Internet Engineering Task Force (IETF) W. George, Ed. Request for Comments: 7439 Time Warner Cable Category: Informational C. Pignataro, Ed. ISSN: 2070-1721 Cisco January 2015 Gap Analysis for Operating IPv6-Only MPLS Networks Abstract This document reviews the Multiprotocol Label Switching (MPLS) protocol suite in the context of IPv6 and identifies gaps that must be addressed in order to allow MPLS-related protocols and applications to be used with IPv6-only networks. This document is intended to focus on gaps in the standards defining the MPLS suite, and is not intended to highlight particular vendor implementations (or lack thereof) in the context of IPv6-only MPLS functionality. In the data plane, MPLS fully supports IPv6, and MPLS labeled packets can be carried over IPv6 packets in a variety of encapsulations. However, support for IPv6 among MPLS control-plane protocols, MPLS applications, MPLS Operations, Administration, and Maintenance (OAM), and MIB modules is mixed, with some protocols having major gaps. For most major gaps, work is in progress to upgrade the relevant protocols. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7439.
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Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Gap Analysis . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. MPLS Data Plane . . . . . . . . . . . . . . . . . . . . . 6 3.2. MPLS Control Plane . . . . . . . . . . . . . . . . . . . 6 3.2.1. Label Distribution Protocol (LDP) . . . . . . . . . . 6 3.2.2. Multipoint LDP (mLDP) . . . . . . . . . . . . . . . . 6 3.2.3. RSVP - Traffic Engineering (RSVP-TE) . . . . . . . . 7 188.8.131.52. Interior Gateway Protocol (IGP) . . . . . . . . . 8 184.108.40.206. RSVP-TE Point-to-Multipoint (P2MP) . . . . . . . 8 220.127.116.11. RSVP-TE Fast Reroute (FRR) . . . . . . . . . . . 8 3.2.4. Path Computation Element (PCE) . . . . . . . . . . . 8 3.2.5. Border Gateway Protocol (BGP) . . . . . . . . . . . . 9 3.2.6. Generalized Multi-Protocol Label Switching (GMPLS) . 9 3.3. MPLS Applications . . . . . . . . . . . . . . . . . . . . 9 3.3.1. Layer 2 Virtual Private Network (L2VPN) . . . . . . . 9 18.104.22.168. Ethernet VPN (EVPN) . . . . . . . . . . . . . . . 10 3.3.2. Layer 3 Virtual Private Network (L3VPN) . . . . . . . 10 22.214.171.124. IPv6 Provider Edge/IPv4 Provider Edge (6PE/4PE) . 11 126.96.36.199. IPv6 Virtual Private Extension/IPv4 Virtual Private Extension (6VPE/4VPE) . . . . . . . . . . 11 188.8.131.52. BGP Encapsulation Subsequent Address Family Identifier (SAFI) . . . . . . . . . . . . . . . . 12 184.108.40.206. Multicast in MPLS/BGP IP VPN (MVPN) . . . . . . . 12 3.3.3. MPLS Transport Profile (MPLS-TP) . . . . . . . . . . 13 3.4. MPLS Operations, Administration, and Maintenance (MPLS OAM) . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.4.1. Extended ICMP . . . . . . . . . . . . . . . . . . . . 14 3.4.2. Label Switched Path Ping (LSP Ping) . . . . . . . . . 15 3.4.3. Bidirectional Forwarding Detection (BFD) . . . . . . 16 3.4.4. Pseudowire OAM . . . . . . . . . . . . . . . . . . . 16 3.4.5. MPLS Transport Profile (MPLS-TP) OAM . . . . . . . . 16 3.5. MIB Modules . . . . . . . . . . . . . . . . . . . . . . . 17 4. Gap Summary . . . . . . . . . . . . . . . . . . . . . . . . . 17 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Normative References . . . . . . . . . . . . . . . . . . 19 6.2. Informative References . . . . . . . . . . . . . . . . . 20 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 26 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction IPv6 [RFC2460] is an integral part of modern network deployments. At the time when this document was written, the majority of these IPv6 deployments were using dual-stack implementations, where IPv4 and IPv6 are supported equally on many or all of the network nodes, and single-stack primarily referred to IPv4-only devices. Dual-stack deployments provide a useful margin for protocols and features that are not currently capable of operating solely over IPv6, because they can continue using IPv4 as necessary. However, as IPv6 deployment and usage becomes more pervasive, and IPv4 exhaustion begins driving changes in address consumption behaviors, there is an increasing likelihood that many networks will need to start operating some or all of their network nodes either as primarily IPv6 (most functions use IPv6, a few legacy features use IPv4), or as