NetworkInternet Engineering Task Force
MPLS Working Group Daniel O. Awduche
Category: Informational Joe Malcolm
Expiration Date: April, December 1999 Johnson Agogbua
UUNET (MCI Worldcom)
Requirements for Traffic Engineering Over MPLS
Status of this Memo
This document is an Internet-Draft. Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-Drafts. Internet-
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts Internet- Drafts as reference
material or to cite them other than as "work in progress."
To view the entire
The list of current Internet-Drafts, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts can be accessed at
The list of Internet-Draft Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
(Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
(US West Coast). can be accessed at
This document presents a set of requirements for Traffic Engineering
over Multiprotocol Label Switching (MPLS). It identifies the
functional capabilities required to implement policies that
facilitate efficient and reliable network operations in an MPLS
domain. These capabilities can be used to optimize the utilization of
network resources, resources and to enhance traffic oriented performance
Table of Contents
1.0 Introduction .................................... 3
1.1 Terminology .................................... 4
1.2 Document Organization .................................... 4
2.0 Traffic Engineering ...................................... 4
2.1 Traffic Engineering Performance Objectives ............... 4 5
2.2 Traffic and Resource Control ............................. 6
2.3 Limitations of Current IGP Control Mechanisms ............ 6 7
3.0 MPLS and Traffic Engineering ............................. 7 8
3.1 Induced MPLS Graph ....................................... 8 9
3.2 The Fundamental Problem of Traffic Engineering Over MPLS . 9 10
4.0 Augmented Capabilities for Traffic Engineering Over MPLS . 10
5.0 Traffic Trunk Attributes and Characteristics ........... 10 11
5.1 Bidirectional Traffic Trunks ............................. 11 12
5.2 Basic Operations on Traffic Trunks ....................... 12 13
5.3 Accounting and Performance Monitoring .................... 12 13
5.4 Basic Attributes of Traffic Trunks ....................... 13
5.5 Traffic Parameter Attributes ............................ 13 14
5.6 Generic Path Selection and Management Attributes ......... 14 15
5.6.1 Administratively Specified Explicit Paths ................ 15
5.6.2 Hierarchy of Preference Rules for Multi-paths ............ 15 16
5.6.3 Resource Class Affinity Attributes ....................... 16
5.6.4 Adaptivity Attribute ..................................... 16 17
5.6.5 Load Distribution Across Parallel Traffic Trunks ......... 18
5.7 Priority Attribute ....................................... 18 19
5.8 Preemption Attribute ..................................... 18 19
5.9 Resilience Attribute ..................................... 19 20
5.10 Policing Attribute ...................................... 20 21
6.0 Resource Attributes ...................................... 21 22
6.1 Maximum Allocation Multiplier ............................ 21 22
6.2 Resource Class Attribute ................................ 22
7.0 Constraint Based Constraint-Based Routing ................................ 22 23
7.1 Basic Features of Constraint Based Constraint-Based Routing ............... 23 24
7.2 Implementation Considerations ............................ 24 25
8.0 Conclusions Conclusion ............................................. 25 26
9.0 Security Considerations .................................. 27
10.0 References ............................................. 26
10.0 Acknowledgments .......................................... 27
11.0 Acknowledgments .......................................... 28
12.0 Author's Address ......................................... 27 28
Multiprotocol Label Switching (MPLS) [1,2] integrates a label
swapping framework with network layer routing. The basic idea
involves assigning short fixed length labels to packets at the
ingress to an MPLS cloud, based cloud (based on the concept of forwarding
equivalence classes [1,2]. [1,2]). Throughout the interior of the MPLS
domain, the labels attached to packets are used to make forwarding
decisions (usually without recourse to the original packet headers.
From this relatively simple paradigm, a headers).
A set of powerful constructs
can be devised that to address a number of many critical issues in the
emerging differentiated services Internet. Internet can be devised from this
relatively simple paradigm. One of the most significant initial
applications of MPLS will be in Traffic Engineering. This aspect The importance
of this application is already well recognized well-recognized (see [1,2,3,9]). [1,2,3]).
This manuscript focuses is exclusively focused on the Traffic Engineering
applications of MPLS. Specifically, the goal of this document is to
highlight the issues and requirements for Traffic Engineering in a
large Internet backbone, in the backbone. The expectation is that the MPLS
specifications, or implementations derived therefrom, will address
the realization of these objectives. A description of the basic
capabilities and functionality required of an MPLS implementation to
accommodate the requirements is also presented.
It should be noted that even though we the focus is on Internet
backbones, the capabilities described here in this document are equally
applicable to Traffic Engineering in enterprise networks; in short networks. In general,
the capabilities can be applied to any label switched network under
a single technical administration in which at least two paths exist
between two nodes.
There has been a number of
Some recent manuscripts that focus have focused on the considerations pertaining
to Traffic Engineering and Traffic management under MPLS; MPLS, most
notably the works of Li and Rekhter , and
Vaananen and Ravikanth . others. In , an
architecture is proposed which employs MPLS and RSVP to provide
scalable differentiated services and Traffic Engineering in the
Internet. In , a
general framework is described that introduces traffic management
capabilities into MPLS. The present manuscript complements the aforementioned efforts, and
similar efforts. It reflects the authors' operational experience in
managing a large Internet backbone.
The reader is assumed to be familiar with the MPLS terminology as
defined in .
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 .
1.2 Document Organization
The remainder of this document is organized as follows: Section 2
discusses the basic functions of Traffic Engineering in the Internet.
Section 3, gives provides an overview of the traffic Engineering potentials
of MPLS. Sections 1 to 3 can be regarded as are essentially background material. Section
4 presents an overview of the fundamental requirements for Traffic
Engineering over MPLS. Section 5 describes the desirable attributes
and characteristics of traffic trunks which are pertinent to Traffic
Engineering. Section 6 presents a set of attributes which can be
associated with resources to constrain the routability of traffic
trunks and LSPs through them. Section 7
argues in favor of advocates
the introduction of a "constraint based "constraint-based routing" framework in MPLS
domains. Finally, Section 8 contains our concluding remarks.
