Internet DRAFT - draft-ietf-issll-intserv-atmapping
draft-ietf-issll-intserv-atmapping
INTERNET-DRAFT Marty Borden,
Bay Networks,
Mark W. Garrett,
Bellcore.
September, 1996.
Interoperation of Controlled-Load and Guaranteed-Service with ATM
<draft-issll-intserv-atmapping-00.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. Both Internet
and ATM technologies have well-defined service architectures. In
each, a small number of services are identified, with behavioral
descriptions and related parameters to quantify network traffic and
Quality of Service (QoS).
This draft provides mappings between the services of each technology,
in order to facilitate effective end-to-end Quality of Service in the
case where ATM subnetwork technology occurs in the path between
Internet end systems. Specifically, it identifies the types of ATM
Virtual Circuits (VCs) which can be utilized for each of the two
currently defined IP services. A detailed discussion is given of the
accompanying parameters and options, and the interactions between the
two models. Some of the text may be considered preliminary
discussion and is expected to be refined as this draft evolves into a
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proper specification.
1.0 Introduction
We consider the problem of providing IP Integrated Services [RFC1633]
with an ATM subnetwork. We assume the use of the rsvp protocol
[RSVP] for IP-level resource reservation. In the ATM network, we
consider ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [UNI3.0,
UNI3.1, UNI4.0]. The latter uses the more complete service model of
The ATM Forum's TM 4.0 specification [TM40, ATMsvc].
This is a complex problem with many facets. In this draft, 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] and Controlled
Load Service [CLS]. 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 [RFC1755], 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
In the case of UNI 3.0 and 3.1 signaling, where these service are not
all clearly distinguishable, we identify equivalent services where
possible.
The service mappings which follow most naturally from their
definitions are as follows:
Guaranteed Service -> CBR or rtVBR
Controlled Load -> nrtVBR or ABR (with a minimum cell rate)
Best Effort -> UBR or ABR
However, 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. 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.
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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 gives detailed VC setup parameters for
Guaranteed Service, and considers the effect of using each of the ATM
service categories. Section 5 provides a similar treatment for
Controlled Load Service. Section 6 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. 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 case of multicast (or point-to-multipoint) flows.
1.1 General System Architecture
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,
any destination host may 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).
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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
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, a longer section on VC
management will be written, either in as part of this draft or
another one from the issll working group at some point.
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 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
based on an empiric of changing TSpecs. In this case we can assume
that the additional resources are still specifiable in the form of a
TSpec, which can be mapped using the same algorithm. (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.
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 draft, 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.
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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 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,
RFC 1755,
RFC 1821,
draft-crawley-rsvp-over-atm,
draft-birman-ipatm-rsvpatm,
draft-onvural-srinivasan-rsvp-atm.
1.3 Abbreviations
ABR Available Bit Rate
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
CLS Controlled Load Service
CLP Cell Loss Priority (bit)
CLR Cell Loss Ratio
CTD Cell Transfer Delay
GS Guaranteed Service
IWF Interworking Function
MBS Maximum Burst Size
MCR Minimum Cell Rate
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PCR Peak Cell Rate
SCR Sustained Cell Rate
UBR Unspecified Bit Rate
VBR Variable Bit Rate
nrtVBR Non-real-time VBR
rtVBR Real-time VBR
2.0 Discussion of Relevant 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 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
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.
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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 Capabilty and the Best Effort indication
flag.
In principle, it is possible to support any forseeable 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.)
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
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
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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 (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
measures approximately 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 CLS rate parameter would
correspond to the SCR, while the PCR should be set to the line rate,
as for Guaranteed Service.
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The remaining service categories are inappropriate for CLS. 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 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
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. 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. 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.
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),
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
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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''
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 (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
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, 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 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.
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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 must be supported as
specified 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
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
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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.
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
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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
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
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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.
3.0 Discussion of Miscellaneous Items
3.1 Units Conversion
In the integrated services domain, buckets 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.
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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.)
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).
4.0 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 non-conforming traffic to
best effort is desired. For this reason, we prefer a rtVBR service
in which tagging is supported. Another good match is to use CBR with
special handling of any non-conforming traffic.
The encodings assume a point-to-multipoint connection. For a unicast
connection, the backward parameters would be equal to the forward
parameters.