IPv6-only (no IPv4 provisioned on the device). This transition toward IPv6-only operation exposes any gaps where features, protocols, or implementations are still reliant on IPv4 for proper function. To that end, and in the spirit of the recommendation in RFC 6540 [RFC6540] that implementations need to stop requiring IPv4 for proper and complete function, this document reviews the MPLS protocol suite in the context of IPv6 and identifies gaps that must be addressed in order to allow MPLS-related protocols and applications to be used with IPv6-only networks and networks that are primarily IPv6 (hereafter referred to as IPv6-primary). This document is intended to focus on gaps in the standards defining the MPLS suite, and not to highlight particular vendor implementations (or lack thereof) in the context of IPv6-only MPLS functionality. 2. Use Case This section discusses some drivers for ensuring that MPLS completely supports IPv6-only operation. It is not intended to be a comprehensive discussion of all potential use cases, but rather a discussion of one use case to provide context and justification to undertake such a gap analysis. IP convergence is continuing to drive new classes of devices to begin communicating via IP. Examples of such devices could include set-top boxes for IP video distribution, cell tower electronics (macro or micro cells), infrastructure Wi-Fi access points, and devices for machine-to-machine (M2M) or Internet of Things (IoT) applications. In some cases, these classes of devices represent a very large deployment base, on the order of thousands or even millions of devices network-wide. The scale of these networks, coupled with the increasingly overlapping use of RFC 1918 [RFC1918] address space within the average network and the lack of globally routable IPv4 space available for long-term growth, begins to drive the need for
many of the endpoints in this network to be managed solely via IPv6. Even if these devices are carrying some IPv4 user data, it is often encapsulated in another protocol such that the communication between the endpoint and its upstream devices can be IPv6-only without impacting support for IPv4 on user data. As the number of devices to manage increases, the operator is compelled to move to IPv6. Depending on the MPLS features required, it is plausible to assume that the (existing) MPLS network will need to be extended to these IPv6-only devices. Additionally, as the impact of IPv4 exhaustion becomes more acute, more and more aggressive IPv4 address reclamation measures will be justified. Many networks are likely to focus on preserving their remaining IPv4 addresses for revenue-generating customers so that legacy support for IPv4 can be maintained as long as necessary. As a result, it may be appropriate for some or all of the network infrastructure, including MPLS Label Switching Routers (LSRs) and Label Edge Routers (LERs), to have its IPv4 addresses reclaimed and transition toward IPv6-only operation. 3. Gap Analysis This gap analysis aims to answer the question of what fails when one attempts to use MPLS features on a network of IPv6-only devices. The baseline assumption for this analysis is that some endpoints, as well as Label Switching Routers (Provider Edge (PE) and Provider (P) routers), only have IPv6 transport available and need to support the full suite of MPLS features defined as of the time of this document's writing at parity with the support on an IPv4 network. This is necessary whether they are enabled via the Label Distribution Protocol (LDP) [RFC5036], RSVP - Traffic Engineering (RSVP-TE) [RFC3209], or Border Gateway Protocol (BGP) [RFC3107], and whether they are encapsulated in MPLS [RFC3032], IP [RFC4023], Generic Routing Encapsulation (GRE) [RFC4023], or Layer 2 Tunneling Protocol Version 3 (L2TPv3) [RFC4817]. It is important when evaluating these gaps to distinguish between user data and control-plane data, because while this document is focused on IPv6-only operation, it is quite likely that some amount of the user payload data being carried in the IPv6-only MPLS network will still be IPv4. A note about terminology: Gaps identified by this document are characterized as "Major" or "Minor". Major gaps refer to significant changes necessary in one or more standards to address the gap due to existing standards language having either missing functionality for IPv6-only operation or explicit language requiring the use of IPv4 with no IPv6 alternatives defined. Minor gaps refer to changes necessary primarily to clarify existing standards language. Usually