2.0 Traffic Engineering
This section describes the basic functions of Traffic Engineering in
an Autonomous System in the contemporary Internet. The limitations of
current IGPs with respect to traffic and resource control are
highlighted. This section serves as motivation for the requirements
Traffic Engineering (TE) is concerned with performance optimization
of operational networks. Specifically, In general, it encompasses the application
of technology and scientific principles to the measurement, modeling,
characterization, and control of Internet traffic, and the
application of such knowledge and techniques to achieve specific
performance objectives. The aspects of Traffic Engineering that are
of interest concerning MPLS are measurement and control.
A major goal of Internet Traffic Engineering is to facilitate
efficient and reliable network operations, and at
the same time optimize the utilization of operations while simultaneously
optimizing network resources. resource utilization and traffic performance.
Traffic Engineering is becoming has become an indispensable function in many
large Autonomous Systems because of the high cost of network assets, assets
and the commercial and competitive nature of the Internet. These
factors emphasize the need for maximal operational efficiency.
2.1 Traffic Engineering Performance Objectives
The key performance objectives associated with traffic engineering
can be classified as being either:
1. traffic oriented or
2. resource oriented.
Traffic oriented performance objectives include those the aspects that
enhance the QoS of traffic streams. In a single class, best effort
Internet service model, the key traffic oriented performance
objectives include: minimization of packet loss, minimization of
delay, maximization of throughput, and enforcement of service level
agreements. Under a single class best effort Internet service model,
minimization of packet loss is one of the most important traffic
oriented performance objectives. Statistically bounded traffic
oriented performance objectives (such as peak to peak packet delay
variation, loss ratio, and maximum packet transfer delay) might
become useful in the forthcoming differentiated services Internet.
Resource oriented performance objectives include those the aspects
pertaining to the optimization of resource utilization. Efficient
management of network resources is the vehicle for the attainment of
resource oriented performance objectives. In particular, it is
generally desirable to ensure that subsets of network resources do
not become over utilized and congested, congested while other subsets along
alternate feasible paths remain underutilized. Bandwidth is a crucial and scarce
resource in contemporary networks.
Therefor, Therefore, a central function of
Traffic Engineering is
efficient management of to efficiently manage bandwidth resources.
Minimizing congestion is a major primary traffic and resource oriented
performance objective. The interest here is not on congestion problems
that are prolonged rather than on transient congestion resulting from
instantaneous bursts, but rather on
congestion problems that are more prolonged. bursts. Congestion typically manifests under two
1. When network resources are insufficient or inadequate to
accommodate offered load.
2. When traffic streams are inefficiently mapped onto available
resources; causing subsets of network resources to become
over-utilized while others remain underutilized.
The first type of congestion problem can be addressed through: by either: (i)
expansion and of capacity, or (ii) application of classical congestion
control techniques, or (iii) both. Classical congestion control
which attempt to regulate the demand, such demand so that it the traffic fits
onto available resources. Classical techniques for congestion control
include: rate limiting, window flow control, router queue management,
schedule-based control, and others; (see  and the references
The second type of congestion problems, namely those resulting from
inefficient resource allocation, can usually be addressed through
In general, congestion resulting from inefficient resource allocation
can be reduced by adopting load balancing policies. The objective of
such strategies is to minimize maximum congestion or alternatively to
minimize maximum resource utilization, through efficient resource
allocation. When congestion is minimized through efficient resource
allocation, packet loss decreases, transit delay decreases, and
aggregate throughput increases. Thereby, the perception of network
service quality experienced by end users becomes significantly
Clearly, load balancing is an important network performance
optimization policy, policy. Nevertheless, the capabilities provided for
Traffic Engineering should be flexible enough, enough so that network
administrators can implement other policies which take into account of
the prevailing cost structure, structure and the utility or revenue model.
2.2 Traffic and Resource Control
Performance optimization of operational networks is fundamentally a
control problem. The In the traffic engineering process model, the
Traffic Engineer Engineer, or a suitable automaton, acts as the controller in
an adaptive feedback control system, which system. This system includes a set of
interconnected network elements, a network performance monitoring
system, and a set of network configuration management tools. The
Traffic Engineer formulates a control policy, observes the state of
the network through the monitoring system, characterizes the traffic,
and applies control actions to drive the network to a desired state,
in accordance with the control policy. This can be
reactively by taking action in response to the current state of the
network, or pro-actively by using forecasting techniques to
anticipate future trends and applying action to obviate the predicted
undesirable future states.
Ideally, control actions should involve:
1. modifying Modification of traffic management parameters,
2. modifying Modification of parameters associated with routing, and
3. modifying Modification of attributes and constraints associated with
To the extent possible, it is desirable to minimize the
The level of manual intervention involved in the traffic engineering process.
process should be minimized whenever possible. This can be
accomplished by automating aspects of the control actions described
above, in a distributed and scalable fashion.
2.3 Limitations of Current IGP Control Mechanisms
This subsection reviews some of the well known limitations of current
IGPs with regard to Traffic Engineering.
The control capabilities offered by existing Internet interior
gateway protocols are quite inadequate not adequate for Traffic Engineering. This
makes it difficult to actualize effective policies that to address network
performance problems. Indeed, IGPs based on shortest path algorithms
contribute significantly to congestion problems in Autonomous Systems
within the Internet. SPF algorithms generally optimize based on a
simple additive metric. Because these These protocols are topology driven, so
bandwidth availability and traffic characteristics are not taken into account factors
considered in making routing decisions. Consequently, congestion frequently
1. the shortest paths of multiple traffic streams converge on
specific links or router interfaces, or
2. a given traffic stream is routed through a link or router
interface which does not have enough bandwidth to accommodate
These scenarios manifest even when feasible alternate paths with
excess capacity exist. It is this aspect of congestion problems (-- a
symptom of suboptimal resource allocation) that Traffic Engineering
aims to vigorously obviate. Equal cost path load sharing can be used
to address case (2) the second cause for congestion listed above with some
degree of success,
but however it is generally not case (1), especially helpful in alleviating
congestion due to the first cause listed above and particularly not
in large networks with dense topology.
