Devil's Advocate - Segment Routing, Why?

Sabri Berisha sabri at cluecentral.net
Sat Jun 20 22:54:06 UTC 2020


----- On Jun 20, 2020, at 2:27 PM, Mark Tinka <mark.tinka at seacom.mu> wrote: 

Hi Mark,

> On 20/Jun/20 00:41, Anoop Ghanwani wrote:

>> One of the advantages cited for SRv6 over MPLS is that the packet contains a
>> record of where it has been.

> I can't see how advantageous that is, 

That will be very advantageous in a datacenter environment, or any other
environment dealing with a lot of ECMP paths. 

I can't tell you how often during my eBay time I've been troubleshooting
end-to-end packetloss between hosts in two datacenters where there were at least
10 or more layers of up to 16 way ECMP between them. Having a record of which
path is being taken by a packet is very helpful to determine the one with a crappy
transceiver.

> or how possible it would be to implement,

That work is already underway, albeit not specifically for MPLS. For example,
I've worked with an experimental version of In-Band Network Telemetry (INT)
as described in this draft: https://tools.ietf.org/html/draft-kumar-ippm-ifa-02

I even demonstrated a very basic implementatoin during SuperCompute 19 in Denver
last year. Most people who were interested in the demo were academics however,
probably because it wasn't a real networking event.

Note that there are several caveats that come with this draft and previous
versions, and that it is still very much work in progress. But the potential is
huge, at least in the DC.

> especially for inter-domain traffic.

That's a different story, but not entirely impossible. A probe packet can
be sent across AS borders, and as long as the two NOCs are cooperating, the
entire path can be reconstructed.

Thanks, 

Sabri


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