INTERNET-DRAFT Mark W. Garrett,
Bellcore
Expires 26 September 1997
Marty Borden,
New Oak Communications
26 March 1997
Interoperation of Controlled-Load and Guaranteed Services with ATM
<draft-ietf-issll-atm-mapping-02.txt>
Status of this Memo
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Abstract
Service mappings are an important aspect of effective interoperation
between Internet Integrated Services and ATM networks. This document
provides guidelines for ATM virtual connection features and
parameters to be used in support of the IP integrated services
protocols. The specifications include IP Guaranteed Service,
Controlled-Load Service and ATM Forum UNI specification, versions
3.0, 3.1 and 4.0.
These service mappings are intended to facilitate effective end-to-
end Quality of Service for IP networks containing ATM subnetworks.
We discuss the various features of the IP and ATM protocols, and
identify solutions and difficult issues of compatibility and
interoperation.
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Table of Contents
0.0 What's New in This Version ......................................... 3
1.0 Introduction ....................................................... x
1.1 General System Architecture .................................... x
1.2 Related Documents .............................................. x
2.0 Discussion of ATM Protocol Features ................................ x
2.1 Service Category and Bearer Capability ......................... x
2.1.1 Service Categories for Guaranteed Service ................ x
2.1.2 Service Categories for Controlled Load ................... x
2.1.3 Service Categories for Best Effort ....................... x
2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions .... x
2.3 ATM Adaptation Layer ........................................... x
2.4 Broadband Low Layer Information ................................ x
2.5 Traffic Descriptors ............................................ x
2.5.1 Translating Traffic Descriptors for Guaranteed Service ... x
2.5.2 Translating Traffic Descriptors for Controlled Load Service x
2.5.3 Translating Traffic Descriptors for Best Effort Service .... x
2.6 QoS Classes and Parameters ..................................... x
2.7 Additional Parameters -- Frame Discard Mode .................... x
3.0 Discussion of IP-IS Protocol Features .............................. x
3.1 Handling of Excess Traffic ..................................... x
3.2 Use of AdSpec in Guaranteed Service with ATM ................... x
4.0 Discussion of Miscellaneous Items .................................. x
4.1 Units Conversion ............................................... x
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service .......... x
5.1 Encoding GS Using Real-Time VBR ................................ x
5.2 Encoding GS Using CBR .......................................... x
5.3 Encoding GS Using Non-Real-Time VBR ............................ x
5.4 Encoding GS Using ABR .......................................... x
5.5 Encoding GS Using UBR .......................................... x
5.6 Encoding GS Using UNI 3.0 and UNI 3.1. ......................... x
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service ..... x
6.1 Encoding CLS Using ABR ......................................... x
6.2 Encoding CLS Using Non-Real-Time VBR ........................... x
6.3 Encoding CLS Using Real-Time VBR ............................... x
6.4 Encoding CLS Using CBR ......................................... x
6.5 Encoding CLS Using UBR ......................................... x
6.6 Encoding CLS Using UNI 3.0 and UNI 3.1. ........................ x
7.0 Summary of ATM VC Setup Parameters for Best Effort Service ......... x
7.1 Encoding Best Effort Service Using UBR ......................... x
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7.2 Encoding Best Effort Service Using Other ATM Service Categories x
8.0 Acknowledgements ................................................... x
Appendix 1 Abbreviations .............................................. x
REFERENCES ............................................................. x
AUTHORS' ADDRESSES ..................................................... x
0.0 What's New in This Version
Corrections to VC setup parameter tables.
Deleted specific QoS parameter values in tables.
Section 3.1 on handling of excess traffic.
1.0 Introduction
We consider the problem of providing IP Integrated Services [1] with
an ATM subnetwork. This document is intended to be consistent with
the rsvp protocol [2] for IP-level resource reservation (although it
is, strictly speaking, independent of rsvp, since GS and CLS services
can be supported through other mechanisms). In the ATM network, we
consider ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [3, 4,
5]. The latter uses the more complete service model of The ATM
Forum's TM 4.0 specification [6, 7].
This is a complex problem with many facets. In this document, we
focus on the service types, parameters and signalling elements needed
for service interoperation. The resulting service mappings can be
used to provide effective end-to-end Quality of Service (QoS) for IP
traffic that traverses ATM networks.
The IP services considered are Guaranteed Service (GS) [8] and
Controlled Load Service (CLS) [9]. We also treat the default Best
Effort Service (BE) in parallel with these. Our recommendations for
BE are intended to be consistent with RFC 1755 [10], and its revision
(in progress) [11], which defines how ATM VCs can be used in support
of normal BE IP service. The ATM services we consider are:
CBR Constant Bit Rate
rtVBR Real-time Variable Bit Rate
nrtVBR Non-real-time Variable Bit Rate
UBR Unspecified Bit Rate
ABR Available Bit Rate
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(Note, Appendix 1 provides definitions for all abbreviations.) In
the case of UNI 3.0 and 3.1 signaling, where these service are not
all clearly distinguishable, we identify the appropriate available
services.
