INTERNET-DRAFT Kathleen Nichols
Diffserv Working Group Bay Networks
Expires: January 1999 Steven Blake
Torrent Networking Technologies
Fred Baker
Cisco Systems
David L. Black
The Open Group
July 1998
Definition of the Differentiated Services Field (DS Field)
in the IPv4 and IPv6 Headers
<draft-ietf-diffserv-header-01.txt>
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Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
Differentiated services are intended to provide 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. Services can be constructed by a combination of:
- setting bits in an IP header field at network boundaries and
administrative boundaries,
- 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
and is 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].
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 done 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 queueing 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. These
behaviors and the 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
premature. Further, much of the details of service construction are
covered by legal agreements between different business entities and
we avoid this as 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 recommended values 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 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 draft 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 IP 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. We present
examples of class selector compliant PHB implementations in the
Appendix.
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 boundary in a particular direction. The terms "aggregate"
and "behavior aggregate" are used interchangeably in this document.
Boundary: the edge of a DS domain, where classifiers and traffic
conditioners are likely to be deployed. A 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 boundary typically is found at the ingress to
the first-hop differentiated services-compliant router (or network
node) a host's packets traverse, or at the egress of the last-hop
differentiated services-compliant router or network node packets
traverse before arriving at a host. This is sometimes referred to as
the boundary at a leaf router. A boundary might be co-located with a
host, subject to local policy.
Classifier: an entity which selects packets based on the content of
packet headers according to defined rules.
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 Service-compliant: in compliance with the requirements
specified in this document.
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 a per-hop behavior according to a
particular algorithm.
Microflow: a single instance of an application-to-application flow of
packets which is identified by source address, source port,
destination address, destination port and protocol id.
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.
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 service level 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.
Traffic conditioner: an entity which performs traffic conditioning
functions and which may contain meters, policers, shapers, and
markers. Traffic conditioners are typically deployed in boundary
nodes only.
Service: a description of the overall treatment of a customer's
traffic within a particular domain, within 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 hop 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 domain and 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 DS field classifier which can
select a differential output queue servicing discipline (or PHB)
based on the codepoint in the DS field SHOULD be in all network
nodes of a differentiated services domain. The classifiers
and traffic conditioners are configured in accordance with some
service specification, a matter of administrative policy outside the
scope of this document.
Additional differentiated services definitions are in [ARCH].
3. Differentiated Services Field Definition
A new 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 may be assigned later, e.g., for explicit
congestion notification. The value of the CU bits MUST be
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
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 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, but
operators MAY choose to use different codepoints, either in addition
to or in place of the recommended default. Note that if operators do
so choose, re-marking of DS fields will 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 recommended default codepoint for a PHB must be unique
for codepoints in the standard space (see Sec. 6).
The exceptions to configurability are for codepoints 'xxx000' and are
noted in Sec. 4.
Packets received with an unrecognized codepoint SHOULD be forwarded
as if they were marked for the Default behavior (see Sec. 4). Such
packets MUST NOT cause the network node to crash.
The structure of the DS field shown above is incompatible with the
existing definition of the IPv4 TOS octet in [RFC791, RFC1349]. The
presumption is that differentiated services domains protect
themselves by deploying re-marking boundary nodes, as should networks
using the RFC 791 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 network boundaries is sensible in any case since an
upstream node can easily set it to any random value. Differentiated
services domains which are not suitably isolated by a re-marking
boundary node 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
network 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, described in this section. Backwards compatibility is
addressed in two ways. First, per-hop behaviors that are
already in widespread use MUST be included in a DS-compliant
node. 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 [RFC791], though the specific differentiated services
PHBs mapped to by those codepoints MAY have additional
specifications.
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" or else the network provider might become quite
unpopular. This permits senders that are not differentiated
services-aware to continue to use the network in the same manner as
today and to have the same kinds of expectations from the network
regarding service. The RECOMMENDED codpoint 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, RFC1349].
