Internet DRAFT - draft-ash-gcac-algorithm-spec
draft-ash-gcac-algorithm-spec
Network Working Group G. Ash
Internet Draft AT&T
Intended status: Experimental D. McDysan
<draft-ash-gcac-algorithm-spec-04.txt> Verizon
Expires: August, 2012 February 24, 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 Internet Protocol
(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, DSTE, PCE, 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 QoS for specified QoS constraints.
The objective is that the source is able to compute a source route
with high likelihood that MPLS GCAC via elements along the selected
path will in fact admit the request. In some cases (e.g., multiple
AS) this objective cannot always be met, but the 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.
Status of this Memo
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This Internet-Draft will expire on August 24, 2012.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. MPLS GCAC Reference Model & Algorithm Summary . . . . . . . . 4
2.1 Inputs to MPLS GCAC . . . . . . . . . . . . . . . . . . . 6
2.2 MPLS GCAC Algorithm Summary . . . . . . . . . . . . . . . 6
3. MPLS GCAC Algorithm . . . . . . . . . . . . . . . . . . . . . 9
3.1 Bandwidth Allocation Parameters . . . . . . . . . . . . . 10
3.2 GCAC Algorithm . . . . . . . . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
7. Normative References . . . . . . . . . . . . . . . . . . . . . 17
8. Informative References . . . . . . . . . . . . . . . . . . . . 17
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
Appendix A: Example MPLS GCAC Implementation Including Path
Selection, Bandwidth Management, QoS Signaling, &
Queuing . . . . . . . . . . . . . . . . . . . . . . . 19
A.1 Example of Path Selection & Bandwidth Management
Implementation . . . . . . . . . . . . . . . . . . . . . . 20
A.2 Example of QoS Signaling Implementation . . . . . . . . . 26
A.3 Example of Queuing Implementation . . . . . . . . . . . . 27
1. Introduction
This document presents a generic connection admission control (GCAC)
reference model and algorithm for IP/MPLS-based networks. Service
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provider (SP) IP/MPLS networks need an MPLS GCAC mechanism, as one
motivational example, to reject voice over Internet Protocol (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 MPLS GCAC via
elements along the selected path will in fact admit the request.
MPLS GCAC includes nested CAC actions, such as RSVP aggregation,
nested 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, 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
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 infrastucture, 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
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experimental configuration will address interoperability issues, such
as, for example, 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 blocking 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.
2. MPLS GCAC Reference Model & Algorithm Summary
Figure 1 illustrates the reference model for the MPLS GCAC algorithm:
,-. ,-.
,--+ `--+--'- --'\
+----+_____+------+ { +----+ +----+ `. +------+
|GEF1| | |______| P |___| P |______| |
| |-----| PE1 | { +----+ +----+ /+| PE2 |
| | | |==========================>| ASBR |
+-:--+ | |<==========================| |
_|..__ +------+ { DSTE/MAR Tunnels ; +------+
_,' \-| ./ -'._ !|
| Access \ / +----+ \, !|
| Network | \_ | P | | !|
| / `| +----+ / !|
`--. ,.__,| | IP/MPLS Network / !|
'`' '' ' .._,,'`.__ _/ '---' !|
| '`''' !|
C1 !|
,-. ,-. !|
,--+ `--+--'- --'\ !|
+----+_____+------+ { +----+ +----+ `. +------+
|GEF2| | |______| P |___| P |______| |
| |-----| PE4 | { +----+ +----+ /+| PE3 |
Ash, McDysan <draft-ash-gcac-algorithm-spec-04.txt> [Page 4]
Internet Draft GCAC Algorithm Specification February 2012
| | | |==========================>| ASBR |
+-:--+ | |<==========================| |
_|..__ +------+ { DSTE/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 GCF functions
LEGEND:
---------
ASBR: autonomous system border router
BW: bandwidth
COS: class of service
DSTE: 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 and individual-usage SIP/RTP end devices
(hard and soft SIP phones, IADs), and 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.
