Re: Microloop protection as discussed in today's meeting

["Ahmed Bashandy (bashandy)" <[e-mail]>]

Stewart,

You are stating the most obvious:)
For a feature to work, e.g. encapsulating into an RSVP tunnel, the entry 
point of the packet must be able to support that feature. That is not a 
problem. That how the entire world works:)

SR-based ti-uloop avoidance is no different. So if a packet arrives at 
node "D" for example, then node "D" must be able to steer the packet 
such that it guarantees loop-freeness.
Once "D" source-routes the packet into the loop-free path towards the 
destination "Z" , downstream routers from "D" towards "Z" need NOT have 
the ability to do uloop avoidance or even know that packets was 
source-routed by node "D"

As for the need for using "strict" vs "loose" source routing, that is 
topology dependent. The same applies for ti-lfa, rLFA, or even directly 
connected LFA.

One last thing, if you think that source routing can reduced to a 
"tunnel" we might as well shutdown SPRING WG:)

Ahmed




On 7/19/2016 7:48 AM, Stewart Bryant wrote:
> Appologies if this is a duplicate.
>
> The RFC that had all of the generic methods that were known at the 
> time of publication is RFC5715.
>
> The two phase methods were in section 6.2 (near-side) and section 6.3 
> (far-side). The first describing tunnelling the traffic towards the 
> repair (and continue to use the repair), the second describing 
> tunnelling traffic towards the destination. For the purposes of this 
> discussion source routing of all flavours can be considered a type of 
> tunnel.
>
> If this approach is a new  genetric  two phase method, it would be 
> useful to articulate it in general terms.
>
> The type of topology that I was trying to explain in the meeting is as 
> follows
>
> A-B-C-D-E-F-H-I-J.....W
> |                     |
> |                     |
> +------X-Y-Z----------+
>
> All costs are 1 and Y has failed.
>
> Traffic to Z can enter enywhere, and is protected by X.
>
> When the network starts to converge ALL the routers A..J will need to 
> update their fib to forward towards Z via W rather than towards X via A.
>
> If they do this in a random order as would be the case without LF 
> convergence then you may precipitate microlooping.
>
> What you need to do is to force the packets toward either X or Z using 
> a tunnel, or a source routed path, and as far as I can see you need to 
> do that at every point of potential entry into the network, in the 
> above case A..J, else you risk a microloop.
>
> Now I suppose that if ALL packets were source routed, then you could 
> consider that the network was constantly in the first phase, but I 
> think that you would need to use strict source routing, rather than 
> loose source routing else it reduces the the problem I describe above.
>
>
> - Stewart
>
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