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.
As Mark Smith said in relation to draft-carpenter-6man-flow-update-02
>> Secondly, for IPv6 packets with a number of extension headers before
>> the transport layer header, I think this rule could impact forwarding
>> performance. While I'm not sure if it is that practical, however it'd
>> be good if flow classification could be done using only fixed headers
>> in the IPv6 packet, or a fixed portion of the fixed header bits.
The problem is, only the protocol number is diagnostic of an individual
flow. The earlier next headers are not guaranteed to be the same for
all packets in a transport session, and they might be the same for packets
in different transport sessions between the same two hosts.
So it seems to me that we are stuck with identifying IPv6 flows by
the 5-tuple, even though it means following the linked list to the
end. Or we can forget about identifying individual transport flows,
and identify all traffic between the same two hosts via the 4-tuple
{source address, dest address, source port, dest port}.
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?
--
Regards
Brian Carpenter
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