Francois Le Faucheur, Editor
Cisco Systems, Inc.
IETF Internet Draft
Expires: April, 2003
Document: draft-ietf-tewg-diff-te-proto-02.txt October, 2002
Protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering
Status of this Memo
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Abstract
This document specifies the IGP and RSVP-TE signaling extensions
(beyond those already specified for existing MPLS Traffic
Engineering) for support of Diff-Serv-aware MPLS Traffic Engineering
(DS-TE). These extensions address the Requirements for DS-TE spelt
out in [DSTE-REQ].
Summary for Sub-IP related Internet Drafts
RELATED DOCUMENTS:
draft-ietf-tewg-diff-te-reqts-06.txt
WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
This ID is a Working Group document of the TE Working Group.
WHY IS IT TARGETED AT THIS WG(s)
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TEWG is responsible for specifying protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering.
JUSTIFICATION
The TEWG charter states that "This will entail verification and
review of the Diffserv requirements in the WG Framework document and
initial specification of how these requirements can be met through
use and potentially expansion of existing protocols."
In line with this, the TEWG is progressing this Working Group
document specifying protocol extensions for Diff-Serv-aware MPLS
Traffic Engineering.
1. Introduction
[DSTE-REQ] presents the Service Providers requirements for support of
Diff-Serv-aware MPLS Traffic Engineering (DS-TE). This includes the
fundamental requirement to be able to enforce different bandwidth
constraints for different classes of traffic.
This document specifies the IGP and RSVP-TE signaling extensions
(beyond those already specified for existing MPLS Traffic Engineering
[OSPF-TE][ISIS-TE][RSVP-TE]) for support of the DS-TE requirements
spelt out in [DSTE-REQ] in environments relying on distributed
Constraint Based Routing (i.e. path computation involving Head-end
LSRs).
[DSTE-REQ] provides a definition and examples of Bandwidth Constraint
Models. This document does not specify nor assume a particular
Bandwidth Constraints model. Specific Bandwidth Constraints model are
outside the scope of this document. While the extensions for DS-TE
specified in this document may not be sufficient to support all the
conceivable Bandwidth Constraints models, they do support the
specific Bandwidth Constraints models envisioned so far for DS-TE
(such as the "Russian Dolls" model further discussed in [DSTE-
RUSSIAN] and the "Maximum Allocation" model introduced in [DSTE-
REQ]).
2. Contributing Authors
This document was the collective work of several. The text and
content of this document was contributed by the editor and the co-
authors listed below. (The contact information for the editor appears
in Section 16, and is not repeated below.)
Jim Boyle Kireeti Kompella
Protocol Driven Networks, Inc. Juniper Networks, Inc.
1381 Kildaire Farm Road #288 1194 N. Mathilda Ave.
Cary, NC 27511, USA Sunnyvale, CA 94099
Phone: (919) 852-5160 Email: kireeti@juniper.net
Email: jboyle@pdnets.com
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Protocols for Diff-Serv-aware TE October 2002
William Townsend Thomas D. Nadeau
Tenor Networks Cisco Systems, Inc.
100 Nagog Park 250 Apollo Drive
Acton, MA 01720 Chelmsford, MA 01824
Phone: +1-978-264-4900 Phone: +1-978-244-3051
Email: Email: tnadeau@cisco.com
btownsend@tenornetworks.com
Darek Skalecki
Nortel Networks
3500 Carling Ave,
Nepean K2H 8E9
Phone: +1-613-765-2252
Email: dareks@nortelnetworks.com
3. Definitions
[DSTE-REQ] discusses how a Head-end LSR may split the set of Traffic
Aggregates from the traffic to a given Tail-end into multiple Traffic
Trunks. Each Traffic Trunk is transported over a separate LSP which
is Constraint Based Routed individually.
For readability a number of definitions from [DSTE-REQ] are repeated
here:
Traffic Trunk: an aggregation of traffic flows of the same class
[i.e. which are to be treated equivalently from the DS-TE
perspective] which are placed inside a Label Switched Path.
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.
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.
4. Configurable Parameters
This section only discusses the differences with the configurable
parameters supported for MPLS Traffic Engineering as per [TE-REQ],
[ISIS-TE], [OSPF-TE], and [RSVP-TE]. All other parameters are
unchanged.
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4.1. Link Parameters
4.1.1. Bandwidth Constraints (BCs)
[DSTE-REQ] states that "Regardless of the Bandwidth Constraint Model,
the DS-TE solution must allow support for up to 8 BCs."
For DS-TE, the existing "Maximum Reservable link bandwidth" parameter
is retained but its semantic is generalized and interpreted as BC0.
Additionally, on every link, a DS-TE implementation MUST provide for
configuration of up to 7 additional link parameters which are the
seven other potential Bandwidth Constraints i.e. BC1, BC2 , ... BC7.
The LSR MUST interpret these Bandwidth Constraints in accordance with
the supported Bandwidth Constraint Model (i.e. what bandwidth
constraint applies to what Class-Type and how).
Where the Bandwidth Constraint Model imposes some relationship among
the values to be configured for these Bandwidth Constraints, the LSR
MUST enforce those at configuration time. For example, when the
"Russian Doll" Bandwidth Constraints Model is used (See [DSTE-REQ]),
the LSR must ensure that BCi is configured smaller or equal to BCj,
where i is greater than j.
4.1.2. per-CT Local Overbooking Multipliers (LOMs)
DS-TE enables a network administrator to apply different overbooking
(or underbooking) ratios for different CTs.
The principal method to achieve this is the same as historically used
in existing TE deployment, which is :
(i) to take into account the over-booking/underbooking ratio
appropriate for the OA/CT associated with the considered LSP
at the time of establishing the bandwidth size of a given
LSP, AND/OR
(ii) to take into account the overbooking/underbooking ratio at
the time of configuring the Maximum Reservable
Bandwidth/Bandwidth Constraints and/or the Maximum Link
Bandwidth (which effectively controls the maximum size of an
individual LSP) and use values which are larger(overbooking)
or smaller(underbooking) than the actual link.
We refer to this method as the "LSP/link size overbooking" method.
The "LSP/link size overbooking" method is expected to be often
sufficient in many DS-TE environments and requires no additional
configurable parameters.
However, in the particular DS-TE environments where, for a given CT,
the overbooking ratio needs to be tweaked differently on different
links and where a very fine accounting of overbooking cross-effect
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across Class-Types is required, a DS-TE implementation MAY optionally
support the "local overbooking" method as a complement to the
"LSP/link size overbooking" method. The "local overbooking" method
relies on optional "per-CT Local Overbooking Multipliers" (LOMs)
which are configurable, on every link, for every CT. The per-CT Local
Overbooking Multiplier effectively allows the network operator to
increase/decrease", on some links, the overbooking ratio already
enforced by the "LSP/link size overbooking" method. This is achieved
by factoring the per-CT LOM in all local bandwidth accounting for the
purposes of admission control and IGP advertisement of unreserved
bandwidths. This is discussed in more details in section 10.3.