The service mappings which follow most naturally from the service
definitions are as follows:
Guaranteed Service -> CBR or rtVBR
Controlled Load -> nrtVBR or ABR (with a minimum cell rate)
Best Effort -> UBR or ABR
For completeness we provide detailed mappings for all service
combinations and identify how each meets or fails to meet the
requirements of the higher level IP services. The reason for not
restricting mappings to the most obvious or natural ones is that we
cannot assume now that these services will always be ubiquitously
available. A number of details, such as treatment of packets in
excess of the flow traffic descriptor, make service mapping a
complicated subject, which cannot be expressed briefly and accurately
at the same time.
The remainder of this introduction provides a general discussion of
the system configuration and other assumptions. Section 2 considers
the relevant ATM protocol elements and their effects as related to
Guaranteed, Controlled Load and Best Effort services (the latter
being the default "service"). Section 3 discusses a number of
important features of the IP services and how they can be handled on
an ATM subnetwork. Section 4 addresses a few miscellaneous problems
which are neither distinctly IP nor ATM. Section 5 gives detailed VC
setup parameters for Guaranteed Service, and considers the effect of
using each of the ATM service categories. Section 6 provides a
similar treatment for Controlled Load Service. Section 7 considers
Best Effort service.
This document is only a part of the total solution to providing the
interworking of IP integrated services with ATM subnetworks. The
important issue of VC management, including flow aggregation, is
considered in [12]. We do not consider how routing -- QoS sensitive
or not -- interacts with the use of VCs, especially in the case of
multicast (or point-to-multipoint) flows. We expect that a
considerable degree of implementation latitude will exist, even
within the guidelines presented here. Many aspects of interworking
between IP and ATM will depend on economic factors, and will not be
subject to standardization.
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1.1 General System Architecture
We assume that the reader has a general working knowledge of IP, rsvp
and ATM protocols. The network architecture we consider is
illustrated in Figure 1, below. An IP-attached host may send unicast
datagrams to another host, or may use an IP multicast address to send
packets to all of the hosts which have "joined" the multicast "tree".
In either case, a destination host may then use RSVP to establish
resource reservation in routers along the internet path for the data
flow.
An ATM network lies in the path (chosen by the IP routing), and
consists of one or many ATM switches. It uses VCs 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 act as both IP routers and ATM
interfaces, capable of initiating and managing VCs across the ATM
user-to-network (UNI) interface. The edge devices are assumed to be
fully functional in both the IP int-serv/RSVP protocols and the ATM
UNI protocols, as well as translating between them.
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).
The edge devices may be considered part of the IP internet or part of
the ATM cloud, or both. This is not an issue 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 initiate connections by signaling, and to accept or
refuse connections signaled to them. They police and schedule cells
going into the ATM cloud. An IP-level reservation (RESV message)
triggers the edge device to translate the RSVP service requirements
into ATM VC (UNI) semantics.
A range of VC management policies are possible, which determine
whether a flow should initiate a new VC or join an existing one. VCs
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are managed according to a combination of standards and local policy
rules, which are specific to either the implementation (equipment) or
the operator (network service provider). Point-to-multipoint
connections within the ATM cloud can be used to support general IP
multicast flows. In ATM, a point to multipoint connection can be
controlled by the source (or root) node, or a leaf initiated join
(LIJ) feature in ATM may be used. Note, the topic of VC management
and mapping of flows onto VCs is considered at length in another
issll working group draft [12].
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.)
IP ATM
____________________
| IWF |
| |
admission <--> | service mapping | <--> ATM
control | VC management | signalling &
| address resolution | admission
|....................| control
| |
classification/ |ATM Adaptation 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 illustrative examples 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
appropriately. (ii) The VC management function may choose to
allocate extra resources in anticipation of further reservations or
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based on an empiric of changing TSpecs. (iii) There must exist a
path for best effort flows and for sending the rsvp control messages.
How this interacts with the establishment of VCs for QoS traffic may
alter the characteristics required of those VCs. See [12] for
further details on VC management.
Therefore, in discussing the service-mapping problem, we will assume
that the VC management function of the IWF can always express its
result in terms of an IP-level service with some QoS and TSpec. The
service mapping algorithm, which is the subject of this document, can
then identify the appropriate VC parameters, whether the resulting
action is initiation of a new VC, the addition/deletion of a leaf to
an existing multipoint tree, or the modification of an existing VC to
one of another description.
1.2 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 working group. 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 the work of the rsvp
[2], int-serv [1, 8, 9, 13, 14] and ion working groups [10, 11] of
the IETF. Rsvp defines the resource reservation protocol (which is
analogous to signaling in ATM). Int-serv defines the behavior and
semantics of particular services (analogous e.g., to the Traffic
Management working group in the ATM Forum). Ion defines interworking
of IP and ATM for traditional Best Effort service, and covers all
issues related to routing and addressing.
A large number of ATM signaling details are covered in RFC 1755 [10],
e.g., differences between UNI 3.0 and UNI 3.1, encapsulation, frame-
relay interworking, etc. These considerations generally extend to IP
over ATM with QoS as well. Any description given in this document of
IP Best Effort service (i.e. the default behavior) over ATM is
intended to be consistent with RFC 1755 and it's extension for UNI
4.0 [11], and those documents are to be considered definitive. In
some instances with non-best-effort services, certain IP/ATM features
will diverge from the following RFC 1755. The authors have attempted
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to note such differences explicitly. (For example, best effort VCs
are taken down on timeout by either edge device, while QoS VCs are
only removed by the upstream edge device when the corresponding rsvp
reservation is deleted.)
