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Re: Design decisions made at the interim SHIM6 WG meeting
Hi Iljitsch,
El 27/10/2005, a las 14:42, Iljitsch van Beijnum escribió:
On 19-okt-2005, at 6:11, Geoff Huston wrote:
The following is a list of SHIM6 design decisions
I don't think "decisions" is the right word here... These are more
like suggestions by the chairs.
sorry, but i disagree here.
The points below were accepted by most of the people in the room (i
certainly accepted most of them)
(perhaps there is some points where there may be some misunderstanding
on the final outcome, like the point about the preferences that Erik
brought up (my understanding was similar to Erik's one) but this is a
natural problem when summarizing long discussions as the ones we had,
and they are being clarified through this discussion)
1. The specification for initial contact will use RFC3484, as
modified by
this draft in terms of source address ordering.
Well, all IPv6 hosts are expected to use RFC 3484 address selection,
nothing new there. In fact, we have to break RFC 3484 from time to
time because otherwise we'd be selecting non-working addresses.
The real point is what happens when the address pair selected by
either the application or RFC 3484 doesn't work. How do we handle this
case?
the idea, (and i think this is was this decision point is about) is
that we need an update to rfc 3484 to deal with this.
i am currently working on a document about this (as we agreed in the
meeting) with some proposals of how to update rfc3484 to deal with
this.
2. Use an 8 byte IP SHIM6 header in the base protocol specification
for
packets that require specific SHIM6 processing by the receiver,
and
allow optimizations on this, including that of a zero-length
header, to
be an experimental protocol extension.
I have no problem with a shim header for demultiplexing in cases where
demultiplexing would otherwise be very hard or impossible. For
instance, in the case of several extension headers, an explicit shim
header makes it possible to indicate which headers see modified
addresses and which headers see unmodified addresses unambiguously.
However, I think it's a very bad idea to have a shim header in EVERY
packet with rewritten addresses, because there are cases where the
shim context can be determined from information that's already in the
packet unambiguously so an extra header is unnecessary.
Mandating a shim header first and then optionally allow it to be
suppressed is useless in practice: we need have the capability to have
the shim header suppressed
so far i think that we all agree
as a mandatory part of the inital base spec.
this is the point of debate: whether this extension should be mandatory
in the base spec or not.
my opinion about this, is that this would be indeed quite complex. I
mean i like the idea but i am afraid that when we try to define the
details, the result will be complex. If not i would certainly support
this
I think that the best approach is what (i understood that it) was
decided in the meeting:
- for now the base spec does not includes this.
- a document is produced to understand what it would take to support
this extension. that is a full fleshed proposal of what would be
required to do this. (my recall is that you and Pekka? were to produce
this?)
4. Use a 32 bit context field with no checksum, and 15 reserved bits
and a
1 bit flag to indicate control / payload. Note potential DOS risks
If we know there are risks today, maybe it makes more sense to make
the tag variable size (to be negotiated between the peers) rather than
fixed?
what about making it fixed of 47 bits from the start?
Remember, there are no second chances: if we botch the initial packet
layouts we'll have to live with them for a long time.
* Adopt HIP parameter format for options; HIP parameter format
defines length in bytes but guarantees 64-bit alignment.
I don't want this alignment. It wastes space, it's an implementation
headache and it buys us nothing.
i think that this is a small price to pay to allow future convergence
with hip that is anther protocol somehow related with shim6
11. Proposed to drop specific mechanism for locator test and response
???
13. [What are privacy requirements for locator lists? Also integrity
- this
protocol is currently "in the clear".]
One person's privacy is the next person's debugging nightmare.
There is no privacy in today's multihoming either.
14. View forking as a unidirectional context state fork (based on a
ULP
signal) that assumes that the forked context state may then use a
different outgoing locator pair.
Forking is a bad idea, it increases the complexity of the shim
manifold while pretty much the same functionality can also be provided
in a different way.
i guess it depends on how you handle it... i mean for me, i see it just
as multiple contexts with the same ulid pair, but with different
context tag. In this view, it doesn't seems very complex, it is just
multiple contexts..., am i missing something?
16. SHIM6 control message sequence numbers are not needed here.
[control
messages]
My reachability/failure detection draft has sequence numbers of its
own in the reachability probes.
right
locator sets also have the generation number which is a seq number
i guess that the point is that the protocol as a whole does not have
seq numbers, like first packet I1 with seq number 1, next packet I2 seq
number 2 and so on...
18. Use a statically specified in the initial protocol specification
of
(10) seconds.
This means that senders must transmit something (data, keepalive)
every 10 seconds, right? So the receiver needs to wait a bit LONGER
than 10 seconds to time out.
as i recall it, 10 seconds was the time of 3 packets i.e. a node is
expected to send packets every 3 seconds, so if 3 packets are missing,
then we have a problem detected.
Note that the tradeoff is:
< 240 seconds: we MUST repair the problem before TCP times out
> 180 seconds: wait for BGP (90 - 180 second timeout) to fix the
problem
< 90 & > 40 seconds: don't wait for BGP, but do wait for OSPF (40
second timeout) to fix the problem
< 40 seconds: don't wait for OSPF
are you suggesting that we change this to a higher value? i think that
this was one proposal, and i think that it can be easily changed...
regards, marcelo