We refer to the Local Overbooking Multiplier for CTc (0 <= c <=7) as
LOM(c).
Since the per-CT Local Overbooking Multipliers are factored in the
IGP advertisement of unreserved bandwidth by the local LSR, a remote
LSR computing a path for a DS-TE tunnel need not be aware that local
overbooking is used on the considered link. In fact, the remote LSR
does not even need to support the optional local overbooking method.
In any case, the remote LSR will compute a path that effectively
takes into account the LOMs on the considered link because it bases
its computation on advertised unreserved bandwidth which do factor in
the LOMs for that link.
4.2. LSR Parameters
4.2.1. TE-Class Mapping
In line with [DSTE-REQ], the preemption attributes defined in [TE-
REQ] are retained with DS-TE and applicable across all Class Types.
The preemption attributes of setup priority and holding priority
retain existing semantics, and in particular these semantics are not
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.
For DS-TE, LSRs MUST allow configuration of a TE-Class mapping
whereby the Class-Type and preemption level are configured for each
of (up to) 8 TE-Classes.
This mapping is referred to as :
TE-Class[i] <--> < CTc , preemption p >
Where 0 <= i <= 7, 0 <= c <= 7, 0 <= p <= 7
Two TE-Classes must not be identical (i.e. have both the same Class-
Type and the same preemption priority).
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Where the network administrator uses less than 8 TE-Classes, the DS-
TE LSR MUST allow remaining ones to be configured as "Unused".
There are no other restrictions on how any of the 8 Class-Types can
be paired up with any of the 8 preemption priorities to form a TE-
class. In particular, one given preemption priority can be paired up
with two (or more) different Class-Types to form two (or more) TE-
classes. Similarly, one Class-Type can be paired up with two (or
more) different preemption priorities to form two (or more) TE-
Classes. Also, there is no mandatory ordering relationship between
the TE-Class index (i.e. "i" above) and the Class-Type (i.e. "c"
above) or the preemption priority (i.e. "p" above) of the TE-Class.
To ensure coherent DS-TE operation, the network administrator MUST
configure exactly the same TE-Class Mapping on all LSRs of the DS-TE
domain.
When the TE-class mapping needs to be modified in the DS-TE domain,
care must be exercised during the transient period of reconfiguration
during which some DS-TE LSRs may be configured with the new TE-class
mapping while others are still configured with the old TE-class
mapping. It is recommended that active tunnels do not use any of the
TE-classes which are being modified during such a transient
reconfiguration period.
4.3. LSP Parameters
4.3.1. Class-Type
With DS-TE, LSRs MUST support, for every LSP, an additional
configurable parameter which indicates the Class-Type of the Traffic
Trunk transported by the LSP.
There is one and only one Class-Type configured per LSP.
The configured Class-Type indicates, in accordance with the supported
Bandwidth Constraint Model, what are the Bandwidth Constraints
applicable to that LSP.
4.3.2. Setup and Holding Preemption Priorities
As per existing TE, DS-TE assumes that every DS-TE LSP is configured
with a setup and holding priority, each with a value between 0 and 7.
4.3.3. Class-Type/Preemption Relationship
With DS-TE, the preemption priority configured for the setup priority
of a given LSP and the Class-Type configured for that LSP must be
such that, together, they form one of the (up to) 8 TE-Classes
configured in the TE-Class Mapping specified is section 4.2.1 above.
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The LSR MUST enforce this rule at configuration time.
The preemption priority configured for the holding priority of a
given LSP and the Class-Type configured for that LSP must also be
such that, together, they form one of the (up to) 8 TE-Classes
configured in the TE-Class Mapping specified is section 4.2.1 above.
The LSR MUST enforce this rule at configuration time.
4.4. Examples of Parameters Configuration
For illustrative purposes, we now present a few examples of how these
configurable parameters may be used. All these examples assume that
different bandwidth constraints need to be enforced for different
sets of Traffic Trunks (e.g. for Voice and for Data) so that two, or
more, Class-Types must be used.
4.4.1. Example 1
The Network Administrator of a first network using two Class Types
(CT1 for Voice and CT0 for Data), may elect to configure the
following TE-Class Mapping to ensure that Voice LSPs are never driven
away from their shortest path because of Data LSPs:
TE-Class[0] <--> < CT1 , preemption 0 >
TE-Class[1] <--> < CT0 , preemption 1 >
TE-Class[i] <--> unused, for 2 <= i <= 7
Voice LSPs would then be configured with:
- CT=CT1, set-up priority =0, holding priority=0
Data LSPs would then be configured with:
- CT=CT0, set-up priority =1, holding priority=1
A new Voice LSP would then be able to preempt an existing Data LSP in
case they contend for resources. A Data LSP would never preempt a
Voice LSP. A Voice LSP would never preempt another Voice LSP. A Data
LSP would never preempt another Data LSP.
4.4.2. Example 2
The Network Administrator of another network may elect to configure
the following TE-Class Mapping in order to optimize global network
resource utilization by favoring placement of large LSPs closer to
their shortest path:
TE-Class[0] <--> < CT1 , preemption 0 >
TE-Class[1] <--> < CT0 , preemption 1 >
TE-Class[2] <--> < CT1 , preemption 2 >
TE-Class[3] <--> < CT0 , preemption 3 >
TE-Class[i] <--> unused, for 4 <= i <= 7
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Large size Voice LSPs could be configured with:
- CT=CT1, set-up priority =0, holding priority=0
Large size Data LSPs could be configured with:
- CT=CT0, set-up priority = 1, holding priority=1
Small size Voice LSPs could be configured with:
- CT=CT1, set-up priority = 2, holding priority=2
Small size Data LSPs could be configured with:
- CT=CT0, set-up priority = 3, holding priority=3.
A new large size Voice LSP would then be able to preempt a small size
Voice LSP or any Data LSP in case they contend for resources.
A new large size Data LSP would then be able to preempt a small size
Data LSP or a small size Voice LSP in case they contend for
resources, but it would not be able to preempt a large size Voice
LSP.
4.4.3. Example 3
The Network Administrator of another network may elect to configure
the following TE-Class Mapping in order to ensure that Voice LSPs are
never driven away from their shortest path because of Data LSPs while
also achieving some optimization of global network resource
utilization by favoring placement of large LSPs closer to their
shortest path:
TE-Class[0] <--> < CT1 , preemption 0 >
TE-Class[1] <--> < CT1 , preemption 1 >
TE-Class[2] <--> < CT0 , preemption 2 >
TE-Class[3] <--> < CT0 , preemption 3 >
TE-Class[i] <--> unused, for 4 <= i <= 7
Large size Voice LSPs could be configured with:
- CT=CT1, set-up priority = 0, holding priority=0.
