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Re: Design decisions made at the interim SHIM6 WG meeting
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.
1. The specification for initial contact will use RFC3484, as
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
2. Use an 8 byte IP SHIM6 header in the base protocol
packets that require specific SHIM6 processing by the receiver,
allow optimizations on this, including that of a zero-length
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 as a mandatory part of the inital
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
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.
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
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.
16. SHIM6 control message sequence numbers are not needed here.
My reachability/failure detection draft has sequence numbers of its
own in the reachability probes.
18. Use a statically specified in the initial protocol
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.
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
< 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
19. Continued exploration to see if a better locator pair is available
following identification of a viable locator is considered to
experimental protocol extension. The exploration in the base
specification will terminate once a viable 9reachability
locator pair has been discovered.
In my draft I continue to search for "better" address pairs as
defined by traffic engineering parameters. So if all locators have
equal preference, there is no further exploration. If the current
address pair is worse (= at least one of the two addresses has a
lower preference than another potential source/dest address) the
exploration continues for "better" pairs.