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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 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?
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 as a mandatory part of the inital base spec.
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?
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 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.
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.
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. 

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


19. Continued exploration to see if a better locator pair is available
following identification of a viable locator is considered to be an experimental protocol extension. The exploration in the base protocol specification will terminate once a viable 9reachability confirmed) 
    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.