MPLS Working Group R. Li Internet-Draft Q. Zhao Intended status: Standards Track Huawei Technologies Expires: January 8, 2013 C. Jacquenet France Telecom Orange July 7, 2012 Receiver-Driven Multicast Traffic-Engineered Label-Switched Paths draft-lzj-mpls-receiver-driven-multicast-rsvp-te-01.txt Abstract This document describes extensions to Resource Reservation Protocol - Traffic Engineering (RSVP-TE) for the setup of Receiver-Driven Traffic-Engineered point-to-multipoint (P2MP) and multipoint-to- multipoint (MP2MP)Label Switched Paths (LSPs) in Multi-Protocol Label Switching (MPLS) and Generalized MPLS (GMPLS)networks. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 8, 2013. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Li, et al. Expires January 8, 2013 [Page 1] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Li, et al. Expires January 8, 2013 [Page 2] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Receiver-Driven mRSVP-TE LSP Examples . . . . . . . . . . . . 7 2.1. P2MP Example . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. MP2MP Example . . . . . . . . . . . . . . . . . . . . . . 9 3. Signaling Protocol Extensions . . . . . . . . . . . . . . . . 10 3.1. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.1. Sessions . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.2. L2S Sub-LSPs . . . . . . . . . . . . . . . . . . . . . 11 3.1.3. Path Originator and Data Receiver . . . . . . . . . . 12 3.1.4. Explicit Routing . . . . . . . . . . . . . . . . . . . 13 3.2. Path Messages . . . . . . . . . . . . . . . . . . . . . . 13 3.3. Resv Messages . . . . . . . . . . . . . . . . . . . . . . 15 3.4. PathErr Messages . . . . . . . . . . . . . . . . . . . . . 15 3.5. ResvErr Message . . . . . . . . . . . . . . . . . . . . . 15 3.6. PathTear Messages . . . . . . . . . . . . . . . . . . . . 16 4. New and Updated Objects . . . . . . . . . . . . . . . . . . . 16 4.1. SESSION Objects . . . . . . . . . . . . . . . . . . . . . 16 4.1.1. P2MP LSP for IPv4 SESSION Objects . . . . . . . . . . 16 4.1.2. MP2MP LSP for IPv4 SESSION Objects . . . . . . . . . . 17 4.1.3. P2MP LSP for IPv6 SESSION Objects . . . . . . . . . . 17 4.1.4. MP2MP LSP for IPv6 SESSION Objects . . . . . . . . . . 17 4.2. SENDER_TEMPLATE Objects . . . . . . . . . . . . . . . . . 18 4.2.1. Multicast LSP IPv4 SENDER_TEMPLATE Objects . . . . . . 18 4.2.2. Multicast LSP IPv6 SENDER_TEMPLATE Objects . . . . . . 18 4.3. L2S_SUB_LSP Objects . . . . . . . . . . . . . . . . . . . 19 4.3.1. L2S_SUB_LSP IPv4 Objects . . . . . . . . . . . . . . . 19 4.3.2. L2S_SUB_LSP IPv6 Objects . . . . . . . . . . . . . . . 19 4.4. FILTER_SPEC Objects . . . . . . . . . . . . . . . . . . . 20 4.4.1. mRSVP-TE LSP_IPv4 FILTER_SPEC Objects . . . . . . . . 20 4.4.2. mRSVP-TE LSP_IPv6 FILTER_SPEC Objects . . . . . . . . 20 5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1. Interwork with PIM . . . . . . . . . . . . . . . . . . . . 20 5.2. Multicast VPN . . . . . . . . . . . . . . . . . . . . . . 21 6. Fast Re-Route Considerations . . . . . . . . . . . . . . . . . 21 7. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 21 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 10. Security Considerations . . . . . . . . . . . . . . . . . . . 22 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11.1. Normative References . . . . . . . . . . . . . . . . . . . 23 11.2. Informative References . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Li, et al. Expires January 8, 2013 [Page 3] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 1. Introduction Multiparty multimedia applications are getting greater attention in the telecom and datacom world. Such applications are QoS-demanding and can therefore benefit from the activation of MPLS traffic engineering capabilities that lead to the dynamic computation and establishment of MPLS LSPs whose characteristics comply with application-specific QoS requirements. P2MP-TE [RFC4875] defines a procedure to set up point-to-multipoint LSPs from sender to receivers. Sometimes multicast data streams are required to get transported over both IP networks and MPLS networks, which require PIM must interwork with RSVP-TE. On other times, PIM bootstraping messages need to transport over an intermediate MPLS domain. This document extends RSVP-TE for the dynamic computation of receiver- driven P2MP and MP2MP LSP tree structures. 