MPLS Working Group Rajiv Asati
Internet Draft Cisco
Updates: 5036 (if approved)
Intended status: Standards Track Vishwas Manral
Expires: February 23, 2012 Hewlett-Packard, Inc.
Rajiv Papneja
Isocore
Carlos Pignataro
Cisco
August 23, 2011
Updates to LDP for IPv6
draft-ietf-mpls-ldp-ipv6-05
Status of this Memo
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Abstract
The Label Distribution Protocol (LDP) specification defines
procedures to exchange label bindings over either IPv4, 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.....................................................3
1.1.1. Topology Scenarios...................................3
1.1.2. LDP TTL Security.....................................4
2. Specification Language.........................................5
3. LSP Mapping....................................................5
4. LDP Identifiers................................................6
5. Peer Discovery.................................................7
5.1. Basic Discovery Mechanism.................................7
5.2. Extended Discovery Mechanism..............................8
6. LDP Session Establishment and Maintenance......................8
6.1. Transport connection establishment........................8
6.2. Maintaining Hello Adjacencies............................10
6.3. Maintaining LDP Sessions.................................10
7. Label Distribution............................................10
8. LDP TTL Security..............................................11
9. IANA Considerations...........................................12
10. Security Considerations......................................12
11. Acknowledgments..............................................12
12. References...................................................14
12.1. Normative References....................................14
12.2. Informative References..................................14
Author's Addresses...............................................15
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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 multicast
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 TTL Security: No rule for built-in Generalized TTL Security
Mechanism (GTSM) in LDP
6) LDP Label Distribution: No rule for advertising IPv4 or/and IPv6
FEC-label bindings over an LDP session
This document addresses the above deficiencies by specifying the
desired behavior/rules/details for using LDP in IPv6 enabled
networks. It also clarifies the scope (section 1.1).
Note that this document updates RFC5036.
1.1. Scope
1.1.1. Topology Scenarios
The following scenarios in which the LSRs may be inter-connected via
one or more dual-stack interfaces (figure 1), or two or more single-
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stack interfaces (figure 2 and figure 3) are addressed by this
document:
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
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 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, say), even though the IPv4 LDP session may
already be established between the LSRs over the existing interface.
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).
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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.
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
LDPv4 - LDP for enabling IPv4 MPLS forwarding
LDPv6 - LDP for enabling IPv6 MPLS forwarding
LDPoIPv4 - LDP over IPv4 transport session
LDPoIPv6 - LDP over IPv6 transport session
FEC - Forwarding Equivalence Class
TLV - Type Length Value
LSR - Label Switch Router
LSP - Label Switched Path
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."
Suffice to say, this rule is correct for IPv4, but not for IPv6,
since an IPv6 router may not have any /32 address.
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This document proposes to modify this rule by also including a /128
address (for IPv6). In fact, it should be reasonable to just 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."
While the above rule mentions 'Address Prefix FEC', it is also
applicable to 'Typed WildCard prefix FEC' [RFC5918].
Additionally, it is desirable that a packet is forwarded to an LSP
of an egress router, only if LSP's address-family matches with that
of the LDP hello adjacency on the next-hop interface.
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). Additionally, section 2.5.2 of
[RFC5036] implicitly prohibits using the same label space for both
IPv4 and IPv6 FEC-label bindings.
The first four octets of the LDP identifier, the 32-bit LSR Id,
identify the LSR and is a globally unique value. This is regardless
of the address family used for the LDP session. In other words, this
document preserves the usage of 32-bit LSR Id on an IPv6 only LSR.
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 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.
The first sentence of last paragraph of Section 2.5.2 of [RFC5036]
is qualified per address family and therefore updated to the
following: "For a given address family over which a Hello is sent,
and a given label space, an LSR MUST advertise the same transport
address." This rightly enables the per-platform label space to be
shared between IPv4 and IPv6.
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In summary, this document not only allows the usage of a common LDP
identifier i.e. same LSR-Id, but also the common Label space id for
both IPv4 and IPv6 on a dual-stack LSR.
