MPLS Working Group Rajiv Asati
Internet Draft Cisco
Updates: 5036 (if approved)
Intended status: Standards Track Vishwas Manral
Expires: August 2014
Hewlett-Packard, Inc.
Rajiv Papneja
Huawei
Carlos Pignataro
Cisco
February 5, 2014
Updates to LDP for IPv6
draft-ietf-mpls-ldp-ipv6-12
Status of this Memo
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Abstract
The Label Distribution Protocol (LDP) specification defines
procedures to exchange label bindings over either IPv4, or IPv6 or
both networks. This document corrects and clarifies the LDP behavior
when IPv6 network is used (with or without IPv4). This document
updates RFC 5036.
Table of Contents
1. Introduction...................................................3
1.1. Scope.....................................................4
1.1.1. Topology Scenarios...................................4
1.1.2. LDP TTL Security.....................................5
2. Specification Language.........................................6
3. LSP Mapping....................................................6
4. LDP Identifiers................................................7
5. Peer Discovery.................................................7
5.1. Basic Discovery Mechanism.................................8
5.1.1. Maintaining Hello Adjacencies........................9
5.2. Extended Discovery Mechanism..............................9
6. LDP Session Establishment and Maintenance......................9
6.1. Transport connection establishment........................9
6.2. LDP Sessions Maintenance.................................11
7. Label Distribution............................................12
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8. LDP Identifiers and Next Hop Addresses........................12
9. LDP TTL Security..............................................13
10. IANA Considerations..........................................14
11. Security Considerations......................................14
12. Acknowledgments..............................................14
13. Additional Contributors......................................14
14. References...................................................16
14.1. Normative References....................................16
14.2. Informative References..................................16
Appendix A.......................................................18
A.1. LDPv6 and LDPv4 Interoperability Safety Net..............18
A.2. Why 32-bit value even for IPv6 LDP Router ID.............18
Author's Addresses...............................................19
1. Introduction
The LDP [RFC5036] specification defines procedures and messages for
exchanging FEC-label bindings over either IPv4 or IPv6 or both (e.g.
dual-stack) networks.
However, RFC5036 specification has the following deficiencies in
regards to IPv6 usage:
1) LSP Mapping: No rule defined for mapping a particular packet to a
particular LSP that has an Address Prefix FEC element containing
IPv6 address of the egress router
2) LDP Identifier: No details specific to IPv6 usage
3) LDP Discovery: No details for using a particular IPv6 destination
(multicast) address or the source address (with or without IPv4
co-existence)
4) LDP Session establishment: No rule for handling both IPv4 and
IPv6 transport address optional objects in a Hello message, and
subsequently two IPv4 and IPv6 transport connections
5) LDP Label Distribution: No rule for advertising IPv4 or/and IPv6
FEC-label bindings over an LDP session, and denying the co-
existence of IPv4 and IPv6 FEC Elements in the same FEC TLV
6) Next Hop Address & LDP Identifier: No rule for accommodating the
usage of duplicate link-local IPv6 addresses
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7) LDP TTL Security: No rule for built-in Generalized TTL Security
Mechanism (GTSM) in LDP
This document addresses the above deficiencies by specifying the
desired behavior/rules/details for using LDP in IPv6 enabled
networks (IPv6-only or Dual-stack networks).
Note that this document updates RFC5036.
1.1. Scope
1.1.1. Topology Scenarios
Two LSRs may involve basic and/or extended LDP discovery in IPv6
and/or IPv4 address-families in various topology scenarios.
This document addresses the following 3 topology scenarios in which
the LSRs may be connected via one or more dual-stack interfaces
(figure 1), or one or more single-stack interfaces (figure 2 and
figure 3):
R1------------------R2
IPv4+IPv6
Figure 1 LSRs connected via a Dual-stack Interface
IPv4
R1=================R2
IPv6
Figure 2 LSRs connected via two single-stack Interfaces
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R1------------------R2---------------R3
IPv4 IPv6
Figure 3 LSRs connected via a single-stack Interface
Note that the topology scenario illustrated in figure 1 also covers
the case of a single-stack interface (IPv4, say) being converted to
a dual-stacked interface by enabling IPv6 routing as well as IPv6
LDP, even though the IPv4 LDP session may already be established
between the LSRs.
