RFC2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers英文版

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Network Working Group K. Nichols
Request for Comments: 2474 Cisco Systems
Obsoletes: 1455, 1349 S. Blake
Category: Standards Track Torrent Networking Technologies
F. Baker
Cisco Systems
D. Black
EMC Corporation
December 1998

Definition of the Differentiated Services Field (DS Field)
in the IPv4 and IPv6 Headers

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 (1998). All Rights Reserved.

Abstract

Differentiated services enhancements to the Internet protocol are
intended to enable scalable service discrimination in the Internet
without the need for per-flow state and signaling at every hop. A
variety of services may be built from a small, well-defined set of
building blocks which are deployed in network nodes. The services
may be either end-to-end or intra-domain; they include both those
that can satisfy quantitative performance requirements (e.g., peak
bandwidth) and those based on relative performance (e.g., "class"
differentiation). Services can be constrUCted by a combination of:

- setting bits in an IP header field at network boundaries
(autonomous system boundaries, internal administrative boundaries,
or hosts),
- using those bits to determine how packets are forwarded by the
nodes inside the network, and
- conditioning the marked packets at network boundaries in accordance
with the requirements or rules of each service.

The requirements or rules of each service must be set through
administrative policy mechanisms which are outside the scope of this
document. A differentiated services-compliant network node includes
a classifier that selects packets based on the value of the DS field,
along with buffer management and packet scheduling mechanisms capable
of delivering the specific packet forwarding treatment indicated by
the DS field value. Setting of the DS field and conditioning of the
temporal behavior of marked packets need only be performed at network
boundaries and may vary in complexity.

This document defines the IP header field, called the DS (for
differentiated services) field. In IPv4, it defines the layout of
the TOS octet; in IPv6, the Traffic Class octet. In addition, a base
set of packet forwarding treatments, or per-hop behaviors, is
defined.

For a more complete understanding of differentiated services, see
also the differentiated services architecture [ARCH].

Table of Contents

1. Introduction ................................................. 3
2. Terminology Used in This Document ............................ 5
3. Differentiated Services Field Definition ..................... 7
4. Historical Codepoint Definitions and PHB Requirements ........ 9
4.1 A Default PHB ............................................. 9
4.2 Once and Future IP Precedence Field Use ................... 10
4.2.1 IP Precedence History and Evolution in Brief .......... 10
4.2.2 Subsuming IP Precedence into Class Selector .......... 11
Codepoints
4.2.2.1 The Class Selector Codepoints ..................... 11
4.2.2.2 The Class Selector PHB Requirements ............... 11
4.2.2.3 Using the Class Selector PHB Requirements ......... 12
for IP Precedence Compatibility
4.2.2.4 Example Mechanisms for Implementing Class ......... 12
Selector Compliant PHB Groups
4.3 Summary ................................................... 13
5. Per-Hop Behavior Standardization Guidelines .................. 13
6. IANA Considerations .......................................... 14
7. Security Considerations ...................................... 15
7.1 Theft and Denial of Service ............................... 15
7.2 IPsec and Tunneling Interactions .......................... 16
8. Acknowledgments .............................................. 17
9. References ................................................... 17
Authors‘ Addresses ............................................... 19
Full Copyright Statement ......................................... 20

1. Introduction

Differentiated services are intended to provide a framework and
building blocks to enable deployment of scalable service
discrimination in the Internet. The differentiated services approach
aims to speed deployment by separating the architecture into two
major components, one of which is fairly well-understood and the
other of which is just beginning to be understood. In this, we are
guided by the original design of the Internet where the decision was
made to separate the forwarding and routing components. Packet
forwarding is the relatively simple task that needs to be performed
on a per-packet basis as quickly as possible. Forwarding uses the
packet header to find an entry in a routing table that determines the
packet‘s output interface. Routing sets the entries in that table
and may need to reflect a range of transit and other policies as well
as to keep track of route failures. Routing tables are maintained as
a background process to the forwarding task. Further, routing is the
more complex task and it has continued to evolve over the past 20
years.

Analogously, the differentiated services architecture contains two
main components. One is the fairly well-understood behavior in the
forwarding path and the other is the more complex and still emerging
background policy and allocation component that configures parameters
used in the forwarding path. The forwarding path behaviors include
the differential treatment an individual packet receives, as
implemented by queue service disciplines and/or queue management
disciplines. These per-hop behaviors are useful and required in
network nodes to deliver differentiated treatment of packets no
matter how we construct end-to-end or intra-domain services. Our
focus is on the general semantics of the behaviors rather than the
specific mechanisms used to implement them since these behaviors will
evolve less rapidly than the mechanisms.

