Subject: RSVP hello extension

From: "Hong Liao" <[email-masked]>
Date: Mon, 21 May 2001 15:41:04 -0400

Hi,

On RSVP -lsp-tunnel-08, section 5.5, I think the clause  "or will response with an "Unknown Object Class" will be dropped,
but it is still there in the latest draft, then I still have following questions.

>so my questions, is this error message put in PathErr message?  Since there is
>no individual error message for Hello message.

This following is email I got from Achieve mail which was post last year.

[Lou's response for my questions]
>>Good point.  The clause "or will response with an "Unknown Object Class"
>>error."  should be dropped.

>>Lou

[My post question last year on Oct.]
>Hello,
>
>On Rsvp-TE page 57, Section 5.5 Compatibility, it says that "Depending on the
>implementation, implementations that do not support the extension will either
>silently discard Hello messages or will response with an "Unknown Object
>Class"
>error."
>
>Hello message  ------->    LSR(implemented not supporting Hello extension)
>                                 <-------     no response(discard hello
> message)
>OR  response with an "Unknown object class" error
>
>so my questions, is this error message put in PathErr message?  Since there is
>not one individual error message for Hello message.
>
>Thanks in advance.
>
>Julia

Thanks,

Julia

---

From: David Charlap <[email-masked]>
Date: Mon, 21 May 2001 19:30:52 -0400

> On RSVP -lsp-tunnel-08, section 5.5, I think the clause  "or will
> response with an "Unknown Object Class" will be dropped,

As far as I can tell, this "unknown object class" response should never
happen.  HELLO objects are only specified within the context of Hello
messages.  If a router understands Hello messages, then it will also
understand the HELLO object.  If it does not understand Hello messages,
it will silently drop the message - as RSVP routers are supposed to do
for all unknown messages.

The only time I can imagine a router sending back this message would be
if a HELLO object was included in a Path or Resv message.  But this
shouldn't happen, because the draft has never specified theis behavior.

I suppose you could come up with a contrived case where a router
understood Hello messages but not HELLO objects.  In which case, sending
back an error message would make sense.  But as you have already pointed
out, there is no such thing as a HelloErr message to carry the error
in.  You can't use PathErr or ResvErr, because you don't have any of the
other required objects (like SESSION, SENDER_TEMPLATE or FILTER_SPEC.) 
So even then, the only possible choice is to silently drop the Hello
message.

> so my questions, is this error message put in PathErr message?
> Since there is no individual error message for Hello message.

A router that doesn't understand Hello objects, and receives one in a
Path or Resv message (from a broken router, obviously) will respond with
PathErr or ResvErr as appropriate.

I can't think of any other situation where it would make sense to
generate such an error.  Neither can I think of a sitation where it
would be possible to send back a well-formed error message, even if you
wanted to.

Hence the reason why this possible response will be dropped from the
spec.  There isn't any situation, (other than the contrived case of a
router illegally putting a HELLO object into a Path or Resv message),
where it would even be possible to generate the error message.

-- David