Internet DRAFT - draft-borden-intserv-atm-mapping
draft-borden-intserv-atm-mapping
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INTERNET-DRAFT Marty Borden,
Bay Networks,
Mark W. Garrett,
Bellcore.
June, 1996.
Interoperation of Controlled-Load and Guaranteed-Service with ATM
<draft-borden-intserv-atm-mapping-00.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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Abstract
Service mappings are one aspect of providing interoperability between
the IP Integrated Services and ATM networks. These encompass the
means of dealing with the different paradigms of each network type to
provide effective end-to-end Quality of Service. This draft is an
initial exploration of how to provide the QoS of an IP integrated
service with ATM subnetworks. It focuses on how to best match ATM
service with the IP integrated services. The work here is
preliminary and is presented as a baseline for discussion.
1.0 Introduction
We are interested in the problem of providing IP Integrated Services
with an ATM subnetwork: This is a complex problem with many facets.
In this draft, we focus on and specify the parameters and signalling
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elements needed for service interoperation between internet
Integrated Services protocols and ATM Traffic Management
capabilities. The mappings provided of service types, parameters and
features between the two technologies can be used to provide
effective end-to-end Quality of Service (QoS) for IP traffic that
traverses ATM networks.
This document is only a part of the total solution to providing the
interworking of IP integrated services with an ATM subnetwork. We do
not consider the important issues of when ATM VCs should be created
or destroyed, how they should be used or coordinated, or of how
routing-- QoS sensitive or not-- interacts with the use of VCs,
especially in the problem area of multicast (or point-to-multipoint)
flows. Instead we concentrate on the mapping of service attributes
between IP integrated services and ATM capabilities.
The goal of this draft is to provide sufficient information and
guidance that it is possible to use the ideas herein to formulate the
general solution. At times, we present in-depth discussions of
interoperation issues beyond what may be appropriate for a final
specification. We assume, for the moment, that this is useful for
developing consensus, and that a concise specification document will
be derived later.
The network architecture we consider is summarized in Figure 1,
below. Here, IP-attached hosts use RSVP to establish resource
reservation in routers along the internet path for the data flow.
How this path is chosen is outside of the scope of this document
(although the service mappings we discuss may play a part), but we
assume that an ATM network lies in the path, consisting of one or
many ATM switches; it uses connections to provide both resources and
QoS within the ATM cloud. These connections are set up, added to (in
the case of multipoint trees), torn down, and controlled by the edge
devices, which are devices capable of IP routing and ATM user-to-
network (UNI) interfaces. Consider these edge devices as fully
functional in both the IP int-serv/RSVP areas and the ATM UNI areas,
as well as translating between them.
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ATM Cloud
------------------
H ---\ ( ) /------- H
H ---- R -- R -- E --( ATM SW -- ATM SW ) -- E -- R -- R -- H
H ---/ | ( ) \
| ------------------ \------- H
H ----------R
Figure 1: Network Architecture with hosts (H),
Routers (R) and Edge Devices (E).
Whether edge devices are considered part of the IP internet or part
of the ATM cloud, or both, is not an issue in the current problem
space, since they must provide capabilities of both environments.
The edge devices have normal RSVP capability to process RSVP
messages, reserve resources, and maintain soft state (in the control
path), and to classify and schedule packets (in the data path). They
also have the normal ATM capabilities to signal (and refuse)
connections, and to police and schedule cells.
In addition, a reservation setup (RESV message) must trigger the edge
device to translate the RSVP service requirements from RSVP semantics
and syntax to the ATM VC (UNI) semantics, and vice-versa. The
difficulty of this must not be minimized.
Point-to-multipoint connections within the ATM cloud are used to
support general IP multicast flows. Different ATM mechanisms may be
available to support this; it may be that the root of the point-to-
multipoint tree controls the tree, which requires coordination
between the edge devices, or the leaf initiated join (LIJ) may be
appropriate.
Thus, VCs are managed according to a combination of standards and
local rules, which may be implementation specific; this management
may decide to multiplex the requested flow onto an existing VC or may
decide that a new VC is warranted.
