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IETF RFC 6601
Last modified on Monday, April 30th, 2012
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Internet Engineering Task Force (IETF) G. Ash, Ed.
Request for Comments: 6601 AT&T
Category: Experimental D. McDysan
ISSN: 2070-1721 Verizon
April 2012
Generic Connection Admission Control (GCAC) Algorithm Specification
for IP/MPLS Networks
 Abstract
This document presents a generic connection admission control (GCAC)
reference model and algorithm for IP-/MPLS-based networks. Service
provider (SP) IP/MPLS networks need an MPLS GCAC mechanism, as one
motivational example, to reject Voice over IP (VoIP) calls when
additional calls would adversely affect calls already in progress.
Without MPLS GCAC, connections on congested links will suffer
degraded quality. The MPLS GCAC algorithm can be optionally
implemented in vendor equipment and deployed by service providers.
MPLS GCAC interoperates between vendor equipment and across multiple
service provider domains. The MPLS GCAC algorithm uses available
standard mechanisms for MPLS-based networks, such as RSVP, Diffserv-
aware MPLS Traffic Engineering (DS-TE), Path Computation Element
(PCE), Next Steps in Signaling (NSIS), Diffserv, and OSPF. The MPLS
GCAC algorithm does not include aspects of CAC that might be
considered vendor proprietary implementations, such as detailed path
selection mechanisms. MPLS GCAC functions are implemented in a
distributed manner to deliver the objective Quality of Service (QoS)
for specified QoS constraints. The objective is that the source is
able to compute a source route with high likelihood that via-elements
along the selected path will in fact admit the request. In some
cases (e.g., multiple Autonomous Systems (ASes)), this objective
cannot always be met, but this document summarizes methods that
partially meet this objective. MPLS GCAC is applicable to any
service or flow that must meet an objective QoS (delay, jitter,
packet loss rate) for a specified quantity of traffic.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6601.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Table of Contents
1. Introduction ....................................................4
1.1. Conventions Used in This Document ..........................5
2. MPLS GCAC Reference Model and Algorithm Summary .................6
2.1. Inputs to MPLS GCAC ........................................8
2.2. MPLS GCAC Algorithm Summary ................................9
3. MPLS GCAC Algorithm ............................................12
3.1. Bandwidth Allocation Parameters ...........................12
3.2. GCAC Algorithm ............................................15
4. Security Considerations ........................................18
5. Acknowledgements ...............................................20
6. Normative References ...........................................20
7. Informative References .........................................21
Appendix A: Example MPLS GCAC Implementation Including Path
Selection, Bandwidth Management, QoS Signaling, and
Queuing ...............................................24
A.1 Example of Path Selection and Bandwidth Management
Implementation .............................................26
A.2 Example of QoS Signaling Implementation ....................32
A.3 Example of Queuing Implementation ..........................34
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
1. Introduction
This document presents a generic connection admission control (GCAC)
reference model and algorithm for IP-/MPLS-based networks. Service
provider (SP) IP/MPLS networks need an MPLS GCAC mechanism, as one
motivational example, to reject Voice over IP (VoIP) calls when
additional calls would adversely affect calls already in progress.
Without MPLS GCAC, connections on congested links will suffer
degraded quality. Given the capital constraints in some SP networks,
over-provisioning is not acceptable. MPLS GCAC supports all access
technologies, protocols, and services while meeting performance
objectives with a cost-effective solution and operates across routing
areas, autonomous systems, and service provider boundaries.
This document defines an MPLS GCAC reference model, algorithm, and
functions implemented in one or more types of network elements in
different domains that operate together in a distributed manner to
deliver the objective QoS for specified QoS constraints, such as
bandwidth. With MPLS GCAC, the source router/server is able to
compute a source route with high likelihood that via-elements along
the selected path will in fact admit the request. MPLS GCAC includes
nested CAC actions, such as RSVP aggregation, nested RSVP - Traffic
Engineering (RSVP-TE) for scaling between provider edge (PE) routers,
and pseudowire (PW) CAC within traffic-engineered tunnels. MPLS GCAC
focuses on MPLS and PW-level CAC functions, rather than application-
level CAC functions.
MPLS GCAC is applicable to any service or flow that must meet an
objective QoS (latency, delay variation, loss) for a specified
quantity of traffic. This would include, for example, most real-
time/RTP services (voice, video, etc.) as well as some non-real-time
services. Real-time/RTP services are typically interactive,
relatively persistent traffic flows. Other services subject to MPLS
GCAC could include, for example, manually provisioned label switched
paths (LSPs) or PWs and automatic bandwidth assignment for
applications that automatically build LSP meshes among PE routers.
MPLS GCAC is applicable to both access and backbone networks, for
example, to slow-speed access networks and to broadband DSL, cable,
and fiber access networks.
This document is Experimental. It is intended that service providers
and vendors experiment with the GCAC concept and the algorithm
described in this document in a controlled manner to determine the
benefits of such a mechanism. That is, they should first experiment
with the GCAC algorithm in their laboratories and test networks.
When testing in live networks, they should install the GCAC algorithm
on selected routers in only part of their network, and they should
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
carefully monitor the effects. The installation should be managed
such that the routers can quickly be switched back to normal
operation if any problem is seen.
Since application of GCAC is most likely in Enterprise VPNs and/or
internal TE infrastructure, it is RECOMMENDED that the experiment be
conducted in such applications, and it is NOT RECOMMENDED that the
experiment be conducted in the Internet. If possible, the
experimental configuration will address interoperability issues, such
as, for example, the use of different constraint models across
different traffic domains.
The experiment can monitor various measures of quality of service
before and after deployment of GCAC, particularly when the
experimental network is under stress during an overload or failure
condition. These quality-of-service measures might include, for
example, dropped packet rate and end-to-end packet delay. The
results of such experiments may be fed back to the IETF community to
refine this document and to move it to the Standards Track (probably
within the MPLS working group) if the experimental results are
positive.
It should be noted that the algorithm might have negative effects on
live deployments if the experiment is a failure. Effects might
include blockage of traffic that would normally be handled or
congestion caused by allowing excessive traffic on a link. For these
reasons, experimentation in production networks needs to be treated
with caution as described above and should only be carried out after
successful simulation and experimentation in test environments. In
Section 2, we describe the MPLS GCAC reference model, and in Section
3, we specify the MPLS GCAC algorithm based on the principles in the
reference model and requirements. Appendix A gives an example of
MPLS GCAC implementation including path selection, bandwidth
management, QoS signaling, and queuing implementation.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
2. MPLS GCAC Reference Model and Algorithm Summary
Figure 1 illustrates the reference model for the MPLS GCAC algorithm:
,-. ,-.