RFC 1821 [15], represents an early discussions of issues involved
with interoperating IP and ATM protocols for integrated services and
QoS.
2.0 Discussion of 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
three 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 CLS to ATM services. Following
the discussion of the service categories, we discuss the tagging and
conformance definitions for IP and ATM, since the policing method is
implicit in the call setup. We then continue with mappings of the
other parameters and information elements.
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
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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 and/or 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 Capability and the Best Effort indication
flag.
In principle, it is possible to support any foreseeable service
through the use of BCOB-A/CBR. This is because the CBR 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 [15].)
2.1.1 Service Categories for 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
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UNI 3.x, the bearer class BCOB-A (or an equivalent BCOB-X
formulation) must be used. In TM/UNI 4.0 either CBR or rtVBR is
appropriate. In both cases, GS would use a value of CLR
appropriately low for the link (i.e., such that congestion losses are
dominated by losses due to bit errors). The use of rtVBR may
encourage recovery of allocated bandwidth left unused by a source.
It also accomm odates more bursty sources with a larger bucket
parameter, and permits the use of tagging for excess traffic (see
Section 2.2).
Neither the BCOB-C bearer class, nor nrtVBR, UBR, ABR are good
matches for the GS service. These provide no delay estimates and
cannot guarantee consistently low delay for every packet.
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 (accounting for overhead),
of the GS TSpec. For both of these, the network provides a nominal
clearing rate of PCR with jitter 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).
2.1.2 Service Categories for Controlled Load
There are three possible mappings for CLS:
CBR (BCOB-A)
ABR
nrtVBR (BCOB-C)
Note that under UNI 3.x, only the first and third choices are
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 will probably be wasteful of network resources.
The ABR category with a positive MCR aligns with the CLS 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
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measures approximately the amount of buffer required at the IWF.
The nrtVBR/BCOB-C category can also be used. The rtVBR category can
be used, although the edge device must choose a value for CTD and CDV
as a matter of local policy.
The UBR category does not provide enough capability for Controlled
Load. The point of CLS is to allow an allocation of resources, which
is facilitated by the token bucket traffic descriptor, and is
unavailable in 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). 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 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 necessarily
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
bandwidth allocation which may be useful for billing purposes. An
ABR connection could similarly be used to support Best Effort
traffic. The support of data communications protocols such as TCP/IP
is the explicit 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 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
An ATM header carries the Cell Loss Priority (CLP) bit. Cells with
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CLP=1 are said to be ``tagged'' 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.
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.
(UBR and ABR have network-specific conformance definitions. Note,
the term "compliance" in ATM is used to describe the behavior of a
connection.)
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 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'' non-
conformant cells. However, the term ``best effort'' seems to be used
with two distinguishable meanings in the int-serv specs. The first
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
(such as the ATM CLP bit) would be more likely to be dropped, but
would not normally 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 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 PCR to
the aggregate CLP=0+1 cells, and another GCRA with SCR to the CLP=0
cells. Thus this conformance definition should always be used in
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support of 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; it may be a hint of network congestion.
Once an ATM connection is established, and the particular conformance
definition is determined, the 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 be 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. See Section 4.1.
It is always better fo 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
carry some cells of packets which are worthless, because some other
cells from those packet are dropped due to non-conformance.
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.
2.3 ATM Adaptation Layer
The AAL type 5 encoding must be used, as specified in RFC 1483 and
RFC 1755. AAL5 requires specification of the maximum SDU size in both
the forward and reverse directions. Both GS and CLS specify a maximum
packet size as part of the TSpec and this value shall 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 aggregated into a single VC,
the TSpecs are merged to yield the largest packet size. 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 [16] must be
supported as specified in RFC 1577 and RFC 1755. Additional
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encapsulations are discussed in RFC 1755 and we refer to the
discussion there.
2.5 Traffic Descriptors
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
maximum burst size (MBS). The SCR and MBS form a leaky bucket pair
(rate, depth), while the bucket depth parameter for PCR is CDVT.
Note that CDVT is not signaled explicitly, but is determined by the
network operator, and serves as a measure of the jitter imposed by
the network.
Since CDVT is not signaled, and is presumed to be small, the leaky
bucket traffic descriptor (TSpec) of the Internet service cannot
always be directly mapped into PCR/CDVT parameters. Additional
buffering is needed at the IWF to account for the depth of the
bucket.
The Burst Tolerance is related to MBS (see TM 4.0 for details).
Roughly, they are both expressions of the bucket depth parameter that
goes with SCR. The units of BT is time while the units of MBS is
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 4.2 below).
In ATM UNI SIG 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. Very roughly it works like this,
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 sensing the existence of both minimal and
requested parameters, 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 dependent.
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
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likelihood of call admission. Whether anything more specific about
Minimal Traffic Descriptors needs to be said here is left for further
study (FFS). 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
are also part of the signaled information element (IE) containing the
traffic descriptor. In the UNI SIG 4.0 traffic descriptor IE there
is an additional parameter, the Frame Discard indicator (see Section
2.7).