Figure 2 shows the functions of an edge device, summarizing the work
not part of IP or ATM abstractly as an InterWorking Function (IWF),
and segregating the control and data planes. (Note: for
expositional convenience, policy control and other control functions
are included as part of the admission control in the diagram.)
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IP ATM
____________________
| IWF |
| |
admission <--> | service mapping | <--> ATM
control | VC management | signalling &
| address resolution | admission
|....................| control
| |
classification/ | ATM Adaption Layer | cell
policing & <--> | Segmentation and | <--> scheduling/
scheduling | Reassembly | shaping
| Buffering |
____________________
Figure 2: Edge Device Functions showing the IWF
In the logical view of Figure 2, some functions, such as scheduling,
are shown separately, since these functions are required of both the
IP and ATM sides. However it may be possible in an integrated
implementation to combine such functions.
It is not possible to completely separate the service mapping and VC
management functions. Several examples of why this is so come to
mind. (i) Multiple integrated-services flows may be aggregated to
use one point-to-multipoint VC; in this case we assume the IP flows
are of the same service type and their parameters have been merged.
(ii) The VC management function may choose to allocate extra
resources in anticipation of further reservations or based on a
empiric of changing TSpecs; in this case we assume that the
additional resources continue to be specified in the form of an
appropriate TSpec. (iii) There must exist a path for best effort
flows and for sending the TSpecs; how this interacts with the
establishment of VCs for QoS traffic may alter the characteristics
required of the VC.
Therefore, in discussing the service-mapping problem, we will assume
that the VC management function of the IWF presents to the service
mapping the need for a VC to support a certain integrated-service
type with a given TSpec. The VC required may be a new one or the
addition/deletion of a leaf to an existing multipoint tree. We
examine the service requirements of a VC supporting this service and
TSpec.
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Having outlined the scope, we now outline the remainder of this
document. The first part of this work, section 2.0, examines each
ATM signalling semantic or feature, and discusses it with respect to
each of the three IP integrated services -- Guaranteed Service (GS),
Controlled Load (CL) and Best Effort (BE) (which is, by default, a
service). Each of the ATM specifications, UNI 3.0, UNI 3.1 and 4.0
are considered here. In this regard, we are able to clearly
recommend one or more choices for each parameter/feature to match
each service; sometimes the choice is obvious, other times there is
room for implementation experience or policy guidelines before making
any decisions. The following sections (3.0 - 5.0) are specific to
each IP integrated service. They provide a summary of the work of
section 2.0 with the result of a recommended approach for ATM
signalling.
1.1 Related documents
Earlier ATM Forum documents were called UNI 3.0 and UNI 3.1. The 3.1
release was used to correct errors and fix alignment with the ITU.
Unfortunately UNI 3.0 and 3.1 are incompatible. However this is in
terms of actual codepoints, not semantics. Therefore, descriptions
of parameter values can generally be used for both.
After 3.1, the ATM Forum decided to release documents separately for
each technical subcommittee. The Traffic Management and Signalling
4.0 documents are available publically at ftp.atmforum.com/pub. We
refer to the combination of traffic management and signalling as
TM/UNI 4.0, although specific references may be made to the TM 4.0
specification or the UNI SIG 4.0 specification.
Within the IETF area, related material includes:
RSVP functional specification,
Guaranteed Service specification,
Controlled Load service specification,
Int-serv data encoding specification,
RFC 1577,
RFC1755,
RFC 1821,
draft-crawley-rsvp-over-atm,
draft-birman-ipatm-rsvpatm,
draft-onvural-srinivasan-rsvp-atm.
