Francois Le Faucheur, Editor
Cisco Systems, Inc.
Waisum Lai, Co-editor
AT&T
IETF Internet Draft
Expires: August, 2003
Document: draft-ietf-tewg-diff-te-reqts-07.txt February 2003
Requirements for support of
Diff-Serv-aware MPLS Traffic Engineering
Status of this Memo
This document is an 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-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 as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document presents Service Provider requirements for support of
Diff-Serv aware MPLS Traffic Engineering (DS-TE).
Its objective is to provide guidance for the definition, selection
and specification of a technical solution addressing these
requirements. Specification for this solution itself is outside the
scope of this document.
A problem statement is first provided. Then, the document describes
example applications scenarios identified by Service Providers where
existing MPLS Traffic Engineering mechanisms fall short and Diff-
Serv-aware Traffic Engineering can address the needs. The detailed
requirements that need to be addressed by the technical solution are
also reviewed. Finally, the document identifies the evaluation
criteria that should be considered for selection and definition of
the technical solution.
Le Faucheur, et. al 1
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
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].
1. Introduction
1.1. Problem Statement
Diff-Serv is used by some Service Providers to achieve scalable
network designs supporting multiple classes of services.
In some such Diff-Serv networks where optimization of transmission
resources on a network-wide basis is not sought, MPLS Traffic
Engineering (TE) mechanisms may simply not be used.
In other networks, where optimization of transmission resources is
sought, Diff-Serv mechanisms [DIFF-MPLS] may be complemented by
MPLS Traffic Engineering mechanisms [TE-REQ] [ISIS-TE] [OSPF-TE]
[RSVP-TE] [CR-LDP] which operate on an aggregate basis across all
Diff-Serv classes of service. In this case, Diff-Serv and MPLS TE
both provide their respective benefits.
Where fine-grained optimization of transmission resources is sought,
it may be desirable to perform traffic engineering at a per-class
level instead of an aggregate level, in order to further enhance
networks in performance and efficiency as discussed in [TEWG-FW]. By
mapping the traffic from a given Diff-Serv class of service on a
separate LSP, it allows this traffic to utilize resources available
to the given class on both shortest paths and non-shortest paths and
follow paths that meet engineering constraints which are specific to
the given class. This is what we refer to as "Diff-Serv-aware
Traffic Engineering (DS-TE)".
This document focuses exclusively on the specific environments which
would benefit from DS-TE. Some examples include:
- networks where bandwidth is scarce (e.g. transcontinental
networks)
- networks with significant amounts of delay-sensitive traffic
- networks where the relative proportion of traffic across
classes of service is not uniform
This document focuses on intra-domain operation. Inter-domain
operation is not considered.
1.2. Definitions
Le Faucheur et. al 2
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
For the convenience of the reader, relevant Diff-Serv ([DIFF-ARCH],
[DIFF-NEW] and [DIFF-PDB]) definitions are repeated herein.
Behavior Aggregate (BA): a collection of packets with the same
(Diff-Serv) codepoint crossing a link in a particular direction.
Per-Hop-Behavior (PHB): the externally observable forwarding
behavior applied at a DS-compliant node to a Diff-Serv behavior
aggregate.
PHB Scheduling Class (PSC): A PHB group for which a common
constraint is that ordering of at least those packets belonging
to the same microflow must be preserved.
Ordered Aggregate (OA): a set of BAs that share an ordering
constraint. The set of PHBs that are applied to this set of
Behavior Aggregates constitutes a PHB scheduling class.
Traffic Aggregate (TA): a collection of packets with a codepoint
that maps to the same PHB, usually in a DS domain or some subset
of a DS domain. A traffic aggregate marked for the foo PHB is
referred to as the "foo traffic aggregate" or "foo aggregate"
interchangeably. This generalizes the concept of Behavior
Aggregate from a link to a network.
Per-Domain Behavior (PDB): the expected treatment that an
identifiable or target group of packets will receive from "edge-
to-edge" of a DS domain. A particular PHB (or, if applicable,
list of PHBs) and traffic conditioning requirements are
associated with each PDB.
We also repeat the following definition from [TE-REQ]:
Traffic Trunk: an aggregation of traffic flows of the same class
which are placed inside a Label Switched Path.
In the context of the present document, "flows of the same class" is
to be interpreted as "flows from the same Forwarding Equivalence
Class which are to be treated equivalently from the DS-TE
perspective".
