WGLC for draft-rtgwg-mrt-frr-architecture

[Stewart Bryant <[e-mail]>]

Hi,

I am sorry that this review is late, but a number of personal matters
needed priority. Hopefully you can accept it still.

Firstly my high order bit, and I suspect I will be in the rough on this.
I am not convinced that this is the best solution that we can produce to
this problem, and I am concerned about the operational issues that
result from the non-intuitive repair paths when compared to other
methods. I am also concerned about our ability to get a solution as
complicated as this right first time in all of the corner cases.
Thus I think that rather than recommend this to the industry
as a standard, we should issue it as informational and see how it works
out at scale.

===========


  1.  Introduction

    This document gives a complete solution for IP/LDP fast-reroute
    [RFC5714].  MRT-FRR creates two alternate trees separate from the
    primary next-hop forwarding used during stable operation.  These two
    trees are maximally diverse from each other, providing link and node
    protection for 100% of paths and failures as long as the failure does
    not cut the network into multiple pieces.

SB> I am concerned that this is an overstatement.
SB> Firstly you cannot handle multiple concurrent failures in
SB> even simple pathological cases. If the network is two
SB> connected and you have both a red and blue failure
SB> that you need to traverse, you (say) commit to the red
SB> the you will fail at blue transition, even though the network is
SB> is still connected at the link level.

SB> The text should start with the invariants and assumptions to
SB> correctly scope the claims in the introduction.

SB> I think you have to say where the trees are routed since only
SB> two mean you need a root, and you need to note that this is
SB> different from the underlying IGP in which there is a tree
SB> per node.



                  Summary Comparison of IP/LDP FRR Methods

    +---------+-------------+-------------+-----------------------------+
    |  Method |   Coverage  |  Alternate  |    Computation (in SPFs)    |
    |         |             |   Looping?  |                             |
    +---------+-------------+-------------+-----------------------------+
    | MRT-FRR |     100%    |     None    |         less than 3         |
    |         |  Link/Node  |             |                             |
    |         |             |             |                             |
    |   LFA   |   Partial   |   Possible  |         per neighbor        |
    |         |  Link/Node  |             |                             |
    |         |             |             |                             |
    |  Remote |   Partial   |   Possible  |    per neighbor (link) or   |
    |   LFA   |  Link/Node  |             |  neighbor's neighbor (node) |
    |         |             |             |                             |
    | Not-Via |     100%    |     None    |      per link and node      |
    |         |  Link/Node  |             |                             |
    |         |             |             |                             |
    |  TI-LFA |     100%    |   Possible  |    per neighbor (link) or   |
    |         |  Link/Node  |             |  neighbor's neighbor (node) |
    +---------+-------------+-------------+-----------------------------+

SB> I really wonder how important the computational complexity is
SB> given that we are moving to an SDN world where these calculations
SB> can be offloaded?
SB>
SB> I think that it is fair to note that the other method use the
SB> existing routing protocol and calculate alternate paths using
SB> the methods of that routing protocol. Whereas with MRT you
SB> really have a new routing protocol with all of the associated
SB> operations overhead.

============

  1.1.  Importance of 100% Coverage

    Fast-reroute is based upon the single failure assumption - that the
    time between single failures is long enough for a network to
    reconverge and start forwarding on the new shortest paths.  That does
    not imply that the network will only experience one failure or
    change.

SB> The above needs to be much earlier to qualify "complete"


===========

  

4.  Maximally Redundant Trees (MRT)



SB> Please remind me is there one red topology or one per failure.
SB> There is just one - correct  - so it would be useful to
SB> explain what happens in a number of instances of failure.

============

5.  Maximally Redundant Trees (MRT) and Fast-Reroute


    When there is a link or node failure affecting, but not partitioning,
SB> that should be "a single link or single node"

============

6.  Unicast Forwarding with MRT Fast-Reroute

    As mentioned before, MRT FRR needs multi-topology forwarding.
    Unfortunately, neither IP nor LDP provides extra bits for a packet to
    indicate its topology.  Once the MRTs are computed, the two sets of
    MRTs can be used as two additional forwarding topologies.  The same
    considerations apply for forwarding along the MRTs as for handling
    multiple topologies.

SB> It would be good to explain how you solve the problem mentioned
SB> in the previous para. Maybe a forward ref to a later section?

