Re: [Roll] using the flow label instead of hop by hop

[Philip Levis <[e-mail]>]

On Oct 23, 2012, at 10:49 AM, Pascal Thubert (pthubert) wrote:

> Phil,
> 
> The flow label is not protected by any IPSec so it cannot be used as a backchannel end-to-end. Now that would be a hack. 

What do you mean, a backchannel? 

> And it is not for node to node communication but for node to network communication. That, in fact, would be a bad idea when destination options can be used safely.
> Over the core internet, 6MAN has specified how the flow label is supposed to be set and it is the responsibility of the root that an outgoing packet.

I'm not sure I understand what you're saying. Can you summarize how 6MAN has specified it is supposed to be set? I thought I understood their recommendations (from when we last had this discussion) but maybe it has changed. My concern is that a node sets the flow label to communicate some information and RPL throws it away. At the very least, we now need RPL to somehow figure out how to set the flow label for packets leaving the LLN: this is a form of protocol translation and so, while not impossible or impermissible, worries me.

> On incoming packets, a normal host ignores the flow label that it cannot trust anyway. The RPL root will flush it and use it as RPL requires within the RPL domain.

Just because a field doesn't have integrity protection doesn't mean that it can't and won't be used. I mean, are you suggesting that the recommendations for the flow label no longer follow RFC3697: 

   "The Flow Label value set by the source MUST be delivered unchanged to the destination node(s)."

Phil