As per Relevance of the word backward, we have this rfc below:
Network Working Group M.
Request for Comments: 2381
Category: Standards Track M.
Bay
August 1998
Interoperation of Controlled-Load
and Guaranteed Service with
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1998). All Rights Reserved
This document provides guidelines for mapping service classes,
traffic management features and parameters between Internet and
technologies. The service mappings are useful for
effective interoperation and end-to-end Quality of Service for
Integrated Services networks containing ATM subnetworks
The discussion and specifications given here support the
integrated services protocols for Guaranteed Service (GS),
Controlled-Load Service (CLS) and the ATM Forum UNI specification
versions 3.0, 3.1 and 4.0. Some discussion of IP best effort
over ATM is also included
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119 [1]. (Note
in many cases the use of "MUST" or "REQUIRED" reflects
interpretation of the requirements of a related standard, e.g.,
Forum UNI 4.0, rsvp, etc.)
Garrett & Borden Standards Track [Page 1]
RFC 2381 Interoperation of CLS and GS with ATM August 1998
Table of
1.0 Introduction .................................................... 3
1.1 General System Architecture ................................. 4
1.2 Related Documents ........................................... 7
2.0 Major Protocol Features for Traffic Management and QoS .......... 8
2.1 Service Category and Bearer Capability ...................... 8
2.1.1 Service Categories for Guaranteed Service ............. 9
2.1.2 Service Categories for Controlled Load ................ 10
2.1.3 Service Categories for Best Effort .................... 11
2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions . 11
2.3 ATM Adaptation Layer ........................................ 13
2.4 Broadband Low Layer Information ............................. 13
2.5 Traffic Descriptors ......................................... 13
2.5.1 Translating Traffic Descriptors for Guaranteed Service. 15
2.5.2 Translating Traffic Descriptors for Controlled
Service .............................................. 18
2.5.3 Translating Traffic Descriptors for Best Effort Service 19
2.6 QoS Classes and Parameters .................................. 19
2.7 Additional Parameters -- Frame Discard Mode ................. 22
3.0 Additional IP-Integrated Services Protocol Features ............. 22
3.1 Path Characterization Parameters for IP Integrated Services . 22
3.2 Handling of Excess Traffic .................................. 24
3.3 Use of Guaranteed Service Adspec Parameters and Slack Term .. 25
4.0 Miscellaneous Items ............................................. 26
4.1 Units Conversion ............................................ 26
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service ....... 27
5.1 Encoding GS Using Real-Time VBR ............................. 28
5.2 Encoding GS Using CBR ....................................... 29
5.3 Encoding GS Using Non-Real-Time VBR ......................... 30
5.4 Encoding GS Using ABR ....................................... 30
5.5 Encoding GS Using UBR ....................................... 30
5.6 Encoding GS Using UNI 3.0 and UNI 3.1. ...................... 31
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service .. 32
6.1 Encoding CLS Using Non-Real-Time VBR ........................ 32
6.2 Encoding CLS Using ABR ...................................... 33
6.3 Encoding CLS Using CBR ...................................... 35
6.4 Encoding CLS Using Real-Time VBR ............................ 35
6.5 Encoding CLS Using UBR ...................................... 35
6.6 Encoding CLS Using UNI 3.0 and UNI 3.1. ..................... 35
7.0 Summary of ATM VC Setup Parameters for Best Effort Service ...... 36
7.1 Encoding Best Effort Service Using UBR ...................... 37
8.0 Security Considerations ......................................... 38
9.0 Acknowledgements ................................................ 38
Appendix 1 Abbreviations ........................................... 39
References .......................................................... 40
Authors' Addresses .................................................. 42
Full Copyright Statement ............................................ 43
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
1.0
We consider the problem of providing IP Integrated Services [2]
an ATM subnetwork. This document is intended to be consistent
the rsvp protocol [3] for IP-level resource reservation, although
applies also in the general case where GS and CLS services
supported through other mechanisms. In the ATM network, we
ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [4, 5, 6].
latter uses the more complete service model of the ATM Forum's TM 4.0
specification [7, 8].
This is a complex problem with many facets. In this document,
focus on the service types, parameters and signalling elements
for service interoperation. The resulting service mappings can
used to provide effective end-to-end Quality of Service (QoS) for
traffic that traverses ATM networks
The IP services considered are Guaranteed Service (GS) [9]
Controlled Load Service (CLS) [10]. We also discuss the default
Effort Service (BE) in parallel with these. Our recommendations
BE are intended to be consistent with RFC 1755 [11], and [12],
define how ATM VCs can be used in support of normal BE IP service
The ATM services we consider are
CBR Constant Bit
rtVBR Real-time Variable Bit
nrtVBR Non-real-time Variable Bit
UBR Unspecified Bit
ABR Available Bit
In the case of UNI 3.x signalling, where these service are not
clearly distinguishable, we identify the appropriate
services
We recommend the following service mappings, since they follow
naturally from the service definitions
Guaranteed Service -> CBR or
Controlled Load -> nrtVBR or ABR (with a
cell rate
Best Effort -> UBR or
For completeness, however, we provide detailed mappings for
service combinations in Sections 5, 6, 7 and identify how each
or fails to meet the requirements of the higher level IP services
The reason for not restricting mappings to the most obvious
natural ones is that we cannot predict how widely available all
these services will be as ATM deployment evolves. A number
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
differences in service definitions, such as the treatment of
in excess of the flow traffic descriptor, make service mapping
relatively complicated subject
The remainder of this introduction provides a general discussion
the system configuration and other assumptions. Section 2
the relevant ATM protocol elements and the corresponding features
Guaranteed, Controlled Load and Best Effort services (the
being the default "service"). Section 3 discusses a number
remaining features of the IP services and how they can be handled
an ATM subnetwork. Section 4 addresses the conversion of
descriptors to account for ATM-layer overheads. Section 5 gives
detailed VC setup parameters for Guaranteed Service, and
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
interworking of IP integrated services with ATM subnetworks.
important issue of VC management, including flow aggregation,
considered in [13, 14, 15]. We do not consider how routing,
sensitive or not, interacts with the use of ATM VCs. We expect
a considerable degree of implementation latitude will exist,
within the guidelines presented here. Many aspects of
between IP and ATM will depend on economic factors, and will not
subject to standardization
1.1 General System
We assume that the reader has a general working knowledge of IP,
and ATM protocols. The network architecture we consider
illustrated in Figure 1. An IP-attached host may send
datagrams to another host, or may use an IP multicast address to
packets to all of the hosts which have "joined" the multicast "tree".
