Francois Le Faucheur
Thomas D. Nadeau
Cisco Systems, Inc.
Angela Chiu
AT&T
William Townsend
Tenor Networks
Darek Skalecki
Nortel Networks
IETF Internet Draft
Expires: January, 2001
Document: draft-lefaucheur-diff-te-reqts-00.txt July, 2000
Requirements for support of
Diff-Serv-aware MPLS Traffic Engineering
Status of this Memo
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Abstract
This document defines the requirements for support of Diff-Serv-
aware MPLS Traffic Engineering on a per-Class-Type basis, as
discussed in the Traffic Engineering Working Group Framework
document [TEWG-FW].
A companion document [DIFF-TE-EXT] proposes actual extensions to
OSPF, ISIS, RSVP and CR-LDP in order to meet those requirements.
Le Faucheur, et. al 1
Requirements for Diff-Serv Traffic Engineering July 2000
1. Introduction
As Diff-Serv becomes prominent in providing scalable multi-class of
services in IP networks, performing traffic engineering at a per-
class level instead of an aggregated level is needed to further
enhance networks in performance and efficiency. By mapping a traffic
trunk in a given class on a separate LSP, it allows the traffic
trunk to utilize resources available on both shortest path(s) and
non-shortest paths and follow paths that meet constraints which are
specific to the given class. It also allows each class to select the
proper protection/restoration mechanism(s) that satisfy its
survivability requirements in a cost effective manner.
Besides the set of parameters defined for the general aggregate TE
[TE-REQ], a new set of per-class parameters needs to be provided at
each LSR interface and propagated via extensions to the IGP
(ISIS/OSPF) [TEWG-FW]. Furthermore, the per-class parameters can be
aggregated into per-Class-Type parameters. The main motivation for
grouping a set of classes into a Class-Type is to improve the
scalability of the IGP link state advertisements by propagating
information on a per-Class-Type basis instead of on a per-class
basis. This approach also has the benefit of allowing better
bandwidth sharing between classes in the same Class-Type.
A Class-Type [TEWG-FW] is defined as a set of classes that satisfy
the following two conditions:
1) Classes in the same Class-Type possess common aggregate maximum
and minimum bandwidth requirements to guarantee the required
performance level.
2) There is no maximum or minimum bandwidth requirement to be
enforced at the level of an individual class within the Class-
Type. One can still implement some "priority" policies for
classes within the same Class-Type in terms of accessing the
Class-Type bandwidth (e.g. via the use of preemption
priorities).
An example of Class-Type comprising multiple Diff-Serv classes is a
low-loss Class-Type that includes both AF1-based and AF2-based
Ordering Aggregates.
Note that with per Class-Type TE, Constraint-Based Routing is
performed with bandwidth constraints on a per Class-Type basis but
LSPs may carry a single Diff-Serv class (Ordered Aggregate) with
Diff-Serv scheduling (i.e. PHB) performed separately for each class.
Diff-Serv scheduling parameters for a given class within a Class-
Type may be automatically adjusted by the LSRs based on the
bandwidth of all LSPs currently established for each class within
the Class-Type.
Le Faucheur et. al 2
Requirements for Diff-Serv Traffic Engineering July 2000
In this document, we will only discuss "per Class-Type TE" because
"per Class TE" can be viewed as a special case of per Class-Type TE
(where each Class-Type is degenerated into a single Diff-Serv
class).
This document focuses on intra-domain operations. Inter-domain
operations is for further study.
The following sections detail the requirements on OSPF/ISIS, RSVP/
CR-LDP, Constraint Based Routing and MPLS MIBs for support of MPLS
Traffic Engineering on a per-Class-Type basis.
2. Requirements for ISIS/OSPF Extensions
[OSPF-TE] and [ISIS-TE] define extensions to OSPF and ISIS for
support of (aggregate) MPLS Traffic Engineering. In this section we
define the requirements on OSPF and ISIS for support of Diff-Serv
Traffic Engineering on a per-Class-Type basis. These requirements
are expected to require further extensions to OSPF and ISIS. Such
extensions are proposed in [DIFF-TE-EXT].
Given that there are hard limits imposed by ISIS/OSPF TLVs, the TLV
space must be used frugally. An additional concern is that the
amount of information advertised by the IGP directly affects the
scalability of the solution. These considerations strongly influence
the requirements defined in this section.
As pointed out in [TEWG-FW], the IGP needs to advertise separate "TE
information" for each Class-Type. We focus now on detailing what
this "TE information" should be.