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4.1 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)
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.
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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.
4.2 Encoding GS as a constant bit rate service
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
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)
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
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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.
4.3 Encoding GS as a non-real-time variable bit rate service
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 CL using nrtVBR, except that the Forward PCR
would be derived from the Tspec peak rate. See section 5.2 below.
4.4 Encoding GS as an ABR service
The authors feel that 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 point of GS.
4.5 Encoding GS as an UBR service
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.
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5.0 Controlled Load Service over ATM
This section describes how to create ATM VCs appropriately matched
for Controlled Load. CL traffic is partly delay tolerant and of
variable rate. We see nrtVBR and ABR (for TM/UNI 4.0 only) as
possible choices in supporting CL.
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.
5.1 Encoding CL using an ABR service
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 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
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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.
5.2 Encoding CL using a non-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 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 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
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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.
5.3 Encoding CL using a real-time variable bit rate service
The encoding of CL 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 CL service specifies and may result in
less utilization of the network.
If rtVBR is used to encode CL, then the encoding is essentially the
same as that for GS. The exceptions are that the Forward PCR is
derived from the line rate and probably a different value of the
transit delay and CDV will be specified. See section 3.1.
5.4 Encoding CL using a constant bit rate service
The encoding of CL 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 CL, 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.
5.5 Encoding CL using a UBR service
This encoding gives no QoS guarantees and would be done in the same
way as for BE traffic. See section 5.1.
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6.0 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.
5.1 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
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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1. REFERENCES
[RFC1633]
R. Braden, D. Clark and S. Shenker, "Integrated Services in the
Internet Architecture: an Overview", RFC 1633, June 1994.
[RFC1755]
M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman and A.
Malis, "ATM Signlaing Support for IP over ATM", RFC 1755, Febru-
ary 1995.
[IPATM]
M. Perez and A. Mankin, "ATM Signalling Support for IP over ATM
- UNI 4.0 Update", Internet Draft, June 1996, <draft-ietf-ion-
sig-uni4.0-oo.txt>
[RFC1821]
M. Borden, E. Crawley, B. Davie and S. Batsell, "Integration of
Real-time Sevices in an IP-ATM Network Architecture", "IP
Authentication Header", RFC 1821, August 1995.
[RSVP]R. Braden, L. Zhang, S. Berson, S. Herzog and S. Jamin,
"Resource ReSevVation Protocol (RSVP) - Version 1 Functional
Specification", Internet Draft, May 1996, <draft-ietf-rsvp-
spec-12.txt>
[GS] S. Shenker, C. Partridge and R. Guerin, "Specification of
Guaranteed Quality of Service", Internet Draft, August 1996,
<draft-ietf-intserv-guaranteed-svc-06.txt>
[CLS]J. Wroclawski, "Specification of the Controlled-Load Network
Element Service", Internet Draft, August 1996, draft-ietf-
intserv-ctrl-load-svc-03.txt
[USE-RSVP-IS]
J. Wroclawski, "The Use of RSVP with IETF Integrated Services",
Internet Draft, August 1996, <draft-ietf-intserv-use-00.txt>
[TEMPLATE]
S. Shenker and J. Wroclawski, "Network Element Service Specifi-
cation Template", Internet Draft, November 1995, <draft-ietf-
intserv-svc-template-02.txt>
[UNI3.0]
The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.0", Prentice Hall, Englewood Cliffs NJ, 1993.
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INTERNET DRAFT Interoperation of CLS and GS with ATM September, 1996
[UNI3.1]
The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.1", Prentice Hall, Upper Saddle River NJ, 1995.
[UNI4.0]
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. The ATM
Forum, "ATM Traffic Management Specification, Version 4.0",
Prentice Hall, Upper Saddle River NJ; specification finalized
April 1996; expected publication, late 1996; available at
ftp://ftp.atmforum.com/pub.
[ATMsvc]
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.
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.
AUTHORS' ADDRESSES
Marty Borden Mark W. Garrett
Bay Networks Bellcore
3 Federal Street 445 South Street
Billerica, MA 01821 Morristown, NJ 07960
USA USA
phone: +1 508 436-3903 phone: +1 201 829-4439
email: mborden@baynetworks.com email: mwg@bellcore.com
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