We refer to the set of TAs corresponding to the set of PHBs of a
given PSC, as a {TA}PSC. A given {TA}PSC will receive the treatment
of the PDB associated with the corresponding PSC. In this document,
we also loosely refer to a {TA}PSC as a "Diff-Serv class of service",
or a "class of service". As an example, the set of packets within a
DS domain with a codepoint that maps to the EF PHB may form one
{TA}PSC in that domain. As another example, the set of packets within
a DS domain with a codepoint that maps to the AF11 or AF12 or AF13
PHB may form another {TA}PSC in that domain.
Le Faucheur et. al 3
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
We refer to the collection of packets which belong to a given Traffic
Aggregate and are associated with a given MPLS Forwarding Equivalence
Class (FEC) ([MPLS-ARCH]) as a <FEC/TA>.
We refer to the set of <FEC/TA> whose TAs belong to a given {TA}PSC
as a <FEC/{TA}PSC>.
1.3. Mapping of traffic to LSPs
A network may have multiple Traffic Aggregates (TAs) it wishes to
service. Recalling from [DIFF-MPLS], there are several options on
how the set of <FEC/{TA}PSC> of a given FEC can be split into
Traffic Trunks for mapping onto LSPs when running MPLS Traffic
Engineering.
One option is to not split this set of <FEC/{TA}PSC> so that each
Traffic Trunk comprises traffic from all the {TA}/PSC . This option
is typically used when aggregate traffic engineering is deployed
using current MPLS TE mechanisms. In that case, all the
<FEC/{TA}PSC> of a given FEC are routed collectively according to a
single shared set of constraints and will follow the same path. Note
that the LSP transporting such a Traffic Trunk is, by definition, an
E-LSP as defined in [DIFF-MPLS].
Another option is to split the different <FEC/{TA}PSC> of a given
FEC into multiple Traffic Trunks on the basis of the {TA}PSC. In
other words, traffic from a given node to another given node, is
split based on the "classes of service", into multiple Traffic
Trunks which are transported over separate LSPs, which can
potentially follow a different path through the network. DS-TE
precisely takes advantage of this fact and indeed computes a
separate path for each LSP. In so doing, DS-TE can take into account
the specific requirements of the Traffic Trunk transported on each
LSP (e.g. bandwidth requirement, preemption priority). Moreover DS-
TE can take into account specific engineering constraints to be
enforced for these sets of Traffic Trunks (e.g. limit all Traffic
Trunks transporting a particular {TA}PSC to x% of link capacity). In
brief, DS-TE achieves per LSP constraint based routing with paths
that better match the specific objectives of the traffic forming the
corresponding Traffic Trunk.
For simplicity, and because this is the specific topic of this
document, the above paragraphs in this section only considered
splitting traffic of a given FEC into multiple Traffic Aggregates on
the basis of {TA}PSC. However, it is to be noted that, in addition
to this, traffic from every {TA}PSC may also be split into multiple
Traffic Trunks for load balancing purposes.
2. Contributing Authors
Le Faucheur et. al 4
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
This document was the collective work of several. The text and
content of this document was contributed by the editors and the co-
authors listed below. (The contact information for the editors
appears in Section 9, and is not repeated below.)
Martin Tatham Thomas Telkamp
BT Global Crossing
Adastral Park, Martlesham Heath, Oudkerkhof 51, 3512 GJ Utrecht
Ipswich IP5 3RE, UK The Netherlands
Phone: +44-1473-606349 Phone: +31 30 238 1250
Email: martin.tatham@bt.com Email: telkamp@gblx.net
David Cooper Jim Boyle
Global Crossing Protocol Driven Networks, Inc.
960 Hamlin Court 1381 Kildaire Farm Road #288
Sunnyvale, CA 94089, USA Cary, NC 27511, USA
Phone: (916) 415-0437 Phone: (919) 852-5160
Email: dcooper@gblx.net Email: jboyle@pdnets.com
Luyuan Fang Gerald R. Ash
AT&T Labs AT&T Labs
200 Laurel Avenue 200 Laurel Avenue
Middletown, New Jersey 07748, USA Middletown, New Jersey 07748,USA
Phone: (732) 420-1921 Phone: (732) 420-4578
Email: luyuanfang@att.com Email: gash@att.com
Pete Hicks Angela Chiu
CoreExpress, Inc AT&T Labs-Research
12655 Olive Blvd, Suite 500 200 Laurel Ave. Rm A5-1F13
St. Louis, MO 63141, USA Middletown, NJ 07748, USA
Phone: (314) 317-7504 Phone: (732) 420-9061
Email: pete.hicks@coreexpress.net Email: chiu@research.att.com
William Townsend Thomas D. Nadeau
Tenor Networks Cisco Systems, Inc.