these changes are needed in order to explicitly codify IPv6 support in places where it is either implicit or omitted today, but the omission is unlikely to prevent IPv6-only operation. 3.1. MPLS Data Plane MPLS labeled packets can be transmitted over a variety of data links [RFC3032], and MPLS labeled packets can also be encapsulated over IP. The encapsulations of MPLS in IP and GRE, as well as MPLS over L2TPv3, support IPv6. See Section 3 of RFC 4023 [RFC4023] and Section 2 of RFC 4817 [RFC4817], respectively. Gap: None. 3.2. MPLS Control Plane 3.2.1. Label Distribution Protocol (LDP) The Label Distribution Protocol (LDP) [RFC5036] defines a set of procedures for distribution of labels between Label Switching Routers that can use the labels for forwarding traffic. While LDP was designed to use an IPv4 or dual-stack IP network, it has a number of deficiencies that prevent it from working in an IPv6-only network. LDP-IPv6 [LDP-IPv6] highlights some of the deficiencies when LDP is enabled in IPv6-only or dual-stack networks and specifies appropriate protocol changes. These deficiencies are related to Label Switched Path (LSP) mapping, LDP identifiers, LDP discovery, LDP session establishment, next-hop address, and LDP Time To Live (TTL) security [RFC5082] [RFC6720]. Gap: Major; update to RFC 5036 in progress via [LDP-IPv6], which should close this gap. 3.2.2. Multipoint LDP (mLDP) Multipoint LDP (mLDP) is a set of extensions to LDP for setting up Point-to-Multipoint (P2MP) and Multipoint-to-Multipoint (MP2MP) LSPs. These extensions are specified in RFC 6388 [RFC6388]. In terms of IPv6-only gap analysis, mLDP has two identified areas of interest: 1. LDP Control Plane: Since mLDP uses the LDP control plane to discover and establish sessions with the peer, it shares the same gaps as LDP (Section 3.2.1) with regards to control plane (discovery, transport, and session establishment) in an IPv6-only network.
2. Multipoint (MP) Forwarding Equivalence Class (FEC) Root Address: mLDP defines its own MP FECs and rules, different from LDP, to map MP LSPs. An mLDP MP FEC contains a Root Address field that is an IP address in IP networks. The current specification allows specifying the root address according to the Address Family Identifier (AFI), and hence covers both IPv4 or IPv6 root addresses, requiring no extension to support IPv6-only MP LSPs. The root address is used by each LSR participating in an MP LSP setup such that root address reachability is resolved by doing a table lookup against the root address to find corresponding upstream neighbor(s). This will pose a problem if an MP LSP traverses IPv4-only and IPv6-only nodes in a dual-stack network on the way to the root node. For example, consider following setup, where R1/R6 are IPv4-only, R3/ R4 are IPv6-only, and R2/R5 are dual-stack LSRs: ( IPv4-only ) ( IPv6-only ) ( IPv4-only ) R1 -- R2 -- R3 -- R4 -- R5 -- R6 Leaf Root Assume R1 to be a leaf node for a P2MP LSP rooted at R6 (root node). R1 uses R6's IPv4 address as the root address in MP FEC. As the MP LSP signaling proceeds from R1 to R6, the MP LSP setup will fail on the first IPv6-only transit/branch LSRs (R3) when trying to find IPv4 root address reachability. RFC 6512 [RFC6512] defines a recursive- FEC solution and procedures for mLDP when the backbone (transit/ branch) LSRs have no route to the root. The proposed solution is defined for a BGP-free core in a VPN environment, but a similar concept can be used/extended to solve the above issue of the IPv6-only backbone receiving an MP FEC element with an IPv4 address. The solution will require a border LSR (the one that is sitting on the border of an IPv4/IPv6 island (namely, R2 and R5 in this example)) to translate an IPv4 root address to an equivalent IPv6 address (and vice versa) through procedures similar to RFC 6512. Gap: Major; update in progress for LDP via [LDP-IPv6], may need additional updates to RFC 6512. 3.2.3. RSVP - Traffic Engineering (RSVP-TE) RSVP-TE [RFC3209] defines a set of procedures and enhancements to establish LSP tunnels that can be automatically routed away from network failures, congestion, and bottlenecks. RSVP-TE allows establishing an LSP for an IPv4 or IPv6 prefix, thanks to its LSP_TUNNEL_IPv6 object and subobjects. Gap: None.