Per-hop behaviors and mechanisms to select them on a per-packet basis
can be deployed in network nodes today and it is this aspect of the
differentiated services architecture that is being addressed first.
In addition, the forwarding path may require that some monitoring,
policing, and shaping be done on the network traffic designated for
"special" treatment in order to enforce requirements associated with
the delivery of the special treatment. Mechanisms for this kind of
traffic conditioning are also fairly well-understood. The wide
deployment of such traffic conditioners is also important to enable
the construction of services, though their actual use in constructing
services may evolve over time.

The configuration of network elements with respect to which packets
get special treatment and what kinds of rules are to be applied to
the use of resources is much less well-understood. Nevertheless, it
is possible to deploy useful differentiated services in networks by
using simple policies and static configurations. As described in
[ARCH], there are a number of ways to compose per-hop behaviors and
traffic conditioners to create services. In the process, additional
eXPerience is gained that will guide more complex policies and
allocations. The basic behaviors in the forwarding path can remain
the same while this component of the architecture evolves.
Experiences with the construction of such services will continue for
some time, thus we avoid standardizing this construction as it is
premature. Further, much of the details of service construction are
covered by legal agreements between different business entities and
we avoid this as it is very much outside the scope of the IETF.

This document concentrates on the forwarding path component. In the
packet forwarding path, differentiated services are realized by
mapping the codepoint contained in a field in the IP packet header to
a particular forwarding treatment, or per-hop behavior (PHB), at each
network node along its path. The codepoints may be chosen from a set
of mandatory values defined later in this document, from a set of
recommended values to be defined in future documents, or may have
purely local meaning. PHBs are expected to be implemented by
employing a range of queue service and/or queue management
disciplines on a network node‘s output interface queue: for example
weighted round-robin (WRR) queue servicing or drop-preference queue
management.

Marking is performed by traffic conditioners at network boundaries,
including the edges of the network (first-hop router or source host)
and administrative boundaries. Traffic conditioners may include the
primitives of marking, metering, policing and shaping (these
mechanisms are described in [ARCH]). Services are realized by the
use of particular packet classification and traffic conditioning
mechanisms at boundaries coupled with the concatenation of per-hop
behaviors along the transit path of the traffic. A goal of the
differentiated services architecture is to specify these building
blocks for future extensibility, both of the number and type of the
building blocks and of the services built from them.

Terminology used in this memo is defined in Sec. 2. The
differentiated services field definition (DS field) is given in Sec.
3. In Sec. 4, we discuss the desire for partial backwards
compatibility with current use of the IPv4 Precedence field. As a
solution, we introduce Class Selector Codepoints and Class Selector

Compliant PHBs. Sec. 5 presents guidelines for per-hop behavior
standardization. Sec. 6 discusses guidelines for allocation of
codepoints. Sec. 7 covers security considerations.

This document is a concise description of the DS field and its uses.
It is intended to be read along with the differentiated services
architecture [ARCH].

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].

2. Terminology Used in This Document

Behavior Aggregate: a collection of packets with the same codepoint
crossing a link in a particular direction. The terms "aggregate" and
"behavior aggregate" are used interchangeably in this document.

Classifier: an entity which selects packets based on the content of
packet headers according to defined rules.

Class Selector Codepoint: any of the eight codepoints in the range ‘
xxx000‘ (where ‘x‘ may equal ‘0‘ or ‘1‘). Class Selector Codepoints
are discussed in Sec. 4.2.2.

Class Selector Compliant PHB: a per-hop behavior satisfying the Class
Selector PHB Requirements specified in Sec. 4.2.2.2.

Codepoint: a specific value of the DSCP portion of the DS field.
Recommended codepoints SHOULD map to specific, standardized PHBs.
Multiple codepoints MAY map to the same PHB.

Differentiated Services Boundary: the edge of a DS domain, where
classifiers and traffic conditioners are likely to be deployed. A
differentiated services boundary can be further sub-divided into
ingress and egress nodes, where the ingress/egress nodes are the
downstream/upstream nodes of a boundary link in a given traffic
direction. A differentiated services boundary typically is found at
the ingress to the first-hop differentiated services-compliant router
(or network node) that a host‘s packets traverse, or at the egress of
the last-hop differentiated services-compliant router or network node
that packets traverse before arriving at a host. This is sometimes
referred to as the boundary at a leaf router. A differentiated
services boundary may be co-located with a host, subject to local
policy. Also DS boundary.

