Re: [mpls] Clarification on the motivation of draft-xu-spring-islands-connection-over-ip-05

<bruno.decraene@orange.com> Wed, 03 August 2016 10:33 UTC

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From: <bruno.decraene@orange.com>
To: Xuxiaohu <xuxiaohu@huawei.com>
Thread-Topic: Clarification on the motivation of draft-xu-spring-islands-connection-over-ip-05
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Date: Wed, 3 Aug 2016 10:33:26 +0000
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Subject: Re: [mpls] Clarification on the motivation of draft-xu-spring-islands-connection-over-ip-05
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Xiaohu,

> From: spring [mailto:spring-bounces@ietf.org] On Behalf Of Xuxiaohu > Sent: Wednesday, August 03, 2016 10:54 AM
> 
> Hi MPLS WG and SPRING WG,
> 
> I have been told by many people that connecting MPLS segment routing (SR) islands/nodes
> over IP as described in (https://tools.ietf.org/html/draft-xu-spring-islands-connection-over-
> ip-05) is very useful for the incremental deployment of MPLS SR. However, the remote
> label distribution peer mode for MPLS SR (which is used for connecting MPLS SR
> islands/nodes) conflicts with RFC3031 (see Section 3.22. Lack of Outgoing Label). I
> discussed with Bruno (SPRPING WG co-chair) offline about the next step of this draft at
> IETF96. he suggested that it'd better to update RFC3031 so as to take the remote label
> distribution peer mode for MPLS SR into account.

I think that you are referring to a private conversation that we had during the Bits-N-Bites, Thursday evening. To be clear, on my side, I assumed that this was a private conversation over a few beers. (and thanks Juniper for the beers).

Then, this is not what I said.

Coming back to the technical point:
First of all, it's not clear to me what, according to you, is missing in the current IETF specifications, to allow for your use case.
- On the data plane standpoint, multiple forms of IP tunnels are capable of carrying MPLS packets. Presumably to be able to send MPLS packets between MPLS nodes which are not adjacent but separated by IP routers. Do we agree on this, or is something missing?
- On the control plane standpoint, the draft is very light on this, but according to the discussion, both tunnels end-points are MPLS segment routing capable and advertise their labels (index) in the IGP. So we do have the labels of the IP tunnel egress/remote end point. Is there something missing?
- On the specification standpoint, your draft is informational, so is not going to change an existing standard track document.

So it looks like to me that you could locally do what you want, e.g. assume that the IP tunnel is a virtual MPLS link/shortcut to the egress (even though not advertised in the IGP), and send the MPLS SR packet over that tunnel.

Then you replied that, even if it's a local behavior, a MPLS RFC prohibited this. Then I replied that this looks like a work for the MPLS WG. e.g. if a MPLS RFC prohibits this behavior, you need to discuss this in the MPLS WG, and if needed, have this MPLS RFC updated.


> I would like to hear more suggestions
> and comments from MPLS and SPRING WGs on the necessity of RFC3031bis

IMHO this part seems primarily for the MPLS WG

> and the best way to standardize the remote label distribution peer mode for MPLS SR.

IMHO this part seems primarily for the SPRING WG.

MPLS SR uses routing protocols to advertise labels, more specifically link state IGP (OSPF, IS-IS) or BGP. Can you elaborate on what is missing? Again, the draft is light on this (to say the least). According to our discussion, nodes share the same IGP, hence they should already have the labels. In all cases, your half-page draft is not proposing something new on the label distribution standpoint.