1.1. Motivation IP multicast distribution trees are receiver-initiated and dynamic by nature. IP multicast-enabled applications are also bandwidth savvy, especially in the area of residential IPTV services, where the delivery of multicast contents to several hundreds of thousands of IPTV receivers assumes the appropriate level of quality. Current source-driven P2MP LSP establishment, as defined as in [RFC4875], assumes a priori knowledge of receiver locations, and the LSP signalling is initiated and driven by the data sender(headend). The priori knowledge of receiver locations is obtained either through static configuration or by using another protocol to discover such receivers. On the other hand, there is no straightfoward way to support MP2MP applications by using P2MP LSP unless full-meshed P2MP LSPs are set up. The receiver-driven extension to RSVP-TE defined in this document will support both P2MP LSPs and MP2MP LSPs. Moreover, it does not require the sender to know all the receivers' locations a priori. The protocols for discovery of receivers are not needed. It provides a natural mechanism to interwork with PIM dynamically. 1.2. Terminology The following terms are used in this document: o Sender: Sender refers to the Originator (and hence the Sender) of the content/payload, as defined in [RFC2205]. o Receiver: Receiver refers to the Receiver of the content/payload, as defined in [RFC2205]. Li, et al. Expires January 8, 2013 [Page 4] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 o Upstream: The direction of flow from content Receiver toward content Sender, as defined in [RFC2205]. o Downstream: The direction of flow from content Sender toward content Receiver, as defined in [RFC2205]. o Path-Sender: The sender of RSVP PATH messages, with no correlation to the direction of content/payload flows. Its flow direction is irrelevant to that of Sender defined above. All other control messages discussed in this document will use this as the reference. o Path-Receiver: The receiver of RSVP PATH messages, with no correlation to the direction of content/payload flows. o Path-Initiator: The Path-Sender that originated a RSVP PATH message. This is different from Path-Sender in that an intermediate node can be a Path-Sender, but such an intermediate node cannot create and initiate the RSVP PATH message. A Path- Initator is a Path-Sender, but a Path-Sender doesn't have to be a Path-Initiator. o Path-Terminator: The Path-Receiver that does NOT propagate the Path message any further. This is different from Path-Receiver in that an intermediate node can be a Path-Receiver, but such an intermediate node will propagate the Path message to the next hop. o Root: A router where a multcast LSP tree is rooted at. Data enters the root and then is distributed to leaves along the P2MP/ MP2MP LSP. 1.3. Overview Although the receiver-driven extensions to RSVP-TE as defined in this document use the existing sender-driven syntax, there are important semantic differences that need to be defined for correct interpretation and interoperability. In the receiver-driven context, we inverted the semantics of RSVP-TE messages, while keeping the syntax unchanged as much as possible. We will use mRSVP-TE to represent the RSVP-TE with receiver-driven extensions described in this document. The following are some key differences that are specific to the receiver-driven paradigm: o The leaf router: the router that receives data/content/payload. In this document, the leaf router will initiate PATH messages. In some sense, the leaf router and the receiver mean the same thing. Li, et al. Expires January 8, 2013 [Page 5] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 The term "receiver-driven" also means "leaf-driven". o L2S Destinations: routers where user data payload traffic