Network Working Group F. Le Faucheur, Editor Request for Comments: 3270 L. Wu Category: Standards Track B. Davie Cisco Systems S. Davari PMC-Sierra Inc. P. Vaananen Nokia R. Krishnan Axiowave Networks P. Cheval Alcatel J. Heinanen Song Networks May 2002
Multi-Protocol Label Switching (MPLS) Support of Differentiated Services
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a flexible solution for support of Differentiated Services (Diff-Serv) over Multi-Protocol Label Switching (MPLS) networks.
This solution allows the MPLS network administrator to select how Diff-Serv Behavior Aggregates (BAs) are mapped onto Label Switched Paths (LSPs) so that he/she can best match the Diff-Serv, Traffic Engineering and protection objectives within his/her particular network. For instance, this solution allows the network administrator to decide whether different sets of BAs are to be mapped onto the same LSP or mapped onto separate LSPs.
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In an MPLS domain [MPLS_ARCH], when a stream of data traverses a common path, a Label Switched Path (LSP) can be established using MPLS signaling protocols. At the ingress Label Switch Router (LSR), each packet is assigned a label and is transmitted downstream. At each LSR along the LSP, the label is used to forward the packet to the next hop.
In a Differentiated Service (Diff-Serv) domain [DIFF_ARCH] all the IP packets crossing a link and requiring the same Diff-Serv behavior are said to constitute a Behavior Aggregate (BA). At the ingress node of the Diff-Serv domain, the packets are classified and marked with a Diff-Serv Code Point (DSCP) which corresponds to their Behavior Aggregate. At each transit node, the DSCP is used to select the Per Hop Behavior (PHB) that determines the scheduling treatment and, in some cases, drop probability for each packet.
This document specifies a solution for supporting the Diff-Serv Behavior Aggregates whose corresponding PHBs are currently defined (in [DIFF_HEADER], [DIFF_AF], [DIFF_EF]) over an MPLS network. This solution also offers flexibility for easy support of PHBs that may be defined in the future.
This solution relies on the combined use of two types of LSPs:
- LSPs which can transport multiple Ordered Aggregates, so that the EXP field of the MPLS Shim Header conveys to the LSR the PHB to be applied to the packet (covering both information about the packet's scheduling treatment and its drop precedence).
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- LSPs which only transport a single Ordered Aggregate, so that the packet's scheduling treatment is inferred by the LSR exclusively from the packet's label value while the packet's drop precedence is conveyed in the EXP field of the MPLS Shim Header or in the encapsulating link layer specific selective drop mechanism (ATM, Frame Relay, 802.1).
As mentioned in [DIFF_HEADER], "Service providers are not required to use the same node mechanisms or configurations to enable service differentiation within their networks, and are free to configure the node parameters in whatever way that is appropriate for their service offerings and traffic engineering objectives". Thus, the solution defined in this document gives Service Providers flexibility in selecting how Diff-Serv classes of service are Routed or Traffic Engineered within their domain (e.g., separate classes of services supported via separate LSPs and Routed separately, all classes of service supported on the same LSP and Routed together).
Because MPLS is path-oriented it can potentially provide faster and more predictable protection and restoration capabilities in the face of topology changes than conventional hop by hop routed IP systems. In this document we refer to such capabilities as "MPLS protection". Although such capabilities and associated mechanisms are outside the scope of this specification, we note that they may offer different levels of protection to different LSPs. Since the solution presented here allow Service Providers to choose how Diff-Serv classes of services are mapped onto LSPs, the solution also gives Service Providers flexibility in the level of protection provided to different Diff-Serv classes of service (e.g., some classes of service can be supported by LSPs which are protected while some other classes of service are supported by LSPs which are not protected).
Furthermore, the solution specified in this document achieves label space conservation and reduces the volume of label set-up/tear-down signaling where possible by only resorting to multiple LSPs for a given Forwarding Equivalent Class (FEC) [MPLS_ARCH] when useful or required.
This specification allows support of Differentiated Services for both IPv4 and IPv6 traffic transported over an MPLS network. This document only describes operations for unicast. Multicast support is for future study.
The solution described in this document does not preclude the signaled or configured use of the EXP bits to support Explicit Congestion Notification [ECN] simultaneously with Diff-Serv over MPLS. However, techniques for supporting ECN in an MPLS environment are outside the scope of this document.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
The reader is assumed to be familiar with the terminology of [MPLS_ARCH], [MPLS_ENCAPS], [MPLS_ATM], [MPLS_FR], including the following:
The reader is assumed to be familiar with the terminology of [DIFF_ARCH], [DIFF_HEADER], [DIFF_AF], [DIFF_EF], including the following:
AF Assured Forwarding
BA Behavior Aggregate
CS Class Selector
DF Default Forwarding
DSCP Differentiated Services Code Point
EF Expedited Forwarding
PHB Per Hop Behavior
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The reader is assumed to be familiar with the terminology of [DIFF_NEW], including the following:
OA Ordered Aggregate. The set of Behavior Aggregates which share an ordering constraint.
PSC PHB Scheduling Class. The set of one or more PHB(s) that are applied to the Behavior Aggregate(s) belonging to a given OA. For example, AF1x is a PSC comprising the AF11, AF12 and AF13 PHBs. EF is an example of PSC comprising a single PHB, the EF PHB.
The following acronyms are also used:
CLP Cell Loss Priority
DE Discard Eligibility
SNMP Simple Network Management Protocol
Finally, the following acronyms are defined in this specification:
A single LSP can be used to support one or more OAs. Such LSPs can support up to eight BAs of a given FEC, regardless of how many OAs these BAs span. With such LSPs, the EXP field of the MPLS Shim Header is used by the LSR to determine the PHB to be applied to the packet. This includes both the PSC and the drop preference.
We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since the PSC of a packet transported on this LSP depends on the EXP field value for that packet.
The mapping from the EXP field to the PHB (i.e., to PSC and drop precedence) for a given such LSP, is either explicitly signaled at label set-up or relies on a pre-configured mapping.
Detailed operations of E-LSPs are specified in section 3 below.
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A separate LSP can be established for a single <FEC, OA> pair. With such LSPs, the PSC is explicitly signaled at the time of label establishment, so that after label establishment, the LSR can infer exclusively from the label value the PSC to be applied to a labeled packet. When the Shim Header is used, the Drop Precedence to be applied by the LSR to the labeled packet, is conveyed inside the labeled packet MPLS Shim Header using the EXP field. When the Shim Header is not used (e.g., MPLS Over ATM), the Drop Precedence to be applied by the LSR to the labeled packet is conveyed inside the link layer header encapsulation using link layer specific drop precedence fields (e.g., ATM CLP).
We refer to such LSPs as "Label-Only-Inferred-PSC LSPs" (L-LSP) since the PSC can be fully inferred from the label without any other information (e.g., regardless of the EXP field value). Detailed operations of L-LSPs are specified in section 4 below.
For a given FEC, and unless media specific restrictions apply as identified in the sections 7, 8 and 9 below, this specification allows any one of the following combinations within an MPLS Diff-Serv domain:
- zero or any number of E-LSPs, and
- zero or any number of L-LSPs.
The network administrator selects the actual combination of LSPs from the set of allowed combinations and selects how the Behavior Aggregates are actually transported over this combination of LSPs, in order to best match his/her environment and objectives in terms of Diff-Serv support, Traffic Engineering and MPLS Protection. Criteria for selecting such a combination are outside the scope of this specification.
For a given FEC, there may be more than one LSP carrying the same OA, for example for purposes of load balancing of the OA; However in order to respect ordering constraints, all packets of a given microflow, possibly spanning multiple BAs of a given Ordered Aggregate, MUST be transported over the same LSP. Conversely, each LSP MUST be capable of supporting all the (active) BAs of a given OA.
Examples of deployment scenarios are provided for information in APPENDIX A.
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[MPLS_ARCH] states in section `2.1. Overview' that: `Some routers analyze a packet's network layer header not merely to choose the packet's next hop, but also to determine a packet's "precedence" or "class of service". They may then apply different discard thresholds or scheduling disciplines to different packets. MPLS allows (but does not require) the precedence or class of service to be fully or partially inferred from the label. In this case, one may say that the label represents the combination of a FEC and a precedence or class of service.'
In line with this, we observe that:
- With E-LSPs, the label represents the combination of a FEC and the set of BAs transported over the E-LSP. Where all the supported BAs are transported over an E-LSP, the label then represents the complete FEC.
- With L-LSPs, the label represents the combination of a FEC and an OA.
Regardless of which label binding protocol is used, E-LSPs and L-LSPs may be established with or without bandwidth reservation.
Establishing an E-LSP or L-LSP with bandwidth reservation means that bandwidth requirements for the LSP are signaled at LSP establishment time. Such signaled bandwidth requirements may be used by LSRs at establishment time to perform admission control of the signaled LSP over the Diff-Serv resources provisioned (e.g., via configuration, SNMP or policy protocols) for the relevant PSC(s). Such signaled bandwidth requirements may also be used by LSRs at establishment time to perform adjustment to the Diff-Serv resources associated with the relevant PSC(s) (e.g., adjust PSC scheduling weight).
Note that establishing an E-LSP or L-LSP with bandwidth reservation does not mean that per-LSP scheduling is required. Since E-LSPs and L-LSPs are specified in this document for support of Differentiated Services, the required forwarding treatment (scheduling and drop policy) is defined by the appropriate Diff-Serv PHB. This forwarding treatment MUST be applied by the LSR at the granularity of the BA and MUST be compliant with the relevant PHB specification.
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When bandwidth requirements are signaled at the establishment of an L-LSP, the signaled bandwidth is obviously associated with the L- LSP's PSC. Thus, LSRs which use the signaled bandwidth to perform admission control may perform admission control over Diff-Serv resources, which are dedicated to the PSC (e.g., over the bandwidth guaranteed to the PSC through its scheduling weight).
When bandwidth requirements are signaled at the establishment of an E-LSP, the signaled bandwidth is associated collectively with the whole LSP and therefore with the set of transported PSCs. Thus, LSRs which use the signaled bandwidth to perform admission control may perform admission control over global resources, which are shared by the set of PSCs (e.g., over the total bandwidth of the link).
Examples of scenarios where bandwidth reservation is not used and scenarios where bandwidth reservation is used are provided for information in APPENDIX B.
2. Label Forwarding Model for Diff-Serv LSRs and Tunneling Models
Since different Ordered Aggregates of a given FEC may be transported over different LSPs, the label swapping decision of a Diff-Serv LSR clearly depends on the forwarded packet's Behavior Aggregate. Also, since the IP DS field of a forwarded packet may not be directly visible to an LSR, the way to determine the PHB to be applied to a received packet and to encode the PHB into a transmitted packet, is different than a non-MPLS Diff-Serv Router.