Note that the topology scenario illustrated in figure 2 also covers
the case of two routers getting connected via an additional single-
stack interface (IPv6 routing and IPv6 LDP), even though the IPv4
LDP session may already be established between the LSRs over the
existing interface(s).
This document also addresses the scenario in which the LSRs do
extended discovery in IPv6 and/or IPv4 address-families:
IPv4
R1-------------------R2
IPv6
Figure 4 LSRs involving IPv4 and IPv6 address-families
1.1.2. LDP TTL Security
LDP TTL Security mechanism specified by this document applies only
to single-hop LDP peering sessions, but not to multi-hop LDP peering
sessions, in line with Section 5.5 of [RFC5082] that describes
Generalized TTL Security Mechanism (GTSM).
As a consequence, any LDP feature that relies on multi-hop LDP
peering session would not work with GTSM and will warrant
(statically or dynamically) disabling GTSM. Please see section 8.
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2. Specification Language
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 [RFC2119].
Abbreviations:
LDP - Label Distribution Protocol
LDPoIPv4 - LDP over IPv4 transport session
LDPoIPv6 - LDP over IPv6 transport session
FEC - Forwarding Equivalence Class
TLV - Type Length Value
LSR - Label Switching Router
LSP - Label Switched Path
LSPv4 - IPv4-signaled Label Switched Path [RFC4798]
LSPv6 - IPv6-signaled Label Switched Path [RFC4798]
AFI - Address Family Identifier
LDP Id - LDP Identifier
3. LSP Mapping
Section 2.1 of [RFC5036] specifies the procedure for mapping a
particular packet to a particular LSP using three rules. Quoting the
3rd rule from RFC5036:
"If it is known that a packet must traverse a particular egress
router, and there is an LSP that has an Address Prefix FEC element
that is a /32 address of that router, then the packet is mapped to
that LSP."
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This rule is correct for IPv4, but not for IPv6, since an IPv6
router may even have a /64 or /96 or /128 (or whatever prefix
length) address. Hence, it is reasonable to say IPv4 or IPv6 address
instead of /32 or /128 addresses as shown below in the updated rule:
"If it is known that a packet must traverse a particular egress
router, and there is an LSP that has an Address Prefix FEC element
that is an IPv4 or IPv6 address of that router, then the packet is
mapped to that LSP."
4. LDP Identifiers
Section 2.2.2 of [RFC5036] specifies formulating at least one LDP
Identifier, however, it doesn't provide any consideration in case of
IPv6 (with or without dual-stacking).
The first four octets of the LDP identifier, the 32-bit LSR Id (e.g.
(i.e. LDP Router Id), identify the LSR and is a globally unique
value within the MPLS network. This is regardless of the address
family used for the LDP session. Hence, this document preserves the
usage of 32-bit (unsigned non-zero integer) LSR Id on an IPv6 only
LSR.
This document also qualifies the first sentence of last paragraph of
Section 2.5.2 of [RFC5036] to be per address family and therefore
updates that sentence to the following:
"For a given address family, an LSR MUST advertise the same
transport address in all Hellos that advertise the same label
space."
This rightly enables the per-platform label space to be shared
between IPv4 and IPv6.
In summary, this document allows the usage of a common LDP
identifier (same LSR Id aka LDP Router Id as well as a common Label
space id) for both IPv4 and IPv6 on a dual-stack LSR.
5. Peer Discovery
If an LSR is enabled with both IPv4 and IPv6 LDP, then the LSR MUST
include the same LDP Identifier (assuming per-platform label space
usage) in both IPv6 and IPv4 LDP Link or targeted Hellos.