=============



6.1.1.1.  Topology-scoped FEC encoded using a single label (Option 1A)

    [RFC7307] provides a mechanism to distribute FEC-Label bindings
    scoped to a given MPLS topology (represented by MPLS MT-ID).  To use
    multi-topology LDP to create MRT forwarding topologies, we associate
    two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
    in addition to the default shortest path forwarding topology with MT-
    ID=0.

SB> Presumably you could MRT a topology other than default?

SB> To be clear - you need 3 * the number of delivery labels in the
SB> network, and in some cases these labels identify prefix or a
SB> VPN exit point.
SB> This is a lot more labels than some of the competing schemes
SB> and so I think we really need a label table size comparison
SB> in the comparison section earlier in the document.

=============

6.1.1.2.  Topology and FEC encoded using a two label stack (Option 1B)

    With this forwarding mechanism, a two label stack is used to encode
    the topology and the FEC of the packet.  The top label (topology-id
    label) identifies the MRT forwarding topology, while the second label
    (FEC label) identifies the FEC.  The top label would be a new FEC
    type with two values corresponding to MRT Red and Blue topologies.

    When an MRT transit router receives a packet with a topology-id
    label, the router pops the top label and uses that it to guide the
    next-hop selection in combination with the next label in the stack
    (the FEC label).  The router then swaps the FEC label, using the FEC-
    label bindings learned through normal LDP mechanisms.  The router
    then pushes the topology-id label for the next-hop.

    As with Option 1A, this forwarding mechanism also has the useful
    property that the FEC associated with the packet is maintained in the
    labels at each hop along the MRT.

    This forwarding mechanism has minimal usage of additional labels,
    memory and LDP communication.  It does increase the size of packets
    and the complexity of the required label operations and look-ups.

    This forwarding option is consistent with context-specific label
    spaces, as described in [RFC 5331].  However, the precise LDP
    behavior required to support this option for MRT has not been
    specified.

SB> So I wonder if this should be normative text given that the underlying
SB> control plane behavior is currently unknown?


SB> You should comment on the impact on forwarding moving from
SB> a one to a two label action at every hop along the repair path.
SB> This is not an issue that the competing approaches have and so
SB> should feature in the comparison section.


============

6.1.2.  MRT IP tunnels (Options 2A and 2B)

    IP tunneling can also be used as an MRT transit forwarding mechanism.
    Each router supporting this MRT transit forwarding mechanism
    announces two additional loopback addresses and their associated MRT
    color.  Those addresses are used as destination addresses for MRT-
    blue and MRT-red IP tunnels respectively.  The special loopback
    addresses allow the transit nodes to identify the traffic as being
    forwarded along either the MRT-blue or MRT-red topology to reach the
    tunnel destination.  Announcements of these two additional loopback
    addresses per router with their MRT color requires IGP extensions,
    which have not been defined.

SB> So help me understand here. How do you do this with just two
SB> loopback addresses. If I have red to a, red to b etc how
SB> do I distinguish them. Presumably you have to do this by
SB> looking inside the tunnel to find the payload. Unlike MPLS
SB> this is a much larger departure from the standard forwarding
SB> process isn't it? Again this needs to go in the comparision.

    Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the
    tunneling mechanism.

    Note that the two forwarding mechanisms using LDP Label options do
    not require additional loopbacks per router, as is required by the IP
    tunneling mechanism.  This is because LDP labels are used on a hop-
    by-hop basis to identify MRT-blue and MRT-red forwarding topologies.

6.2.  Forwarding LDP Unicast Traffic over MRT Paths

    In the previous section, we examined several options for providing
    MRT transit forwarding functionality, which is independent of the
    type of traffic being carried.  We now look at the MRT ingress
    functionality, which will depend on the type of traffic being carried
    (IP or LDP).  We start by considering LDP traffic.

    We also simplify the initial discussion by assuming that the network
    consists of a single IGP area, and that all routers in the network
    participate in MRT.  Other deployment scenarios that require MRT
    egress functionality are considered later in this document.

    In principle, it is possible to carry LDP traffic in MRT IP tunnels.
    However, for LDP traffic, it is very desirable to avoid tunneling.
    Tunneling LDP traffic to a remote node requires knowledge of remote
    FEC-label bindings so that the LDP traffic can continue to be
    forwarded properly when it leaves the tunnel.  This requires targeted
    LDP sessions which can add management complexity.

SB> I have been thinking a lot about this problem just recently
SB> whilst gardening, and you could take page from the SR playbook and use the
SB> known offset approach. I guess that is a big change, but
SB> I think we might be able to use it to avoid the longstanding distribution
SB> issue - i.e. convert the problem to offset math.