enters the LSP. L2S means Leaf-to-Source. The source is the sender or root of a multicast stream. o RSVP P2MP PATH messages traverse from receivers to the root. o RSVP P2MP RESV messages traverse from the root to the leaf routers of the P2MP tree strcuture. o A RSVP RESV message received by a router is interpreted as a successful resource reservation made by the upstream node for the establishment of the P2MP tree structure. o A RSVP RESV message received by a router is interpreted as successful resource reservation made by the downstream node for the establishment of an MP2MP tree structure. o Label allocation on incoming interfaces is done prior to sending RSVP PATH messages upstream for P2MP tree structures. o Label allocation on incoming interfaces is done prior to sending RSVP RESV messages upstream for MP2MP tree structures. o For P2MP LSP tree structures, a node receiving a RSVP PATH message first decides if this RSVP PATH message will make the said node a branch LSR or not. If it is not a branch LSR, it is a transit LSR. In the case that it will become a transit LSR because of this PATH message, it will, before sending the RSVP PATH message upstream, allocate required bandwidth on the interface on which the RSVP PATH message is received. The upstream node can send traffic soon after successfully reserving resources on the downstream link, on which the RSVP PATH message SHOULD be received. In the case that the node is already a branch or a transit node before it receives the PATH message, then it will allocate required bandwidth on the interface on which the RSVP PATH message is received, and send the RESV message to the node which sends the PATH message without propagating the PATH message further to the upstream node. For P2MP LSPs, a label is carried by the PATH message and should be used by the upstream node when distributing the data from upstream to downstream. o For MP2MP LSP tree structures, a node will allocate required bandwidth on the interface through which the RSVP PATH message is sent before sending the RSVP PATH message upstream. A node receiving a RSVP PATH message MUST first decide if this RSVP PATH message will make the said node a branch LSR or not. In the case Li, et al. Expires January 8, 2013 [Page 6] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 it will become a transit LSR because of this PATH message, then it will allocate required bandwidth on the interface on which the RSVP PATH message is received and will allocate required bandwidth on the interface through which the RSVP PATH message is sent, before sending the RSVP PATH message upstream. The downstream node can send traffic soon after successfully reserving bandwidth on the upstream link through which the RSVP PATH message SHOULD be sent. The upstream node can send traffic soon after successfully reserving bandwidth on the downstream link on which the RSVP PATH message SHOULD be received. In the case that the node is already a branch or a transit node before it receives the PATH message, then it will allocate required resources on the interface on which the RSVP PATH message is received, and send the RESV message to the node which sends the PATH message without propagating the PATH message further to the upstream node. The label carried by the PATH message should be used by the Path-Receiver node to forward data from the Path-Receiver node to the Path-Sender node, and the label carried by RESV messages should be used by its corresponding Path-Sender node to send data from the Path-Sender node to the Path-Receiver node. o For the sake of readability, from now on all mRSVP-TE LSPs will be used to represent all P2MP and/or MP2MP LSPs in receiver-driven (RD) multicast P2MP/MP2MP MPLS environments. We will sometimes use RD P2MP TE LSP or RD MP2MP TE LSP to represent such receiver- driven multicast LSPs. 2. Receiver-Driven mRSVP-TE LSP Examples In what follows we describe two examples to show how P2MP and MP2MP are set up, respectively. In both of such examples, Path messages are initiated by data receivers. For the P2MP example, a Path message carries