Re: [spring] to drop or to forward unlabelled (Re: Question on RFC8660)

[Martin Horneffer <[e-mail]>]

Hi Tarek,

1) it's a good question what to do label stack has more than one label 
left, e.g. more SR-MPLS segments. Or a service label. Or both...
So far I was considering two possibilities:
a) Only one label is dumped. In case of SR-MPLS this means that one 
sgement can be skipped. In case of a folloowing service label, this 
could theoretically put the packet in a wrong context.
b) The packets is only forwarded if the label was the last, i.e. 
bottom-of-stack set.
Any of the two possibilities would help in the relevant case of an IP 
packet with a single label (which is the transport label).

2) My assumption would be that the last path information is being used.
I.e. if a packets arrives at a router R with a single label. Based on 
the label R determines a next-hop. If the next-hop does not provide a 
label for this FEC, then the router effectively performs a POP 
operations, but still forwards.
If the packets arrives at R as plain IP, and the routes can successfully 
do a route lookup and determine a next-hop, then it sends the packets 
WITHOUT doing a PUSH operation.
If I understood it correctly, this is the way LDP behaves.

Best regards, Martin


Am 30.08.20 um 02:36 schrieb Tarek Saad:
> Hi Martin,
>
> See inline for some comments.
>
> On 8/27/20, 6:35 AM, "spring on behalf of Martin Horneffer" <[e-mail] on behalf of [e-mail]> wrote:
>
>      Hello everyone,
>
>      may I come back the the question below? Or rather let me update it a little:
>
>      In case an SR-MPLS path is broken, should a node rather drop the packet,
>      or forward it?
>
>      This can happen whenever the IGP points to a certain next hop, but that
>      neither supplies a valid SID, nor allows LDP-stitching for whatever
>      reason. For PUSH as well as for CONTINUE.
>
>      We have been using MPLS transport and a BGP free core since about two
>      decades now, using LDP. In the analog case, LDP creates "unlabelled"
>      entries in the LFIB, does the equivalent of a POP operation and forwards
>      the packet to the next-hop as chosen by the IGP.
>
>      This behavior obviously breaks any traffic that relies on a service
>      label, but it can protect some traffic.
>      In our case a huge percentage of all traffic still is public IPv4. This
>      needs MPLS only for a transport label, be it LDP or SR-MPLS. If this
>      traffic gets forwarded unlabelled, it follows an IGP default route to a
>      central device, where it is 1) redirected to the correct destination and
>      2) counted in a way that operators can quickly see whether and where
>      this kind of failure occurs at some point in the network.
>
> [TS]: The SR Path may be composed of multiple SIDs (i.e. label stack) -- where the top SID destination is not the SR Path endpoint.. I sense from "gets forwarded unlabelled " as you will discard the full label stack (?) and forward the packet unlabeled to the central device? Or are you encapsulating the remaining MPLS packet over IP and IP routing packet to the central device to be inspected? In either case, I expect somehow the packet to be ultimately delivered to the original SR Path endpoint/egress?
>
>      After more operational experience and several internal discussions we
>      agreed that we want packets to be forwarded unlabelled rather than
>      dropped. Anyone to share, or oppose this position?
> [TS]: again, I'm not clear on when you say "forward unlabelled" - do you mean packets will follow the top SID's the IP IGP route? Or are you peaking into the destination IP (public IPv4) of packet encapsulated in MPLS to determine how to forward it?
>
> Regards,
> Tarek
>
>      Best regards, Martin
>
>
>      Am 31.01.20 um 16:50 schrieb Martin Horneffer:
>      > Hello everyone,
>      >
>      > again it seems the interesting questions only show up when applying
>      > something to the live network...
>      >
>      > We ran into something that poses a question related to RFC8660: What
>      > is the exact meaning of section 2.10.1, "Forwarding for PUSH and
>      > CONTINUE of Global SIDs", when the chosen neighbor doesn't provide a
>      > valid MPLS path?
>      >
>      > The relevant sections reads:
>      >
>      >       -  Else, if there are other usable next hops, use them to forward
>      >          the incoming packet.  The method by which the router "R0"
>      >          decides on the possibility of using other next hops is beyond
>      >          the scope of this document.  For example, the MCC on "R0" may
>      >          chose the send an IPv4 packet without pushing any label to
>      >          another next hop.
>      >
>      > Does the part "send an IPv4 packet without pushing any label" apply to
>      > PUSH and CONTINUE, or just to PUSH?
>      > Does R0 have to validate that neighbor N can correctly process to
>      > packet? Or can it forward the packet regardless?
>      >
>      > The reason for asking is that we are now seeing issues similar to ones
>      > we had when starting with LDP based MPLS about two decades ago:
>      > traffic being black holed even though a path to the destination
>      > exists, because the MPLS path is interrupted somewhere in the middle.
>      >
>      > With LDP we know the case of LFIBentries called "unlabelled". While
>      > this does break connectivity for many kinds of service, e.g. those
>      > relying on an additional service labels, it still works for plain
>      > IP(v4) traffic. In our cases, this works perfectly fine for all
>      > internal routing and control traffic. And even for IPv4 traffic that
>      > gets collected by a central router that injects a default route.
>      >
>      > However, depending on the exact interpretation of the above paragraph,
>      > an implementor might feel obliged to chose the next paragraph:
>      >
>      >       -  Otherwise, drop the packet.
>      >
>      > Which is, at least in our case, very unfortunate...
>      >
>      > Any advice or opinion appreciated!
>      >
>      >
>      > Best regards, Martin
>      >
>      >
>      >
>      >
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