rfc2381.txt

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        CBR (BCOB-A)
        nrtVBR (BCOB-C)
        ABR

   Note that under UNI 3.x, there are equivalent services to CBR and
   nrtVBR, but not ABR.  The first, with a CBR/BCOB-A connection,
   provides a higher level of QoS than is necessary, but it may be
   convenient to simply allocate a fixed-rate "pipe", which we expect to
   be ubiquitously supported in ATM networks.  However unless this is
   the only choice available, it would probably be wasteful of network
   resources.

   The nrtVBR/BCOB-C category is perhaps the best match, since it
   provides for allocation of bandwidth and buffers with an additional
   peak rate indication, similar to the CLS TSpec.  Excess traffic can
   be handled by CLP bit tagging with VBR.

   The ABR category with a positive MCR aligns with the CLS idea of
   "best effort with a floor."  The ATM network agrees to forward cells
   with a rate of at least MCR, which MUST be directly converted from
   the token bucket rate of the receiver TSpec.  The bucket size
   parameter measures approximately the amount of buffer necessary at
   the IWF.  This buffer serves to absorb the bursts allowed by the
   token bucket, since they cannot be passed directly into an ABR VC.

   The rtVBR category can be used, although the edge device MUST then
   determine values for CTD and CDV.  Since there are no corresponding
   IP-level parameters, their values are set as a matter of local
   policy.





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RFC 2381         Interoperation of CLS and GS with ATM       August 1998


   The UBR category does not provide enough capability for Controlled
   Load.  The point of CLS is to allow an allocation of resources.  This
   is facilitated by the token bucket traffic descriptor, which is
   unavailable with UBR.

2.1.3 Service Categories for Best Effort

   All of the service categories have the capability to carry Best
   Effort service, but the natural service category is UBR (or, in UNI
   3.x, BCOB-C or BCOB-X, with the best effort indication set).  CBR or
   rtVBR clearly could be used, and since the service is not real-time,
   a nrtVBR connection could also be used.  In these cases the rate
   parameter used reflects a bandwidth allocation in support of the
   ingress edge device's best effort connectivity to the egress edge
   router.  It would be normal for traffic from many source/destination
   pairs to be aggregated on this connection; indeed, since Best Effort
   is the default IP behavior, the individual flows are not normally
   identified or accounted for.  CBR may be a preferred solution in the
   case where best effort traffic is sufficiently highly aggregated that
   a simple fixed-rate pipe is efficient.  Both CBR and nrt-VBR provide
   explicit bandwidth allocation which may be useful for billing
   purposes.  In the case of UBR, the network operator SHOULD allocate
   bandwidth for the overall service through the admission control
   function, although such allocation is not done explicitly per VC.

   An ABR connection could similarly be used to support Best Effort
   traffic.  Indeed, the support of data communications protocols such
   as TCP/IP is the explicit purpose for which ABR was designed.  It is
   conceivable that a separate ABR connection would be made for each IP
   flow, although the normal case would probably have all IP Best Effort
   traffic with a common egress router sharing a single ABR connection.

   The rt-VBR service category may be considered less suitable, simply
   because both the real-time delay constraint and the use of SCR/BT add
   unnecessary complexity.

   See specifications from the IETF ion working group [10, 11] for
   related work on support of Best Effort service with ATM.

2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions

   Each ATM cell header carries a Cell Loss Priority (CLP) bit.  Cells
   with CLP=1 are said to be "tagged" or "marked" and have lower
   priority.  This tagging may be done by the source, to indicate
   relative priority within the VC, or by a switch, to indicate traffic
   in violation of policing parameters.  Options involving the use of
   tagging are decided at call setup time.




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RFC 2381         Interoperation of CLS and GS with ATM       August 1998


   A Conformance Definition is a rule that determines whether a cell is
   conforming to the traffic descriptor of the VC.  The conformance
   definition is given in terms of a Generic Cell Rate Algorithm (GCRA),
   also known as a "leaky bucket" algorithm, for CBR and VBR services.
   The conformance definition also specifies rules for tagging traffic
   in excess of the {SCR, MBS} GCRA traffic descriptor.  (Note, the term
   "compliance" in ATM is used to describe the behavior of a connection,
   as opposed to "conformance", which applies to a single cell.)

   The network may tag cells that are non-conforming, rather than
   dropping them if the VC set-up requests tagging and the network
   supports the tagging option.  When tagging is used and congestion
   occurs, a switch MUST attempt to discard tagged cells in preference
   to discarding CLP=0 cells.  However, the mechanism for doing this is
   completely implementation specific.  The behavior that best meets the
   requirements of IP Integrated Services is where tagged cells are
   treated as "best effort" in the sense that they are transported when
   bandwidth is available, queued when buffers are available, and
   dropped when resources are overcommitted.  ATM standards, however, do
   not explicitly specify treatment of tagged traffic.  Providers of GS
   and CLS service with ATM subnetworks SHOULD ascertain the actual
   behavior of ATM implementation with respect to tagged cells.

   Since GS and CLS services REQUIRE excess traffic to be treated as
   best effort, the tagging option SHOULD always be chosen (if
   supported) in the VC setup as a means of "downgrading" the cells
   comprising non-conformant packets.  The term "best effort" can be
   interpreted in two ways.  The first is as a service class that, for
   example, may be implemented as a separate queue.  The other sense is
   more generic, meaning that the network makes a best effort to
   transport the traffic.  A reasonable interpretation of this is that a
   network with no contending traffic would transport the packet, while
   a very congested network would drop the packet.  A mechanism that
   tags best effort packets with lower loss priority (such as with the
   ATM CLP bit) would drop some of these packets, but would not reorder
   the remaining ones with respect to the conforming portion of the
   flow.  The "best effort" mechanism for excess traffic does not
   necessarily have to be the same as that for best effort "service", as
   long as it fits this generic sense of best effort.