a label for the use of sending data downstream. And for the MP2MP example, both Path message and Resv message carries a label for sending data downstream and upstream. Li, et al. Expires January 8, 2013 [Page 7] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 2.1. P2MP Example Sender/Source/Path Terminator/Ingress Router +---------+ | R1 | +-----+---+ _ \ \ /\ \ \ \ Path Message w/ Label OBJECT Resv \ \ \ (msg2) Message \ \ \ (msg3) \ \ \ _\/ \ \ +----------------+ Path Remerge | R3 | Creates Branch Point +----------------+ _ _ / / /\ \ \ /\ / / / \ \ \ Path Message (msg1) Resv Message / / / msg4 \ \ \ w/ Label OBJECT (msg6) / / / \ \ \ / / /Path Msg \ \ \ / / / (msg5) \ \ \ \/_ / / w/Label OBJ _\/ \ \ +----------+ +---+-----+ | R4 | | R5 | +----------+ +---------+ Path Initiator Path Initiator Originator ID = R4 Originator ID = R5 L2S Destination = R1 L2S Destination = R1 Session = S Session = S Figure 1: P2MP Example In Figure 1, when R5 is added as the first leaf of a mulitcast distribution tree (multicast LSP), the message flow goes as follows: R5->msg1->R3->msg2->R1->msg3->R3->msg4->R5. When the leaf R4 is added, the message flow goes from R4->msg5->R3->msg6->R4. In this case, when R3 receives msg5, R3 finds out that a multicast LSP has already been set up for the same session and the same source. Therefore, R3 finds itself a branch node for leaf R4 and R5, so it will terminate the PATH message and build the corresponding RESV message and send it back to R4. The association of the LSP initiated by R4 to the existing multicast LSP is determined based on the Li, et al. Expires January 8, 2013 [Page 8] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 processing of the SESSION object and L2S_SUB_LSP object from the mRSVP-TE message. The SESSION object and the L2S_SUB_LSP objects are documented later in this draft. 2.2. MP2MP Example Root/Path Terminator/Ingress Router +---------+ | R1 | +-----+---+ _ \ \ /\ \ \ \ Path-mp2mp Message w/ Label OBJECT Resv \ \ \ (msg2) Message \ \ \ (msg3) \ \ \ w/ Label OBJECT _\/ \ \ +----------------+ Path-mp2mp | R3 | (Branch Point) +----------------+ _ _ / / /\ \ \ /\ / / / \ \ \ Path-mp2mp Message (msg1) Resv Message / / / msg4 \ \ \ (msg1) (msg6) / / / \ \ \ w/ Label OBJECT w/ Label OBJECT/ / /Path-mp2mp \ \ \ / / / Message \ \ \ / / / (msg5) \ \ \ \/_ / / w/ Label OBJ _\/ \ \ +----------+ +---+-----+ | R4 | | R5 | +----------+ +---------+ Path-mp2mp Initiator Path-mp2mp Initiator Originator ID = R4 Originator ID = R5 L2S Destination = R1 L2S Destination = R1 Session = S Session = S Figure 2: MP2MP Example In Figure 2, when R5 is added as the first leaf (as both a sender and a receiver) of an MP2MP multicast LSP, the message flow goes from R5->msg1->R3->msg2->R1->msg3->R3->msg4->R5. When the leaf R4 ( as both a sender and a receiver)is added, the message flow goes from R4->msg5->R3->msg6->R4. In this case, when R3 receives msg5, R3 finds out that an MP2MP mulitcast LSP has already been set up for the same session and the same root and R3 will become the branch LSR for Li, et al. Expires January 8, 2013 [Page 9] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 the leaf R4 and R5, so it will terminate the PATH message, build a RESV message and send the RESV message back to R4. The association of the LSP initiated by R4 to the existing MP2MP LSP is determined based on the processing of the SESSION object and the S2L_SUB_LSP from the mRSVP-TE message. The SESSION objects and the L2S_SUB_LSP objects are further documented later in this draft. 