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Network Working Group                                         K. NicholsRequest for Comments: 2474                                 Cisco SystemsObsoletes:1455,1349                                           S. BlakeCategory: Standards Track                Torrent Networking Technologies                                                                F. Baker                                                           Cisco Systems                                                                D. Black                                                         EMC Corporation                                                           December 1998Definition of the Differentiated Services Field (DS Field)in the IPv4 and IPv6 HeadersStatus 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.Nichols, et. al.            Standards Track                     [Page 1]

RFC 2474             Differentiated Services Field         December 1998   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 Contents1.  Introduction .................................................32.  Terminology Used in This Document ............................53.  Differentiated Services Field Definition .....................74.  Historical Codepoint Definitions and PHB Requirements ........94.1  A Default PHB .............................................94.2  Once and Future IP Precedence Field Use ...................104.2.1  IP Precedence History and Evolution in Brief ..........104.2.2  Subsuming IP Precedence into Class Selector  ..........11              Codepoints4.2.2.1  The Class Selector Codepoints .....................114.2.2.2  The Class Selector PHB Requirements ...............114.2.2.3  Using the Class Selector PHB Requirements .........12                  for IP Precedence Compatibility4.2.2.4  Example Mechanisms for Implementing Class .........12                  Selector Compliant PHB Groups4.3  Summary ...................................................135.  Per-Hop Behavior Standardization Guidelines ..................136.  IANA Considerations ..........................................147.  Security Considerations ......................................157.1  Theft and Denial of Service ...............................157.2  IPsec and Tunneling Interactions ..........................168.  Acknowledgments ..............................................179.  References ...................................................17   Authors' Addresses ...............................................19   Full Copyright Statement .........................................20Nichols, et. al.            Standards Track                     [Page 2]

RFC 2474             Differentiated Services Field         December 19981.  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.Nichols, et. al.            Standards Track                     [Page 3]

RFC 2474             Differentiated Services Field         December 1998   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 SelectorNichols, et. al.            Standards Track                     [Page 4]

RFC 2474             Differentiated Services Field         December 1998   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.Nichols, et. al.            Standards Track                     [Page 5]

RFC 2474             Differentiated Services Field         December 1998   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 byNichols, et. al.            Standards Track                     [Page 6]

RFC 2474             Differentiated Services Field         December 1998   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 unusedNichols, et. al.            Standards Track                     [Page 7]

RFC 2474             Differentiated Services Field         December 1998   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 theRFC 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 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.Nichols, et. al.            Standards Track                     [Page 8]

RFC 2474             Differentiated Services Field         December 1998   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 theseNichols, et. al.            Standards Track                     [Page 9]

RFC 2474             Differentiated Services Field         December 1998   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 wereNichols, et. al.            Standards Track                    [Page 10]

RFC 2474             Differentiated Services Field         December 1998   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 '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   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 CodepointNichols, et. al.            Standards Track                    [Page 11]

RFC 2474             Differentiated Services Field         December 1998   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 ofNichols, et. al.            Standards Track                    [Page 12]

RFC 2474             Differentiated Services Field         December 1998   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 groupNichols, et. al.            Standards Track                    [Page 13]

RFC 2474             Differentiated Services Field         December 1998   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 preferentiallyNichols, et. al.            Standards Track                    [Page 14]

RFC 2474             Differentiated Services Field         December 1998   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.Nichols, et. al.            Standards Track                    [Page 15]

RFC 2474             Differentiated Services Field         December 1998   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 DSNichols, et. al.            Standards Track                    [Page 16]

RFC 2474             Differentiated Services Field         December 1998   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",RFC 2402, November 1998.   [ARCH]      Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.               and W. Weiss, "An Architecture for Differentiated               Services",RFC 2475, 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",RFC 2434, October               1998.   [DRR]       M. Shreedhar and G. Varghese, Efficient Fair Queueing               using Deficit Round Robin", Proc. ACM SIGCOMM 95, 1995.Nichols, et. al.            Standards Track                    [Page 17]

RFC 2474             Differentiated Services Field         December 1998   [ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security               Payload (ESP)",RFC 2406, 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",RFC 2460, December 1998.   [RFC791]    Postel, J., Editor, "Internet Protocol", STD 5,RFC 791,               September 1981.   [RFC1122]   Braden, R., "Requirements for Internet hosts -               communication layers", STD 3,RFC 1122, October 1989.   [RFC1812]   Baker, F., Editor, "Requirements for IP Version 4               Routers",RFC 1812, June 1995.   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, 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.Nichols, et. al.            Standards Track                    [Page 18]

RFC 2474             Differentiated Services Field         December 1998Authors' 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.comNichols, et. al.            Standards Track                    [Page 19]

RFC 2474             Differentiated Services Field         December 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Nichols, et. al.            Standards Track                    [Page 20]