For Constraint Based Routing to be able to compute paths which
satisfy different bandwidth constraints for each Class-Type, the IGP
needs to advertise different "Unreserved Bandwidth" information for
each Class-Type. Moreover, we propose that the preemption attribute
defined in [TE-REQ] be retained for all Class-Types. Thus, the IGP
needs to advertise "Unreserved Bandwidth" at each preemption level
for each Class-Type.
For the bandwidth constraints to be effectively different for each
Class-Type, LSRs need to allow configuration for every link of a
"Maximum Reservable Bandwidth" for each Class-Type. Clearly, the
"Unreserved Bandwidth" advertised for each Class-Type takes into
account the "Maximum Reservable Bandwidth" configured for the
corresponding Class-Type. Consequently, Constraint Based Routing can
compute paths for the different Class-Types without receiving the
"Maximum Reservable Bandwidth" for each Class-Type from the IGP.
Thus we feel that the IGP need not advertise the Maximum Reservable
Bandwidth for each Class-Type. We note that the Maximum Reservable
Bandwidth for each Class-Type could have been used by Constraint
Based Routing to enhance route computation in some situations (e.g.
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Requirements for Diff-Serv Traffic Engineering July 2000
as a tie breaker), but we feel this does not justify the extra
overhead in IGP advertisement.
Current IGP extensions for (aggregate) TE [OSPF-TE][ISIS-TE] specify
advertisement of the Maximum Reservable Bandwidth for (aggregate)
TE. Note that this document does not propose that this be changed.
Other TE attributes already advertised by the IGP (i.e.
resource/color) need not be advertised per Class-Type as those will
be applicable to all Class-Types.
It is desirable to be able to avoid under-utilizing aggregate
resource. To achieve this, it is necessary to allow the sum of the
configurable Maximum Reservable Bandwidth of all Class-Types be
larger than a configurable Maximum Reservable Aggregate Bandwidth
(i.e. aggregate across all Class-Types). At the same time, it is
desirable to be able to avoid over-utilizing the aggregate resource.
To achieve this, it is necessary to be able to enforce this Maximum
Reservable Aggregate Bandwidth; in other words it is necessary to
ensure that the sum of all LSPs across all Class-Types never exceeds
the Maximum Reservable Aggregate Bandwidth.
For example, a 10Gb/s link may be configured to allow:
- Class-Type 0 (BE) to reserve up to 9 Gb/s
- Class-Type 1 (e.g. real time including EF) to reserve up to 5
Gb/s
- Class-Type 2 (eg low loss including AF1 and AF2) to reserve up
to 8 Gb/s
and at the same may be configured to allow:
- on an aggregate basis, the sum of all Class-Types to reserve up
to 10 Gb/s.
Therefore, a path computed by the Constraint Based Routing for an
LSP of Class-Type N must ensure that this LSP fits within the
remaining Class-Type N bandwidth AND that this LSP fits within the
remaining Aggregate bandwidth.
One way to achieve this, would be:
- for each Class-Type, that IGP uses the "Unreserved Bandwidth for
Class-Type N" to advertise the Class-Type N bandwidth currently
unreserved (i.e. the difference between the Maximum Reservable
Bandwidth for Class-Type N and the bandwidth reserved by
existing Class-Type N LSPs),
- in addition, that IGP separately advertises the "Unreserved
Aggregate Bandwidth" (i.e. the difference between the Maximum
Reservable Aggregate Bandwidth and the bandwidth reserved by
existing LSPs of all Class-Types)
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Requirements for Diff-Serv Traffic Engineering July 2000
- have Constraint Based Routing ensure that a new Class-Type N LSP
fits both in the received "Unreserved Bandwidth for Class-Type
N" and in the "Unreserved Aggregate Bandwidth".
Such an approach has the drawbacks that it would require that N+1
"unreserved bandwidth" information be advertised by the IGP when N
Class-Types are supported, and that it requires the node performing
Constraint Based Routing to meet a double bandwidth constraints.
Instead we propose that:
- for each Class-Type, that IGP uses the "Unreserved Bandwidth for
Class-Type N" to directly advertise the amount of bandwidth that
is effectively useable by Class-Type N. This is computed as the
smaller of these two values:
o The Class-Type N bandwidth currently unreserved (i.e. the
difference between the Maximum Reservable Bandwidth for
Class-Type N and the bandwidth reserved by existing Class-
Type N LSPs).
o The aggregate bandwidth currently unreserved (i.e. the
difference between the Maximum Reservable Aggregate
Bandwidth and the bandwidth reserved by existing LSPs of
all Class-Types).