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5.1. Basic Discovery Mechanism
Section 2.4.1 of [RFC5036] defines the Basic Discovery mechanism for
directly connected LSRs. Following this mechanism, LSRs periodically
send LDP Link Hellos destined to "all routers on this subnet" group
multicast IP address.
Interesting enough, per the IPv6 addressing architecture [RFC4291],
IPv6 has three "all routers on this subnet" multicast addresses:
FF01:0:0:0:0:0:0:2 = Interface-local scope
FF02:0:0:0:0:0:0:2 = Link-local scope
FF05:0:0:0:0:0:0:2 = Site-local scope
[RFC5036] does not specify which particular IPv6 'all routers on
this subnet' group multicast IP address should be used by LDP Link
Hellos.
This document specifies the usage of link-local scope e.g.
FF02:0:0:0:0:0:0:2 as the destination multicast IP address in IPv6
LDP Link Hellos. An LDP Hello packet received on any of the other
destination addresses MUST be dropped. Additionally, the link-local
IPv6 address MUST be used as the source IP address in IPv6 LDP Link
Hellos.
Also, the LDP Link Hello packets MUST have their IPv6 Hop Limit set
to 255, be checked for the same upon receipt (before any LDP
specific processing) and be handled as specified in Generalized TTL
Security Mechanism (GTSM) section 3 of [RFC5082]. The built-in
inclusion of GTSM automatically protects IPv6 LDP from off-link
attacks.
More importantly, if an interface is a dual-stack LDP interface
(e.g. enabled with both IPv6 and IPv4 LDP), then the LSR MUST
periodically send both IPv6 and IPv4 LDP Link Hellos (using the same
LDP Identifier per section 4) on that interface and be able to
receive them. This facilitates discovery of IPv6-only, IPv4-only and
dual-stack peers on the interface's subnet and ensures successful
subsequent peering using the appropriate (address family) transport
on a multi-access or broadcast interface.
An implementation MUST send IPv6 LDP link Hellos before sending IPv4
LDP Link Hellos on a dual-stack interface.
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5.1.1. Maintaining Hello Adjacencies
In case of dual-stack LDP interface, the LSR SHOULD maintain link
Hello adjacencies for both IPv4 and IPv6 address families. This
document, however, allows an LSR to maintain Rx-side Link Hello
adjacency for the address family that has been used for the
establishment of the LDP session (either IPv4 or IPv6).
5.2. Extended Discovery Mechanism
The extended discovery mechanism (defined in section 2.4.2 of
[RFC5036]), in which the targeted LDP Hellos are sent to a pre-
configured (unicast) destination IPv6 address, requires only one
IPv6 specific consideration: the link-local IPv6 addresses MUST NOT
be used as the targeted LDP hello packet's source or destination
addresses.
6. LDP Session Establishment and Maintenance
Section 2.5.1 of [RFC5036] defines a two-step process for LDP
session establishment, once the peer discovery has completed (LDP
Hellos have been exchanged):
1. Transport connection establishment
2. Session initialization
The forthcoming sub-section 6.1 discusses the LDP consideration for
IPv6 and/or dual-stacking in the context of session establishment,
whereas sub-section 6.2 discusses the LDP consideration for IPv6
and/or dual-stacking in the context of session maintenance.
6.1. Transport connection establishment
Section 2.5.2 of [RFC5036] specifies the use of an optional
transport address object (TLV) in LDP Hello message to convey the
transport (IP) address, however, it does not specify the behavior of
LDP if both IPv4 and IPv6 transport address objects (TLV) are sent
in a Hello message or separate Hello messages. More importantly, it
does not specify whether both IPv4 and IPv6 transport connections
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should be allowed, if there were both IPv4 and IPv6 Hello
adjacencies.
This document specifies that:
1. An LSR MUST NOT send a Hello message containing both IPv4 and
IPv6 transport address optional objects. In other words, there
MUST be at most one optional Transport Address object in a
Hello message. An LSR MUST include only the transport address
whose address family is the same as that of the IP packet
carrying Hello message.