In either case, a destination host may then use RSVP to
resource reservation in routers along the internet path for the
flow
An ATM network lies in the path (chosen by the IP routing),
consists of one or more ATM switches. It uses SVCs to provide
resources and QoS within the ATM cloud. These connections are
up, added to (in the case of multipoint trees), torn down,
controlled by the edge devices, which act as both IP routers and
interfaces, capable of initiating and managing VCs across the
user-to-network (UNI) interface. The edge devices are assumed to
fully functional in both the IP int-serv/RSVP protocols and the
UNI protocols, as well as translating between them
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ATM
-----------------
H ----\ ( ) /-------
H ---- R -- R -- E-( X -- X -- X )-E -- R -- R --
H ----/ | ( ) \
| ----------------- \------
H ----------
Figure 1: Network Architecture with Hosts (H),
Routers (R), Edge Devices (E) and
Switches (X).
When considering the edge devices with respect to traffic
from source to destination, the upstream edge device is called
"ingress" point and the downstream device the "egress" point.
edge devices may be considered part of the IP internet or part of
ATM cloud, or both. They process RSVP messages, reserve resources
and maintain soft state (in the control path), and classify
schedule packets (in the data path). They also initiate
connections by signalling, and accept or refuse connections
to them. They police and schedule cells going into the ATM cloud
The service mapping function occurs when an IP-level
(RESV message) triggers the edge device to translate the RSVP
requirements into ATM VC (UNI) semantics
A range of VC management policies are possible, which
whether a flow initiates a new VC or joins an existing one. VCs
managed according to a combination of standards and local
rules, which are specific to either the implementation (equipment)
the operator (network service provider). Point-to-
connections within the ATM cloud can be used to support general
multicast flows. In ATM, a point to multipoint connection can
controlled by the source (or root) node, or a leaf initiated
(LIJ) feature in ATM may be used. The topic of VC management
considered at length in other ISSLL documents [13, 14, 15].
Figure 2 shows the functions of an edge device, summarizing the
not part of IP or ATM abstractly as an InterWorking Function (IWF),
and segregating the control and data planes
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
IP
____________________
| IWF |
| |
admission and <--> | service mapping | <-->
policy control | VC management | signalling &
| address resolution |
|....................|
| |
classification, |ATM Adaptation Layer|
policing & <--> | Segmentation and | <--> scheduling
scheduling | Reassembly |
| Buffering |
____________________
Figure 2: Edge Device Functions showing the
In the logical view of Figure 2, some functions, such as scheduling
are shown separately, since these functions are present on both
IP and ATM sides. However it may be possible in an
implementation to combine such functions
The service mapping and VC management functions can be
interdependent. For example: (i) Multiple integrated-services
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
parameters have been merged appropriately. (ii) The VC
function may choose to allocate extra resources in anticipation
further reservations or based on an empiric of changing TSpecs
(iii) There MUST exist a path for best effort flows and for
the rsvp control messages. How this interacts with the
of VCs for QoS traffic may alter the desired characteristics of
VCs. See [13, 14, 15] for further details on VC management
Therefore, in discussing the service mapping problem, we will
that the VC management function of the IWF can always express
result in terms of an IP-level service with some QoS and TSpec.
service mapping algorithm can then identify the appropriate
parameters as if a new VC were to be created for this service.
VC management function can then use this information consistent
its own policy, which determines whether the resulting action
new or existing VCs
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1.2 Related
Earlier ATM Forum documents combined signalling, traffic
and other areas into a single document, e.g., UNI 3.0 [4] and UNI 3.1
[5]. The 3.1 release was used to correct errors and fix
with the ITU. While UNI 3.0 and 3.1 are incompatible in terms
actual codepoints, the semantics are generally the same. Therefore
we will often refer to UNI 3.x to mean either version of the
protocol
After 3.1, the ATM Forum released documents separately for
technical working group. The UNI Signalling 4.0 [6] and
Management 4.0 [7] documents represent a consistent overall
protocol, and we will sometime refer to the combination as TM/
4.0.
Within the IETF, related material includes the work of the rsvp [3],
int-serv [2, 9, 10, 16, 17] and ion working groups [11, 12].
defines the resource reservation protocol (which is analogous
signalling in ATM). Int-serv defines the behavior and semantics
particular services (analogous to the Traffic Management
group in the ATM Forum). Ion defines interworking of IP and ATM
traditional Best Effort service, and generally covers issues
to IP/ATM routing and addressing
A large number of ATM signalling 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 extend to
over ATM with QoS as well. The description given in this document
IP Best Effort service (i.e. the default behavior) over ATM
intended to be consistent with RFC 1755 and it's extension for
4.0 [11], and those documents are to be considered definitive.
non-best-effort services, certain IP/ATM features will diverge
the following RFC 1755. We have attempted to note such
explicitly. (For example, best effort VCs may be taken down
timeout by either edge device, while QoS VCs are only removed by
upstream edge device when the corresponding rsvp reservation
deleted.)