,--+ `--+--'- --'\
+----+_____+------+ { +----+ +----+ `. +------+
|GEF1| | |______| P |___| P |______| |
| |-----| PE1 | { +----+ +----+ /+| PE2 |
| | | |==========================>| ASBR |
+-:--+ | |<==========================| |
_|..__ +------+ { DS-TE/MAR Tunnels ; +------+
_,' \-| ./ -'._ !|
| Access \ / +----+ \, !|
| Network | \_ | P | | !|
| / `| +----+ / !|
`--. ,.__,| | IP/MPLS Network / !|
'`' '' ' .._,,'`.__ _/ '---' !|
| '`''' !|
C1 !|
,-. ,-. !|
,--+ `--+--'- --'\ !|
+----+_____+------+ { +----+ +----+ `. +------+
|GEF2| | |______| P |___| P |______| |
| |-----| PE4 | { +----+ +----+ /+| PE3 |
| | | |==========================>| ASBR |
+-:--+ | |<==========================| |
_|..__ +------+ { DS-TE/MAM Tunnels ; +------+
_,' \-| ./ -'._
| Access \ / +----+ \,
| Network | \_ | P | |
| / `| +----+ /
`--. ,.__,| | IP/MPLS Network /
'`' '' ' .._,,'`.__ _/ '---'
| '`'''
C2
CUSTOMER I/F PARAMETERS: BW, QoS, CoS, priority
NOTE: PE, P, ASBR, GEF elements all support GCFs
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
LEGEND:
---------
ASBR: Autonomous System Border Router
BW: bandwidth
CoS: class of service
DS-TE: Diffserv-aware MPLS Traffic Engineering
GCAC: generic connection admission control
GCF: GCAC core function
GEF: GCAC edge function
I/F: interface
MAM: Maximum Allocation Model
MAR: Maximum Allocation with Reservation
P: provider router
PE: provider edge router
--- connection signaling
___ bearer/media flows
Figure 1: MPLS GCAC Reference Model
MPLS GCAC is applicable to any service or flow for which MPLS GCAC is
required to meet a given QoS. As such, the reference model applies
to most real-time/RTP services (voice, video, etc.) as well as some
non-real-time services. Real-time/RTP services are typically
interactive, relatively persistent traffic flows. Non-real-time
applications subject to MPLS GCAC could include, for example,
manually provisioned LSPs or PWs and automatic bandwidth assignment
for applications that automatically build LSP meshes among PE
routers. The reference model also applies to MPLS GCAC when MPLS is
used in access networks, which include, for example, slow-speed
access networks and broadband DSL, cable, and fiber access networks.
Endpoints will be IP/PBXs (Private Branch Exchanges) and individual-
usage SIP/RTP end devices (hard and soft SIP phones, Integrated
Access Devices (IADs)). This traffic will enter and leave the core
via possibly bandwidth-constrained access networks, which may also be
MPLS aware but may use some other admission control technology.
The basic elements considered in the reference model are the MPLS
GCAC edge function (GEF), GCAC core functions (GCFs), the PE routers,
Autonomous System Border Routers (ASBRs), and provider (P) routers.
As illustrated in Figure 1, the GEF interfaces to the application at
the source and destination PE, and the GCF exists at the PE, P, and
ASBR routers. GEF has an end-to-end focus and deals with whether
individual connection requests fit within an MPLS tunnel, and GCF has
a hop-by-hop focus and deals with whether an MPLS tunnel can be
established across specific core network elements on a path. The GEF
functionality may be implemented in the PE, ASBR, or a stand-alone
network element. The source/destination routers (or external devices
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
through a router interface) support both GEF and GCF, while internal
routers (or external devices through a router interface) support GCF.
In Figure 1, the GEF handles both signaling and bearer control.
2.1. Inputs to MPLS GCAC
Inputs to the GEF and GCF include the following, where most are
inputs to both GEF and GCF, except as noted. Most of the parameters
apply to the specific flow/LSP being calculated, while some
parameters, such as request type, apply to the calculation method.
Required inputs are marked with (*); all other inputs are optional:
Traffic Description
* Bandwidth per DS-TE class type [RFC 4124] (GEF, GCF)
* Bandwidth for LSP from [RFC 3270] (GEF, GCF)
* Aggregated RSVP bandwidth requirements from [RFC 4804] (GEF)
Variance Factor (GEF, GCF)
Class of Service (CoS) and Quality of Service (QoS)
* Class Type (CT) from [RFC 4124] (GEF, GCF)
Signaled or configured Traffic Class (TC) [RFC 5462] to Per Hop
Behavior (PHB) mapping from [RFC 3270] (GEF, GCF)
Signaled or configured PHB from [RFC 3270] (GEF, GCF)
QoS requirements from NSIS/Y.1541 [RFC 5971][RFC 5974][RFC 5975]
[RFC 5976] (GEF)
Priority
Admission priority (high, normal, best effort) from NSIS/Y.1541
[RFC 5971][RFC 5974][RFC 5975][RFC 5976] (GEF, GCF)
Preemption priority from [RFC 4124] (GEF, GCF)
Request type
Primary tunnel (GEF, GCF)
Backup tunnel and fraction of capacity reserved for backup (GEF,
GCF)
Oversubscription method (see [RFC 3270])
Over/undersubscribe requested capacity (GEF, GCF)
Over/undersubscribe available bandwidth (GEF, GCF)
These inputs can be received by the GEF and GCF from a signaling
interface (such as SIP or H.323), RSVP, or an NMS. They can also be
derived from measured traffic levels or from elsewhere.
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2.2. MPLS GCAC Algorithm Summary
Figure 1 is a reference model for MPLS GCAC and illustrates the GEF
to GEF MPLS GCAC algorithm to determine whether there is sufficient
bandwidth to complete a connection. The originating GEF receives a
connection request including the above input parameters over the
input interface, for example, via an RSVP bandwidth request as
specified in [RFC 4804]. The GEF a) determines whether there is
enough bandwidth on the route between the originating and terminating
GEFs via routing and signaling communication with the GCFs at the P,
PE, and ASBR network elements along the path to accommodate the
connection, b) communicates the accept/reject decision on the input
interface for the connection request, and c) keeps account of network
resource allocations by tracking bandwidth use and allocations per
CoS. Optionally, the GEF may dynamically adjust the tunnel size by
signaling communication with the GCFs at nodes along the candidate
paths. For example, the GEF could a) maintain per-CoS tunnel
capacity based on aggregated connection requests and respond on a
connection-by-connection basis based on the available capacity, b)
periodically adjust the tunnel capacity upward, when needed, and
downward when spare capacity exists in the tunnel, and c) use a 'make
before break' mechanism to adjust tunnel capacity in order to
minimize disruption to the bearer traffic.
In the reference model, DS-TE [RFC 4124] tunnels are configured
between the GEFs based on the traffic forecast and current network
utilization. These guaranteed bandwidth DS-TE tunnels are created
using RSVP-TE [RFC 3209]. DS-TE bandwidth constraints models are
applied uniformly within each domain, such as the Maximum Allocation
with Reservation (MAR) Bandwidth Constraints Model [RFC 4126], the
Maximum Allocation Model (MAM) [RFC 4125], and the Russian Dolls Model
(RDM) [RFC 4127]. An IGP such as OSPF or IS-IS is used to advertise
bandwidth availability by CT for use by the GCF to determine MPLS
tunnel bandwidth allocation and admission on core (backbone) links.
These DS-TE tunnels are configured based on the forecasted traffic
load, and when needed, LSPs for different CTs can take different
paths.
As described in Section 3, the unreserved link bandwidth on CTc on
link k (ULBCck) is the only bandwidth allocation parameter that must
be available to the MPLS GCAC algorithm. In the case that a
connection is set up across multiple service provider networks, i.e.,
across multiple routing domains/autonomous systems (ASes), there are
several options to enable MPLS GCAC to be implemented:
1. Use [OIF-E-NNI] to advertise ULBCck parameters to the originating
GEF, for the full topology of adjacent domains/areas/ASes, as
described in Section 3.3.2.1.2 of [OIF-E-NNI]. Note that the
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
option of abstract node summarization described in [OIF-E-NNI]
will not suffice since the process of summarization results in
loss of topology and capacity usage information. In this manner,
the originating GEF can implement the MPLS GCAC algorithm
described in Section 3 across multiple domains/areas/ASes.
2. Use [BGP-TE] to advertise ULBCck parameters via BGP to the
originating GEF for the full topology of adjacent
domains/areas/ASes. In this manner, the originating GEF can
implement the MPLS GCAC algorithm described in Section 3 across
multiple domains/areas/ASes. However, network providers may be
reluctant to divulge full topology and capacity usage information
to other providers. Furthermore, [BGP-TE] was never intended to
provide full TE topology distribution across ASBRs. Such a
mechanism would be neither stable nor scalable.