2. An LSR SHOULD accept the Hello message that contains both IPv4
and IPv6 transport address optional objects, but MUST use only
the transport address whose address family is the same as that
of the IP packet carrying the Hello message. An LSR SHOULD
accept only the first transport object for a given Address
family in the received Hello message, and ignore the rest, if
the LSR receives more than one transport object.
3. An LSR MUST send separate Hello messages (each containing
either IPv4 or IPv6 transport address optional object) for each
IP address family, if LDP was enabled for both IP address
families.
4. An LSR MUST use a global unicast IPv6 address in IPv6 transport
address optional object of outgoing targeted Hellos, and check
for the same in incoming targeted hellos (i.e. MUST discard the
hello, if it failed the check).
5. An LSR MUST prefer using a global unicast IPv6 address in IPv6
transport address optional object of outgoing Link Hellos, if
it had to choose between global unicast IPv6 address and
unique-local or link-local IPv6 address.
6. An LSR SHOULD NOT create (or honor the request for creating) a
TCP connection for a new LDP session with a remote LSR, if they
already have an LDP session (for the same LDP Identifier)
established over whatever IP version transport.
This means that only one transport connection is established
regardless of IPv6 or/and IPv4 Hello adjacencies presence
between two LSRs.
7. An LSR SHOULD prefer the LDP/TCP connection over IPv6 for a new
LDP session with a remote LSR, if it is able to determine the
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dual-stack presence (e.g. they have both IPv4 and IPv6 Hello
adjacencies). This applies to the section 2.5.2 of RFC5036.
Each LSR, assuming an active role for whichever address
family(s), SHOULD enforce the LDP/TCP connection over IPv6
preference for a time-period (default value is 5 seconds),
after which LDP/TCP connection over IPv4 SHOULD be attempted.
This enforcement is independent of whether the LSR is assuming
the active role for IPv4. This timer is started upon receiving
the first (IPv4 or IPv6) Hello from the neighbor.
An implementation may provide an option to favor one AFI (IPv4, say)
over another AFI (IPv6, say) for the TCP transport connection, so as
to use the favored IP version for the LDP session, and force
deterministic active/passive roles.
6.2. LDP Sessions Maintenance
This document specifies that two LSRs maintain a single LDP session
regardless of number of Link or Targeted Hello adjacencies between
them, as described in section 6.1. This is independent of whether:
- they are connected via a dual-stack LDP enabled interface or via
two single-stack LDP enabled interfaces;
- a single-stack interface is converted to a dual-stack interface
(e.g. figure 1) on either LSR;
- an additional single-stack or dual-stack interface is added or
removed between two LSRs (e.g. figure 2).
The procedures defined in section 6.1 SHOULD result in preferring
LDPoIPv6 session only after the loss of an existing LDP session
(because of link failure, node failure, reboot etc.).
If the last hello adjacency for a given address family goes down
(e.g. due to dual-stack interfaces being converted into a single-
stack interfaces on one LSR etc.), and that address family is the
same as the one used in the transport connection, then the transport
connection (LDP session) SHOULD be reset. Otherwise, the LDP session
SHOULD stay intact.
If the LDP session is torn down for whatever reason (LDP disabled
for the corresponding transport, hello adjacency expiry etc.), then
the LSRs SHOULD initiate establishing a new LDP session as per the
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procedures described in section 6.1 of this document along with
RFC5036.
7. Label Distribution
An LSR MUST NOT allocate and MUST NOT advertise FEC-Label bindings
for link-local IPv6 address, and ignore such bindings, if ever
received. An LSR MUST treat the IPv4-mapped IPv6 address, defined in
section 2.5.5.2 of [RFC4291], the same as that of a global IPv6
address and not mix it with the 'corresponding' IPv4 address.
Additionally, to ensure backward compatibility (and interoperability
with IPv4-only LDP implementations) in light of section 3.4.1.1 of
RFC5036, as rationalized in the Appendix section A.1 later, this
document specifies that -
1. An LSR MUST NOT send a label mapping message with a FEC TLV
containing two or more Prefix FEC Elements of different address
families. This means that a FEC TLV in the label mapping
message must contain all the Prefix FEC Elements belonging to
IPv6 address family or IPv4 address family, but not both.