Another related document is RFC 1821 [17], which represents an
discussion of issues involved with interoperating IP and
protocols for integrated services and QoS
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2.0 Major Protocol Features for Traffic Management and
The ATM Call Setup is sent by the ingress edge device to the
network to establish end-to-end (ATM) service. This setup
the following information
Service Category/Broadband Bearer
AAL
Broadband Low Layer
Calling and Called Party Addressing
Traffic
QoS Class and/or
Additional Parameters of TM/UNI 4.0
In this section, we discuss each of these items as they relate
creating ATM VCs suitable for GS, CLS and BE services. We do
discuss routing and addressing at all, since they are (at
presently) independent of QoS. Following the section on
categories, we discuss tagging and conformance definitions for IP
ATM. These do not appear explicitly as set-up parameters in
above list, since they are implied by the policing method used
2.1 Service Category and Bearer
The highest level of abstraction distinguishing features of ATM
is in the service category or bearer capability. Service
were introduced in TM/UNI 4.0; previously the bearer capability
used to discriminate at this level
These elements indicate the general properties of a VC: whether
is a real-time delay constraint, whether the traffic is constant
variable rate, the applicable traffic and QoS description
and (implicitly) the complexity of some supporting switch
(e.g., ABR).
For UNI 3.0 and UNI 3.1, there are only two distinct options
bearer capabilities (in our context):
BCOB-A: constant rate, timing required, unicast/multipoint
BCOB-C: variable rate, timing not required, unicast/multipoint
A third capability, BCOB-X, can be used as a substitute for the
two capabilities, with its dependent parameters (traffic type
timing requirement) set appropriately. The distinction between
BCOB-X formulation and the "equivalent" (for our purposes) BCOB-A
BCOB-C constructs is whether the ATM network is to provide pure
relay service or interwork with other technologies (
interoperable signalling), such as narrowband ISDN. Where
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
distinction is applicable, the appropriate code SHOULD be used (
[5] and related ITU specs, e.g., I.371).
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
to TM/UNI 4.0. The ABR service is also new to TM/UNI 4.0.
exists in all specifications, although it is called "best effort"
UNI 3.x. In either case it is indicated by the "best effort
indication flag (and the QoS Class set to 0).
The Service Category in TM/UNI 4.0 is encoded into the same
Information Element (IE) as the Bearer Capability in UNI 3.x, for
purpose of backward compatibilty. Thus, we use the convention
referring to Service Category (CBR, rtVBR, nrtVBR, UBR, ABR)
TM/UNI 4.0 (where the bearer capability is implicit). When we
to the Bearer Capability explicitly (BCOB-A, BCOB-C, BCOB-X), we
describing a UNI 3.x signalling message
In principle, it is possible to support any service through the
of BCOB-A/CBR. This is because the CBR service is equivalent
having a "pipe" of a specified bandwidth. However, it may
significantly more efficient to use the other ATM services
applicable and available [17].
2.1.1 Service Categories for Guaranteed
There are two possible mappings for GS
CBR (BCOB-A
Real-time support is REQUIRED for GS. Thus in UNI 3.x, the
Class BCOB-A (or an equivalent BCOB-X formulation) MUST be used.
TM/UNI 4.0 either CBR or rtVBR is appropriate. The use of rtVBR
encourage recovery of allocated bandwidth left unused by a source
It also accommodates more bursty sources with a larger token
burst parameter, and permits the use of tagging for excess
(see Section 2.2).
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Neither the BCOB-C Bearer Class, nor nrtVBR, UBR, ABR are
matches for the GS service. These provide no delay estimates
cannot guarantee consistently low delay for every packet
For BCOB-A or CBR, specification of a peak cell rate (PCR)
REQUIRED by ATM standards. In these cases, PCR is the
clearing rate with a nominal jitter toleration (bucket size), CDVT
When rtVBR is specifed, two rates, PCR and SCR are REQUIRED (by
standards). This models bursty traffic with specified peak
sustainable rates. The corresponding ATM token bucket depth
are CDVT, and CDVT+BT, respectively
2.1.2 Service Categories for Controlled
There are three possible good mappings for CLS
CBR (BCOB-A
nrtVBR (BCOB-C
Note that under UNI 3.x, there are equivalent services to CBR
nrtVBR, but not ABR. The first, with a CBR/BCOB-A connection
provides a higher level of QoS than is necessary, but it may
convenient to simply allocate a fixed-rate "pipe", which we expect
be ubiquitously supported in ATM networks. However unless this
the only choice available, it would probably be wasteful of
resources
The nrtVBR/BCOB-C category is perhaps the best match, since
provides for allocation of bandwidth and buffers with an
peak rate indication, similar to the CLS TSpec. Excess traffic
be handled by CLP bit tagging with VBR
The ABR category with a positive MCR aligns with the CLS idea
"best effort with a floor." The ATM network agrees to forward
with a rate of at least MCR, which MUST be directly converted
the token bucket rate of the receiver TSpec. The bucket
parameter measures approximately the amount of buffer necessary
the IWF. This buffer serves to absorb the bursts allowed by
token bucket, since they cannot be passed directly into an ABR VC
The rtVBR category can be used, although the edge device MUST
determine values for CTD and CDV. Since there are no
IP-level parameters, their values are set as a matter of
policy
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
The UBR category does not provide enough capability for
Load. The point of CLS is to allow an allocation of resources.
is facilitated by the token bucket traffic descriptor, which
unavailable with UBR
2.1.3 Service Categories for Best
All of the service categories have the capability to carry
Effort service, but the natural service category is UBR (or, in
3.x, BCOB-C or BCOB-X, with the best effort indication set). CBR
rtVBR clearly could be used, and since the service is not real-time
a nrtVBR connection could also be used. In these cases the
parameter used reflects a bandwidth allocation in support of
ingress edge device's best effort connectivity to the egress
router. It would be normal for traffic from many source/
pairs to be aggregated on this connection; indeed, since Best
is the default IP behavior, the individual flows are not
identified or accounted for. CBR may be a preferred solution in
case where best effort traffic is sufficiently highly aggregated
a simple fixed-rate pipe is efficient. Both CBR and nrt-VBR
explicit bandwidth allocation which may be useful for
purposes. In the case of UBR, the network operator SHOULD
bandwidth for the overall service through the admission
function, although such allocation is not done explicitly per VC
An ABR connection could similarly be used to support Best
traffic. Indeed, the support of data communications protocols
as TCP/IP is the explicit purpose for which ABR was designed. It
conceivable that a separate ABR connection would be made for each
flow, although the normal case would probably have all IP Best
traffic with a common egress router sharing a single ABR connection
The rt-VBR service category may be considered less suitable,
because both the real-time delay constraint and the use of SCR/BT
unnecessary complexity
See specifications from the IETF ion working group [10, 11]
related work on support of Best Effort service with ATM
2.2 Cell Loss Priority Bit, Tagging and Conformance
Each ATM cell header carries a Cell Loss Priority (CLP) bit.