A packet initially marked for the Default behavior MAY be re-marked
with another codepoint as it passes a boundary into another DS domain
so that it will be forwarded using a different PHB within the 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 quite similarly to the use proposed
here for the DSCP field. Thus, a simple prototype diffserv
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 service, early commercial
routers and UNIX IP forwarding code generally did not. As networks
became more complex and customer requirements grew, commercial
routers 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 looked at IP
addresses, IP protocol numbers, TCP or UDP ports, and other header
fields. IP Precedence was and is among the options such filters can
look at.
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 existing forwarding treatments selected by the IP
Precedence field. In addition, we give a set of codepoints that MUST
map to PHBs meeting 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 the 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
The DS field values of 'xxx000|xx', 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 meet the Class
Selector PHB requirements in addition to preserving the Default PHB
requirement on codepoint '000000'.
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. 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. Although a particular packet marked with a
lower relative order Class Selector codepoint might be observed to be
forwarded earlier than a particular packet marked with a higher
relative order, sample observations that take place over a reasonable
period of network history will conform to this requirement. A
discarded packet is considered to be an extreme case of untimely
forwarding.
Further, PHBs selected by distinct Class Selector codepoints SHOULD
be independently forwarded; that is, packets marked with different
Class Selector codepoints MAY be reordered. A network node MAY have
limits on the amount of the node's resources that can be used by each
of these PHBs.
PHB groups whose specification meets these requirements are referred
to as Class Selector Compliant PHBs.
It is easy to see that the Class Selector PHB Requirements on
codepoint '000000' are compatible with those listed in Sec. 4.1
for the Default PHB.
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 might 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, WFQ or variants
[HPFQA], weighted round-robin queueing (WRR), 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 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 availble in a set
of PHBs that did no more than meet the minimum Class Selector PHB
requirements. More examples are given in the Appendix.
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
Thirty-two DS codepoints are reserved for standardization as
RECOMMENDED codepoints, and 32 codepoints are reserved for
experimental or local use (EXP/LU) (see Sec. 6 for further details).
Codepoint space Usage Policy
--------------- ------------
xxxxx0 Reserved for RECOMMENDED codepoints
xxxxx1 EXP/LU
The behavioral characteristics of a PHB are to be standardized, and
not the 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 combined with WFQ or variants [HPFQA], weighted round-robin
queueing, or CBQ [CBQ].
It is RECOMMENDED that PHB implementations do not introduce any
packet reordering within a microflow. Where PHBs are defined as a
group, their definitions MUST specify any possible packet re-ordering
implications which may occur if different packets within a microflow
are marked for different PHBs within the group.
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.
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. This
specification has left room in the codepoint space to allow for
evolution, thus the defined space is intentionally sparse.
6. IANA Considerations
The DSCP field in 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 defines the assignment of eight 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 out 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.
More extensive treatment of the security concerns raised by the overall
differentiated services architecture are discussed in [ARCH].
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 network boundaries
with security and integrity of the network infrastructure within a
DS domain. Network boundary nodes MUST ensure that all traffic entering
the domain has codepoint values appropriate to the traffic and the domain,
remarking the traffic with new codepoint values if necessary.
These 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 in 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 to prevent the arrival of traffic with
inappropriate codepoints 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 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
tunneling 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 tunneling 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
[ACTIVE] B. Braden, et. al., "Recommendations on Queue Management
and Congestion Avoidance in the Internet", Internet RFC
2309, April 1998.
[AH] S. Kent and R. Atkinson, "IP Authentication Header",
Internet Draft <draft-ietf-ipsec-auth-header-07.txt>,
July 1998.
[ARCH] Differentiated Services Architecture Document (work in
preparation).
[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.
[CLARK] D. Clark and W. Fang, "Explicit Allocation of Best
Effort Packet Delivery Service",
http://diffserv.lcs.mit.edu/Papers/exp-alloc-ddc-wf.pdf
[CONS] T. Narten and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", Internet Draft
<draft-iesg-iana-considerations-03.txt>, March 1998.
[CCBES] C. Lefelhocz, B. Lyles, S. Shenker, and L. Zhang,
"Congestion Control for Best-Effort Service: Why We Need
a New Paradigm", IEEE Network, Vol. 10, no. 1, January
1996.
