<- RFC Index (4101..4200)
RFC 4126
Network Working Group J. Ash
Request for Comments: 4126 AT&T
Category: Experimental June 2005
Max Allocation with Reservation Bandwidth Constraints Model for
Diffserv-aware MPLS Traffic Engineering & Performance Comparisons
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document complements the Diffserv-aware MPLS Traffic Engineering
(DS-TE) requirements document by giving a functional specification
for the Maximum Allocation with Reservation (MAR) Bandwidth
Constraints Model. Assumptions, applicability, and examples of the
operation of the MAR Bandwidth Constraints Model are presented. MAR
performance is analyzed relative to the criteria for selecting a
Bandwidth Constraints Model, in order to provide guidance to user
implementation of the model in their networks.
Table of Contents
1. Introduction ....................................................2
1.1. Specification of Requirements ..............................3
2. Definitions .....................................................3
3. Assumptions & Applicability .....................................5
4. Functional Specification of the MAR Bandwidth
Constraints Model ...............................................6
5. Setting Bandwidth Constraints ...................................7
6. Example of MAR Operation ........................................8
7. Summary .........................................................9
8. Security Considerations ........................................10
9. IANA Considerations ............................................10
10. Acknowledgements ..............................................10
A. MAR Operation & Performance Analysis ..........................11
B. Bandwidth Prediction for Path Computation ......................19
Normative References ..............................................20
Informative References ............................................20
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1. Introduction
Diffserv-aware MPLS traffic engineering (DS-TE) requirements and
protocol extensions are specified in [DSTE-REQ, DSTE-PROTO]. A
requirement for DS-TE implementation is the specification of
Bandwidth Constraints Models for use with DS-TE. The Bandwidth
Constraints Model provides the 'rules' to support the allocation of
bandwidth to individual class types (CTs). CTs are groupings of
service classes in the DS-TE model, which are provided separate
bandwidth allocations, priorities, and QoS objectives. Several CTs
can share a common bandwidth pool on an integrated, multiservice
MPLS/Diffserv network.
This document is intended to complement the DS-TE requirements
document [DSTE-REQ] by giving a functional specification for the
Maximum Allocation with Reservation (MAR) Bandwidth Constraints
Model. Examples of the operation of the MAR Bandwidth Constraints
Model are presented. MAR performance is analyzed relative to the
criteria for selecting a Bandwidth Constraints Model, in order to
provide guidance to user implementation of the model in their
networks.
Two other Bandwidth Constraints Models are being specified for use in
DS-TE:
1. Maximum Allocation Model (MAM) [MAM] - the maximum allowable
bandwidth usage of each CT is explicitly specified.
2. Russian Doll Model (RDM) [RDM] - the maximum allowable bandwidth
usage is done cumulatively by grouping successive CTs according to
priority classes.
MAR is similar to MAM in that a maximum bandwidth allocation is given
to each CT. However, through the use of bandwidth reservation and
protection mechanisms, CTs are allowed to exceed their bandwidth
allocations under conditions of no congestion but revert to their
allocated bandwidths when overload and congestion occurs.
All Bandwidth Constraints Models should meet these objectives:
1. applies equally when preemption is either enabled or disabled
(when preemption is disabled, the model still works 'reasonably'
well),
2. bandwidth efficiency, i.e., good bandwidth sharing among CTs under
both normal and overload conditions,
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3. bandwidth isolation, i.e., a CT cannot hog the bandwidth of
another CT under overload conditions,
4. protection against QoS degradation, at least of the high-priority
CTs (e.g., high-priority voice, high-priority data, etc.), and
5. reasonably simple, i.e., does not require additional IGP
extensions and minimizes signaling load processing requirements.
In Appendix A, modeling analysis is presented that shows the MAR
Model meets all of these objectives and provides good network
performance, relative to MAM and full-sharing models, under normal
and abnormal operating conditions. It is demonstrated that MAR
simultaneously achieves bandwidth efficiency, bandwidth isolation,
and protection against QoS degradation without preemption.
In Section 3 we give the assumptions and applicability; in Section 4
a functional specification of the MAR Bandwidth Constraints Model;
and in Section 5 we give examples of its operation. In Appendix A,
MAR performance is analyzed relative to the criteria for selecting a
Bandwidth Constraints Model, in order to provide guidance to user
implementation of the model in their networks. In Appendix B,
bandwidth prediction for path computation is discussed.
1.1. Specification of Requirements
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 [RFC2119].
2. Definitions
For readability a number of definitions from [DSTE-REQ, DSTE-PROTO]
are repeated here:
Traffic Trunk: an aggregation of traffic flows of the same class
(i.e., treated equivalently from the DS-TE
perspective), which is placed inside a Label
Switched Path (LSP).
