|Subject:||Re: Comments on draft-ietf-rtgwg-ipfrr-spec-base-09|
|Date:||Thu, 25 Oct 2007 13:33:52 -0400|
On 10/12/07, Rüdiger A Martin <[email protected]> wrote:
Dear Alia, dear Alex,
True - The first place a node protecting LFA is mentioned, I've added a reference to the section with the definition.
I don't see a clean way of defining it at/before that location.
- There are several kinds of loop-free alternates. But the suggested
Yes, we didn't have consensus about the order of preferences - and it depends
on what other types of protection are available in the network. I agree that this
would be useful. How does the following added section (after 3.6) sound?
"3.7. LFA types and Trade-offs
LFAs can provide different amounts of protection and the decision
about which type to prefer is dependent upon network topology and
other techniques in use in the network. This section describes the
different protection levels and the trade-offs associated with each.
1. Primary Next-hop: When there are equal-cost primary next-hops,
using one as an alternate is guaranteed to not cause micro-loops
involving S. Traffic flows across the paths that the network will
converge to, but congestion may be experienced on the primary
paths since traffic is sent across fewer. All primary next-hops
are downstream paths.
2. Downstream Paths: A downstream path, unlike an LFA, is guaranteed
not to cause a micro-loop involving S regardless of the actual
failure detected. However, the expected coverage of such
alternates in a network is expected to be poor. All downstream
paths are LFAs.
3. LFA: An LFA can have good coverage of a network, depending on
topology. However, it is possible to get micro-loops involving S
if a more severe failure occurs than is protected against.
The different types of protection are abbreviated as LP (link-
protecting), NP (node-protecting) and SP (SRLG-protecting).
a. LP, NP, and SP: If such an alternate exists, it gives protection
against all failures.
b. LP and NP: Many networks may handle SRLG failures via another
method or may focus on node and link failures as being more
c. LP: A network may handle node failures via a high availability
technique and be concerned primarily about protecting the more
common link failure case.
d. NP: These only exist on interfaces that aren't point-to-point.
If link protection is handled in a different layer, then an NP
alternate may be acceptable.
I'd welcome suggestions on how to improve this.
- Page 7: "Since the funcionality of link and node protecting LFAs is
Added the clarification of "link-protecting downstream paths".
- A few comments on the algorithm on page 16 (besides I agree with
True - this is intended to reflect the behavior for the network once it has converged.
I could instead embed the preference at Step 16 - but that flexibility is there already.
* The output of the algorithm depends on the order of the
Ah - nope - it is due to a missing step - added between step 10 & 11.
"If cand_type is not PRIMARY, P_i.alt_type is PRIMARY and the router prefers other primary next-
hops for use as the alternate, then continue to the next candidate next-hop"
Besides we really appreciate this document and support the progression
Ok - so at the end of the abstract, I've added
"This simple approach does not require any support from other routers. The
extent to which this goal can be met by this specification is
dependent on the topology of the network."
Thanks for the comments and suggestions!
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