Move SCMP to CP draft

[nicorusti]

We went through the DP draft with Eliot at IETF121. He was overall positive, but mentioned that SCMP is a CP topic and (for now) move it to the CP

Ideas:

• IS SCMP used anywhere by the control service? (@jiceatscion )
• SCMP is an additional signaling mechanism between endpoints and SCION routers (?). It could be a new section before security considerations in the CP draft
• Add SCMP to the definitions in the CP draft
• Add cross references from DP (handling link failures) to the CP draft
• Clarify path revocation

[jiceatscion]

I'll do the move, as I have a small fix to add to the DP anyway. Kevin or I can do a second pass to attend to the breadcrumbs

[jiceatscion]

Is SCMP used by the CS?:

• Yes!
• https://github.com/scionproto/scion/blob/dd852aa7a8e6eb99cb677851a7c55eb33bf64cb2/control/cmd/control/main.go#L217
• Link-down messages are used to revoke segments.
• The public goes "Ooooo"

[jiceatscion]

Move done per the PR The cross-referencing probably needs some more work.

[nicorusti]

@jiceatscion is the CP really revoking paths based on SCMP? I thought this was not the case since the 2018 redesign.

I feel (after merging a PR, sigh!) that there are still some open points:

• Right now we have a section about Handling Link Failures in the DP but not in the CP. Shall we move it to the CP (but then we break the flow, as we talk about BFD in the DP)
• This wording in the CP might have to be updated if we include revocation (I'd rather not):

  Note that link removal is not part of path discovery in SCION. For scheduled removal of links, operators let path segments expire. On link failures, endpoints route around the failed link by switching to different paths in the data plane.