A popular means of circumventing approach to circumvent the inadequacies of current IGPs is
through the use of an overlay model, using such as IP over ATM or IP over
frame relay. The overlay model extends the design space by enabling
arbitrary virtual topologies to be provisioned atop the network's
physical topology. The virtual topology is constructed from virtual
circuits which appear as physical links to the IGP routing protocols.
The overlay model also provides many additional important services which to support
traffic and resource control, including: (1) constraint based constraint-based routing
at the VC level, (2) support for administratively configurable
explicit VC paths, (3) path compression, (4) call admission control
functions, (5) traffic shaping and traffic policing functions, and
(6) survivability of VCs. These capabilities enable the actualization
of a variety of Traffic Engineering policies. For example, virtual
circuits can easily be rerouted to move traffic from over-utilized
resources onto relatively underutilized ones.
For Traffic Engineering in large dense networks, it is desirable to
equip MPLS with a level of functionality at least commensurate with
current overlay models. Fortunately, this can be done in a fairly
straight forward manner.
3.0 MPLS and Traffic Engineering
This section provides an overview of the applicability of MPLS to
Traffic Engineering. Subsequent sections elaborate on discuss the set of
capabilities required to meet the Traffic Engineering requirements.
MPLS is strategically significant for Traffic Engineering because it
can potentially provide most of the functionality available from the
overlay model, in an integrated manner, and at a lower cost than the
currently competing alternatives. Equally importantly, MPLS offers
the possibility to automate aspects of the Traffic Engineering
function. This later last consideration is left for requires further study investigation and
is beyond the scope of this manuscript.
A note on terminology: The concept of MPLS traffic trunks is used
extensively in the remainder of this document. According to Li and
Rekhter , a traffic trunk is an aggregation of traffic flows of
the same class which are placed inside a Label Switched Path.
Essentially, a traffic trunk is an abstract representation of traffic
to which specific characteristics can be associated. It is useful to
view traffic trunks as atomic objects which that can be routed; that is, the path
through which a traffic trunk traverses can be changed. In this
respect, traffic trunks are similar to virtual circuits in ATM and
Frame Relay networks. It is important important, however, to emphasize that
there is a fundamental distinction between a traffic trunk and the LSP
path, and indeed the LSP, through which it traverses. An LSP is a
specification of the label switched path through which the traffic
traverses. In practice, the terms LSP and traffic trunk are often
used synonymously. Additional characteristics of traffic trunks as
used in this manuscript worth
noting are summarized in section 5.0.
The attractiveness of MPLS for Traffic Engineering can be attributed
to the following factors: (1) explicit label switched paths which are
not constrained by the destination based forwarding paradigm can be
easily created through manual administrative action or through
automated action by the underlying protocols, (2) LSPs can
potentially be efficiently maintained, (3) traffic trunks can be
instantiated and mapped onto LSPs, (4) a set of attributes can be
associated with traffic trunks which modulate their behavioral
characteristics, (5) a set of attributes can be associated with
resources which constrain the placement of LSPs and traffic trunks
across them, (6) MPLS allows for both traffic aggregation and
disaggregation whereas classical destination only based IP forwarding
permits only aggregation, (7) it is relatively easy to integrate a "constraint based
"constraint-based routing" framework with MPLS, (8) a good
implementation of MPLS can offer significantly lower overhead than
competing alternatives for Traffic Engineering. Furthermore,
Additionally, through explicit routes, label switched paths, MPLS permits a
quasi circuit switching capability to be superimposed on the current
Internet routing model. Many of the existing proposals for Traffic
Engineering over MPLS focus only on the potential to create explicit
LSPs. Although this capability is fundamental for Traffic
Engineering, it is not really sufficient. Additional augmentations
are required to foster the actualization of policies that lead leading to
performance optimization of large operational networks. Some of the
necessary augmentations are described in this manuscript.
3.1 Induced MPLS Graph
This subsection introduces the concept of an "induced MPLS graph"
which is central to Traffic Engineering in MPLS domains. An induced
MPLS graph is analogous to a virtual topology in an overlay model,
and model. It
is logically mapped onto the physical network through the selection
of LSPs for traffic trunks.
An induced MPLS graph consists of a set of LSRs which comprise the
nodes of the graph and a set of LSPs which provide logical point to
point connectivity between the LSRs, and hence serve as the links of
the induced graph. Using the concept of label stacks
(see ), it may be possible to construct hierarchical
graphs. graphs based on the concept of label stacks (see ).
Induced MPLS graphs are important because the basic problem of
bandwidth management in an MPLS domain concerns is the issue of how to
efficiently map an induced MPLS graph onto the physical network
topology. The induced MPLS graph abstraction is formalized below.
Let G = (V, E, c) be a capacitated graph depicting the physical
topology of the network. Here, V is the set of nodes in the network
and E is the set of links; that is, for v and w in V, the object
(v,w) is in E if v and w are directly connected under G. The
parameter "c" is a set of capacity and other constraints associated
with E and V. We will refer to G as the "base" network topology.
Let H = (U, F, d) be the induced MPLS graph, where U is a subset of
V representing the set of LSRs in the network, or more precisely the
set of LSRs that are the endpoints of at least one LSP. Here, F is
the set of LSPs, so that for x and y in U, the object (x, y) is in F
if there is an LSP with x and y as endpoints. The parameter "d" is
the set of demands and restrictions associated with F. Evidently, H
is a directed graph. It can be seen that H depends on the
transitivity characteristics of G.
3.2 The Fundamental Problem of Traffic Engineering Over MPLS
There are basically three fundamental problems that relate to Traffic
Engineering over MPLS.
- The first problem concerns how to map packets onto forwarding
- The second problem concerns how to map forwarding equivalence
classes onto traffic trunks.
- The third problem concerns how to map traffic trunks onto the
physical network topology through label switched paths.
Here, we do
This document is not concern ourselves with focusing on the first two aspects of
the problem problems listed.
(even-though they are quite important). Instead, the remainder of
this manuscript will focus on the capabilities that permit the third
mapping function to be performed in a manner that
results resulting in efficient
and reliable network operations. This is really the problem of
mapping an induced MPLS graph (H) onto the "base" network topology
4.0 Augmented Capabilities for Traffic Engineering Over MPLS
The previous sections reviewed the basic functions of Traffic
Engineering in the contemporary Internet. The applicability of MPLS
to that activity was also discussed. The remaining sections of this
manuscript describe the functional capabilities required to fully
support Traffic Engineering over MPLS in large networks.
