RFC 3270
Multi-Protocol Label Switching (MPLS) Support of Differentiated Services
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 EXP-Inferred-PSC LSPs (E-LSP) . . . . . . . . . . . . . . . . . 6 1.3 Label-Only-Inferred-PSC LSPs (L-LSP). . . . . . . . . . . . . . 7 1.4 Overall Operations. . . . . . . . . . . . . . . . . . . . . . . 7 1.5 Relationship between Label and FEC. . . . . . . . . . . . . . . 8 1.6 Bandwidth Reservation for E-LSPs and L-LSPs . . . . . . . . . . 8 2. Label Forwarding Model for Diff-Serv LSRs and Tunneling Models . 9 2.1 Label Forwarding Model for Diff-Serv LSRs . . . . . . . . . . . 9 2.2 Incoming PHB Determination. . . . . . . . . . . . . . . . . . .10 2.3 Outgoing PHB Determination With Optional Traffic Conditioning .11 2.4 Label Forwarding. . . . . . . . . . . . . . . . . . . . . . . .11 2.5 Encoding Diff-Serv Information Into Encapsulation Layer . . . .13 2.6 Diff-Serv Tunneling Models over MPLS. . . . . . . . . . . . . .13 3. Detailed Operations of E-LSPs. . . . . . . . . . . . . . . . . .22 3.1 E-LSP Definition. . . . . . . . . . . . . . . . . . . . . . . .22 3.2 Populating the `Encaps-->PHB mapping' for an incoming E-LSP . .23 3.3 Incoming PHB Determination On Incoming E-LSP. . . . . . . . . .23 3.4 Populating the `Set of PHB-->Encaps mappings' for an outgoing E-LSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 3.5 Encoding Diff-Serv information into Encapsulation Layer On Outgoing E-LSP. . . . . . . . . . . . . . . . . . . . . . . . .26 3.6 E-LSP Merging . . . . . . . . . . . . . . . . . . . . . . . . .27 4. Detailed Operation of L-LSPs. . . . . . . . . . . . . . . . . .28 4.1 L-LSP Definition. . . . . . . . . . . . . . . . . . . . . . . .28 4.2 Populating the `Encaps-->PHB mapping' for an incoming L-LSP . .28 4.3 Incoming PHB Determination On Incoming L-LSP. . . . . . . . . .30 4.4 Populating the `Set of PHB-->Encaps mappings' for an outgoing L-LSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 4.5 Encoding Diff-Serv Information into Encapsulation Layer on Outgoing L-LSP. . . . . . . . . . . . . . . . . . . . . . . . .33 4.6 L-LSP Merging . . . . . . . . . . . . . . . . . . . . . . . . .34 5. RSVP Extension for Diff-Serv Support . . . . . . . . . . . . . .34 5.1 Diff-Serv related RSVP Messages Format. . . . . . . . . . . . .34 5.2 DIFFSERV Object . . . . . . . . . . . . . . . . . . . . . . . .35 5.3 Handling DIFFSERV Object. . . . . . . . . . . . . . . . . . . .37 5.4 Non-support of the DIFFSERV Object. . . . . . . . . . . . . . .40 5.5 Error Codes For Diff-Serv . . . . . . . . . . . . . . . . . . .40 5.6 Intserv Service Type. . . . . . . . . . . . . . . . . . . . . .41 6. LDP Extensions for Diff-Serv Support . . . . . . . . . . . . . .41 6.1 Diff-Serv TLV . . . . . . . . . . . . . . . . . . . . . . . . .42 6.2 Diff-Serv Status Code Values. . . . . . . . . . . . . . . . . .44 6.3 Diff-Serv Related LDP Messages. . . . . . . . . . . . . . . . .44 6.4 Handling of the Diff-Serv TLV . . . . . . . . . . . . . . . . .46 6.5 Non-Handling of the Diff-Serv TLV . . . . . . . . . . . . . . .49 6.6 Bandwidth Information . . . . . . . . . . . . . . . . . . . . .49
7. MPLS Support of Diff-Serv over PPP, LAN, Non-LC-ATM and Non-LC-FR Interfaces . . . . . . . . . . . . . . . . . . . . . .49 8. MPLS Support of Diff-Serv over LC-ATM Interfaces . . . . . . . .50 8.1 Use of ATM Traffic Classes and Traffic Management mechanisms. .50 8.2 LSR Implementation With LC-ATM Interfaces . . . . . . . . . . .50 9. MPLS Support of Diff-Serv over LC-FR Interfaces. . . . . . . . .51 9.1 Use of Frame Relay Traffic parameters and Traffic Management mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . .51 9.2 LSR Implementation With LC-FR Interfaces. . . . . . . . . . . .51 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . .52 11. Security Considerations . . . . . . . . . . . . . . . . . . . .52 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .52 Appendix A. Example Deployment Scenarios. . . . . . . . . . . . . .53 Appendix B. Example Bandwidth Reservation Scenarios . . . . . . . .58 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . .62 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . .641. Introduction
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).
- 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.
1.1 Terminology
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: FEC Forwarding Equivalency Class FTN FEC-To-NHLFE Map ILM Incoming Label Map LC-ATM Label Switching Controlled-ATM (interface) LC-FR Label Switching Controlled-Frame Relay (interface) LSP Label Switched Path LSR Label Switch Router MPLS Multi-Protocol Label Switching NHLFE Next Hop Label Forwarding Entry 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
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: E-LSP EXP-Inferred-PSC LSP L-LSP Label-Only-Inferred-PSC LSP1.2 EXP-Inferred-PSC LSPs (E-LSP)
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.
1.3 Label-Only-Inferred-PSC LSPs (L-LSP)
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.1.4 Overall Operations
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.
1.5 Relationship between Label and FEC
[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.1.6 Bandwidth Reservation for E-LSPs and L-LSPs
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.
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
2.1 Label Forwarding Model for Diff-Serv LSRs
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.
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.
2.2 Incoming PHB Determination
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
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".2.4 Label Forwarding
[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.
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.
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.2.6 Diff-Serv Tunneling Models over MPLS
2.6.1 Diff-Serv Tunneling Models
[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.
- 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.
2.6.2 Pipe Model
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.
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
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").
2.6.2.1 Short Pipe Model
The Short Pipe Model is an optional variation of the Pipe Model
described above. The only difference is that, with the Short Pipe
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
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:
- 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).
2.6.3 Uniform Model
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
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.
- 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.
2.6.4 Hierarchy
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: +--------Swap--...---+ / (outmost header) \ / \ Push(2).................(2)Pop / (outer header) \ / \ >>---Push(1)........................(1)Pop-->> (inner header) (1) Tunneling Model 1 (2) Tunneling Model 2 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.
(page 22 continued on part 2)