Small size Voice LSPs could be configured with:
- CT=CT1, set-up priority = 1, holding priority=1.
Large size Data LSPs could be configured with:
- CT=CT0, set-up priority = 2, holding priority=2.
Small size Data LSPs could be configured with:
- CT=CT0, set-up priority = 3, holding priority=3.
A Voice LSP could preempt a Data LSP if they contend for resources. A
Data LSP would never preempt a Voice LSP. A Large size Voice LSP
could preempt a small size Voice LSP if they contend for resources. A
Large size Data LSP could preempt a small size Data LSP if they
contend for resources.
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4.4.4. Example 4
The Network Administrator of another network may elect to configure
the following TE-Class Mapping in order to ensure that no preemption
occurs in the DS-TE domain:
TE-Class[0] <--> < CT1 , preemption 0 >
TE-Class[1] <--> < CT0 , preemption 0 >
TE-Class[i] <--> unused, for 2 <= i <= 7
Voice LSPs would then be configured with:
- CT=CT1, set-up priority =0, holding priority=0
Data LSPs would then be configured with:
- CT=CT0, set-up priority =0, holding priority=0
No LSP would then be able to preempt any other LSP.
4.4.5. Example 5
The Network Administrator of another network may elect to configure
the following TE-Class Mapping in view of increased network stability
through a more limited use of preemption:
TE-Class[0] <--> < CT1 , preemption 0 >
TE-Class[1] <--> < CT1 , preemption 1 >
TE-Class[2] <--> < CT0 , preemption 1 >
TE-Class[3] <--> < CT0 , preemption 2 >
TE-Class[i] <--> unused, for 4 <= i <= 7
Large size Voice LSPs could be configured with:
- CT=CT1, set-up priority = 0, holding priority=0.
Small size Voice LSPs could be configured with:
- CT=CT1, set-up priority = 1, holding priority=0.
Large size Data LSPs could be configured with:
- CT=CT0, set-up priority = 2, holding priority=1.
Small size Data LSPs could be configured with:
- CT=CT0, set-up priority = 2, holding priority=2.
A new large size Voice LSP would be able to preempt a Data LSP in
case they contend for resources, but it would not be able to preempt
any Voice LSP even a small size Voice LSP.
A new small size Voice LSP would be able to preempt a small size Data
LSP in case they contend for resources, but it would not be able to
preempt a large size Data LSP or any Voice LSP.
A Data LSP would not be able to preempt any other LSP.
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5. IGP Extensions for DS-TE
This section only discusses the differences with the IGP
advertisement supported for (aggregate) MPLS Traffic Engineering as
per [OSPF-TE] and [ISIS-TE]. The rest of the IGP advertisement is
unchanged.
5.1. Bandwidth Constraints
As detailed above in section 4.1.1, up to 8 Bandwidth Constraints (
BCb, 0 <= b <= 7) are configurable on any given link.
With DS-TE, the existing "Maximum Reservable Bw" sub-TLV is retained
with a generalized semantic so that it is now interpreted as
Bandwidth Constraint 0 (BC0).
This document also defines the following new optional sub-TLV to
advertise the eight potential Bandwidth Constraints (BC0 to BC7):
"Bandwidth Constraints" sub-TLV:
Bandwidth Constraint Model Id (1 octet)
Bandwidth Constraints (Nx4 octets)
Where:
- With OSPF, the sub-TLV is a sub-TLV of the "Link TLV" and its
sub-TLV type is TBD. See IANA Considerations section below.
- With ISIS, the sub-TLV is a sub-TLV of the "extended IS
reachability TLV" and its sub-TLV type is TBD. See IANA
Considerations section below.
- Bandwidth Constraint Model Id: 1 octet identifier for the
Bandwidth Constraints Model currently in use by the LSR
initiating the IGP advertisement.
Value 0 identifies the Russian Doll Bandwidth Constraint
Model defined in [DSTE-REQ] (and detailed in [DSTE-RUSSIAN]).
Value 1 identifies the Maximum Allocation Bandwidth
Constraint Model defined in [DSTE-REQ].
- Bandwidth Constraints: contains BC0, BC1,... BCN-1.
Each Bandwidth Constraint is encoded in 32 bits in IEEE
floating point format. The units are bytes (not bits!) per
second. It is recommended that only the Bandwidth Constraints
corresponding to active CTs be advertised in order to
minimize the impact on IGP scalability.
A DS-TE LSR MAY optionally advertise Bandwidth Constraints.
A DS-TE LSR which does advertise Bandwidth Constraints MUST use the
new "Bandwidth Constraints" sub-TLV to do so. For example,
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considering the case where a Service Provider deploys DS-TE with two
active CTs and where two Bandwidth Constraints per link apply : a DS-
TE LSR which does advertise Bandwidth Constraint would include the
"Bandwidth Constraints" sub-TLV in the IGP advertisement and this
should contain only BC0 and BC1.
A DS-TE LSR which does advertise Bandwidth Constraints, MAY also
include the existing "Maximum Reservable Bw" sub-TLV. This may be
useful in migration situations where some LSRs in the network are not
DS-TE capable (see Appendix C) and thus do not understand the new
"Bandwidth Constraints" sub-TLV. In that case, the DS-TE LSR MUST set
the value of the "Maximum Reservable Bw" sub-TLV to the same value as
the one for BC0 encoded in the "Bandwidth Constraints" sub-TLV.
A DS-TE LSR receiving both the old "Maximum Reservable Bw" sub-TLV
and the new "Bandwidth Constraints" sub-TLV for a given link MAY
ignore the "Maximum Reservable Bw" sub-TLV.
A DS-TE LSR receiving the "Bandwidth Constraints" sub-TLV with a
Bandwidth Constraint Model Id which does not match the Bandwidth
Constraint Model it currently uses, MAY generate a warning to the
operator reporting the inconsistency between Bandwidth Constraint
Models used on different links. Also, in that case, if the DS-TE LSR
does not support the Bandwidth Constraint Model designated by the
Bandwidth Constraint Model Id, or if the DS-TE LSR does not support
operations with multiple simultaneous Bandwidth Constraint Models,
the DS-TE LSR MAY discard the corresponding TLV.
5.2. Unreserved Bandwidth
With DS-TE, the existing "Unreserved Bandwidth" sub-TLV is retained
as the only vehicle to advertise dynamic bandwidth information
necessary for Constraint Based Routing on Head-ends, except that it
is used with a generalized semantic. The Unreserved Bandwidth sub-TLV
still carries eight bandwidth values but they now correspond to the
unreserved bandwidth for each of the TE-Class (instead of for each
preemption priority as per existing TE).