1.2 Abbreviations
CBR Constant Bit Rate
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VBR Variable Bit Rate
rtVBR Real-time VBR
nrtVBR Non-real-time VBR
UBR Unspecified Bit Rate
ABR Available Bit Rate
BCOB Broadband Connectioned Oriented Bearer
BCOB-{A,C,X} Bearer Class A, C, or X
PCR Peak Cell Rate
SCR Sustained Cell Rate
MBS Maximum Burst Size
CDV Cell Delay Variation
CTD Cell Transfer Delay
CDVT Cell Delay Variation Tolerance
BT Burst Tolerance
MCR Minimum Cell Rate
CLP Cell Loss Priority (bit)
CLR Cell Loss Ratio
CL Controlled Load
GS Guaranteed Service
BE Best Effort
2.0 ATM Protocol Features
In this section, we discuss each of the items that must be specified
in the setup of an ATM VC. For each of these we discuss which
specified items and values may be most appropriate for each of the
integrated services.
The ATM Call Setup is sent by the edge device to the ATM network to
establish end-to-end [ATM] service. This setup contains the
following information.
Service Category/Broadband Bearer Capability
AAL Parameters
Broadband Low Layer Information
Calling and Called Party Addressing Information
Traffic Descriptors
QoS Parameters
Additional Parameters of TM/UNI 4.0
We will discuss each of these, except addressing information, as they
relate to the translation of GS and CL to ATM services. We also
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discuss the tagging and conformance definitions for IP and ATM, since
the policing method is implicit in the call setup.
2.1 Service Category and Bearer Capability
The highest level of abstraction distinguishing features of ATM VCs
is in the service category or bearer capability. Service categories
were introduced in TM/UNI 4.0; previously the bearer capability was
used to discriminate at this level.
In each version of the ATM specifications, these indicate the general
properties required of a VC: whether there is a real-time delay
constraint, whether the traffic is constant or variable rate, the
applicable traffic and QoS description parameters and (implicitly)
the complexity of some supporting switch mechanisms.
For UNI 3.0 and UNI 3.1, there are only two distinct options for
bearer capabilities (in our context):
BCOB-A: constant rate, timing required, unicast/multipoint;
BCOB-C: variable rate, timing not required, unicast/multipoint.
There is a third capability, BCOB-X, but in the case of AAL5 (which
we require -- see below) it can be used interchangeably and
consistently with the above two capabilities.
In TM/UNI 4.0 the service categories are:
Constant Bit Rate (CBR)
Real-time Variable Bit Rate (rtVBR)
Non-real-time Variable Bit Rate (nrtVBR)
Unspecified Bit Rate (UBR)
Available Bit Rate (ABR)
The first two of these are real-time services, so that rtVBR is new
to TM/UNI 4.0. The ABR service is also new to TM/UNI 4.0. UBR
exists in all specifications, except perhaps in name, through the
``best effort'' indication flag or, in UNI 3.x, the QoS Class 0.
The encoding used in 4.0 is consistent with the earlier versions.
For example, the Service Category is indicated solely by the
combination of the Bearer Capabilty and the Best Effort indication
flag.
In principle, it is possible to support any forseeable integrated
service through the use of BCOB-A/CBR. This is because the CBR
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service is equivalent to having a ``pipe'' with specified
bandwidth/timing. However, it may be desirable to make better use of
the ATM network's resources by using other, less demanding, services
when available. (See RFC 1821 for a discussion of this.)
Guaranteed Service
There are two possible mappings for GS:
CBR (BCOB-A)
rtVBR
GS requires real-time support, that is, timing is required. Thus in
UNI 3.x, the bearer class BCOB-A (or an equivalent BCOB-X
formulation) must be used. In TM/UNI 4.0 either of CBR or rtVBR is
appropriate, the latter allowing the network to possibly take
advantage of the statistical multiplexing gain of variable rate flows
and to use tagging (see section 2.2).
Neither the BCOB-C bearer class, nor nrtVBR, UBR, ABR are matches for
the GS service. These provide no delay estimates and one cannot
expect low, predictable, or consistent delays.
Specification of BCOB-A or CBR requires specification of a PCR. The
PCR should be specified as the the token bucket rate parameter, with
appropriate conversion from bytes to cells (and overhead), of the GS
TSpec. For both of these, the network provides a nominal clearing
rate of PCR with toleration (bucket size) CDVT, specified in a
network specific manner (see below).
