On Wed, Apr 14, 2010 at 7:16 PM, Bob Hinden <[email protected]> wrote:
> Brian,
>
> On Apr 14, 2010, at 3:26 PM, Brian E Carpenter wrote:
>
>> Hi,
>>
>> Common practice in network monitoring and in QoS technologies
>> is to identify a flow of packets by the 5-tuple
>> {source address, dest address, source port, dest port, protocol #}.
>> This is relatively trivial at line speed in IPv4 since
>> these things are at fixed locations in the header. But in IPv6,
>> the protocol number is at the end of a linked list of "next
>> headers." Even if the normal case is only one item in the linked
>> list, any implementation (hardware or software) that extracts
>> the 5-tuple has to follow the linked list to the end.
>
> Do we have any data on what percentage of packets have the next header a
> transport header vs. something else? It would be good to know how often it's
> the non-trivial case.
>
> I agree that an implementation will have to know how to at least check what
> the next header is.
>
most high-end routers today that do v6 only deal with a limited number
(3 or so) extension headers anyway, so they've figured out how to do
line-rate 100-ge with ipv6 ...
this is, also, one of the reason whenever someone says: "We should
make an extension header for this!" I say: "Extension headers are the
devil, they should all die a horrible death, don't add to an already
horrendous problem."
>>
>> As Mark Smith said in relation to draft-carpenter-6man-flow-update-02
>> Or we can strongly recommend that all hosts set the flow label, so
>> that we can use the 3-tuple {source address, dest address, flow label}.
>>
>> What do people think?
>
> That would make a lot of sense to me. It would also have an advantage of
> allowing a set of TCP connections between two hosts to be associated as a
> single flow where the hosts wanted them to be treated in the same manner.
> It's hard to do that with IPv4 as the port numbers will be different for each
> TCP connection.
This seems fine, until the 2 sessions are larger (combined) than parts
of the pathways between the 2 hosts. ECMP/LAG are nice tools, and
necessary in today's internet, if you can't hash the flows reasonably
and 2 large flows end up hashed to the same smaller link bad things
for the hosts' traffic will happen.
-Chris
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