More precisely, a DS-TE LSR MUST support the Unreserved Bandwidth
sub-TLV with a definition which is generalized into the following:
The Unreserved Bandwidth sub-TLV specifies the amount of bandwidth
not yet reserved for each of the eight TE-classes, in IEEE floating
point format arranged in increasing order of TE-Class index, with
unreserved bandwidth for TE-Class [0] occurring at the start of the
sub-TLV, and unreserved bandwidth for TE-Class [7] at the end of the
sub-TLV. The unreserved bandwidth value for TE-Class [i] ( 0 <= i <=
7) is referred to as "Unreserved TE-Class [i]". It indicates the
bandwidth that is available, for reservation, to an LSP which :
- transports a Traffic Trunk from the Class-Type of TE-
Class[i], and
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- has a setup priority corresponding to the preemption priority
of TE-Class[i].
The units are bytes per second.
Since the bandwidth values are now ordered by TE-class index and thus
can relate to different CTs with different bandwidth constraints and
can relate to any arbitrary preemption priority, a DS-TE LSR MUST NOT
assume any ordered relationship among these bandwidth values.
With existing TE, since all preemption priorities reflect the same
(and only) bandwidth constraints and since bandwidth values are
advertised in preemption priority order, the following relationship
is always true, and is often assumed by TE implementations:
If i < j , then "Unreserved Bw [i]" >= "Unreserved Bw [j]"
With DS-TE, no relationship is to be assumed so that:
If i < j , then
"Unreserved TE-Class [i]" = "Unreserved TE-Class [j]"
OR
"Unreserved TE-Class [i]" > "Unreserved TE-Class [j]"
OR
"Unreserved TE-Class [i]" < "Unreserved TE-Class [j]".
Rules for computing "Unreserved TE-Class [i]" are specified in
section 10.
If TE-Class[i] is unused, the value advertised by the IGP in
"Unreserved TE-Class [i]" MUST be set to zero.
5.3. Local Overbooking Multiplier
The following additional optional sub-TLV is defined for DS-TE:
"Local Overbooking Multiplier" sub-TLV:
per-CT Local Overbooking Multipliers (N x 2 octets)
Where:
- With OSPF, the sub-TLV is a sub-TLV of the "Link TLV" and its
sub-TLV type is TBD. See IANA Considerations section below.
- With ISIS, the sub-TLV is a sub-TLV of the "extended IS
reachability TLV" and its sub-TLV type is TBD. See IANA
Considerations section below.
- per-CT Local Overbooking Multipliers: contains LOM(0), LOM(1)
..., LOM(N-1) and where N is the number of per-CT Local
Overbooking Multipliers actually advertised. It is
recommended that only the LOMs corresponding to active CTs be
included, in order to minimize the impact on IGP scalability.
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Each LOM is encoded as an integer in the range
[ 0 , (2^16 -1) ] that represents LOM expressed in
percentage. For example, a LOM of 1 (i.e. 100%) is encoded as
100, and represents no overbooking/underbooking. A LOM of 2
(i.e. 200%) is encoded as 200, and represents an overbooking
of 2. A LOM of 0.5 (i.e. 50%) is encoded as 50 and represents
an overbooking of 0.5 (or, in other words, an underbooking of
2).
A DS-TE LSR supporting the optional local overbooking method MAY
optionally advertise LOMs. A DS-TE LSR which does advertise LOMs MUST
use the "Local Overbooking Multiplier" sub-TLV to do so.
For example, where a Service Provider only deploys DS-TE with two CTs
and makes use of the Local Overbooking method, the "Local Overbooking
Multiplier" sub-TLV may optionally be used and would then contain
only LOM(0) and LOM(1).
Note that the use of this sub-TLV is only optional even when the
optional Local Overbooking method is actually used (and thus when the
Local Overbooking Multipliers parameters are actually configured
locally on some or all links). Its use may assist in head-end
prediction of network response to LSP establishment.
6. RSVP-TE Extensions for DS-TE
In this section we describe extensions to RSVP-TE for support of
Diff-Serv-aware MPLS Traffic Engineering. These extensions are in
addition to the extensions to RSVP defined in [RSVP-TE] for support
of (aggregate) MPLS Traffic Engineering and to the extensions to RSVP
defined in [DIFF-MPLS] for support of Diff-Serv over MPLS.
6.1. DS-TE related RSVP Messages Format
One new RSVP Object is defined in this document: the CLASSTYPE
Object. Detailed description of this Object is provided below. This
new Object is applicable to Path messages. This specification only
defines the use of the CLASSTYPE Object in Path messages used to
establish LSP Tunnels in accordance with [RSVP-TE] and thus
containing a Session Object with a C-Type equal to LSP_TUNNEL_IPv4
and containing a LABEL_REQUEST object.
Restrictions defined in [RSVP-TE] for support of establishment of LSP
Tunnels via RSVP-TE are also applicable to the establishment of LSP
Tunnels supporting DS-TE. For instance, only unicast LSPs are
supported and Multicast LSPs are for further study.
This new CLASSTYPE object is optional with respect to RSVP so that
general RSVP implementations not concerned with MPLS LSP set up do
not have to support this object.
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Protocols for Diff-Serv-aware TE October 2002
An LSR supporting DS-TE MUST support the CLASSTYPE Object.
6.1.1. Path Message Format
The format of the Path message is as follows:
<Path Message> ::= <Common Header> [ <INTEGRITY> ]
<SESSION> <RSVP_HOP>
<TIME_VALUES>
[ <EXPLICIT_ROUTE> ]
<LABEL_REQUEST>
[ <SESSION_ATTRIBUTE> ]
[ <DIFFSERV> ]
[ <CLASSTYPE> ]
[ <POLICY_DATA> ... ]
[ <sender descriptor> ]
<sender descriptor> ::= <SENDER_TEMPLATE> [ <SENDER_TSPEC> ]
[ <ADSPEC> ]
[ <RECORD_ROUTE> ]
6.2. CLASSTYPE Object
The CLASSTYPE object format is shown below.
6.2.1. CLASSTYPE object
class = TBD, C_Type = 1 (need to get an official class num from the
IANA with the form 0bbbbbbb). See IANA Considerations section below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | CT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved : 29 bits
This field is reserved. It must be set to zero on transmission
and must be ignored on receipt.
CT : 3 bits
Indicates the Class-Type. Values currently allowed are 1, 2, ...
, 7.
6.3. Handling CLASSTYPE Object
To establish an LSP tunnel with RSVP, the sender LSR creates a Path
message with a session type of LSP_Tunnel_IPv4 and with a
LABEL_REQUEST object as per [RSVP-TE]. The sender LSR may also
include the DIFFSERV object as per [DIFF-MPLS].
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Protocols for Diff-Serv-aware TE October 2002
If the LSP is associated with Class-Type 0, the sender LSR MUST NOT
include the CLASSTYPE object in the Path message.
If the LSP is associated with Class-Type N (1 <= N <=7), the sender
LSR MUST include the CLASSTYPE object in the Path message with the
Class-Type (CT) field set to N.