This document reserves 0.0.0.0 as the LSR-Id, and prohibits its
usage.
5. Peer Discovery
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
sends 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 for IPv6
LDP Link Hellos. An LDP Hello packet received on any of the other
addresses must be dropped.
Also, the 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 specified in Generalized TTL Security Mechanism
(GTSM)[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 IPv4 and IPv6 LDP), then the LSR must
periodically send both IPv4 and IPv6 LDP Link Hellos (using the same
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LDP Identifier per section 4) and must separately maintain the Hello
adjacency for IPv4 and IPv6 on that interface.
Needless to say, the IPv4 and IPv6 LDP Link Hellos must carry the
same LDP identifier (assuming per-platform label space usage).
5.2. Extended Discovery Mechanism
Suffice to say, the extended discovery mechanism (defined in section
2.4.2 of [RFC5036]) doesn't require any additional IPv6 specific
consideration, since the targeted LDP Hellos are sent to a pre-
configured destination IPv6 address.
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-sections discuss the LDP consideration for IPv6
and/or dual-stacking in the context of session establishment and
maintenance.
6.1. Transport connection establishment
Section 2.5.2 of [RFC5036] specifies the use of an optional
transport address object (TLV) in LDP Link 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 should be allowed, if there were Hello
adjacencies for both IPv4 and IPv6 whether over a single interface
or multiple interfaces.
This document specifies that:
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- An LSR should not send a Hello containing both IPv4 and IPv6
transport address optional objects. In other words, there
should be at most one optional Transport Address object in a
Hello message. An LSR should include only the transport address
whose address family is the same as that of the IP packet
carrying Hello.
- An LSR should accept the Hello message that contains both IPv4
and IPv6 transport address optional objects, but use only the
transport address whose address family is the same as that of
the IP packet carrying Hello.
- An LSR must send separate Hellos (each containing either IPv4
or IPv6 transport address optional object) for each IP address-
family, if LDP was enabled for both IP address-families.
- 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 should be established, even if
there are two Hello adjacencies (one for IPv4 and another for
IPv6). This is independent of whether the Hello Adjacencies are
created over a single interface (scenarios 1 in section 1.1) or
multiple interfaces (scenario 2 in section 1.1) between two
LSRs.
- An LSR should prefer the LDP/TCP connection over IPv6 for a new
LDP session with a remote LSR, if it has both IPv4 and IPv6
hello adjacencies for the same LDP Identifier (over a dual-
stack interface, or two or more single-stack IPv4 and IPv6
interfaces). This applies to the section 2.5.2 of RFC5036.
- An LSR should prefer the LDP/TCP connection over IPv6 for a new
LDP session with a remote LSR, if they attempted two TCP
connections using IPv4 and IPv6 transport addresses
simultaneously.
This document allows for the implementation to provide a
configuration option to override the above stated preference from
IPv6 to IPv4 on a per-peer basis. Suffice to say that such option
must be set on both LSRs.
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6.2. Maintaining Hello Adjacencies
As outlined in section 2.5.5 of RFC5036, this draft suggests that if
an LSR has a dual-stack interface, which is enabled with both IPv4
and IPv6 LDP, then the LSR must periodically send both IPv4 and IPv6
LDP Link Hellos and must separately maintain the Hello adjacency for
IPv4 and IPv6 on that interface.
This ensures successful labeled IPv4 and labeled IPv6 traffic
forwarding on a dual-stacked interface, as well as successful LDP
peering using the appropriate transport on a multi-access
interface (even if there are IPv4-only, IPv6-only and dual-stack
LSRs connected to that multi-access interface).
6.3. Maintaining LDP Sessions
Two LSRs maintain a single LDP session between them, as described in
section 6.1, whether they are connected via a dual-stack LDP enabled
interface or via two single-stack LDP enabled interfaces. This is
also true when a single-stack interface is converted to a dual-stack
interface, or when another interface is added between two LSRs.
