-----Original Message-----
From: Richard Rabbat [mailto:rabbat@fla.fujitsu.com]
Sent: 10 September
2003 20:11
To: ccamp@ops.ietf.org; Zafar Ali
Cc: Vishal Sharma; 'Richard
Rabbat'
Subject: Comparison of restoration
requirements between transport and packet networks
Hi Zafar, CCAMP,
During discussions with several colleagues within the CCAMP
WG, it has become clear that it would be useful to clarify some of the
fundamental differences between restoration in packet networks and that in
transport networks.
This is because this difference, together with the time
criticality of restoration at the transport layer, requires the development of
techniques for time-bounded notification. It would then be useful to discuss
the solutions proposed in draft-rabbat-fault-notification-protocol-03 for such
notification.
We are in the process of preparing a contribution on this
subject, but thought it would be useful to highlight a few key points on the
mailing list, so that we can elicit feedback and comments from the WG.
In normal packet networks (MPLS networks) one can pre-signal
*and* pre-configure a backup LSP for a working LSP. This is because selecting a
label at a node for a backup LSP is sufficient to be able to switch traffic for
that LSP when that traffic arrives. If resources are required for the backup LSP
(buffers and bandwidth), they too can be reserved in advance (during the LSP
signaling phase), but can still be used by low-priority or extra-traffic LSPs
as long as there is no failure on the working LSP.
This is true even for shared mesh restoration in MPLS
networks. In that case, multiple labels would be assigned, one for each of the
backup LSPs (corresponding to link and/or node disjoint working LSPs)
transiting a node on the shared backup path, but only one set of resources
(buffers, bandwidth) would be reserved (if such resource reservation was
needed).
In transport networks, however, one can pre-signal but not
pre-configure a backup LSP (unless one was doing just 1+1 protection). This is
because, in transport networks, if an LSP is established (that is, it is
cross-connected) then the full bandwidth of the LSP is automatically
*consumed*, irrespective of whether traffic actually flows on this LSP.
For this reason, to implement shared restoration schemes in transport
networks (and allow extra-traffic) a backup LSP cannot be cross-connected until
*after* the specific failure for which this backup LSP was pre-signaled has
occured.
Now, if signaling-based notification is used in transport
networks, an *additional phase of signaling* is required along the backup path
to enable nodes along that path to reconfigure themselves (this is
well-described in the functional specification document
of the P&R Design Team). This lengthens the time to
recover from the failure. Depending on the layer at which recovery is being
performed this may or may not be acceptable.
In the specific case of transport networks, restoration is
typically a time-critical activity, so this round-trip signaling delay could be
unacceptable when time-bounded notification and recovery is desired.
In addition, signaling individual LSPs or individual LSP
bundles may create buffering problems that makes signaling time unbounded.
If instead, the information about a failure is flooded to
all the network nodes, and the backup paths are selected intelligently (as
described in draft-rabbat-fault-notification-protocol-03.txt), this additional
signaling hand-shake delay can be eliminated. This is because by flooding
the information about a fault on a working LSP, one can inform, in parallel,
all the nodes lying along the path of the backup LSP. Thus, the repair point(s)
upon learning of the fault holds off activating the backup LSP(s) for an
appropriate time in which all nodes along the corresponding backup path(s) will
have reconfigured themselves.
We would also like to get feedback on a suitable protocol
that could implement time-critical flooding notification.
Comments, thoughts and questions are welcome!
--
Richard Rabbat, Ph.D.
Member of Research Staff, Fujitsu Labs of America
1240 E Arques Ave, MS 345, Sunnyvale, CA 94085
Phone: 408-530-4537. Fax: 408-530-4581. Cell: 650-714-7618