If a path message contains multiple CLASSTYPE objects, only the first
one is meaningful; subsequent CLASSTYPE object(s) MUST be ignored and
MUST not be forwarded.
Each LSR along the path MUST record the CLASSTYPE object, when
present, in its path state block.
If the CLASSTYPE object is not present in the Path message, the LSR
MUST associate the Class-Type 0 to the LSP.
The destination LSR responding to the Path message by sending a Resv
message MUST NOT include a CLASSTYPE object in the Resv message
(whether the Path message contained a CLASSTYPE object or not).
During establishment of an LSP corresponding to the Class-Type N, the
LSR MUST perform admission control over the bandwidth available for
that particular Class-Type.
An LSR that recognizes the CLASSTYPE object and that receives a path
message which contains the CLASSTYPE object but which does not
contain a LABEL_REQUEST object or which does not have a session type
of LSP_Tunnel_IPv4, MUST send a PathErr towards the sender with the
error code 'Diff-Serv-aware TE Error' and an error value of
'Unexpected CLASSTYPE object'. Those are defined below in section
6.5.
An LSR receiving a Path message with the CLASSTYPE object, which
recognizes the CLASSTYPE object but does not support the particular
Class-Type, MUST send a PathErr towards the sender with the error
code 'Diff-Serv-aware TE Error' and an error value of 'Unsupported
Class-Type'. Those are defined below in section 6.5.
An LSR receiving a Path message with the CLASSTYPE object, which
recognizes the CLASSTYPE object but determines that the Class-Type
value is not valid (i.e. Class-Type value 0), MUST send a PathErr
towards the sender with the error code 'Diff-Serv-aware TE Error' and
an error value of 'Invalid Class-Type value'. Those are defined below
in section 6.5.
An LSR receiving a Path message with the CLASSTYPE object, which:
- recognizes the CLASSTYPE object,
- supports the particular Class-Type, but
- determines that the tuple formed by (i) this Class-Type and
(ii) the set-up priority signaled in the same Path message,
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Protocols for Diff-Serv-aware TE October 2002
is not one of the eight TE-classes configured in the TE-class
mapping,
MUST send a PathErr towards the sender with the error code 'Diff-
Serv-aware TE Error' and an error value of 'CT and setup priority do
not form a configured TE-Class'. Those are defined below in section
6.5.
An LSR receiving a Path message with the CLASSTYPE object, which:
- recognizes the CLASSTYPE object,
- supports the particular Class-Type, but
- determines that the tuple formed by (i) this Class-Type and
(ii) the holding priority signaled in the same Path message,
is not one of the eight TE-classes configured in the TE-class
mapping,
MUST send a PathErr towards the sender with the error code 'Diff-
Serv-aware TE Error' and an error value of 'CT and holding priority
do not form a configured TE-Class'. Those are defined below in
section 6.5.
An LSR receiving a Path message with the CLASSTYPE object and with
the DIFFSERV object for an L-LSP, which:
- recognizes the CLASSTYPE object,
- has local knowledge of the relationship between Class-Types
and PSC (e.g. via configuration)
- based on this local knowledge, determines that the PSC
signaled in the DIFFSERV object is inconsistent with the
Class-Type signaled in the CLASSTYPE object,
MUST send a PathErr towards the sender with the error code 'Diff-
Serv-aware TE Error' and an error value of 'Inconsistency between
signaled PSC and signaled CT'. Those are defined below in section
6.5.
An LSR receiving a Path message with the CLASSTYPE object and with
the DIFFSERV object for an E-LSP, which:
- recognizes the CLASSTYPE object,
- has local knowledge of the relationship between Class-Types
and PHBs (e.g. via configuration)
- based on this local knowledge, determines that the PHBs
signaled in the MAP entries of the DIFFSERV object are
inconsistent with the Class-Type signaled in the CLASSTYPE
object,
MUST send a PathErr towards the sender with the error code 'Diff-
Serv-aware TE Error' and an error value of 'Inconsistency between
signaled PHBs and signaled CT'. Those are defined below in section
6.5.
An LSR MUST handle the situations where the LSP can not be accepted
for other reasons than those already discussed in this section, in
accordance with [RSVP-TE] and [DIFF-MPLS] (e.g. a reservation is
rejected by admission control, a label can not be associated).
6.4. Non-support of the CLASSTYPE Object
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Protocols for Diff-Serv-aware TE October 2002
An LSR that does not recognize the CLASSTYPE object Class-Num MUST
behave in accordance with the procedures specified in [RSVP] for an
unknown Class-Num whose format is 0bbbbbbb (i.e. it must send a
PathErr with the error code 'Unknown object class' toward the
sender).
An LSR that recognizes the CLASSTYPE object Class-Num but does not
recognize the CLASSTYPE object C-Type, MUST behave in accordance with
the procedures specified in [RSVP] for an unknown C-type (i.e. it
must send a PathErr with the error code 'Unknown object C-Type'
toward the sender).
In both situations, this causes the path set-up to fail. The sender
SHOULD notify management that a LSP cannot be established and
possibly might take action to retry reservation establishment without
the CLASSTYPE object.
6.5. Error Codes For Diff-Serv-aware TE
In the procedures described above, certain errors must be reported as
a 'Diff-Serv-aware TE Error'. The value of the 'Diff-Serv-aware TE
Error' error code is (TBD). See IANA Considerations section below.
The following defines error values for the Diff-Serv-aware TE Error:
Value Error
1 Unexpected CLASSTYPE object
2 Unsupported Class-Type
3 Invalid Class-Type value
4 CT and setup priority do not form a configured TE-Class
5 CT and holding priority do not form a configured
TE-Class
6 Inconsistency between signaled PSC and signaled CT
7 Inconsistency between signaled PHBs and signaled CT
7. Constraint Based Routing
Let us consider the case where a path needs to be computed for an LSP
whose Class-Type is configured to CTc and whose set-up preemption
priority is configured to p.
Then the pair of CTc and p will map to one of the TE-Classes defined
in the TE-Class mapping. Let us assume that this is the i-th TE-Class
i.e. TE-Class[i].
The Constraint Based Routing algorithm of a DS-TE LSR is still only
required to perform path computation satisfying a single bandwidth
constraint which is to fit in "Unreserved TE-Class [i]" as advertised
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Protocols for Diff-Serv-aware TE October 2002
by the IGP for every link. Thus, no changes are required to the
existing TE Constraint Based Routing algorithm itself.
The Constraint Based Routing algorithm MAY also optionally take into
account, when used, the optional additional information advertised in
IGP which are the Bandwidth Constraints and the Local Overbooking
Multipliers. As an example, the Bandwidth Constraints MIGHT be used
as a tie-breaker criteria in situations where multiple paths,
otherwise equally attractive, are possible.