Thus, in order to describe Label Forwarding by Diff-Serv LSRs, we model the LSR Diff-Serv label switching behavior, comprised of four stages:
- Incoming PHB Determination (A)
- Outgoing PHB Determination with Optional Traffic Conditioning(B)
- Label Forwarding (C)
- Encoding of Diff-Serv information into Encapsulation Layer (EXP, CLP, DE, User_Priority) (D)
Each stage is described in more detail in the following sections.
Obviously, to enforce the Diff-Serv service differentiation the LSR MUST also apply the forwarding treatment corresponding to the Outgoing PHB.
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This model is illustrated below:
--Inc_label(s)(*)------------------------>I===I--Outg_label(s)(&)--> \ I I \ \---->I===I I C I \-->I===I--Encaps-> I A I I===I--Outg_PHB->I===I I D I (&) -Encaps->I===I--Inc_PHB->I B I \ /->I===I (*) I===I \--------+ \----Forwarding--> Treatment (PHB)
"Encaps" designates the Diff-Serv related information encoded in the MPLS Encapsulation layer (e.g., EXP field, ATM CLP, Frame Relay DE, 802.1 User_Priority)
(*) when the LSR behaves as an MPLS ingress node, the incoming packet may be received unlabelled.
(&) when the LSR behaves as an MPLS egress node, the outgoing packet may be transmitted unlabelled.
This model is presented here to describe the functional operations of Diff-Serv LSRs and does not constrain actual implementation.
This stage determines which Behavior Aggregate the received packet belongs to.
2.2.1 Incoming PHB Determination Considering a Label Stack Entry
Sections 3.3 and 4.3 provide the details on how to perform incoming PHB Determination considering a given received label stack entry and/or received incoming MPLS encapsulation information depending on the incoming LSP type and depending on the incoming MPLS encapsulation.
Section 2.6 provides the details of which label stack entry to consider for the Incoming PHB Determination depending on the supported Diff-Serv tunneling mode.
2.2.2 Incoming PHB Determination Considering IP Header
Section 2.6 provides the details of when the IP Header is to be considered for incoming PHB determination, depending on the supported Diff-Serv tunneling model. In those cases where the IP header is to
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be used, this stage operates exactly as with a non-MPLS IP Diff-Serv Router and uses the DS field to determine the incoming PHB.
2.3 Outgoing PHB Determination With Optional Traffic Conditioning
The traffic conditioning stage is optional and may be used on an LSR to perform traffic conditioning including Behavior Aggregate demotion or promotion. It is outside the scope of this specification. For the purpose of specifying Diff-Serv over MPLS forwarding, we simply note that the PHB to be actually enforced and conveyed to downstream LSRs by an LSR (referred to as "outgoing PHB"), may be different to the PHB which had been associated with the packet by the previous LSR (referred to as "incoming PHB").
When the traffic conditioning stage is not present, the "outgoing PHB" is simply identical to the "incoming PHB".
[MPLS_ARCH] describes how label swapping is performed by LSRs on incoming labeled packets using an Incoming Label Map (ILM), where each incoming label is mapped to one or multiple NHLFEs. [MPLS_ARCH] also describes how label imposition is performed by LSRs on incoming unlabelled packets using a FEC-to-NHLFEs Map (FTN), where each incoming FEC is mapped to one or multiple NHLFEs.
A Diff-Serv Context for a label is comprised of:
- `LSP type (i.e., E-LSP or L-LSP)'
- `supported PHBs'
- `Encaps-->PHB mapping' for an incoming label
- `Set of PHB-->Encaps mappings' for an outgoing label
The present specification defines that a Diff-Serv Context is stored in the ILM for each incoming label.
[MPLS_ARCH] states that the `NHLFE may also contain any other information needed in order to properly dispose of the packet'. In accordance with this, the present specification defines that a Diff- Serv Context is stored in the NHLFE for each outgoing label that is swapped or pushed.
This Diff-Serv Context information is populated into the ILM and the FTN at label establishment time.
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If the label corresponds to an E-LSP for which no `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, the `supported PHBs' is populated with the set of PHBs of the preconfigured `EXP<-->PHB mapping', which is discussed below in section 3.2.1.
If the label corresponds to an E-LSP for which an `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, the `supported PHBs' is populated with the set of PHBs of the signaled `EXP<-->PHB mapping'.
If the label corresponds to an L-LSP, the `supported PHBs' is populated with the set of PHBs forming the PSC that is signaled at LSP set-up.
The details of how the `Encaps-->PHB mapping' or `Set of PHB-->Encaps mappings' are populated are defined below in sections 3 and 4.
[MPLS_ARCH] also states that:
"If the ILM [respectively, FTN] maps a particular label to a set of NHLFEs that contain more than one element, exactly one element of the set must be chosen before the packet is forwarded. The procedures for choosing an element from the set are beyond the scope of this document. Having the ILM [respectively, FTN] map a label [respectively, a FEC] to a set containing more than one NHLFE may be useful if, e.g., it is desired to do load balancing over multiple equal-cost paths."
In accordance with this, the present specification allows that an incoming label [respectively FEC] may be mapped, for Diff-Serv purposes, to multiple NHLFEs (for instance where different NHLFEs correspond to egress labels supporting different sets of PHBs). When a label [respectively FEC] maps to multiple NHLFEs, the Diff-Serv LSR MUST choose one of the NHLFEs whose Diff-Serv Context indicates that it supports the Outgoing PHB of the forwarded packet.
When a label [respectively FEC] maps to multiple NHLFEs which support the Outgoing PHB, the procedure for choosing one among those is outside the scope of this document. This situation may be encountered where it is desired to do load balancing of a Behavior Aggregate over multiple LSPs. In such situations, in order to respect ordering constraints, all packets of a given microflow MUST be transported over the same LSP.
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2.5 Encoding Diff-Serv Information Into Encapsulation Layer
This stage determines how to encode the fields which convey Diff-Serv information in the transmitted packet (e.g., MPLS Shim EXP, ATM CLP, Frame Relay DE, 802.1 User_Priority).
2.5.1 Encoding Diff-Serv Information Into Transmitted Label Entry
Sections 3.5 and 4.5 provide the details on how to perform Diff-Serv information encoding into a given transmitted label stack entry and/or transmitted MPLS encapsulation information depending on the corresponding outgoing LSP type and depending on the MPLS encapsulation.
Section 2.6 provides the details in which label stack entry to perform Diff-Serv information encoding into depending on the supported Diff-Serv tunneling mode.
2.5.2 Encoding Diff-Serv Information Into Transmitted IP Header
To perform Diff-Serv Information Encoding into the transmitted packet IP header, this stage operates exactly as with a non-MPLS IP Diff- Serv Router and encodes the DSCP of the Outgoing PHB into the DS field.
Section 2.6 provides the details of when Diff-Serv Information Encoding is to be performed into transmitted IP header depending on the supported Diff-Serv tunneling mode.
[DIFF_TUNNEL] considers the interaction of Differentiated Services with IP tunnels of various forms. MPLS LSPs are not a form of "IP tunnels" since the MPLS encapsulating header does not contain an IP header and thus MPLS LSPs are not considered in [DIFF_TUNNEL]. However, although not a form of "IP tunnel", MPLS LSPs are a form of "tunnel".
From the Diff-Serv standpoint, LSPs share a number of common characteristics with IP Tunnels:
- Intermediate nodes (i.e., Nodes somewhere along the LSP span) only see and operate on the "outer" Diff-Serv information.
- LSPs are unidirectional.
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- The "outer" Diff-Serv information can be modified at any of the intermediate nodes.
However, from the Diff-Serv standpoint, LSPs also have a distinctive property compared to IP Tunnels:
- There is generally no behavior analogous to Penultimate Hop Popping (PHP) used with IP Tunnels. Furthermore, PHP results in the "outer" Diff-Serv information associated with the LSP not being visible to the LSP egress. In situations where this information is not meaningful at the LSP Egress, this is obviously not an issue at all. In situations where this information is meaningful at the LSP Egress, then it must somehow be carried in some other means.
The two conceptual models for Diff-Serv tunneling over IP Tunnels defined in [DIFF_TUNNEL] are applicable and useful to Diff-Serv over MPLS but their respective detailed operations is somewhat different over MPLS. These two models are the Pipe Model and the Uniform Model. Their operations over MPLS are specified in the following sections. Discussion and definition of alternative tunneling models are outside the scope of this specification.
With the Pipe Model, MPLS tunnels (aka LSPs) are used to hide the intermediate MPLS nodes between LSP Ingress and Egress from the Diff-Serv perspective.
In this model, tunneled packets must convey two meaningful pieces of Diff-Serv information:
- the Diff-Serv information which is meaningful to intermediate nodes along the LSP span including the LSP Egress (which we refer to as the "LSP Diff-Serv Information"). This LSP Diff-Serv Information is not meaningful beyond the LSP Egress: Whether Traffic Conditioning at intermediate nodes on the LSP span affects the LSP Diff-Serv information or not, this updated Diff-Serv information is not considered meaningful beyond the LSP Egress and is ignored.
- the Diff-Serv information which is meaningful beyond the LSP Egress (which we refer to as the "Tunneled Diff-Serv Information"). This information is to be conveyed by the LSP Ingress to the LSP Egress. This Diff-Serv information is not meaningful to the intermediate nodes on the LSP span.
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Operation of the Pipe Model without PHP is illustrated below:
========== LSP =============================>
---Swap--(M)--...--Swap--(M)--Swap---- / (outer header) \ (M) (M) / \ >--(m)-Push.................(m).....................Pop--(m)--> I (inner header) E (M*)
(M) represents the "LSP Diff-Serv information" (m) represents the "Tunneled Diff-Serv information" (*) The LSP Egress considers the LSP Diff-Serv information received in the outer header (i.e., before the pop) in order to apply its Diff-Serv forwarding treatment (i.e., actual PHB) I represents the LSP ingress node E represents the LSP egress node
With the Pipe Model, the "LSP Diff-Serv Information" needs to be conveyed to the LSP Egress so that it applies its forwarding treatment based on it. The "Tunneled Diff-Serv information" also needs to be conveyed to the LSP Egress so it can be conveyed further downstream.
Since both require that Diff-Serv information be conveyed to the LSP Egress, the Pipe Model operates only without PHP.
