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Re: I-D ACTION:draft-van-beijnum-shim6-suppress-header-00.txt

Hello;

On Jul 3, 2006, at 9:10 AM, Iljitsch van Beijnum wrote:


On 3-jul-2006, at 14:33, Brian E Carpenter wrote:
The author is of the opinion that such an increase in overhead on top of the increase in overhead incurred by IPv6 in general over IPv4, makes the shim6 multihoming solution less attractive, so it's worthwhile to 
try and suppress the shim6 header where possible.

Has anyone looked at shim6/rohc interaction?

I haven't, but I'd assume that it would be relatively simple to  
compress the shim header too in cases where other headers are being  
compressed.

Also, has anyone estimated what fraction of traffic would carry
shim6 headers, on reasonable assumptions about the prevalence
of multihoming events?

Generally, outages are fairly rare (don't forget that many people  
don't even multihome...), based on common SLA targets I'd say that  
at any given time about 1 - 3 out of a thousand last mile links are  
down. Although very short outages are more frequent than longer  
ones, long ones account for the majority of the downtime. In these  
cases, a shim6 enabled host will only use addresses associated with  
the links that are down if it had already established a context  
before the outage. So in practice, the number of packets that are  
shimmed because of outages will likely be something well below 1 in  
1000, say around 1 in 10,000 to 1 in 100,000.

I think that these estimates may be much too small. For example,  
where my house is (Fairfax County, Virginia), the local cable company  
Internet is severely oversubscribed
mid-afternoon, and long periods (one hour or more) of 50%+ packet  
loss are common, which, looked at in detail are a few seconds of no  
packet loss followed by a few seconds of near total loss, repeated.  
So, if Shim6 was functional, basically every one of my packets would  
be shimmed every afternoon (assuming that the other providers  
available also have the same sort of problems).

Don't forget too that there is a filter effect likely here - the more  
common outages are,
the more likely you are to use something like Shim6, so the sampling  
is not unbiased, and it is probable that Shim6 users will have higher  
than usual packet losses.

Both of these things, of course, support your basic premise.


BTW, none of the local residential providers here provide SLA's,  
although do I know of

one person who has gotten one by request.

Regards
Marshall


However, shim6 has the potential to make outages visible that are  
hidden today. For instance, if I sit between major exchange  
locations A and B, and my ISP gives me addresses that are routed  
over A and addresses that are routed over B (even though I may not  
be multihomed), I could end up with non-working addresses part of  
the time where such an outage would be invisible today. Doing this  
allows the user to "see" the two different paths and use whichever  
works best, as opposed to the current situation where a routing  
protocol selects one of the paths and the other is ignored.

But the clincher would be shimming for traffic engineering. Since  
it's hard to get people to connect to the "right" address at the  
beginning of a context, it's likely that some content suppliers  
will want to rehome a context immediately after it has been set up  
for traffic engineering purposes. This has the potential to  
generate a LOT of shimmed traffic so this is the main reason I  
wrote the draft.