2.5.1 Translating Traffic Descriptors for Guaranteed Service
For Guaranteed Service there is a peak rate, p, a source Tspec rate,
r_s, a receiver Tspec rate r_r, and an Rspec rate, R. The two Tspec
rates are intended to support receiver heterogeneity, in the sense
that different receivers can accept different rates representing
subsets of the sender's traffic. In this document we leave this
feature for further study (FFS), and assume the two Tspec rates are
always identical. The Tspec rate describes the traffic itself, and
is used for policing, while the Rspec rate (which cannot be smaller)
is the allocated service rate. A receiver increases R over r to
reduce the delay.
When mapping Guaranteed Service onto a rtVBR VC, the ATM traffic
descriptor parameters (PCR, SCR, MBS) can be set within the following
bounds:
R <= PCR <= min(p, line rate)
r <= SCR <= PCR
0 <= MBS <- b.
Note that a receiver can choose R > p to lower the delay. This
leaves the first equation somewhat subject to interpretation. If a
receiver chooses R > line rate, it seems clear that the admission
control would simply reject the reservation.
The edge device has a buffer preceding the ATM network which must be
sufficient to absorb bursts arriving faster than they can be admitted
into the ATM network. For example, parameters may be set as PCR = R,
SCR = r, MBS = b. The edge device buffer of size b would absorb a
burst sent at any IP-level peak rate. Although this buffer exists,
the ATM network must accept bursts at rate PCR, at least R, to ensure
that the edge device delay is no greater than b/R. Since this buffer
is not in the ATM network, its delay is not included in D_ATM.
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For GS over CBR, the service rate is mapped to the PCR parameter,
using the same constraint for PCR given above. The edge device again
requires adequate buffering to accommodate the TSpec bucket depth and
ensure delay before entering the ATM network of no more than b/R. If
PCR is greater than R, the buffer requirement may be relaxed
accordingly.
2.5.2 Translating Traffic Descriptors for Controlled Load Service
Controlled Load service has a peak rate, p, a Tspec rate, r, and a
corresponding bucket depth parameter, b. The ATM traffic parameters
for nrtVBR service category are constrained by
r <= SCR <= PCR <= min(p, line rate)
0 <= MBS <- b.
For ABR VCs, the Tspec 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, so the edge device must have a
buffer of at least b bytes.
For CBR, the Tspec rate sets a lower bound on PCR, and again, the
available buffering in the edge device must be adequate to
accommodate possible bursts.
2.5.3 Translating Traffic Descriptors for Best Effort Service
For Best Effort service, there is no traffic description. The UBR
service category allows negotiation of PCR, simply to allow the
source to discover 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
Severely Errored Cell Block Ratio, but these are less well understood
and accepted compared to the basic loss, delay and jitter parameters
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mentioned here. The ITU has recently included a standard set of
parameter values for a (small) number of QoS classes in the latest
version of Recommendation I.356, October 1996. The network provider
may choose to define further network-specific QoS classes in addition
to these. 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.
Note that the QoS class definitions in the new I.356 version may not
align with this model.
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
CLS 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 CLS 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 Discussion of IP-IS Protocol Features
3.1 Handling of Excess Traffic
(Placeholder for text.)
Reiterate that whole packets should be tagged, See Section
2.2.
3.2 Use of AdSpec in Guaranteed Service with ATM
The AdSpec is a feature of Guaranteed Service which allows a receiver
to calculate the worst-case delay associated with a GS flow. Three
quantities, C, D, and MPL, are accumulated (by simple addition of
components, one for each network element) in the PATH message from
source to receiver. The resulting values can be different for each
unique receiver. The maximum delay is then found by
delay <= b/R + C/R + D + MPL
The Maximum Path Latency (MPL) includes propagation delay and any
other unavoidable system delays. (We neglect the effect of maximum
packet size and peak rate here; see the GS specification [8] for the
more detailed equation.) The service rate requested by the receiver,
R, can be greater than the sender's Tspec rate, r. The effect of the
larger R is to allocate more bandwidth and, through this equation,
lower the packet delay. The burst size, b, is the leaky bucket
parameter from the Tspec, and is not changed by the receiver in the
Rspec.
The values of C and D which a router advertise will depend on both
the particular packet scheduling algorithm used in the router, and
the characteristics of the subnet attached to the router. We assume
here that each router (or the source host) takes responsibility for
its downstream subnet only. If the subnet is a simple point-to-point
link, then the subnet-specific parts of C and D will account for the
link transmission rate and MTU. An ATM subnet is more complex.
The edge router will always have an internal packet scheduler, which
will contribute to C and D. For this discussion we consider only the
ATM subnet-specific components. We further assume that the ATM
network will be represented as a "pure delay" element, contributing a
component to D, but not to C. The reason for this is that C would
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depend on details of the cell scheduling algorithm inside the ATM
switches, which is not known by the edge device, where the AdSpec
parameters are accumulated. (In the special case where the edge
device does have enough information to modify C, it would not be
precluded.) Generally the delay behavior of the whole ATM cloud may
be expressed abstractly as a fixed constant D_ATM.