Class-Type (CT): the set of Traffic Trunks crossing a link that is
governed by a specific set of bandwidth
constraints. CT is used for the purposes of link
bandwidth allocation, constraint-based routing,
and admission control. A given Traffic Trunk
belongs to the same CT on all links.
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Up to 8 CTs (MaxCT = 8) are supported. They are
referred to as CTc, 0 <= c <= MaxCT-1 = 7. Each
CT is assigned either a Bandwidth Constraint, or
a set of Bandwidth Constraints. Up to 8
Bandwidth Constraints (MaxBC = 8) are supported
and they are referred to as BCc, 0 <= c <=
MaxBC-1 = 7.
TE-Class: A pair of: a) a CT, and b) a preemption priority
allowed for that CT. This means that an LSP,
transporting a Traffic Trunk from that CT, can
use that preemption priority as the set-up
priority, the holding priority, or both.
MAX_RESERVABLE_BWk: 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
[OSPF-TE].
BCck: bandwidth constraint for CTc on link k =
allocated (minimum guaranteed) bandwidth for CTc
on link k (see Section 4).
RBW_THRESk: reservation bandwidth threshold for link k (see
Section 4).
RESERVED_BWck: reserved bandwidth-in-progress on CTc on link k
(0 <= c <= MaxCT-1), RESERVED_BWck = total amount
of the bandwidth reserved by all the established
LSPs that belong to CTc.
UNRESERVED_BWk: unreserved link bandwidth on link k specifies the
amount of bandwidth not yet reserved for any CT,
UNRESERVED_BWk = MAX_RESERVABLE_BWk - sum
[RESERVED_BWck (0 <= c <= MaxCT-1)].
UNRESERVED_BWck: unreserved link bandwidth on CTc on link k
specifies the amount of bandwidth not yet
reserved for CTc, UNRESERVED_BWck =
UNRESERVED_BWk - delta0/1(CTck) * RBW-THRESk
where
delta0/1(CTck) = 0 if RESERVED_BWck < BCck
delta0/1(CTck) = 1 if RESERVED_BWck >= BCck
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A number of recovery mechanisms under investigation in the IETF take
advantage of the concept of bandwidth sharing across particular sets
of LSPs. "Shared Mesh Restoration" in [GMPLS-RECOV] and "Facility-
based Computation Model" in [MPLS-BACKUP] are example mechanisms that
increase bandwidth efficiency by sharing bandwidth across backup LSPs
protecting against independent failures. To ensure that the notion
of RESERVED_BWck introduced in [DSTE-REQ] is compatible with such a
concept of bandwidth sharing across multiple LSPs, the wording of the
definition provided in [DSTE-REQ] is generalized. With this
generalization, the definition is compatible with Shared Mesh
Restoration defined in [GMPLS-RECOV], so that DS-TE and Shared Mesh
Protection can operate simultaneously, under the assumption that
Shared Mesh Restoration operates independently within each DS-TE
Class-Type and does not operate across Class-Types. For example,
backup LSPs protecting primary LSPs of CTc also need to belong to
CTc; excess traffic LSPs that share bandwidth with backup LSPs of CTc
also need to belong to CTc.
3. Assumptions & Applicability
In general, DS-TE is a bandwidth allocation mechanism for different
classes of traffic allocated to various CTs (e.g., voice, normal
data, best-effort data). Network operation functions such as
capacity design, bandwidth allocation, routing design, and network
planning are normally based on traffic-measured load and forecast
[ASH1].
As such, the following assumptions are made according to the
operation of MAR:
1. Connection admission control (CAC) allocates bandwidth for network
flows/LSPs according to the traffic load assigned to each CT,
based on traffic measurement and forecast.
2. CAC could allocate bandwidth per flow, per LSP, per traffic trunk,
or otherwise. That is, no specific assumption is made about a
specific CAC method, except that CT bandwidth allocation is
related to the measured/forecasted traffic load, as per assumption
#1.
3. CT bandwidth allocation is adjusted up or down according to
measured/forecast traffic load. No specific time period is
assumed for this adjustment, it could be short term (seconds,
minutes, hours), daily, weekly, monthly, or otherwise.
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4. Capacity management and CT bandwidth allocation thresholds (e.g.,
BCc) are designed according to traffic load, and are based on
traffic measurement and forecast. Again, no specific time period
is assumed for this adjustment, it could be short term (hours),
daily, weekly, monthly, or otherwise.
5. No assumption is made on the order in which traffic is allocated
to various CTs; again traffic allocation is assumed to be based
only on traffic load as it is measured and/or forecast.
6. If link bandwidth is exhausted on a given path for a
flow/LSP/traffic trunk, alternate paths may be attempted to
satisfy CT bandwidth allocation.
Note that the above assumptions are not unique to MAR, but are
generic, common assumptions for all BC Models.
4. Functional Specification of the MAR Bandwidth Constraints Model
A DS-TE Label Switching Router (LSR) that implements MAR MUST support
enforcement of bandwidth constraints, in compliance with the
specifications in this section.
