RE: GID clarifications

At 08:22 PM 12/7/2003 -0800, H.K. Jerry Chu wrote:
>Mike,
>
> >At 06:04 PM 12/4/2003 -0800, bill wrote:
> >>Ok, let me ask a few questions to lead us toward concensus
> >
> >As the author of the IB spec for chapter 4, I'll attempt to provide some
> >clarifications.
> >
> >>1) Does anyone give a compelling reason to use anything but GID0 for IPoIB
> >>- what is the benefit of doing this ?
> >
> >Multiple GIDs were provided as a method to:
> >
> >- Provide traffic segregation between subnets conceptually similar to
> >multi-LID usage within a single subnet.
>
>According to IBA 1.0a spec (i don't have 1.1 spec at hand), "A subnet shall
>have only one assigned GID prefix at any given time (at GID index 0)."
>It seems to me unless a CA port can be simultaneously connected to more
>than one IB subnet, the unique subnet prefix plus the default GID prefix
>(the link-local 0xFF80 prefix) are the only two possible values to have
>for the upper 64 bits of a GID.

Correct.  One can have either the default or an assigned thus all GID share 
one or the other.

>  I wonder what the traffic seggregation between subnets you have in mind.

The basic idea was one could encode in the lower 64 bits (these are admin 
assigned if not the port EUI64 value) or through a combination of other GRH 
attributes (e.g. TClass, Flow Label, etc.) one could segregate traffic to 
take different paths when passing through a router. The path selection 
would be implemented via selecting an egress port and assigning an 
appropriate next router hop / endnode LID.

> >- Provide a method to support software / OS virtualization and device
> >sharing via using a unique global identifier independent of the number of
> >LIDs assigned (though one can take a similar approach if the LMC value is
> >greater than 0.
> >
> >I will note that there are many OSVs offering or starting to offer
> >virtualization services and while some initial implementations may be
> >focused on GID 0, this may not hold true in the coming few years.
>
>Not sure what flavor of "virtualization" you're alluding to. If the
>virtualization is achieved through the use of multiple IB partitions,
>different IB partitions can safely share the same GID much like
>different Ethernet VLAN sharing the same MAC address. I don't see the
>need for multiple GIDs.

Virtualization is nominally a function of address while partitions are a 
function of segregation enforcement ala 802.1Q.  While one can use 
partitions, they are not seen by the underlying software while a GRH is 
seen thus depending upon how it is implemented, how the remote peer "sees" 
each target peer, etc. a GID is more flexible and may align better with 
application usage.  Please keep in mind I'm explaining how the motivation 
for multiple GID from an IBA perspective.  Now, can one use a single GID 
with multiple P_Key and then have a unique IP-to-QP mapping per resulting 
partition to achieve something similar?  The answer is yes, it is possible 
but that was not the intent of the IBTA when this was first being debated.

>Perhaps you meant virtualization like the hosting of multiple IP
>addressses with the same subnet prefix but different host numbers
>on the same NIC, this can be achieved through the use of multiple
>locally-assigned EUI-64s in IB, and I agree it is a useful feature
>to support.
>
> >
> >- Provide a method to constrain out some data flows are routed - local vs.
> >non-local subnet access given a GID can use either the default or the SM
> >assigned subnet prefix.
>
>Note that the draft currently recommends the use of the IB subnet
>prefix for the GID in IPoIB. Although the real requirement is that the
>GID must be reachable (i.e. routable) at the IB layer from any node
>within the scope of the IP link it belongs to, recommending the IB
>subnet prefix is less error prone as it continues to work even after
>the underlying IP link has been expanded to cover multiple IB subnets.

Agreed.


> >
> >>2) Does anyone have an implementation that uses anything but GID0 for 
> IPoIB ?
> >
> >I believe it would be a mistake to limit support to just GID 0.  IB
> >technology was designed to support a diverse set of usage models that will
> >evolve over time.  The only potential problem was a DLPI application raw
> >level usage model which is (a) somewhat outside of the scope of the IP over
> >IB specification and (b) rather limited in terms of usage within the
> >industry.  I don't really want to debate the merits of raw DLPI but do
> >acknowledge it is in use today on Ethernet and other layer 2 interconnects.
> >
> >>If I do not hear any counter arguments by next Friday - I would like to
> >>ask for concensus on MUST use GID0
> >
> >Please consider this a counter argument and a vote against such a movement.
>
>You are opposing requiring GID0, but are you ok with the current wording
>of RECOMMENDING the GID formed by the IB subnet prefix and the portGUID
>(the one assigned by the manufacturer)?

I can live with RECOMMENDING not with MUST.

>BTW, I'm confused by the use of GID0 as I thought in IBA spec. it means
>the one with the link-local prefix but I thought people wanted to use
>the one with the IB subnet prefix.

GID 0 is just the guaranteed GID to be created via the combination of the 
port GUID (manufacturer assigned GUID) and the subnet prefix.

Mike


>Jerry
>
> >
> >Mike
> >
> >
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>
>
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