- have Constraint Based Routing ensure that a new Class-Type N LSP
simply fits in the received "Unreserved Bandwidth for Class-Type
N".
Such an approach only requires that N "unreserved bandwidth"
information be advertised by the IGP when N Class-Types are
supported, and only requires that the node performing Constraint
Based Routing meets a single bandwidth constraints.
We propose to begin by allowing a total of 4 Class-Types (i.e., 3
beyond the existing one aka. Class-Type 0). This is expected to be
sufficient for practical deployments in the foreseeable future. As
an example, a total of three Class-Types already allow support of
separate bandwidth control for Real-Time, Low-Loss and Best Effort,
while allowing multiple classes within each Class-Type (e.g. AF1 and
AF2 flavors of "Low-Loss"). More Class-Types could be defined in the
future if required.
Implementations of Diff-Serv Traffic Engineering in compliance with
this specification MUST support at least a total of 2 Class-Types
and MAY support a total of 3 or 4 Class-Types.
The IGP must be able to only advertise the Bandwidth Information for
the subset of Class-Types actually used in the network (i.e. not
always advertise the Unreserved Bandwidth information for all the
new Class-Types).
It may be desirable to prevent a Class-Type from being starved by
others. In the example given above where we defined three Class-
Types, it may be useful to be able to always ensure that some amount
of Class-Type 0 LSPs can be routed over that link (i.e. to prevent
Le Faucheur et. al 5
Requirements for Diff-Serv Traffic Engineering July 2000
Class-Type 1 LSPs and Class-Type 2 LSPs from reserving up to 100% of
the maximum reservable aggregate bandwidth which would result in
Class-Type 0 LSPs not having any access to the capacity of that
link). Such capability would require the ability from the IGP to
advertise an optional "minimum reservable bandwidth" per Class-Type.
This is not seen as an immediate requirement but could be defined in
the future if required.
3. Requirements for RSVP/CR-LDP Extensions
[RSVP-TE] and [CR-LDP] define extensions to RSVP and LDP for support
of (aggregate) MPLS Traffic Engineering. [DIFF-MPLS] defines the
extensions to RSVP and LDP for support of Diff-Serv over MPLS. In
this section we define the requirements on RSVP and CR-LDP for
support of Diff-Serv Traffic Engineering on a per-Class-Type basis.
These requirements are expected to require further extensions to
RSVP and CR-LDP. Such extensions are proposed in [DIFF-TE-EXT].
In order for an LSR to perform resource availability checking for an
LSP that belongs to a certain Class-Type, the LSR needs to be made
aware through RSVP/CR-LDP signaling of the Class-Type associated
with the LSP.
To that end, we propose that RSVP/CR-LDP be extended to be able to
signal the Class-Type.
We identify the following backward compatibility requirements for
the RSVP/CR-LDP extensions:
- operations in heterogeneous environments need to be supported
for smooth migration, where some LSRs are Diff-Serv-TE-capable
(as defined in this specification) while some other LSRs are not
Diff-Serv-TE-capable (i.e. support (aggregate) TE only)
- in heterogeneous environments, the solution needs to allow
establishment of Class-Type 0 LSPs across paths combining Diff-
Serv-TE-capable LSRs and non-Diff-Serv-TE-capable LSRs
- in heterogeneous environments, the solution needs to ensure that
a non-Diff-Serv-TE-capable LSR would reject establishment of a
Class-Type N (N=1,2,3) LSP.
The admission control algorithm implemented for LSP establishment
must locally maintain different variables which keep track of the
currently unreserved bandwidth for each Class-Type. These unreserved
bandwidth variables must be updated in accordance with the approach
discussed in the previous section for enforcement of the Maximum
Reservable Aggregate Bandwidth across all Class-Types, if so
configured on an LSR. In particular, when admitting a Class-Type N
LSP, the LSR must take into account this Class-Type N LSP to update
the variables tracking the unreserved bandwidth for Class-Type N, as
well as to potentially update the variables tracking the unreserved
bandwidth for the other Class-Types (since the new LSP eats-up
Le Faucheur et. al 6
Requirements for Diff-Serv Traffic Engineering July 2000
Aggregate bandwidth which in turn may reduce the amount of LSP that
may be established in other Class-Types).