An LSR may constrain the advertisement of FEC-label bindings for a
particular address family by negotiating the IP Capability for a
given AFI, as specified in [IPPWCap] document. This allows an LSR
pair to neither advertise nor receive the undesired FEC-label
bindings on a per AFI basis.
8. LDP Identifiers and Next Hop Addresses
RFC5036 section 2.7 specifies the logic for mapping the IP routing
next-hop (of a given FEC) to an LDP peer so as to find the correct
label entry for that FEC. The logic involves using the IP routing
next-hop address as an index into the (peer Address) database (which
is populated by the Address message containing mapping between each
peer's local addresses and its LDP Identifier) to determine the LDP
peer.
However, this logic is insufficient to deal with duplicate IPv6
(link-local) next-hop addresses used by two or more peers. The
reason is that all interior IPv6 routing protocols (can) use link-
local IPv6 addresses as the IP routing next-hops, and 'IPv6
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Addressing Architecture [RFC4291]' allows a link-local IPv6 address
to be used on more than one links.
Hence, this logic is extended by this specification to use not only
the IP routing next-hop address, but also the IP routing next-hop
interface to uniquely determine the LDP peer(s). The next-hop
address-based LDP peer mapping is to be done through LDP peer
address database (populated by Address messages received from the
LDP peers), whereas next-hop interface-based LDP peer mapping is to
be done through LDP hello adjacency/interface database (populated by
hello messages from the LDP peers).
This extension solves the problem of two or more peers using the
same link-local IPv6 address (in other words, duplicate peer
addresses) as the IP routing next-hops.
Lastly, for better scale and optimization, an LSR may advertise only
the link-local IPv6 addresses in the Address message, assuming that
the peer uses only the link-local IPv6 addresses as static and/or
dynamic IP routing next-hops.
9. LDP TTL Security
This document recommends enabling Generalized TTL Security Mechanism
(GTSM) for LDP, as specified in [RFC6720], for the LDP/TCP transport
connection over IPv6 (i.e. LDPoIPv6). The GTSM inclusion is intended
to automatically protect IPv6 LDP peering session from off-link
attacks.
[RFC6720] allows for the implementation to statically
(configuration) and/or dynamically override the default behavior
(enable/disable GTSM) on a per-peer basis. Suffice to say that such
an option could be set on either LSR (since GTSM negotiation would
ultimately disable GTSM between LSR and its peer(s)).
LDP Link Hello packets MUST have their IPv6 Hop Limit set to 255,
and be checked for the same upon receipt before any further
processing, as per section 3 of [RFC5082].
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10. IANA Considerations
None.
11. Security Considerations
The extensions defined in this document only clarify the behavior of
LDP, they do not define any new protocol procedures. Hence, this
document does not add any new security issues to LDP.
While the security issues relevant for the [RFC5036] are relevant
for this document as well, this document reduces the chances of off-
link attacks when using IPv6 transport connection by including the
use of GTSM procedures [RFC5082]. Please see section 9 for LDP TTL
Security details.
Moreover, this document allows the use of IPsec [RFC4301] for IPv6
protection, hence, LDP can benefit from the additional security as
specified in [RFC4835] as well as [RFC5920].
12. Acknowledgments
We acknowledge the authors of [RFC5036], since some text in this
document is borrowed from [RFC5036].
Thanks to Bob Thomas for providing critical feedback to improve this
document early on.
Many thanks to Eric Rosen, Lizhong Jin, Bin Mo, Mach Chen, Shane
Amante, Pranjal Dutta, Mustapha Aissaoui, Matthew Bocci, Mark Tinka,
Tom Petch, Kishore Tiruveedhula, Manoj Dutta, Vividh Siddha, Qin Wu,
and Loa Andersson for thoroughly reviewing this document, and
providing insightful comments and multiple improvements.