The proposed capabilities consist of:
1. A set of attributes associated with traffic trunks which
collectively specify their behavioral characteristics.
Some of these attributes have already been suggested in
 within the context of traffic management for MPLS.
2. A set of attributes associated with resources which constrain
the placement of traffic trunks through them. These can also be
viewed as topology attribute constraints.
3. A "constraint based "constraint-based routing" (QoS routing) framework which is used to select
paths for traffic trunks subject to constraints imposed by items
1) and 2) above. The constraint based constraint-based routing framework need does not
have to be part of MPLS. However, the two need to be tightly
The attributes associated with traffic trunks and resources, as well
as parameters associated with routing, collectively represent the
control variables which can be modified either through administrative
action or automatically through automated agents to drive the network to a desired
In an operational network, it is highly desirable that these
attributes can be dynamically modified online by an operator without
adversely disrupting network operations.
5.0 Traffic Trunk Attributes and Characteristics
This section describes the desirable attributes which can be
associated with traffic trunks to influence their behavioral
First, the basic properties of traffic trunks as (as used in this
manuscript) are summarized below:
- A traffic trunk is an *aggregate* of traffic flows belonging
to the same class. In some contexts, it may be desirable to
relax this definition and allow traffic trunks to include
multi-class traffic aggregates.
- In a single class service model, such as the current Internet,
a traffic trunk could encapsulate all of the traffic between an
ingress LSR and an egress LSR, or subsets thereof.
- Traffic trunks are routable objects (similar to ATM VCs) VCs).
- A traffic trunk is distinct from the LSP through which it
traverses. In operational contexts, a traffic trunk can be
moved from one path onto another.
- A traffic trunk is unidirectional.
In practice, a traffic trunk can be characterized by its ingress and
egress LSRs, the forwarding equivalence class which is mapped onto
it, and a set of attributes which determine its behavioral
Two basic issues are of particular significance: (1) parameterization
of traffic trunks and (2) path placement and maintenance rules for
5.1 Bidirectional Traffic Trunks
Although traffic trunks are conceptually unidirectional, in many
practical contexts, it is useful to simultaneously instantiate two
traffic trunks with the same endpoints, but which carry packets in
opposite directions. The two traffic trunks are logically coupled
together. That is, one One trunk, called the forward trunk, carries traffic from
an originating node to a destination node, while the node. The other trunk, called
the backward trunk, carries traffic from the destination node to the
originating node. We refer to the amalgamation of two such traffic
trunks as one bidirectional traffic trunk (BTT) if the following two
- Both traffic trunks are instantiated through an atomic action at
one LSR, called the originator node. node, or through an atomic action
at a network management station.
- Neither of the composite traffic trunks can exist without the
other. That is, both are instantiated and destroyed together.
It is also useful to consider the
The topological properties of BTTs. BTTs should also be considered. A BTT
can be topologically symmetric or topologically asymmetric. A BTT is
said to be "topologically symmetric" if its constituent traffic
trunks are routed through the same physical path, even-though even though they
operate in opposite directions. Otherwise, if If, however, the component traffic
trunks are routed through different physical paths, then the BTT is
said to be "topologically asymmetric."
It should be noted that bidirectional traffic trunks are merely an
administrative convenience. In practice, most of the traffic engineering
functions can be implemented using only unidirectional traffic
5.2 Basic Operations on Traffic Trunks
In the following, we summarize the
The basic operations on traffic trunks which are significant for to Traffic
Engineering purposes. purposes are summarized below.
- Establish: Create To create an instance of a traffic trunk.
- Activate: Cause To cause a traffic trunk to start passing traffic.
The establishment and activation of a traffic trunk are
logically separate events. Although, in practice they can They may, however, be implemented
or invoked as one atomic action.
- Deactivate: Cause To cause a traffic trunk to stop passing traffic.
- Modify Attributes: Cause To cause the attributes of a traffic trunk
to be modified.
- Reroute: Cause To cause a traffic trunk to change its route. This
can be done through administrative action or automatically
by the underlying protocols.
- Destroy: Remove To remove an instance of a traffic trunk from the
network and reclaim all resources allocated to it. Such
resources include label space, space and possibly available bandwidth.
The above are considered the basic operations on traffic trunks.
Additional operations are also possible such as policing, traffic
shaping, policing and so forth. traffic
5.3 Accounting and Performance Monitoring
Accounting and performance monitoring capabilities are very important for purposes of
to the billing and traffic characterization. The billing aspect of accounting was
discussed in . From a Traffic Engineering perspective,
performance characterization functions. Performance
statistics obtained from an accounting system and performance monitoring
systems can be used for traffic characterization, performance
optimization, and capacity planning. planning within the Traffic Engineering
The capability to obtain statistics at the traffic trunk level is so
important that it should be considered an essential requirement for
Traffic Engineering over MPLS.
5.4 Basic Traffic Engineering Attributes of Traffic Trunks
An attribute of a traffic trunk is a parameter assigned to it which
influences its behavioral characteristics.
Attributes can be explicitly assigned to traffic trunks through
administration action or they can be implicitly assigned by the
underlying protocols when packets are classified and mapped into
equivalence classes at the ingress to an MPLS domain. Regardless of
how the attributes were originally assigned, for Traffic Engineering
purposes, it should be possible to administratively modify such
The basic attributes of traffic trunks which are particularly significant for
Traffic Engineering are itemized below. Some, of these attributes
have already been included under the traffic management framework
- Traffic parameter attributes
- Generic Path selection and maintenance attributes
- Priority attribute
- Preemption attribute
- Resilience attribute
- Policing attribute
The combination of traffic parameters and policing attributes is
analogous to usage parameter control in ATM networks. Also, most Most of the above
attributes listed above have analogs in well established
technologies. Consequently, it should be relatively straight forward
to map the traffic trunk attributes onto many existing switching and
Priority and preemption can be regarded as relational attributes
because they express certain binary relations between traffic trunks.