Specification of rtVBR requires the specification of two rates, SCR
and PCR. This models bursty traffic with specified peak and average
rates. With rtVBR, it is appropriate to map the PCR to the line rate
of incoming traffic and the SCR to the GS TSpec bucket rate. The ATM
bucket sizes are CDVT, in a network specific manner, and CDVT+BT,
respectively for the PCR and SCR parameters (see below).
Controlled Load
There are three possible mappings for CL:
CBR (BCOB-A)
ABR
nrtVBR (BCOB-C)
Note that under UNI 3.x, only the first and third choices are
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applicable. 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 should be ubiquitously
supported in ATM networks. However unless this is the only choice
available, this appears to be wasteful of network resources.
The ABR category with a positive MCR aligns with the CL idea of
``best effort with floor.'' The ATM network agrees to forward cells
with a rate of at least MCR, which should be directly converted from
the token bucket rate of the TSpec. The bucket size parameter
measures the amount of buffer required at the IWF.
The nrtVBR/BCOB-C category can also be used. It does introduce some
unaligned complexity in the conformance definition (see section 2.2)
by the use of two leaky buckets. The CL rate parameter would
correspond to the SCR, while the PCR should be set to the line rate,
as for Guaranteed Service.
The remaining service categories are inappropriate for CL. The rtVBR
category adds complexity without providing useful features: there is
no need for tightly constrained delays, and the double-rate traffic
description is not needed. The UBR category does not provide enough
capability for Controlled Load. The point of CL is to allow an
allocation of resources, which is facilitated by the token bucket
traffic descriptor, and is unavailable in UBR.
Best Effort
Any of the service categories has 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 indicator flag). A 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 edge
device's best effort connectivity to the far edge router. It would
be normal for many flows to be aggregated on this connection; indeed,
since Best Effort is the default IP behavior, the individual flows
are not necessarily identified or accounted for. An ABR connection
could similarly be used to support Best Effort traffic. This is the
purpose for which ABR was specifically designed. It is conceivable
that a separate ABR connection would be made for different IP flows,
although the normal case would probably have all IP Best Effort
traffic with a common exit router sharing a single ABR connection.
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2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions
An ATM header carries the Cell Loss Priority (CLP) bit. Cells with
bit CLP=1 are said to have been tagged and have lower priority. This
tagging may have been done by the source or an upstream switch.
Options involving the use of tagging are decided at call setup time.
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),
which may be implemented using a leaky bucket algorithm, for CBR and
VBR services. (UBR and ABR have network specific conformance
definitions.)
The network may tag cells which are non-conforming, rather than
dropping them only if the VC is set up to request tagging and the
network supports the tagging option. When congestion occurs, a
switch must attempt to discard tagged cells in preference to the
discarding of CLP=0 cells. However, the mechanism for doing this is
completely implementation specific. Tagged cells are treated with a
behavior which is Best Effort in the sense that they are transported
when bandwidth is available, queued when buffers are available, and
dropped when the resources are overcommitted.
Since GS and CL 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'' nonconformant cells.
However, we wish to point out that the term ``best effort'' seems to
be used with two distinguishable meanings in the int-serv specs. The
first interpretation is that of a service class that, in some
typical scheduler implementations, would correspond to a separate
queue. Placing excess traffic in best effort in this sense would be
giving it lower delay priority. The other sense is more generic,
meaning that the network would make a best effort to transport the
traffic. A reasonable expectation is that a network with no
contending traffic would transport the packet, while a very congested
network would drop the packet. A packet that could be tagged with
lower loss priority would not be as likely to be transported out of
order with respect to the conforming portion of the flow. Such a
mechanism would agree with the latter definition of best effort, but
not the former.
In TM/UNI 4.0 tagging does not apply to the CBR or ABR services.
However, there are three subcategories 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 PCR to the aggregate
CLP=0+1 cells, and another GCRA with SCR to the CLP=0 cells. Thus
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this conformance definition should always be used. For UBR service,
conformance definition UBR.2 supports the use of tagging, but a CLP=1
cell does not imply non-conformance; it is a hint of network
congestion.