3. Use individual AS control and MPLS crankback [RFC 4920] to retain
originating GEF control. For example, in Figure 1, if a
connection crosses the two ASes shown (call them AS1 and AS2),
the source GEF1 applies the GCAC algorithm described in Section 3
to the links in AS1, that is, between PE1 and PE2/ASBR in Figure
1. Then, in AS2, the GCF in PE3/ASBR applies the MPLS GCAC
algorithm to the links in AS2, that is, between PE3 and PE4 in
Figure 1. If the flow is rejected in AS2, crankback signaling is
used to inform GEF1. In routing a connection across multiple
ASes, e.g., across AS1-->AS2-->AS3, if the flow is rejected, say
in AS2, the originating GEF1 can seek an alternate route, perhaps
AS1-->AS4-->AS3. This option does not achieve full originating
GEF control with the desired full topology visibility across ASes
but avoids possible issues with obtaining full topology
visibility across ASes.
4. Use Path Computation Elements (PCEs) [RFC 4655] across multiple
ASes. PCEs could potentially execute the GCAC algorithm within
each AS and communicate/interwork across domains to determine
which high-level path can supply the requested bandwidth.
In the reference model, the GEFs implement RSVP aggregation [RFC 4804]
for scalability. The GEF RSVP aggregator keeps a running total of
bandwidth usage on the DS-TE tunnel, adding the bandwidth
requirements during connection setup and subtracting during
connection teardown. The aggregator determines whether or not there
is sufficient bandwidth for the connection from that originating GEF
to the destination GEF. The destination GEF also checks whether
there is enough bandwidth on the DS-TE tunnel from the destination
GEF to the originating GEF. The aggregate bandwidth usage on the DS-
TE tunnel is also available to the DS-TE bandwidth constraints model.
If the available bandwidth is insufficient, then the GEF sends a PATH
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
message through the tunnel to the other end, requesting bandwidth
using GCFs, and if successful, the source would then complete a new
explicit route with a PATH message along the path with increased
bandwidth, again invoking GCFs on the path. If the size of the DS-TE
tunnel cannot be increased on the primary and alternate LSPs, then
when the DS-TE tunnel bandwidth is exhausted, the GEF aggregator
sends a message to the endpoint denying the reservation. If the DS-
TE tunnels are underutilized, the tunnel bandwidth may be reduced
periodically to an appropriate level. In the case of a basic single
class TE scenario, there is a single TE tunnel rather than multiple-
CT DS-TE tunnels; otherwise, the above GCAC functions remain the
same.
Optionally, the reference model implements separate queues with
Diffserv based on Traffic Class (TC) bits [RFC 5462]. For example,
these queues may include two Expedited Forwarding (EF) priority
queues, with the highest priority assigned to Emergency
Telecommunications Service (ETS) traffic and the second priority
assigned to normal-priority real-time traffic (alternatively, there
could be a single EF queue with dual policers [RFC 5865]). Several
Assured Forwarding (AF) queues may be used for various data traffic,
for example, premium private data traffic and premium public data
traffic. A separate best-effort queue may be used for the best-
effort traffic. Several DS-TE tunnels may share the same physical
link and therefore share the same queue.
The MPLS GCAC algorithm increases the likelihood that the route
selected by the GEF will succeed, even when the LSP traverses
multiple service provider networks.
Path computation is not part of the GCAC algorithm; rather, it is
considered as a vendor proprietary function, although standard
IP/MPLS functions may be included in path computation, such as the
following:
a) Path Computation Element (PCE) [RFC 4655][RFC 4657][RFC 5440] to
implement inter-area/inter-AS/inter-SP path selection algorithms,
including alternate path selection, path reoptimization, backup
path computation to protect DS-TE tunnels, and inter-area/inter-
AS/inter-SP traffic engineering.
b) Backward-Recursive PCE-Based Computation (BRPC) [RFC 5441].
c) Per-Domain Path Computation [RFC 5152].
d) MPLS fast reroute [RFC 4090] to protect DS-TE LSPs against
failure.
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e) MPLS crankback [RFC 4920] to trigger alternate path selection and
enable explicit source routing.
3. MPLS GCAC Algorithm
MPLS GCAC is performed at the GEF during the connection setup attempt
phase to determine if a connection request can be accepted without
violating existing connections' QoS and throughput requirements. To
enable routing to produce paths that will likely be accepted, it is
necessary for nodes to advertise some information about their
internal CAC states. Such advertisements should not require nodes to
expose detailed and up-to-date CAC information, which may result in
an unacceptably high rate of routing updates. MPLS GCAC advertises
CAC information that is generic (e.g., independent of the actual path
selection algorithms used) and rich enough to support any CAC.
MPLS GCAC defines a set of parameters to be advertised and a common
admission interpretation of these parameters. This common
interpretation is in the form of an MPLS GCAC algorithm to be
performed during MPLS LSP path selection to determine if a link or
node can be included for consideration. The algorithm uses the
advertised MPLS GCAC parameters (available from the topology
database) and the characteristics of the connection being requested
(available from QoS signaling) to determine if a link/node will
likely accept or reject the connection. A link/node is included if
the MPLS GCAC algorithm determines that it will likely accept the
connection and excluded otherwise.
3.1. Bandwidth Allocation Parameters
MPLS GCAC bandwidth allocation parameters for each DS-TE CT are as
defined within DS-TE [RFC 4126], OSPF-TE extensions [RFC 4203], and IS-
IS-TE extensions [RFC 5307]. The following parameters are available
from DS-TE/TE extensions, advertised by the IGP, and available to the
GEF and GCF [RFC 4124]. Note that the approach presented in this
section is adapted from [PNNI], Appendix B.
MRBk Maximum reservable bandwidth on link k specifies the maximum
bandwidth that may be reserved; this may be greater than the
maximum link bandwidth, in which case the link may be
oversubscribed.
BWCck Bandwidth constraint for CTc on link k = allocated (minimum
guaranteed) bandwidth for CTc on link k.
ULBCck Unreserved link bandwidth on CTc on link k specifies the
amount of bandwidth not yet reserved for CTc.
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Note that BWCck and ULBCck are the only DS-TE parameters flooded by
the IGP [RFC 4124][RFC 4203][RFC 5307]. For example, when bandwidth
reservation is used [RFC 4126], ULBCck is calculated and flooded by
the IGP as follows:
RBTk Reservation bandwidth threshold for link k.
ULBCck Unreserved link bandwidth on CTc on link k specifies the
amount of bandwidth not yet reserved for CTc, taking RBTk
into account,
ULBCck = ULBk - delta0/1(CTck) * RBTk
where
delta0/1(CTck) = 0 if RBWck < BWCck
delta0/1(CTck) = 1 if RBWck >= BWCck
Also derivable at the GEF and GCF is MRBCck, the maximum reservable
link bandwidth for CTc. For example, when bandwidth reservation is
used [RFC 4126], MRBCck is calculated as follows:
MRBCck Maximum reservable link bandwidth for CTc on link k specifies
the amount of bandwidth not yet reserved for CTc.
MRBCck = MRBk - delta0/1(CTck) * RBTk
where
delta0/1(CTck) = 0 if RBWck < BWCck
delta0/1(CTck) = 1 if RBWck >= BWCck
Note that these bandwidth parameters must be configured in a
consistent way within domains and across domains. GEF routing of
LSPs is based on ULBCck, where ULBk is available and RBTk can be
accounted for by configuration, e.g., RBTk typically = .05 * MRBk.
Also available are administrative weight (denoted as "link cost" in
[RFC 2328]), TE metric [RFC 3630], and administrative group (also
called color) 4-octet mask [RFC 3630].