100 Nagog Park 250 Apollo Drive
Acton, MA 01720, USA Chelmsford, MA 01824, USA
Phone: +1 978-264-4900 Phone: (978) 244-3051
Email:btownsend@tenornetworks.com Email: tnadeau@cisco.com
Darek Skalecki
Nortel Networks
3500 Carling Ave,
Nepean K2H 8E9,
Phone: (613) 765-2252
Email: dareks@nortelnetworks.com
3. Application Scenarios
Le Faucheur et. al 5
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
3.1. Scenario 1: Limiting Proportion of Classes on a Link
An IP/MPLS network may need to carry a significant amount of VoIP
traffic compared to its link capacity. For example, 10,000
uncompressed calls at 20ms packetization result in about 1Gbps of IP
traffic, which is significant on an OC-48c based network. In case of
topology changes such as link/node failure, VoIP traffic levels can
even approach the full bandwidth on certain links.
For delay/jitter reasons, some network administrators see it as
undesirable to carry more than a certain percentage of VoIP traffic
on any link. The rest of the available link bandwidth can be used to
route other "classes of service" corresponding to delay/jitter
insensitive traffic (e.g. Best Effort Internet traffic). The exact
determination of this "certain" percentage is outside the scope of
this requirements document.
During normal operations, the VoIP traffic should be able to preempt
other "classes of service" (if these other classes are designated as
preemptable and they have lower preemption priority),
so that it will be able to use the shortest available path, only
constrained by the maximum defined link utilization ratio/percentage
of the VoIP class.
Existing TE mechanisms only allow constraint based routing of
traffic based on a single bandwidth constraint common to all
"classes of service", which does not satisfy the needs described
here. This leads to the requirement for DS-TE to be able to enforce
a different bandwidth constraint for different "classes of service".
In the above example, the bandwidth constraint to be enforced for
VoIP traffic may be the "certain" percentage of each link capacity,
while the bandwidth constraint to be enforced for the rest of the
"classes of service" might have their own constraints or have access
to the rest of the link capacity.
3.2. Scenario 2: Maintain relative proportion of traffic
Suppose an IP/MPLS network supports 3 "classes of service". The
network administrator wants to perform Traffic Engineering to
distribute the traffic load. Assume also that proportion across
"classes of service" varies significantly depending on the
source/destination POPs.
With existing TE mechanisms, the proportion of traffic from each
"class of service" on a given link will vary depending on multiple
factors including:
- in which order the different TE-LSPs are established
- the preemption priority associated with the different TE-LSPs
- link/node failure situations
This may make it difficult or impossible for the network
administrator to configure the Diff-Serv PHBs (e.g. queue bandwidth)
Le Faucheur et. al 6
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
to ensure that each "class of service" gets the appropriate
treatment. This leads again to the requirement for DS-TE to be able
to enforce a different bandwidth constraint for different "classes
of service". This could be used to ensure that, regardless of the
order in which tunnels are routed, regardless of their preemption
priority and regardless of the failure situation, the amount of
traffic of each "class of service" routed over a link matches the
Diff-Serv scheduler configuration on that link for the corresponding
class (e.g. queue bandwidth).
As an illustration of how DS-TE would address this scenario, the
network administrator may configure the service rate of Diff-Serv
queues to (45%,35%,20%) for "classes of service" (1,2,3)
respectively. The administrator would then split the traffic into
separate Traffic Trunks for each "class of service" and associate a
bandwidth to each LSP transporting those Traffic Trunks. The network
administrator may also want to configure preemption priorities of
each LSP in order to give highest restoration priority to the
highest priority "class of service" and medium priority to the
medium "class of service". Then DS-TE could ensure that after a
failure, "class of service" 1 traffic would be rerouted with first
access at link capacity but without exceeding its service rate of
45% of the link bandwidth. "Class of service" 2 traffic would be
rerouted with second access at the link capacity but without
exceeding its allotment. Note that where "class of service" 3 is the
Best-Effort service, the requirement on DS-TE may be to ensure that
the total amount of traffic routed across all "classes of service"
does not exceed the total link capacity of 100% (as opposed to
separately limiting the amount of Best Effort traffic to 20 even if
there was little "class of service" 1 and "class of service" 2
traffic).
In this scenario, DS-TE would allow for the maintenance of a more
steady distribution of "classes of service", even during rerouting.
This would rely on the required capability of DS-TE to adjust the
amount of traffic of each "class of service" routed on a link based
on the configuration of the scheduler and the amount of bandwidth
available for each "class of service".
Alternatively, some network administrators may want to solve the
problem by having the scheduler dynamically adjusted based on the
amount of bandwidth of the LSPs admitted for each "class of
service". This is an optional additional requirement on the DS-TE
solution.