Conceptually, these binary relations determine the manner in which
traffic trunks interact with each other as they compete for network
resources during path establishment and path maintenance.
5.5 Traffic parameter attributes
Traffic parameters can be used to capture the characteristics of the
traffic streams (or more precisely the forwarding equivalence class) which are
to be transported through the traffic trunk. Such characteristics might may
include peak rates, average rates, permissible burst size, etc. From
a traffic engineering perspective, the traffic parameters are
significant because they indicate the resource requirements of the
traffic trunk. This is useful for resource allocation and congestion
avoidance through anticipatory policies.
For purposes the purpose of bandwidth allocation, a single canonical value of
bandwidth requirements can be computed from a traffic trunk's traffic
parameters. Techniques for performing such these computations are already well known; for
known. One example of this is the theory of effective
bandwidth, and such like. bandwidth.
5.6 Generic Path Selection and Management Attributes
Generic path selection and management attributes define the rules for
selecting the route taken by a traffic trunk, and trunk as well as the rules for
maintenance of paths that are already established.
Paths can either be computed automatically by the underlying routing
protocols or they can be defined administratively by a network
operator. If there are no resource requirements or restrictions are
associated with a traffic trunk, then a topology driven protocol can
be used to select its path. However, if there are resource requirements or
policy restrictions, restrictions exist, then a constraint based constraint-based routing scheme must
should be used for path selection.
In Section 7, we describe a constraint based constraint-based routing framework which can
automatically compute paths subject to a set of constraints. constraints is
described. Issues that pertain pertaining to explicit paths instantiated through
administrative action are discussed in Section 5.6.1 below.
Path management concerns all aspects that pertain pertaining to the maintenance of
paths traversed by traffic trunks. In some operational contexts, it
is desirable that an MPLS implementation should be able to can dynamically reconfigure
itself, to adapt to some notion of change in "system state."
Adaptivity and resilience are aspects of dynamic path management.
To guide the path selection and management process, a set of
attributes are required. In the remainder of this sub-section,
we describe the The basic attributes and behavioral
characteristics associated with traffic trunk path selection and management.
management are described in the remainder of this sub-section.
5.6.1 Administratively Specified Explicit Paths
An administratively specified explicit path for a traffic trunk is
one which is configured through operator action. An administratively
specified path can be completely specified or partially specified. A
path is completely specified if all of the required hops between the
endpoints are indicated. A path is partially specified if only a
subset of intermediate hops are indicated. In this case, the
underlying protocols are required to complete the path. Due to
operator errors, an administratively specified path can be
inconsistent or illogical. The underlying protocols should be able to
detect such inconsistencies and provide appropriate feedback.
A "path preference rule" attribute should be associated with
administratively specified explicit paths. A path preference rule
attribute is a binary variable which indicates whether the
administratively configured explicit path is "mandatory" or
If an administratively specified explicit path is selected with a
"mandatory attribute, then that path (and only that path) must be
used. If a mandatory path is topological infeasible (e.g. the two
endpoints are topologically partitioned), or if the path cannot be
instantiated because the available resources are inadequate, then the
path setup process fails. In other words, if a path is specified as
mandatory, then an alternate path cannot be used whatsoever; regardless of
prevailing circumstance. A mandatory path which is successfully
instantiated is also implicitly pinned. Once the path is instantiated
it cannot be changed except through deletion and instantiation of a
On the other hand,
However, if an administratively specified explicit path is selected
with a "non-mandatory" preference rule attribute value, then the path
should be used if feasible. Otherwise, an alternate path can be
chosen instead by the underlying protocols.
5.6.2 Hierarchy of Preference Rules For Multi-Paths
In some practical contexts, it is can be useful to be able have the ability to
administratively specify a set of candidate explicit paths for a
given traffic trunk and define a hierarchy of preference relations on
the paths. During path establishment, the preference rules are
applied to select a suitable path from the candidate list. Also,
under failure scenarios the preference rules are applied to select an
alternate path from the candidate list.
5.6.3 Resource Class Affinity Attributes
Resource class affinity attributes associated with a traffic trunk
can be used to specify the class of resources (see Section 6) which
are to be explicitly included or excluded from the path of the
traffic trunk. These are policy attributes which can be used to
impose additional constraints on the path traversed by a given
traffic trunk. Resource class affinity attributes for a traffic can
be specified as a sequence of tuples:
<resource-class, affinity>; <resource-class, affinity>; ..
The resource-class parameter identifies a resource class for which an
affinity relationship is defined with respect to the traffic trunk.
The affinity parameter indicates the affinity relationship; that is,
whether members of the resource class are to be included or excluded
from the path of the traffic trunk. Specifically, the affinity
parameter is MAY be a binary variable which takes one of the following
values: (1) explicit inclusion, and (2) explicit exclusion.
If the affinity attribute is a binary variable, it is also may be possible to
use Boolean expressions to specify the resource class affinities
associated with a given traffic trunk.
If no resource class affinity attributes are specified, then a "don't
care" affinity relationship is assumed to hold between the traffic
trunk and all resources. That is, there is no requirement to
explicitly include or exclude any resources from the traffic trunk's
path. This should be the default in practice.
Resource class affinity attributes are very useful and powerful
constructs because they can be used to implement a variety of
policies. For example, they can be used to contain certain traffic
trunks within specific topological regions of the network.
A "constraint based "constraint-based routing" framework (see section 7.0) can be used
to compute an explicit path for a traffic trunk subject to resource
class affinity constraints in the following manner:
1. For explicit inclusion, prune all resources which do not belong belonging
to the specified classes prior to performing path computation.
2. For explicit exclusion, prune all resources which belong belonging to the
specified classes before performing path placement computations.
5.6.4 Adaptivity Attribute
Network characteristics and state change over time. For example, new
resources become available, failed resources become reactivated, and
allocated resources become deallocated; in general deallocated. In general, sometimes more
efficient paths become available. Therefor, Therefore, from a Traffic
Engineering perspective, it is necessary to have administrative
control parameters that can be used to specify how traffic trunks
respond to this dynamism. In some scenarios, it might be desirable to
dynamically change the paths of some certain traffic trunks in response to
changes in network state. This process is called re-optimization. In many
other scenarios, re-optimization might be highly very undesirable.
