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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.