3.3. Scenario 3: Guaranteed Bandwidth Services
In addition to the Best effort service, an IP/MPLS network operator
may desire to offer a point-to-point "guaranteed bandwidth" service
whereby the provider pledges to provide a given level of performance
(bandwidth/delay/loss...) end-to-end through its network from an
ingress port to an egress port. The goal is to ensure that all the
Le Faucheur et. al 7
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
"guaranteed" traffic under the scope of a subscribed service level
specification, will be delivered within the tolerances of this
service level specification.
One approach for deploying such "guaranteed" service involves:
- dedicating a Diff-Serv PHB (or a Diff-Serv PSC as defined in
[DIFF-NEW]) to the "guaranteed" traffic
- policing guaranteed traffic on ingress against the traffic
contract and marking the "guaranteed" packets with the
corresponding DSCP/EXP value
Where very high level of performance is targeted for the
"guaranteed" service, it may be necessary to ensure that the amount
of "guaranteed" traffic remains below a given percentage of link
capacity on every link. Where the proportion of "guaranteed" traffic
is high, constraint based routing can be used to enforce such a
constraint.
However, the network operator may also want to simultaneously
perform Traffic Engineering of the rest of the traffic (i.e. non-
guaranteed traffic) which would require that constraint based
routing is also capable of enforcing a different bandwidth
constraint, which would be less stringent than the one for
guaranteed traffic.
Again, this combination of requirements can not be addressed with
existing TE mechanisms. DS-TE mechanisms allowing enforcement of a
different bandwidth constraint for guaranteed traffic and for non-
guaranteed traffic are required.
4. Detailed Requirements for DS-TE
This section specifies the functionality that the above scenarios
require out of the DS-TE solution. Actual technical protocol
mechanisms and procedures to achieve such functionality are outside
the scope of this document.
4.1. DS-TE Compatibility
Since DS-TE may impact scalability (as discussed later in this
document) and operational practices, DS-TE is expected to be used
when existing TE mechanisms combined with Diff-Serv cannot address
the network design requirements (i.e. where constraint based routing
is required and where it needs to enforce different bandwidth
constraints for different "classes of service", such as in the
scenarios described above in section 3). Where the benefits of DSTE
are only required in a topological subset of their network, some
network operators may wish to only deploy DS-TE in this topological
subset..
Thus, the DS-TE solution MUST be developed in such a way that:
Le Faucheur et. al 8
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
(i) it raises no interoperability issues with existing deployed
TE mechanisms.
(ii) it allows DS-TE deployment to the required level of
granularity and scope (e.g. only in a subset of the
topology, or only for the number of classes required in the
considered network)
4.2. Class-Types
The fundamental requirement for DS-TE is to be able to enforce
different bandwidth constraints for different sets of Traffic
Trunks.
[TEWG-FW] introduces the concept of Class-Types when discussing
operations of MPLS Traffic Engineering in a Diff-Serv environment.
We refine this definition into the following:
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.
Note that different LSPs transporting Traffic Trunks from the same
CT may be using the same or different preemption priorities as
explained in more details in section 4.4 below.
Mapping of {TA}PSC to Class-Types is flexible. Different {TA}PSC can
be mapped to different CTs, multiple {TA}PSC can be mapped to the
same CT and one {TA}PSC can be mapped to multiple CTs.
For illustration purposes, let's consider the case of a network
running 4 Diff-Serv PDBs which are respectively based on the EF PHB
[EF], the AF1x PSC [AF], the AF2x PSC and the Default (i.e. Best-
Effort) PHB [DIFF-FIELD]. The network administrator may decide to
deploy DS-TE in the following way:
o from every DS-TE Head-end to every DS-TE Tail-end, split
traffic into 4 Traffic Trunks: one for traffic of each
{TA}PSC
o because the QoS objectives for the AF1x PDB and for the AF2x
PDB may be of similar nature (e.g. both targeting low loss
albeit at different levels perhaps), the same (set of)
Bandwidth Constraint(s) may be applied collectively over the
AF1x Traffic Trunks and the AF2x Traffic Trunks. Thus, the
network administrator may only define three CTs: one for the
EF Traffic Trunks, one for the AF1x and AF2x Traffic Trunks
and one for the Best Effort Traffic Trunks.
As another example of mapping of {TA}PSC to CTs, a network operator
may split the traffic from the {TA}PSC associated with EF into two
Le Faucheur et. al 9
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
different sets of traffic trunks, so that each set of traffic trunks
is subject to different constraints on the bandwidth it can access.