8. Diff-Serv scheduling
The Class-Type signaled at LSP establishment MAY optionally be used
by DS-TE LSRs to dynamically adjust the resources allocated to the
Class-Type by the Diff-Serv scheduler. In addition, the Diff-Serv
information (i.e. the PSC) signaled by the TE-LSP signaling protocols
as specified in [DIFF-MPLS], if used, MAY optionally be used by DS-TE
LSRs to dynamically adjust the resources allocated to a PSC/OA within
a Class Type by the Diff-Serv scheduler.
9. Existing TE as a Particular Case of DS-TE
We observe that existing TE can be viewed as a particular case of
DS-TE where:
(i) a single Class-Type is used, all 8 preemption priorities
are allowed for that Class-Type and the following TE-Class
Mapping is used:
TE-Class[i] <--> < CT0 , preemption i >
Where 0 <= i <= 7.
(ii) optional per-CT Local Overbooking Multipliers are not
used.
In that case, DS-TE behaves as existing TE.
As with existing TE, the IGP advertises:
- Unreserved Bandwidth for each of the 8 preemption priorities
- Max Link Bandwidth
As with existing TE, the IGP may also optionally advertise BC0 =
Maximum Reservable Bandwidth. But note that, unlike with existing TE,
the IGP will also advertise the Bandwidth Constraint sub-TLV (which
will contain BC0).
Since all LSPs transport traffic from CT0, LSP Signaling is done
without explicit signaling of the Class-Type (which is only used for
other Class-Types than CT0 as explained in section 6) as with
existing TE.
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Protocols for Diff-Serv-aware TE October 2002
10. Computing "Unreserved TE-Class [i]" and Admission Control Rules
10.1. Computing "Unreserved TE-Class [i]"
We first observe that, for existing TE, details on admission control
algorithms for TE LSPs, and consequently details on formulas for
computing the unreserved bandwidth, are outside the scope of the
current IETF work. This is left for vendor differentiation. Note that
this does not compromise interoperability across various
implementations since the TE schemes rely on LSRs to advertise their
local view of the world in terms of Unreserved Bw to other LSRs. This
way, regardless of the actual local admission control algorithm used
on one given LSR, Constraint Based Routing on other LSRs can rely on
advertised information to determine whether an additional LSP will be
accepted or rejected by the given LSR. The only requirement is that
an LSR advertises unreserved bandwidth values which are consistent
with its specific local admission control algorithm and take into
account the holding preemption priority of established LSPs.
In the context of DS-TE, again, details on admission control
algorithms are left for vendor differentiation and formulas for
computing the unreserved bandwidth for TE-Class[i] are outside the
scope of this specification. However, DS-TE places the additional
requirement on the LSR that the unreserved bandwidth values
advertised MUST reflect all of the Bandwidth Constraints relevant to
the CT associated with TE-Class[i] in accordance with the Bandwidth
Constraints Model. Thus, formulas for computing "Unreserved TE-Class
[i]" depend on the Bandwidth Constraints model in use and MUST
reflect how bandwidth constraints apply to CTs.
As with existing TE, DS-TE LSRs MUST consider the holding preemption
priority of established LSPs (as opposed to their set-up preemption
priority) for the purpose of computing the unreserved bandwidth for
TE-Class [i].
10.2. Admission Control Rules
A DS-TE LSR MUST support the following admission control rule:
Regardless of how the admission control algorithm actually computes
the unreserved bandwidth for TE-Class[i] for one of its local link,
an LSP of bandwidth B, of set-up preemption priority p and of Class-
Type CTc is admissible on that link iff:
B <= unreserved bandwidth for TE-Class[i], AND
B <= Max Link Bandwidth
Where
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Protocols for Diff-Serv-aware TE October 2002
- TE-Class [i] maps to < CTc , p > in the LSR's configured TE-
Class mapping
- Max Link Bandwidth is the maximum link bandwidth configured
on the link and advertised in IGP.
10.3. Support of Optional Local Overbooking Method
We remind the reader that, as discussed in section 4.1.2, the
"LSP/link size overbooking" method (which does not use the Local
Overbooking Multipliers) is expected to be sufficient in many DS-TE
environments. It is expected that the optional Local Overbooking
method (and LOMs) would only be used in specific environments, in
particular where different overbooking ratios need to be enforced on
different links of the DS-TE domain and cross-effect of overbooking
across CTs needs to be accounted for very accurately.
This section discusses the impact of the optional local overbooking
on IGP advertisements as well as on admission control rules. This is
only applicable in the cases where the optional local overbooking
method is indeed supported by the DS-TE LSRs and actually deployed.
Let us define "Reserved(CTc)" as the sum of the bandwidth reserved by
all established LSPs which belong to CTc. When LOMs are not used, the
bandwidth constraints relevant to CTc according to the Bandwidth
Constraints model in use effectively apply to "Reserved(CTc)".
Let us define "Normalised(CTc)" as "Reserved(CTc)/LOM(c)". When LOMs
are used, the bandwidth constraints relevant to CTc according to the
Bandwidth Constraints model in use now effectively apply to
"Normalised(CTc)".
10.3.1. Bandwidth Constraints
When the optional Local Overbooking method is supported, the
relationship among the Bandwidth Constraints values to be enforced by
the LSR is not modified. The LSR MUST still ensure that BCi is
configured smaller or equal to BCj, where i is greater than j.
When the Bandwidth Constraints are optionally advertised in the
Bandwidth Constraints sub-TLV, the values advertised are the BC
values as configured (i.e. LOMs do not affect the advertisement of
BCs).
10.3.2. Computing "Unreserved TE-Class [i]"
As discussed above, details on formulas for computing the unreserved
bandwidth, are outside the scope of the current IETF work. However,
when the Local Overbooking method is supported, the advertised
unreserved bandwidth values MUST take into account the per-CT Local
Overbooking Multipliers in order to :
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Protocols for Diff-Serv-aware TE October 2002
(i) reflect the fact that Bandwidth Constraints are now
applied to "Normalised(CTc)" instead of
"Reserved(CTc)" as explained above
(ii) express bandwidth in a way which is directly
comparable to the "raw" bandwidth configured for LSPs
on their Headend (and which obviously does not take
into account the LOMs that may be configured on
various links along the path).
Again, because the formulas for computing "Unreserved TE-Class [i]"
MUST reflect all the relevant bandwidth constraints, these formulas
depend on the Bandwidth Constraints model in use.
Note that factoring in LOMs may result in the Unreserved Bandwidth
values advertised for a particular CT being larger than one or more
of the advertised BCs relevant to this CT (since advertised BCs value
are not affected by LOMs).
10.3.3. Max Link Bandwidth
The Max Link bandwidth parameter effectively controls the maximum
size of a single LSP. The value advertised in the Max Link Bandwidth
sub-TLV is the value as configured locally (i.e. LOMs do not affect
the advertisement of the Max Link Bandwidth).