On the other hand, if a dual-stack interface is converted to a
single-stack interface (by disabling IPv4 or IPv6 routing), then the
LDP session should be torn down ONLY if the disabled IP version was
the same as that of the transport connection. 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
procedures described in section 6.1 of this document and RFC5036.
7. Label Distribution
This document specifies that an LSR should advertise and receive
both IPv4 and IPv6 label bindings from and to the peer, only if it
has valid IPv4 and IPv6 Hello Adjacencies for that peer, as
specified in section 6.2.
This means that the LSR must not advertise any IPv6 label bindings
to a peer over an IPv4 LDP session, if no IPv6 Hello Adjacency
existed for that peer (and vice versa).
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8. LDP TTL Security
This document also specifies that the LDP/TCP transport connection
over IPv6 (i.e. LDPoIPv6) must follow the Generalized TTL Security
Mechanism (GTSM) procedures (Section 3 of [RFC5082]) for an LDP
session peering established between the adjacent LSRs using Basic
Discovery, by default.
In other words, GTSM is enabled by default for an IPv6 LDP peering
session using Basic Discovery. This means that the 'IP Hop Limit' in
IPv6 packet is set to 255 upon sending, and checked to be 255 upon
receipt. The IPv6 packet must be dropped failing such a check upon
receipt.
The reason GTSM is enabled for Basic Discovery by default, but not
for Extended Discovery is that the usage of Basic Discovery
typically results in a single-hop LDP peering session, whereas the
usage of Extended Discovery typically results in a multi-hop LDP
peering session. While the latter is deemed out of scope (section
1.2), in line with GTSM [RFC5082], it is worth clarifying the
following exceptions that may occur with Basic or Extended Discovery
usage:
a) Two adjacent LSRs (i.e. back-to-back PE routers) forming a
single-hop LDP peering session after doing an Extended Discovery
(for Pseudowire, say)
b) Two adjacent LSRs forming a multi-hop LDP peering session after
doing a Basic Discovery, due to the way IP routing is setup
between them (temporarily or permanently)
c) Two adjacent LSRs (i.e. back-to-back PE routers) forming a
single-hop LDP peering session after doing both Basic and
Extended Discovery
In (a), GTSM is not enabled for the LDP peering session by default,
hence, it would not do any harm or good.
In (b), GTSM is enabled by default for the LDP peering session by
default and enforced, hence, it would prohibit the LDP peering
session from getting established.
In (c), GTSM is enabled by default for Basic Discovery and enforced
on the subsequent LDP peering. However, if each LSR uses the same
IPv6 transport address object value in both Basic and Extended
discoveries, then it would result in a single LDP peering session
and that would be enabled with GTSM. Otherwise, GTSM would not be
enforced on the 2nd LDP peering session corresponding to the
Extended Discovery.
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This document allows for the implementation to provide an option to
statically (configuration) and/or dynamically override the default
behavior (i.e. disable GTSM) on a per-peer basis. This would also
address the exception (b) above. Suffice to say that such an option
could be set on either LSR (since GTSM negotiation would ultimately
disable GTSM between an LSR and its peer(s)).
The built-in GTSM inclusion is intended to automatically protect
IPv6 LDP peering session from off-link attacks.
9. IANA Considerations
None.
10. 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].
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].
11. Acknowledgments
We acknowledge the authors of [RFC5036], since the text in this
document is borrowed from [RFC5036].
Thanks to Bob Thomas for providing critical feedback to improve this
document early on. Thanks to Kamran Raza, Eric Rosen, Lizhong Jin,
Bin Mo, Mach Chen, and Kishore Tiruveedhula for reviewing this
document. The authors also acknowledge the help of Manoj Dutta and
Vividh Siddha.
This document was prepared using 2-Word-v2.0.template.dot.
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12. References
12.1. 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.
12.2. 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.
[RFC5918] Asati, R. Minei, I., and Thomas, B., "Label Distribution
Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
(FEC)", RFC 5918, April 2010.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
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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
ISOCORE
12359 Sunrise Valley Dr, STE 100
Reston, VA 20190
Email: rpapneja@isocore.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
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