In this case, two distinct CTs are defined for the EF {TA}PSC
traffic: one for the traffic subset subject to the first (set of)
bandwidth constraint(s), the other for the traffic subset subject to
the second (set of) bandwidth constraint(s).
The DS-TE solution MUST support up to 8 CTs. Those are referred to
as CTc, 0 <= c <= MaxCT-1 = 7.
The DS-TE solution MUST be able to enforce a different set of
Bandwidth Constraints for each CT.
A DS-TE implementation MUST support at least 2 CTs, and MAY support
up to 8 CTs.
In a given network, the DS-TE solution MUST NOT require the network
administrator to always deploy the maximum number of CTs. The DS-TE
solution MUST allow the network administrator to deploy only the
number of CTs actually utilized.
4.3. Bandwidth Constraints
We refer to a Bandwidth Constraint Model as the set of rules
defining:
- the maximum number of Bandwidth Constraints; and
- which CTs each Bandwidth Constraint applies to and how.
By definition of CT, each CT is assigned either a Bandwidth
Constraint, or a set of Bandwidth Constraints.
We refer to the Bandwidth Constraints as BCb, 0 <= b <= MaxBC-1
For a given Class-Type CTc, 0 <= c <= MaxCT, let us define
"Reserved(CTc)" as the sum of the bandwidth reserved by all
established LSPs which belong to CTc.
Different models of Bandwidth Constraints are conceivable for
control of the CTs.
For example, a model with one separate Bandwidth Constraint per CT
could be defined. This model is referred to as the "Maximum
Allocation Model" and is defined by:
- MaxBC= MaxCT
- for each value of b in the range 0 <= b <= (MaxCT - 1):
Reserved (CTb) <= BCb,
For illustration purposes, on a link of 100 unit of bandwidth where
three CTs are used, the network administrator might then configure
BC0=20, BC1= 50, BC2=30 such that:
- All LSPs supporting Traffic Trunks from CT2 use no more than 30
(e.g. Voice <= 30)
- All LSPs supporting Traffic Trunks from CT1 use no more than 50
(e.g. Premium Data <= 50)
Le Faucheur et. al 10
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
- All LSPs supporting Traffic Trunks from CT0 use no more than 20
(e.g. Best Effort <= 20)
As another example, a "Russian Doll" model of Bandwidth Constraints
may be defined whereby:
- MaxBC= MaxCT
- for each value of b in the range 0 <= b <= (MaxCT - 1):
SUM (Reserved (CTc)) <= BCb,
for all "c" in the range b <= c <= (MaxCT - 1)
For illustration purposes, on a link of 100 units of bandwidth where
three CTs are used, the network administrator might then configure
BC0=100, BC1= 80, BC2=60 such that:
- All LSPs supporting Traffic Trunks from CT2 use no more than 60
(e.g. Voice <= 60)
- All LSPs supporting Traffic Trunks from CT1 or CT2 use no more
than 80 (e.g. Voice + Premium Data <= 80)
- All LSPs supporting Traffic Trunks from CT0 or CT1 or CT2 use no
more than 100 (e.g. Voice + Premium Data + Best Effort <= 100).
Other Bandwidth Constraints model can also be conceived. Those could
involve arbitrary relationships between BCb and CTc. Those could
also involve additional concepts such as associating minimum
reservable bandwidth to a CT.
The DS-TE technical solution MUST have the capability to support
multiple Bandwidth Constraints models. The DS-TE technical solution
MUST specify at least one bandwidth constraint model and MAY specify
multiple Bandwidth Constraints models. Additional Bandwidth
Constraints models MAY also be specified at a later stage if deemed
useful based on operational experience from DS-TE deployments. The
choice of which (or which set of) Bandwidth Constraints model(s) is
to be supported by a given DS-TE implementation, is an
implementation choice. For simplicity, a network operator may elect
to use the same Bandwidth Constraints Model on all the links of
his/her network. However, if he/she wishes/needs to do so, the
network operator may elect to use different Bandwidth Constraints
models on different links in a given network.
Regardless of the Bandwidth Constraint Model, the DS-TE solution
MUST allow support for up to 8 BCs.
4.4. Preemption and TE-Classes
[TEWG-FW] defines the notion of preemption and preemption priority.
The DS-TE solution MUST retain full support of such preemption.
However, a network administrator preferring not to use preemption
for user traffic MUST be able to disable the preemption mechanisms
described below.
The preemption attributes defined in [TE-REQ] MUST be retained and
applicable across all Class Types. The preemption attributes of
Le Faucheur et. al 11
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
setup priority and holding priority MUST retain existing semantics,
and in particular these semantics MUST not be affected by the
Ordered Aggregate transported by the LSP or by the LSP's Class Type.