In the MAR Bandwidth Constraints Model, the bandwidth allocation
control for each CT is based on estimated bandwidth needs, bandwidth
use, and status of links. The Label Edge Router (LER) makes needed
bandwidth allocation changes, and uses [RSVP-TE], for example, to
determine if link bandwidth can be allocated to a CT. Bandwidth
allocated to individual CTs is protected as needed, but otherwise it
is shared. Under normal, non-congested network conditions, all
CTs/services fully share all available bandwidth. When congestion
occurs for a particular CTc, bandwidth reservation prohibits traffic
from other CTs from seizing the allocated capacity for CTc.
On a given link k, a small amount of bandwidth RBW_THRESk (the
reservation bandwidth threshold for link k) is reserved and governs
the admission control on link k. Also associated with each CTc on
link k are the allocated bandwidth constraints BCck to govern
bandwidth allocation and protection. The reservation bandwidth on a
link (RBW_THRESk) can be accessed when a given CTc has bandwidth-in-
use (RESERVED_BWck) below its allocated bandwidth constraint (BCck).
However, if RESERVED_BWck exceeds its allocated bandwidth constraint
(BCck), then the reservation bandwidth (RBW_THRESk) cannot be
accessed. In this way, bandwidth can be fully shared among CTs if
available, but is otherwise protected by bandwidth reservation
methods.
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Bandwidth can be accessed for a bandwidth request = DBW for CTc on a
given link k based on the following rules:
Table 1: Rules for Admitting LSP Bandwidth Request = DBW on Link k
For LSP on a high priority or normal priority CTc:
If RESERVED_BWck <= BCck: admit if DBW <= UNRESERVED_BWk
If RESERVED_BWck > BCck: admit if DBW <= UNRESERVED_BWk - RBW_THRESk;
or, equivalently:
If DBW <= UNRESERVED_BWck, admit the LSP.
For LSP on a best-effort priority CTc:
allocated bandwidth BCck = 0;
Diffserv queuing admits BE packets only if there is available link
bandwidth.
The normal semantics of setup and holding priority are applied in the
MAR Bandwidth Constraints Model, and cross-CT preemption is permitted
when preemption is enabled.
The bandwidth allocation rules defined in Table 1 are illustrated
with an example in Section 6 and simulation analysis in Appendix A.
5. Setting Bandwidth Constraints
For a normal priority CTc, the bandwidth constraints BCck on link k
are set by allocating the maximum reservable bandwidth
(MAX_RESERVABLE_BWk) 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/[sum {TRAF_LOAD_BWck, c=0, MaxCT-1}]
X MAX_RESERVABLE_BWk
For normal priority CTc:
BCck = PROPORTIONAL_BWck
For a high priority CT, the bandwidth constraint BCck is set to a
multiple of the proportional bandwidth. That is:
For high priority CTc:
BCck = FACTOR X 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' of the maximum reservable bandwidth, and gives priority
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to the high priority CTs. Normally the bandwidth allocated to high
priority CTs should be a relatively small fraction of the total link
bandwidth, with a maximum of 10-15 percent being a reasonable
guideline.
As stated in Section 4, the bandwidth allocated to a best-effort
priority CTc should be set to zero. That is:
For best-effort priority CTc:
BCck = 0
6. Example of MAR Operation
In the example, assume there are three class-types: CT0, CT1, CT2.
We consider a particular link with
MAX-RESERVABLE_BW = 100
And with the allocated bandwidth constraints set as follows:
BC0 = 30
BC1 = 20
BC2 = 20
These bandwidth constraints are based on the normal traffic loads, as
discussed in Section 5. With MAR, any of the CTs is allowed to
exceed its bandwidth constraint (BCc) as long a there are at least
RBW_THRES (reservation bandwidth threshold on the link) units of
spare bandwidth remaining. Let's assume
RBW_THRES = 10
So under overload, if
RESERVED_BW0 = 50
RESERVED_BW1 = 30
RESERVED_BW2 = 10
Therefore, for this loading
UNRESERVED_BW = 100 - 50 - 30 - 10 = 10
CT0 and CT1 can no longer increase their bandwidth on the link,
because they are above their BC values and there is only RBW_THRES=10
units of spare bandwidth left on the link. But CT2 can take the
additional bandwidth (up to 10 units) if the demand arrives, because
it is below its BC value.
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As also discussed in Section 4, if best effort traffic is present, it
can always seize whatever spare 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.
Let's say an LSP arrives for CT0 needing 5 units of bandwidth (i.e.,
DBW = 5). We need to decide, based on Table 1, whether to admit this
LSP or not. Since for CT0
RESERVED_BW0 > BC0 (50 > 30), and
DBW > UNRESERVED_BW - RBW_THRES (i.e., 5 > 10 - 10)
Table 1 says the LSP is rejected/blocked.