The following quantities can be derived from information advertised
by the IGP and otherwise available to the GEF and GCF:
RBWck Reserved bandwidth on CTc on link k (0 <= c <= MaxCT-1).
RBWck = total amount of bandwidth reserved by all established
LSPs that belong to CTc
RBWck = BWCck - ULBCck.
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ULBk Unreserved link bandwidth on link k specifies the amount of
bandwidth not yet reserved for any CT.
ULBk = MRBk - sum [RBWck (0 <= c <= MaxCT-1)].
The GCAC algorithm assumes that a DS-TE bandwidth constraints model
is used uniformly within each domain (e.g., MAR [RFC 4126], MAM
[RFC 4125], or RDM [RFC 4127]). European Advanced Networking Test
Center (EANTC) testing [EANTC] has shown that interoperability is
problematic when different DS-TE bandwidth constraints models are
used by different network elements within a domain. Specific testing
of MAM and RDM across different vendor equipment showed the
incompatibility. However, while the characteristics of the 3 DS-TE
bandwidth constraints models are quite different, it is necessary to
specify interworking between them even though it could be complex.
The following parameters are also defined and available to GCF and
are assumed to be locally configured to be a consistent value across
all nodes and domain(s):
SBWck Sustained bandwidth for CTc on link k (aggregate of existing
connections).
SBWck = factor * RBWck where factor is configured based on
standard 'demand overbooking' factors.
VFck Variance factor for CTc on link k; VFck is BWMck normalized
by variance of SBWck. VFck is configured based on typical
traffic variability statistics.
In many implementations of the Private Network-Network Interface
(PNNI) GCAC algorithm, the variance factor is not included, or
equivalently, VFck is assumed to be zero. A simplified MPLS GCAC
algorithm is also derived assuming VFck = 0.
Note that different demand overbooking factors can be specified for
each CT, e.g., no overbooking might be used for constant bitrate
services, while a large overbooking factor might be used for bursty
variable bitrate services. We specify demand overbooking rather than
link overbooking for the GCAC algorithm; one advantage is the demand
overbooking is compatible with source routing used by the GCAC
algorithm.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Also defined is
BWMck bandwidth margin for CTc on link k; BWMck = RBWck - SBWck
GEF uses BWCck, RBWck, ULBCck, SBWck, BWMck, and VFck for LSP/IGP
routing. GEF also needs to track per-CT LSP bandwidth allocation and
reserved bandwidth parameters, which are defined as follows:
RBWLcl reserved bandwidth for CTc on LSP l
UBWLcl unreserved bandwidth for CTc on LSP l
3.2. GCAC Algorithm
The assumption behind the MPLS GCAC is that the ratio between the
bandwidth margin that the node is putting above the sustained
bandwidth and the standard deviation of the sustained bandwidth does
not change significantly as one new aggregate flow is added on the
link. Any ingress node doing path selection can then compute the new
standard deviation of the aggregate rate (from the old value and the
aggregate flow's traffic descriptors) and an estimate of the new
BWMck. From this, the increase in bandwidth required to carry the
new aggregate flow can be computed and compared to BWCck.
To expand on the discussion above, let RBWck denote the reserved
bandwidth capacity, i.e., the amount of bandwidth that has been
allocated to existing aggregate flows for CTc on link k by the actual
CAC used in the node. BWMck is the difference between RBWck and the
aggregate sustained bandwidth (SBWck) of the existing aggregate
flows. SBWck can be either the sum of existing aggregate flows'
declared sustainable bandwidth (SBWi for aggregate flow i) or a
smaller (possibly measured or estimated) value. Let MRBCck denote
the maximum reservable bandwidth that is usable by aggregate flows
for CTc on link k. The following diagram illustrates the
relationship among MRBCck, RBWck, BWMck, SBWck, and ULBCck:
|<-- BWMck-->|<----- ULBCck ----->|
|---------------|------------|--------------------|
0 SBWck RBWck MRBCck
The assumption is that BWMck is proportional to some measure of the
burstiness of the traffic generated by the existing aggregate flows,
this measure being the standard deviation of the aggregate traffic
rate defined as the square root of the sum of SBWi(PBWi - SBWi) over
all existing aggregate flows, where SBWi and PBWi are declared
sustainable and peak bandwidth for aggregate flow i, respectively.
This assumption is based on the simple argument that RBWck needs to
be some multiple of the standard deviation above the mean aggregate
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traffic rate to guarantee some level of packet loss ratio and packet
queuing time. Depending on the actual CAC used, the BWMck-to-
standard-deviation ratio may vary as aggregate flows are established
and taken down. It is reasonable to assume, however, that with a
sufficiently large value of RBWck, this ratio will not vary
significantly. What this means is a link can advertise its current
BWMck-to-standard-deviation ratio (actually in the form of VF, which
is the square of this number), and the MPLS GCAC algorithm can use
this number to estimate how much bandwidth is required to carry an
additional aggregate flow.
Following the derivation given in [PNNI], Appendix B, the MPLS GCAC
algorithm is derived as follows. Consider an aggregate flow
bandwidth change request DBWi with peak bandwidth PBWi and
sustainable bandwidth SBWi and a link with the following MPLS GCAC
parameters: ULBCck, BWMck, and VFck for CTc and link k. Denote the
variance (i.e., square of standard deviation) of the aggregate
traffic rate by VARk (not advertised). Denote other unadvertised
MPLS GCAC quantities by RBWck and SBWck. Then,
VARk = SUM SBWi*(PBWi-SBWi) (1)
over existing
aggregate flows i
and
BWMck**2
VFck = ---------- (2)
VARk
Using the above equation, VARk can be computed from the advertised
VFck and BWMck as:
VARk = (BWMck**2)/VFck.
Let DBWi be the additional bandwidth capacity needed to carry the
flow within aggregate sustainable bandwidth SBWi. The MPLS GCAC
algorithm basically computes DBWi from the advertised MPLS GCAC
parameters and the new aggregate flow's traffic descriptors, and
compares it with ULBCck. If ULBCck >= DBWi, then the link is
included for path selection consideration; otherwise, it is excluded,
i.e.,
If (ULBCck >= DBWi), then include link k; else exclude link k (3)
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Let BWMcknew denote the bandwidth margin if the new aggregate flow
were accepted. Denote other 'new' quantities by RBWcknew, SBWcknew,
and VARnew. Then,
DBWi = BWMcknew - BWMck + SBWi (4)
since BWMcknew = RBWcknew - SBWcknew, BWMck = RBWck - SBWck, and
SBWcknew - SBWck = SBWi. Substituting (4) into (3), rearranging
terms, and squaring both sides yield:
If ((ULBCck+BWMck-SBWck)**2 >= BWMcknew**2), then include link k;
else exclude link k (5)
Using the MPLS GCAC assumption made earlier, BWMcknew**2 can be
computed as:
BWMcknew**2 = VFck * VARnew, (6)
Where
VARnew = VARk + SBWck * (PBWi-SBWi). (7)
Substituting (2), (6) and (7) into (5) yields:
If ((ULBCck+BWMck-SBWi)**2 >= BWMck**2 + VFck*SBWi(PBWi-SBWi)),
then include link k;
else exclude link k (8)
and moving BWMck**2 to the left-hand side and rearranging terms yield
If ((ULBCck-SBWi) * (ULBCck-SBWi+2*BWMck) >= VFck*SBWi(PBWi-SBWi),
then include link k;
else exclude link k (9)
Equation (9) represents the Constrained Shortest Path First (CSPF)
method implemented by most vendors and deployed by most service
providers in MPLS networks. In general, DBWi is between SBWi and
PBWi. So, the above test is not necessary for the cases ULBCck >=
PBWi and ULBCck < SBWi. In the former case, the link is included; in
the latter case, the link is excluded.