Once an ATM connection is established, the use of the conformance
definition and resulting policing action is mandatory. Since the
conformance algorithm operates on cells, when mapping rates and
bucket sizes from IP services to corresponding ATM parameters, a
correction needs to made (at call setup time) for the ATM
segmentation overhead. Unfortunately this overhead, as a ratio,
depends on packet length, with the overhead largest for small
packets. Thus the appropriate correction could be based on minimum
packet size, expected packet size, or otherwise in a network specific
manner, determined at the edge device IWF.
2.3 ATM Adaptation Layer
The AAL type 5 encoding must be used, as in RFC 1483 and RFC 1755.
AAL5 requires specification of the maximum SDU size in both the
forward and reverse directions. Both GS and CL specify a maximum
packet size as part of the TSpec and this value should be used as the
maximum SDU in each direction for unicast connections, but only in
one direction for point-to-multipoint connections, which are
unidirectional. When more than one flow is to use the same VC, the
TSpecs can be merged to yield the largest packet sizes. In no case
can this exceed 65535 (or, of course, the MTU of the link).
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 default encapsulation LLC/SNAP must be supported as
in RFC 1577 and RFC 1755. Additional encapsulations are discussed in
RFC 1755 and we refer to the discussion there.
2.5 Traffic Descriptor
The ATM traffic descriptor always contains specification of a peak
cell rate (PCR) (in each direction). For variable rate services it
also contains specification of a sustainable cell rate (SCR) and
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maximum burst size (MBS).
The Best Effort indicators and Tagging indicators are also part of
the traffic descriptors in the signalling sense. In UNI SIG 4.0
there is an additional parameter, the Frame Discard indicator in the
traffic descriptor. The latter is used to indicate the request that
if a cell is to be dropped, then all subsequent cells of a frame be
dropped up to the End of Message (EOM) cell (AAL 5); see section 2.7.
In ATM UNI SIG 4.0 there are also the notions of Alternative Traffic
Descriptors and Minimal Traffic Descriptors. Alternative Traffic
Descriptors enumerate other acceptable choices for traffic
descriptors and do not seem to be relevant here. Minimal Traffic
Descriptors are used in ``negotiation,'' a term which when
interpreted colloquially will lead to confusion. Very roughly it
works like this, e.g., for PCR. 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 sensing the existence of both minimal and requested
parameters, intermediate switches along the path may reduce the
requested PCR to a comfortable level. If at any point the requested
PCR falls below the minimal PCR then the call is cleared. This is a
very rough sketch, but we do see potential to make use of Minimal
Traffic Descriptors in future versions of this draft in order to
present an acceptable range for parameters and have higher liklihood
of call admission. Minimal Traffic Descriptors are not explored
further in this version of the draft.
The Traffic Management viewpoint, which we examine next, is more
concerned with the value of the PCR, SCR and MBS parameters after
call setup.
PCR and CDVT are used in the CBR and VBR conformance definitions as
parameters for a leaky bucket. However CDVT is not signalled and is
determined by the network operator as a measure of the ``clumping''
done by the network. This makes it difficult to map any leaky bucket
description of a TSpec to the PCR-CDVT leaky bucket. Additional
buffering will be needed at the IWF to account for the depth of the
bucket.
The SCR and MBS are used with the VBR services. They are used in an
implementation specific manner to allocate resources. The Burst
Tolerance (BT) is derived from MBS (see TM 4.0) to be used in a
second SCR-BT leaky bucket. Since both parameters are available to
be signalled, this leaky bucket has the potential to be used in the
same way as the integrated services bucket. Note that the
segmentation overhead and minimum policed unit need to be taken into
account when translating the bucket parameters.
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For Guaranteed Service there is a bucket rate, r and a service rate,
R. The bucket rate describes the traffic, and can be used for
policing, while the service rate (which cannot be smaller) is the
allocated service rate. When mapping Guaranteed Service onto a rtVBR
VC, the mapping is straightforward. The bucket rate maps to the SCR
and the peak rate maps to PCR. The bucket depth parameter maps to
MBS. The minimum policed unit may need to be taken into account when
translating the leaky bucket parameters. Note that due to cell
segmentation, the ATM traffic parameters will increase due to the
additional headers. The minimum packet size can be used to identify
the worst case situation.