This document was prepared using 2-Word-v2.0.template.dot.
13. Additional Contributors
The following individuals contributed to this document:
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Kamran Raza
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, ON K2K-3E8, Canada
Email: skraza@cisco.com
Nagendra Kumar
Cisco Systems, Inc.
SEZ Unit, Cessna Business Park,
Bangalore, KT, India
Email: naikumar@cisco.com
Andre Pelletier
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, ON K2K-3E8, Canada
Email: apelleti@cisco.com
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14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 4291, February 2006.
[RFC5036] Andersson, L., Minei, I., and Thomas, B., "LDP
Specification", RFC 5036, October 2007.
[RFC5082] Pignataro, C., Gill, V., Heasley, J., Meyer, D., and
Savola, P., "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
14.2. Informative References
[RFC4301] Kent, S. and K. Seo, "Security Architecture and Internet
Protocol", RFC 4301, December 2005.
[RFC4835] Manral, V., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4835, April 2007.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC4798] De Clercq, et al., "Connecting IPv6 Islands over IPv4 MPLS
Using IPv6 Provider Edge Routers (6PE)", RFC 4798,
February 2007.
[IPPWCap] Raza, K., "LDP IP and PW Capability", draft-ietf-mpls-ldp-
ip-pw-capability, June 2011.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[RFC6286] E. Chen, and J. Yuan, "Autonomous-System-Wide Unique BGP
Identifier for BGP-4", RFC 6286, June 2011.
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[RFC6720] R. Asati, and C. Pignataro, "The Generalized TTL Security
Mechanism (GTSM) for the Label Distribution Protocol
(LDP)", RFC 6720, August 2012.
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Appendix A.
A.1. LDPv6 and LDPv4 Interoperability Safety Net
It is naive to assume that RFC5036 compliant implementations have
supported IPv6 address family (IPv6 FEC processing, in particular)
in label advertisement all along. And if that assumption turned out
to be not true, then section 3.4.1.1 of RFC5036 would cause LSRs to
abort processing the entire label mapping message and generate an
error.
This would result in LDPv6 to be somewhat undeployable in existing
production networks.
The change proposed in section 7 of this document provides a good
safety net and makes LDPv6 incrementally deployable without making
any such assumption on the routers' support for IPv6 FEC processing
in current production networks.
A.2. Why 32-bit value even for IPv6 LDP Router ID
Please note that 32-bit LSR Id value would not map to any IPv4-
address in an IPv6 only LSR (i.e., single stack), nor would there be
an expectation of it being IP routable, nor DNS-resolvable. In IPv4
deployments, the LSR Id is typically derived from an IPv4 address,
generally assigned to a loopback interface. In IPv6 only
deployments, this 32-bit LSR Id must be derived by some other means
that guarantees global uniqueness within the MPLS network, similar
to that of BGP Identifier [RFC6286] and OSPF router ID [RFC5340].
This document reserves 0.0.0.0 as the LSR Id, and prohibits its
usage with IPv6, in line with OSPF router Id in OSPF version 3
[RFC5340].
Asati, et. al Expires August 5, 2014 [Page 18]
Internet-Draft draft-ietf-mpls-ldp-ipv6 February 5, 2014
Author's Addresses
Vishwas Manral
Hewlet-Packard, Inc.
19111 Pruneridge Ave., Cupertino, CA, 95014
Phone: 408-447-1497
Email: vishwas.manral@hp.com
Rajiv Papneja
Huawei Technologies
2330 Central Expressway
Santa Clara, CA 95050
Phone: +1 571 926 8593
EMail: rajiv.papneja@huawei.com
Rajiv Asati
Cisco Systems, Inc.
7025 Kit Creek Road
Research Triangle Park, NC 27709-4987
Email: rajiva@cisco.com
Carlos Pignataro
Cisco Systems, Inc.
7200 Kit Creek Road
Research Triangle Park, NC 27709-4987
Email: cpignata@cisco.com
Asati, et. al Expires August 5, 2014 [Page 19]