Differentiated Services-Compliant: in compliance with the
requirements specified in this document. Also DS-compliant.

Differentiated Services Domain: a contiguous portion of the Internet
over which a consistent set of differentiated services policies are
administered in a coordinated fashion. A differentiated services
domain can represent different administrative domains or autonomous
systems, different trust regions, different network technologies
(e.g., cell/frame), hosts and routers, etc. Also DS domain.

Differentiated Services Field: the IPv4 header TOS octet or the IPv6
Traffic Class octet when interpreted in conformance with the
definition given in this document. Also DS field.

Mechanism: The implementation of one or more per-hop behaviors
according to a particular algorithm.

Microflow: a single instance of an application-to-application flow of
packets which is identified by source address, destination address,
protocol id, and source port, destination port (where applicable).

Per-hop Behavior (PHB): a description of the externally observable
forwarding treatment applied at a differentiated services-compliant
node to a behavior aggregate. The description of a PHB SHOULD be
sufficiently detailed to allow the construction of predictable
services, as documented in [ARCH].

Per-hop Behavior Group: a set of one or more PHBs that can only be
meaningfully specified and implemented simultaneously, due to a
common constraint applying to all PHBs in the set such as a queue
servicing or queue management policy. Also PHB Group.

Traffic Conditioning: control functions that can be applied to a
behavior aggregate, application flow, or other operationally useful
subset of traffic, e.g., routing updates. These MAY include
metering, policing, shaping, and packet marking. Traffic
conditioning is used to enforce agreements between domains and to
condition traffic to receive a differentiated service within a domain
by marking packets with the appropriate codepoint in the DS field and
by monitoring and altering the temporal characteristics of the
aggregate where necessary. See [ARCH].

Traffic Conditioner: an entity that performs traffic conditioning
functions and which MAY contain meters, policers, shapers, and
markers. Traffic conditioners are typically deployed in DS boundary
nodes (i.e., not in interior nodes of a DS domain).

Service: a description of the overall treatment of (a subset of) a
customer‘s traffic across a particular domain, across a set of
interconnected DS domains, or end-to-end. Service descriptions are
covered by administrative policy and services are constructed by

applying traffic conditioning to create behavior aggregates which
experience a known PHB at each node within the DS domain. Multiple
services can be supported by a single per-hop behavior used in
concert with a range of traffic conditioners.

To summarize, classifiers and traffic conditioners are used to select
which packets are to be added to behavior aggregates. Aggregates
receive differentiated treatment in a DS domain and traffic
conditioners MAY alter the temporal characteristics of the aggregate
to conform to some requirements. A packet‘s DS field is used to
designate the packet‘s behavior aggregate and is subsequently used to
determine which forwarding treatment the packet receives. A behavior
aggregate classifier which can select a PHB, for example a
differential output queue servicing discipline, based on the
codepoint in the DS field SHOULD be included in all network nodes in
a DS domain. The classifiers and traffic conditioners at DS
boundaries are configured in accordance with some service
specification, a matter of administrative policy outside the scope of
this document.

Additional differentiated services definitions are given in [ARCH].

3. Differentiated Services Field Definition

A replacement header field, called the DS field, is defined, which is
intended to supersede the existing definitions of the IPv4 TOS octet
[RFC791] and the IPv6 Traffic Class octet [IPv6].

Six bits of the DS field are used as a codepoint (DSCP) to select the
PHB a packet experiences at each node. A two-bit currently unused
(CU) field is reserved and its definition and interpretation are
outside the scope of this document. The value of the CU bits are
ignored by differentiated services-compliant nodes when determining
the per-hop behavior to apply to a received packet.

The DS field structure is presented below:

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
DSCP CU 
+---+---+---+---+---+---+---+---+

DSCP: differentiated services codepoint
CU: currently unused

In a DSCP value notation ‘xxxxxx‘ (where ‘x‘ may equal ‘0‘ or ‘1‘)
used in this document, the left-most bit signifies bit 0 of the DS
field (as shown above), and the right-most bit signifies bit 5.

Implementors should note that the DSCP field is six bits wide. DS-
compliant nodes MUST select PHBs by matching against the entire 6-bit
DSCP field, e.g., by treating the value of the field as a table index
which is used to select a particular packet handling mechanism which
has been implemented in that device. The value of the CU field MUST
be ignored by PHB selection. The DSCP field is defined as an
unstructured field to facilitate the definition of future per-hop
behaviors.