This means that if LSP1 contends with LSP2 for resources, LSP1 may
preempt LSP2 if LSP1 has a higher set-up preemption priority (i.e.
lower numerical priority value) than LSP2's holding preemption
priority regardless of LSP1's OA/CT and LSP2's OA/CT.
We introduce the following definition:
TE-Class: A pair of:
(i) a Class-Type
(ii) a preemption priority allowed for that Class-
Type. This means that an LSP transporting a
Traffic Trunk from that Class-Type can use that
preemption priority as the set-up priority, as
the holding priority or both.
Note that by definition:
- for a given Class-Type, there may be one or multiple TE-classes
using that Class-Type, each using a different preemption priority
- for a given preemption priority, there may be one or multiple TE-
Class(es) using that preemption priority, each using a different
Class-Type.
The DS-TE solution MUST allow all LSPs transporting Traffic Trunks
of a given Class-Type to use the same preemption priority. In other
words, the DS-TE solution MUST allow a Class-Type to be used by
single TE-Class. This effectively allows the network administrator
to ensure that no preemption happens within that Class-Type, when so
desired.
As an example, the DS-TE solution MUST allow the network
administrator to define a Class-Type comprising a single TE-class
using preemption 0.
The DS-TE solution MUST allow two LSPs transporting Traffic Trunks
of the same Class-Type to use different preemption priorities, and
allow the LSP with higher (numerically lower) set-up priority to
preempt the LSP with lower (numerically higher) holding priority
when they contend for resources. In other words, the DS-TE solution
MUST allow multiple TE-Classes to be defined for a given Class-Type.
This effectively allows the network administrator to enable
preemption within a Class-Type, when so desired.
As an example, the DS-TE solution MUST allow the network
administrator to define a Class-Type comprising three TE-Classes;
one using preemption 0, one using preemption 1 and one using
preemption 4.
The DS-TE solution MUST allow two LSPs transporting Traffic Trunks
from different Class-Types to use different preemption priorities,
Le Faucheur et. al 12
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
and allow the LSP with higher setup priority to preempt the one with
lower holding priority when they contend for resources.
As an example, the DS-TE solution MUST allow the network
administrator to define two Class-Types (CT0 and CT1) each
comprising two TE-Classes where say:
-one TE-Class groups CT0 and preemption 0
-one TE-Class groups CT0 and preemption 2
-one TE-Class groups CT1 and preemption 1
-one TE-Class groups CT1 and preemption 3
The network administrator would then, in particular, be able to :
- transport a CT0 Traffic Trunk over an LSP with setup priority=0
and holding priority=0
- transport a CT0 Traffic Trunk over an LSP with setup priority=2
and holding priority=0
- transport a CT1 Traffic Trunk over an LSP with setup priority=1
and holding priority=1
- transport a CT1 Traffic Trunk over an LSP with setup priority=3
and holding priority=1.
The network administrator would then, in particular, NOT be able
to :
- transport a CT0 Traffic Trunk over an LSP with setup priority=1
and holding priority=1
- transport a CT1 Traffic Trunk over an LSP with setup priority=0
and holding priority=0
The DS-TE solution MUST allow two LSPs transporting Traffic Trunks
from different Class-Types to use the same preemption priority. In
other words, the DS-TE solution MUST allow TE-classes using
different CTs to use the same preemption priority. This effectively
allows the network administrator to ensure that no preemption
happens across Class-Types, if so desired.
As an example, the DS-TE solution MUST allow the network
administrator to define three Class-Types (CT0, CT1 and CT2) each
comprising one TE-Class which uses preemption 0. In that case, no
preemption will ever occur.
Since there are 8 preemption priorities and up to 8 Class-Types,
there could theoretically be up to 64 TE-Classes in a network. This
is felt to be beyond current practical requirements. The current
practical requirement is that the DS-TE solution MUST allow support
for up to 8 TE-classes. The DS-TE solution MUST allow these TE-
classes to comprise any arbitrary subset of 8 (or less) from the
(64) possible combinations of (8) Class-Types and (8) preemption
priorities.
As with existing TE, an LSP which gets preempted is torn down at
preemption time. The Head-end of the preempted LSP may then attempt
to reestablish that LSP, which involves re-computing a path by
Le Faucheur et. al 13
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
Constraint Based Routing based on updated available bandwidth
information and then signaling for LSP establishment along the new
path. It is to be noted that there may be cases where the preempted
LSP cannot be reestablished (e.g. no possible path satisfying LSP
bandwidth constraints as well as other constraints). In such cases,
the Head-end behavior is left to implementation. It may involve
periodic attempts at reestablishing the LSP, relaxing of the LSP
constraints, or other behaviors.
