Margaret,
The flow label specification should define the default usage of the flow label, and
allow for future specification of more specific usages. IPv6 WG needs to reach
consensus on the default usage, document that in the flow label specification, and not
set any unnecessary or ideological restrictions on the usages to be defined later.
Enabling usage for "load balancing" is within default usage of the current draft. I
think we need now have a discussion on what exactly this means for the spec.
You wrote:
> Load balancing routers want to make balancing decisions based on the
> value of the flow label field, with no other signaling required. A
> very simple mechanism (such as hashing the flow label field) will
Does "load balancing" here mean selection of one of the parallel paths (with the same
metric), or selection of one of parallel links to the next hop?
In any case the source and destination addresses should be included in the hash.
IMO the address fields alone would be enough for "core" routers, where it is unlikely
that a single host pair can dominate the traffic.
> allow load balancing routers to consistently send packets from a
> single flow over the same path. This offers a couple of big
> advantages:
>
> - It reduces packet reordering, potentially resulting in
> significant performance increases.
We need to understand how significant issue this is.
First, this may be an performance issue for the final destination only, as routers do
not reassemble flows.
If the selection of the path can have a significant effect on the overall packet
delivery time, the load balancing decision should not be based on a blind flow
identity only.
IP has never promised to not reorder packets. I do not think we need to build in any
guarantees now either.
So, exactly when could we expect to get significant performance increases?
> - It makes the PMTU mechanism work better, since packets
> for a single connection will take a consistent
> path.
>
In practice, is or will path load balancing be made over paths where the PMTU is
different among the parallel paths?
Is it forbidden for e.g. TCP to share PMTU information between different connections,
if those connections are between same addresses?
> But, for this simple mechanism to work, load balancing routers have to
> be able to rely on the fact that the flow label will label a _flow_
> (one direction of a TCP, UDP or SCTP connection).
>
Also this is an issue to be discussed. For the load balancing to be effective, the
bandwidth of the identified (and "balanced") unit should be of small enough portion of
link/path capacity. There is no guarantee that a single TCP connection is "low
bandwidth" compared to some other aggregate of packets between a pair of addresses. On
the contrary, TCP tries to adapt to the path and get the maximum throughput (e.g. for
a file transfer).
It seems that if a given TCP connection is consistently mapped to the same flow
identity, the simple path load balancing will work the same regardless the granularity
of the aggregate from the source host. But note that a Flow Label alone does NOT
identify the flow, you need to consider the source and destination addresses also
(which is also very clearly spelled out in the current draft :-)
Jarno
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