Now let's say an LSP arrives for CT2 needing 5 units of bandwidth
(i.e., DBW = 5). We need to decide based on Table 1 whether to admit
this LSP or not. Since for CT2
RESERVED_BW2 < BC2 (10 < 20), and
DBW < UNRESERVED_BW (i.e., 5 < 10)
Table 1 says to admit the LSP.
Hence, in the above example, in the current state of the link and in
the current CT loading, CT0 and CT1 can no longer increase their
bandwidth on the link, because they are above their BCc values and
there is only RBW_THRES=10 units of spare bandwidth left on the link.
But CT2 can take the additional bandwidth (up to 10 units) if the
demand arrives, because it is below its BCc value.
7. Summary
The proposed MAR Bandwidth Constraints Model includes the following:
1. allocation of bandwidth to individual CTs,
2. protection of allocated bandwidth by bandwidth reservation
methods, as needed, but otherwise full sharing of bandwidth,
3. differentiation between high-priority, normal-priority, and best-
effort priority services, and
4. provision of admission control to reject connection requests, when
needed, in order to meet performance objectives.
The modeling results presented in Appendix A show that MAR bandwidth
allocation achieves a) greater efficiency in bandwidth sharing while
still providing bandwidth isolation and protection against QoS
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degradation, and b) service differentiation for high-priority,
normal-priority, and best-effort priority services.
8. Security Considerations
Security considerations related to the use of DS-TE are discussed in
[DSTE-PROTO]. They apply independently of the Bandwidth Constraints
Model, including the MAR specified in this document.
9. IANA Considerations
[DSTE-PROTO] defines a new name space for "Bandwidth Constraints
Model Id". The guidelines for allocation of values in that name
space are detailed in Section 13.1 of [DSTE-PROTO]. In accordance
with these guidelines, the IANA has assigned a Bandwidth Constraints
Model Id for MAR from the range 0-239 (which is to be managed as per
the "Specification Required" policy defined in [IANA-CONS]).
Bandwidth Constraints Model Id 2 was allocated by IANA to MAR.
10. Acknowledgements
DS-TE and Bandwidth Constraints Models have been an active area of
discussion in the TEWG. I would like to thank Wai Sum Lai for his
support and review of this document. I also appreciate helpful
discussions with Francois Le Faucheur.
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Appendix A. MAR Operation & Performance Analysis
A.1. MAR Operation
In the MAR Bandwidth Constraints Model, the bandwidth allocation
control for each CT is based on estimated bandwidth needs, bandwidth
use, and status of links. The LER makes needed bandwidth allocation
changes, and uses [RSVP-TE], for example, to determine if link
bandwidth can be allocated to a CT. Bandwidth allocated to
individual CTs is protected as needed, but otherwise it is shared.
Under normal, non-congested network conditions, all CTs/services
fully share all available bandwidth. When congestion occurs for a
particular CTc, bandwidth reservation acts to prohibit traffic from
other CTs from seizing the allocated capacity for CTc. Associated
with each CT is the allocated bandwidth constraint (BCc) which
governs bandwidth allocation and protection; these parameters are
illustrated with examples in this Appendix.
In performing MAR bandwidth allocation for a given flow/LSP, the LER
first determines the egress LSR address, service-identity, and CT.
The connection request is allocated an equivalent bandwidth to be
routed on a particular CT. The LER then accesses the CT priority,
QoS/traffic parameters, and routing table between the LER and egress
LSR, and sets up the connection request using the MAR bandwidth
allocation rules. The LER selects a first-choice path and determines
if bandwidth can be allocated on the path based on the MAR bandwidth
allocation rules given in Section 4. If the first choice path has
insufficient bandwidth, the LER may then try alternate paths, and
again applies the MAR bandwidth allocation rules now described.
MAR bandwidth allocation is done on a per-CT basis, in which
aggregated CT bandwidth is managed to meet the overall bandwidth
requirements of CT service needs. Individual flows/LSPs are
allocated bandwidth in the corresponding CT according to CT bandwidth
availability. A fundamental principle applied in MAR bandwidth
allocation methods is the use of bandwidth reservation techniques.
Bandwidth reservation gives preference to the preferred traffic by
allowing it to seize idle bandwidth on a link more easily than the
non-preferred traffic. Burke [BUR] first analyzed bandwidth
reservation behavior from the solution of the birth-death equations
for the bandwidth reservation model. Burke's model showed the
relative lost-traffic level for preferred traffic, which is not
subject to bandwidth reservation restrictions, as compared to non-
preferred traffic, which is subject to the restrictions. Bandwidth
reservation protection is robust to traffic variations and provides
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significant dynamic protection of particular streams of traffic. It
is widely used in large-scale network applications [ASH1, MUM, AKI,
KRU, NAK].
