[frr] [PATCH net-next 0/4] net: mpls: Allow users to configure more labels per route

Jeff Tantsura jefftant at gmail.com
Sun Mar 26 16:16:03 EDT 2017


Hi Thomas,

Good points.

There’s no free lunch in fast path universe, packet recalculation has price associated with it, the most obvious things are increased latency and reduced throughput, there’s more.
Sorry for repeating myself - not being linux kernel expert myself - I’d appreciate pros/cons analysis of taking different approaches, the impact of adding new code and system behavior with it.

Those who expect the underlying platform to be not X86 only (Cumulus?) what are your expectation from HAL/ HW SDK prospective?  

Cheers,
Jeff


> On Mar 26, 2017, at 12:02, Thomas Morin <thomas.morin at orange.com> wrote:
> 
> Hi David,
> 
> [adding my colleague Bruno to the list, he may correct things I might have oversimplified on segment routing, or have a idea about 12]
> 
> 2017-03-25, David Ahern:
>> Eric's question below is basically adding labels at tunnel ingress vs
>> while traversing the LSP. I was generically increasing both to more than
>> 2 labels. Opinions?
> 
> An MPLS packet may in transit receive additional labels. I most cases (all?), this will be most properly seen as a LSP hierarchy (tunneling one LSP into another LSP), so closer to a notion of ingress rather than something related to the initial LSP. But I don't know if the distinction is of importance.
> 
> The cases that comes to mind would be:
> - tunneling into a fast-reroute bypass LSP (possibly a segment routing LSP, see segment routing TI LFA)
> - seamless MPLS
> - carrier's carrier type of deployment
> 
> In these cases a router could receive an MPLS packet, and possibly after popping the topmost, push a stack of labels onto the packet.
> 
> About the email below:
> - how did 12 end up being considered "covering all currently known segment routing use cases" ? it seems that SR could use an arbitrary number of labels  (not saying 12 is a bad number, but...)
> - I'm not sure what Eric's idea of "running the packet a couple of times through the mpls table to get all of the desired labels applied" would mean: after the first lookup, what data would be used as key for the following lookup ?
> - back to your question, which seems to imply one could possibly increase number of labels for ingress without increasing number of labels for transit: isn't the same datastructure used in both to represent an mpls next hop (in RFC3031, both the ILM and FTN point to NHLFE entries, but I haven't digged enough to identify how these maps to the kernel implementation)
> - would a concept of a linked list of mpls_nh make sense, each with one label to impose, make sense, so that no hard limit is put on the label stack depth?
> 
> -Thomas
> 
> 
> 
>> -------- Forwarded Message --------
>> Subject: Re: [PATCH net-next 0/4] net: mpls: Allow users to configure
>> more labels per route
>> Date: Sat, 25 Mar 2017 14:15:54 -0500
>> From: Eric W. Biederman <ebiederm at xmission.com>
>> To: David Ahern <dsa at cumulusnetworks.com>
>> CC: netdev at vger.kernel.org, roopa at cumulusnetworks.com, rshearma at brocade.com
>> 
>> David Ahern <dsa at cumulusnetworks.com> writes:
>> 
>>> Bump the maximum number of labels for MPLS routes from 2 to 12. To keep
>>> memory consumption in check the labels array is moved to the end of mpls_nh
>>> and mpls_iptunnel_encap structs as a 0-sized array. Allocations use the
>>> maximum number of labels across all nexthops in a route for LSR and the
>>> number of labels configured for LWT.
>>> 
>>> The mpls_route layout is changed to:
>>> 
>>>   +----------------------+
>>>   | mpls_route           |
>>>   +----------------------+
>>>   | mpls_nh 0            |
>>>   +----------------------+
>>>   | alignment padding    |   4 bytes for odd number of labels; 0 for even
>>>   +----------------------+
>>>   | via[rt_max_alen] 0   |
>>>   +----------------------+
>>>   | alignment padding    |   via's aligned on sizeof(unsigned long)
>>>   +----------------------+
>>>   | ...                  |
>>> 
>>> Meaning the via follows its mpls_nh providing better locality as the
>>> number of labels increases. UDP_RR tests with namespaces shows no impact
>>> to a modest performance increase with this layout for 1 or 2 labels and
>>> 1 or 2 nexthops.
>>> 
>>> The new limit is set to 12 to cover all currently known segment
>>> routing use cases.
>> 
>> How does this compare with running the packet a couple of times through
>> the mpls table to get all of the desired labels applied?
>> 
>> I can certainly see the case in an mpls tunnel ingress where this might
>> could be desirable.    Which is something you implement in your last
>> patch.  However is it at all common to push lots of labels at once
>> during routing?
>> 
>> I am probably a bit naive but it seems absurd to push more
>> than a handful of labels onto a packet as you are routing it.
>> 
>> Eric
>> 
> 





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