10.3.4. Admission Control Rules
When the optional Local Overbooking method is supported, the DS-TE
LSR MUST support the same admission control rules as without LOM:
Regardless of how the admission control algorithm actually computes
the unreserved bandwidth for TE-Class[i] for one of its local link,
an LSP of bandwidth B, of set-up preemption priority p and of Class-
Type CTc is admissible on that link iff:
(i) B <= unreserved bandwidth for TE-Class[i], AND
(ii) B <= Max Link Bandwidth
Where
- TE-Class [i] maps to < CTc , p > in the LSR's configured TE-
Class mapping
- Max Link Bandwidth is the maximum link bandwidth configured
on the link and advertised in IGP.
Condition (i) above applies to B [and not B/LOM(c) ] because the LOM
is already factored in the unreserved bandwidth values.
Condition (ii) above also applies to B [and not B/LOM(c) ]. Condition
(ii) controls the maximum bandwidth that a single LSP can grab on its
own, by limiting it to the Maximum Link Bandwidth. Since
"overbooking" is usually only (fully) achievable when a sufficient
Le Faucheur et. al 21
Protocols for Diff-Serv-aware TE October 2002
number of LSPs share the Link Bandwidth, it is not appropriate to
factor in the LOM when assessing the maximum bandwidth that a single
LSP can grab on a link.
10.3.5. Example Usage of LOM
To illustrate how the local overbooking method affects bandwidth
accounting as described above, let's consider the trivial case where
a single CT (CT0) is used.
Let's assume that the TE-Class mapping is the following:
TE-Class <--> CT, preemption
==============================
0 CT0, 0
rest unused
Let's assume that on a given link, BCs and LOMs are configured in the
following way:
BC0 = 200
LOM(0) = 4 (i.e. = 400%)
Let's further assume that the DS-TE LSR uses the basic admission
control rule where exact bandwidth of each established LSP is simply
deducted form available bandwidth.
If there is no established LSP on the considered link, the LSR will
advertise for that link in IGP :
Unreserved TE-Class [0] = 200 x 4 = 800
Note that the value advertised for Unreserved Bandwidth is larger
than the advertised value of BC0 (200).
If there is only a single established LSP, with CT=CT0 and BW=100,
the LSR will advertise:
Unreserved TE-Class [0] = 700
11. Security Considerations
The solution is not expected to add specific security requirements
beyond those of Diff-Serv and existing TE. The security mechanisms
currently used with Diff-Serv and existing TE can be used with this
solution.
12. Acknowledgments
We thank Martin Tatham, Angela Chiu and Pete Hicks for their earlier
contribution in this work. We also thank Sanjaya Choudhury for his
thorough review and suggestions.
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Protocols for Diff-Serv-aware TE October 2002
13. IANA Considerations
This document defines a number of objects with implications for IANA.
This document defines in section 5.1 a new sub-TLV, the "Bandwidth
Constraints" sub-TLV, for the OSPF "Link" TLV [OSPF-TE]. A sub-TLV
Type in the range 10 to 32767 needs to be assigned by Expert Review.
This sub-TLV Type also needs to be registered by IANA.
This document defines in section 5.3 another new sub-TLV, the "Local
Overbooking Multiplier" sub-TLV, for the OSPF "Link" TLV [OSPF-TE]. A
sub-TLV Type in the range 10 to 32767 needs to be assigned by Expert
Review. This sub-TLV Type also needs to be registered by IANA.
This document defines in section 5.1 a new sub-TLV, the "Bandwidth
Constraints" sub-TLV, for the ISIS "Extended IS Reachability" TLV
[ISIS-TE]. A sub-TLV Type needs to be assigned by Expert Review. This
sub-TLV Type also needs to be registered by IANA.
This document defines in section 5.3 another new sub-TLV, the "Local
Overbooking Multiplier" sub-TLV, for the ISIS "Extended IS
Reachability" TLV [ISIS-TE]. A sub-TLV Type needs to be assigned by
Expert Review. This sub-TLV Type also needs to be registered by IANA.
This document defines in section 5.3 a "Bandwidth Constraint Model
Id" field within the "Bandwidth Constraints" sub-TLV. This document
also defines in section 5.3 two values for this field (0 and 1).
Future allocations of values in this space and in the range 2 to 127
should be handled by IANA using the First Come First Served policy
defined in [IANA]. Values in the range 128 to 255 are for
experimental use.
This document defines in section 6.2.1 a new RSVP object, the
CLASSTYPE object. This object requires a number from the space
defined in [RSVP] for those objects which, if not understood, cause
the entire RSVP message to be rejected with an error code of "Unknown
Object Class". Such objects are identified by a zero in the most
significant bit of the class number. Within that space, this object
requires a number to be allocated by IANA from the "IETF Consensus"
space.
This document defines in section 6.5 a new RSVP error code, the
"Diff-Serv-aware TE Error". This new Error code needs to be allocated
by IANA. This document defines values 1 through 7 of the value field
to be used within the ERROR_SPEC object for the "Diff-Serv-aware TE
error" code. Future allocations of values in this space should be
handled by IANA using the First Come First Served policy defined in
[IANA].
14. Normative References
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Protocols for Diff-Serv-aware TE October 2002
[DSTE-REQ] Le Faucheur et al, Requirements for support of Diff-Serv-
aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-reqts-06.txt,
September 2002.
[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
katz-yeung-ospf-traffic-08.txt, September 2002.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-04.txt, October 2001.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RSVP] Braden et al, "Resource ReSerVation Protocol (RSVP) - Version
1 Functional Specification", RFC 2205, September 1997.
[DIFF-MPLS] Le Faucheur et al, "MPLS Support of Diff-Serv", RFC3270,
May 2002.
15. Informative References
[DSTE-RUSSIAN] Le Faucheur F., "Russian Dolls Bandwidth Constraints
Model for DS-TE", draft-ietf-tewg-diff-te-russian-00.txt, October
2002.
16. Editor's 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
Appendix A - Prediction for Multiple Path Computation
There are situations where a Head-End needs to compute paths for
multiple LSPs over a short period of time. There are potential
advantages for the Head-end in trying to predict the impact of the n-
th LSP on the unreserved bandwidth when computing the path for the
(n+1)-th LSP, before receiving updated IGP information. One example
would be to perform better load-distribution of the multiple LSPs
across multiple paths. Another example would be to avoid CAC
rejection when the (n+1)-th LSP would no longer fit on a link after
establishment of the n-th LSP. While there are also a number of
conceivable scenarios where doing such predictions might result in a
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Protocols for Diff-Serv-aware TE October 2002
worse situation, it is more likely to improve the situation. As a
matter of fact, a number of network administrators have elected to
use such predictions when deploying existing TE.
Such predictions are local matters, are optional and are outside the
scope of this specification.
Where such predictions are not used, the optional Bandwidth
Constraint sub-TLV and the optional Local Overbooking Multiplier sub-
TLV need not be advertised in IGP since the information contained in
the Unreserved Bw sub-TLV is all that is required by Head-Ends to
perform Constraint Based Routing.
