Opened 8 years ago

Closed 8 years ago

#69 closed defect (fixed)

Dave Thaler comments on draft-ietf-pcp-port-set-04

Reported by: dthaler@… Owned by: draft-ietf-pcp-port-set@…
Priority: major Milestone: milestone1
Component: port-set Version: 1.0
Severity: In WG Last Call Keywords:


Le 2013-10-30 18:33, Dave Thaler a écrit :

1) The claim that a port set makes code simpler needs an explanation

justify it.

E.g., if an app really needs 16 ports, and it fails to get 16 back
in its first request, it still has to handle that and say make
multiple separate


Two observations:

  • Wouldn't that behaviour be a tiny bit evil? It reminds me of

applications that create multiple flows in an attempt to trick an
SFQ-based QoS into allocating them more bandwidth.

  • Often, apps that ask for N ports don't really need N ports. Ideally

they would like to get N ports, but they can live with <N ports. No
need for single- port fallback in that case.

So if it already has to have
code to deal with multiple separate requests, why is it simpler to
add port set functionality to and implement both? This seems

to a reader.

That would only apply to apps that absolutely need N ports to
function, and don't need them to be contiguous. Those apps need to
fall back on single- port requests as you describe. But I would argue
that these apps are really quite rare.

We can add text describing these concerns in the next revision.

Sure, but the claim in section 2 is that the client side logic *is* simpler.
Not "often" simpler, or "may be simpler". You can’t assert this when
sections 6.4 and 6.3 both leave some major questions up to the implementation. So it might be more complicated not simpler. Suggest just deleting the "Client-side simplicity" bullet.

The text added in section 6.4 looks fine to me, except for one editorial suggestion on this text:

Suppose a PCP client asks for 16 ports and receives 8. What should

it do? Should it consider this a final answer? Should it try a
second request, asking for 8 more ports? Should it fall back to 8
individual MAP requests? This document does not try to answer those
questions , but describes issues to be considered when doing so.

Suggest changing last sentence to
"This document leaves the answers to be implementation-specific, but describes issues to be considered when answering them."

That is, it answers them by saying it's implementation-specific.

Section 6.3 now looks fine to me, except for a typo:

which port set mapping requests would be arbitrated, Alternatively,

Change first comma to period.

4) Section 4.1 says if the response does not include a PORT_SET, in
a response it assumes the server doesn't support it, and sends

MAP requests for the rest of the ports it needs.

But section 4.2 says if the server does support PORT_SET but can
only return 1 in its first response, It must omit the PORT_SET
option. I think

that's wrong since it will confuse the client as noted above.

If it's included with a Port Set Size of 1, the client knows it can
keep trying using PORT_SET for subsequent blocks, which might get

than 1 port in each response.

Agreed. I'll modify the draft accordingly unless others disagree.

I expected the draft to be modified accordingly as I didn't see any disagreement on the list or in the meeting notes (let me know if I missed something).
However a different change was made to the draft which I think isn't as good.

Specifically, per the "Agreed" above, I expected it to say the server will return a port set option with size of 1, and the client then knows the server supports port-sets and can choose to ask for another port-set if it wants.
And if the response has no port-set option, then it knows the server doesn't support it and can send other requests accordingly.

Instead, -04 changed to keep the statement:

If the PCP server ends up mapping only a single port, for any reason,
the PORT_SET option MUST NOT be present in the response.

And instead added this text:

If no PORT_SET option is present in the response, the PCP client

cannot conclude that the PCP server does not support the PORT_SET
option. It may just be that the PCP server does support PORT_SET but
decided to allocate only a single port, for reasons that are its own.
If the client wishes to obtain more ports, it MAY send additional MAP
requests (see Section 6.4),

I didn't see it explained why this is better than what I thought we agreed to on the list.

Also related to the above is the following from IETF 88 meeting minutes:

Dan: Server can be upgraded/changed to support/not support PORT_SET.

Is there any harm to always request PORT_SET?

Dave: If I get an indication that the server doesn't currently support

PORT_SET, I'll send multiple requests in parallel.

Just keep the above in mind when addressing the earlier issue.

Change History (1)

comment:1 Changed 8 years ago by dthaler@…

  • Resolution set to fixed
  • Severity changed from Active WG Document to In WG Last Call
  • Status changed from new to closed

Addressed in -06

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