Bandwidth reservation is used in MAR bandwidth allocation to control
sharing of link bandwidth across different CTs. On a given link, a
small amount of bandwidth (RBW_THRES) is reserved (perhaps 1% of the
total link bandwidth), and the reservation bandwidth can be accessed
when a given CT has reserved bandwidth-in-progress (RESERVED_BW)
below its allocated bandwidth (BC). That is, if the available link
bandwidth (unreserved idle link bandwidth UNRESERVED_BW) exceeds
RBW_THRES, then any CT is free to access the available bandwidth on
the link. However, if UNRESERVED_BW is less than RBW_THRES, then the
CT can utilize the available bandwidth only if its current bandwidth
usage is below the allocated amount (BC). In this way, bandwidth can
be fully shared among CTs if available, but it is protected by
bandwidth reservation if below the reservation level.
Through the bandwidth reservation mechanism, MAR bandwidth allocation
also gives preference to high-priority CTs, in comparison to normal-
priority and best-effort priority CTs.
Hence, bandwidth allocated to each CT is protected by bandwidth
reservation methods, as needed, but otherwise shared. Each LER
monitors CT bandwidth use on each CT, and determines if connection
requests can be allocated to the CT bandwidth. For example, for a
bandwidth request of DBW on a given flow/LSP, the LER determines the
CT priority (high, normal, or best-effort), CT bandwidth-in-use, and
CT bandwidth allocation thresholds, and uses these parameters to
determine the allowed load state threshold to which capacity can be
allocated. In allocating bandwidth DBW to a CT on given LSP (for
example, A-B-E), each link in the path is checked for available
bandwidth in comparison to the allowed load state. If bandwidth is
unavailable on any link in path A-B-E, another LSP could be tried,
such as A-C-D-E. Hence, determination of the link load state is
necessary for MAR bandwidth allocation, and two link load states are
distinguished: available (non-reserved) bandwidth (ABW_STATE), and
reserved-bandwidth (RBW_STATE). Management of CT capacity uses the
link state and the allowed load state threshold to determine if a
bandwidth allocation request can be accepted on a given CT.
A.2. Analysis of MAR Performance
In this Appendix, modeling analysis is presented in which MAR
bandwidth allocation is shown to provide good network performance,
relative to full sharing models, under normal and abnormal operating
conditions. A large-scale Diffserv-aware MPLS traffic engineering
simulation model is used, in which several CTs with different
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priority classes share the pool of bandwidth on a multiservice,
integrated voice/data network. MAR methods have also been analyzed
in practice for networks that use time division multiplexing (i.e.,
TDM-based networks) [ASH1], and in modeling studies for IP-based
networks [ASH2, ASH3, E.360].
All Bandwidth Constraints Models should meet these objectives:
1. applies equally when preemption is either enabled or disabled
(when preemption is disabled, the model still works 'reasonably'
well),
2. bandwidth efficiency, i.e., good bandwidth sharing among CTs under
both normal and overload conditions,
3. bandwidth isolation, i.e., a CT cannot hog the bandwidth of
another CT under overload conditions,
4. protection against QoS degradation, at least of the high-priority
CTs (e.g., high-priority voice, high-priority data, etc.), and
5. reasonably simple, i.e., does not require additional IGP
extensions and minimizes signaling load processing requirements.
The use of any given Bandwidth Constraints Model has significant
impacts on the performance of a network, as explained later.
Therefore, the criteria used to select a model need to enable us to
evaluate how a particular model delivers its performance, relative to
other models. Lai [LAI, DSTE-PERF] has analyzed the MAM and RDM
Models and provided valuable insights into the relative performance
of these models under various network conditions.
In environments where preemption is not used, MAM is attractive
because a) it is good at achieving isolation, and b) it achieves
reasonable bandwidth efficiency with some QoS degradation of lower
classes. When preemption is used, RDM is attractive because it can
achieve bandwidth efficiency under normal load. However, RDM cannot
provide service isolation under high load or when preemption is not
used.
Our performance analysis of MAR bandwidth allocation methods is based
on a full-scale, 135-node simulation model of a national network,
combined with a multiservice traffic demand model to study various
scenarios and tradeoffs [ASH3, E.360]. Three levels of traffic
priority -- high, normal, and best effort -- are given across 5 CTs:
normal priority voice, high priority voice, normal priority data,
high priority data, and best effort data.
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The performance analyses for overloads and failures include a) the
MAR Bandwidth Constraints Model, as specified in Section 4, b) the
MAM Bandwidth Constraints Model, and c) the No-DSTE Bandwidth
Constraints Model.