The Pipe Model is particularly appropriate for environments in which:
- the cloud upstream of the incoming interface of the LSP Ingress and the cloud downstream of the outgoing interface of the LSP Egress are in Diff-Serv domains which use a common set of Diff- Serv service provisioning policies and PHB definitions, while the LSP spans one (or more) Diff-Serv domain(s) which use(s) a different set of Diff-Serv service provisioning policies and PHB definitions
- the outgoing interface of the LSP Egress is in the (last) Diff- Serv domain spanned by the LSP.
As an example, consider the case where a service provider is offering an MPLS VPN service (see [MPLS_VPN] for an example of MPLS VPN architecture) including Diff-Serv differentiation. Say that a collection of sites is interconnected via such an MPLS VPN service. Now say that this collection of sites is managed under a common administration and is also supporting Diff-Serv service differentiation. If the VPN site administration and the Service
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Provider are not sharing the exact same Diff-Serv policy (for instance not supporting the same number of PHBs), then operation of Diff-Serv in the Pipe Model over the MPLS VPN service would allow the VPN Sites Diff-Serv policy to operate consistently throughout the ingress VPN Site and Egress VPN Site and transparently over the Service Provider Diff-Serv domain. It may be useful to view such LSPs as linking the Diff-Serv domains at their endpoints into a single Diff-Serv region by making these endpoints virtually contiguous even though they may be physically separated by intermediate network nodes.
The Pipe Model MUST be supported.
For support of the Pipe Model over a given LSP without PHP, an LSR performs the Incoming PHB Determination and the Diff-Serv information Encoding in the following manner:
- when receiving an unlabelled packet, the LSR performs Incoming PHB Determination considering the received IP Header.
- when receiving a labeled packet, the LSR performs Incoming PHB Determination considering the outer label entry in the received label stack. In particular, when a pop operation is to be performed for the considered LSP, the LSR performs Incoming PHB Determination BEFORE the pop.
- when performing a push operation for the considered LSP, the LSR:
o encodes Diff-Serv Information corresponding to the OUTGOING PHB in the transmitted label entry corresponding to the pushed label.
o encodes Diff-Serv Information corresponding to the INCOMING PHB in the encapsulated header (swapped label entry or IP header).
- when performing a swap-only operation for the considered LSP, the LSR encodes Diff-Serv Information in the transmitted label entry that contains the swapped label
- when performing a pop operation for the considered LSP, the LSR does not perform Encoding of Diff-Serv Information into the header exposed by the pop operation (i.e., the LSR leaves the exposed header "as is").
The Short Pipe Model is an optional variation of the Pipe Model described above. The only difference is that, with the Short Pipe
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Model, the Diff-Serv forwarding treatment at the LSP Egress is applied based on the "Tunneled Diff-Serv Information" (i.e., Diff- Serv information conveyed in the encapsulated header) rather than on the "LSP Diff-Serv information" (i.e., Diff-Serv information conveyed in the encapsulating header).
Operation of the Short Pipe Model without PHP is illustrated below:
========== LSP =============================>
---Swap--(M)--...--Swap--(M)--Swap---- / (outer header) \ (M) (M) / \ >--(m)-Push.................(m).....................Pop--(m)--> I (inner header) E
(M) represents the "LSP Diff-Serv information" (m) represents the "Tunneled Diff-Serv information" I represents the LSP ingress node E represents the LSP egress node
Since the LSP Egress applies its forwarding treatment based on the "Tunneled Diff-Serv Information", the "LSP Diff-Serv information" does not need to be conveyed by the penultimate node to the LSP Egress. Thus the Short Pipe Model can also operate with PHP.
Operation of the Short Pipe Model with PHP is illustrated below:
=========== LSP ============================>
---Swap--(M)--...--Swap------ / (outer header) \ (M) (M) / \ >--(m)-Push.................(m).............Pop-(m)--E--(m)--> I (inner header) P (M*)
(M) represents the "LSP Diff-Serv information" (m) represents the "Tunneled Diff-Serv information" (*) The Penultimate LSR considers the LSP Diff-Serv information received in the outer header (i.e., before the pop) in order to apply its Diff-Serv forwarding treatment (i.e., actual PHB) I represents the LSP ingress node P represents the LSP penultimate node E represents the LSP egress node
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The Short Pipe Model is particularly appropriate for environments in which:
- the cloud upstream of the incoming interface of the LSP Ingress and the cloud downstream of the outgoing interface of the LSP Egress are in Diff-Serv domains which use a common set of Diff- Serv service provisioning policies and PHB definitions, while the LSP spans one (or more) Diff-Serv domain(s) which use(s) a different set of Diff-Serv service provisioning policies and PHB definitions
- the outgoing interface of the LSP Egress is in the same Diff-Serv domain as the cloud downstream of it.
Since each outgoing interface of the LSP Egress is in the same Diff- Serv domain as the cloud downstream of it, each outgoing interface may potentially be in a different Diff-Serv domain, and the LSP Egress needs to be configured with awareness of every corresponding Diff-Serv policy. This operational overhead is justified in some situations where the respective downstream Diff-Serv policies are better suited to offering service differentiation over each egress interface than the common Diff-Serv policy used on the LSP span. An example of such a situation is where a Service Provider offers an MPLS VPN service and where some VPN users request that their own VPN Diff-Serv policy be applied to control service differentiation on the dedicated link from the LSP Egress to the destination VPN site, rather than the Service Provider's Diff-Serv policy.
The Short Pipe Model MAY be supported.
For support of the Short Pipe Model over a given LSP without PHP, an LSR performs the Incoming PHB Determination and the Diff-Serv information Encoding in the same manner as with the Pipe Model with the following exception:
- when receiving a labeled packet, the LSR performs Incoming PHB Determination considering the header (label entry or IP header) which is used to do the actual forwarding. In particular, when a pop operation is to be performed for the considered LSP, the LSR performs Incoming PHB Determination AFTER the pop.
For support of the Short Pipe Model over a given LSP with PHP, an LSR performs Incoming PHB Determination and Diff-Serv information Encoding in the same manner as without PHP with the following exceptions:
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- the Penultimate LSR performs Incoming PHB Determination considering the outer label entry in the received label stack. In other words, when a pop operation is to be performed for the considered LSP, the Penultimate LSR performs Incoming PHB Determination BEFORE the pop.
Note that the behavior of the Penultimate LSR in the Short Pipe Mode with PHP, is identical to the behavior of the LSP Egress in the Pipe Mode (necessarily without PHP).
With the Uniform Model, MPLS tunnels (aka LSPs) are viewed as artifacts of the end-to-end path from the Diff-Serv standpoint. MPLS Tunnels may be used for forwarding purposes but have no significant impact on Diff-Serv. In this model, any packet contains exactly one piece of Diff-Serv information which is meaningful and is always encoded in the outer most label entry (or in the IP DSCP where the IP packet is transmitted unlabelled for instance at the egress of the LSP). Any Diff-Serv information encoded somewhere else (e.g., in deeper label entries) is of no significance to intermediate nodes or to the tunnel egress and is ignored. If Traffic Conditioning at intermediate nodes on the LSP span affects the "outer" Diff-Serv information, the updated Diff-Serv information is the one considered meaningful at the egress of the LSP.
Operation of the Uniform Model without PHP is illustrated below:
========== LSP =============================>
---Swap--(M)--...-Swap--(M)--Swap---- / (outer header) \ (M) (M) / \ >--(M)--Push...............(x).......................Pop--(M)-> I (inner header) E
(M) represents the Meaningful Diff-Serv information encoded in the corresponding header. (x) represents non-meaningful Diff-Serv information. I represents the LSP ingress node E represents the LSP egress node
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Operation of the Uniform Model with PHP is illustrated below:
========== LSP =========================>
---Swap-(M)-...-Swap------ / (outer header) \ (M) (M) / \ >--(M)--Push..............(x)............Pop-(M)--E--(M)-> I (inner header) P
(M) represents the Meaningful Diff-Serv information encoded in the corresponding header. (x) represents non-meaningful Diff-Serv information. I represents the LSP ingress node P represents the LSP penultimate node E represents the LSP egress node
The Uniform Model for Diff-Serv over MPLS is such that, from the Diff-Serv perspective, operations are exactly identical to the operations if MPLS was not used. In other words, MPLS is entirely transparent to the Diff-Serv operations.
Use of the Uniform Model allows LSPs to span Diff-Serv domain boundaries without any other measure in place than an inter-domain Traffic Conditioning Agreement at the physical boundary between the Diff-Serv domains and operating exclusively on the "outer" header, since the meaningful Diff-Serv information is always visible and modifiable in the outmost label entry.
The Uniform Model MAY be supported.
For support of the Uniform Model over a given LSP, an LSR performs Incoming PHB Determination and Diff-Serv information Encoding in the following manner:
- when receiving an unlabelled packet, the LSR performs Incoming PHB Determination considering the received IP Header.
- when receiving a labeled packet, the LSR performs Incoming PHB Determination considering the outer label entry in the received label stack. In particular, when a pop operation is to be performed for the considered LSP, the LSR performs Incoming PHB Determination BEFORE the pop.
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- when performing a push operation for the considered LSP, the LSR encodes Diff-Serv Information in the transmitted label entry corresponding to the pushed label. The Diff-Serv Information encoded in the encapsulated header (swapped label entry or IP Header) is of no importance.
- when performing a swap-only operation for the considered LSP, the LSR encodes Diff-Serv Information in the transmitted label entry that contains the swapped label.
- when PHP is used, the Penultimate LSR needs to be aware of the "Set of PHB-->Encaps mappings" for the label corresponding to the exposed header (or the `PHB-->DSCP mapping') in order to perform Diff-Serv Information Encoding. Methods for providing this mapping awareness are outside the scope of this specification. As an example, the "PHB-->DSCP mapping" may be locally configured. As another example, in some environments, it may be appropriate for the Penultimate LSR to assume that the "Set of PHB-->Encaps mappings" to be used for the outgoing label in the exposed header is the "Set of PHB-->Encaps mappings" that would be used by the LSR if the LSR was not doing PHP. Note also that this specification assumes that the Penultimate LSR does not perform label swapping over the label entry exposed by the pop operation (and in fact that it does not even look at the exposed label). Consequently, restrictions may apply to the Diff-Serv Information Encoding that can be performed by the Penultimate LSR. For example, this specification does not allow situations where the Penultimate LSR pops a label corresponding to an E-LSP supporting two PSCs, while the header exposed by the pop contains label values for two L-LSPs each supporting one PSC, since the Diff-Serv Information Encoding would require selecting one label or the other.