With some exceptions noted below, the mapping of codepoints to PHBs
MUST be configurable. A DS-compliant node MUST support the logical
equivalent of a configurable mapping table from codepoints to PHBs.
PHB specifications MUST include a recommended default codepoint,
which MUST be unique for codepoints in the standard space (see Sec.
6). Implementations should support the recommended codepoint-to-PHB
mappings in their default configuration. Operators may choose to use
different codepoints for a PHB, either in addition to or in place of
the recommended default. Note that if operators do so choose, re-
marking of DS fields may be necessary at administrative boundaries
even if the same PHBs are implemented on both sides of the boundary.

See [ARCH] for further discussion of re-marking.

The exceptions to general configurability are for codepoints ‘xxx000‘
and are noted in Secs. 4.2.2 and 4.3.

Packets received with an unrecognized codepoint SHOULD be forwarded
as if they were marked for the Default behavior (see Sec. 4), and
their codepoints should not be changed. Such packets MUST NOT cause
the network node to malfunction.

The structure of the DS field shown above is incompatible with the
existing definition of the IPv4 TOS octet in [RFC791]. The
presumption is that DS domains protect themselves by deploying re-
marking boundary nodes, as should networks using the RFC791
Precedence designations. Correct operational procedure SHOULD follow
[RFC791], which states: "If the actual use of these precedence
designations is of concern to a particular network, it is the
responsibility of that network to control the Access to, and use of,
those precedence designations." Validating the value of the DS field
at DS boundaries is sensible in any case since an upstream node can
easily set it to any arbitrary value. DS domains that are not
isolated by suitably configured boundary nodes may deliver
unpredictable service.

Nodes MAY rewrite the DS field as needed to provide a desired local
or end-to-end service. Specifications of DS field translations at DS
boundaries are the subject of service level agreements between
providers and users, and are outside the scope of this document.
Standardized PHBs allow providers to build their services from a
well-known set of packet forwarding treatments that can be expected
to be present in the equipment of many vendors.

4. Historical Codepoint Definitions and PHB Requirements

The DS field will have a limited backwards compatibility with current
practice, as described in this section. Backwards compatibility is
addressed in two ways. First, there are per-hop behaviors that are
already in widespread use (e.g., those satisfying the IPv4 Precedence
queueing requirements specified in [RFC1812]), and we wish to permit
their continued use in DS-compliant nodes. In addition, there are
some codepoints that correspond to historical use of the IP
Precedence field and we reserve these codepoints to map to PHBs that
meet the general requirements specified in Sec. 4.2.2.2, though the
specific differentiated services PHBs mapped to by those codepoints
MAY have additional specifications.

No attempt is made to maintain backwards compatibility with the "DTR"
or TOS bits of the IPv4 TOS octet, as defined in [RFC791].

4.1 A Default PHB

A "default" PHB MUST be available in a DS-compliant node. This is
the common, best-effort forwarding behavior available in existing
routers as standardized in [RFC1812]. When no other agreements are
in place, it is assumed that packets belong to this aggregate. Such
packets MAY be sent into a network without adhering to any particular
rules and the network will deliver as many of these packets as
possible and as soon as possible, subject to other resource policy
constraints. A reasonable implementation of this PHB would be a
queueing discipline that sends packets of this aggregate whenever the
output link is not required to satisfy another PHB. A reasonable
policy for constructing services would ensure that the aggregate was
not "starved". This could be enforced by a mechanism in each node
that reserves some minimal resources (e.g, buffers, bandwidth) for
Default behavior aggregates. This permits senders that are not
differentiated services-aware to continue to use the network in the
same manner as today. The impact of the introduction of
differentiated services into a domain on the service expectations of
its customers and peers is a complex matter involving policy
decisions by the domain and is outside the scope of this document.
The RECOMMENDED codepoint for the Default PHB is the bit pattern ‘
000000‘; the value ‘000000‘ MUST map to a PHB that meets these

specifications. The codepoint chosen for Default behavior is
compatible with existing practice [RFC791]. Where a codepoint is not
mapped to a standardized or local use PHB, it SHOULD be mapped to the
Default PHB.

A packet initially marked for the Default behavior MAY be re-marked
with another codepoint as it passes a boundary into a DS domain so
that it will be forwarded using a different PHB within that domain,
possibly subject to some negotiated agreement between the peering
domains.