For GS over CBR, the bucket rate can be mapped to the PCR parameter.
As noted above, the edge device may need to ensure that adequate
buffering exists at the ATM network ingress to accommodate the TSpec
bucket depth. If the available buffering is not sufficient, then a
VC may have to be set up using the IP peak rate parameter mapping to
PCR. It is probably inadvisable to try to set the PCR to a value
between the bucket rate and the peak rate, since such a value would
depend on assumptions about the statistical properties of the source.
Controlled Load service has a single bucket rate and corresponding
depth parameter. The minimum policed unit and maximum packet size
play the same roles in mapping parameters as for Guaranteed Service.
When using nrtVBR, the bucket rate and depth map to SCR and MBS,
while the PCR parameter can be set to the line rate as a worst case
value. For ABR VCs, the bucket rate would be used to set the minimum
cell rate (MCR) parameter. The bucket depth parameter does not map
directly to a signalled ATM parameter, but the edge device should
check that the buffering at the ATM ingress is sufficient to account
for the size of bursts allowed by that parameter. Finally for CBR,
the bucket rate sets the PCR, and again, the available buffering in
the edge device must be adequate to accommodate possible bursts.
For Best Effort service, there is no traffic description. The UBR
service category allows negotiation of PCR, simply to identify to the
source the smallest physical bottleneck along the path.
2.6 QoS Classes and Parameters
In TM/UNI 4.0 the three QoS parameters may be individually signalled.
These parameters are the Cell Loss Ratio (CLR), Cell Transfer Delay
(CTD), and Cell Delay Variation (CDV). In UNI 3.x the setup message
includes only the QoS Class, which is essentially an index to a
network specific table of values for these three parameters. A
network provider may choose to associate other parameters, such as
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Severely Errored Cell Block Ratio, but these are less well understood
and accepted compared to the basic loss, delay and jitter parameters
mentioned here. The ITU may include a standard set of parameter
values for a number (probably four) of QoS classes. In that case,
the network provider could define further network-specific QoS
classes in addition. The problem of agreement between network
providers as to the definition of QoS classes is completely
unaddressed to date. We will adopt a convention expressed in UNI
3.x, that assumes that QoS class 1 is appropriate for low-delay,
low-loss CBR connections, and QoS class 3 is appropriate for variable
rate connections with loss and delay roughly appropriate for non-
real-time data applications.
Since no IP layer counterparts to these ATM QoS parameters exist in
any of the IP services, they must be set by policy of the edge
device. The QoS classes can be chosen relatively easily. QoS class
1 should be used with Guaranteed Service and QoS class 3 should be
used with Controlled Load Service. Best Effort Service always gets
QoS class 0, which is unspecified QoS by definition. There are two
issues which amount to the same thing: First, the choice of
individually signalled parameter values (under TM/UNI 4.0) for GS and
CL is the edge device policy. The second issue is choosing parameter
values for the two QoS classes, which is the ATM network policy. If
the same network operator controls both, then these problems are
identical; if not, an agreement to make the values identical would be
extremely desirable.
Note that we have mapped QoS class 1 and 3 onto Guaranteed and
Controlled Load service respectively. This is regardless of what
service category is used. So when running CL over a CBR pipe, it
would not be inappropriate to use QoS class 3. This leaves the delay
unspecified (or much looser than with QoS 1). These comments should
be taken as preliminary, as these issues are far from clear, and
industry consensus should be sought.
2.7 Additional Parameters -- Frame Discard Mode
In TM/UNI 4.0 ATM allows the user to choose a mode where a dropped
cell causes all cells up to the last remaining in the AAL5 PDU to be
also dropped. This improves efficiency and the behavior of end-to-
end protocols such as TCP, since the remaining cells of a damaged PDU
are useless to the receiver. For IP over ATM, Frame Discard should
always be used in both directions, if available, for all services.