3. Signaling Protocol Extensions The RSVP-TE with receiver-driven extensions (mRSVP-TE) is similar to the RSVP-TE protocol as specified in [RFC4875], [RFC3473] and [RFC3209], but differs in that the data receivers of an LSP tunnel initiate the Path messages toward the data sender (or the root of a mulitcast LSP). Compared with [RFC4875], mRSVP-TE can also be used to set up MP2MP LSPs. In the context of the receiver-driven RSVP-TE, the Receiver is the Path-Originator. The Path messages go from the Receivers towards the Sender. The Resv messages flow in the opposite direction as compared to the Path messages, i.e. Resv messages are generated by the Sender or a branch LSR. Path messages flow in opposite directions as cmpared with those of the multicast stream distributions, while Resv messages flow in the same directions as the multicast streams. In the context of the receiver-driven RSVP-TE, a Path message will be terminated at the "root" of the multicast distribution tree (multicast LSP) or at an intermediate node if the intermediate node has received another Path message from another receiver for the same multicast distribution tree. When an intermediate node receives two or more Path messages for the same multicast distribution tree, the intermediate node will merge them together. Whether two Path messages should be merged depends on the information encoded in the SESSION and L2S-SUB-LSP objects. The SESSION object encodes multicast group information and the L2S-SUB-LSP (leaf-to-source sub- lsp) object encodes the multicast source or multicast root information. The following sections describe the receiver-driven extensions to the RSVP-TE protocol. When there is no difference in the protocol, the usage of [RFC4875] is assumed. 3.1. Mechanisms 3.1.1. Sessions As specified in [RFC2205], a session is a data flow with a particular destination and transport-layer protocol. In the context of Li, et al. Expires January 8, 2013 [Page 10] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 multicast, the data flow is essentially a multicast distribution tree rooted at the P2MP source or MP2MP root. For the sake of reliability, two or more sources/roots may be deployed to distribute the same multicast streams. A mulitcast stream is often represented by a mulitcast group address. In this document, we will encode the mulitcast group address in the SESSION object and the mulitcast source/root address in the leaf-to-source sub-LSP object. Note that the same session can have different sources/roots, and the same sources/roots can have different sessions. In the context of the receiver-driven mRSVP-TE, the processing of SESSION objects is different from that of SESSION objects in sender- driven RSVP-TE [RFC4875]. In order to distinguish them, we will employ different C-Types of SESSIONs. In this document we will document SESSION objects for native IPv4/IPv6 multicast applications. For new and more applications, new types of SESSION objects will be added. Following the method used by RSVP-TE and P2MP RSVP-TE, this draft documents the use of some new SESSION C-Type as follows: Class Name = SESSION C-Type XX+0 mRSVP_TE_P2MP_LSP_TUNNEL_IPv4 C-Type XX+1 mRSVP_TE_P2MP_LSP_TUNNEL_IPv6 C-Type XX+2 mRSVP_TE_MP2MP_LSP_TUNNEL_IPv4 C-Type XX+3 mRSVP_TE_MP2MP_LSP_TUNNEL_IPv6 C-Type Where XX is a number to be allocated by IANA. Figure 3: New C-Types of SESSIONs The new SESSION C-Type MUST be used in all receiver-driven P2MP RSVP-TE messages. 3.1.2. L2S Sub-LSPs A multicast LSP is composed of one or more leaf-to-source sub-LSPs, which are merged together at the branch nodes. There are two ways to identify each such sub-LSP: o From the Sender's perspective, each sub-LSP is identified by the SESSION object, the SENDER_TEMPLATE object and S2L_SUB_LSP object, as specified in [RFC 4875]. The SESSION object encodes P2MP ID, Li, et al. Expires January 8, 2013 [Page 11] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 Tunnel ID, and Extended Tunnel ID. The P2MP ID is unique within the scope of the sender (ingress LSR) and remains constant throughout the lifetime of the P2MP tree structure. The Extended Tunnel ID, which remains constant throughout the lifetime of the P2MP tree structure, and which should contain the sender's address to make sure the identifier is globally unique. Finally, the Tunnel ID, also remains constant throughout the lifetime of the P2MP tree structure. The SENDER_TEMPLATE object contains the ingress LSR source address. The S2L_SUB_LSP contains the destination address of the sub-LSP. o From the Receiver's perspective, each sub-LSP is identified by a new SESSION object, a new SENDER_TEMPLATE