retained, it will continue to be incorrect. Both solutions have a
mechanism to housekeep and free the unwanted state after
resynchronization is complete. [RFC3478] may be better at
eradicating incorrect forwarding state, because it replays all
message exchanges that caused the state to be populated.
In [RFC3478], no more data than the forwarding state needs to have
been saved by the recovering node. All LDP state may be relearned by
message exchanges with peers. Whether those exchanges may cause the
same incorrect state to arise on the recovering node is an obvious
concern.
In [RFC3479], the forwarding state must be supplemented by a small
amount of state specific to the protocol extensions. LDP state may
be retained directly or reconstructed from the forwarding state. The
same issues apply when reconstructing state but are mitigated by the
fact that this is likely a different code path. Errors in the
retained state specific to the protocol extensions will persist.
7.12. Interoperability and Backward Compatibility
It is important that new additions to LDP interoperate with existing
implementations at least in provision of the existing levels of
function.
Both [RFC3478] and [RFC3479] do this through rules for handling the
absence of the FT optional negotiation object during session
initialization.
Additionally, [RFC3478] is able to perform limited recovery (i.e.,
redistribution of state) even when only one of the participating LSRs
supports the procedures. This may offer considerable advantages in
interoperation with legacy implementations.
7.13. Interaction With Other Label Distribution Mechanisms
Many LDP LSRs also run other label distribution mechanisms. These
include management interfaces for configuration of static label
mappings, other distinct instances of LDP, and other label
distribution protocols. The last example includes traffic
engineering label distribution protocol that are used to construct
tunnels through which LDP LSPs are established.
As with re-use of individual labels by LDP within a restarting LDP
system, care must be taken to prevent labels that need to be retained
by a restarting LDP session or protocol component from being used by
another label distribution mechanism. This might compromise data
security, amongst other things.
It is a matter for implementations to avoid this issue through the
use of techniques, such as a common label management component or
segmented label spaces.
7.14. Applicability to CR-LDP
CR-LDP [RFC3212] utilizes Downstream-On-Demand label distribution.
[RFC3478] describes Downstream-On-Demand as an area for future study
and is therefore not applicable for CR-LDP. [RFC3479] is suitable
for use in a network entirely based on CR-LDP or in one that is mixed
between LDP and CR-LDP.
8. Security Considerations
This document is informational and introduces no new security
concerns.
The security considerations pertaining to the original LDP protocol
[RFC3036] remain relevant.
[RFC3478] introduces the possibility of additional denial-of- service
attacks. All of these attacks may be countered by use of an
authentication scheme between LDP peers, such as the MD5-based scheme
outlined in [LDP].
In MPLS, a data mis-delivery security issue can arise if an LSR
continues to use labels after expiration of the session that first
caused them to be used. Both [RFC3478] and [RFC3479] are open to
this issue.
9. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF’s procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
10. References
10.1. Normative References
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC3478] Leelanivas, M., Rekhter, Y. and R. Aggarwal, "Graceful
Restart Mechanism for LDP", RFC 3478, February 2003.
[RFC3479] Farrel, A., Editor, "Fault Tolerance for the Label
Distribution Protocol (LDP)", RFC 3479, February 2003.
10.2. Informative References
[RFC2547] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
March 1999.
[RFC3212] Jamoussi, B., Editor, Andersson, L., Callon, R., Dantu,
R., Wu, L., Doolan, P., Worster, T., Feldman, N.,
Fredette, A., Girish, M., Gray, E., Heinanen, J., Kilty,
T. and A. Malis, "Constraint-Based LSP Setup using LDP",
RFC 3212, January 2002.
[RFC3469] Sharma, V., Ed., and F. Hellstrand, Ed., "Framework for
Multi-Protocol Label Switching (MPLS)-based Recovery",
RFC 3469, February 2003.
11. Acknowledgements
The author would like to thank the authors of [RFC3478] and [RFC3479]
for their work on fault tolerance of LDP. Many thanks to Yakov
Rekhter, Rahul Aggarwal, Manoj Leelanivas and Andrew Malis for their
considered input to this applicability statement.
12. Author’s Address
Adrian Farrel
Old Dog Consulting
Phone: +44 (0) 1978 860944
EMail: adrian@olddog.co.uk
13. Full Copyright Statement
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