Note that LSR behaviors for the Pipe, the Short Pipe and the Uniform Model only differ when doing a push or a pop. Thus, Intermediate LSRs which perform swap only operations for an LSP, behave in exactly the same way, regardless of whether they are behaving in the Pipe, Short Pipe or the Uniform model. With a Diff-Serv implementation supporting multiple Tunneling Models, only LSRs behaving as LSP Ingress, Penultimate LSR or LSP Egress need to be configured to operate in a particular Model. Signaling to associate a Diff-Serv tunneling model on a per-LSP basis is not within the scope of this specification.
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Through the label stack mechanism, MPLS allows LSP tunneling to nest to any depth. We observe that with such nesting, the push of level N+1 takes place on a subsequent (or the same) LSR to the LSR doing the push for level N, while the pop of level N+1 takes place on a previous (or the same) LSR to the LSR doing the pop of level N. For a given level N LSP, the Ingress LSR doing the push and the LSR doing the pop (Penultimate LSR or LSP Egress) must operate in the same Tunneling Model (i.e., Pipe, Short Pipe or Uniform). However, there is no requirement for consistent tunneling models across levels so that LSPs at different levels may be operating in different Tunneling Models.
Hierarchical operations are illustrated below in the case of two levels of tunnels:
Tunneling Model 2 may be the same as or may be different from Tunneling Model 1.
For a given LSP of level N, the LSR must perform the Incoming PHB Determination and the Diff-Serv information Encoding as specified in section 2.6.2, 2.6.2.1 and 2.6.3 according to the Tunneling Model of this level N LSP and independently of the Tunneling Model of other level LSPs.
Within a given MPLS Diff-Serv domain, all the E-LSPs relying on the pre-configured mapping are capable of transporting the same common set of 8, or fewer, BAs. Each of those E-LSPs may actually transport this full set of BAs or any arbitrary subset of it.
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For a given FEC, two given E-LSPs using a signaled `EXP<-->PHB mapping' can support the same or different sets of Ordered Aggregates.
3.2 Populating the `Encaps-->PHB mapping' for an incoming E-LSP
This section defines how the `Encaps-->PHB mapping' of the Diff-Serv Context is populated for an incoming E-LSP in order to allow Incoming PHB determination.
The `Encaps-->PHB mapping' for an E-LSP is always of the form `EXP-->PHB mapping'.
If the label corresponds to an E-LSP for which no `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, the `EXP-->PHB mapping' is populated based on the Preconfigured `EXP<-->PHB mapping' which is discussed below in section 3.2.1.
If the label corresponds to an E-LSP for which an `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, the `EXP-->PHB mapping' is populated as per the signaled `EXP<-->PHB mapping'.
LSRs supporting E-LSPs which use the preconfigured `EXP<-->PHB mapping' must allow local configuration of this `EXP<-->PHB mapping'. This mapping applies to all the E-LSPs established on this LSR without a mapping explicitly signaled at set-up time.
The preconfigured `EXP<-->PHB mapping' must either be consistent at every E-LSP hop throughout the MPLS Diff-Serv domain spanned by the LSP or appropriate remarking of the EXP field must be performed by the LSR whenever a different preconfigured mapping is used on the ingress and egress interfaces.
In case, the preconfigured `EXP<-->PHB mapping' has not actually been configured by the Network Administrator, the LSR should use a default preconfigured `EXP<-->PHB mapping' which maps all EXP values to the Default PHB.
This section defines how Incoming PHB Determination is carried out when the considered label entry in the received label stack corresponds to an E-LSP. This requires that the `Encaps-->PHB mapping' is populated as defined in section 3.2.
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When considering a label entry corresponding to an incoming E-LSP for Incoming PHB Determination, the LSR:
- determines the `EXP-->PHB mapping' by looking up the `Encaps-->PHB mapping' of the Diff-Serv Context associated in the ILM with the considered incoming E-LSP label.
- determines the incoming PHB by looking up the EXP field of the considered label entry in the `EXP-->PHB mapping' table.
3.4 Populating the `Set of PHB-->Encaps mappings' for an outgoing E-LSP
This section defines how the `Set of PHB-->Encaps mappings' of the Diff-Serv Context is populated at label setup for an outgoing E-LSP in order to allow Encoding of Diff-Serv information in the Encapsulation Layer.
An outgoing E-LSP must always have a `PHB-->EXP mapping' as part of the `Set of PHB-->Encaps mappings' of its Diff-Serv Context.
If the label corresponds to an E-LSP for which no `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, this `PHB-->EXP mapping' is populated based on the Preconfigured `EXP<-->PHB mapping' which is discussed above in section 3.2.1.
If the label corresponds to an E-LSP for which an `EXP<-->PHB mapping' has been explicitly signaled at LSP setup, the `PHB-->EXP mapping' is populated as per the signaled `EXP<-->PHB mapping'.
If the LSP is egressing over an ATM interface which is not label switching controlled, then one `PHB-->CLP mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing LSP. This `PHB-->CLP mapping' is populated in the following way:
- it is a function of the PHBs supported on this LSP, and may use the relevant mapping entries for these PHBs from the Default `PHB-->CLP mapping' defined in section 3.4.2.1. Mappings other than the one defined in section 3.4.2.1 may be used. In particular, if a mapping from PHBs to CLP is standardized in the future for operations of Diff-Serv over ATM, such a standardized mapping may then be used.
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For example if the outgoing label corresponds to an LSP supporting the AF1 PSC, then the `PHB-->CLP mapping' may be populated with:
PHB CLP Field
AF11 ----> 0 AF12 ----> 1 AF13 ----> 1 EF ----> 0
Notice that in this case the `Set of PHB-->Encaps mappings' contains both a `PHB-->EXP mapping' and a `PHB-->CLP mapping'.
If the LSP is egressing over a Frame Relay interface which is not label switching controlled, one `PHB-->DE mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing LSP and is populated in the following way:
- it is a function of the PHBs supported on this LSP, and may use the relevant mapping entries for these PHBs from the Default `PHB-->DE mapping' defined in section 3.4.3.1. Mappings other than the one defined in section 3.4.3.1 may be used. In particular, if a mapping from PHBs to DE is standardized in the future for operations of Diff-Serv over Frame Relay, such a standardized mapping may then be used.
Notice that in this case the `Set of PHB-->Encaps mappings' contains both a `PHB-->EXP mapping' and a `PHB-->DE mapping'.
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If the LSP is egressing over a LAN interface on which multiple 802.1 Traffic Classes are supported as per [IEEE_802.1], then one `PHB-->802.1 mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing LSP. This `PHB-->802.1 mapping' is populated in the following way:
- it is a function of the PHBs supported on this LSP, and uses the relevant mapping entries for these PHBs from the Preconfigured `PHB-->802.1 mapping' defined in section 3.4.4.1.
Notice that the `Set of PHB-->Encaps mappings' then contains both a `PHB-->EXP mapping' and a `PHB-->802.1 mapping'.
At the time of producing this specification, there are no standardized mapping from PHBs to 802.1 Traffic Classes. Consequently, an LSR supporting multiple 802.1 Traffic Classes over LAN interfaces must allow local configuration of a `PHB-->802.1 mapping'. This mapping applies to all the outgoing LSPs established by the LSR on such LAN interfaces.
3.5 Encoding Diff-Serv information into Encapsulation Layer On Outgoing E-LSP
This section defines how to encode Diff-Serv information into the MPLS encapsulation Layer for a given transmitted label entry corresponding to an outgoing E-LSP. This requires that the `Set of PHB-->Encaps mappings' be populated as defined in section 3.4.
The LSR first determines the `Set of PHB-->Encaps mappings' of the Diff-Serv Context associated with the corresponding label in the NHLFE.
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If the `Set of PHB-->Encaps mappings' contains a mapping of the form `PHB-->EXP mapping', then the LSR:
- determines the value to be written in the EXP field of the corresponding level label entry by looking up the "outgoing PHB" in this `PHB-->EXP mapping' table.
If the `Set of PHB-->Encaps mappings' contains a mapping of the form `PHB-->CLP mapping', then the LSR:
- determines the value to be written in the CLP field of the ATM encapsulation header, by looking up the "outgoing PHB" in this `PHB-->CLP mapping' table.
If the `Set of PHB-->Encaps mappings' contains a mapping of the form `PHB-->DE mapping', then the LSR:
- determines the value to be written in the DE field of the Frame Relay encapsulation header, by looking up the "outgoing PHB" in this `PHB-->DE mapping' table.
If the `Set of PHB-->Encaps mappings' contains a mapping of the form `PHB-->802.1 mapping', then the LSR:
- determines the value to be written in the User_Priority field of the Tag Control Information of the 802.1 encapsulation header [IEEE_802.1], by looking up the "outgoing PHB" in this 'PHB-- >802.1 mapping' table.
In an MPLS domain, two or more LSPs can be merged into one LSP at one LSR. E-LSPs are compatible with LSP Merging under the following condition:
E-LSPs can only be merged into one LSP if they support the exact same set of BAs.
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For E-LSPs using a signaled `EXP<-->PHB mapping', the above merge condition MUST be enforced by LSRs through explicit checking at label setup that the exact same set of PHBs is supported on the merged LSPs.
For E-LSPs using the preconfigured `EXP<-->PHB mapping', since the PHBs supported over an E-LSP is not signaled at establishment time, an LSR can not rely on signaling information to enforce the above merge. However all E-LSPs using the preconfigured `EXP<-->PHB mapping' are required to support the same set of Behavior Aggregates within a given MPLS Diff-Serv domain. Thus, merging of E-LSPs using the preconfigured `EXP<-->PHB mapping' is allowed within a given MPLS Diff-Serv domain.
4.2 Populating the `Encaps-->PHB mapping' for an incoming L-LSP
This section defines how the `Encaps-->PHB mapping' of the Diff-Serv Context is populated at label setup for an incoming L-LSP in order to allow Incoming PHB determination.
If the LSR terminates the MPLS Shim Layer over this incoming L-LSP and the L-LSP ingresses on an interface which is not ATM nor Frame Relay, then the `Encaps-->PHB mapping' is populated in the following way:
- it is actually a `EXP-->PHB mapping'
- this mapping is a function of the PSC which is carried on this LSP, and must use the relevant mapping entries for this PSC from the Mandatory `EXP/PSC-->PHB mapping' defined in Section 4.2.1.1.
For example if the incoming label corresponds to an L-LSP supporting the AF1 PSC, then the `Encaps-->PHB mapping' will be populated with:
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An LSR, supporting L-LSPs over PPP interfaces and LAN interfaces, is an example of an LSR terminating the Shim layer over ingress interfaces which are not ATM nor Frame Relay.
