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   cells received on a link with a particular label, and the label
   switching semantics govern the next-hop selection, allowing a traffic
   stream to follow a specially engineered path through the network.
   This improved granularity comes at the cost of additional management
   and configuration requirements to establish and maintain the label
   switched paths.  In addition, the amount of forwarding state
   maintained at each node scales in proportion to the number of edge
   nodes of the network in the best case (assuming multipoint-to-point





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   label switched paths), and it scales in proportion with the square of
   the number of edge nodes in the worst case, when edge-edge label
   switched paths with provisioned resources are employed.

   The Integrated Services/RSVP model relies upon traditional datagram
   forwarding in the default case, but allows sources and receivers to
   exchange signaling messages which establish additional packet
   classification and forwarding state on each node along the path
   between them [RFC1633, RSVP].  In the absence of state aggregation,
   the amount of state on each node scales in proportion to the number
   of concurrent reservations, which can be potentially large on high-
   speed links.  This model also requires application support for the
   RSVP signaling protocol.  Differentiated services mechanisms can be
   utilized to aggregate Integrated Services/RSVP state in the core of
   the network [Bernet].

   A variant of the Integrated Services/RSVP model eliminates the
   requirement for hop-by-hop signaling by utilizing only "static"
   classification and forwarding policies which are implemented in each
   node along a network path.  These policies are updated on
   administrative timescales and not in response to the instantaneous
   mix of microflows active in the network.  The state requirements for
   this variant are potentially worse than those encountered when RSVP
   is used, especially in backbone nodes, since the number of static
   policies that might be applicable at a node over time may be larger
   than the number of active sender-receiver sessions that might have
   installed reservation state on a node.  Although the support of large
   numbers of classifier rules and forwarding policies may be
   computationally feasible, the management burden associated with
   installing and maintaining these rules on each node within a backbone
   network which might be traversed by a traffic stream is substantial.

   Although we contrast our architecture with these alternative models
   of service differentiation, it should be noted that links and nodes
   employing these techniques may be utilized to extend differentiated
   services behaviors and semantics across a layer-2 switched
   infrastructure (e.g., 802.1p LANs, Frame Relay/ATM backbones)
   interconnecting DS nodes, and in the case of MPLS may be used as an
   alternative intra-domain implementation technology.  The constraints
   imposed by the use of a specific link-layer technology in particular
   regions of a DS domain (or in a network providing access to DS
   domains) may imply the differentiation of traffic on a coarser grain
   basis.  Depending on the mapping of PHBs to different link-layer
   services and the way in which packets are scheduled over a restricted
   set of priority classes (or virtual circuits of different category
   and capacity), all or a subset of the PHBs in use may be supportable
   (or may be indistinguishable).




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2.  Differentiated Services Architectural Model

   The differentiated services architecture is based on a simple model
   where traffic entering a network is classified and possibly
   conditioned at the boundaries of the network, and assigned to
   different behavior aggregates.  Each behavior aggregate is identified
   by a single DS codepoint.  Within the core of the network, packets
   are forwarded according to the per-hop behavior associated with the
   DS codepoint.  In this section, we discuss the key components within
   a differentiated services region, traffic classification and
   conditioning functions, and how differentiated services are achieved
   through the combination of traffic conditioning and PHB-based
   forwarding.

2.1  Differentiated Services Domain

   A DS domain is a contiguous set of DS nodes which operate with a
   common service provisioning policy and set of PHB groups implemented
   on each node.  A DS domain has a well-defined boundary consisting of
   DS boundary nodes which classify and possibly condition ingress
   traffic to ensure that packets which transit the domain are
   appropriately marked to select a PHB from one of the PHB groups
   supported within the domain.  Nodes within the DS domain select the
   forwarding behavior for packets based on their DS codepoint, mapping
   that value to one of the supported PHBs using either the recommended
   codepoint->PHB mapping or a locally customized mapping [DSFIELD].
   Inclusion of non-DS-compliant nodes within a DS domain may result in
   unpredictable performance and may impede the ability to satisfy
   service level agreements (SLAs).

   A DS domain normally consists of one or more networks under the same
   administration; for example, an organization's intranet or an ISP.
   The administration of the domain is responsible for ensuring that
   adequate resources are provisioned and/or reserved to support the
   SLAs offered by the domain.

2.1.1  DS Boundary Nodes and Interior Nodes

   A DS domain consists of DS boundary nodes and DS interior nodes.  DS
   boundary nodes interconnect the DS domain to other DS or non-DS-
   capable domains, whilst DS interior nodes only connect to other DS
   interior or boundary nodes within the same DS domain.

   Both DS boundary nodes and interior nodes must be able to apply the
   appropriate PHB to packets based on the DS codepoint; otherwise
   unpredictable behavior may result.  In addition, DS boundary nodes
   may be required to perform traffic conditioning functions as defined
   by a traffic conditioning agreement (TCA) between their DS domain and



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   the peering domain which they connect to (see Sec. 2.3.3).

   Interior nodes may be able to perform limited traffic conditioning
   functions such as DS codepoint re-marking.  Interior nodes which
   implement more complex classification and traffic conditioning
   functions are analogous to DS boundary nodes (see Sec. 2.3.4.4).