with CLP=1 are said to be "tagged" or "marked" and have
priority. This tagging may be done by the source, to
relative priority within the VC, or by a switch, to indicate
in violation of policing parameters. Options involving the use
tagging are decided at call setup time
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
A Conformance Definition is a rule that determines whether a cell
conforming to the traffic descriptor of the VC. The
definition is given in terms of a Generic Cell Rate Algorithm (GCRA),
also known as a "leaky bucket" algorithm, for CBR and VBR services
The conformance definition also specifies rules for tagging
in excess of the {SCR, MBS} GCRA traffic descriptor. (Note, the
"compliance" in ATM is used to describe the behavior of a connection
as opposed to "conformance", which applies to a single cell.)
The network may tag cells that are non-conforming, rather
dropping them if the VC set-up requests tagging and the
supports the tagging option. When tagging is used and
occurs, a switch MUST attempt to discard tagged cells in
to discarding CLP=0 cells. However, the mechanism for doing this
completely implementation specific. The behavior that best meets
requirements of IP Integrated Services is where tagged cells
treated as "best effort" in the sense that they are transported
bandwidth is available, queued when buffers are available,
dropped when resources are overcommitted. ATM standards, however,
not explicitly specify treatment of tagged traffic. Providers of
and CLS service with ATM subnetworks SHOULD ascertain the
behavior of ATM implementation with respect to tagged cells
Since GS and CLS services REQUIRE excess traffic to be treated
best effort, the tagging option SHOULD always be chosen (
supported) in the VC setup as a means of "downgrading" the
comprising non-conformant packets. The term "best effort" can
interpreted in two ways. The first is as a service class that,
example, may be implemented as a separate queue. The other sense
more generic, meaning that the network makes a best effort
transport the traffic. A reasonable interpretation of this is that
network with no contending traffic would transport the packet,
a very congested network would drop the packet. A mechanism
tags best effort packets with lower loss priority (such as with
ATM CLP bit) would drop some of these packets, but would not
the remaining ones with respect to the conforming portion of
flow. The "best effort" mechanism for excess traffic does
necessarily have to be the same as that for best effort "service",
long as it fits this generic sense of best effort
There are three conformance definitions of VBR service (for
rtVBR and nrtVBR) to consider. In VBR, only the
definition VBR.3 supports tagging and applies the GCRA with rate
to the aggregate CLP=0+1 cells, and another GCRA with rate SCR to
CLP=0 cells. This conformance definition SHOULD always be used
a VBR service supporting IP integrated services. For UBR service
conformance definition UBR.2 supports the use of tagging, but a CLP=1
cell does not imply non-conformance; rather, it may be used by
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
network to indicate congestion
In TM/UNI 4.0 tagging is not a feature of the conformance
for the CBR or ABR service categories. (Since
definitions are generally network specific, some implementations
or ABR may, in fact, use tagging in some way.) Wherever an
network does support tagging, in the sense of transporting CLP=1
cells on a "best effort" basis, it is a useful and
mechanism for handling excess traffic
It is always better for the IWF to tag cells when it can
that the ATM network would do so. This is because the IWF knows
IP packet boundaries and can tag all of the cells corresponding to
packet. If left to the ATM layer UPC, the network would
drop some of the cells of a packet while carrying others, which
then be dropped by the receiver. Therefore, the IWF, knowing the
GCRA parameters, SHOULD always anticipate the cells which will
tagged by the ATM UPC and tag all of the cells uniformly across
affected packet. See Section 3.2 for further discussion of
traffic
2.3 ATM Adaptation
The AAL type 5 encoding SHOULD be used, as specified in RFC 1483
RFC 1755. For AAL-5, specification of the maximum SDU size in
the forward and reverse directions is REQUIRED. Both GS and
specify a maximum packet size, M, as part of the TSpec and this
SHOULD be used (corrected for AAL headers) as the maximum SDU in
direction for unicast connections, and for unidirectional point-to
multipoint connections. When multiple flows are aggregated into
single VC, the M parameters of the receiver TSpecs are
according to rules given in the GS and CLS specs
2.4 Broadband Low Layer
The B-LLI Information Element is transferred transparently by the
network between the edge devices and is used to specify
encapsulation method. Multiple B-LLI IEs may be sent as part
negotiation. The LLC/SNAP encapsulation [18] SHOULD be supported
the default, but "null" or "VC encapsulation" MAY also be allowed
Implementations SHOULD follow RFC 1577 [19] and RFC 1755 [10]
BLLI usage
2.5 Traffic
The ATM traffic descriptor always contains a peak cell rate (PCR
(for each direction). For VBR services it also contains
sustainable cell rate (SCR) and maximum burst size (MBS). The
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
and MBS form a leaky bucket pair (rate, depth), while the
depth parameter for PCR is CDVT. Note that CDVT is not
explicitly, but is determined by the network operator, and can
viewed as a measure of the jitter imposed by the network
Since CDVT is generally presumed to be small (equivalent to a
cells of token bucket depth), and cannot be set independently
each connection, it cannot be used to account for the
permitted by b of the IP-layer TSpec. Additional buffering may
needed at the IWF to account for the depth of the token bucket
The ATM Burst Tolerance (BT) is equivalent to MBS (see TM 4.0 [6]
the exact equation). They are both expressions of the bucket
parameter associated with SCR. The units of BT are time while
units of MBS are cells. Since both SCR and MBS are signalled,
can be computed directly from the IP layer traffic description.
specific manner in which resources are allocated from the
description is implementation specific. Note that when
the traffic parameters, the segmentation overhead and minimum
unit need to be taken into account (see Section 4.1 below).
In ATM UNI Signalling 4.0 there are the notions of
Traffic Descriptors and Minimal Traffic Descriptors.