The allocated bandwidth constraints for MAR are described in Section
5 as:
Normal priority CTs: BCck = PROPORTIONAL_BWk,
High priority CTs: BCck = FACTOR X PROPORTIONAL_BWk
Best-effort priority CTs: BCck = 0
In the MAM Bandwidth Constraints Model, the bandwidth constraints for
each CT are set to a multiple of the proportional bandwidth
allocation:
Normal priority CTs: BCck = FACTOR1 X PROPORTIONAL_BWk,
High priority CTs: BCck = FACTOR2 X PROPORTIONAL_BWk
Best-effort priority CTs: BCck = 0
Simulations show that for MAM, the sum (BCc) should exceed
MAX_RESERVABLE_BWk for better efficiency, as follows:
1. The normal priority CTs and the BCc values need to be over-
allocated to get reasonable performance. It was found that over-
allocating by 100% (i.e., setting FACTOR1 = 2), gave reasonable
performance.
2. The high priority CTs can be over-allocated by a larger multiple
FACTOR2 in MAM and this gives better performance.
The rather large amount of over-allocation improves efficiency, but
somewhat defeats the 'bandwidth protection/isolation' needed with a
BC Model, because one CT can now invade the bandwidth allocated to
another CT. Each CT is restricted to its allocated bandwidth
constraint BCck, which is the maximum level of bandwidth allocated to
each CT on each link, as in normal operation of MAM.
In the No-DSTE Bandwidth Constraints Model, no reservation or
protection of CT bandwidth is applied, and bandwidth allocation
requests are admitted if bandwidth is available. Furthermore, no
queuing priority is applied to any of the CTs in the No-DSTE
Bandwidth Constraints Model.
Table 2 gives performance results for a six-times overload on a
single network node at Oakbrook, Illinois. The numbers given in the
table are the total network percent lost (i.e., blocked) or delayed
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traffic. Note that in the focused overload scenario studied here,
the percentage of lost/delayed traffic on the Oakbrook node is much
higher than the network-wide average values given.
Table 2
Performance Comparison for MAR, MAM, & No-DSTE
Bandwidth Constraints (BC) Models
6X Focused Overload on Oakbrook
(Total Network % Lost/Delayed Traffic)
Class Type MAR BC MAM BC No-DSTE BC
Model Model Model
NORMAL PRIORITY VOICE 0.00 1.97 10.30
HIGH PRIORITY VOICE 0.00 0.00 7.05
NORMAL PRIORITY DATA 0.00 6.63 13.30
HIGH PRIORITY DATA 0.00 0.00 7.05
BEST EFFORT PRIORITY DATA 12.33 11.92 9.65
Clearly the performance is better with MAR bandwidth allocation, and
the results show that performance improves when bandwidth reservation
is used. The reason for the poor performance of the No-DSTE Model,
without bandwidth reservation, is due to the lack of protection of
allocated bandwidth. If we add the bandwidth reservation mechanism,
then performance of the network is greatly improved.
The simulations showed that the performance of MAM is quite sensitive
to the over-allocation factors discussed above. For example, if the
BCc values are proportionally allocated with FACTOR1 = 1, then the
results are much worse, as shown in Table 3:
Table 3
Performance Comparison for MAM Bandwidth Constraints Model
with Different Over-allocation Factors
6X Focused Overload on Oakbrook
(Total Network % Lost/Delayed Traffic)
Class Type (FACTOR1 = 1) (FACTOR1 = 2)
NORMAL PRIORITY VOICE 31.69 1.97
HIGH PRIORITY VOICE 0.00 0.00
NORMAL PRIORITY DATA 31.22 6.63
HIGH PRIORITY DATA 0.00 0.00
BEST EFFORT PRIORITY DATA 8.76 11.92
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Table 4 illustrates the performance of the MAR, MAM, and No-DSTE
Bandwidth Constraints Models for a high-day network load pattern with
a 50% general overload. The numbers given in the table are the total
network percent lost (i.e., blocked) or delayed traffic.
Table 4
Performance Comparison for MAR, MAM, & No-DSTE
Bandwidth Constraints (BC) Models
50% General Overload (Total Network % Lost/Delayed Traffic)
Class Type MAR BC MAM BC No-DSTE BC
Model Model Model
NORMAL PRIORITY VOICE 0.02 0.13 7.98
HIGH PRIORITY VOICE 0.00 0.00 8.94
NORMAL PRIORITY DATA 0.00 0.26 6.93
HIGH PRIORITY DATA 0.00 0.00 8.94
BEST EFFORT PRIORITY DATA 10.41 10.39 8.40
Again, we can see the performance is always better when MAR bandwidth
allocation and reservation is used.
Table 5 illustrates the performance of the MAR, MAM, and No-DSTE
Bandwidth Constraints Models for a single link failure scenario (3
OC-48). The numbers given in the table are the total network percent
lost (blocked) or delayed traffic.