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3.0 Guaranteed Service over ATM
This section describes how to create ATM VCs appropriately matched
for Guaranteed Service. The key points differentiating among ATM
choices are that real-time timing is required, that the data flow may
have a variable rate, and that demotion of nonconforming traffic to
best effort is desired. For this reason, we prefer a rtVBR service
in which tagging is supported.
The encodings assume a point-to-multipoint connection. For a unicast
connection, the backward parameters would be equal to the forward
parameters.
Encoding GS as a real-time variable bit rate service
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size 0
Mode 1 (Message mode) Note 1
SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR CLP=0+1 From TSpec peak rate
Backward PCR CLP=0+1 0
Forward SCR CLP=0 From TSpec token bucket rate
Backward SCR CLP=0 0
Forward MBS (CLP=0) From TSpec bucket size param
Backward MBS (CLP=0) 0
BE indicator NOT included
Forward Frame Discard bit 1 Note 2
Backward Frame Discard bit 1 Note 2
Tagging Forward bit 1 (Tagging requested) Note 2
Tagging Backward bit 0 (No Tagging) Note 2
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3
ATM Transfer Capability 9 Note 2
Traffic Type 010 (Variable Bit Rate)
Timing Requirements 01 (Timing Required)
Susceptible to Clipping 00 (Not susceptible)
User Plane Configuration 01 (For pt-to-mpt)
Broadband Low Layer Information
Layer 2 protocol 12 (ISO 8802/2)
Layer 3 protocol 204 (ISO/IEC TR 9577)
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QoS Class
QoS Class Forward 1 Note 4
QoS Class Backward 1 Note 4
QoS Parameters
Transit Delay 100ms Notes 2,5
Forward CLR (CLP=0) 1.0e-6 Notes 2,5
Backward CLR (CLP=0) 1.0e-6 Notes 2,5
Forward CDV 30ms Notes 2,5
Backward CDV 30ms Notes 2,5
Note 1: Only included for UNI 3.0.
Note 2: Only included in TM/UNI 4.0.
Note 3: Value 1 (BCOB-A) can also be used.
Note 4: Optional in TM/UNI 4.0.
Note 5: Values chosen to initiate discussion.
It is also possible to support GS using a CBR ``pipe.'' The
advantage of this is that CBR is probably supported; the disadvantage
is that data flows may not fill the pipe (utilization loss) and there
is no tagging option available.
Encoding GS as a constant bit rate service
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size parameter M of TSpec
Mode 1 (Message mode) Note 1
SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR 0+1 From TSpec token bucket rate
Backward PCR 0+1 0
BE indicator NOT included
Forward Frame Discard bit 1 Note 2
Backward Frame Discard bit 1 Note 2
Tagging Forward bit 0 (No Tagging) Note 2
Tagging Backward bit 0 (No Tagging) Note 2
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3
ATM Transfer Capability 7 Note 2
Traffic Type 001 (Constant Bit Rate)
Timing Requirements 01 (Timing Required)
Susceptible to Clipping 00 (Not susceptible)
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User Plane Configuration 01 (For pt-to-mpt)
Broadband Low Layer Information
Layer 2 protocol 12 (ISO 8802/2)
Layer 3 protocol 204 (ISO/IEC TR 9577)
QoS Class
QoS Class Forward 1 Note 4
QoS Class Backward 1 Note 4
QoS Parameters
Transit Delay 100ms Notes 2,5
Forward CLR (CLP=0) 1.0e-6 Notes 2,5
Backward CLR (CLP=0) 1.0e-6 Notes 2,5
Forward CDV 30ms Notes 2,5
Backward CDV 30ms Notes 2,5
Note 1: Only included for UNI 3.0.
Note 2: Only included in TM/UNI 4.0.
Note 3: Value 1 (BCOB-A) can also be used.
Note 4: Optional in TM/UNI 4.0.
Note 5: Values chosen to initiate discussion.
4.0 Controlled Load Service over ATM
This section describes how to create ATM VCs appropriately matched
for Controlled Load. CL traffic is delay tolerant and variable rate.
We see nrtVBR and ABR (for TM/UNI 4.0 only) as possible choices in
supporting CL.