Exclude Include
|<--- link ---->|<-- Test (9)-->|<--- link ----->|
|---------------|---------------|----------------| ULBCck
SBWi PBWi
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Note that VF and BWM are frequently not implemented; equivalently,
these quantities are assumed to be zero, in which case Equation (9)
becomes
If (ULBCck >= SBWi), then include link k; else exclude link k (10)
Exclude Include
|<--- link ---->|<--- link ----->|
|---------------|----------------| ULBCck
SBWi PBWi
PNNI GCAC implementations often do not incorporate the variance
factor VF, in which case Equation (10) is used.
MPLS GCAC must not reject a best-effort (BE, unassigned bandwidth)
aggregate flow request based on bandwidth availability, but it may
reject based on other reasons such as the number of BE flows
exceeding a chosen threshold. MPLS GCAC defines only one parameter
for the BE service category -- maximum bandwidth (MBW) -- to
advertise how much capacity is usable for BE flows. The purpose of
advertising this parameter is twofold: MBW can be used for path
optimization, and MBW = 0 is used to indicate that a link is not
accepting any (additional) BE flows.
Demand overbooking of LSP bandwidth is employed and must be compliant
with [RFC 4124] and [RFC 3270] to over-/undersubscribe requested
capacity. It is simplest to use only one oversubscription method,
i.e., the GCAC algorithm assumes oversubscription of demands per CT,
both within domains and for interworking between domains. The
motivation is that interworking may be infeasible between domains if
different overbooking models are used. Note that the same assumption
was made for DS-TE bandwidth constraints models, in that the GCAC
algorithm assumes a consistent DS-TE bandwidth constraints model is
used within each domain and interoperability of bandwidth constraints
models across domains.
4. Security Considerations
It needs to be clearly understood that all routers contain local and
implementation-specific rules (or algorithms) to help them determine
what to do with traffic that exceeds capacity and how to admit new
flows. If these rules are poorly designed or implemented with
defects, then problems may be observed in the network. Furthermore,
the implementation of such algorithms provides a mechanism for
attacking the delivery of traffic within the network. In view of
this, routers and their software are usually extensively tested
before deployment, router vendors are extended a degree of trust, and
a "compromised router" (i.e., one on which an attacker has installed
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
their own code) is considered a weak spot in the system. Note that
if a router is compromised, it can be made to do substantially more
problematic things than simply modifying the admission control
algorithm. Implementers are RECOMMENDED to ensure that software
modifications to routers are fully secured, and operators are
RECOMMENDED to apply security measures (that are outside the scope of
this document) to prevent unauthorized updates to router software.
Nothing in this document suggests any change to normal software
security practices.
The use of a GCAC priority parameter raises possibilities for theft-
of-service attacks because users could claim an emergency priority
for their flows without real need, thereby effectively preventing
serious emergency calls to get through. Several options exist for
countering such user attacks at the interface to the user, for
example:
- Only some user groups (e.g., police) are authorized to set the
emergency priority bit using a policy applied to RSVP-TE
signaling.
- Any user is authorized to employ the emergency priority bit for
particular destination addresses (e.g., police) using a policy
applied to RSVP-TE signaling.
- If an attack occurs, the user/group and actions taken should be
logged to trace the attack.
- [RFC 5069] identifies a number of security threats against
emergency call marking and mapping. Section 6 of [RFC 5069] lists
security requirements to counter these threats, and those
requirements should be followed by implementations of this
document.
- The security requirements listed in Section 11 of [RFC 4412] should
be followed. These requirements apply to use of the
Communications Resource Priority Header for the Session Initiation
Protocol (SIP) and concern aspects of authentication and
authorization, confidentiality and privacy requirements,
protection against denial-of-service attacks, and anonymity.
Within the network, the policy and integrity mechanisms already
present in RSVP-TE [RFC 3209] can be used to ensure that the MPLS LSP
has the right policy and security credentials to assume the signaled
priority and bandwidth. Further discussion of this topic for the
signaling of priority levels using RSVP can be found in [RFC 6401].
Some similarities may also be drawn to the security issues
Ash & McDysan Experimental PAGE 19
RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
surrounding the placement of emergency calls in Internet multimedia
systems [RFC 5069] although the concepts are only comparable at the
highest levels.
Like any algorithm, the algorithm specified in this document operates
on data that is supplied as input parameters. That data is assumed
to be collected and stored locally (i.e., on the router performing
the algorithm). It is a fundamental assumption of the secure
operation of any router that the data stored on that router cannot be
externally modified. In this particular case, it is important that
the input parameters to the algorithm cannot be influenced by an
outside party. Thus, as with all configuration parameters on a
router, the implementer MUST supply and the operator is RECOMMENDED
to use security mechanisms to protect writing of the configuration
parameters for this algorithm.
5. Acknowledgements
The authors greatly appreciate Adrian Farrel's support in serving as
the sponsoring Area Director for this document and for his valuable
comments and suggestions on the document. The authors also greatly
appreciate Young Lee serving as the document shepherd and his
valuable comments and suggestions. Finally, Robert Sparks' thorough
review and helpful suggestions are sincerely appreciated.
6. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC 3031,
January 2001.
[RFC 3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC 3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
"Multi-Protocol Label Switching (MPLS) Support of
Differentiated Services", RFC 3270, May 2002.
[RFC 3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003.
Ash & McDysan Experimental PAGE 20
RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
[RFC 4124] Le Faucheur, F., Ed., "Protocol Extensions for Support
of Diffserv-aware MPLS Traffic Engineering", RFC 4124,
June 2005.
[RFC 4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, October 2005.
[RFC 4804] Le Faucheur, F., Ed., "Aggregation of Resource
ReSerVation Protocol (RSVP) Reservations over MPLS
TE/DS-TE Tunnels", RFC 4804, February 2007.
[RFC 4920] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,
N., and G. Ash, "Crankback Signaling Extensions for MPLS
and GMPLS RSVP-TE", RFC 4920, July 2007.
[RFC 5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 5307, October 2008.
7. Informative References
[BGP-TE] Gredler, H., Farrel, A., Medved, J., and S. Previdi,
"North-Bound Distribution of Link-State and TE
Information using BGP", Work in Progress, March 2012.
[EANTC] "Multi-vendor Carrier Ethernet Interoperability Event",
Carrier Ethernet World Congress 2006, Madrid Spain,
September 2006.
[FEEDBACK] Ashwood-Smith, P., Jamoussi, B., Fedyk, D., and D.
Skalecki, "Improving Topology Data Base Accuracy with
Label Switched Path Feedback in Constraint Based Label
Distribution Protocol", Work in Progress, June 2003.
[OIF-E-NNI] Optical Interworking Forum (OIF), "External Network-
Network Interface (E-NNI) OSPFv2-based Routing - 2.0
(Intra-Carrier) Implementation Agreement", IA # OIF-
ENNI-OSPF-02.0, July 13, 2011.
[PNNI] ATM Forum Technical Committee, "Private Network-Network
Interface Specification Version 1.1 (PNNI 1.1)",
af-pnni-0055.002, April 2002.
[RFC 2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
Ash & McDysan Experimental PAGE 21
RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
[RFC 3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le
Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RFC 4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC
4090, May 2005.
[RFC 4125] Le Faucheur, F. and W. Lai, "Maximum Allocation
Bandwidth Constraints Model for Diffserv-aware MPLS
Traffic Engineering", RFC 4125, June 2005.
[RFC 4126] Ash, J., "Max Allocation with Reservation Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering & Performance Comparisons", RFC 4126, June
2005.
[RFC 4127] Le Faucheur, F., Ed., "Russian Dolls Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4127, June 2005.
[RFC 4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)", RFC
4412, February 2006.
[RFC 4655] Farrel, A., Vasseur, JP., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC 4657] Ash, J., Ed., and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC 5069] Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069, January
2008.