4.5. Mapping of Traffic to LSPs
The DS-TE solution MUST allow operation over E-LSPs onto which a
single <FEC/{TA}PSC> is transported.
The DS-TE solution MUST allow operation over L-LSPs.
The DS-TE solution MAY allow operation over E-LSPs onto which
multiple <FEC/{TA}PSC> of a given FEC are transported, under the
condition that those multiple <FEC/{TA}PSC> can effectively be
treated by DS-TE as a single atomic traffic trunk (in particular
this means that those multiple <FEC/{TA}PSC> are routed as a whole
based on a single collective bandwidth requirement, a single
affinity attribute, a single preemption level, a single Class-Type,
...). In that case, it is also assumed that the multiple {TA}PSCs
are grouped together in a consistent manner throughout the DS-TE
domain (e.g. if <FECx/{TA}PSC1> and <FECx/{TA}PSC2> are transported
together on an E-LSP, then there will not be any L-LSP transporting
<FECy/{TA}PSC1> or <FECy/{TA}PSC2> on its own, and there will not be
any E-LSP transporting <FECz/{TA}PSC1> and/or <FECz/{TA}PSC2> with
<FECz/{TA}PSC3>).
4.6. Dynamic Adjustment of Diff-Serv PHBs
As discussed in section 3.2, the DS-TE solution MAY support
adjustment of Diff-Serv PHBs parameters (e.g. queue bandwidth) based
on the amount of TE-LSPs established for each OA/Class-Type. Such
dynamic adjustment is optional for DS-TE implementations.
Where this dynamic adjustment is supported, it MUST allow for
disabling via configuration (thus reverting to PHB treatment with
static scheduler configuration independent of DS-TE operations). It
MAY involve a number of configurable parameters which are outside
the scope of this specification. Those MAY include configurable
parameters controlling how scheduling resources (e.g. service rates)
need to be apportioned across multiple OAs when those belong to the
same Class-Type and are transported together on the same E-LSP.
Where supported, the dynamic adjustment MUST take account of the
performance requirements of each PDB when computing required
adjustments.
4.7. Overbooking
Le Faucheur et. al 14
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
Existing TE mechanisms allow overbooking to be applied on LSPs for
Constraint Based Routing and admission control. Historically this
has been achieved in TE deployment through factoring overbooking
ratios at the time of sizing the LSP bandwidth and/or at the time of
configuring the Maximum Reservable Bandwidth on links.
The DS-TE solution MUST also allow overbooking and MUST effectively
allow different overbooking ratios to be enforced for different CTs.
The DS-TE solution SHOULD optionally allow the effective overbooking
ratio of a given CT to be tweaked differently in different parts of
the network.
4.8. Restoration
With existing TE, restoration policies use standard priority
mechanisms such as, for example, the preemption priority to
effectively control the order/importance of LSPs for restoration
purposes.
The DS-TE solution MUST ensure that similar application of the use
of standard priority mechanisms for implementation of restoration
policy are not prevented since those are expected to be required for
achieving the survivability requirements of DS-TE networks.
Further discussion of restoration requirements are presented in the
output document of the TEWG Requirements Design Team [SURVIV-REQ].
5. Solution Evaluation Criteria
A range of solutions is possible for the support of the DS-TE
requirements discussed above. For example, some solutions may
require that all current TE protocols syntax (IGP, RSVP-TE, CR-LDP)
be extended in various ways. For instance, current TE protocols
could be modified to support multiple bandwidth constraints rather
than the existing single aggregate bandwidth constraint.
Alternatively, other solutions may keep the existing TE protocols
syntax unchanged but modify their semantic to allow for the multiple
bandwidth constraints.
This section identifies the evaluation criteria that MUST be used to
assess potential DS-TE solutions for selection.
5.1. Satisfying detailed requirements
The solution MUST address all the scenarios described in section 2
and satisfy all the requirements listed in section 3.
5.2. Flexibility
Le Faucheur et. al 15
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
- number of Class-Types that can be supported, compared to
number identified in Requirements section
- number of PDBs within a Class-Type
5.3. Extendibility
- how far can the solution be extended in the future if
requirements for more Class-Types are identified in the
future.
5.4. Scalability
- impact on network scalability in what is propagated,
processed, stored and computed (IGP signaling, IGP
processing, IGP database, TE-Tunnel signaling ,...).