   There are three conformance definitions of VBR service (for both
   rtVBR and nrtVBR) to consider.  In VBR, only the conformance
   definition VBR.3 supports tagging and applies the GCRA with rate PCR
   to the aggregate CLP=0+1 cells, and another GCRA with rate SCR to the
   CLP=0 cells.  This conformance definition SHOULD always be used with
   a VBR service supporting IP integrated services.  For UBR service,
   conformance definition UBR.2 supports the use of tagging, but a CLP=1
   cell does not imply non-conformance; rather, it may be used by the



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RFC 2381         Interoperation of CLS and GS with ATM       August 1998


   network to indicate congestion.

   In TM/UNI 4.0 tagging is not a feature of the conformance definitions
   for the CBR or ABR service categories.  (Since conformance
   definitions are generally network specific, some implementations CBR
   or ABR may, in fact, use tagging in some way.)  Wherever an ATM
   network does support tagging, in the sense of transporting CLP=1
   cells on a "best effort" basis, it is a useful and preferable
   mechanism for handling excess traffic.

   It is always better for the IWF to tag cells when it can anticipate
   that the ATM network would do so.  This is because the IWF knows the
   IP packet boundaries and can tag all of the cells corresponding to a
   packet.  If left to the ATM layer UPC, the network would inevitably
   drop some of the cells of a packet while carrying others, which would
   then be dropped by the receiver.  Therefore, the IWF, knowing the VC
   GCRA parameters, SHOULD always anticipate the cells which will be
   tagged by the ATM UPC and tag all of the cells uniformly across each
   affected packet.  See Section 3.2 for further discussion of excess
   traffic.

2.3 ATM Adaptation Layer

   The AAL type 5 encoding SHOULD be used, as specified in RFC 1483 and
   RFC 1755.  For AAL-5, specification of the maximum SDU size in both
   the forward and reverse directions is REQUIRED.  Both GS and CLS
   specify a maximum packet size, M, as part of the TSpec and this value
   SHOULD be used (corrected for AAL headers) as the maximum SDU in each
   direction for unicast connections, and for unidirectional point-to-
   multipoint connections.  When multiple flows are aggregated into a
   single VC, the M parameters of the receiver TSpecs are merged
   according to rules given in the GS and CLS specs.

2.4 Broadband Low Layer Information

   The B-LLI Information Element is transferred transparently by the ATM
   network between the edge devices and is used to specify the
   encapsulation method.  Multiple B-LLI IEs may be sent as part of
   negotiation.  The LLC/SNAP encapsulation [18] SHOULD be supported as
   the default, but "null" or "VC encapsulation" MAY also be allowed.
   Implementations SHOULD follow RFC 1577 [19] and RFC 1755 [10] for
   BLLI usage.

2.5 Traffic Descriptors

   The ATM traffic descriptor always contains a peak cell rate (PCR)
   (for each direction).  For VBR services it also contains a
   sustainable cell rate (SCR) and maximum burst size (MBS).  The SCR



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   and MBS form a leaky bucket pair (rate, depth), while the bucket
   depth parameter for PCR is CDVT.  Note that CDVT is not signalled
   explicitly, but is determined by the network operator, and can be
   viewed as a measure of the jitter imposed by the network.

   Since CDVT is generally presumed to be small (equivalent to a few
   cells of token bucket depth), and cannot be set independently for
   each connection, it cannot be used to account for the burstiness
   permitted by b of the IP-layer TSpec.  Additional buffering may be
   needed at the IWF to account for the depth of the token bucket.

   The ATM Burst Tolerance (BT) is equivalent to MBS (see TM 4.0 [6] for
   the exact equation).  They are both expressions of the bucket depth
   parameter associated with SCR.  The units of BT are time while the
   units of MBS are cells.  Since both SCR and MBS are signalled, they
   can be computed directly from the IP layer traffic description.  The
   specific manner in which resources are allocated from the traffic
   description is implementation specific.  Note that when translating
   the traffic parameters, the segmentation overhead and minimum policed
   unit need to be taken into account (see Section 4.1 below).

   In ATM UNI Signalling 4.0 there are the notions of Alternative
   Traffic Descriptors and Minimal Traffic Descriptors.  Alternative
   Traffic Descriptors enumerate other acceptable choices for traffic
   descriptors and are not considered here.  Minimal Traffic Descriptors
   are used in "negotiation," which refers to the specific way in which
   an ATM connection is set up.  To illustrate, roughly, taking PCR as
   an example: A minimal PCR and a requested PCR are signalled, the
   requested PCR being the usual item signalled, and the minimal PCR
   being the absolute minimum that the source edge device will accept.
   When both minimal and requested parameters are present, the
   intermediate switches along the path may reduce the requested PCR to
   a "comfortable" level.  This choice is part of admission control, and
   is therefore implementation specific.  If at any point the requested
   PCR falls below the minimal PCR then the call is cleared.  Minimal
   Traffic Descriptors can be used to present an acceptable range for
   parameters and ensure a higher likelihood of call admission.  In
   general, our discussion of connection parameters assumes the values
   resulting from successful connection setup.

   The Best Effort indicator (used only with UBR) and Tagging indicators
   (see Section 2.2) are also part of the signalled information element
   (IE) containing the traffic descriptor.  In the UNI 4.0 traffic
   descriptor IE there is an additional parameter, the Frame Discard
   indicator, which is discussed below in Section 2.7.






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RFC 2381         Interoperation of CLS and GS with ATM       August 1998