[ESP] S. Kent and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", Internet Draft
<draft-ietf-ipsec-esp-v2-06.txt>, July 1998.
[HPFQA] J. Bennett and Hui Zhang, "Hierarchical Packet Fair
Queueing Algorithms", Proc. ACM SIGCOMM 96, August 1996.
[IPv6] S. Deering and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", Internet Draft
<draft-ietf-ipngwg-ipv6-spec-v2-01.txt>, November 1997.
[RFC791] Information Sciences Institute, "Internet Protocol",
Internet RFC 791, September 1981.
[RFC1122] RFC 1122, "Requirements for Internet hosts -
communication layers". R.T. Braden. Oct-01-1989.
[RFC1349] P. Almquist, "Type of Service in the Internet Protocol
Suite", Internet RFC 1349, July 1992.
[RFC1812] F. Baker, editor, "Requirements for IP Version 4
Routers", Internet RFC 1812, June 1995.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", Internet RFC 2119, March 1997.
[2BIT] K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit
Differentiated Services Architecture for the Internet",
ftp://ftp.ee.lbl.gov/papers/dsarch.pdf.
Appendix. Examples of Class Selector Compliant PHB Implementations
This appendix shows how three different mechanisms can be used to
implement Class Selector Compliant PHBs.
A.1 Priority Queues
This approach employs eight queues where each of the eight codepoints
maps to a different queue. Queues are ranked in priority order so
that each queue, from the perspective of the next lower priority
queue, implements a "low loss, low delay" forwarding behavior. As an
alternative, one can employ four queues, where bits 0 and 1 of the
codepoint are used to select the queue. Bit 2 of the codepoint is
used as a drop preference within the queue. RED is used within the
queues according to its usual parameters, but with in-profile traffic
having a higher min-threshold and max-threshold than out-of-profile
traffic, and therefore experiencing a higher probability of timely
delivery. Out-of-profile traffic should consider the presence of
lower order in-profile traffic in the calculation of drop
probability [CLARK].
The strength of this approach is that order is maintained within each
precedence queue, but higher ordered traffic may be sent before lower
ordered traffic. It has a weakness, however, in that apart from
admission control and policing, it affords lower precedence traffic
no assurance of eventual transmission.
A.2 Round Robin Queuing
Like the previous example, this approach employs a queue per
codepoint, but each queue is emptied at some non-zero rate, in round-
robin order, rather than being given simple priority service.
The strength of this approach is that higher ordered traffic is, on
average, queued for a shorter duration than lower ordered traffic.
It also avoids the lockout issue that priority queuing systems
experience. A counter-intuitive scenario can occur, however, if a
high rate queue is heavily utilized while a lower rate queue is
under-utilized; a packet directed to the lower rate queue can
actually be better protected from loss and variation in delay when
placed in an empty or very short queue.
A.3 Virtual Circuit or Virtual Channel Selection
The difference between this approach and Round Robin Queuing is
somewhat academic. If one has a serial line to a routing neighbor,
and manages the link using a load sharing algorithm, the load sharing
algorithm in some sense emulates the way the line would behave if it
were in reality a number of different lines, or if it were one
channelized line. In a virtual circuit selection model, the
emulation becomes reality - one deploys a set of rate-limited VCs to
a routing neighbor, and uses them in the same way one would otherwise
have used round-robin queues.
The strengths and weaknesses are very similar to those of Round Robin
Queuing, except that this allows one to capitalize on the
capabilities of a link layer such as ATM or Frame Relay.
Authors' Addresses
Kathleen Nichols
Bay Networks Architecture Lab
4401 Great America Parkway
SC01-04
Santa Clara, CA 95052-8185
Phone: +1-408-495-3252
Fax: +1-408-495-1299
E-mail: knichols@baynetworks.com
Steven Blake
Torrent Networking Technologies (effective 7/15)
E-mail: slblake@intercenter.net
Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, California 93111
Phone: (408) 526-4257
Email: fred@cisco.com
David L. Black
The Open Group
11 Cambridge Center
Cambridge, MA 02142
Phone: (617) 621-7347
Email: d.black@opengroup.org
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