Since the AdSpec values are incremented before any reservation is
made, the edge device must have some knowledge about the VC which
would be set up in case a reservation were made. This does not
really add to the complexity of the device, since it must also have
this information in order to make an intelligent VC setup request.
For example, the edge device may have a cached table with the
propagation delay and a reasonable additional delay budget, from
which it composes a value of CTD for the VC setup. The device may
learn such information through VC setup negotiation, and, indeed,
there may be no other way to obtain that information. However, it
seems reasonable that these values would be cached for later use when
new VCs to the same egress router need to be established.
Therefore, we will presume a table with values of MPL (which includes
propagation delay) and expected queueing delays for each possible
egress edge device. (How such a table is maintained is
implementation specific.) The latter quantity is simply D_ATM, the
value added to the AdSpec D term to account for the ATM network.
When a RESV message arrives, causing a VC to be set up, the requested
value for CTD should then be given by
CTD = D_ATM + MPL + S_ATM.
The last term, S_ATM is the portion of the slack term applied to the
ATM portion of the path. Recall that the slack term [8] is positive
when the receiver can afford more delay than that computed from the
AdSpec. The ATM edge device may take part (or all) of the slack term
to relax the delay constraint on the ATM VC. The distribution of
delay slack among the nodes and subnets is network specific.
An important detail to note is the relationship between the b/R term
of the (Internet) delay and the corresponding MBS/SCR in the ATM
network, when using a VBR VC. The term b/R accounts for the delay
experienced by the last byte of a burst, of size b, which encounters
a congested node. In the simple ideal case, where the scheduling
algorithm emulates a fixed rate server, at rate R, the delay of the
last byte is b/R. Once this occurs, the stream has been smoothed,
and such a delay will not occur at later congested nodes, as long as
they also serve at rate R. The form of the delay equation expresses
this ideal behavior with C and D acting as error terms. Now, since
the delay which smooths the burst can occur outside of the ATM cloud,
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the b/R term cannot include any delay within the ATM cloud. However,
a burst of size MBS is permitted to enter the ATM network, and it may
be served at a rate no greater than SCR. We might reasonably expect
a queueing delay of MBS/SCR to occur at a congested ATM switch. If
the ATM network will impose this delay, then it must be included in
the value of D_ATM advertised. If the ATM network can increase its
bandwidth allocation (e.g., due to CTD being lower than MBS/SCR), to
decrease this delay, then this behavior should be reflected in the
value of D_ATM. So, the information from which the edge device
determines D_ATM must reflect an accurate abstraction of the actual
behavior of the ATM network. To the extent that D_ATM is approximate
(and it must be an upper bound on the actual delay), it reduces the
chance that the VC setup will succeed, and/or increases its cost.
4.0 Discussion of Miscellaneous Items
4.1 Units Conversion
In the integrated services domain, bucket sizes and rates are
measured in bytes and bytes/sec, respectively, whereas for ATM, they
are measured in cells and cells/sec.
Packets are segmented into 53 byte cells of which the first 5 bytes
are header information. For
B = number of Bytes,
C = number of cells,
a rough approximation between the token bucket parameters (rate and
bucket depth) is
C = B/48.
This is actually a lower bound on C and does not take into account
the extra padding at the end of a partially filled cell, or the 8
byte trailer in the last cell of an AAL5 encoding. The actual
relationship between the number of cells and bytes of one packet is
C = 1 + int(B/48) + x,
where x = 1 if B mod 48 > 41
0 otherwise.
where int() is the rounding down operation. The third term is 0 or
1 and is 1 only when the remainder of B/48 is 41 or more. (An
additional cell is needed because the 41 bytes plus 8 byte trailer
will not fit in a cell.)
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The above formula is not particularly amenable to engineering
considerations. By equating the number of bytes before and after
segmentation we have
48 C = B + 8 + A,
where A is the additional padding used in the last 2 cells and has
the range 0 <= A <= 47. From this we obtain a number of useful
observations.
For example, if one believes that the packet lengths are uniformly
distributed mod 48, then on average, 48 C = B + 8 + 47/2, or C = B/48
+ .65625.
We can also make use of the upper bound on A to state that 48 C <= B
+ 55. This is true for any one packet. Considering the number of
bytes in a stream of P packets, we have
48 C <= B + 55 P.
The number of packets P may not be a readily available quantity.
However, in terms of the minimum policed unit m, we know that P * m
<= B. Hence P <= B/m and 48 C <= B ( 1 + 55/m). That is,
C <= B/48 * (1 + 55/m).
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service
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 non-conforming traffic to
best effort is required to be in agreement with the definition of GS.
For this reason, we prefer an rtVBR service in which tagging is
supported. Another good match is to use CBR with special handling of
any non-conforming traffic.
Note, in all cases the encodings assume point to multipoint
connections, where the backward channel is not used. This is done to
be consistent with rsvp, which generally assumes a multicast
scenerio. If a specific situation does not involve multicast, then
the IWF may make use of the backward channel in a point-to-point VC,
provided that the QoS parameters are mapped consistently for the
service provided.