If the LSR terminates the MPLS Shim Layer over this incoming L-LSP and the L-LSP ingresses on an ATM or Frame Relay interface, then the `Encaps-->PHB mapping' is populated in the following way:
- it should actually be a `EXP-->PHB mapping'. Alternative optional ways of populating the `Encaps-->PHB mapping' might be defined in the future (e.g., using a 'CLP/EXP--> PHB mapping' or a 'DE/EXP-->PHB mapping') but are outside the scope of this document.
- when the `Encaps-->PHB mapping' is an `EXP-->PHB mapping', this `EXP-->PHB mapping' mapping is a function of the PSC which is carried on the L-LSP, and must use the relevant mapping entries for this PSC from the Mandatory `EXP/PSC-->PHB mapping' defined in Section 4.2.1.1.
An Edge-LSR of an ATM-MPLS domain or of a FR-MPLS domain is an example of an LSR terminating the shim layer over an ingress ATM/FR interface.
If the LSR does not terminate an MPLS Shim Layer over this incoming label and uses ATM encapsulation (i.e., it is an ATM-LSR), then the `Encaps-->PHB mapping' for this incoming L-LSP is populated in the following way:
- it is actually a `CLP-->PHB mapping'
- the mapping is a function of the PSC, which is carried on this LSP, and should use the relevant mapping entries for this PSC from the Default `CLP/PSC-->PHB mapping' defined in Section 4.2.2.1.
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For example if the incoming label corresponds to an L-LSP supporting the AF1 PSC, then the `Encaps-->PHB mapping' should be populated with:
If the LSR does not terminate an MPLS Shim Layer over this incoming label and uses Frame Relay encapsulation (i.e., it is a FR-LSR), then the `Encaps-->PHB mapping' for this incoming L-LSP is populated in the following way:
- it is actually a `DE-->PHB mapping'
- the mapping is a function of the PSC which is carried on this LSP, and should use the relevant mapping entries for this PSC from the Default `DE/PSC-->PHB mapping' defined in Section 4.2.3.1.
This section defines how Incoming PHB determination is carried out when the considered label entry in the received label stack corresponds to an L-LSP. This requires that the `Encaps-->PHB mapping' is populated as defined in section 4.2.
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When considering a label entry corresponding to an incoming L-LSP for Incoming PHB Determination, the LSR first determines the `Encaps-->PHB mapping' associated with the corresponding label.
If the `Encaps-->PHB mapping' is of the form `DE-->PHB mapping', then the LSR:
- determines the incoming PHB by looking at the DE field of the Frame Relay encapsulation and by using the `DE-->PHB mapping'.
4.4 Populating the `Set of PHB-->Encaps mappings' for an outgoing L-LSP
This section defines how the `Set of PHB-->Encaps mappings' of the Diff-Serv Context is populated at label setup for an outgoing L-LSP in order to allow Encoding of Diff-Serv Information.
If the LSR uses an MPLS Shim Layer over this outgoing L-LSP, then one `PHB-->EXP mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing L-LSP. This `PHB-->EXP mapping' is populated in the following way:
- it is a function of the PSC supported on this LSP, and must use the mapping entries relevant for this PSC from the Mandatory `PHB-->EXP mapping' defined in section 4.4.1.1.
For example, if the outgoing label corresponds to an L-LSP supporting the AF1 PSC, then the following `PHB-->EXP mapping' is added into the `Set of PHB-->Encaps mappings':
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If the L-LSP is egressing on an ATM interface (i.e., it is an ATM-LSR or it is a frame-based LSR sending packets on an LC-ATM interface or on an ATM interface which is not label switching controlled), then one `PHB-->CLP mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing L-LSP.
If the L-LSP is egressing over an ATM interface which is not label- controlled, the `PHB-->CLP mapping' is populated as per section 3.4.2.
If the L-LSP is egressing over an LC-ATM interface, the `PHB-->CLP mapping' is populated in the following way:
- it is a function of the PSC supported on this LSP, and should use the relevant mapping entries for this PSC from the Default `PHB-->CLP mapping' defined in section 3.4.2.1.
Notice that if the LSR is a frame-based LSR supporting an L-LSP egressing over an ATM interface, then the `Set of PHB-->Encaps mappings' contains both a `PHB-->EXP mapping' and a `PHB-->CLP mapping'. If the LSR is an ATM-LSR supporting an L-LSP, then the `Set of PHB-->Encaps mappings' only contains a `PHB-->CLP mapping'.
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If the L-LSP is egressing over a Frame Relay interface (i.e., it is an LSR sending packets on an LC-FR interface or on a Frame Relay interface which is not label switching controlled), one `PHB-->DE mapping' is added to the `Set of PHB-->Encaps mappings' for this outgoing L-LSP.
If the L-LSP is egressing over a FR interface which is not label switching controlled, the `PHB-->DE mapping' is populated as per section 3.4.3.
If the L-LSP is egressing over an LC-FR interface, the `PHB-->DE mapping' is populated in the following way:
- it is a function of the PSC supported on this LSP, and should use the relevant mapping entries for this PSC from the Default `PHB-->DE mapping' defined in section 3.4.3.1.
Notice that if the LSR is an Edge-LSR supporting an L-LSP egressing over a LC-FR interface, then the `Set of PHB-->Encaps mappings' contains both a `PHB-->EXP mapping' and a `PHB-->DE mapping'. If the LSR is a FR-LSR supporting an L-LSP, then the `Set of PHB-->Encaps mappings' only contains a `PHB-->DE mapping'.
If the LSP is egressing over a LAN interface on which multiple 802.1 Traffic Classes are supported, as defined in [IEEE_802.1], then one `PHB-->802.1 mapping' is added as per section 3.4.4.
4.5 Encoding Diff-Serv Information into Encapsulation Layer on Outgoing L-LSP
This section defines how to encode Diff-Serv information into the MPLS encapsulation Layer for a transmitted label entry corresponding to an outgoing L-LSP. This requires that the `Set of PHB-->Encaps mappings' is populated as defined in section 4.4.
The LSR first determines the `Set of PHB-->Encaps mappings' of the Diff-Serv Context associated with the corresponding label in the NHLFE and then performs corresponding encoding as specified in sections 3.5.1, 3.5.2, 3.5.3 and 3.5.4.
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In an MPLS domain, two or more LSPs can be merged into one LSP at one LSR. L-LSPs are compatible with LSP Merging under the following condition:
L-LSPs can only be merged into one L-LSP if they support the same PSC.
The above merge condition MUST be enforced by LSRs, through explicit checking at label setup, that the same PSC is supported on the merged LSPs.
Note that when L-LSPs merge, the bandwidth that is available for the PSC downstream of the merge point must be sufficient to carry the sum of the merged traffic. This is particularly important in the case of EF traffic. This can be ensured in multiple ways (for instance via provisioning, or via bandwidth signaling and explicit admission control).
The MPLS architecture does not assume a single label distribution protocol. [RSVP_MPLS_TE] defines the extension to RSVP for establishing LSPs in MPLS networks. This section specifies the extensions to RSVP, beyond those defined in [RSVP_MPLS_TE], to establish LSPs supporting Differentiated Services in MPLS networks.
One new RSVP Object is defined in this document: the DIFFSERV 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 DIFFSERV Object in Path messages used to establish LSP Tunnels in accordance with [RSVP_MPLS_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_MPLS_TE] for support of the establishment of LSP Tunnels via RSVP are also applicable to the establishment of LSP Tunnels supporting Diff-Serv: for instance, only unicast LSPs are supported and Multicast LSPs are for further study.
This new DIFFSERV 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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The DIFFSERV Object is optional for support of LSP Tunnels as defined in [RSVP_MPLS_TE]. A Diff-Serv capable LSR supporting E-LSPs using the preconfigured `EXP<-->PHB mapping' in compliance with this specification MAY support the DIFFSERV Object. A Diff-Serv capable LSR supporting E-LSPs using a signaled `EXP<-->PHB mapping' in compliance with this specification MUST support the DIFFSERV Object. A Diff-Serv capable LSR supporting L-LSPs in compliance with this specification MUST support the DIFFSERV Object.
The DIFFSERV object formats are shown below. Currently there are two possible C_Types. Type 1 is a DIFFSERV object for an E-LSP. Type 2 is a DIFFSERV object for an L-LSP.
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Reserved : 13 bits This field is reserved. It must be set to zero on transmission and must be ignored on receipt.
EXP : 3 bits This field contains the value of the EXP field for the `EXP<-->PHB mapping' defined in this MAP entry.
PHBID : 16 bits This field contains the PHBID of the PHB for the `EXP<-->PHB mapping' defined in this MAP entry. The PHBID is encoded as specified in [PHBID].
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To establish an LSP tunnel with RSVP, the sender creates a Path message with a session type of LSP_Tunnel_IPv4 and with a LABEL_REQUEST object as per [RSVP_MPLS_TE].
To establish an E-LSP tunnel with RSVP, which uses the Preconfigured `EXP<-->PHB mapping', the sender creates a Path message:
- with a session type of LSP_Tunnel_IPv4,
- with the LABEL_REQUEST object, and
- without the DIFFSERV object.
To establish an E-LSP tunnel with RSVP, which uses the Preconfigured `EXP<-->PHB mapping', the sender MAY alternatively create a Path message:
- with a session type of LSP_Tunnel_IPv4,
- with the LABEL_REQUEST object, and
- with the DIFFSERV object for an E-LSP containing no MAP entries.
To establish an E-LSP tunnel with RSVP, which uses a signaled `EXP<-->PHB mapping', the sender creates a Path message:
- with a session type of LSP_Tunnel_IPv4,
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- with the LABEL_REQUEST object,
- with the DIFFSERV object for an E-LSP containing one MAP entry for each EXP value to be supported on this E-LSP.
To establish with RSVP an L-LSP tunnel, the sender creates a Path message:
- with a session type of LSP_Tunnel_IPv4,
- with the LABEL_REQUEST object,
- with the DIFFSERV object for an L-LSP containing the PHB Scheduling Class (PSC) supported on this L-LSP.
If a path message contains multiple DIFFSERV objects, only the first one is meaningful; subsequent DIFFSERV object(s) must be ignored and not forwarded.
Each LSR along the path records the DIFFSERV object, when present, in its path state block.
If a DIFFSERV object is not present in the Path message, the LSR SHOULD interpret this as a request for an E-LSP using the Preconfigured `EXP<-->PHB mapping'. However, for backward compatibility purposes, with other non-Diff-Serv Quality of Service options allowed by [RSVP_MPLS_TE] such as Integrated Services Controlled Load or Guaranteed Services, the LSR MAY support a configurable "override option". When this "override option" is configured, the LSR interprets a path message without a Diff-Serv object as a request for an LSP with such non-Diff-Serv Quality of Service.