4. Requirements for Constraint Based Routing Extensions
In order for Constraint Based Routing to support Diff-Serv TE on a
per-Class-Type basis, the Constraint Based Routing algorithm need to
be capable of taking into account the "Unreserved Bandwidth for
Class-Type N" when computing a path for a Class-Type N LSP.
5. Requirements for MIB Extensions
In order for an LSR to support the configuration and monitoring of
Diff-Serv Traffic Engineering certain enhancements to some of the
existing MPLS Management Information Bases (MIBs) will be required.
[LSRMIB] defines the MPLS Label Switch Router MIB (LSR MIB) which
contains objects useful for the management and configuration of MPLS
LSPs. [TE MIB] defines the MPLS Traffic Engineering MIB (TE MIB)
which contains objects useful for the management and configuration
of MPLS Traffic Engineered Tunnels.
In particular, the MIB extensions need to:
- track for each MPLS interface, the Maximum Reservable Bandwidth
configured for each Class-Type.
- track for each MPLS interface, the Maximum Reservable Aggregate
Bandwidth configured.
- track for each LSP, the Class-Type associated with the LSP. On
the Head-End LSRs, the Class-Type is configured as part of the
tunnel configuration. On other LSRs, the Class-Type is
associated with the LSP at establishment time based on signaled
information.
Additional details of these changes will be provided in forthcoming
versions of this draft. It is the authors' intent to transfer these
MIB requirements to future versions of the MPLS TE and the MPLS LSR
MIBs. It is not the intent of this document to define the SMI
required for the MIB enhancements; rather, it is to flesh out and
define the details of these changes in the context of this document.
6. Security Considerations
The solution developed to address the requirements defined in this
document must address security aspects.
7. Acknowledgments
This document has benefited from discussions with Carol Iturralde
and Rob Goguen.
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Requirements for Diff-Serv Traffic Engineering July 2000
References
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over
MPLS, RFC2702, September 1999.
[TEWG-FW] Awduche et al, A Framework for Internet Traffic
Engineering, draft-ietf-tewg-framework-01.txt, May 2000.
[DIFF-TE-EXT] Le Faucheur et al, Extensions to IS-IS, OSPF, RSVP,
CR-LDP and MPLS MIBs for support of Diff-Serv-aware MPLS Traffic
Engineering, draft-lefaucheur-diff-te-ext-00.txt, July 2000.
[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF,
draft-katz-yeung-ospf-traffic-01.txt, April 2000.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-01.txt, May 1999.
[RSVP-TE] Awduche et al, "Extensions to RSVP for LSP Tunnels",
draft-ietf-mpls-rsvp-lsp-tunnel-05.txt, February 2000.
[DIFF-MPLS] Le Faucheur et al, "MPLS Support of Diff-Serv", draft-
ietf-mpls-diff-ext-05.txt, June 2000
[LDP] Andersson et al., "LDP Specification", draft-ietf-mpls-ldp-
06.txt, October 1999
[CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
draft-ietf-mpls-cr-ldp-03.txt, October 1999
[TEMIB] Srinivansan, C., and A. Viswanathan, "MPLS Traffic
Engineering Management Information Base Using SMIv2", draft-ietf-
mpls-te-mib-03.txt, March 10, 2000.
[LSRMIB] Srinivansan, C., Viswanathan, A., and T. Nadeau "MPLS
Label Switch Router Management Information Base Using SMIv2", draft-
ietf-mpls-lsr-mib-04.txt, April 26, 2000.
Authors' Address:
Francois Le Faucheur
Cisco Systems, Inc.
Petra B - Les Lucioles - 291, rue Albert Caquot - 06560 Valbonne -
France
Phone: +33 4 92 96 75 64
Email: flefauch@cisco.com
Angela Chiu
AT&T Labs
Le Faucheur et. al 8
Requirements for Diff-Serv Traffic Engineering July 2000
Rm 4-204,100 Schulz Dr., Red Bank, NJ 07701
USA
Phone: +1 (732) 345-3441
Email: alchiu@att.com
William Townsend
Tenor Networks
100 Nagog Park
Acton, MA 01720
Phone: +1-978-264-4900
Email: btownsend@tenornetworks.com
Thomas D. Nadeau
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
Phone: +1-978-244-3051
Email: tnadeau@cisco.com
Darek Skalecki
Nortel Networks
3500 Carling Ave,
Nepean K2H 8E9
Phone: +1-613-765-2252
Email: dareks@nortelnetworks.com
Le Faucheur et. al 9