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5.1 Encoding GS Using Real-Time VBR (ATM Forum TM/UNI 4.0)
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size 0
SSCS Type 0 (Null SSCS)
Traffic Descriptor
Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0
Forward SCR CLP=0 Note 1
Backward SCR CLP=0 0
Forward MBS (CLP=0) Note 1
Backward MBS (CLP=0) 0
BE indicator NOT included
Forward Frame Discard bit 1
Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 9 (Real time VBR) Note 3
Susceptible to Clipping 00 (bit encoding for Not
susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 1 Note 5
QoS Class Backward 1 Note 5
QoS Parameters Note 6
Acceptable Forward CDV
Acceptable Forward CLR
Forward Max CTD
Note 1: See discussion Section 2.5.1.
Note 2: Value 3 (BCOB-C) can also be used.
Note 3: The ATC value 19 is not used. The value 19 implies CLR
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objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess
traffic in the case of IP.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
Note 5: Cf ITU I.365 (Oct 1996) for new definition.
Note 6: See section 2.6 for the values to be used The cumulative CDV
is also provided, but it depends on local implementation, and
not on values mapped from IP level service parameters.
5.2 Encoding GS Using CBR (ATM Forum TM/UNI 4.0)
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.
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 0 Note 1
Backward PCR 0
Forward PCR 0+1 Note 1
Backward PCR 0+1 0
BE indicator NOT included
Forward Frame Discard bit 1
Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 5 (CBR) Note 3, 4
Susceptible to Clipping 00 (bit encoding for Not
susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information
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User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 5
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 1 Note 6
QoS Class Backward 1 Note 6
QoS Parameters Note 7
Acceptable Forward CDV
Acceptable Forward CLR
Forward Max CTD
Note 1: See discussion Section 2.5.1.
Note 2: Value 1 (BCOB-A) can also be used.
Note 3: If bearer class A is chosen the ATC field must be absent.
Note 4: The ATC value 7 is not used. The value 7 implies CLR
objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess
traffic in the case of IP.
Note 5: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
Note 6: Cf ITU I.365 (Oct 1996) for new definition.
Note 7: See section 2.6 for the values to be used The cumulative CDV
is also provided, but it depends on local implementation, and
not on values mapped from IP level service parameters.
5.3 Encoding GS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)
The remaining ATM service categories, including nrtVBR, do not
provide delay guarantees and cannot be recommended as the best fits.
However in some circumstances, the best fits may not be available.
If nrtVBR is used, no hard delay can be given. However by using a
variable rate service with low utilization, delay may be
`reasonable', but not controlled. The encoding of GS as nrtVBR is
the same as that for CLS using nrtVBR, except that the Forward PCR
would be derived from the Tspec peak rate. See Section 6.2 below.
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5.4 Encoding GS Using ABR (ATM Forum TM/UNI 4.0)
This is a very unlikely combination. The objective of the ABR
service is to provide `low' loss rates which, via flow control, can
result in delays. The introduction of delays is contrary to the
design objectives of GS. If ABR were used for GS, the VC parameters
would follow as for CLS over ABR. See Section 6.1.
5.5 Encoding GS Using UBR (ATM Forum TM/UNI 4.0)
The UBR service is the default lowest common denominator of the
services. It cannot provide delay or loss guarantees. However if it
is used for GS, it will be encoded in the same way as Best Effort
over UBR, with the exception that the PCR would be determined from
the peak rate of the Tspec. See Section 5.1.
5.6 Encoding GS Using ATM Forum UNI 3.0/3.1 Specifications
It is not recommended to support GS using VBR for the following
reasons. The Class C bearer class does not represent real-time
behavior. Appendix F of UNI 3.1 specification precludes the
specification of traffic type "VBR" with the timing requirement "End
to End timing Required" in conjunction with bearer class X.
It is possible to support GS using a CBR ``pipe.'' The following
table specifies the support of GS using CBR.
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 Note 2
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Backward PCR 0
Forward PCR 0+1 Note 2
Backward PCR 0+1 0
BE indicator NOT included
Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3
Traffic Type 001 (bit encoding for Constant Bit
Rate)
Timing Requirements 01 (bit encoding for Timing
Required)
Susceptible to Clipping 00 (bit encoding for Not
susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 1
QoS Class Backward 1
QoS Parameters
Parameters are implied by the QOS Class
Note 1: Only included for UNI 3.0.
Note 2: See discussion, Section 2.5.1.
Note 3: Value 1 (BCOB-A) can also be used. If BCOB-A is used Traffic
Type and Timing Requirements fields are not included.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service
This section describes how to create ATM VCs appropriately matched
for Controlled Load. CLS traffic is partly delay tolerant and of
variable rate. NrtVBR and ABR (for TM/UNI 4.0 only) are the possible
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choices in supporting CLS.
Generally we prefer to use point-to-multipoint connections. However
this is not yet available in ABR. Other than in ABR, the encodings
assume a point-to-multipoint connection. For a unicast connection,
the backward parameters would be equal to the forward parameters.
6.1 Encoding CLS Using ABR (ATM Forum TM/UNI 4.0)
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
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 1
ATM Transfer Capability 12 (ABR)
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
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 2
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 3 Note 3
QoS Class Backward 3 Note 3
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QoS Parameters Note 4
Acceptable Forward CDV
Acceptable Forward CLR
Forward Max CTD
ABR Setup Parameters Note 5 ABR
Additional Parameters Note 5
Note 1: Value 3 (BCOB-C) can also be used.