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The basic elements considered in the reference model are the MPLS
GCAC edge function (GEF), GCAC core functions (GCF), the PE routers,
autonomous system border routers (ASBR), and provider (P) routers.
As illustrated in Figure 1, the GEF interfaces to the application at
the source and destination PE, and the GCF exist 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 in a stand-alone
network element. The source/destination routers (or external devices
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 DSTE class type [RFC4124] (GEF, GCF)
* Bandwidth for LSP from [RFC3270] (GEF, GCF)
* Aggregated RSVP bandwidth requirements from [RFC4804] (GEF)
Variance Factor (GEF, GCF)
Service Class/CoS & QoS
* Class Type (CT) from [RFC4124] (GEF, GCF)
Signaled or configured TC-PHB mapping from [RFC3270] (GEF, GCF)
Signaled or configured PHB from [RFC3270] (GEF, GCF)
QoS requirements from NSIS/Y.1541 [RFC5971, RFC5974, RFC5975,
RFC5976] (GEF)
Priority
Admission priority (high, normal, best effort) from NSIS/Y.1541
[RFC5971, RFC5974, RFC5975, RFC5976] (GEF, GCF)
Preemption priority from [RFC4124] (GEF, GCF)
Request type
Primary tunnel (GEF, GCF)
Backup tunnel and fraction of capacity reserved for backup (GEF,
GCF)
Oversubscription method (see [RFC3270])
Over/under-subscribe requested capacity (GEF, GCF)
Over/under-subscribe 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, from an NMS, be derived from
measured traffic levels, or from elsewhere.
2.2 MPLS GCAC Algorithm Summary
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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 [RFC4804]. 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 GCF functions
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 GCF functions 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, DSTE [RFC4124] tunnels are configured
between the GEFs based on the traffic forecast and current network
utilization. These guaranteed bandwidth DSTE tunnels are created
using RSVP-TE [RFC3209]. DSTE bandwidth constraints models are
applied uniformly within each domain, such as the maximum allocation
with reservation bandwidth constraints model (MAR) [RFC4126],
maximum allocation bandwidth constraints model (MAM) [RFC4125], and
the Russian dolls bandwidth constraints model (RDM) [RFC4127]. An
IGP such as OSPF or ISIS 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 DSTE
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 (AS's),
there are several options to enable MPLS GCAC to be implemented in
this case:
1. Use [OIF E-NNI] to advertise ULBCck parameters to the originating
GEF, for the full topology of adjacent domains/areas/AS's, as
described in Section 3.3.2.1.2 of [OIF E-NNI]. Note that the 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
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Section 3 across multiple domains/areas/AS's.
2. Use [BGP-TE] to advertise ULBCck parameters via BGP to the
originating GEF for the full topology of adjacent domains/areas/AS's.
In this manner the originating GEF can implement the MPLS GCAC
algorithm described in Section 3 across multiple domains/areas/AS's.
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 [RFC4920] to retain
originating GEF control. For example, in Figure 1 if a connection
crosses the two AS's 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 function 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 AS's, e.g., across
AS1-->AS2-->AS3, if the flow is rejected, say in AS2, the originating
GEF1 can seek an alternate route perhaps through, say,
AS1-->AS4-->AS3. This option does not achieve full originating GEF
control with the desired full topology visibility across AS's, but
avoids possible issues with obtaining full topology visibility across
AS's.