An Adaptivity attribute is a part of the path maintenance parameters
associated with traffic trunks. The adaptivity attribute associated
with a traffic trunk indicates whether the trunk is subject to
optimization. That is, an adaptivity attribute is a binary variable
which takes one of the following values: (1) permit re-optimization
and (2) disable re-optimization.
If re-optimization is enabled, then a traffic trunk can be rerouted
through different paths by the underlying protocols in response to
changes in network state (primarily changes in resource
availability). Conversely, if re-optimization is disabled, then the
traffic trunk is "pinned" to its established path and cannot be
rerouted in response to changes in network state.
Stability is a major concern when re-optimization is permitted. To
promote stability, an MPLS implementation should not be too reactive
to the evolutionary dynamics of the network. At the same time, it
must adapt fast enough so that optimal use can be made of network
assets. This necessarily implies that the frequency of re-optimization should be
administratively configurable to allow for tuning.
It is to be noted that re-optimization is distinct from resilience. A
different attribute is used to specify the resilience characteristics
of a traffic trunk (see section 5.9). In practice, it would seem
reasonable to expect traffic trunks which
are subject to re-optimization to be
implicitly resilient to failures along their paths. However, a
traffic trunk which is not subject to re-optimization and whose path
is not administratively specified with a "mandatory" attribute can
also be required to be resilient to link and node failures along its
Formally, it can be stated that adaptivity to state evolution through
re-optimization implies resilience to failures, whereas resilience to
failures does not imply general adaptivity through re-optimization to
5.6.5 Load Distribution Across Parallel Traffic Trunks
Load distribution across multiple parallel traffic trunks between two
nodes is an important consideration. In many practical contexts, the
aggregate traffic between two nodes might may be such that no single link
(hence no single path) can carry the load. However, the aggregate
flow might be less than the maximum permissible flow across a "min-cut" "min-
cut" that partitions the two nodes. In this case, the only feasible
solution is to appropriately divide the aggregate traffic into sub-streams sub-
streams and route the sub-streams through multiple paths between the
In an MPLS domain, this problem can be addressed by instantiating
multiple traffic trunks between the two nodes, such that each traffic
trunk carries a proportion of the aggregate traffic. Therefor, Therefore, a
flexible means of load assignment to multiple parallel traffic trunks
carrying traffic of the same class between a pair of nodes is required.
Specifically, from an operational perspective, in situations where
parallel traffic trunks are warranted, it would be useful to have
some attribute that can be used to indicate the relative proportion
of traffic to be carried by each traffic trunk. The underlying
protocols will then map the load onto the traffic trunks according to
the specified proportions. It is also, generally desirable to
maintain packet ordering between packets belong to the same micro-flow micro-
flow (same source address, destination address, and port number).
5.7 Priority attribute
The priority attribute defines the relative importance of traffic
trunks. If a constraint based constraint-based routing framework is used with MPLS,
then priorities become very important because they can be used to
determine the order in which path selection is done for traffic
trunks at connection establishment and under fault scenarios.
Priorities are also important in implementations that permit
preemption because they can be used to impose a partial order on the
set of traffic trunks according to which preemptive policies can be
5.8 Preemption attribute
The preemption attribute determines whether a traffic trunk can
preempt another traffic trunk from a given path, and whether another
traffic trunk can preempt a specific traffic trunk. Preemption is
useful for both traffic oriented and resource oriented performance
objectives. Within a differentiated services environment, preemption Preemption can used to assure that high priority traffic
trunks can always be routed through relatively favorable paths. paths within
a differentiated services environment.
Preemption can also be used to implement various prioritized
restoration policies following fault events.
The preemption attribute can be used to specify four preempt modes
for a traffic trunk: (1) preemptor enabled, (2) non-preemptor, (3)
preemptable, and (4) non-preemptable. A preemptor enabled traffic
trunk can preempt lower priority traffic trunks
which are designated as
preemptable. A traffic trunk which is specified as non-preemptable cannot be
preempted by any other trunks, regardless of relative priorities. A
traffic trunk which is designated as preemptable can be preempted by higher
priority trunks which are preemptor enabled.
It is trivial to see that some of the preempt modes are mutually
exclusive. Using the numbering scheme depicted above, the feasible
preempt mode combinations for a given traffic trunk are as follows:
(1, 3), (1, 4), (2, 3), and (2, 4). The (2, 4) combination should be
A traffic trunk, say "A", can preempt another traffic trunk, say "B",
only if *all* of the following five conditions hold: (i) "A" has a
relatively higher priority than "B", (ii) "A" contends for a resource
utilized by "B", (iii) the resource cannot concurrently accommodate
"A" and "B" based on some certain decision criteria, (iv) "A" is preemptor
enabled, and (v) "B" is preemptable.
Although useful, preemption
Preemption is not considered a mandatory attribute under the current
best effort Internet service model. model although it is useful. However, in
a differentiated services scenario, the need for preemption becomes
more compelling. Moreover, in the emerging optical internetworking
architectures, where some protection and restoration functions
might may be
migrated from the optical layer to data network elements (such as
gigabit and terabit label switching routers) to reduce costs, preemption
preemptive strategies can be used to reduce the restoration time for
high priority traffic trunks under fault conditions.
5.9 Resilience Attribute
The resilience attribute determines the behavior of a traffic trunk
under fault conditions. That is, when a fault occurs along the path
through which the traffic trunk traverses. The following are the basic
problems that need to be addressed under such circumstances: (1) fault
detection, (2) failure notification, (3) recovery and service
restoration. Obviously, an MPLS implementation will have to
incorporate mechanisms to address these issues.
Many recovery policies can be specified for traffic trunks whose
established paths are impacted by faults. The following are a
of feasible schemes:
1. Do not reroute the traffic trunk. For example, a survivability
scheme might may already be in place, provisioned through an
alternate mechanism, which guarantees service continuity
under failure scenarios without the need to reroute traffic
trunks. An example of such an alternate scheme (certainly there
many others), others exist), is a situation whereby multiple parallel
label switched paths are provisioned between two nodes, and
function in a manner such that failure of one LSP causes the
traffic trunk placed on it to be dispersed between mapped onto the remaining LSPs. LSPs
according to some well defined policy.
