[openib-general] Problem is routing CM REQ

[Michael Krause]

At 03:55 PM 2/13/2007, Sean Hefty wrote:
> >A LID is subnet local on that we can all agree.   The CM Req contains
> >either the LID of a local subnet CA or the LID a local router which will
> >move the packet to the next hop to the destination.   12.7.11 is basically
> >saying that the remote LID is the router's LID of the local subnet's router
> >Port.   12.7.21 also refers to the remote LID but in each subnet that is
> >either the router Port's LID or the destination CA.
>
>This isn't my interpretation.
>
>12.7.11 Local Port LID:  When local and remote ports are on different subnets,
>this field must be the LID of the router that the *passive* side will 
>target for
>the return path.
>
>The CM REQ carries the LIDs for the remote (passive side) subnet.  This is 
>what
>the passive side needs to configure the QP, not the active side LID 
>information.
>(See address vector information for 11.2.4.2 - page 574.)
>
>So, the CM REQ is _sent_ to either the LID of the local subnet CA or the 
>LID of
>a local router port, but _contains_ the LIDs from the remote subnet.

  In volume 1, version 1.2, page 574 it states:

Emacs!


12.7.11
Emacs!

Both of these statements refer to the local subnet's LID for the router 
port being used by the local CA to communicate to a remote subnet.   The IB 
architecture is built upon the concept that no subnet local information 
knowledge is required beyond the subnet itself to establish communication 
across subnets.   Perhaps the various wordings are a bit confusing but the 
CM protocol should not be concerned with a remote subnet's LID or any 
validation of such remote subnet information.   All it needs to do is 
communicate what is global so that a remote endnode can respond 
correctly.  It is up to the router and the associated router protocol to 
perform any global to subnet local mapping which includes the LID and LRH 
generation.   The router must work with each subnet's SM / SA to provide 
the necessary global to subnet local mappings which are then queried by the 
CM agent to find the appropriate router Port.   There is no requirement in 
the specification to ever communicate across a subnet anything that is 
strictly subnet local.  LID is a strictly subnet local value and is not 
shared.  Again, the passive here refers to the subnet local router LID and 
not the remote subnet's LID.

Mike 
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