Where such predictions are used on Head-Ends, the optional Bandwidth
Constraint sub-TLV (and the optional Local Overbooking Multiplier
sub-TLV if different overbooking ratios need to be supported on
different links) MAY be advertised in IGP. This is in order for the
Head-ends to predict as accurately as possible how an LSP affects
unreserved bandwidth values for subsequent LSPs.
Remembering that actual admission control algorithms are left for
vendor differentiation, we observe that predictions can only be
performed effectively when the Head-end LSR predictions are based on
the same (or a very close) admission control algorithm as used by
other LSRs.
Appendix B - Solution Evaluation
1. Satisfying Detailed Requirements
This DS-TE Solution addresses the scenarios presented in [DSTE-REQ].
It also satisfies all the detailed requirements presented in [DSTE-
REQ].
2. Flexibility
This DS-TE solution supports 8 CTs. It is entirely flexible as to how
Traffic Trunks are grouped together into a CT.
3. Extendibility
A maximum of 8 CTs is considered by the authors of this document as
more than comfortable. However, this solution could be extended to
support more CTs if deemed necessary in the future. However, this
would necessitate additional IGP extensions beyond those specified in
this document.
4. Scalability
This DS-TE solution is expected to have a very small scalability
impact compared to existing TE.
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From an IGP viewpoint, the amount of mandatory information to be
advertised is identical to existing TE. Two additional sub-TLVs have
been specified, but their use is optional and those contained a
limited amount of static information (at most 8 Bandwidth Constraints
and 8 LOMs).
We expect no noticeable impact on LSP Path computation since, as with
existing TE, this solution only require CSPF to consider a single
unreserved bandwidth value for any given LSP.
From a signaling viewpoint we expect no significant impact due to
this solution since it only requires processing of one additional
information (the Class-Type) and does not significantly increase the
likelihood of CAC rejection. Note that DS-TE has some inherent impact
on LSP signaling in the sense that it assumes that different classes
of traffic are split over different LSPs so that more LSPs need to be
signaled; but this is due to the DS-TE concept itself and not to the
actual DS-TE solution discussed here.
5. Backward Compatibility/Migration
This solution is expected to allow smooth migration from existing TE
to DS-TE. This is because existing TE can be supported exactly as
today as a particular configuration of DS-TE. This means that an
"upgraded" LSR with a DS-TE implementation can directly interwork
with an "old" LSR supporting existing TE only.
This solution is expected to allow smooth migration when increasing
the number of CTs actually deployed since it only requires
configuration changes. however, these changes must be performed in a
coordinated manner across the DS-TE domain.
Appendix C - Interoperability with non DS-TE capable LSRs
This DSTE solution allows operations in a hybrid network where some
LSRs are DS-TE capable while some LSRs and not DS-TE capable, which
may occur during migration phases. This Appendix discusses the
constraints and operations in such hybrid networks.
We refer to the set of DS-TE capable LSRs as the DS-TE domain. We
refer to the set of non DS-TE capable (but TE capable) LSRs as the
TE-domain.
Hybrid operations requires that the TE-class mapping in the DS-TE
domain is configured so that:
- a TE-class exist for CT0 for every preemption priority
actually used in the TE domain
- the index in the TE-class mapping for each of these TE-
classes is equal to the preemption priority.
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Protocols for Diff-Serv-aware TE October 2002
For example, imagine the TE domain uses preemption 2 and 3. Then, DS-
TE can be deployed in the same network by including the following TE-
classes in the TE-class mapping:
i <---> CT preemption
====================================
2 CT0 2
3 CT0 3
Another way to look at this is to say that, the whole TE-class
mapping does not have to be consistent with the TE domain, but the
subset of this TE-Class mapping applicable to CT0 must effectively be
consistent with the TE domain.
In such hybrid networks :
- CT0 LSPs can be established by both DS-TE capable LSRs and
non-DSTE capable LSRs
- CT0 LSPs can transit via (or terminate at) both DS-TE capable
LSRs and non-DSTE capable LSRs
- LSPs from other CTs can only be established by DS-TE capable
LSRs
- LSPs from other CTs can only transit via (or terminate at)
DS-TE capable LSRs
Note that, for such hybrid operation, non DS-TE capable LSRs need to
be able to accept Unreserved Bw sub-TLVs containing non decreasing
bandwidth values (ie with Unreserved [p] < Unreserved [q] with p <q).
Also, the Bandwidth Constraints sub-TLV needs to be advertised in the
DS-TE domain and the Maximum Reservable Bandwidth sub-TLV needs to be
advertised in the TE-domain and the DS-TE domain.
Let us consider, the following example to illustrate operations:
LSR0--------LSR1----------LSR2
Link01 Link12
Where:
LSR0 is a non-DS-TE capable LSR
LSR1 and LSR2 are DS-TE capable LSRs
Let's assume again that preemption 2 and 3 are used in the TE-domain
and that the following TE-class mapping is configured on LSR1 and
LSR2:
i <---> CT preemption
====================================
0 CT1 0
1 CT1 1
2 CT0 2
3 CT0 3
rest unused
LSR0 is configured with a Max Reservable bandwidth=m for Link01.
LSR1 is configured with a BC0=x0 and a BC1=x1(possibly=0) for Link01.
Le Faucheur et. al 27
Protocols for Diff-Serv-aware TE October 2002
LSR0 will advertise in IGP for Link01:
- Max Reservable Bw sub-TLV = <m>
- Unreserved Bw sub-TLV =
<CT0/0,CT0/1,CT0/2,CT0/3,CT0/4,CT0/5,CT0/6,CT0/7>
On receipt of such advertisement, LSR1 will:
- understand that LSR0 is not DS-TE capable because it
advertised a Max Reservable Bw sub-TLV and no Bandwidth
Constraint sub-TLV
- conclude that only CT0 LSPs can transit via LSR0 and that
only the values CT0/2 and CT0/3 are meaningful in the
Unreserved Bw sub-TLV. LSR1 may effectively behave as if the
six other values contained in the Unreserved Bw sub-TLV were
set to zero.
LSR1 will advertise in IGP for Link01:
- Max Reservable Bw sub-TLV = <x0>
- Bandwidth Constraint sub-TLV = <BC Model ID=0, x0,x1>
- Unreserved Bw sub-TLV = <CT1/0,CT1/1,CT0/2,CT0/3,0,0,0,0>
On receipt of such advertisement, LSR0 will:
- Ignore the Bandwidth Constraint sub-TLV (unrecognized)
- Correctly process CT0/2 and CT0/3 in the Unreserved Bw sub-
TLV and use these values for CTO LSP establishment
- Incorrectly believe that the other values contained in the
Unreserved Bw sub-TLV relates to other preemption priorities
for CT0, but will actually never use those since we assume
that only preemption 2 and 3 are used in the TE domain.
Le Faucheur et. al 28