Note 2: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
Note 3: Cf ITU I.365 (Oct 1996) for new definition.
Note 4: See section 2.6 for the values to be used. The cumulative CDV
is also provided, but it depends on local implementation, and
not on values mapped from IP level service parameters.
Note 5: Discussion of these parameters is beyond the scope of this draft.
6.2 Encoding CLS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)
AAL
Type 5
Forward CPCS-SDU Size parameter M of TSpec
Backward CPCS-SDU Size 0
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
Forward Frame Discard bit 1
Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 1
ATM Transfer Capability 10 (Non-real time VBR) Note 2, 3
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Susceptible to Clipping 00 (bit encoding Not susceptible)
User Plane Configuration 01 (bit encoding pt-to-mpt)
Broadband Low Layer Information
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 3 Note 5
QoS Class Backward 3 Note 5
QoS Parameters Note 6
Acceptable Forward CDV
Acceptable Forward CLR
Forward Max CTD
Note 1: Value 3 (BCOB-C) can also be used.
Note 2: If bearer class C is used, the ATC field must be absent
Note 3: The ATC value 11 is not used. The value 11 implies CLR
objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess
traffic in the case of IP.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
Note 5: Cf ITU I.365 (Oct 1996) for new definition.
Note 6: See section 2.6 for the values to be used. The cumulative CDV
is also provided, but it depends on local implementation, and
not on values mapped from IP level service parameters.
6.3 Encoding CLS Using Real-Time VBR (ATM Forum TM/UNI 4.0)
The encoding of CLS using rtVBR imposes a hard limit on the delay,
which is specified as an end-to-end delay in the ATM network. This
is more stringent than the CLS service specifies and may result in
less utilization of the network.
If rtVBR is used to encode CLS, then the encoding is essentially the
same as that for GS. The exceptions are that the Forward PCR is
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derived from the line rate and probably a different value of the
transit delay and CDV will be specified. See Section 3.1.
6.4 Encoding CLS Using CBR (ATM Forum TM/UNI 4.0)
The encoding of CLS using CBR is more stringent than using rtVBR
since it does not take into account the variable rate of the data.
Consequently there may be even lower utilization of the network.
To use CBR for CLS, the same encoding as in Section 3.2 would be
used. However a different set of values of the QoS parameters will
likely be used.
6.5 Encoding CLS Using UBR (ATM Forum TM/UNI 4.0)
This encoding gives no QoS guarantees and would be done in the same
way as for BE traffic. See Section 5.1.
6.6 Encoding CLS Using Non-Real-Time VBR as in UNI 3.0/3.1
Specifications
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
Tagging Forward bit 1 (Tagging requested)
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Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2
Traffic Type 010 (bit encoding for Variable Bit
Rate)
Timing Requirements 00 (bit encoding for No Indication)
Susceptible to Clipping 00 (bit encoding for Not
susceptible)
User Plane Configuration 01 (bit encoding for For pt-to-mpt)
Broadband Low Layer Information
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 3
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 3
QoS Class Backward 3
QoS Parameters
Parameters are implied by the QOS Class
Note 1: Only included for UNI 3.0.
Note 2: Value 3 (BCOB-C) can also be used. If BCOB-C is used Traffic
Type and Timing Requirements fields are not included.
Note 3: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
7.0 Summary of ATM VC Setup Parameters for Best Effort Service
This section describes how to create ATM VCs appropriately matched for
Best Effort. The BE service does not need a reservation of resources.
The following subsections are for information only. See the IETF ION
working group draft on ATM signalling support for IP over ATM using UNI
4.0 [11] for recommendations.
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7.1 Encoding Best Effort Service Using UBR (ATM Forum TM/UNI 4.0)
This section is for information only. For recommendation, see the
IETF ION working group draft on ATM signalling support for IP over
ATM using UNI 4.0 [11].
AAL
Type 5
Forward CPCS-SDU Size MTU of link
Backward CPCS-SDU Size MTU of link
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
Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 1
ATM Transfer Capability 10 (Non-real time VBR) Note 2
Susceptible to Clipping 00 (bit encoding for Not susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information
User Information Layer 2
Protocol 12 (ISO 8802/2)
User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 3
ISO/IEC TR 9577 IPI 204
QoS Class
QoS Class Forward 0
QoS Class Backward 0
Note 1: Value 3 (BCOB-C) can also be used.
Note 2: If bearer class C is used, the ATC field must be absent
Note 3: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755.
.fi
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7.2 Encoding Best Effort Service Using Other ATM Service Categories
See the IETF ION working group draft on ATM signalling support for IP
over ATM using UNI 4.0 [11].
8.0 Security
Some security issues are raised in the rsvp specification [2], which
would apply here as well. There are no additional security
considerations raised in this document.
9.0 Acknowledgements
The authors would like to thank the members of the ISSLL working
group for their input. In particular, thanks to Jon Bennett of Fore
Systems, Roch Guerin of IBM and Susan Thomson of Bellcore.