Controlled Load Support using ABR
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size parameter M of TSpec
SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR CLP=0+1 From line rate
Backward PCR CLP=0+1 From line rate
Forward MCR CLP 0+1 From TSpec token bucket rate
Backward MCR CLP 0+1 From TSpec token bucket rate
BE indicator NOT included
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Forward Frame Discard bit 1
Backward Frame Discard bit 1
Tagging Forward bit 0 (Tagging not requested)
Tagging Backward bit 0 (Tagging not requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3
ATM Transfer Capability 12
Traffic Type 010 (Variable Bit Rate)
Timing Requirements 10 (Timing Not Required)
Susceptible to Clipping 00 (Not susceptible)
User Plane Configuration 00 (For pt-to-pt)
Broadband Low Layer Information
Layer 2 protocol 12 (ISO 8802/2)
Layer 3 protocol 204 (ISO/IEC TR 9577)
QoS Class
QoS Class Forward 3 Note 4
QoS Class Backward 3 Note 4
ABR Setup Parameters For Further Study
ABR Additional Parameters For Further Study
Note 3: Value 3 (BCOB-C) can also be used.
Note 4: Optional in TM/UNI 4.0.
Controlled Load support using nrtVBR
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size 0
Mode 1 (Message mode) Note 1
SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR CLP=0+1 From line rate
Backward PCR CLP=0+1 0
Forward SCR CLP=0 From TSpec token bucket rate
Backward SCR CLP=0 0
Forward MBS (CLP=0) From TSpec bucket size param
Backward MBS (CLP=0) 0
BE indicator NOT included
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Forward Frame Discard bit 1 Note 2
Backward Frame Discard bit 1 Note 2
Tagging Forward bit 1 (Tagging requested) Note 2
Tagging Backward bit 0 (No Tagging) Note 2
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3
ATM Transfer Capability Absent Note 2
Traffic Type 010 (Variable Bit Rate)
Timing Requirements 10 (Timing Not Required)
Susceptible to Clipping 00 (Not susceptible)
User Plane Configuration 01 (For pt-to-mpt)
Broadband Low Layer Information
Layer 2 protocol 12 (ISO 8802/2)
Layer 3 protocol 204 (ISO/IEC TR 9577)
QoS Class
QoS Class Forward 3 Note 4
QoS Class Backward 3 Note 4
QoS Parameters
Forward CLR (CLP=0) 1.0e-6 Notes 2,5
Backward CLR (CLP=0) 1.0e-6 Notes 2,5
Note 1: Only included for UNI 3.0.
Note 2: Only included in TM/UNI 4.0.
Note 3: Value 3 (BCOB-C) can also be used.
Note 4: Optional in TM/UNI 4.0.
Note 5: Values chosen to initiate discussion.
5.0 Best Effort Service over ATM
This section describes how to create ATM VCs appropriately matched
for Best Effort. The BE service does not need a reservation of
resources.
Best Effort Service using UBR
AAL
Type 5
Forward CPCS-SDU Size MTU of link
Backward CPCS-SDU Size MTU of link
Mode 1 (Message mode) Note 1
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SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR CLP=0+1 From line rate
Backward PCR CLP=0+1 0
BE indicator included
Forward Frame Discard bit 1 Note 2
Backward Frame Discard bit 1 Note 2
Tagging Forward bit 1 (Tagging requested) Note 2
Tagging Backward bit 0 (no tagging) Note 2
Broadband Bearer Capability
Bearer Class 16 (BCOB-X)
Traffic Type 010 (Variable Bit Rate)
Timing Requirements 10 (Timing not required)
Susceptible to Clipping 00 (Not susceptible)
User Plane Configuration 01 (For pt-to-mpt)
Broadband Low Layer Information
Layer 2 protocol 12 (ISO 8802/2)
Layer 3 protocol 204 (ISO/IEC TR 9577)
QoS Class
QoS Class Forward 0
QoS Class Backward 0
Note 1: Only included for UNI 3.0.
Note 2: Only included in TM/UNI 4.0.
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