If a DIFFSERV object for an E-LSP containing no MAP entry is present in the Path message, the LSR MUST interpret this as a request for an E-LSP using the Preconfigured `EXP<-->PHB mapping'. In particular, this allows an LSR with the "override option" configured to support E-LSPs with Preconfigured `EXP<-->PHB mapping', simultaneously with LSPs with non-Diff-Serv Quality of Service.
If a DIFFSERV object for an E-LSP containing at least one MAP entry is present in the Path message, the LSR MUST interpret this as a request for an E-LSP with signaled `EXP<-->PHB mapping'.
If a DIFFSERV object for an L-LSP is present in the Path message, the LSR MUST interpret this as a request for an L-LSP.
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The destination LSR of an E-LSP or L-LSP responds to the Path message containing the LABEL_REQUEST object by sending a Resv message:
- with the LABEL object
- without a DIFFSERV object.
Assuming the label request is accepted and a label is allocated, the Diff-Serv LSRs (sender, destination, intermediate nodes) must:
- update the Diff-Serv Context associated with the established LSPs in their ILM/FTN as specified in previous sections (incoming and outgoing label),
- install the required Diff-Serv forwarding treatment (scheduling and dropping behavior) for this NHLFE (outgoing label).
An LSR that recognizes the DIFFSERV object and that receives a path message which contains the DIFFSERV object but which does not contain a LABEL_REQUEST object or which does not have a session type of LSP_Tunnel_IPv4, sends a PathErr towards the sender with the error code `Diff-Serv Error' and an error value of `Unexpected DIFFSERV object'. Those are defined below in section 5.5.
An LSR receiving a Path message with the DIFFSERV object for E-LSP, which recognizes the DIFFSERV object but does not support the particular PHB encoded in one, or more, of the MAP entries, sends a PathErr towards the sender with the error code `Diff-Serv Error' and an error value of `Unsupported PHB'. Those are defined below in section 5.5.
An LSR receiving a Path message with the DIFFSERV object for E-LSP, which recognizes the DIFFSERV object but determines that the signaled `EXP<-->PHB mapping' is invalid, sends a PathErr towards the sender with the error code `Diff-Serv Error' and an error value of Invalid `EXP<-->PHB mapping'. Those are defined below in section 5.5. `The EXP<-->PHB mapping' signaled in the DIFFSERV Object for an E-LSP is invalid when:
- the MAPnb field is not within the range 0 to 8 or
- a given EXP value appears in more than one MAP entry, or
- the PHBID encoding is invalid.
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An LSR receiving a Path message with the DIFFSERV object for L-LSP, which recognizes the DIFFSERV object but does not support the particular PSC encoded in the PSC field, sends a PathErr towards the sender with the error code `Diff-Serv Error' and an error value of `Unsupported PSC'. Those are defined below in section 5.5.
An LSR receiving a Path message with the DIFFSERV object, which recognizes the DIFFSERV object but that is unable to allocate the required per-LSP Diff-Serv context sends a PathErr with the error code "Diff-Serv Error" and the error value "Per-LSP context allocation failure". Those are defined below in section 5.5.
A Diff-Serv LSR MUST handle the situations where the label request can not be accepted for reasons other than those already discussed in this section, in accordance with [RSVP_MPLS_TE] (e.g., reservation rejected by admission control, a label can not be associated).
An LSR that does not recognize the DIFFSERV 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 recognize the DIFFSERV object Class-Num but does not recognize the DIFFSERV 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 L-LSP cannot be established and should possibly take action to retry LSP establishment without the DIFFSERV object (e.g., attempt to use E-LSPs with Preconfigured `EXP<-->PHB mapping' as a fall-back strategy).
Both E-LSPs and L-LSPs can be established with or without bandwidth reservation.
As specified in [RSVP_MPLS_TE], to establish an E-LSP or an L-LSP with bandwidth reservation, Int-Serv's Controlled Load service (or possibly Guaranteed Service) is used and the bandwidth is signaled in the SENDER_TSPEC (respectively FLOWSPEC) of the path (respectively Resv) message.
As specified in [RSVP_MPLS_TE],to establish an E-LSP or an L-LSP without bandwidth reservation, the Null Service specified in [NULL] is used.
Note that this specification defines usage of E-LSPs and L-LSPs for support of the Diff-Serv service only. Regardless of the Intserv service (Controlled Load, Null Service, Guaranteed Service,...) and regardless of whether the reservation is with or without bandwidth reservation, E-LSPs and L-LSPs are defined here for support of Diff- Serv services. Support of Int-Serv services over an MPLS Diff-Serv backbone is outside the scope of this specification.
Note also that this specification does not concern itself with the DCLASS object defined in [DCLASS], since this object conveys information on DSCP values, which are not relevant inside the MPLS network.
The MPLS architecture does not assume a single label distribution protocol. [LDP] defines the Label Distribution Protocol and its usage for establishment of label switched paths (LSPs) in MPLS networks. This section specifies the extensions to LDP to establish LSPs supporting Differentiated Services in MPLS networks.
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One new LDP TLV is defined in this document:
- the Diff-Serv TLV
Detailed description of this TLV is provided below.
The new Diff-Serv TLV is optional with respect to LDP. A Diff-Serv capable LSR supporting E-LSPs which uses the Preconfigured `EXP<-- >PHB mapping' in compliance with this specification MAY support the Diff-Serv TLV. A Diff-Serv capable LSR supporting E-LSPs which uses the signaled `EXP<-->PHB mapping' in compliance with this specification MUST support the Diff-Serv TLV. A Diff-Serv capable LSR supporting L-LSPs in compliance with this specification MUST support the Diff-Serv TLV.
Reserved : 13 bits This field is reserved. It must be set to zero on transmission and must be ignored on receipt.
EXP : 3 bits This field contains the value of the EXP field for the `EXP<-->PHB mapping' defined in this MAP entry.
PHBID : 16 bits This field contains the PHBID of the PHB for the `EXP<-->PHB mapping' defined in this MAP entry. The PHBID is encoded as specified in [PHBID].
6.4.1 Handling of the Diff-Serv TLV in Downstream Unsolicited Mode
This section describes operations when the Downstream Unsolicited Mode is used.
When allocating a label for an E-LSP which is to use the preconfigured `EXP<-->PHB mapping', a downstream Diff-Serv LSR issues a Label Mapping message without the Diff-Serv TLV.
When allocating a label for an E-LSP which is to use a signaled `EXP<-->PHB mapping', a downstream Diff-Serv LSR issues a Label Mapping message with the Diff-Serv TLV for an E-LSP which contains one MAP entry for each EXP value to be supported on this E-LSP.
When allocating a label for an L-LSP, a downstream Diff-Serv LSR issues a Label Mapping message with the Diff-Serv TLV for an L-LSP which contains the PHB Scheduling Class (PSC) to be supported on this L-LSP.
Assuming the label set-up is successful, the downstream and upstream LSRs must:
- update the Diff-Serv Context associated with the established LSPs in their ILM/FTN as specified in previous sections (incoming and outgoing label),
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- install the required Diff-Serv forwarding treatment (scheduling and dropping behavior) for this NHLFE (outgoing label).
An upstream Diff-Serv LSR receiving a Label Mapping message with multiple Diff-Serv TLVs only considers the first one as meaningful. The LSR must ignore and not forward the subsequent Diff-Serv TLV(s).
An upstream Diff-Serv LSR which receives a Label Mapping message, with the Diff-Serv TLV for an E-LSP and does not support the particular PHB encoded in one or more of the MAP entries, must reject the mapping by sending a Label Release message which includes the Label TLV and the Status TLV with a Status Code of `Unsupported PHB'.
An upstream Diff-Serv LSR receiving a Label Mapping message with the Diff-Serv TLV for an E-LSP and determining that the signaled `EXP<-->PHB mapping' is invalid, must reject the mapping by sending a Label Release message which includes the Label TLV and the Status TLV with a Status Code of Invalid `EXP<-->PHB mapping'. The `EXP<-->PHB mapping' signaled in the DIFFSERV Object for an E-LSP is invalid when:
- the MAPnb field is not within the range 1 to 8, or
- a given EXP value appears in more than one MAP entry, or
- the PHBID encoding is invalid
An upstream Diff-Serv LSR receiving a Label Mapping message with the Diff-Serv TLV for an L-LSP containing a PSC value which is not supported, must reject the mapping by sending a Label Release message which includes the Label TLV and the Status TLV with a Status Code of `Unsupported PSC'.
6.4.2 Handling of the Diff-Serv TLV in Downstream on Demand Mode
This section describes operations when the Downstream on Demand Mode is used.
When requesting a label for an E-LSP which is to use the preconfigured `EXP<-->PHB mapping', an upstream Diff-Serv LSR sends a Label Request message without the Diff-Serv TLV.
When requesting a label for an E-LSP which is to use a signaled `EXP<-->PHB mapping', an upstream Diff-Serv LSR sends a Label Request message with the Diff-Serv TLV for an E-LSP which contains one MAP entry for each EXP value to be supported on this E-LSP.
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When requesting a label for an L-LSP, an upstream Diff-Serv LSR sends a Label Request message with the Diff-Serv TLV for an L-LSP which contains the PSC to be supported on this L-LSP.
A downstream Diff-Serv LSR sending a Label Mapping message in response to a Label Request message for an E-LSP or an L-LSP must not include a Diff-Serv TLV in this Label Mapping message. Assuming the label set-up is successful, the downstream and upstream LSRs must:
- update the Diff-Serv Context associated with the established LSPs in their ILM/FTN as specified in previous sections (incoming and outgoing label),
- install the required Diff-Serv forwarding treatment (scheduling and dropping behavior) for this NHLFE (outgoing label).
An upstream Diff-Serv LSR receiving a Label Mapping message containing a Diff-Serv TLV in response to its Label Request message, must reject the label mapping by sending a Label Release message which includes the Label TLV and the Status TLV with a Status Code of `Unexpected Diff-Serv TLV'.
A downstream Diff-Serv LSR receiving a Label Request message with multiple Diff-Serv TLVs only considers the first one as meaningful. The LSR must ignore and not forward the subsequent Diff-Serv TLV(s).
A downstream Diff-Serv LSR which receives a Label Request message with the Diff-Serv TLV for an E-LSP and does not support the particular PHB encoded in one (or more) of the MAP entries, must reject the request by sending a Notification message which includes the Status TLV with a Status Code of `Unsupported PHB'.