Table 5
Performance Comparison for MAR, MAM, & No-DSTE
Bandwidth Constraints (BC) Models
Single Link Failure (2 OC-48)
(Total Network % Lost/Delayed Traffic)
Class Type MAR BC MAM BC No-DSTE BC
Model Model Model
NORMAL PRIORITY VOICE 0.00 0.62 0.63
HIGH PRIORITY VOICE 0.00 0.31 0.32
NORMAL PRIORITY DATA 0.00 0.48 0.50
HIGH PRIORITY DATA 0.00 0.31 0.32
BEST EFFORT PRIORITY DATA 0.12 0.72 0.63
Again, we can see the performance is always better when MAR bandwidth
allocation and reservation is used.
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Table 6 illustrates the performance of the MAR, MAM, and No-DSTE
Bandwidth Constraints Models for a multiple link failure scenario (3
links with 3 OC-48, 3 OC-3, 4 OC-3 capacity, respectively). The
numbers given in the table are the total network percent lost
(blocked) or delayed traffic.
Table 6
Performance Comparison for MAR, MAM, & No-DSTE
Bandwidth Constraints (BC) Models
Multiple Link Failure
(3 Links with 2 OC-48, 2 OC-12, 1 OC-12, Respectively)
(Total Network % Lost/Delayed Traffic)
Class Type MAR BC MAM BC No-DSTE BC
Model Model Model
NORMAL PRIORITY VOICE 0.00 0.91 0.92
HIGH PRIORITY VOICE 0.00 0.44 0.44
NORMAL PRIORITY DATA 0.00 0.70 0.72
HIGH PRIORITY DATA 0.00 0.44 0.44
BEST EFFORT PRIORITY DATA 0.14 1.03 1.04
Again, we can see the performance is always better when MAR bandwidth
allocation and reservation is used.
Lai's results [LAI, DSTE-PERF] show the trade-off between bandwidth
sharing and service protection/isolation, using an analytic model of
a single link. He shows that RDM has a higher degree of sharing than
MAM. Furthermore, for a single link, the overall loss probability is
the smallest under full sharing and largest under MAM, with RDM being
intermediate. Hence, on a single link, Lai shows that the full
sharing model yields the highest link efficiency, while MAM yields
the lowest; and that full sharing has the poorest service protection
capability.
The results of the present study show that, when considering a
network context in which there are many links and multiple-link
routing paths are used, full sharing does not necessarily lead to
maximum, network-wide bandwidth efficiency. In fact, the results in
Table 4 show that the No-DSTE Model not only degrades total network
throughput, but also degrades the performance of every CT that should
be protected. Allowing more bandwidth sharing may improve
performance up to a point, but it can severely degrade performance if
care is not taken to protect allocated bandwidth under congestion.
Both Lai's study and this study show that increasing the degree of
bandwidth sharing among the different CTs leads to a tighter coupling
between CTs. Under normal loading conditions, there is adequate
capacity for each CT, which minimizes the effect of such coupling.
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Under overload conditions, when there is a scarcity of capacity, such
coupling can cause severe degradation of service, especially for the
lower priority CTs.
Thus, the objective of maximizing efficient bandwidth usage, as
stated in Bandwidth Constraints Model objectives, needs to be
exercised with care. Due consideration also needs to be given to
achieving bandwidth isolation under overload, in order to minimize
the effect of interactions among the different CTs. The proper
tradeoff of bandwidth sharing and bandwidth isolation needs to be
achieved in the selection of a Bandwidth Constraints Model.
Bandwidth reservation supports greater efficiency in bandwidth
sharing, while still providing bandwidth isolation and protection
against QoS degradation.
In summary, the proposed MAR Bandwidth Constraints Model includes the
following: a) allocation of bandwidth to individual CTs, b)
protection of allocated bandwidth by bandwidth reservation methods,
as needed, but otherwise full sharing of bandwidth, c)
differentiation between high-priority, normal-priority, and best-
effort priority services, and d) provision of admission control to
reject connection requests, when needed, in order to meet performance
objectives.
In the modeling results, the MAR Bandwidth Constraints Model compares
favorably with methods that do not use bandwidth reservation. In
particular, some of the conclusions from the modeling are as follows:
o MAR bandwidth allocation is effective in improving performance over
methods that lack bandwidth reservation; this allows more bandwidth
sharing under congestion.
o MAR achieves service differentiation for high-priority, normal-
priority, and best-effort priority services.
o Bandwidth reservation supports greater efficiency in bandwidth
sharing while still providing bandwidth isolation and protection
against QoS degradation, and is critical to stable and efficient
network performance.
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Appendix B. Bandwidth Prediction for Path Computation
As discussed in [DSTE-PROTO], there are potential advantages for a
Head-end when predicting the impact of an LSP on the unreserved
bandwidth for computing the path of the LSP. One example would be to
perform better load-distribution of multiple LSPs across multiple
paths. Another example would be to avoid CAC rejection when the LSP
no longer fits on a link after establishment.