220.127.116.11. Interior Gateway Protocol (IGP) RFC 3630 [RFC3630] specifies a method of adding traffic engineering capabilities to OSPF Version 2. New TLVs and sub-TLVs were added in RFC 5329 [RFC5329] to extend TE capabilities to IPv6 networks in OSPF Version 3. RFC 5305 [RFC5305] specifies a method of adding traffic engineering capabilities to IS-IS. New TLVs and sub-TLVs were added in RFC 6119 [RFC6119] to extend TE capabilities to IPv6 networks. Gap: None. 18.104.22.168. RSVP-TE Point-to-Multipoint (P2MP) RFC 4875 [RFC4875] describes extensions to RSVP-TE for the setup of Point-to-Multipoint (P2MP) LSPs in MPLS and Generalized MPLS (GMPLS) with support for both IPv4 and IPv6. Gap: None. 22.214.171.124. RSVP-TE Fast Reroute (FRR) RFC 4090 [RFC4090] specifies Fast Reroute (FRR) mechanisms to establish backup LSP tunnels for local repair supporting both IPv4 and IPv6 networks. Further, [RFC5286] describes the use of loop-free alternates to provide local protection for unicast traffic in pure IP and MPLS networks in the event of a single failure, whether link, node, or shared risk link group (SRLG) for both IPv4 and IPv6. Gap: None. 3.2.4. Path Computation Element (PCE) The Path Computation Element (PCE) defined in RFC 4655 [RFC4655] is an entity that is capable of computing a network path or route based on a network graph and applying computational constraints. A Path Computation Client (PCC) may make requests to a PCE for paths to be computed. The PCE Communication Protocol (PCEP) is designed as a communication protocol between PCCs and PCEs for path computations and is defined in RFC 5440 [RFC5440]. The PCEP specification [RFC5440] is defined for both IPv4 and IPv6 with support for PCE discovery via an IGP (OSPF [RFC5088] or IS-IS [RFC5089]) using both IPv4 and IPv6 addresses. Note that PCEP uses identical encoding of subobjects, as in RSVP-TE defined in RFC 3209 [RFC3209] that supports both IPv4 and IPv6.
The extensions to PCEP to support confidentiality [RFC5520], route exclusions [RFC5521], monitoring [RFC5886], and P2MP TE LSPs [RFC6006] have support for both IPv4 and IPv6. Gap: None. 3.2.5. Border Gateway Protocol (BGP) RFC 3107 [RFC3107] specifies a set of BGP protocol procedures for distributing the labels (for prefixes corresponding to any address family) between label switch routers so that they can use the labels for forwarding the traffic. RFC 3107 allows BGP to distribute the label for IPv4 or IPv6 prefix in an IPv6-only network. Gap: None. 3.2.6. Generalized Multi-Protocol Label Switching (GMPLS) The Generalized Multi-Protocol Label Switching (GMPLS) specification includes signaling functional extensions [RFC3471] and RSVP-TE extensions [RFC3473]. The gap analysis in Section 3.2.3 applies to these. RFC 4558 [RFC4558] specifies Node-ID Based RSVP Hello Messages with capability for both IPv4 and IPv6. RFC 4990 [RFC4990] clarifies the use of IPv6 addresses in GMPLS networks including handling in the MIB modules. The second paragraph of Section 5.3 of RFC 6370 [RFC6370] describes the mapping from an MPLS Transport Profile (MPLS-TP) LSP_ID to RSVP- TE with an assumption that Node_IDs are derived from valid IPv4 addresses. This assumption fails in an IPv6-only network, given that there would not be any IPv4 addresses. Gap: Minor; Section 5.3 of RFC 6370 [RFC6370] needs to be updated. 3.3. MPLS Applications 3.3.1. Layer 2 Virtual Private Network (L2VPN) L2VPN [RFC4664] specifies two fundamentally different kinds of Layer 2 VPN services that a service provider could offer to a customer: Virtual Private Wire Service (VPWS) and Virtual Private LAN Service (VPLS). RFC 4447 [RFC4447] and RFC 4762 [RFC4762] specify the LDP protocol changes to instantiate VPWS and VPLS services, respectively, in an MPLS network using LDP as the signaling protocol. This is complemented by RFC 6074 [RFC6074], which specifies a set of procedures for instantiating L2VPNs (e.g., VPWS, VPLS) using BGP as a
