Network Working Group F. Le Faucheur, Ed. Request for Comments: 4127 Cisco Systems, Inc. Category: Experimental June 2005 Russian Dolls Bandwidth Constraints Model for Diffserv-aware MPLS Traffic Engineering Status of This Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2005).
AbstractThis document provides specifications for one Bandwidth Constraints Model for Diffserv-aware MPLS Traffic Engineering, which is referred to as the Russian Dolls Model. 1. Introduction ....................................................2 1.1. Specification of Requirements ..............................2 2. Contributing Authors ............................................3 3. Definitions .....................................................4 4. Russian Dolls Model Definition ..................................5 5. Example Formulas for Computing "Unreserved TE-Class [i]" with Russian Dolls Model .............................................7 6. Receiving Both Maximum Reservable Bandwidth and Bandwidth Constraints sub-TLVs ............................................8 7. Security Considerations .........................................8 8. IANA Considerations .............................................8 9. Acknowledgements ................................................9 Appendix A: Addressing [DSTE-REQ] Scenarios .......................10 Normative References ..............................................11 Informative References ............................................12
DSTE-REQ] presents the Service Providers requirements for support of Diffserv-aware MPLS Traffic Engineering (DS-TE). This includes the fundamental requirement to be able to enforce different Bandwidth Constraints for different classes of traffic. [DSTE-REQ] also defines the concept of Bandwidth Constraints Model for DS-TE and states that "The DS-TE technical solution MUST specify at least one Bandwidth Constraints Model and MAY specify multiple Bandwidth Constraints Models". This document provides a detailed description of one particular Bandwidth Constraints Model for DS-TE which is introduced in [DSTE-REQ] and called the Russian Dolls Model (RDM). [DSTE-PROTO] specifies the Interior Gateway Protocol (IGP) and RSVP- TE signaling extensions for support of DS-TE. These extensions support RDM. RFC2119].
DSTE-REQ] are repeated here: 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 setup priority, the holding priority, or both. A number of recovery mechanisms under investigation or specification in the IETF take advantage of the concept of bandwidth sharing across particular sets of LSPs. "Shared Mesh Restoration" in [GMPLS-RECOV] and "Facility-based Computation Model" in [MPLS-BACKUP] are example mechanisms that increase bandwidth efficiency by sharing bandwidth across backup LSPs protecting against independent failures. To ensure that the notion of "Reserved (CTc)" introduced in [DSTE-REQ] is compatible with such a concept of bandwidth sharing across multiple LSPs, the wording of the "Reserved (CTc)" definition provided in [DSTE-REQ] is generalized into the following: Reserved (CTc): For a given Class-Type CTc ( 0 <= c <= MaxCT ), let us define "Reserved(CTc)" as the total amount of the bandwidth reserved by all the established LSPs which belong to CTc. With this generalization, the Russian Dolls Model definition provided in this document is compatible with Shared Mesh Restoration defined in [GMPLS-RECOV], so that DS-TE and Shared Mesh Protection can operate simultaneously. This assumes that Shared Mesh Restoration operates independently within each DS-TE Class-Type and does not operate across Class-Types (for example, backup LSPs protecting Primary LSPs of CTx also need to belong to CTx; Excess Traffic LSPs sharing bandwidth with Backup LSPs of CTx also need to belong to CTx).
We also introduce the following definition: Reserved(CTb,q): Let us define "Reserved(CTb,q)" as the total amount of the bandwidth reserved by all the established LSPs that belong to CTb and have a holding priority of q. Note that if q and CTb do not form one of the 8 possible configured TE-Classes, then there cannot be any established LSPs that belongs to CTb and has a holding priority of q; therefore, in this case, Reserved(CTb,q) = 0.
Purely for illustration purposes, the diagram below represents the Russian Dolls Bandwidth Constraints Model in a pictorial manner when 3 Class-Types are active: I------------------------------------------------------I I-------------------------------I I I--------------I I I I CT2 I CT2+CT1 I CT2+CT1+CT0 I I--------------I I I I-------------------------------I I I------------------------------------------------------I I-----BC2------> I----------------------BC1------> I------------------------------BC0=Max Reservable Bw---> While simpler Bandwidth Constraints models or, conversely, more flexible/sophisticated Bandwidth Constraints models can be defined, the Russian Dolls Model is attractive in some DS-TE environments for the following reasons: - Although it is a little less intuitive than the Maximum Allocation Model (see [DSTE-MAM]), RDM is still a simple model to conceptualize. - RDM can be used simultaneously to ensure bandwidth efficiency and to protect against QoS degradation of all CTs, whether preemption is used or not. - RDM can be used in conjunction with preemption to simultaneously achieve (i) isolation across CTs (so that each CT is guaranteed its share of bandwidth no matter the level of contention by other classes), (ii) bandwidth efficiency, and (iii) protection against QoS degradation of all CTs. - RDM only requires limited protocol extensions such as the ones defined in [DSTE-PROTO]. RDM may not be attractive in some DS-TE environments for the following reasons: - if the usage of preemption is precluded for some administrative reason, while RDM can still ensure bandwidth efficiency and protection against QoS degradation of all CTs, RDM cannot guarantee isolation across Class-Types. Additional considerations on the properties of RDM can be found in [BC-CONS] and [BC-MODEL].