Where such predictions are used on Head-ends, the optional Bandwidth
Constraints sub-TLV and the optional Maximum Reservable Bandwidth
sub-TLV MAY be advertised in the IGP. This can be used by Head-ends
to predict how an LSP affects unreserved bandwidth values. Such
predictions can be made with MAR by using the unreserved bandwidth
values advertised by the IGP, as discussed in Sections 2 and 4:
UNRESERVED_BWck = MAX_RESERVABLE_BWk - UNRESERVED_BWk -
delta0/1(CTck) * RBW-THRESk
where
delta0/1(CTck) = 0 if RESERVED_BWck < BCck
delta0/1(CTck) = 1 if RESERVED_BWck >= BCck
Furthermore, the following estimate can be made for RBW_THRESk:
RBW_THRESk = RBW_% * MAX_RESERVABLE_BWk,
where RBW_% is a locally configured variable, which could take on
different values for different link speeds. This information could
be used in conjunction with the BC sub-TLV, MAX_RESERVABLE_BW sub-
TLV, and UNRESERVED_BW sub-TLV to make predictions of available
bandwidth on each link for each CT. Because admission control
algorithms are left for vendor differentiation, predictions can only
be performed effectively when the Head-end LSR predictions are based
on the same (or a very close) admission control algorithm used by
other LSRs.
LSPs may occasionally be rejected when head-ends are establishing
LSPs through a common link. As an example, consider some link L, and
two head-ends H1 and H2. If only H1 or only H2 is establishing LSPs
through L, then the prediction is accurate. But if both H1 and H2
are establishing LSPs through L at the same time, the prediction
would not work perfectly. In other words, the CAC will occasionally
run into a rejected LSP on a link with such 'race' conditions. Also,
as mentioned in Appendix A, such a prediction is optional and outside
the scope of the document.
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Normative References
[DSTE-REQ] Le Faucheur, F. and W. Lai, "Requirements for Support
of Differentiated Services-aware MPLS Traffic
Engineering", RFC 3564, July 2003.
[DSTE-PROTO] Le Faucheur, F., Ed., "Protocol Extensions for Support
of Diffserv-aware MPLS Traffic Engineering," RFC 4124,
June 2005.
[RFC2119] Bradner, S., "Key words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IANA-CONS] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
Informative References
[AKI] Akinpelu, J. M., "The Overload Performance of
Engineered Networks with Nonhierarchical & Hierarchical
Routing," BSTJ, Vol. 63, 1984.
[ASH1] Ash, G. R., "Dynamic Routing in Telecommunications
Networks," McGraw-Hill, 1998.
[ASH2] Ash, G. R., et al., "Routing Evolution in Multiservice
Integrated Voice/Data Networks," Proceeding of ITC-16,
Edinburgh, June 1999.
[ASH3] Ash, G. R., "Performance Evaluation of QoS-Routing
Methods for IP-Based Multiservice Networks," Computer
Communications Magazine, May 2003.
[BUR] Burke, P. J., Blocking Probabilities Associated with
Directional Reservation, unpublished memorandum, 1961.
[DSTE-PERF] Lai, W., "Bandwidth Constraints Models for
Differentiated Services-aware MPLS Traffic Engineering:
Performance Evaluation", RFC 4128, June 2005.
[E.360] ITU-T Recommendations E.360.1 - E.360.7, "QoS Routing &
Related Traffic Engineering Methods for Multiservice
TDM-, ATM-, & IP-Based Networks".
[GMPLS-RECOV] Lang, J., et al., "Generalized MPLS Recovery Functional
Specification", Work in Progress.
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RFC 4126 MAR Bandwidth Constraints Model for DS-TE June 2005
[KRU] Krupp, R. S., "Stabilization of Alternate Routing
Networks", Proceedings of ICC, Philadelphia, 1982.
[LAI] Lai, W., "Traffic Engineering for MPLS, Internet
Performance and Control of Network Systems III
Conference", SPIE Proceedings Vol. 4865, pp. 256-267,
Boston, Massachusetts, USA, 29 July-1 August 2002.
[MAM] Le Faucheur, F., Lai, W., "Maximum Allocation Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4125, June 2005.
[MPLS-BACKUP] Vasseur, J. P., et al., "MPLS Traffic Engineering Fast
Reroute: Bypass Tunnel Path Computation for Bandwidth
Protection", Work in Progress.
[MUM] Mummert, V. S., "Network Management and Its
Implementation on the No. 4ESS, International Switching
Symposium", Japan, 1976.
[NAK] Nakagome, Y., Mori, H., Flexible Routing in the Global
Communication Network, Proceedings of ITC-7, Stockholm,
1973.
[OSPF-TE] Katz, D., Kompella, K. and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003.
[RDM] Le Faucheur, F., Ed., "Russian Dolls Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4127, June 2005.
[RSVP-TE] 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.
Author's Address
Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1 732-420-4578
EMail: gash@att.com
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