- how does scalability impact evolve with number of Class-
Types/PDBs actually deployed in a network. In
particular, is it possible to keep overhead small for a
large networks which only use a small number of Class-
Types/PDBs, while allowing higher number of Class-
Types/PDBs in smaller networks which can bear higher
overhead)
5.5. Backward compatibility/Migration
- backward compatibility/migration with/from existing TE
mechanisms
- backward compatibility/migration when
increasing/decreasing the number of Class-Types actually
deployed in a given network.
5.6. Bandwidth Constraints Model
Work is currently in progress to investigate the performance and
trade-offs of different operational aspects of Bandwidth Constraints
models (for example see [BC-MODEL], [BC-CONS] and [MAR]). In this
investigation, at least the following criteria are expected to be
considered:
(1) addresses the scenarios in Section 2
(2) works well under both normal and overload conditions
(3) applies equally when preemption is either enabled or disabled
(4) minimizes signaling load processing requirements
(5) maximizes efficient use of the network
(6) Minimizes implementation and deployment complexity.
In selection criteria (2), "normal condition" means that the network
is attempting to establish a volume of DS-TE LSPs for which it is
designed; "overload condition" means that the network is attempting
to establish a volume of DS-TE LSPs beyond the one it is designed
for; "works well" means that under these conditions, the network
Le Faucheur et. al 16
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
should be able to sustain the expected performance, e.g., under
overload it is x times worse than its normal performance.
6. Security Considerations
The solution developed to address the DS-TE requirements defined in
this document MUST address security aspects. DS-TE does not raise
any specific additional security requirements beyond the existing
security requirements of MPLS TE and Diff-Serv. The solution MUST
ensure that the existing security mechanisms (including those
protecting against DOS attacks) of MPLS TE and Diff-Serv are not
compromised by the protocol/procedure extensions of the DS-TE
solution or otherwise MUST provide security mechanisms to address
this.
7. Acknowledgment
We thank David Allen for his help in aligning with up-to-date
Diff-Serv terminology.
8. Normative References
[AF] Heinanen, J et al., "Assured Forwarding PHB Group", RFC2597
[DIFF-ARCH] Blake et al., "An Architecture for Differentiated
Services", RFC2475.
[DIFF-FIELD] Nichols et al., "Definition of the Differentiated
Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC2474.
[MPLS-ARCH] Rosen et al., "Multiprotocol Label Switching
Architecture", RFC3031.
[DIFF-MPLS] Le Faucheur et al, "Multi-Protocol Label Switching
(MPLS) Support of Differentiated Services", RFC3270, May 2002.
[DIFF-NEW] Grossman, " New Terminology and Clarifications for
Diffserv ", RFC3260, April 2002.
[EF] Davie et al., "An Expedited Forwarding PHB (Per-Hop Behavior)",
RFC3246, March 2002.
[TEWG-FW] Awduche et al, Overview and Principles of Internet Traffic
Engineering, RFC3272, May 2002.
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over
MPLS, RFC2702, September 1999.
Le Faucheur et. al 17
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
[RFC2119] S. Bradner, Key words for use in RFCs to Indicate
Requirement Levels, RFC2119, March 1997.
9. Informative References
[CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
RFC3212, January 2002.
[DIFF-PDB] Nichols et al., "Definition of Differentiated Services
Per Domain Behaviors and Rules for their Specification", RFC3086.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-04.txt, December 2002.
[OSPF-TE] Katz et al., Traffic Engineering Extensions to OSPF,
draft-katz-yeung-ospf-traffic-09.txt, October 2002.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[SURVIV-REQ] W.S. Lai et al., "Network Hierarchy and Multilayer
Survivability", RFC3386, November 2002.
[BC-MODEL] W.S. Lai , "Bandwidth Constraint Models for Diffserv-
aware MPLS Traffic Engineering", work in progress, June 2002.
[BC-CONS] F. Le Faucheur, "Considerations on Bandwidth Constraints
Models for DS-TE", work in progress, June 2002.
[MAR] J Ash, "Max Allocation with Reservation BW Constraint Model
for MPLS/DiffServ TE", work in progress, November 2002.
10. Editors' Address:
Francois Le Faucheur
Cisco Systems, Inc.
Village d'Entreprise Green Side - Batiment T3
400, Avenue de Roumanille
06410 Biot-Sophia Antipolis, France
Phone: +33 4 97 23 26 19
Email: flefauch@cisco.com
Wai Sum Lai
AT&T Labs
200 Laurel Avenue
Middletown, New Jersey 07748, USA
Phone: (732) 420-3712
Email: wlai@att.com
Le Faucheur et. al 18
Requirements for Diff-Serv-aware Traffic Engineering Feb 2003
Le Faucheur et. al 19