4.2 Once and Future IP Precedence Field Use

We wish to maintain some form of backward compatibility with present
uses of the IP Precedence Field: bits 0-2 of the IPv4 TOS octet.
Routers exist that use the IP Precedence field to select different
per-hop forwarding treatments in a similar way to the use proposed
here for the DSCP field. Thus, a simple prototype differentiated
services architecture can be quickly deployed by appropriately
configuring these routers. Further, IP systems today understand the
location of the IP Precedence field, and thus if these bits are used
in a similar manner as DS-compliant equipment is deployed,
significant failures are not likely during early deployment. In
other words, strict DS-compliance need not be ubiquitous even within
a single service provider‘s network if bits 0-2 of the DSCP field are
employed in a manner similar to, or subsuming, the deployed uses of
the IP Precedence field.

4.2.1 IP Precedence History and Evolution in Brief

The IP Precedence field is something of a forerunner of the DS field.
IP Precedence, and the IP Precedence Field, were first defined in
[RFC791]. The values that the three-bit IP Precedence Field might
take were assigned to various uses, including network control
traffic, routing traffic, and various levels of privilege. The least
level of privilege was deemed "routine traffic". In [RFC791], the
notion of Precedence was defined broadly as "An independent measure
of the importance of this datagram." Not all values of the IP
Precedence field were assumed to have meaning across boundaries, for
instance "The Network Control precedence designation is intended to
be used within a network only. The actual use and control of that
designation is up to each network." [RFC791]

Although early BBN IMPs implemented the Precedence feature, early
commercial routers and UNIX IP forwarding code generally did not. As
networks became more complex and customer requirements grew,
commercial router vendors developed ways to implement various kinds
of queueing services including priority queueing, which were

generally based on policies encoded in filters in the routers, which
examined IP addresses, IP protocol numbers, TCP or UDP ports, and
other header fields. IP Precedence was and is among the options such
filters can examine.

In short, IP Precedence is widely deployed and widely used, if not in
exactly the manner intended in [RFC791]. This was recognized in
[RFC1122], which states that while the use of the IP Precedence field
is valid, the specific assignment of the priorities in [RFC791] were
merely historical.

4.2.2 Subsuming IP Precedence into Class Selector Codepoints

A specification of the packet forwarding treatments selected by the
IP Precedence field today would have to be quite general; probably
not specific enough to build predictable services from in the
differentiated services framework. To preserve partial backwards
compatibility with known current uses of the IP Precedence field
without sacrificing future flexibility, we have taken the approach of
describing minimum requirements on a set of PHBs that are compatible
with most of the deployed forwarding treatments selected by the IP
Precedence field. In addition, we give a set of codepoints that MUST
map to PHBs meeting these minimum requirements. The PHBs mapped to
by these codepoints MAY have a more detailed list of specifications
in addition to the required ones stated here. Other codepoints MAY
map to these same PHBs. We refer to this set of codepoints as the
Class Selector Codepoints, and the minimum requirements for PHBs that
these codepoints may map to are called the Class Selector PHB
Requirements.

4.2.2.1 The Class Selector Codepoints

A specification of the packet forwarding treatments selected by the
The DS field values of ‘xxx000xx‘, or DSCP = ‘xxx000‘ and CU
subfield unspecified, are reserved as a set of Class Selector
Codepoints. PHBs which are mapped to by these codepoints MUST
satisfy the Class Selector PHB requirements in addition to preserving
the Default PHB requirement on codepoint ‘000000‘ (Sec. 4.1).

4.2.2.2 The Class Selector PHB Requirements

We refer to a Class Selector Codepoint with a larger numerical value
than another Class Selector Codepoint as having a higher relative
order while a Class Selector Codepoint with a smaller numerical value
than another Class Selector Codepoint is said to have a lower
relative order. The set of PHBs mapped to by the eight Class
Selector Codepoints MUST yield at least two independently forwarded
classes of traffic, and PHBs selected by a Class Selector Codepoint

SHOULD give packets a probability of timely forwarding that is not
lower than that given to packets marked with a Class Selector
codepoint of lower relative order, under reasonable operating
conditions and traffic loads. A discarded packet is considered to be
an extreme case of untimely forwarding. In addition, PHBs selected
by codepoints ‘11x000‘ MUST give packets a preferential forwarding
treatment by comparison to the PHB selected by codepoint ‘000000‘ to
preserve the common usage of IP Precedence values ‘110‘ and ‘111‘ for
routing traffic.