Regards,
--Bruno

> 
> Best regards,
> Xiaohu
> 
> > -----Original Message-----
> > From: mpls [mailto:mpls-bounces@ietf.org] On Behalf Of Xuxiaohu
> > Sent: Wednesday, April 06, 2016 11:37 PM
> > To: spring@ietf.org
> > Cc: mpls@ietf.org
> > Subject: [mpls] Clarification on the motivation of
> > draft-xu-spring-islands-connection-over-ip-05
> >
> > Hi all,
> >
> > According to suggestions from SPRING co-chairs, I would clarify the intention of
> > this draft
> > (https://tools.ietf.org/html/draft-xu-spring-islands-connection-over-ip-05) as
> > follows.
> >
> > If I understood the current MPLS-SR specification
> > (https://tools.ietf.org/html/draft-ietf-spring-segment-routing-mpls) correctly,
> > the IGP next-hop for a given /32 or /128 prefix FEC (i.e., node segment prefix) is
> > taken as the next-hop of the received MPLS packet belonging to that FEC. When
> > the IGP next-hop for that FEC is a non-MPLS node, the outgoing label for that
> > FEC is lacked. As a result, the received MPLS packet belonging to that FEC would
> > be dropped BY DEFAULT according to the following specification as quoted from
> > RFC3031:
> >
> > "3.22. Lack of Outgoing Label
> >    When a labeled packet is traveling along an LSP, it may occasionally
> >    happen that it reaches an LSR at which the ILM does not map the
> >    packet's incoming label into an NHLFE, even though the incoming label
> >    is itself valid.  This can happen due to transient conditions, or due
> >    to an error at the LSR which should be the packet's next hop.
> >    It is tempting in such cases to strip off the label stack and attempt
> >    to forward the packet further via conventional forwarding, based on
> >    its network layer header.  However, in general this is not a safe
> >    procedure:
> >       -  If the packet has been following an explicitly routed LSP, this
> >          could result in a loop.
> >       -  The packet's network header may not contain enough information
> >          to enable this particular LSR to forward it correctly.
> >    Unless it can be determined (through some means outside the scope of
> >    this document) that neither of these situations obtains, the only
> >    safe procedure is to discard the packet."
> >
> > In the T-LDP or L-BGP case, the next-hop for the MPLS packet belong to a given
> > prefix FEC is not the IGP next-hop of that FEC anymore. For instance, in the
> > T-LDP case, the next-hop is the T-LDP peer, and in the L-BGP case, the next-hop is
> > the L-BGP peer. As a result, if T-LDP peers or L-BGP peers are not directly
> > connected and are separated by an IP network, the LSP signaled by T-LDP or
> > L-BGP could be transported over that IP network.
> >
> > The situation in MPLS-SR is a little bit complex since the outgoing label for a given
> > /32 or /128 prefix FEC could be learnt either from the IGP next-hop of that FEC
> > or the originator of that FEC due to the IGP flooding property. In the former case,
> > the IGP next-hop for a given FEC is taken as the next-hop of the received MPLS
> > packet belonging to that FEC; in the latter case, the originator of that FEC is taken
> > as the next-hop of the MPLS packet belonging to that FEC. The former case is
> > straightforward to understand and has been described in the current MPLS-SR
> > draft. the latter case is a bit hard to understand and has not been described in
> > the current MPLS-SR drafts. In fact, the latter case belongs to the "remote label
> > distribution peer" case as defined in RFC3031, see below:
> >
> > "When two LSRs are IGP neighbors, we will refer to them as "local
> >       label distribution peers".  When two LSRs may be label distribution
> >       peers, but are not IGP neighbors, we will refer to them as "remote
> >       label distribution peers."
> >
> > In a word, this draft
> > (https://tools.ietf.org/html/draft-xu-spring-islands-connection-over-ip-05)
> > describes the remote label distribution peer mode which is useful for
> > incremental deployment purpose.
> >
> > I would like to hear from SPRING and MPLS WGs whether the remote label
> > distribution peer mode is valid for MPLS-SR and whether we need a draft to
> > describe the remote label distribution peer mode for MPLS-SR.
> >
> > Best regards,
> > Xiaohu
> > _______________________________________________
> > mpls mailing list
> > mpls@ietf.org
> > https://www.ietf.org/mailman/listinfo/mpls
> 
> _______________________________________________
> spring mailing list
> spring@ietf.org
> https://www.ietf.org/mailman/listinfo/spring

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