object and a new L2S_SUB_LSP object. The SESSION object, different from the one used in typical sender-driven environments, contains information to be used as the key to associate different PATH messages originated from different leaves. The SENDER_TEMPLATE object contains the Path-Originator's address, which is actually the Data Receiver. The L2S_SUB_LSP contains the source or root address of the sub-LSP, i.e. the data Sender's address. The SESSION, SENDER_TEMPLATE and L2S_SUB_LSP all together will identify the multicast stream, the multicast stream's source, and a mulitcast stream's receiver This document takes the approach from the Receiver's perspective. The approach from the Sender's perspective is documented in [RFC 4875]. Once an LSR receives a receiver-driven Path message with the SESSION object and L2S_SUB_LSP object, the LSR should be able to use the SESSION object and L2S_SUB_LSP object to determine whether the sub- LSP signaled by this Path message should be merged with existing multicast LSPs. 3.1.3. Path Originator and Data Receiver In the context of the receiver-driven RSVP-TE, a Path Originator is also a Data Receiver. This document will document a new type of SENDER_TEMPLATE object, which contains the Path-Originator's IP address and describes the identity of the Path Originator. In [RFC 2205] and [RFC 4875], the "sender" is both a path originator and a data sender. In the receiver-driven context, path originators and data senders may be different. For P2MP, path originators are actually the data receivers. For MP2MP, path originators are also both the data senders and data receivers. In this document, we will use the same Object Class SENDER_TEMPLATE Li, et al. Expires January 8, 2013 [Page 12] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 with a different C-Type to represent and identify Path Originator. In the case of P2MP LSP, the SENDER_TEMPLATE describes the identify of a data receiver. In the case of MP2MP, the SENDER_TEMPLATE describes the identify of an LSR which work as both a data sender and a data receiver. All of the SESSION object, L2S_SUB_LSP object and SENDER_TEMPLATE object together contained in a Path message will uniquely identify a leaf-to-source sub-LSP. 3.1.4. Explicit Routing An EXPLICIT_ROUTE Object (ERO) is used to optionally specify the explicit route of an L2S sub-LSP. Each signaled ERO corresponds to a particular L2S_SUB_LSP object. Details of explicit route encoding are specified in section 4.5 of [RFC4875], but they are encoded in a reverse order in the receiver-driven context. When a Path message signals a L2S sub-LSP, the EXPLICIT_ROUTE object encodes the path from the leaf to the root LSR. The Path message also includes the L2S_SUB_LSP object for the L2S sub-LSP being signaled. The < [], > tuple represents the L2S sub-LSP and is referred to as the sub-LSP descriptor. The absence of the ERO should be interpreted as requiring hop-by-hop reverse-forwarding for the sub-LSP based on the root address field of the L2S_SUB_LSP object. 3.2. Path Messages The mechanism specified in this document allows a multicast P2MP/ MP2MP LSP to be signaled using one or more Path messages. Each Path message may signal one L2S sub-LSPs. A receiver-driven P2MP MPLS-TE LSP uses the Path message to carry the LABEL object upstream from the Receiver towards the Sender. With a receiver-driven usage of the RSVP PATH messages, the LABEL_REQUEST object carried by the PATH message is no longer mandatory, it becomes optional for receiver-driven PATH messages, as specified in Figure 4: Li, et al. Expires January 8, 2013 [Page 13] Internet-Draft Receiver-Driven Multicast RSVP-TE July 2012 ::= [ ] [ [ | ] ...] [ ] [ ] [ ] [ ] [ ... ] [ ] [ ] [ ] [ ... ] [] Figure 4: Path Message Extensions The SESSION object encodes information about the being-signalled multicast stream. The SESSION object together with L2S_SUB_LSP will be used as the key to associate different sub-LSPs to the same multicast LSP. Using [RFC4875] as the base specification, the LABEL object is added to the as specified in Figure 5: ::= [ ] [ ] [ ] [ ]