Traffic Descriptors enumerate other acceptable choices for
descriptors and are not considered here. Minimal Traffic
are used in "negotiation," which refers to the specific way in
an ATM connection is set up. To illustrate, roughly, taking PCR
an example: A minimal PCR and a requested PCR are signalled,
requested PCR being the usual item signalled, and the minimal
being the absolute minimum that the source edge device will accept
When both minimal and requested parameters are present,
intermediate switches along the path may reduce the requested PCR
a "comfortable" level. This choice is part of admission control,
is therefore implementation specific. If at any point the
PCR falls below the minimal PCR then the call is cleared.
Traffic Descriptors can be used to present an acceptable range
parameters and ensure a higher likelihood of call admission.
general, our discussion of connection parameters assumes the
resulting from successful connection setup
The Best Effort indicator (used only with UBR) and Tagging
(see Section 2.2) are also part of the signalled information
(IE) containing the traffic descriptor. In the UNI 4.0
descriptor IE there is an additional parameter, the Frame
indicator, which is discussed below in Section 2.7.
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RFC 2381 Interoperation of CLS and GS with ATM August 1998
2.5.1 Translating Traffic Descriptors for Guaranteed
For Guaranteed Service the source TSpec contains peak rate, rate
and bucket depth parameters, p_s, r_s, b_s. The receiver
contains corresponding parameters p_r, r_r, b_r. The (receiver
RSpec also has a rate, R. The two different TSpec rates are
to support receiver heterogeneity, in the sense that receivers
accept different rates representing different subsets of the sender'
traffic. Whenever rates from different receivers differ, the
MUST always be merged appropriately before being mapping into
parameters
Note that when the sender and receiver TSpec rates r_s, r_r differ
there is no mechanism specified (in either rsvp or the int-
specs) for indicating which subset of the traffic is to
transported. Implementation of this feature is therefore
network specific. The policing and scheduling mechanisms may
be parameterized with the (lower) receiver rate, resulting in
random loss of traffic sufficient to make up the difference in rates
The receiver TSpec rate describes the traffic for which resources
to be reserved, and may be used for policing, while the RSpec
(which cannot be smaller) is used (perhaps in an
specific way) to modify the allocated service bandwidth in order
reduce the delay
When mapping Guaranteed Service onto a rtVBR VC, the ATM
descriptor parameters (PCR, SCR, MBS) can be set cannonically as
PCR = p_
SCR =
MBS = b_r
There are a number of conditions that may lead to different choices
The following discussion is not intended to set hard requirements
but to provide some interpretation and guidance on the bounds
possible parameter mappings. The ingress edge device
includes a buffer preceding the ATM network interface. This
can be used to absorb bursts that fall within the IP-level TSpec,
not within the ATM traffic descriptor. The minimal REQUIREMENT
guaranteed service is that the delay in this buffer MUST NOT
b/R, and the delays within the ATM network MUST be
accounted for in the values of Adspec parameters C and D
by the ingress router (see Section 3.3 below).
If either an edge device buffer of size b_r exists or the ATM
burst size (MBS) parameter is at least b_r, then the various
parameters will generally exhibit the following relationship
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r_r <= SCR <= R <= PCR <= APB <= line
r_r <= p_r <=
APB refers to the General Characterization Parameter
AVAILABLE_PATH_BANDWIDTH, which is negotiated in the Adspec
of the PATH message. APB reflects the narrowest bottleneck
along the path, and so is always no larger than the local line rate
The receiver SHOULD choose a peak rate no greater than APB for
reservation to be accepted, although the source peak rate, p_s,
be higher, as the source does not know the value of APB. There is
advantage to allocating any rate above APB of course, so it is
upper bound for all the other parameters
We might normally expect to find R <= p_r, as would be necessary
the simple mapping of PCR = p_r, SCR = R given above. However,
receiver is free to choose R > p_r to lower the GS delay [8].
this case, PCR cannot be set below R, because a burst of size
arriving into the buffer MUST be cleared at rate R to keep the
component of GS delay down to b/R. So here we will have PCR = R.
may seem that PCR = p_r would be sufficient to avoid buffer overflow
since data is generated at the source at a rate bounded by p_r
However, setting PCR < R, can result in the delay bound advertised
C and D not being met. Also, traffic is always subject to jitter
the network, and the arrival rate at a network element can exceed p_
for short periods of time
In the case R <= p_r, we may still choose PCR such that R <= PCR <
p_r. The edge device buffer is then necessary (and sufficient)
absorb the bursts (limited to size b_r + C_sum + R D_sum)
arrive faster than they depart. For example, it may be the case
the cost of the ATM VC depends on PCR, while the cost of the
service reservation is not strongly dependent on the IP-level
rate. The user may then have an incentive to set p_r to APB,
the operator of the IP/ATM edge router has an incentive to reduce
as much as possible. This may be a realistic concern, since
charging models of IP and ATM are historically different as far
usage sensitivity, and the value of p_r, if set close to APB,
be many times the nominal GS allocated rate of R. Thus, we can
PCR to R, with a buffer of size b_r + C_sum + R D_sum, with no
of traffic, and no violation of the GS delay bound
A more subtle, and perhaps controversial case is where we set SCR
a value below R. The major feature of the GS service is to allow
receiver to specify the allocated rate R to be larger than the
r_r sufficient to transport the traffic, in order to lower
queueing delay (roughly) from b/r + C_TOT/r + D_TOT to b/R + C_TOT/
+ D_TOT. To effectively allocate bandwidth R to the flow, we set
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to match R. (Note it is unnecessary in any case to set SCR above R
so the relation, SCR <= R, is still true.) It is possible to set
to a value as low as r_r, without violating the delay bounds
overflowing the edge device buffer. With PCR = R, a burst of size
will be buffered and sent into the ATM network at rate R, so the
byte suffers delay only b/R. Any further traffic will be limited
rate r_r, which is SCR, so with the arriving and departing
matched, its delay will also be no more than b/R
While this scenario meets the GS service requirements, the
for allocating SCR = r_r rather than R is that the delay in the
network will have a component of MBS/SCR, which will be b/r
than b/R, contained in the D term advertised for the ATM sub-
(see further discussion in Section 3.3 below). It is also true
allocating r instead of R in a portion of the path is rather
the spirit of GS. As mentioned above, this mapping may however
useful in practice in the case where pricing in the ATM network
to different incentives in the tradeoff between delay and
than those of the user who buys IP integrated services
Another point of view on parameter mapping suggests that SCR
merely reflect the traffic description, hence SCR = r_r, while
service requirement is expressed in the QoS parameter as CDV = b/R
Thus the ATM network may internally allocate bandwidth R, but it
free to use other methods as well to achieve the delay constraint
Mechanisms such as statistical flow/connection aggregation may
implemented in the ATM network and hidden from the user (or
mapping module in the edge router) which sees only the
implemented in the signalled parameters
Note that this discussion considers an edge device buffer size
b_r. In practice, it may be necessary for the AAL/
module to buffer M bytes in converting packets to cells. Also
additional amount of buffer equal to C_sum + R D_sum is
necessary to absorb jitter imposed by the upstream network [8].