4. Use path computation elements (PCEs) [RFC4655] across multiple
ASs. 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 [RFC4804]
for scalability. The GEF RSVP aggregator keeps a running total of
bandwidth usage on the DSTE 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 DSTE tunnel from the destination GEF to the
originating GEF. The aggregate bandwidth usage on the DSTE tunnel is
also available to the DSTE bandwidth constraints model. If the
available bandwidth is insufficient, then the GEF sends a PATH
message through the tunnel to the other end, requesting bandwidth
using GCF functions, and if successful the source would then complete
a new explicit route with a PATH message along the path with
increased bandwidth, again invoking GCF functions on the path. If
the size of the DSTE tunnel cannot be increased on the primary and
alternate LSPs, then when the DSTE tunnel bandwidth is exhausted, the
GEF aggregator sends a message to the endpoint denying the
reservation. If the DSTE tunnels are underutilized, the tunnel
bandwidth may be reduced periodically to an appropriate level.
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In the case of a basic single class TE scenario, there is a single TE
tunnel rather than multiple-CT DSTE tunnels, otherwise the above GCAC
functions remain the same.
Optionally, the reference model implements separate queues with
DiffServ based on TC bits. 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 [RFC5865]). 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
DSTE 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) [RFC4655, RFC4657, RFC5440] to
implement inter-area/inter-AS/inter-SP path selection algorithms,
including alternate path selection, path reoptimization, backup path
computation to protect DSTE tunnels, and
inter-area/inter-AS/inter-SP traffic engineering.
b) Backward recursive path computation (BRPC) [RFC5441].
c) Per domain path computation (PDPC) [RFC5152].
d) MPLS fast reroute (FRR) [RFC4090] to protect DSTE LSPs against
failure.
e) MPLS crankback [RFC4920] 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.
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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 DSTE CT are as
defined within DSTE [RFC4126], OSPF-TE extensions [RFC4203], and
ISIS-TE extensions [RFC5307]. The following parameters are
available from DSTE/TE extensions, advertised by the IGP, and
available to the GEF and GCF [RFC4124]. Note that the approach
presented in this section is adapted from PNNI Appendix B [PNNI].
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
Note that BWCck and ULBCck are the only DSTE parameters flooded by
the IGP [RFC4124, RFC4203, RFC5307]. For example, when bandwidth
reservation is used [RFC4126], 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 account
of RBTk,
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 [RFC4126], MRBCck is calculated as follows:
MRBCck maximum reservable link bandwidth for CTc on link k specifies
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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 [RFC2328], TE metric
[RFC3630], and administrative group (also called color) 4-octet mask
[RFC3630].
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.
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 DSTE bandwidth constraints model is
used uniformly within each domain (e.g., MAR [RFC4126],
MAM [RFC4125], or RDM [RFC4127]). EANTC testing [EANTC] has shown
that interoperability is problematic when different DSTE 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 DSTE 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 PNNI GCAC algorithm, the variance
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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 bit-rate
services, while a large overbooking factor might be used for bursty
variable bit-rate 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.
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 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 CT c 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
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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
traffic rate to guarantee some levels 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 around some 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 CT c 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)
Let BWMcknew denote the bandwidth margin if the new aggregate flow
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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
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)
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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 number of BE flows exceeding a
chosen threshold. MPLS GCAC defines only one parameter for 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 [RFC4124] and [RFC3270] to over/under-subscribe 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
use different overbooking models are used. Note that the same
assumption was made for DSTE bandwidth constraints models, in that
the GCAC algorithm assumes a consistent DSTE 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
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.
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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.
Within the network the policy and integrity mechanisms already
present in RSVP-TE [RFC3209] 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 [RFC6401].
Some similarities may also be drawn to the security issues
surrounding the placement of emergency calls in Internet multimedia
systems [RFC5069] 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. IANA Considerations
There are no IANA Considerations.
6. Acknowledgements
The authors greatly appreciate Adrian Farrel's support in serving as
the sponsoring Area Director for this draft and for his valuable
comments and suggestions on the document. The authors also greatly
appreciate Young Lee's serving as the document shepherd and for his
valuable comments and suggestions.
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7. Normative References
[RFC2328] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC3031] Rosen, E., et al., "Multiprotocol Label Switching
Architecture", RFC 3031, January 2001.