   A host in a network containing a DS domain may act as a DS boundary
   node for traffic from applications running on that host; we therefore
   say that the host is within the DS domain.  If a host does not act as
   a boundary node, then the DS node topologically closest to that host
   acts as the DS boundary node for that host's traffic.

2.1.2  DS Ingress Node and Egress Node

   DS boundary nodes act both as a DS ingress node and as a DS egress
   node for different directions of traffic.  Traffic enters a DS domain
   at a DS ingress node and leaves a DS domain at a DS egress node.  A
   DS ingress node is responsible for ensuring that the traffic entering
   the DS domain conforms to any TCA between it and the other domain to
   which the ingress node is connected.  A DS egress node may perform
   traffic conditioning functions on traffic forwarded to a directly
   connected peering domain, depending on the details of the TCA between
   the two domains.  Note that a DS boundary node may act as a DS
   interior node for some set of interfaces.

2.2  Differentiated Services Region

   A differentiated services region (DS Region) is a set of one or more
   contiguous DS domains.  DS regions are capable of supporting
   differentiated services along paths which span the domains within the
   region.

   The DS domains in a DS region may support different PHB groups
   internally and different codepoint->PHB mappings.  However, to permit
   services which span across the domains, the peering DS domains must
   each establish a peering SLA which defines (either explicitly or
   implicitly) a TCA which specifies how transit traffic from one DS
   domain to another is conditioned at the boundary between the two DS
   domains.

   It is possible that several DS domains within a DS region may adopt a
   common service provisioning policy and may support a common set of
   PHB groups and codepoint mappings, thus eliminating the need for
   traffic conditioning between those DS domains.






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2.3  Traffic Classification and Conditioning

   Differentiated services are extended across a DS domain boundary by
   establishing a SLA between an upstream network and a downstream DS
   domain.  The SLA may specify packet classification and re-marking
   rules and may also specify traffic profiles and actions to traffic
   streams which are in- or out-of-profile (see Sec. 2.3.2).  The TCA
   between the domains is derived (explicitly or implicitly) from this
   SLA.

   The packet classification policy identifies the subset of traffic
   which may receive a differentiated service by being conditioned and/
   or mapped to one or more behavior aggregates (by DS codepoint re-
   marking) within the DS domain.

   Traffic conditioning performs metering, shaping, policing and/or re-
   marking to ensure that the traffic entering the DS domain conforms to
   the rules specified in the TCA, in accordance with the domain's
   service provisioning policy.  The extent of traffic conditioning
   required is dependent on the specifics of the service offering, and
   may range from simple codepoint re-marking to complex policing and
   shaping operations.  The details of traffic conditioning policies
   which are negotiated between networks is outside the scope of this
   document.

2.3.1  Classifiers

   Packet classifiers select packets in a traffic stream based on the
   content of some portion of the packet header.  We define two types of
   classifiers.  The BA (Behavior Aggregate) Classifier classifies
   packets based on the DS codepoint only.  The MF (Multi-Field)
   classifier selects packets based on the value of a combination of one
   or more header fields, such as source address, destination address,
   DS field, protocol ID, source port and destination port numbers, and
   other information such as incoming interface.

   Classifiers are used to "steer" packets matching some specified rule
   to an element of a traffic conditioner for further processing.
   Classifiers must be configured by some management procedure in
   accordance with the appropriate TCA.

   The classifier should authenticate the information which it uses to
   classify the packet (see Sec. 6).

   Note that in the event of upstream packet fragmentation, MF
   classifiers which examine the contents of transport-layer header
   fields may incorrectly classify packet fragments subsequent to the
   first.  A possible solution to this problem is to maintain



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   fragmentation state; however, this is not a general solution due to
   the possibility of upstream fragment re-ordering or divergent routing
   paths.  The policy to apply to packet fragments is outside the scope
   of this document.

2.3.2  Traffic Profiles

   A traffic profile specifies the temporal properties of a traffic
   stream selected by a classifier.  It provides rules for determining
   whether a particular packet is in-profile or out-of-profile.  For
   example, a profile based on a token bucket may look like:

     codepoint=X, use token-bucket r, b

   The above profile indicates that all packets marked with DS codepoint
   X should be measured against a token bucket meter with rate r and
   burst size b.  In this example out-of-profile packets are those
   packets in the traffic stream which arrive when insufficient tokens
   are available in the bucket.  The concept of in- and out-of-profile
   can be extended to more than two levels, e.g., multiple levels of
   conformance with a profile may be defined and enforced.

   Different conditioning actions may be applied to the in-profile
   packets and out-of-profile packets, or different accounting actions
   may be triggered.  In-profile packets may be allowed to enter the DS
   domain without further conditioning; or, alternatively, their DS
   codepoint may be changed.  The latter happens when the DS codepoint
   is set to a non-Default value for the first time [DSFIELD], or when
   the packets enter a DS domain that uses a different PHB group or
   codepoint->PHB mapping policy for this traffic stream.  Out-of-
   profile packets may be queued until they are in-profile (shaped),
   discarded (policed), marked with a new codepoint (re-marked), or