[RFC 5152] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
Per-Domain Path Computation Method for Establishing
Inter-Domain Traffic Engineering (TE) Label Switched
Paths (LSPs)", RFC 5152, February 2008.
[RFC 5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
Computation Element (PCE) Communication Protocol
(PCEP)", RFC 5440, March 2009.
Ash & McDysan Experimental PAGE 22
RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
[RFC 5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
Roux, "A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441,
April 2009.
[RFC 5462] Andersson, L. and R. Asati, "Multiprotocol Label
Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
to "Traffic Class" Field", RFC 5462, February 2009.
[RFC 5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted
Traffic", RFC 5865, May 2010.
[RFC 5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[RFC 5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling", RFC 5974, October 2010.
[RFC 5975] Ash, G., Ed., Bader, A., Ed., Kappler, C., Ed., and D.
Oran, Ed., "QSPEC Template for the Quality-of-Service
NSIS Signaling Layer Protocol (NSLP)", RFC 5975, October
2010.
[RFC 5976] Ash, G., Morton, A., Dolly, M., Tarapore, P., Dvorak,
C., and Y. El Mghazli, "Y.1541-QOSM: Model for Networks
Using Y.1541 Quality-of-Service Classes", RFC 5976,
October 2010.
[RFC 6401] Le Faucheur, F., Polk, J., and K. Carlberg, "RSVP
Extensions for Admission Priority", RFC 6401, October
2011.
[TQO] Ash, G., "Traffic Engineering and QoS Optimization of
Integrated Voice and Data Networks", Elsevier, 2006.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Appendix A: Example MPLS GCAC Implementation Including Path Selection,
Bandwidth Management, QoS Signaling, and Queuing
Figure 2 illustrates an example of the integrated voice/data MPLS
GCAC method in which bandwidth is allocated on an aggregated basis to
the individual DS-TE CTs. In the example method, CTs have different
priorities including high-priority, normal-priority, and best-effort-
priority services CTs. Bandwidth allocated to each CT is protected
by bandwidth reservation methods, as needed, but bandwidth is
otherwise shared among CTs. Each originating GEF monitors CT
bandwidth use on each MPLS LSP [RFC 3031] for each CT, and determines
when CT LSP bandwidth needs to be increased or decreased. In Figure
2, changes in CT bandwidth capacity are determined by GEFs based on
an overall aggregated bandwidth demand for CT capacity (not on a per-
connection/per-flow demand basis). Based on the aggregated bandwidth
demand, GEFs make periodic discrete changes in bandwidth allocation,
that is, they either increase or decrease bandwidth on the LSPs
constituting the CT bandwidth capacity. For example, if aggregate
flow requests are made for CT LSP bandwidth that exceeds the current
DS-TE tunnel bandwidth allocation, the GEF initiates a bandwidth
modification request on the appropriate LSP(s). This may entail
increasing the current LSP bandwidth allocation by a discrete
increment of bandwidth denoted here as DBW, where DBW is the
additional amount needed by the current aggregate flow request. The
bandwidth admission control for each link in the path is performed by
the GCF based on the status of the link using the bandwidth
allocation procedure described below, where we further describe the
role of the different parameters (such as the reserved bandwidth
threshold RBT shown in Figure 2) in the admission control procedure.
Also, the GEF periodically monitors LSP bandwidth use, and if
bandwidth use falls below the current LSP allocation, the GEF
initiates a bandwidth modification request to decrease the LSP
bandwidth allocation to the current level of bandwidth utilization.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
HIGH-PRIORITY-CT LSP
+----+======================+----+======================+----+
|GEF1|NORMAL-PRIORITY-CT LSP| VN | |GEF2|
| |======================| |======================| |
| |LOW-PRIORITY/BE-CT LSP| | | |
+----+======================+----+======================+----+
LEGEND
------
BE - Best Effort
CT - Class Type
GEF - GCAC Edge Function
LSP - Label Switched Path
VN - Via Node
o Distributed bandwidth allocation method applied on a
per-class-type (CT) LSP basis
o GEF allocates bandwidth to each CTc LSP based on demand
- GEF decides CTc LSP bandwidth increase based on
+ aggregate flow sustained bandwidth (SBWi) and variance factor
VFck
+ routing priority (high, normal, best effort)
+ CTc reserved bandwidth (RBWck) and bandwidth constraint
(BWCck)
+ link reserved bandwidth threshold (RBTk) and unreserved
bandwidth (ULBk)
- GEF periodically decreases CTc LSP bandwidth allocation based on
bandwidth use
o VNs send crankback messages to GEF if DS-TE/MAR bandwidth
allocation rules not met
o Link(s) not meeting request excluded from TE topology database
before attempting another explicit route computation
Figure 2: Per-Class-Type (CT) LSP Bandwidth Management
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GEF uses SBWi, VFck, RBWck, BWCck, RBTk, and ULBk for LSP bandwidth
allocation decisions and IGP routing and uses RBWcl and UBWcl to
track per-CT LSP bandwidth allocation and reserved bandwidth. In
making a CT bandwidth allocation modification, the GEF determines the
CT priority (high, normal, or best effort), CT bandwidth-in-use, and
CT bandwidth allocation thresholds. These parameters are used to
determine whether network capacity can be allocated for the CT
bandwidth modification request.
A.1. Example of Path Selection and Bandwidth Management Implementation
In OSPF, link-state flooding is used to make status updates. This is
a state-dependent routing (SDR) method where CSPF is typically used
to alter LSP routing according to the state of the network. In
general, SDR methods calculate a path cost for each connection
request based on various factors such as the load state or congestion
state of the links in the network. In contrast, the example MPLS
GCAC algorithm uses event-dependent routing (EDR), where LSP routing
is updated locally on the basis of whether connections succeed or
fail on a given path choice. In the EDR learning approaches, the
path that was last tried successfully is tried again until congested,
at which time another path is selected at random and tried on the
next connection request. EDR path choices can also be changed with
time in accordance with changes in traffic load patterns. Success-
to-the-top (STT) EDR path selection, illustrated in Figure 3, uses a
simplified decentralized learning method to achieve flexible adaptive
routing. The primary path (path-p) is used first if available, and a
currently successful alternate path (path-s) is used until it is
congested. In the case that path-s is congested (e.g., bandwidth is
not available on one or more links), a new alternate path (path-n) is
selected at random as the alternate path choice for the next
connection request overflow from the primary path. Bandwidth
reservation is used under congestion conditions to protect traffic on
the primary path. STT-EDR uses crankback when an alternate path is
congested at a via node, and the connection request advances to a new
random path choice. In STT-EDR, many path choices can be tried by a
given connection request before the request is rejected.
Figure 3 illustrates the example MPLS GCAC operation of STT-EDR path
selection and admission control combined with per-CT bandwidth
allocation. GEF A monitors CT bandwidth use on each CT LSP and
determines when CT LSP bandwidth needs to be increased or decreased.
Based on the bandwidth demand, GEF A makes periodic discrete changes
in bandwidth allocation, that is, either increases or decreases
bandwidth on the LSPs constituting the CT bandwidth capacity. If
aggregate flow requests are made for CT LSP bandwidth that exceeds
the current LSP bandwidth allocation, GEF A initiates a bandwidth
modification request on the appropriate LSP(s).
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|<----- ULBk <= RBTk ---->|
LSP-p |------------------------|-------------------------|
A B E
|<-- ULBk <= RBTk -->|
LSP-s |---------------|--------------------|-------------|
A C D E
Example of STT-EDR routing method:
1. If node A to node E bandwidth needs to be modified (say
increased by DBW), primary LSP-p (e.g., LSP A-B-E) is tried
first.