[RFC3209] Awduche, D., et al., "Extensions to RSVP for LSP Tunnels",
RFC 3209, December 2001.
[RFC3270] LeFaucheur, F., et al., "Multi-Protocol Label Switching
(MPLS) Support of Differentiated Services", RFC 3270, May
2002.
[RFC3630] Katz, D., et al., "Traffic Engineering (TE) Extensions to
OSPF Version 2", RFC 3630, September 2003.
[RFC4124] LeFaucheur, F., "Protocol Extensions for Support of
Diffserv-Aware MPLS Traffic Engineering", RFC 4124, June
2005.
[RFC4203] Kompella, K., Rekhter, Y., "OSPF Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4203, October 2005.
[RFC4804] LeFaucheur, F., "Aggregation of Resource ReSerVation
Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels",
RFC 4804, February 2007.
[RFC4920] Farrel, A., et al., "Crankback Signaling Extensions for
MPLS and GMPLS RSVP-TE", RFC 4920, July 2007.
[RFC5307] Kompella, K., Rekhter, Y., "IS-IS Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC
5307, October 2008.
8. Informative References
[BGP-TE] Gredler, H., et al., "North-Bound Distribution of Link-State
and TE Information using BGP", work in progress.
[EANTC] "Multi-vendor Carrier Ethernet Interoperability Event,"
Carrier Ethernet World Congress 2006, Madrid Spain,
September 2006.
[FEEDBACK] Ashwood-Smith, P., et al., "Improving Topology Data Base
Accuracy with Label Switched Path Feedback in Constraint
Based Label Distribution Protocol," IETF work in progress.
[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.
[RFC2597] Heinanen, J., et al., "Assured Forwarding PHB Group", RFC
2597, June 1999.
[RFC3246] Davie, B., et al., "An Expedited Forwarding PHB", RFC 3246,
March 2002.
[RFC4090] Pan, P., et al., "Fast Reroute Extensions to RSVP-TE for
LSP Tunnels", May 2005.
[RFC4125] LeFaucheur, F., Lai, W., "Maximum Allocation Bandwidth
Ash, McDysan <draft-ash-gcac-algorithm-spec-04.txt> [Page 17]
Internet Draft GCAC Algorithm Specification February 2012
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4125, June 2005.
[RFC4126] Ash, J., "Max Allocation with Reservation Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering & Performance Comparisons", RFC 4126, June
2005.
[RFC4127] LeFaucheur, F., "Russian Dolls Bandwidth Constraints Model
for Diffserv-aware MPLS Traffic Engineering", RFC 4127,
June 2005.
[RFC4655] Farrel, A., et al., "A Path Computation Element (PCE)-
Based Architecture", RFC 4655, August 2006.
[RFC4657] Ash, J., Le Roux, J.L., "Path Computation Element (PCE)
Communication Protocol Generic Requirements", RFC 4657,
September 2006.
[RFC5069] Taylor, T., et al., "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069, January
2008.
[RFC5152] Vasseur, JL., et al., "A Per-Domain Path Computation Method
for Establishing Inter-Domain Traffic Engineering (TE)
Label Switched Paths (LSPs)", RFC 5152, February 2008.
[RFC5440] Vasseur, JL., Le Roux, J. L., "Path Computation Element
(PCE) Communication Protocol", RFC 5440, March 2009.
[RFC5441] Vasseur, JP., et al., "A Backward-Recursive PCE-Based
Computation (BRPC) Procedure to Compute Shortest
Constrained Inter-Domain Traffic Engineering Label
Switched Paths", RFC 5441, April 2009.
[RFC5865] Baker, F., et al., "A Differentiated Services Code Point
(DSCP) for Capacity-Admitted Traffic", RFC 5865, May 2010.
[RFC5971] Schulzrinne, H., Hancock, R., "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[RFC5974] Manner, J., et al., "NSIS Signaling Layer Protocol (NSLP)
for Quality-of-Service Signaling", RFC 5974, October 2010.