With ATM, it is possible to have little or no buffer in the
router, because the ATM VC can be set to accept bursts at peak rate
This may be unusual, since the edge router normally has enough
to absorb bursts according to the TSpec token bucket parameters.
consider two cases. First, if PCR >= p_r, then MBS can be set to b_
and no buffering is necessary to absorb non-excessive bursts.
extra buffering needed to absorb jitter can also be transferred
MBS. This effectively moves the buffering across the UNI into
ATM network
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For completeness, we consider an edge router with no burst-
buffers and an MBS parameter of approximately zero. In this case
is sufficient to set the rate parameters to PCR = SCR = max (R, p_r).
This amounts to peak-rate allocation of bandwidth, which will
usually be very cost effective. This case may be relevant where
IP routers and ATM switches differ substantially in their
designs. IP-level users may typically specify much larger
parameters than can be handled in the ATM subnet. Peak-
bandwidth allocation provides a means to work around this problem
It is also true that intermediate tradeoffs can be formulated,
the burst-absorbing buffer is less than b bytes, and SCR is set
R and below p_r. Note that jitter-absorbing buffers (C_sum +
D_sum) can not be avoided, generally, by increasing ATM rates,
SCR is set to exceed the physical line rate(s) into the edge
for the flow
For GS over CBR, the value of PCR may be mapped to the RSpec rate R
if the edge device has a buffer of size b_r + C_sum + R D_sum.
little or no burst buffering, the requirements resemble the zero
buffer case above, and we have PCR = max (R, p_r).
buffers sufficient to absorb network jitter, given by C_sum, D_sum
MUST always be provided in the edge router, or in the ATM network
MBS
2.5.2 Translating Traffic Descriptors for Controlled Load
The Controlled Load service TSpec has a peak rate, p, a "
bucket" rate, r, and a corresponding token bucket depth parameter, b
The receiver TSpec values are used to determine resource allocation
and a simple mapping for the nrtVBR service category is given by
PCR = p_
SCR = r_
MBS = b_r
The discussions in the preceding section on using edge device
to reduce PCR and/or MBS apply generally to the CLS over nrtVBR
as well. Extra buffers to accommodate jitter accumulated (beyond
b_r burst size allowed at the source) MUST be provided. For CLS
there are no Adspec parameters C and D, so the dimensioning of
buffers is an implementation design issue
For ABR VCs, the TSpec rate r_r is used to set the minimum cell
(MCR) parameter. Since there is no corresponding signalled
depth parameter, the edge device SHOULD have a buffer of at least b_
bytes, plus additional buffers to absorb jitter. With ABR, the
network can quickly throttle the actual transfer rate down to MCR,
a source transmitting above that rate can experience high loss at
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ingress edge device when the ATM network becomes congested
For CBR, the TSpec rate r_r sets a lower bound on PCR, and again,
available buffering in the edge device SHOULD be adequate
accommodate possible bursts of b_r
The REQUIREMENT for CLS that network delays approximate "best-
service under unloaded conditions", is interpreted here to mean
it would be sufficient to allocate bandwidth resources so that
last byte of a burst of size b_r sees a delay approximately b_r/r_r
For example, a network element with no cross-traffic, a
conserving scheduler and an output link rate of r_L, might
delays in the range from M/r_L to b_r/r_L, that are much lower
b_r/r_r. While the access to the full link bandwidth (r_L),
reflected in this example, is a more literal interpretation of
"under unloaded conditions" for a shared link, an ATM VC may
have access to bandwidth equal to its nominal allocation (
implementation specific function of SCR and PCR).
2.5.3 Translating Traffic Descriptors for Best Effort
For Best Effort service, there is no traffic description. The
service category allows negotiation of PCR simply to allow the
to discover the smallest physical bottleneck along the path.
ingress edge router may set PCR to the ATM line rate, and then
the VC setup is complete, the returned value indicates an upper
on throughput. For UBR service, resources may be allocated for
overall service (i.e., not per-VC) using the (
specific) admission control features of the ATM switches
Often a service provider will statically configure large VCs with
certain bandwidth allocation to handle all best effort
between two edge routers. ABR, CBR or nrtVBR VCs are appropriate
this design, with traffic parameters set to comfortably
the expected traffic load. See IETF ION specifications for IP
ATM signalling [10, 11].
2.6 QoS Classes and
In UNI 3.x the quality of service is indicated by a single
called "QoS Class," which is essentially an index to a
specific table of values for the actual QoS parameters. In TM/
4.0 three QoS parameters may be individually signalled, and
signalled values override those implied by the QoS Class, which
still present. These parameters are the Cell Loss Ratio (CLR),
Transfer Delay (CTD), and Cell Delay Variation (CDV) [6].