2. Reroute through a feasible path with enough resources. If none
exists, then do not reroute.
3. Reroute through any available path regardless of resource
4. Many other schemes are possible; possible including some of which might be
combinations of the above.
A "basic" resilience attribute indicates the recovery procedure to be
applied to traffic trunks whose paths are impacted by faults.
Specifically, a "basic" resilience attribute is a binary variable
which determines whether the target traffic trunk is to be rerouted
when segments of its path fail. "Extended" resilience attributes can
be used to specify detailed actions to be taken under fault
scenarios. For example, an extended resilience attribute might
specify a set of alternate paths to use under fault conditions, and as
well as the rules that govern the relative preference of each
Resilience attributes mandate close interaction between MPLS and
5.10 Policing attribute
The policing attribute determine determines the actions that should be taken by
the underlying protocols when a traffic trunk becomes non-compliant.
That is, when a traffic trunk exceeds its contract as specified in
the traffic parameters. In general, Generally, policing attributes can indicate
whether a non-conformant traffic trunk is to be rate limited, tagged,
or simply forwarded without any policing action. This aspect is discussed in the MPLS traffic management
draft . If policing is
used, then adaptations of established algorithms such as the ATM
Forum's GCRA  can be used to perform this function.
Policing is necessary in many operational scenarios, but is quite
undesirable in many some others. In general, it is usually desirable to
police at the ingress to a network (to enforce compliance with
service level agreements) and to minimize policing within the core,
except when capacity constraints dictate otherwise.
Therefore, from a Traffic Engineering perspective, it is necessary to
be able to administratively enable or disable traffic policing for
each traffic trunk.
6.0 Resource Attributes
Resource attributes are part of the topology state parameters, which
are used to constrain the routing of traffic trunks through specific
6.1 Maximum Allocation Multiplier
The maximum allocation multiplier (MAM) of a resource is an
administratively configurable attribute which determines the
proportion of the resource that is available for allocation to
traffic trunks. This attribute is mostly applicable to link
bandwidth. However, in general, it can also be applied to buffer resources on
LSRs. The concept of MAM is analogous to the concepts of subscription
and booking factors in frame relay and ATM networks.
It is possible to choose
The values of the MAM such can be chosen so that a resource can be under-allocated under-
allocated or over-allocated. A resource is said to be
allocated if the aggregate demands of all traffic trunks (as
expressed in the trunk traffic parameters) that can be allocated to
it is are always less than the capacity of the resource. A resource is
said to be over-allocated if the aggregate demands of all traffic
trunks allocated to it can exceed the capacity of the resource.
Under-allocation can be used to bound the utilization of resources.
However,the situation under MPLS is more complex than in circuit
switched schemes because under MPLS, some flows can be routed via
conventional hop by hop protocols (also via explicit paths) without
consideration for resource constraints.
Over-allocation can be used to take advantage of the statistical
characteristics of traffic in order to implement more efficient
resource allocation policies. In particular, over-allocation can be
used in situations where the peak demands of traffic trunks do not
coincide in time.
6.2 Resource Class Attribute
Resource class attributes are administratively assigned parameters
which express some notion of "class" for resources. Resource class
attributes can be viewed as "colors" which are assigned to
resources, resources such that
the set of resources with the same "color" conceptually belong to the
same class. Resource class attributes can be used to implement a
variety of policies. The key resources of interest here are links.
When applied to links, the resource class attribute effectively
becomes become an aspect of the "link state" parameters.
From a Traffic Engineering perspective, the
The concept of resource class attributes is a powerful abstraction, which abstraction.
From a Traffic Engineering perspective, it can be used to implement a lot of
many policies with regard to both traffic and resource oriented
performance optimization. Specifically, resource class attributes can
be used to:
1. Apply uniform policies to a set of resources which need that do not need
to be in the same topological region.
2. Specify the relative preference of sets of resources for
path placement of traffic trunks.
3. Explicitly restrict the placement of traffic trunks
to specific subsets of resources.
4. Implement generalized inclusion / exclusion policies.
5. Enforce traffic locality containment policies. That is is,
policies that seek to contain local traffic within
specific topological regions of the network.
Additionally, resource class attributes can be used for
In general, a resource can be assigned more than one resource class
attribute. For example, all of the OC-48 links in a given network
be assigned a distinguished resource class attribute, and attribute. The subsets of
OC-48 links might which exist with a given abstraction domain of the
network may be assigned additional resource class attributes in order
to implement specific containment policies, or to architect the
network in a certain manner.
7.0 Constraint Based Constraint-Based Routing
This section discusses the issues that pertain pertaining to constraint based constraint-based
routing in MPLS domains. In contemporary terminology, constraint constraint-
based routing is often referred to as "QoS Routing" see [5,6,7,10].
However, we prefer
This document uses the term "constraint based routing," as "constraint-based routing" however,
because it more
aptly better captures the envisaged functionality; functionality envisioned, which in general
generally encompasses QoS routing as a subset.
constraint-based routing enables a demand driven, resource
reservation aware, routing paradigm to co-exist with current topology
driven hop by hop Internet interior gateway protocols.
A constraint based constraint-based routing framework uses the following as input:
- The attributes associated with traffic trunks.
- The attributes associated with resources.
- Other topology state information.
Based on this information, a constraint based constraint-based routing process on each
node automatically computes explicit routes for each traffic trunk that originates
originating from the node. In this case, an explicit route for each
traffic trunk is a specification of a label switched path that
satisfies the demand requirements expressed in the trunk's
attributes, subject to constraints imposed by resource
administrative policy, and other topology state information.