2. Available bandwidth is tested locally on each link in LSP-p. If
bandwidth not available (e.g., unreserved bandwidth on link BE
is less than the reserved bandwidth threshold and this CT is
above its bandwidth allocation), crankback to node A.
3. If DBW is not available on one or more links of LSP-p, then the
currently successful LSP-s (e.g., LSP A-C-D-E) is tried next.
4. If DBW is not available on one or more links of LSP-s, then a
new LSP is searched by trying additional candidate paths until a
new successful LSP-n is found or the candidate paths are
exhausted.
5. LSP-n is then marked as the currently successful path for the
next time bandwidth needs to be modified.
Figure 3: STT-EDR Path Selection and Per-CT Bandwidth Allocation
For example, in Figure 3, if the LSR-A to LSR-E bandwidth needs to be
modified, say increased by DBW, the primary LSP-p (A-B-E) is tried
first. The bandwidth admission control for each link in the path is
performed based on the status of the link using the bandwidth
allocation procedure described below, where we further describe the
role of the reserved bandwidth RBWck shown in Figure 3 in the
admission control procedure. If the first choice LSP cannot admit
the bandwidth change, node A may then try an alternate LSP. If DBW
is not available on one or more links of LSP-p, then the currently
successful LSP-s A-C-D-E (the 'STT path') is tried next. If DBW is
not available on one or more links of LSP-s, then a new LSP is
searched by trying additional candidate paths (not shown) until a new
successful LSP-n is found or all of the candidate paths held in the
cache are exhausted. LSP-n is then marked as the currently
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
successful path for the next time bandwidth needs to be modified.
DBW is set to the additional amount of bandwidth required by the
aggregate flow request.
If all cached candidate paths are tried without success, the search
then generates a new CSPF path. If a new CSPF calculation succeeds
in finding a new path, that path is made the stored path, and the
bottom cached path falls off the list. If all cached paths fail and
a new CSPF path cannot be found, then the original stored LSP is
retained. New requests go through the same routing algorithm again,
since available bandwidth, etc., has changed and new requests might
be admitted. Also, GEF A periodically monitors LSP bandwidth use
(e.g., once each 2-minute interval), and if bandwidth use falls below
the current LSP allocation, the GEF initiates a bandwidth
modification request to decrease the LSP bandwidth allocation to the
currently used bandwidth level. Bandwidth reservation occurs in STT-
EDR with PATH/RESV messages per application of [RFC 4804].
In the STT-EDR computation, most of the time the primary path and
stored path will succeed, and no CSPF calculation needs to be done.
Therefore, the STT-EDR algorithm achieves good throughput performance
while significantly reducing link-state flooding control load [TQO].
An analogous method was proposed in the MPLS working group
[FEEDBACK], where feedback based on failed path routing attempts is
kept by the TE database and used when running CSPF.
In the example GCAC method, bandwidth allocation to the primary and
alternate LSPs uses the MAR bandwidth allocation procedure, as
described below. Path selection uses a topology database that
includes available bandwidth on each link. From the topology
database pruned of links that do not meet the bandwidth constraint,
the GEF determines a list of shortest paths by using a shortest path
algorithm (e.g., Bellman-Ford or Dijkstra methods). This path list
is determined based on administrative weights of each link, which are
communicated to all nodes within the routing domain (e.g.,
administrative weight = 1 + e x distance, where e is a factor giving
a relatively smaller weight to the distance in comparison to the hop
count). Analysis and simulation studies of a large national network
model show that 6 or more primary and alternate cached paths provide
the best overall performance.
PCE [RFC 4655][RFC 4657][RFC 5440] is used to implement
inter-area/inter-AS/ inter-SP path selection algorithms, including
alternate path selection, path reoptimization, backup path
computation to protect DS-TE tunnels, and inter-area/inter-AS/inter-
SP traffic engineering. The DS-TE tunnels are protected against
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failure by using MPLS Fast Reroute [RFC 4090]. OSPF TE extensions
[RFC 4203] are used to support the TE database (TED) required for
implementation of the above PCE path selection methods.
The example MPLS GCAC method incorporates the MAR bandwidth
constraint model [RFC 4126] incorporated within DS-TE [RFC 4124]. In
DS-TE/MAR, a small amount of reserved bandwidth RBTk governs the
admission control on link k. Associated with each CTc on link k are
the allocated bandwidth constraints BWCck to govern bandwidth
allocation and protection. The reservation bandwidth on a link,
RBTk, can be accessed when a given CTc has reserved bandwidth RBWck
below its allocated bandwidth constraint BWCck. However, if RBWck
exceeds its allocated bandwidth constraint BWCck, then the
reservation bandwidth threshold RBTk cannot be accessed. In this
way, bandwidth can be fully shared among CTs if available but is
otherwise protected by bandwidth reservation methods. Therefore,
bandwidth can be accessed for a bandwidth request = DBW for CTc on a
given link k based on the following rules:
For an LSP on a high-priority or normal-priority CTc:
If RBWck = BWCc, admit if DBW = ULBk
If RBWck > BWCc, admit if DBW = ULBk - RBTk;
or, equivalently:
If DBW = ULBCck, admit the LSP.
where
ULBCck = idle link bandwidth on link k for CTc = ULBk -
delta0/1(CTck) x RBWk
delta0/1(CTck) = 0 if RBWck < BWCck
delta0/1(CTck) = 1 if RBWck = BWCck
For an LSP on a best-effort-priority CTc:
allocated bandwidth BWCc = 0;
Diffserv queuing serves best-effort packets only if there is
available link bandwidth.
In setting the bandwidth constraints for CTck, for a normal-priority
CTc, the bandwidth constraints (BWCck) on link k are set by
allocating the maximum reservable link bandwidth (MRBk) in proportion
to the forecast or measured traffic load bandwidth TRAF_LOAD_BWck for
CTc on link k. That is:
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
PROPORTIONAL_ BWck =
TRAF_LOAD_ BWck/[S (c) {TRAF_LOAD_ BWck, c=0, MaxCT-1}] x MRBk
For a normal-priority CTc:
BWCck = PROPORTIONAL_ BWck
For a high-priority CT, the bandwidth constraint BWCck is set to a
multiple of the proportional bandwidth. That is:
For high-priority CTc:
BWCck = FACTOR * PROPORTIONAL_ BWck
where FACTOR is set to a multiple of the proportional bandwidth
(e.g., FACTOR = 2 or 3 is typical). This results in some over-
allocation ('overbooking') of the link bandwidth and gives priority
to the high-priority CTs. Normally, the bandwidth allocated to high-
priority CTs should be a relatively small fraction of the total link
bandwidth, a maximum of 10-15 percent being a reasonable guideline.
As stated above, the bandwidth allocated to a best-effort-priority
CTc is set to zero. That is:
For a best-effort-priority CTc:
BWCck = 0
Analysis and simulation studies show that the level of reserved
capacity RBTk in the range of 3-5% of link capacity provides the best
overall performance.
We give a simple example of the MAR bandwidth allocation method.
Assume that there are two class types, CT0 and CT1, and a particular
link with
MRB = 100
with the allocated bandwidth constraints set as follows:
BWC0 = 30
BWC1 = 50
These bandwidth constraints are based on the forecasted traffic
loads, as discussed above. Either CT is allowed to exceed its
bandwidth constraint BWCc as long a there is at least RBW units of
spare bandwidth remaining. Assume RBT = 10. So under overload, if
RBW0 = 20
RBW1 = 70
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Then, for this loading
UBW = 100 - 20 - 70 = 10
If a bandwidth increase request = 5 = DBW arrives for Class Type 0
(CT0), then accept for CT0 since RBW0 < BWC0 and DBW (= 5) < ILBW (=
10).
