Re: [Lsr] Open issues with Dynamic Flooding: Including LANs in the Flooding Topology

I agree as well. 
Thanks,
Acee

On 4/2/19, 1:26 PM, "Lsr on behalf of tony.li@tony.li" <lsr-bounces@ietf.org on behalf of tony.li@tony.li> wrote:

    I am in complete agreement with both Les’s extensive analysis and opinion.  ;-)

    Tony

    > On Apr 2, 2019, at 8:37 AM, Les Ginsberg (ginsberg) <ginsberg@cisco.com> wrote:
    > 
    > In reply to my own post, here is my opinion regarding including LANs in the Flooding Topology:
    > 
    > While I think it would be "nice" and simplifying to be able to ignore LANs, I think we are unable to do so because the possibility that LANs are actually in use as transit links in some topologies exists.
    > 
    > NOTE: I am not persuaded by the argument that some operators have LANs that could be operated in point-to-point mode but they simply don't configure the links to do so. If a customer is serious about flooding reduction then they need to also do what they can to reduce unnecessary LSPs/LSAs from even being generated.
    > 
    > Even if we treat LANs as always enabled for flooding , any algorithm to calculate the flooding topology would be sub-optimal if it did not consider the fact that flooding is occurring on the LANs.
    > 
    > Bottom line, unless we are confident that LANs will not exist in the topologies where flooding optimizations will be used, not supporting LANs seems to be an undesirable restriction.
    > 
    >   Les
    > 
    > 
    >> -----Original Message-----
    >> From: Lsr <lsr-bounces@ietf.org> On Behalf Of Les Ginsberg (ginsberg)
    >> Sent: Tuesday, April 02, 2019 8:31 AM
    >> To: tony.li@tony.li; lsr@ietf.org
    >> Subject: [Lsr] Open issues with Dynamic Flooding: Including LANs in the
    >> Flooding Topology
    >> 
    >> (I have altered the subject so we can discuss the two issues in Tony's
    >> previous post separately.)
    >> 
    >> 
    >> There are several  aspects to consider when discussing LAN support in the
    >> context of flooding optimizations:
    >> 
    >> 1)Flooding topology advertisement (centralized mode only)
    >> 
    >> Support for encoding LANs when advertising the flooding topology requires
    >> that
    >> we include not only all routers in the set of "nodes" in the network but also
    >> (to use IS-IS terminology) all “pseudo-nodes” as well. This means when
    >> advertising the set of nodes and associated indeces used in calculating the
    >> flooding topology there needs to be an indication as to whether a given
    >> entry is a node or a pseudo-node. The encoding for this is straightforward
    >> in IS-IS (include pseudo-node ID in the node identifier) but more complex
    >> in OSPF.
    >> 
    >> However, this is a problem with a straightforward solution and is therefore
    >> not a significant consideration.
    >> 
    >> 2)Enablement/disablement of flooding on a LAN
    >> 
    >> Correct operation of flooding on a LAN requires all nodes connected to the
    >> LAN perform their role when the LAN is enabled for flooding and conversely
    >> all nodes suppress flooding via the LAN when flooding is disabled for
    >> flooding. This applies to temporary enablement of flooding on a LAN in the
    >> event of a partitioned flooding topology i.e., if any node connected to the
    >> LAN
    >> signals enablement of temporary flooding on the LAN all nodes connected to
    >> the
    >> LAN MUST honor that request.
    >> 
    >> Selective enablement of flooding on a LAN based on whether it is part
    >> of the calculated flooding topology therefore entails some additional
    >> complexity.
    >> 
    >> Note that this discussion assumes that flooding operation on a LAN
    >> is not altered by the introduction of flooding optimizations. For example
    >> there is no proposal to selectively enable transmission of LSPs/LSAs on
    >> a LAN only by a subset of the nodes connected to the LAN.
    >> 
    >> 3)Use of LANs in flooding topology algorithms
    >> 
    >> When LANs are considered part of the flooding topology, any algorithm
    >> used to compute the flooding topology has to consider how to use LANs.
    >> For example, using a LAN might have an advantage in that by enabling
    >> flooding on a single LAN multiple nodes are now connected to the flooding
    >> topology. This might reduce the number of point-to-point edges required
    >> in the flooding topology and/or decrease the diameter of the flooding
    >> topology.
    >> 
    >> But use of a LAN might either increase the diameter of the flooding topology
    >> and thereby affect convergence or unnecessarily increase the degree of
    >> connectivity of certain nodes to the flooding topology and thereby reduce
    >> the optimization achieved.
    >> 
    >> If LANs are always enabled for flooding but are not included in the set of
    >> nodes considered as part of the flooding topology (see point #1 above) then
    >> "false partitions" might occur during the calculation of the flooding
    >> topology.
    >> 
    >> Whether LANs are considered part of the flooding topology or not, the
    >> presence
    >> of a LAN introduces the possibility that there are "hidden nodes" to which
    >> flooding is actually occurring but which are not explicitly mentioned in
    >> the calculated flooding topology.
    >> 
    >> 4)Deployment Limitations
    >> 
    >> The significance of support for LANs depends upon their existence in a
    >> deployment where the use of flooding optimizations is desired.
    >> 
    >> If all links are point-to-point then the question is irrelevant.
    >> 
    >> If all links are point-to-point but ethernet links have not been configured
    >> to operate in point-to-point mode then lack of support for LANs would
    >> compromise the value of flooding optimizations. A counter argument to this
    >> case is that unnecessary operation in LAN mode itself increases the number
    >> of
    >> LSPs/LSAs that need to be flooded by as much as 50% and therefore is
    >> something which SHOULD be addressed by altering configuration. There
    >> should
    >> then be motivation for network operators to enable point-to-point operation
    >> where possible even if they have not done so before.
    >> 
    >> If LANs with more than 2 connected nodes are present and are used for
    >> transiting traffic then lack of support for LAN circuits for flooding
    >> optimizations will lead to diminished effectiveness of flooding optimizations.
    >> 
    >> Summary:
    >> 
    >> When forming an opinion on whether to include LANs in the flooding
    >> topology
    >> it is prudent to consider the above points.
    >> 
    >> 
    >> 
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