A downstream Diff-Serv LSR receiving a Label Request message with the Diff-Serv TLV for an E-LSP and determining that the signaled `EXP<-->PHB mapping' is invalid, must reject the request by sending a Notification message which includes the Status TLV with a Status Code of Invalid `EXP<-->PHB mapping'. The `EXP<-->PHB mapping' signaled in the DIFFSERV TLV for an E-LSP is invalid when:
- the MAPnb field is not within the range 1 to 8, or
- a given EXP value appears in more than one MAP entry, or
- the PHBID encoding is invalid
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A downstream Diff-Serv LSR receiving a Label Request message with the Diff-Serv TLV for an L-LSP containing a PSC value which is not supported, must reject the request by sending a Notification message which includes the Status TLV with a Status Code of `Unsupported PSC'.
A downstream Diff-Serv LSR that recognizes the Diff-Serv TLV Type in a Label Request message but is unable to allocate the required per- LSP context information, must reject the request sending a Notification message which includes the Status TLV with a Status Code of `Per-LSP context allocation failure'.
A downstream Diff-Serv LSR that recognizes the Diff-Serv TLV Type in a Label Request message and supports the requested PSC but is not able to satisfy the label request for other reasons (e.g., no label available), must send a Notification message in accordance with existing LDP procedures [LDP] (e.g., with a `No Label Resource' Status Code). This Notification message must include the requested Diff-Serv TLV.
An LSR that does not recognize the Diff-Serv TLV Type, on receipt of a Label Request message or a Label Mapping message containing the Diff-Serv TLV, must behave in accordance with the procedures specified in [LDP] for an unknown TLV whose U Bit and F Bit are set to 0 i.e., it must ignore the message, return a Notification message with `Unknown TLV' Status.
Bandwidth information may also be signaled at the establishment time of E-LSP and L-LSP, for instance for the purpose of Traffic Engineering, using the Traffic Parameters TLV as described in [MPLS CR LDP].
7. MPLS Support of Diff-Serv over PPP, LAN, Non-LC-ATM and Non-LC-FR Interfaces
The general operations for MPLS support of Diff-Serv, including label forwarding and LSP setup operations are specified in the previous sections. This section describes the specific operations required for MPLS support of Diff-Serv over PPP interfaces, LAN interfaces, ATM Interfaces which are not label controlled and Frame Relay interfaces which are not label controlled.
On these interfaces, this specification allows any of the following LSP combinations per FEC:
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- Zero or any number of E-LSP, and
- Zero or any number of L-LSPs.
A Diff-Serv capable LSR MUST support E-LSPs which use preconfigured `EXP<-->PHB mapping' over these interfaces.
A Diff-Serv capable LSR MAY support E-LSPs which use signaled `EXP<-->PHB mapping' and L-LSPs over these interfaces.
8. MPLS Support of Diff-Serv over LC-ATM Interfaces
This section describes the specific operations required for MPLS support of Diff-Serv over label switching controlled ATM (LC-ATM) interfaces.
This document allows any number of L-LSPs per FEC within an MPLS ATM Diff-Serv domain. E-LSPs are not supported over LC-ATM interfaces.
8.1 Use of ATM Traffic Classes and Traffic Management mechanisms
The use of the "ATM service categories" specified by the ATM Forum, of the "ATM Transfer Capabilities" specified by the ITU-T or of vendor specific ATM traffic classes is outside of the scope of this specification. The only requirement for compliant implementation is that the forwarding behavior experienced by a Behavior Aggregate forwarded over an L-LSP by the ATM LSR MUST be compliant with the corresponding Diff-Serv PHB specifications.
Since there is only one bit (CLP) for encoding the PHB drop precedence value over ATM links, only two different drop precedence levels are supported in ATM LSRs. Sections 4.2.2 and 4.4.2 define how the three drop precedence levels of the AFn Ordered Aggregates are mapped to these two ATM drop precedence levels. This mapping is in accordance with the requirements specified in [DIFF_AF] for the case when only two drop precedence levels are supported.
To avoid discarding parts of the packets, frame discard mechanisms, such as Early Packet Discard (EPD) (see [ATMF_TM]) SHOULD be enabled in the ATM-LSRs for all PHBs described in this document.
A Diff-Serv capable LSR MUST support L-LSPs over LC-ATM interfaces. This specification assumes that Edge-LSRs of the ATM-LSR domain use the "shim header" encapsulation method defined in [MPLS_ATM]. Operations without the "shim header" encapsulation are outside the scope of this specification.
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9. MPLS Support of Diff-Serv over LC-FR Interfaces
This section describes the specific operations required for MPLS support of Diff-Serv over label switching controlled Frame Relay (LC-FR) interfaces.
This document allows any number of L-LSPs per FEC within an MPLS Frame Relay Diff-Serv domain. E-LSPs are not supported over LC-FR interfaces.
9.1 Use of Frame Relay Traffic parameters and Traffic Management mechanisms
The use of the Frame Relay traffic parameters as specified by ITU-T and Frame Relay-Forum or of vendor specific Frame Relay traffic management mechanisms is outside of the scope of this specification. The only requirement for compliant implementation is that the forwarding behavior experienced by a Behavior Aggregate forwarded over an L-LSP by the Frame Relay LSR MUST be compliant with the corresponding Diff-Serv PHB specifications.
Since there is only one bit (DE) for encoding the PHB drop precedence value over Frame Relay links, only two different drop precedence levels are supported in Frame Relay LSRs. Sections 4.2.3 and 4.4.3 define how the three drop precedence levels of the AFn Ordered Aggregates are mapped to these two Frame Relay drop precedence levels. This mapping is in accordance with the requirements specified in [DIFF_AF] for the case when only two drop precedence levels are supported.
A Diff-Serv capable LSR MUST support L-LSPs over LC-Frame Relay interfaces.
This specification assumes that Edge-LSRs of the FR-LSR domain use the "generic encapsulation" method as recommended in [MPLS_FR]. Operations without the "generic encapsulation" are outside the scope of this specification.
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This document defines a number of objects with implications for IANA.
This document defines in section 5.2 a new RSVP object, the DIFFSERV object. This object required 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 required a number from the "IETF Consensus" space. "65" has been allocated by IANA for the DIFFSERV object.
This document defines in section 5.5 a new RSVP error code, "Diffserv Error". Error code "27" has been assigned by IANA to the "Diffserv Error". This document defines values 1 through 5 of the value field to be used within the ERROR_SPEC object for this error code. Future allocations of values in this space should be handled by IANA using the First Come First Served policy defined in [IANA].
This document defines in section 6.1 a new LDP TLV, the Diffserv TLV. The number for this TLV has been assigned by working group consensus according to the policies defined in [LDP].
This document defines in section 6.2 five new LDP Status Code values for Diffserv-related error conditions. The values for the Status Code have been assigned by working group consensus according to the policies defined in [LDP].
This document does not introduce any new security issues beyond those inherent in Diff-Serv, MPLS and RSVP, and may use the same mechanisms proposed for those technologies.
This document has benefited from discussions with Eric Rosen, Angela Chiu and Carol Iturralde. It has also borrowed from the work done by D. Black regarding Diff-Serv and IP Tunnels interaction.
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This section does not provide additional specification and is only here to provide examples of how this flexible approach for Diff-Serv support over MPLS may be deployed. Pros and cons of various deployment options for particular environments are beyond the scope of this document.
A.1 Scenario 1: 8 (or fewer) BAs, no Traffic Engineering, no MPLS Protection
A Service Provider running 8 (or fewer) BAs over MPLS, not performing Traffic engineering, not using MPLS protection and using MPLS Shim Header encapsulation in his/her network, may elect to run Diff-Serv over MPLS using a single E-LSP per FEC established via LDP. Furthermore the Service Provider may elect to use the preconfigured `EXP<-->PHB mapping'.
Operations can be summarized as follows:
- the Service Provider configures at every LSR, the bi-directional mapping between each PHB and a value of the EXP field (e.g., 000<-->AF11, 001<-->AF12, 010<-->AF13)
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC (e.g., bandwidth allocated to AF1) and the dropping behavior for each PHB (e.g., drop profile for AF11, AF12, AF13)
- LSRs signal establishment of a single E-LSP per FEC using LDP in accordance with the specification above (i.e., no Diff-Serv TLV in LDP Label Request/Label Mapping messages to implicitly indicate that the LSP is an E-LSP and that it uses the preconfigured mapping)
A.2 Scenario 2: More than 8 BAs, no Traffic Engineering, no MPLS Protection
A Service Provider running more than 8 BAs over MPLS, not performing Traffic Engineering, not using MPLS protection and using MPLS Shim encapsulation in his/her network may elect to run Diff-Serv over MPLS using for each FEC:
- one E-LSP established via LDP and using the preconfigured mapping to support a set of 8 (or less) BAs, AND
- one L-LSP per <FEC,OA> established via LDP for support of the other BAs.
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Operations can be summarized as follows:
- the Service Provider configures at every LSR the bi-directional mapping between each PHB and a value of the EXP field for the BAs transported over the E-LSP
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC supported over the E-LSP and the dropping behavior for each corresponding PHB
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC supported over the L-LSPs and the dropping behavior for each corresponding PHB
- LSRs signal establishment of a single E-LSP per FEC for the set of E-LSP transported BAs using LDP as specified above (i.e., no Diff-Serv TLV in LDP Label Request/Label Mapping messages to implicitly indicate that the LSP is an E-LSP and that it uses the preconfigured mapping)
- LSRs signal establishment of one L-LSP per <FEC,OA> for the other BAs using LDP as specified above (i.e., Diff-Serv TLV in LDP Label Request/Label Mapping messages to indicate the L-LSP's PSC).
A Service Provider running 8 (or fewer) BAs over MPLS, performing aggregate Traffic Engineering (i.e., performing a single common path selection for all BAs), using aggregate MPLS protection (i.e., restoring service to all PSCs jointly) and using MPLS Shim Header encapsulation in his/her network, may elect to run Diff-Serv over MPLS using a single E-LSP per FEC established via RSVP [RSVP_MPLS_TE] or CR-LDP [CR-LDP_MPLS_TE] and using the preconfigured mapping.
Operations can be summarized as follows:
- the Service Provider configures at every LSR the bi-directional mapping between each PHB and a value of the EXP field (e.g., 000<-->AF11, 001<-->AF12, 010<-->AF13)
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC (e.g., bandwidth allocated to AF1) and the dropping behavior for each PHB (eg drop profile for AF11, AF12, AF13)
- LSRs signal establishment of a single E-LSP per FEC which will use the preconfigured mapping:
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* using the RSVP protocol as specified above (i.e., no DIFFSERV RSVP Object in the PATH message containing the LABEL_REQUEST Object), OR
* using the CR-LDP protocol as specified above (i.e., no Diff- Serv TLV in LDP Label Request/Label Mapping messages).
