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
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
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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(IETF). It represents the consensus of the IETF community. It has
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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
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.
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
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
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
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.
184.108.40.206. 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
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.
220.127.116.11. 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.
18.104.22.168. 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.
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.
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.
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
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
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.
22.214.171.124. 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)
126.96.36.199. 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.
188.8.131.52. IPv6 Virtual Private Extension/IPv4 Virtual Private Extension
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
184.108.40.206. 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
220.127.116.11. 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.
18.104.22.168.1. 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
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.
22.214.171.124.2. 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 126.96.36.199) 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.
188.8.131.52.3. 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
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.