[RFC5975] Ash, G., et al., "QSPEC Template for the
Quality-of-Service NSIS Signaling Layer Protocol
(NSLP)", RFC 5975, October 2010.
[RFC5976] Ash, G., et al., "Y.1541-QOSM: Model for Networks Using
Y.1541 Quality-of-Service Classes", RFC 5976, October
2010.
[RFC6401] Le Faucheur, F., et al., "RSVP Extensions for Admission
Priority", RFC 6401, October 2011.
[TQO] Ash, G., "Traffic Engineering & QoS Optimization of Integrated
Voice & Data Networks," Elsevier, 2006.
9. Authors' Addresses
Gerald Ash (Editor)
AT&T
Email: gash5107@yahoo.com
Dave McDysan
Verizon
22001 Loudoun County PKWY
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Ashburn, VA 20147
Email: dave.mcdysan@verizon.com
Appendix A: Example MPLS GCAC Implementation Including Path Selection,
Bandwidth Management, QoS Signaling, & 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 DSTE 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 [RFC3031] 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, 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 DSTE 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 function 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 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 down to
the current level of bandwidth utilization.
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
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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 & variance factor VFck
+ routing priority (high, normal, best-effort)
+ CTc reserved bandwidth RBWck & bandwidth constraint BWCck
+ link reserved bandwidth threshold RBTk & unreserved bandwidth
ULBk
- GEF periodically decreases CTc LSP bandwidth allocation based on
bandwidth use
o VNs send crankback message to GEF if DSTE-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
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 & 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 last tried, which is also successful, 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
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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 increase or decrease
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).
|<----- 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 tested locally on each link in LSP-p, if
bandwidth not available (e.g., unreserved bandwidth on link BE
less than reserved bandwidth threshold & 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 & 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
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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
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 down
to the currently used bandwidth level. Bandwidth reservation occurs
in STT-EDR with PATH/RESV messages per application of [RFC4804].
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 earlier in the MPLS working group
[FEEDBACK], where feedback based on failed path routing attempts is
kept by the TE data base 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, 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 [RFC4655, RFC4657, RFC5440] is used to implement
inter-area/inter-AS/ inter-SP path selection algorithms, including
alternate path selection, path reoptimization, backup path
computation to protect DSTE tunnels, and
inter-area/inter-AS/inter-SP traffic engineering. The DSTE tunnels
are protected against failure by using MPLS Fast Reroute [RFC4090].
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OSPF TE extensions [RFC4203] 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 [RFC4126] incorporated within DSTE [RFC4124].
In DSTE/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:
PROPORTIONAL_ BWck =
TRAF_LOAD_ BWck/[S (c) {TRAF_LOAD_ BWck, c=0, MaxCT-1}] x MRBk
For normal priority CTc:
BWCck = PROPORTIONAL_ BWck
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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 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, 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 forecast 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
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).
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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 to admit this request or not. 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 to admit this request or not. 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 [RFC4126] do not change based on traffic
conditions. In particular, these parameters defined in [RFC4126],
as described above, are configured and do not change until
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 DSTE/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
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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) [RFC5971]
o NSIS QoS signaling layer protocol (QoS-NSLP) [RFC5974]
[RFC5975] defines a QoS specification (QSPEC) object, which contains
the QoS parameters required by a QoS model (QOSM) [RFC5976]. 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.
The QSPEC parameters defined in [RFC5975] include, among others:
TRAFFIC DESCRIPTION Parameters:
o <Traffic Model> Parameters
CONSTRAINTS Parameters:
o <Path Latency>, <Path Jitter>, <Path PLR>, <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> & <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 [RFC5976], 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
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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> 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 TC/COS bits. The queuing implementation assumes 3
levels of priority, high, normal, and best effort. These queues
include two EF priority queues [RFC3246, RFC5865], 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 [RFC2597] 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.
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