Further, PHBs selected by distinct Class Selector Codepoints SHOULD
be independently forwarded; that is, packets marked with different
Class Selector Codepoints MAY be re-ordered. A network node MAY
enforce limits on the amount of the node‘s resources that can be
utilized by each of these PHBs.

PHB groups whose specification satisfy these requirements are
referred to as Class Selector Compliant PHBs.

The Class Selector PHB Requirements on codepoint ‘000000‘ are
compatible with those listed for the Default PHB in Sec. 4.1.

4.2.2.3 Using the Class Selector PHB Requirements for IP Precedence
Compatibility

A DS-compliant network node can be deployed with a set of one or more
Class Selector Compliant PHB groups. This document states that the
set of codepoints ‘xxx000‘ MUST map to such a set of PHBs. As it is
also possible to map multiple codepoints to the same PHB, the vendor
or the network administrator MAY configure the network node to map
codepoints to PHBs irrespective of bits 3-5 of the DSCP field to
yield a network that is compatible with historical IP Precedence use.
Thus, for example, codepoint ‘011010‘ would map to the same PHB as
codepoint ‘011000‘.

4.2.2.4 Example Mechanisms for Implementing Class Selector Compliant
PHB Groups

Class Selector Compliant PHBs can be realized by a variety of
mechanisms, including strict priority queueing, weighted fair
queueing (WFQ), WRR, or variants [RPS, HPFQA, DRR], or Class-Based
Queuing [CBQ]. The distinction between PHBs and mechanisms is
described in more detail in Sec. 5.

It is important to note that these mechanisms might be available
through other PHBs (standardized or not) that are available in a
particular vendor‘s equipment. For example, future documents may
standardize a Strict Priority Queueing PHB group for a set of

recommended codepoints. A network administrator might configure
those routers to select the Strict Priority Queueing PHBs with
codepoints ‘xxx000‘ in conformance with the requirements of this
document.

As a further example, another vendor might employ a CBQ mechanism in
its routers. The CBQ mechanism could be used to implement the Strict
Priority Queueing PHBs as well as a set of Class Selector Compliant
PHBs with a wider range of features than would be available in a set
of PHBs that did no more than meet the minimum Class Selector PHB
requirements.

4.3 Summary

This document defines codepoints ‘xxx000‘ as the Class Selector
codepoints, where PHBs selected by these codepoints MUST meet the
Class Selector PHB Requirements described in Sec. 4.2.2.2. This is
done to preserve a useful level of backward compatibility with
current uses of the IP Precedence field in the Internet without
unduly limiting future flexibility. In addition, codepoint ‘000000‘
is used as the Default PHB value for the Internet and, as such, is
not configurable. The remaining seven non-zero Class Selector
codepoints are configurable only to the extent that they map to PHBs
that meet the requirements in Sec. 4.2.2.2.

5. Per-Hop Behavior Standardization Guidelines

The behavioral characteristics of a PHB are to be standardized, and
not the particular algorithms or the mechanisms used to implement
them. A node may have a (possibly large) set of parameters that can
be used to control how packets are scheduled onto an output interface
(e.g., N separate queues with settable priorities, queue lengths,
round-robin weights, drop algorithm, drop preference weights and
thresholds, etc). To illustrate the distinction between a PHB and a
mechanism, we point out that Class Selector Compliant PHBs might be
implemented by several mechanisms, including: strict priority
queueing, WFQ, WRR, or variants [HPFQA, RPS, DRR], or CBQ [CBQ], in
isolation or in combination.

PHBs may be specified individually, or as a group (a single PHB is a
special case of a PHB group). A PHB group usually consists of a set
of two or more PHBs that can only be meaningfully specified and
implemented simultaneously, due to a common constraint applying to
each PHB within the group, such as a queue servicing or queue
management policy. A PHB group specification SHOULD describe
conditions under which a packet might be re-marked to select another
PHB within the group. It is RECOMMENDED that PHB implementations do
not introduce any packet re-ordering within a microflow. PHB group

specifications MUST identify any possible packet re-ordering
implications which may occur for each individual PHB, and which may
occur if different packets within a microflow are marked for
different PHBs within the group.

Only those per-hop behaviors that are not described by an existing
PHB standard, and have been implemented, deployed, and shown to be
useful, SHOULD be standardized. Since current experience with
differentiated services is quite limited, it is premature to
hypothesize the exact specification of these per-hop behaviors.