- protection is activated on all the E-LSPs in order to achieve MPLS protection via mechanisms outside the scope of this document.
A Service Provider running any number of BAs over MPLS, performing per-OA Traffic Engineering (i.e., performing a separate path selection for each OA) and performing per-OA MPLS protection (i.e., performing protection with potentially different levels of protection for the different OAs) in his/her network, may elect to run Diff-Serv over MPLS using one L-LSP per <FEC,OA> pair established via RSVP or CR-LDP.
Operations can be summarized as follows:
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC (e.g., bandwidth allocated to AF1) and the dropping behavior for each PHB (e.g., drop profile for AF11, AF12, AF13)
- LSRs signal establishment of one L-LSP per <FEC,OA>:
* using the RSVP as specified above to signal the L-LSP's PSC (i.e., DIFFSERV RSVP Object in the PATH message containing the LABEL_REQUEST), OR
* using the CR-LDP protocol as specified above to signal the L- LSP PSC (i.e., Diff-Serv TLV in LDP Label Request/Label Mapping messages).
- the appropriate level of protection is activated on the different L-LSPs (potentially with a different level of protection for each PSC) via mechanisms outside the scope of this document.
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A Service Provider running 8 (or fewer) BAs over MPLS, performing per-OA Traffic Engineering (i.e., performing a separate path selection for each OA) and performing per-OA MPLS protection (i.e., performing protection with potentially different levels of protection for the different OAs) in his/her network, may elect to run Diff-Serv over MPLS using one E-LSP per <FEC,OA> pair established via RSVP or CR-LDP. Furthermore, the Service Provider may elect to use the preconfigured mapping on all the E-LSPs.
Operations can be summarized as follows:
- the Service Provider configures at every LSR the bi-directional mapping between each PHB and a value of the EXP field (e.g., 000<-->AF11, 001<-->AF12, 010<-->AF13)
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC (e.g., bandwidth allocated to AF1) and the dropping behavior for each PHB (eg drop profile for AF11, AF12, AF13)
- LSRs signal establishment of one E-LSP per <FEC,OA>:
* using the RSVP protocol as specified above to signal that the LSP is an E-LSP which uses the preconfigured mapping (i.e., no DIFFSERV RSVP Object in the PATH message containing the LABEL_REQUEST), OR
* using the CR-LDP protocol as specified above to signal that the LSP is an E-LSP which uses the preconfigured mapping (i.e., no Diff-Serv TLV in LDP Label Request/Label Mapping messages)
- the Service Provider configures, for each E-LSP, at the head-end of that E-LSP, a filtering/forwarding criteria so that only the packets belonging to a given OA are forwarded on the E-LSP established for the corresponding FEC and corresponding OA.
- the appropriate level of protection is activated on the different E-LSPs (potentially with a different level of protection depending on the PSC actually transported over each E-LSP) via mechanisms outside the scope of this document.
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A.6 Scenario 6: no Traffic Engineering/MPLS Protection on 8 BAs, per-OA Traffic Engineering/MPLS Protection on other BAs.
A Service Provider not performing Traffic Engineering/MPLS Protection on 8 (or fewer) BAs, performing per-OA Traffic Engineering/MPLS Protection on the other BAs (i.e., performing a separate path selection for each OA corresponding to the other BAs and performing MPLS Protection with a potentially different policy for each of these OA) and using the MPLS Shim encapsulation in his/her network may elect to run Diff-Serv over MPLS, using for each FEC:
- one E-LSP using the preconfigured mapping established via LDP to support the set of 8 (or fewer) non-traffic-engineered/non- protected BAs, AND
- one L-LSP per <FEC,OA> pair established via RSVP or CR-LDP for support of the other BAs.
Operations can be summarized as follows:
- the Service Provider configures at every LSR the bi-directional mapping between each PHB and a value of the EXP field for the BAs supported over the E-LSP
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC supported over the E-LSP and the dropping behavior for each corresponding PHB
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC supported over the L-LSPs and the dropping behavior for each corresponding PHB
- LSRs signal establishment of a single E-LSP per FEC for the non- traffic engineered BAs using LDP as specified above (i.e., no Diff-Serv TLV in LDP Label Request/Label Mapping messages)
- LSRs signal establishment of one L-LSP per <FEC,OA> for the other BAs:
* using the RSVP protocol as specified above to signal the L-LSP PSC (i.e., DIFFSERV RSVP Object in the PATH message containing the LABEL_REQUEST Object), OR
* using the CR-LDP protocol as specified above to signal the L- LSP PSC (i.e., Diff-Serv TLV in LDP Label Request/Label Mapping messages).
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- protection is not activated on the E-LSPs.
- the appropriate level of protection is activated on the different L-LSPs (potentially with a different level of protection depending on the L-LSP's PSC) via mechanisms outside the scope of this document.
A.7 Scenario 7: More than 8 BAs, no Traffic Engineering, no MPLS Protection
A Service Provider running more than 8 BAs over MPLS, not performing Traffic engineering, not performing MPLS protection and using MPLS Shim Header encapsulation in his/her network, may elect to run Diff- Serv over MPLS using two E-LSPs per FEC established via LDP and using signaled `EXP<-->PHB mapping'.
Operations can be summarized as follows:
- the Service Provider configures at every LSR, and for every interface, the scheduling behavior for each PSC (e.g., bandwidth allocated to AF1) and the dropping behavior for each PHB (e.g., drop profile for AF11, AF12, AF13)
- LSRs signal establishment of two E-LSPs per FEC using LDP in accordance with the specification above (i.e., Diff-Serv TLV in LDP Label Request/Label Mapping messages to explicitly indicate that the LSP is an E-LSP and its `EXP<-->PHB mapping'). The signaled mapping will indicate the subset of 8 (or less) BAs to be transported on each E-LSP and what EXP values are mapped to each BA on each E-LSP.
APPENDIX B. Example Bandwidth Reservation Scenarios
Consider the case where a network administrator elects to:
- have Diff-Serv resources entirely provisioned off-line (e.g., via Command Line Interface, via SNMP, via COPS,...)
- have Shortest Path Routing used for all the Diff-Serv traffic.
This is the closest model to provisioned Diff-Serv over non-MPLS IP. In that case, E-LSPs and/or L-LSPs would be established without signaled bandwidth.
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B.2 Scenario 2: Bandwidth Reservation for per-PSC Admission Control
Consider the case where a network administrator elects to:
- have Diff-Serv resources entirely provisioned off-line (e.g., via Command Line Interface, via SNMP, via COPS,...)
- use L-LSPs
- have Constraint Based Routing performed separately for each PSC, where one of the constraints is availability of bandwidth from the bandwidth allocated to the relevant PSC.
In that case, L-LSPs would be established with signaled bandwidth. The bandwidth signaled at L-LSP establishment would be used by LSRs to perform admission control at every hop to ensure that the constraint on availability of bandwidth for the relevant PSC is met.
B.3 Scenario 3: Bandwidth Reservation for per-PSC Admission Control and per-PSC Resource Adjustment
Consider the case where a network administrator elects to:
- use L-LSPs
- have Constraint Based Routing performed separately for each PSC, where one of the constraints is availability of bandwidth from the bandwidth allocated to the relevant PSC.
- have Diff-Serv resources dynamically adjusted
In that case, L-LSPs would be established with signaled bandwidth. The bandwidth signaled at L-LSP establishment would be used by LSRs to attempt to adjust the resources allocated to the relevant PSC (e.g., scheduling weight) and then perform admission control to ensure that the constraint on availability of bandwidth for the relevant PSC is met after the adjustment.
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[ATMF_TM] ATM Forum, "Traffic Management Specification Version 4.1", March 1999.
[CR-LDP_MPLS_TE] Jamoussi, B., Editor, Andersson, L., Callon, R. and R. Dantu, "Constraint-Based LSP Setup using LDP", RFC 3212, January 2002.
[DCLASS] Bernet, Y., "Format of the RSVP DCLASS Object", RFC 2996, November 2000.
[DIFF_AF] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, June 1999.
[DIFF_ARCH] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.
[DIFF_EF] Davie, B., Charny, A., Baker, F., Bennet, J., Benson, K., Boudec, J., Chiu, A., Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., Ramakrishnam, K. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
[DIFF_HEADER] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[DIFF_NEW] Grossman, D., "New Terminology and Clarifications for Diffserv", RFC 3260, April 2002.
[DIFF_TUNNEL] Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000.
[ECN] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
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[IEEE_802.1] ISO/IEC 15802-3: 1998 ANSI/IEEE Std 802.1D, 1998 Edition (Revision and redesignation of ISO/IEC 10038:98.
[LDP] Andersson, L., Doolan, D., Feldman, N., Fredette, A. and B. Thomas, "LDP Specification", RFC 3036, January 2001.
[MPLS_ARCH] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001.
[MPLS_ATM] Davie, B., Lawrence, J., McCloghrie, K., Rosen, E., Swallow, G., Rekhter, Y. and P. Doolan, "MPLS using LDP and ATM VC Switching", RFC 3035, January 2001.
[MPLS_ENCAPS] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001.
[MPLS_FR] Conta, A., Doolan, P. and A. Malis, "Use of Label Switching on Frame Relay Networks Specification", RFC 3034, January 2001.
[MPLS_VPN] Rosen, E., "BGP/MPLS VPNs", Work in Progress.
[NULL] Bernet, Y., Smith, A. and B. Davie, "Specification of the Null Service Type", RFC 2997, November 2000.
[PHBID] Black, D., Brim, S., Carpenter, B. and F. Le Faucheur, "Per Hop Behavior Identification Codes" RFC 3140, June 2001.
[RSVP] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource ReSerVation Protocol (RSVP) - Version 1 Functional Specification", RFC 2205, September 1997.
[RSVP_MPLS_TE] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V. and G. Swallow, "Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001.
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Authors' Addresses
Francois Le Faucheur Cisco Systems Village d'Entreprise Green Side - Batiment T3 400, Avenue de Roumanille 06410 Biot-Sophia Antipolis France
Ram Krishnan Axiowave Networks 200 Nickerson Road Marlboro, MA 01752
EMail: ram@axiowave.com
Pierrick Cheval Alcatel 5 rue Noel-Pons 92737 Nanterre Cedex France EMail: pierrick.cheval@space.alcatel.fr
Juha Heinanen Song Networks, Inc. Hallituskatu 16 33200 Tampere, Finland
EMail: jh@song.fi
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