A constraint based constraint-based routing framework can greatly reduce the level of
manual configuration and intervention required to actualize Traffic
In practice, the Traffic Engineer Engineer, an operator, or even an operator automaton
administratively specify the endpoints of a traffic trunk and assign a set of
attributes to the trunk which encapsulate the performance
expectations and behavioral characteristics of the trunk. The
constraint-based routing framework is then expected to find a
feasible path that satisfies to satisfy the expectations. If necessary, the Traffic
Engineer or a traffic engineer engineering support system can then use manually
administratively configured explicit routes to perform fine grained
7.1 Basic Features of Constraint Based Constraint-Based Routing
A constraint based constraint-based routing framework should be able to at least have the
capability to automatically obtain a basic feasible solution to the
traffic trunk path placement problem.
In general, the constraint based constraint-based routing problem is known to be
intractable for most realistic constraints. However, in practice, a
very simple well known heuristic (see e.g. ) ) can be used to find a
feasible path if one exists:
- First prune resources that do not satisfy the requirements of
the traffic trunk attributes.
- Next, run a shortest path algorithm on the residual graph.
It is easy to see that
Clearly, if a feasible path exists, exists for a single traffic trunk, then
the above simple procedure will find it. Additional rules can be
specified to break ties, ties and perform further optimizations. In
general, ties should be broken so that congestion is minimized. When
multiple traffic trunks are to be routed, however, it can be shown
that the above algorithm may not always find a mapping, even when a
feasible mapping exists.
7.2 Implementation Considerations
Many commercial implementations of frame relay and ATM switches
already support some notion of constraint based constraint-based routing. For such
devices or for the novel MPLS centric contraptions devised therefrom,
it should be relatively easy to extend the current constraint based constraint-based
routing implementations to accommodate the peculiar requirements of
For routers that use topology driven hop by hop IGPs, constraint constraint-
based routing can be incorporated in at least one of two ways:
1. Extend By extending the current IGP protocols such as OSPF and IS-IS to
constraint based constraint-based routing. Effort is already underway to
provide such extensions to OSPF (see [5,7]).
2. Add By adding a constraint based constraint-based routing process to each router which
can co-exist with current IGPs. This scenario is depicted
in Figure 1.
| Management Interface |
| | |
------------ ------------------ --------------
| MPLS |<->| Constraint Based Constraint-Based | | Conventional |
| | | Routing Process | | IGP Process |
------------ ------------------ --------------
| Resource Attribute | | Link State |
| Availability Database | | Database |
Figure 1. Constraint Based Constraint-Based Routing Process on Layer 3 LSR
There are many important details associated with implementing
constraint-based routing on Layer 3 devices which we do not discuss
here. These include the following:
- Mechanisms for exchange of topology state information
(resource availability information, link state information,
resource attribute information) between constraint based constraint-based
- Mechanisms for maintenance of topology state information.
- Interaction between constraint based constraint-based routing processes and
conventional IGP processes.
- Mechanisms to accommodate the adaptivity requirements of
- Mechanisms to accommodate the resilience and survivability
requirements of traffic trunks.
In summary, constraint based constraint-based routing assists in performance
optimization of operational networks by automatically finding
feasible paths that satisfy a set of constraints for traffic trunks.
It can drastically reduce the amount of manual administrative explicit path
configuration and manual intervention required to achieve Traffic
This manuscript presented a set of requirements for Traffic
Engineering over MPLS. A number of Many capabilities were described aimed at
enhancing the applicability of MPLS to Traffic Engineering in the
It should be noted that some of the issues described here can be
addressed by incorporating a minimal set of building blocks into
MPLS, and then using a network management superstructure to extend
the functionality in order to realize the requirements. Also, the
constraint-based routing framework need does not have to be part of the
core MPLS specifications. However, MPLS does require some interaction
constraint based constraint-based routing framework in order to meet the
9.0 Security Considerations
This document does not introduce new security issues beyond those
inherent in MPLS and may use the same mechanisms proposed for this
technology. It is, however, specifically important that manipulation
of administratively configurable parameters be executed in a secure
manner by authorized entities.
 E. Rosen, A. Viswanathan, R. Callon, "A Proposed Architecture
for MPLS", Work in Progress.
 R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,
A. Viswanathan, "A Framework for Multiprotocol Label
Switching", Work in Progress.
 T. Li and Y. Rekhter, "Provider Architecture for Differentiated
Services and Traffic Engineering (PASTE)," RFC 2430,
 Y. Rekhter, B. Davie, D. Katz, E. Rosen, G. Swallow, "Cisco
Systems' Tag Switching Architecture - Overview", RFC 2105,
 Z. Zhang, C. Sanchez, B. Salkewicz, E. Crawley "Quality of
Service Extensions to OSPF", Work in Progress.
 E. Crawley, R. Nair, B. Rajagopalan, H. Sandick, "A framework
for QoS Based Routing in the Internet," RFC 2386, August 1998
 R. Guerin, S. Kamat, A. Orda, T. Przygienda, D. Williams,
"QoS Routing Mechanisms and OSPF Extensions," Work in Progress.
 C. Yang and A. Reddy, "A Taxonomy for Congestion Control
Algorithms in Packet Switching Networks," IEEE Network
Magazine, Volume 9, Number 5, July/August 1995,
 P. Vaananen and R. Ravikanth, "A Framework for Traffic
Management in MPLS Networks," Work in Progress.
 W. Lee, M. Hluchyi, and P. Humblet, "Routing Subject to
Quality of Service Constraints in Integrated Communication
Networks," IEEE Network, July 1995, pp 46-55.
 ATM Forum, "Traffic Management Specification: Version 4.0"
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
The authors would like to thank Yakov Rekhter for his review of an
earlier draft of this document. The authors would also like to thank
Louis Mamakos and Bill Barns for their helpful suggestions, and
Curtis Villamizar for providing some useful feedback.
12.0 AUTHORS' ADDRESSES
Daniel O. Awduche Joe Malcolm Johnson Agogbua
UUNET Worldcom UUNET Worldcom UUNET Worldcom
3060 Williams Drive 3060 Williams Drive (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Voice: +1 703-208-5277
UUNET (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Voice: +1 703-206-5895
UUNET (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Voice: +1 703-206-5794
Mike O'Dell Jim McManus
UUNET Worldcom UUNET Worldcom
3060 Williams Drive (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Voice: +1 703-206-5890
UUNET (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Voice: +1 703-206-5607