Each standardized PHB MUST have an associated RECOMMENDED codepoint,
allocated out of a space of 32 codepoints (see Sec. 6). This
specification has left room in the codepoint space to allow for
evolution, thus the defined space (‘xxx000‘) is intentionally sparse.

Network equipment vendors are free to offer whatever parameters and
capabilities are deemed useful or marketable. When a particular,
standardized PHB is implemented in a node, a vendor MAY use any
algorithm that satisfies the definition of the PHB according to the
standard. The node‘s capabilities and its particular configuration
determine the different ways that packets can be treated.

Service providers are not required to use the same node mechanisms or
configurations to enable service differentiation within their
networks, and are free to configure the node parameters in whatever
way that is appropriate for their service offerings and traffic
engineering objectives. Over time certain common per-hop behaviors
are likely to evolve (i.e., ones that are particularly useful for
implementing end-to-end services) and these MAY be associated with
particular EXP/LU PHB codepoints in the DS field, allowing use across
domain boundaries (see Sec. 6). These PHBs are candidates for future
standardization.

It is RECOMMENDED that standardized PHBs be specified in accordance
with the guidelines set out in [ARCH].

6. IANA Considerations

The DSCP field within the DS field is capable of conveying 64
distinct codepoints. The codepoint space is divided into three pools
for the purpose of codepoint assignment and management: a pool of 32
RECOMMENDED codepoints (Pool 1) to be assigned by Standards Action as
defined in [CONS], a pool of 16 codepoints (Pool 2) to be reserved
for experimental or Local Use (EXP/LU) as defined in [CONS], and a
pool of 16 codepoints (Pool 3) which are initially available for
experimental or local use, but which should be preferentially

utilized for standardized assignments if Pool 1 is ever exhausted.
The pools are defined in the following table (where ‘x‘ refers to
either ‘0‘ or ‘1‘):

Pool Codepoint space Assignment Policy
---- --------------- -----------------

1 xxxxx0 Standards Action
2 xxxx11 EXP/LU
3 xxxx01 EXP/LU (*)

(*) may be utilized for future Standards Action allocations as
necessary

This document assigns eight RECOMMENDED codepoints (‘xxx000‘) which
are drawn from Pool 1 above. These codepoints MUST be mapped, not to
specific PHBs, but to PHBs that meet "at least" the requirements set
forth in Sec. 4.2.2.2 to provide a minimal level of backwards
compatibility with IP Precedence as defined in [RFC791] and as
deployed in some current equipment.

7. Security Considerations

This section considers security issues raised by the introduction of
differentiated services, primarily the potential for denial-of-
service attacks, and the related potential for theft of service by
unauthorized traffic (Section 7.1). Section 7.2 addresses the
operation of differentiated services in the presence of IPsec
including its interaction with IPsec tunnel mode and other tunnelling
protocols. See [ARCH] for more extensive treatment of the security
concerns raised by the overall differentiated services architecture.

7.1 Theft and Denial of Service

The primary goal of differentiated services is to allow different
levels of service to be provided for traffic streams on a common
network infrastructure. A variety of techniques may be used to
achieve this, but the end result will be that some packets receive
different (e.g., better) service than others. The mapping of network
traffic to the specific behaviors that result in different (e.g.,
better or worse) service is indicated primarily by the DS codepoint,
and hence an adversary may be able to oBTain better service by
modifying the codepoint to values indicating behaviors used for
enhanced services or by injecting packets with such codepoint values.
Taken to its limits, such theft-of-service becomes a denial-of-
service attack when the modified or injected traffic depletes the
resources available to forward it and other traffic streams.

The defense against this class of theft- and denial-of-service
attacks consists of the combination of traffic conditioning at DS
domain boundaries with security and integrity of the network
infrastructure within a DS domain. DS domain boundary nodes MUST
ensure that all traffic entering the domain is marked with codepoint
values appropriate to the traffic and the domain, remarking the
traffic with new codepoint values if necessary. These DS boundary
nodes are the primary line of defense against theft- and denial-of-
service attacks based on modified codepoints, as success of any such
attack indicates that the codepoints used by the attacking traffic
were inappropriate. An important instance of a boundary node is that
any traffic-originating node within a DS domain is the initial
boundary node for that traffic. Interior nodes in a DS domain rely
on DS codepoints to associate traffic with the forwarding PHBs, and
are NOT REQUIRED to check codepoint values before using them. As a
result, the interior nodes depend on the correct operation of the DS
domain boundary nodes to prevent the arrival of traffic with
inappropriate codepoints or in excess of provisioned levels that
would disrupt operation of the domain.