    The two MRT LDP
    Label forwarding mechanisms have the useful property that the FEC
    associated with the packet is maintained in the labels at each hop



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    along the MRT, as long as an MRT to the originator of the FEC is
    used.  The MRT IP tunneling mechanism does not have this useful
    property.  Therefore, this document only considers the two MRT LDP
    Label forwarding mechanisms for protecting LDP traffic with MRT fast-
    reroute.

SB> I do not understand the preceding text

  ===========

6.2.3.  Other considerations for forwarding LDP traffic using MRT LDP
         Labels

    Note that forwarding LDP traffic using MRT LDP Labels requires that
    an MRT to the originator of the FEC be used.  For example, one might
    find it desirable to have the PLR use an MRT to reach the primary
    next-next-hop for the FEC, and then continue forwarding the LDP
    packet along the shortest path tree from the primary next-next-hop.

SB> Does this mean that the packet looses it's MRT marking at this point?
SB> If so can't this result in a multi-failure repair loops?

=============

6.3.  Forwarding IP Unicast Traffic over MRT Paths

    For IP traffic, there is no currently practical alternative except
    tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
    forwarding topology.

SB> Well isn't there a possible solution based on IPv6 options?

    The choice of tunnel egress MAY be flexible
    since any router closer to the destination than the next-hop can
    work.

SB> Again isn't  there an SRLG or other multiple failure issue with this?

==========



6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option 1B)

    The MRT LDP Label option 1B forwarding mechanism encodes the topology
    and the FEC using a two label stack as described in Section 6.1.1.2.
    When a PLR receives an IP packet that needs to be forwarded on the
    Red MRT to a particular tunnel endpoint, the PLR pushes two labels on
    the IP packet.  The first (inner) label is the normal LDP label
    learned from the next-hop on the Red MRT, associated with a FEC
    originated by the tunnel endpoint.  The second (outer) label is the
    topology-identification label associated with the Red MRT.

    For completeness, we note here a potential optimization.  In order to
    tunnel an IP packet over an MRT to the destination of the IP packet
    (as opposed to an arbitrary tunnel endpoint), then we could just push
    a topology-identification label directly onto the packet.  An MRT
    transit router would need to pop the topology-id label, do an IP
    route lookup in the context of that topology-id , and push the
    topology-id label.

SB> What are you optimizing - certainly not the forwarding cycles.

=============



12.2.  MRT Recalculation

    When a failure event happens, traffic is put by the PLRs onto the MRT
    topologies.  After that, each router recomputes its shortest path
    tree (SPT) and moves traffic over to that.  Only after all the PLRs
    have switched to using their SPTs and traffic has drained from the
    MRT topologies should each router install the recomputed MRTs into
    the FIBs.

    At each router, therefore, the sequence is as follows:

    1.  Receive failure notification

    2.  Recompute SPT

    3.  Install new SPT

    4.  If the network was stable before the failure occured, wait a
        configured (or advertised) period for all routers to be using
        their SPTs and traffic to drain from the MRTs.



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    5.  Recompute MRTs

    6.  Install new MRTs.

    While the recomputed MRTs are not installed in the FIB, protection
    coverage is lowered.  Therefore, it is important to recalculate the
    MRTs and install them quickly.


SB> that is one way of doing it, anothey way is to have n MRTs running
SB> ships in the night as each failure occurs and clean up later. I think
SB> that this makes the migration timing less critical. I an not
SB> sure whether you don't need some approach like "new labels" to make
SB> this unconditionally safe.

  ===========

15.  IANA Considerations

    Please create an MRT Profile registry for the MRT Profile TLV.  The
    range is 0 to 255.  The default MRT Profile has value 0.  Values
    1-200 are by Standards Action.  Values 201-220 are for
    experimentation.  Values 221-255 are for vendor private use.

SB> I am suprised that this is not in a protocol documnet rather than the
SB> architecture since you have no data structures or encodings in here.

16.  Security Considerations

    This architecture is not currently believed to introduce new security
    concerns.

SB> I suppose you could express this in terms of the parameters being
SB> carried in the existing routing and thus you inherit their security
SB> mechanisms, and that the topology boundary is set by their boundary
SB> and thus you inherit their security and privacy characteristics.

SB> I am supprised that the draft does not say anything about the
SB> management of this new approach to FRR and the OAM that you will
SB> need to test it.


===========