If a bandwidth increase request = 5 = DBW arrives for Class Type 1
(CT1), then reject for CT1 since RBW1 > BC1 and DBW (= 5) > ILBW -
RBT = 10 - 10 = 0.
Therefore, CT0 can take the additional bandwidth (up to 10 units) if
the demand arrives, since it is below its BWC value. CT1, however,
can no longer increase its bandwidth on the link, since it is above
its BWC value and there is only RBT=10 units of idle bandwidth left
on the link. If best effort traffic is present, it can always seize
whatever idle bandwidth is available on the link at the moment but is
subject to being lost at the queues in favor of the higher-priority
traffic.
On the other hand, if a request arrives to increase bandwidth for CT1
by 5 units of bandwidth (i.e., DBW = 5), we need to decide whether or
not to admit this request. Since for CT1,
RBW1 > BWC1 (70 > 50), and
DBW > UBW - RBT (i.e., 5 > 10 - 10)
the bandwidth request is rejected by the bandwidth allocation rules
given above. Now let's say a request arrives to increase bandwidth
for CT0 by 5 units of bandwidth (i.e., DBW = 5). We need to decide
whether or not to admit this request. Since for CT0
RBW0 < BWC0 (20 < 30), and
DBW < UBW (i.e., 5 < 10)
The example illustrates that with the current state of the link and
the current CT loading, CT1 can no longer increase its bandwidth on
the link, since it is above its BWC1 value and there is only RBW=10
units of spare bandwidth left on the link. But CT0 can take the
additional bandwidth (up to 10 units) if the demand arrives, since it
is below its BWC0 value.
For the example GCAC, the method for bandwidth additions and
deletions to LSPs in is as follows. The bandwidth constraint
parameters defined in the MAR method [RFC 4126] do not change based on
traffic conditions. In particular, these parameters defined in
[RFC 4126], as described above, are configured and do not change until
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
reconfigured: MRBk, BWCck, and RBTk. However, the reserved bandwidth
variables change based on traffic: RBWck, ULBk, and ULBCck. The
RBWck and bandwidth allocated to each DS-TE/MAR tunnel is dynamically
changed based on traffic: it is increased when the traffic demand
increases (using RSVP aggregation), and it is periodically decreased
when the traffic demand decreases. Furthermore, if tunnel bandwidth
cannot be increased on the primary path, an alternate LSP path is
tried. When LSP tunnel bandwidth needs to be increased to
accommodate a given aggregate flow request, the bandwidth is
increased by the amount of the needed additional bandwidth, if
possible. The tunnel bandwidth quickly rises to the currently needed
maximum bandwidth level, wherein no further requests are made to
increase bandwidth, since departing flows leave a constant amount of
available or spare bandwidth in the tunnel to use for new requests.
Tunnel bandwidth is reduced every 120 seconds by 0.5 times the
difference between the allocated tunnel bandwidth and the current
level of the actually utilized bandwidth (i.e., the current level of
spare bandwidth). Analysis and simulation modeling results show that
these parameters provide the best performance across a number of
overload and failure scenarios.
A.2. Example of QoS Signaling Implementation
The example GCAC method uses Next Steps in Signaling (NSIS)
algorithms for signaling MPLS GCAC QoS requirements of individual
flows. NSIS QoS signaling has been specified by the IETF NSIS
working group and extends RSVP signaling by defining a two-layer QoS
signaling model:
o NSIS Transport Layer Protocol (NTLP) [RFC 5971]
o NSIS Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling [RFC 5974]
[RFC 5975] defines a QoS specification (QSPEC) object, which contains
the QoS parameters required by a QoS model (QOSM) [RFC 5976]. A QOSM
specifies the QoS parameters and procedures that govern the resource
management functions in a QoS-aware router. Multiple QOSMs can be
supported by the QoS-NSLP, and the QoS-NSLP allows stacking of QSPEC
parameters to accommodate different QOSMs being used in different
domains. As such, NSIS provides a mechanism for interdomain QoS
signaling and interworking.
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
The QSPEC parameters defined in [RFC 5975] include, among others:
TRAFFIC DESCRIPTION Parameters:
o <Traffic Model> Parameters
CONSTRAINTS Parameters:
o <Path Latency>, <Path Jitter>, <Path PLR>, and <Path PER>
Parameters
o <PHB Class> Parameter
o <DSTE Class Type> Parameter
o <Y.1541 QoS Class> Parameter
o <Reservation Priority> Parameter
o <Preemption Priority> and <Defending Priority> Parameters
The ability to achieve end-to-end QoS through multiple Internet
domains is also an important requirement. MPLS GCAC end-to-end QoS
signaling ensures that end-to-end QoS is met by applying the
Y.1541-QOSM [RFC 5976], as now illustrated.
The QoS GEF initiates an end-to-end, inter-domain QoS RESERVE message
containing the QoS parameters, including for example, <Traffic
Model>, <Y.1541 QoS Class>, <Reservation Priority>, and perhaps other
parameters for the flow. The RESERVE message may cross multiple
domains; each node on the data path checks the availability of
resources and accumulating the delay, delay variation, and loss ratio
parameters, as described below. If an intermediate node cannot
accommodate the new request, the reservation is denied. If no
intermediate node has denied the reservation, the RESERVE message is
forwarded to the next domain. If any node cannot meet the
requirements designated by the RESERVE message to support a QoS
parameter, for example, it cannot support the accumulation of end-to-
end delay with the <Path Latency> parameter, the node sets a flag
that will deny the reservation. Also, parameter negotiation can be
done, for example, by setting the <Y.1541 QoS Class> to a lower class
than specified in the RESERVE message. When the available <Y.1541
QoS Class> must be reduced from the desired <Y.1541 QoS Class>, say
because the delay objective has been exceeded, then there is an
incentive to respond to the GEF with an available value for delay in
the <Path Latency> parameter. For example, if the available <Path
Latency> is 150 ms (still useful for many applications) and the
desired QoS is 100 ms (according to the desired <Y.1541 QoS Class>
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RFC 6601 GCAC Algorithm for IP/MPLS Networks April 2012
Class 0), then the response would be that Class 0 cannot be achieved
and Class 1 is available (with its 400 ms objective). In addition,
the response includes an available <Path Latency> = 150 ms, making
acceptance of the available <Y.1541 QoS Class> more likely.
A.3. Example of Queuing Implementation
In this MPLS GCAC example, queuing behaviors for the CT traffic
priorities incorporates Diffserv mechanisms and assumes separate
queues based on Traffic Class (TC)/CoS bits. The queuing
implementation assumes 3 levels of priority: high, normal, and best
effort. These queues include two EF priority queues
[RFC 3246][RFC 5865], with the highest priority assigned to emergency
traffic (GETS/ETS/E911) and the second priority assigned to normal-
priority real-time (e.g., VoIP) traffic. Separate AF queues
[RFC 2597] are used for data services, such as premium private data
and premium public data traffic, and a separate best-effort queue is
assumed for the best-effort traffic. All queues have static
bandwidth allocation limits applied based on the level of forecast
traffic on each link, such that the bandwidth limits will not be
exceeded under normal conditions, allowing for some traffic overload.
In the MPLS GCAC method, high-priority, normal-priority, and best-
effort traffic share the same network; under congestion, the Diffserv
priority-queuing mechanisms push out the best-effort-priority traffic
at the queues so that the normal-priority and high-priority traffic
can get through on the MPLS-allocated LSP bandwidth.
Authors' Addresses
Gerald Ash (editor)
AT&T
EMail: gash5107@yahoo.com
Dave McDysan
Verizon
22001 Loudoun County Pkwy
Ashburn, VA 20147
EMail: dave.mcdysan@verizon.com
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RFC TOTAL SIZE: 82005 bytes
PUBLICATION DATE: Monday, April 30th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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