7.2 IPsec and Tunnelling Interactions

The IPsec protocol, as defined in [ESP, AH], does not include the IP
header‘s DS field in any of its cryptographic calculations (in the
case of tunnel mode, it is the outer IP header‘s DS field that is not
included). Hence modification of the DS field by a network node has
no effect on IPsec‘s end-to-end security, because it cannot cause any
IPsec integrity check to fail. As a consequence, IPsec does not
provide any defense against an adversary‘s modification of the DS
field (i.e., a man-in-the-middle attack), as the adversary‘s
modification will also have no effect on IPsec‘s end-to-end security.

IPsec‘s tunnel mode provides security for the encapsulated IP
header‘s DS field. A tunnel mode IPsec packet contains two IP
headers: an outer header supplied by the tunnel ingress node and an
encapsulated inner header supplied by the original source of the
packet. When an IPsec tunnel is hosted (in whole or in part) on a
differentiated services network, the intermediate network nodes
operate on the DS field in the outer header. At the tunnel egress
node, IPsec processing includes removing the outer header and
forwarding the packet (if required) using the inner header. The
IPsec protocol REQUIRES that the inner header‘s DS field not be
changed by this decapsulation processing to ensure that modifications
to the DS field cannot be used to launch theft- or denial-of-service
attacks across an IPsec tunnel endpoint. This document makes no
change to that requirement. If the inner IP header has not been
processed by a DS boundary node for the tunnel egress node‘s DS

domain, the tunnel egress node is the boundary node for traffic
exiting the tunnel, and hence MUST ensure that the resulting traffic
has appropriate DS codepoints.

When IPsec tunnel egress decapsulation processing includes a
sufficiently strong cryptographic integrity check of the encapsulated
packet (where sufficiency is determined by local security policy),
the tunnel egress node can safely assume that the DS field in the
inner header has the same value as it had at the tunnel ingress node.
An important consequence is that otherwise insecure links within a DS
domain can be secured by a sufficiently strong IPsec tunnel. This
analysis and its implications apply to any tunnelling protocol that
performs integrity checks, but the level of assurance of the inner
header‘s DS field depends on the strength of the integrity check
performed by the tunnelling protocol. In the absence of sufficient
assurance for a tunnel that may transit nodes outside the current DS
domain (or is otherwise vulnerable), the encapsulated packet MUST be
treated as if it had arrived at a boundary from outside the DS
domain.

8. Acknowledgements

The authors would like to acknowledge the Differentiated Services
Working Group for discussions which helped shape this document.

9. References

[AH] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC2402, November 1998.

[ARCH] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Services", RFC2475, December 1998.

[CBQ] S. Floyd and V. Jacobson, "Link-sharing and Resource
Management Models for Packet Networks", IEEE/ACM
Transactions on Networking, Vol. 3 no. 4, pp. 365-386,
August 1995.

[CONS] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC2434, October
1998.

[DRR] M. Shreedhar and G. Varghese, Efficient Fair Queueing
using Deficit Round Robin", Proc. ACM SIGCOMM 95, 1995.

[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC2406, November 1998.

[HPFQA] J. Bennett and Hui Zhang, "Hierarchical Packet Fair
Queueing Algorithms", Proc. ACM SIGCOMM 96, August 1996.

[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.

[RFC791] Postel, J., Editor, "Internet Protocol", STD 5, RFC791,
September 1981.

[RFC1122] Braden, R., "Requirements for Internet hosts -
communication layers", STD 3, RFC1122, October 1989.

[RFC1812] Baker, F., Editor, "Requirements for IP Version 4
Routers", RFC1812, June 1995.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RPS] D. Stiliadis and A. Varma, "Rate-Proportional Servers: A
Design Methodology for Fair Queueing Algorithms", IEEE/
ACM Trans. on Networking, April 1998.

Authors‘ Addresses

Kathleen Nichols
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134-1706

Phone: +1-408-525-4857
EMail: kmn@cisco.com

Steven Blake
Torrent Networking Technologies
3000 Aerial Center, Suite 140
Morrisville, NC 27560

Phone: +1-919-468-8466 x232
EMail: slblake@torrentnet.com

Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, CA 93111

Phone: +1-408-526-4257
EMail: fred@cisco.com

David L. Black
EMC Corporation
35 Parkwood Drive
Hopkinton, MA 01748

Phone: +1-508-435-1000 x76140
EMail: black_david@emc.com

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