discovery protocol and LDP, as well as L2TPv3, as a signaling protocol. RFC 4761 [RFC4761] and RFC 6624 [RFC6624] specify BGP protocol changes to instantiate VPLS and VPWS services in an MPLS network, using BGP for both discovery and signaling. In an IPv6-only MPLS network, use of L2VPN represents a connection of Layer 2 islands over an IPv6 MPLS core, and very few changes are necessary to support operation over an IPv6-only network. The L2VPN signaling protocol is either BGP or LDP in an MPLS network, and both can run directly over IPv6 core infrastructure as well as IPv6 edge devices. RFC 6074 [RFC6074] is the only RFC that appears to have a gap for IPv6-only operation. In its discovery procedures (Sections 3.2.2 and 6 of RFC 6074 [RFC6074]), it suggests encoding PE IP addresses in the Virtual Switching Instance ID (VSI-ID), which is encoded in Network Layer Reachability Information (NLRI) and should not exceed 12 bytes (to differentiate its AFI/SAFI (Subsequent Address Family Identifier) encoding from RFC 4761). This means that a PE IP address cannot be an IPv6 address. Also, in its signaling procedures (Section 3.2.3 of RFC 6074 [RFC6074]), it suggests encoding PE_addr in the Source Attachment Individual Identifier (SAII) and the Target Attachment Individual Identifier (TAII), which are limited to 32 bits (AII Type=1) at the moment. RFC 6073 [RFC6073] defines the new LDP Pseudowire (PW) Switching Point PE TLV, which supports IPv4 and IPv6. Gap: Minor; RFC 6074 needs to be updated. 126.96.36.199. Ethernet VPN (EVPN) Ethernet VPN [EVPN] defines a method for using BGP MPLS-based Ethernet VPNs. Because it can use functions in LDP and mLDP, as well as Multicast VPLS [RFC7117], it inherits LDP gaps previously identified in Section 3.2.1. Once those gaps are resolved, it should function properly on IPv6-only networks as defined. Gap: Major for LDP; update to RFC 5036 in progress via [LDP-IPv6] that should close this gap (see Section 3.2.1). 3.3.2. Layer 3 Virtual Private Network (L3VPN) RFC 4364 [RFC4364] defines a method by which a Service Provider may use an IP backbone to provide IP VPNs for its customers. The following use cases arise in the context of this gap analysis: 1. Connecting IPv6 islands over IPv6-only MPLS network 2. Connecting IPv4 islands over IPv6-only MPLS network
Both use cases require mapping an IP packet to an IPv6-signaled LSP. RFC 4364 defines Layer 3 Virtual Private Networks (L3VPNs) for IPv4-only and has references to 32-bit BGP next-hop addresses. RFC 4659 [RFC4659] adds support for IPv6 on L3VPNs, including 128-bit BGP next-hop addresses, and discusses operation whether IPv6 is the payload or the underlying transport address family. However, RFC 4659 does not formally update RFC 4364, and thus an implementer may miss this additional set of standards unless it is explicitly identified independently of the base functionality defined in RFC 4364. Further, Section 1 of RFC 4659 explicitly identifies use case 2 as out of scope for the document. The authors do not believe that there are any additional issues encountered when using L2TPv3, RSVP, or GRE (instead of MPLS) as transport on an IPv6-only network. Gap: Major; RFC 4659 needs to be updated to explicitly cover use case 2 (discussed in further detail below) 188.8.131.52. IPv6 Provider Edge/IPv4 Provider Edge (6PE/4PE) RFC 4798 [RFC4798] defines IPv6 Provider Edge (6PE), which defines how to interconnect IPv6 islands over a MPLS-enabled IPv4 cloud. However, use case 2 is doing the opposite, and thus could also be referred to as IPv4 Provider Edge (4PE). The method to support this use case is not defined explicitly. To support it, IPv4 edge devices need to be able to map IPv4 traffic to MPLS IPv6 core LSPs. Also, the core switches may not understand IPv4 at all, but in some cases they may need to be able to exchange Labeled IPv4 routes from one Autonomous System (AS) to a neighboring AS. Gap: Major; RFC 4798 covers only the "6PE" case. Use case 2 is currently not specified in an RFC. 184.108.40.206. IPv6 Virtual Private Extension/IPv4 Virtual Private Extension (6VPE/4VPE) RFC 4659 [RFC4659] defines IPv6 Virtual Private Network Extension (6VPE), a method by which a Service Provider may use its packet- switched backbone to provide Virtual Private Network (VPN) services for its IPv6 customers. It allows the core network to be MPLS IPv4 or MPLS IPv6, thus addressing use case 1 above. RFC 4364 should work as defined for use case 2 above, which could also be referred to as IPv4 Virtual Private Extension (4VPE), but the RFC explicitly does not discuss this use and defines it as out of scope. Gap: Minor; RFC 4659 needs to be updated to explicitly cover use case 2.