Appendix 1 Abbreviations
AAL ATM Adaptation Layer
ABR Available Bit Rate
ATM Asynchronous Transfer Mode
B-LLI Broadband Low Layer Information
BCOB Broadband Connection-Oriented Bearer Capability
BCOB-{A,C,X} Bearer Class A, C, or X
BE Best Effort
BT Burst Tolerance
CBR Constant Bit Rate
CDV Cell Delay Variation
CDVT Cell Delay Variation Tolerance
CLP Cell Loss Priority (bit)
CLR Cell Loss Ratio
CLS Controlled Load Service
CPCS Common Part Convergence Sublayer
CTD Cell Transfer Delay
EOM End of Message
FFS For Further Study
GCRA Generic Cell Rate Algorithm
GS Guaranteed Service
IE Information Element
IETF Internet Engineering Task Force
IP Internet Protocol
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IS Integrated Services
ISSLL Integrated Services over Specific Link Layers
ITU International Telecommunication Union
IWF Interworking Function
LIJ Leaf Initiated Join
LLC Logical Link Control
MBS Maximum Burst Size
MCR Minimum Cell Rate
MPL Minimum Path Latency
MTU Maximum Transfer Unit
nrtVBR Non-real-time VBR
PCR Peak Cell Rate
PDU Protocol Data Unit
QoS Quality of Service
RESV Reservation Message (of rsvp protocol)
RFC Request for Comment
RSVP Resource Reservation Protocol
Rspec Reservation Specification
rtVBR Real-time VBR
SCR Sustained Cell Rate
SDU Service Data Unit
SIG ATM Signaling (ATM Forum document)
SNAP Subnetwork Attachment Point
SSCS Service-Specific Convergence Sub-layer
Sw Switch
TCP Transport Control Protocol
TM Traffic Management
TSpec Traffic Specification
UBR Unspecified Bit Rate
UNI User-Network Interface
UPC Usage Parameter Control (ATM traffic policing function)
VBR Variable Bit Rate
VC (ATM) Virtual Connection
REFERENCES
[1] R. Braden, D. Clark and S. Shenker, "Integrated Services in the
Internet Architecture: an Overview", RFC 1633, June 1994.
[2] R. Braden, L. Zhang, S. Berson, S. Herzog and S. Jamin,
"Resource ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", Internet Draft, May 1996, <draft-ietf-rsvp-
spec-12.txt>
[3] The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.0", Prentice Hall, Englewood Cliffs NJ, 1993.
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[4] The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.1", Prentice Hall, Upper Saddle River NJ, 1995.
[5] The ATM Forum, "ATM User-Network Interface (UNI) Signalling
Specification, Version 4.0", Prentice Hall, Upper Saddle River
NJ, specification finalized July 1996; expected publication,
late 1996; available at ftp://ftp.atmforum.com/pub.
[6] The ATM Forum, "ATM Traffic Management Specification, Version
4.0", Prentice Hall, Upper Saddle River NJ; specification final-
ized April 1996; expected publication, late 1996; available at
ftp://ftp.atmforum.com/pub.
[7] M. W. Garrett, "A Service Architecture for ATM: From Applica-
tions to Scheduling", IEEE Network Mag., Vol. 10, No. 3, pp. 6-
14, May 1996.
[8] S. Shenker, C. Partridge and R. Guerin, "Specification of
Guaranteed Quality of Service", Internet Draft, August 1996,
<draft-ietf-intserv-guaranteed-svc-06.txt>
[9] J. Wroclawski, "Specification of the Controlled-Load Network
Element Service", Internet Draft, August 1996, draft-ietf-
intserv-ctrl-load-svc-03.txt
[10] M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman and A.
Malis, "ATM Signaling Support for IP over ATM", RFC 1755, Febru-
ary 1995.
[11] M. Perez and A. Mankin, "ATM Signalling Support for IP over ATM
- UNI 4.0 Update", Internet Draft, November 1996, <draft-ietf-
ion-sig-uni4.0-01.txt>
[12] S. Berson, L. Berger, "IP Integrated Services with RSVP over
ATM", Internet Draft, September 1996, <draft-ietf-issll-atm-
support-01.txt>
[13] S. Shenker and J. Wroclawski, "Network Element Service Specifi-
cation Template", Internet Draft, November 1995, <draft-ietf-
intserv-svc-template-02.txt>
[14] J. Wroclawski, "The Use of RSVP with IETF Integrated Services",
Internet Draft, August 1996, <draft-ietf-intserv-use-00.txt>
[15] M. Borden, E. Crawley, B. Davie and S. Batsell, "Integration of
Real-time Services in an IP-ATM Network Architecture", "IP
Authentication Header", RFC 1821, August 1995.
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[16] J. Heinanen, "Multiprotocol Encapsulation over ATM Adaptation
Layer 5", RFC 1483, July 1993.
AUTHORS' ADDRESSES
Mark W. Garrett Marty Borden
Bellcore New Oak Communications, Inc.
445 South Street 42 Nanog Park
Morristown, NJ 07960 Acton MA, 01720
USA USA
phone: +1 201 829-4439 phone: +1 508 266-1011
email: mwg@bellcore.com email: mborden@newoak.com
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Table of Contents
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