experimental or local-use PHBs, which are specific to a network operator and must be registered with IANA.
A PHB is one kind of treatment. Sometimes several related treatments must stay together, called a set. Some treatments are official standards; others are made up for a particular network.
§3The 16-bit Encoding
PHBs are encoded in an unsigned 16-bit field. The encoding has two cases:
Case 1: Standardized PHBs use the recommended DSCP value, left-justified in the 16-bit field, with the lower 10 bits set to zero. For a set of standardized PHBs, the encoding is the smallest DSCP value in the set, with bit 14 set to 1 to mark it as a set (for example, the AF1x class uses the AF11 DSCP with bit 14 set).
Case 2: Non-standardized PHBs use a 12-bit identification code assigned by IANA, left-justified in the field. Bit 15 is set to 1 to distinguish this case from Case 1. Bit 14 indicates whether the code refers to a single PHB (0) or a set (1). Bits 12 and 13 are reserved for future expansion.
The code is just 16 bits long. Standardized kinds use their DSCP number; non-standard ones use a number from an IANA registry. A flag bit tells whether it is one kind or a group of related kinds.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
DSCP | DSCP value | 6 bits | 6-bit DSCP value, left-justified. | |
ZERO-1 | Reserved | 3 bits | Set to zero. | |
SET-FLAG | Set flag | 1 bits | 0 for single PHB, 1 for set of PHBs. | |
ZERO-2 | Reserved | 6 bits | Set to zero. |
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
PHB-CODE | PHB code | 12 bits | 12-bit IANA-assigned identification code. | |
RESERVED | Reserved | 2 bits | Bits 12-13, reserved for future use. | |
SET-FLAG | Set flag | 1 bits | 0 for single PHB, 1 for set of PHBs. | |
CASE | Case | 1 bits | 1 | Set to 1 for non-standardized PHBs. |
§4Usage in Networks
PHB codes are used when bandwidth brokers negotiate QoS guarantees across a network, when MPLS establishes label-switched paths that should receive specific treatment, and when ATM switches need to know what forwarding behavior to apply to a virtual circuit. Instead of exchanging a long description of the desired behavior, endpoints can send the compact 16-bit PHB code.
For example, a bandwidth broker might use a PHB code to tell an ATM switch, "traffic on this circuit should receive low-latency treatment." An RSVP or LDP protocol message can include a PHB code to request that an MPLS tunnel be granted a specific forwarding behavior at each hop.
When a network device asks for a certain kind of treatment (like 'give this low latency'), it sends a 16-bit code instead of describing it in words. Other devices understand what the code means.
§5Registry and Assignments
IANA maintains a registry of Per Hop Behavior Identification Codes for non-standardized PHBs, initially allowing values from 0 to 4095 (12 bits). Assignments require the applicant's identity, a brief description of the PHB, and a reference to a detailed specification. During the first year, IANA forwards requests to the IETF DiffServ Working Group for review. After that, the Transport Area Directors or a designated expert reviews requests.
Modifying a PHB identification code could constitute theft or denial of service, so protocols that use these codes must be adequately protected by encryption and authentication.
New, non-standard treatments are registered with IANA so everyone knows what the codes mean. The code itself does not provide security, so the protocol using it must protect against tampering.