Re: [mpls] I-D Action: draft-rosen-mpls-rfc3107bis-01.txt

Eric C Rosen <erosen@juniper.net> Mon, 20 June 2016 19:07 UTC

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To: Lucy yong <lucy.yong@huawei.com>
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From: Eric C Rosen <erosen@juniper.net>
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Subject: Re: [mpls] I-D Action: draft-rosen-mpls-rfc3107bis-01.txt
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Hi Lucy,

Thanks for reviewing the draft!
>
> ·Sn 3.2.2, the listed local policies seem all related to label 
> swapping actions.
>
>  it is valuable to add a new local policy as follow:
>
> oAdd a single label or a sequence of labels to the NLRI before 
> propagating the route.
>
>      This policy results that, when a node receives a MPLS packet, it 
> will pop out the label(s) and forward the packet to next hop.
>
I think this is a sub-case of "replacing one label with multiple labels".
>
> ·Sn 4 data plane description is not sufficient. When applying mutli 
> labels on data plane, we need to specify the rules to fill the label 
> stack entries beside pushing labels; e.g. TTL, EXP. To backward 
> comparable (RFC3032), need to clarify TTL, EXP processing applying to 
> the top label before and after label process action.
>
3107bis does not convey TTL or TC values, so I think all we need to say 
is that "the setting of the TTL and TC fields in the label stack of a 
data packet is determined by local policy".
>
> ·This feature gives each next hop flexibility to determine how many 
> labels to bind a prefix, which may impact Path MTU. We SHOULD avoid 
> each path segment to fragment labeled packets.
>
MPLS does not have fragmentation, so it is not possible to fragment 
labeled packets ;-)

> Either use PMTU discovery or configuration parameter to determine the 
> proper MTU and limit the fragmentation to be done once. If a next hop 
> decides to advertise different # of labels to different prefix, the 
> case will be more complex. The draft needs describe this.
>
3107bis provides a mechanism that can be used to propagate an 
instruction to push a sequence of labels onto a packet's label stack.  
You're right that a router should not be instructed to push on so many 
labels that the MTU is exceeded.  But this is always an issue with MPLS, 
and is not specifically related to 3107bis.  It is outside the scope of 
this draft to specify procedures for determining the maximum number of 
labels that can be safely pushed.
>
> ·This example in Sn 4 is not correct.
>
>    In this case, if S1 receives an MPLS data packet whose top label is
>
>    L21 and whose second label is L22, S1 will remove both L21 and L22
>
>   from the label stack, and replace them with <L11,L12,...L1k>.  Note
>
>    that the fact that L21 is a context label is known only to S1; other
>
>    BGP speakers do not know how S1 will interpret L21 (or L22).
>
>    The ability to replace one or more labels by one or more labels can
>
>    provide great flexibility, but must be done carefully.  Let's suppose
>
>    again that S1 receives an UPDATE that specifies prefix P, label stack
>
> <L11,L12,...,L1k>, and next hop N1.  And suppose that S1 propagates
>
>    this UPDATE to BGP speaker S2 after setting next hop self and after
>
>    replacing the label field with <L21,L22,...L2k>.  Finally, suppose
>
>    that S1 programs its data plane so that when it processes a received
>
>    MPLS packet whose top label is L21, it replaces L21 with
>
> <L11,L12,...,L1k>, and then tunnels the packet to N1.
>
>    In this case, BGP speaker S2 will have received a route with prefix
>
>    P, label field <L21,L22,...L2k>, and next hop S1.  If S2 decides to
>
>    forward an IP packet according to this route, it will push
>
> <L21,L22,...L2k> onto the packet's label stack, and tunnel the packet
>
>    to S1.  S1 will replace L21 with <L11,L12,...,L1k>, and will tunnel
>
>    the packet to N1.  N1 will receive the packet with the following
>
>    label stack: <L11,L12,...L1k,L22,...L2k>.  While this may be useful
>
>    in certain scenarios, it may provide unintended results in other
>
>    scenarios. -end
>
>    Lucy: Label <L21,L22,L2k> is advertised by S1, it does not make a 
> sense that S1
>
>    programs its data plane so that when it processes a received MPLS 
> packet whose top label
>
>    is L21, it replaces L21 with <L11,L12,..L1k>, and tunnel the packet 
> to N1, i.e.
>
>    N1 will receive the packet with the following
>
>    label stack: <L11,L12,...L1k,L22,...L2k>.
>
Whether this example corresponds to an actual use case is debatable.  
The example merely shows something that can be done with the specified 
mechanisms.

This scenario would only be useful if S1 knows somehow that L22 will 
rise to the top of the packet's label stack at a node to which L22 is 
meaningful.
>
> S1 should replace <L21, L22, ..L2k> with
>
> <L11,L12,...,L1k> in this case.
>
No, 3107bis does not (and should not) modify any of the rules for 
processing the label stack of an incoming packet.

Eric