220.127.116.11. BGP Encapsulation Subsequent Address Family Identifier (SAFI) RFC 5512 [RFC5512] defines the BGP Encapsulation SAFI and the BGP Tunnel Encapsulation Attribute, which can be used to signal tunneling over an IP Core that is using a single address family. This mechanism supports transport of MPLS (and other protocols) over Tunnels in an IP core (including an IPv6-only core). In this context, load balancing can be provided as specified in RFC 5640 [RFC5640]. Gap: None. 18.104.22.168. Multicast in MPLS/BGP IP VPN (MVPN) RFC 6513 [RFC6513] defines the procedure to provide multicast service over an MPLS VPN backbone for downstream customers. It is sometimes referred to as Next Generation Multicast VPN (NG-MVPN) The procedure involves the below set of protocols. 22.214.171.124.1. PE-CE Multicast Routing Protocol RFC 6513 [RFC6513] explains the use of Protocol Independent Multicast (PIM) as a Provider Edge - Customer Edge (PE-CE) protocol, while Section 11.1.2 of RFC 6514 [RFC6514] explains the use of mLDP as a PE-CE protocol. The MCAST-VPN NLRI route-type format defined in RFC 6514 [RFC6514] is not sufficiently covering all scenarios when mLDP is used as a PE-CE protocol. The issue is explained in Section 2 of [mLDP-NLRI] along with a new route type that encodes the mLDP FEC in NLRI. Further, [PE-CE] defines the use of BGP as a PE-CE protocol. Gap: None. 126.96.36.199.2. P-Tunnel Instantiation [RFC6513] explains the use of the below tunnels: o RSVP-TE P2MP LSP o PIM Tree o mLDP P2MP LSP o mLDP MP2MP LSP o Ingress Replication
Gap: Gaps in RSVP-TE P2MP LSP (Section 188.8.131.52) and mLDP (Section 3.2.2) P2MP and MP2MP LSP are covered in previous sections. There are no MPLS-specific gaps for PIM Tree or Ingress Replication, and any protocol-specific gaps not related to MPLS are outside the scope of this document. 184.108.40.206.3. PE-PE Multicast Routing Protocol Section 3.1 of RFC 6513 [RFC6513] explains the use of PIM as a PE-PE protocol, while RFC 6514 [RFC6514] explains the use of BGP as a PE-PE protocol. PE-PE multicast routing is not specific to P-tunnels or to MPLS. It can be PIM or BGP with P-tunnels that are label based or PIM tree based. Enabling PIM as a PE-PE multicast protocol is equivalent to running it on a non-MPLS IPv6 network, so there are not any MPLS- specific considerations and any gaps are applicable for non-MPLS networks as well. Similarly, BGP only includes the P-Multicast Service Interface (PMSI) tunnel attribute as a part of the NLRI, which is inherited from P-tunnel instantiation and considered to be an opaque value. Any gaps in the control plane (PIM or BGP) will not be specific to MPLS. Gap: Any gaps in PIM or BGP as a PE-PE multicast routing protocol are not unique to MPLS, and therefore are outside the scope of this document. It is included for completeness. 3.3.3. MPLS Transport Profile (MPLS-TP) MPLS-TP does not require IP (see Section 2 of RFC 5921 [RFC5921]) and should not be affected by operation on an IPv6-only network. Therefore, this is considered out of scope for this document but is included for completeness. Although not required, MPLS-TP can use IP. One such example is included in Section 3.2.6, where MPLS-TP identifiers can be derived from valid IPv4 addresses. Gap: None. MPLS-TP does not require IP.