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A network provider may choose to associate other parameters, such
Severely Errored Cell Block Ratio, with a QoS Class definition,
these cannot be signalled individually. The ATM Forum UNI 3.0, 3.1
and TM 4.0 specs, following prior ITU specs, give vague
definitions for QoS Classes 1 to 4. (QoS Class 0 is well-defined
"no QoS parameters defined".) Since our mapping is based on
specifications, we generally follow this guidance by setting the
Class value to 0 for UBR and ABR (as REQUIRED), 1 for CBR and
and 3 for nrtVBR. Note that the QoS Class follows the ATM
category, and not the IP service, to avoid combination that
unlikely to be supported. For example, if only nrtVBR is
for GS, then choosing QoS Class = 1 would probably result
connection failure. The QoS Class MUST NOT be interpreted as a
to add real-time behavior to an inherently non-real-time service
The ITU has included a standard set of parameter values for a (small
number of QoS Classes in the latest version of Recommendation I.356
[21]. Network providers may choose to define further network
specific QoS Classes in addition to these. Note that the QoS
definitions in the new I.356 version might not align with the
we follow from the ATM Forum UNI specs. Apart from
definitions, there is no consistent agreement on QoS
definitions among providers in practice
The ATM QoS parameters have no explicitly signalled IP
counterparts. The values that are signalled in the ATM network
determined by the IP service definitions and knowledge of
underlying ATM network characteristics, as explained below
The ingress edge router SHOULD keep a table of QoS information
the set of egress routers that it may establish VCs with. This
may be simplified by using default values, but it will probably
good practice to maintain a table of current data for the
popular egress points. An edge device that initiates VC
generally needs to have some way to propose initial value for CDV
CTD, even if they are changed by negotiation; so by positing such
table, we are not creating any new design burden. Cached
can be updated when VCs are successfully established, and to
extent that IP-layer reservations can wait for VCs to complete,
values can be refined through iterated negotiation
Both GS and CLS REQUIRE that losses of packets due to congestion
minimized, so that the loss rate is approximately the same as for
unloaded network. The characteristic loss behavior of the
medium not due to congestion (e.g., bit errors or fading on
channels) determines the order of the permitted packet loss rate
The ingress edge device MUST choose a value of CLR that provides
appropriate IP-level packet loss rate. The CLR value may be
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over all egress points in the ATM network, or may differ, e.g.,
wireless or satellite ATM links are in some paths. The
of CLR MUST account for the effects of packet size distribution
ATM Frame Discard mode (which can change the effective packet
rate by orders of magnitude [22]).
The ingress router will also tabulate values for the Minimum
Latency (MPL) and estimated queueing delays (D_ATM) for each
point. The latter will be used as part of the Adspec "D"
for GS, but its use here applies to CLS as well (when the VC
includes delay parameters). MPL represents all constant (non
congestion related) delays, including propagation delay. D_
accounts for the variable component of delays in the ATM network
(It may depend on non-signalled parameters such as CDVT.)
these quantities, a new VC can be set up with delay-related
parameters given
CDV = D_
CTD = D_ATM + MPL
(CDV and CTD may be adjusted (increased) by the slack term in GS,
Section 3.3 below.)
It is interesting (and perhaps unfortunate) to note that in a
GS/rtVBR service, the delay bound advertised can contain
components of b/R instead of one. Consider the simple case where
= R is the rate allocated to the flow in both IP routers and
switches along the path, and the buffer allocation is MBS = b
Parekh's theory [23], which is the basis of the GS delay formula [8]
states that the b/R delay term occurs only once, because once a
of size b has been served by a congested node at rate R, the
will not arrive at a subsequent node as a single burst. However,
can't tell a priori if this bottleneck will occur in the ATM
or elsewhere in the IP network, so the declaration of CDV
account for it (i.e., CDV >= b/R). Once CDV is set, the ATM
can impose this delay, whether or not the traffic arrives in a burst
Since the delay b/R can also occur elsewhere, it cannot be
from the first term of the GS delay formula. The ATM b/R
component appears in the third term of the GS delay formula, D_tot
See Section 3.3 below for more on GS Adspec parameters. This
may be mitigated when the ATM network employs more
statistical resource allocation schemes
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2.7 Additional Parameters -- Frame Discard
TM/UNI 4.0 allows the user to choose a mode where the ATM network
aware, for the purpose of congestion management, of PDUs larger
an ATM cell (i.e., AAL PDUs that correspond in our context to
packets). This facilitates implementation of algorithms such
partial packet discard, where a dropped cell causes subsequent
in the same AAL-5 PDU to be dropped as well. Several
applicable buffer management schemes have been proposed [22, 24].
Frame discard can improve the efficiency and performance of end-to
end protocols such as TCP, since the remaining cells of a damaged
are generally useless to the receiver. For IP over ATM,
Discard MUST always be indicated, if available
3.0 Additional IP-Integrated Services Protocol
3.1 Path Characterization Parameters for IP Integrated Services with
This section discusses the setting of General
Parameters (GCPs) at an ATM egress edge router. GCPs are
from IP source to IP destination, and modified by intermediate
using the Adspec portion of PATH messages in rsvp. The GS-
Adspec parameters are discussed below in Section 3.3.
