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doc: Add TI-LFA developer docs

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Signed-off-by: GalaxyGorilla <sascha@netdef.org>
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GalaxyGorilla 2020-11-20 13:56:33 +00:00
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66
doc/developer/ospf-sr.rst
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@ -17,6 +17,7 @@ Supported Features
* Automatic provisioning of MPLS table
* Equal Cost Multi-Path (ECMP)
* Static route configuration with label stack up to 32 labels
* TI-LFA (for P2P interfaces only)
Interoperability
----------------
@ -182,6 +183,71 @@ called. Once check the validity of labels, they are send to ZEBRA layer through
command for deletion. This is completed by a new labelled route through
`ZEBRA_ROUTE_ADD` command, respectively `ZEBRA_ROUTE_DELETE` command.
TI-LFA
^^^^^^
Experimental support for Topology Independent LFA (Loop-Free Alternate), see
for example 'draft-bashandy-rtgwg-segment-routing-ti-lfa-05'. The related
files are `ospf_ti_lfa.c/h`.
The current implementation is rather naive and does not support the advanced
optimizations suggested in e.g. RFC7490 or RFC8102. It focuses on providing
the essential infrastructure which can also later be used to enhance the
algorithmic aspects.
Supported features:
* Link and node protection
* Intra-area support
* Proper use of Prefix- and Adjacency-SIDs in label stacks
* Asymmetric weights (using reverse SPF)
* Non-adjacent P/Q spaces
* Protection of Prefix-SIDs
If configured for every SPF run the routing table is enriched with additional
backup paths for every prefix. The corresponding Prefix-SIDs are updated with
backup paths too within the OSPF SR update task.
Informal High-Level Algorithm Description:
::
p_spaces = empty_list()
for every protected_resource (link or node):
p_space = generate_p_space(protected_resource)
p_space.q_spaces = empty_list()
for every destination that is affected by the protected_resource:
q_space = generate_q_space(destination)
# The label stack is stored in q_space
generate_label_stack(p_space, q_space)
# The p_space collects all its q_spaces
p_spaces.q_spaces.add(q_space)
p_spaces.add(p_space)
adjust_routing_table(p_spaces)
Possible Performance Improvements:
* Improve overall datastructures, get away from linked lists for vertices
* Don't calculate a Q space for every destination, but for a minimum set of
backup paths that cover all destinations in the post-convergence SPF. The
thinking here is that once a backup path is known that it is also a backup
path for all nodes on the path themselves. This can be done by using the
leafs of a trimmed minimum spanning tree generated out of the post-
convergence SPF tree for that particular P space.
* For an alternative (maybe better) optimization look at
https://tools.ietf.org/html/rfc7490#section-5.2.1.3 which describes using
the Q space of the node which is affected by e.g. a link failure. Note that
this optimization is topology dependent.
It is highly recommended to read e.g. `Segment Routing I/II` by Filsfils to
understand the basics of Ti-LFA.
Configuration
-------------

2
doc/user/ospfd.rst
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@ -1245,6 +1245,8 @@ TI-LFA requires a proper Segment Routing configuration.
Configured on the router level. Activates TI-LFA for all interfaces.
Note that so far only P2P interfaces are supported.
Debugging OSPF
==============