As a simple example usage of the "Russian Dolls" Bandwidth Constraints Model, a network administrator, using one CT for Voice (CT1) and one CT for data (CT0), might configure on a given link: - BC0 = Max-Reservable - Bw = 2.5 Gb/s (i.e., Voice + Data is limited to 2.5 Gb/s) - BC1 = 1.5 Gb/s (i.e., Voice is limited to 1.5 Gb/s). DSTE-PROTO], formulas for computing "Unreserved TE- Class [i]" MUST reflect all of the Bandwidth Constraints relevant to the CT associated with TE-Class[i], and thus, depend on the Bandwidth Constraints Model. Thus, a DS-TE LSR implementing RDM MUST reflect the RDM Bandwidth Constraints defined in section 4 above when computing "Unreserved TE-Class [i]". As explained in [DSTE-PROTO], the details of admission control algorithms, as well as formulas for computing "Unreserved TE-Class [i]", are outside the scope of the IETF work. Keeping that in mind, we provide in this section an example for illustration purposes, of how values for the unreserved bandwidth for TE-Class[i] might be computed with RDM. In the example, we assume the basic admission control algorithm, which simply deducts the exact bandwidth of any established LSP from all of the Bandwidth Constraints relevant to the CT associated with that LSP. We assume that: TE-Class [i] <--> < CTc , preemption p> in the configured TE-Class mapping. For readability, formulas are first shown assuming only 3 CTs are active. The formulas are then extended to cover the cases where more CTs are used. If CTc = CT0, then "Unreserved TE-Class [i]" = [ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 2 If CTc = CT1, then "Unreserved TE-Class [i]" = MIN [ [ BC1 - SUM ( Reserved(CTb,q) ) ] for q <= p and 1 <= b <= 2, [ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 2 ]
If CTc = CT2, then "Unreserved TE-Class [i]" = MIN [ [ BC2 - SUM ( Reserved(CTb,q) ) ] for q <= p and 2 <= b <= 2, [ BC1 - SUM ( Reserved(CTb,q) ) ] for q <= p and 1 <= b <= 2, [ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 2 ] The formula can be generalized to 8 active CTs and expressed in a more compact way in the following: "Unreserved TE-Class [i]" = MIN [ [ BCc - SUM ( Reserved(CTb,q) ) ] for q <= p and c <= b <= 7, [ BC(c-1) - SUM ( Reserved(CTb,q) ) ] for q <= p and (c-1)<= b <= 7, . . . [ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 7, ] where: TE-Class [i] <--> < CTc , preemption p> in the configured TE-Class mapping. DSTE-PROTO] states that "A DS-TE LSR, which does advertise BCs, MUST use the new "Bandwidth Constraints" sub-TLV (in addition to the existing Maximum Reservable Bandwidth sub-TLV) to do so." With RDM, BC0 is equal to the Maximum Reservable Bandwidth because they both represent the aggregate constraint across all CTs. Thus, a DS-TE LSR, receiving both the "Maximum Reservable Bw" sub-TLV and the new "Bandwidth Constraints" sub-TLV (which contains BC0) for a given link where the RDM model is used, MAY ignore the "Maximum Reservable Bw" sub-TLV. DSTE-PROTO]. Those apply independently of the Bandwidth Constraints Model, including RDM specified in this document. DSTE-PROTO] defines a new name space for "Bandwidth Constraints Model Id". The guidelines for allocation of values in that name space are detailed in section 13.1 of [DSTE-PROTO]. In accordance
with these guidelines, the IANA has assigned a Bandwidth Constraints Model Id for RDM from the range 0-239 (which is to be managed as per the "Specification Required" policy defined in [IANA-CONS]). Bandwidth Constraints Model Id 0 was allocated by IANA to RDM.
By also configuring: - every CT2/Voice TE-LSP with preemption = 0 - every CT1/Premium Data TE-LSP with preemption = 1 - every CT0/Best-Effort TE-LSP with preemption = 2 DS-TE with the Russian Dolls Model will also ensure that: - CT2 Voice LSPs always have first preemption priority in order to use the CT2 capacity - CT1 Premium Data LSPs always have second preemption priority in order to use the CT1 capacity - Best-Effort can use up to link capacity of what is left by CT2 and CT1. Optional automatic adjustment of Diffserv scheduling configuration could be used for maintaining very strict relationships between the amounts of established traffic of each Class Type and corresponding Diffserv resources. [DSTE-REQ] Le Faucheur, F. and W. Lai, "Requirements for Support of Differentiated Services-aware MPLS Traffic Engineering", RFC 3564, July 2003.
[DSTE-PROTO] Le Faucheur, F., Ed., "Protocol Extensions for Support of Diffserv-aware MPLS Traffic Engineering", RFC 4124, June 2005. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [IANA-CONS] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [BC-CONS] Le Faucheur, F., "Considerations on Bandwidth Constraints Model for DS-TE", Work in Progress, June 2002. [BC-MODEL] Lai, W., "Bandwidth Constraints Models for Differentiated Services (Diffserv)-aware MPLS Traffic Engineering: Performance Evaluation", RFC 4128, June 2005. [DSTE-MAM] Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth Constraints Model for Diffserv-aware MPLS Traffic Engineering", RFC 4125, June 2005. [GMPLS-RECOV] Lang, et al., "Generalized MPLS Recovery Functional Specification", Work in Progress. [MPLS-BACKUP] Vasseur, et al., "MPLS Traffic Engineering Fast Reroute: Bypass Tunnel Path Computation for Bandwidth Protection", Work in Progress.
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