parameters are denoted as where x is the service and y is
parameter number. Service number 1 indicates default or
parameter values. Please refer to [25] for definitions and details
The IS break bit <1,2> MUST, of course, be left alone
implementations following these guidelines (as they are
IS-aware). Similarly, the router MUST always increment IS_
<1,4>. The GS and CLS service-specific break bits, <2,2> and <5,2>
respectively, MUST be set if the support of the service
inadequate. In general GS is adequately supported by CBR (BCOB-A
and rtVBR service categories, and not adequately supported by UBR
ABR and nrtVBR because delays are not controlled. CLS may
adequately supported by all service categories except UBR (or
Effort in UNI 3.x). See Sections 5, 6 for further discussion
For GS, the ATM network MUST meet the delay performance
through the Adspec parameters, MPL, C, and D. If it
predictably meet these requirements, the GS break bit MUST be set
Similarly both break bits MUST be set if reservations are honored
but sufficient resources to avoid congestion loss are not
in practice. If the service break bits are not set, then
corresponding service hop counters, <2,4>, <5,4>, MUST
incremented
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The Available Path Bandwidth (APB) parameters indicate
minimum physical bottleneck rate along the path. This may
discoverable in an ATM network as the negotiated PCR value for
UBR VC along the same path. This value MUST be corrected for AAL
ATM and physical-layer headers, as necessary, to reflect
effective IP datagram bandwidth. With ATM, it is possible that
is some policy limitation on the value of PCR, below the
link bottleneck. In this case, the advertised value of APB (
general, and for each service if the values of APB signalled
service specific) MUST reflect this limit, since excess
beyond this rate will be dropped. (Note that there is no tagging
traffic in excess of PCR for TM/UNI 4.0.) These values
generally be cached by the ingress router for the set of
routers with which it typically needs to establish VCs. The
parameters are only adjusted down, to reflect the minimum as
composed value
In the case of a multipoint VC, several parameters can be
for each egress point, e.g., because the characteristics of
physical links of the VC branches differ. When this occurs, the
at the egress routers MUST correct these values in PATH messages
they exit the ATM network. (We use the word "correct" because
ingress router SHOULD set the parameters to a value that
appropriate for the largest number of branches, or a value that
do the least harm if the egress routers failed to correct
parameters for each branch.) This is the only case where the
router needs to operate on rsvp control messages. (A
correction MUST be implemented for any non-rsvp set-up mechanism).
The parameters for which such correction is REQUIRED are
Available Path Bandwidth (APB), the Minimum Path Latency (MPL),
Path MTU (although for ATM/AAL-5 this may typically be constant),
the ATM-specific components of the GS Adspec parameters C_ATM
D_ATM
The ingress router table SHOULD store values for the ATM-network
for the various egress points. The composed values
formed by addition and forwarded along the path. In the cases
ATM routing chooses different paths, depending on the
category, for VCs to a given egress point, the table will
reflect different values for each service. If the ATM network
very large and complex, it may become difficult to predict the
that VCs will take once they are set up. This could be a
source of misconfiguration, resulting in discrepancies between
delay advertisements and actual results. The RSpec Slack term may
useful in mitigating this problem
AAL-5 will support any message size up to 65,535 bytes, so
the AAL SDU to the receiver TSpec M parameter value (plus 8 bytes
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the LLC/SNAP header) will generally not be an issue. In the
Adspec, however, the PATH_MTU parameter for each
SHOULD be set to 9188 bytes, which is the default MTU for AAL-5 [19].
3.2 Handling of Excess
For IP Integrated Services, network elements will transport
in excess of the TSpec parameters whenever physical
(bandwidth, buffers and processing) are available. (In CLS
"network element MUST attempt to forward the excess traffic on
best-effort basis" under certain conditions; and in GS a
policers "SHOULD relegate non-conforming datagrams to best effort".)
While excess traffic SHOULD be supported on a best effort basis,
MUST NOT interfere with the QoS (delay and loss) of conforming
and GS traffic, nor with normal service of non-reserved best
traffic
There are several solutions with ATM: the most attractive is to use
VBR service category (with an appropriate conformance definition)
tag excess traffic as low priority using the CLP bit. This
reordering of the flow, but necessitates careful design of the
router scheduler. To avoid reordering, the excess traffic can
queued with conforming traffic. A threshold SHOULD be used to
that conforming traffic is not unnecessarily delayed by the excess
Also, for GS, the extra delay that would be incurred due to
traffic in the queue ahead of conforming packets would have to
accurately reflected in the delay advertisement. Note that
ingress router SHOULD tag all cells of each non-conforming packet
rather than letting the ATM network apply tagging due to ATM-
non-conformance
There is no requirement in ATM standards that tagged cells,
either by the user or by policing, be transported if possible
Therefore, the operator of an edge router supporting IP-IS
ascertain the actual behavior of the ATM equipment in the path,
may span multiple administrative domains in the ATM network.
tagged cells are simply dropped at some point, regardless of load
then the operator may consider setting the break bit, at least
CLS service
The other solutions generally involve a separate VC to carry
excess. A distinct VC can be set up for each VC supporting a GS
CLS flow, or, if many flows are aggregated into a single QoS VC,
another VC can handle the excess traffic for that set of flows. A
can be set up to handle all excess traffic from the ingress router
the egress point. Since the QoS of the excess traffic is
particularly constrained, the design is quite flexible. However
using a separate VC may cause misordering of packets within a flow
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The service category for the excess-traffic VC may typically be
or ABR, although one could use CBR or nrtVBR if the excess
were predictable enough to know what rate to allocate. (
wouldn't normally be expected for excess traffic, though.)
Whether a separate VC is used may be influenced by the design of
router scheduler. The CLS spec suggests two
implementations: one where excess traffic shares the Best
class scheduler allocation, but at lower priority than other
effort traffic. The other, where a separate allocation is made.
first would allow excess traffic to use the same VC as normal
effort traffic, and the second would suggest a separate VC
TM/UNI 4.0. does not support tagging of traffic in excess of PCR
Although UNI 3.x does have a separate PCR parameter for CLP=0
only, we do not recommend using this feature for reasons
interoperability with TM/UNI 4.0 equipment. This restricts CBR
to use solutions other than tagging. The value of PCR can be
higher than necessary for conformant traffic, in an effort to
excess traffic on the same VC. In some cases this may be a
solution, such as when there is little additional cost imposed for
high PCR. If PCR can be set as high as APB, then the excess
is fully accommodated
3.3 Use of Guaranteed Service Adspec Parameters and Slack
The Adspec is used by the Guaranteed Service to allow a receiver
calculate the worst-case delay associated with a GS flow.
quantities, C, D, and MPL, are accumulated (by simple addition
components corresponding to each network element) in the PATH
from source to receiver. The resulting delay values can be
for each unique receiver. The maximum delay is computed
delay <= b_r/R + C_TOT/R + D_TOT +
The Minimum Path Latency (MPL) includes propagation delay,
b_r/R accounts for bursts due to the source and C and D include
queueing, scheduling and serialization delays. (We neglect
effect of maximum packet size and peak rate here; see the
specification [8] for a more detailed equation.) The service
requested by the receiver, R, can