Re: GID clarifications

Kanoj,

The port's GUID is it's EUI-64 identifier. A GID, in turn can be formed 
using this GUID, the CA's GUID or some other value. Any GID is unique in 
the IB fabric however only by using the port's GUID/EUI-64 identifier can 
one uniquely identify the port. The other GID's, as noted, can be applied 
to other ports too - though not concurrently. The GID formed using the 
EUI-64 identifier is the 'GID at index 0'. I've made note of your comment 
and can clarify further as needed.

wrt, using only GID at index 0 as in the MAC address, there have been 
proponents of using the other GIDs (non EUI-64 GID).

Vivek
--
Vivek Kashyap
Linux Technology Center, IBM
[email protected] 
[email protected] 
Ph: 503 578 3422 T/L: 775 3422





Kanoj Sarcar <[email protected]>
Sent by: [email protected]
12/02/2003 03:54 PM
Please respond to Kanoj.Sarcar

 
        To:     [email protected]
        cc: 
        Subject:        [Ipoverib] GID clarifications



Hi,

It seems that certain parts of the 
draft-ietf-ipoib-ip-over-infiniband-05.txt draft might
require clarification. For this discussion, please refer to

1. section 9.1 Link Information says:
"The GID, if formed using the GUID, can be used to unambiguously 
identify an endpoint."

2. section 9.1.1 Link Layer Address/Hardware Address, part c) says:
"It is RECOMMENDED that the GID formed by the combination of the
IB subnet prefix and the port's GUID be included in the 
link-layer/hardware address."

Can comment #2 be safely taken to mean that GID index 0 is
being RECOMMENDED? Port GUID seems somewhat ambiguous, since I 
am not sure (after reading IBA 1.1 section 4.1) whether the
term GUID refers only to the manufacturer assigned value that appears
in lower 64 bits of GID index 0, or whether it also includes 
lower 64 bits of SM assigned GIDs at higher indices. If GID index 0
was really intended, the comment should explicitly state so to prevent
confusion. 

Also, to make things simple, is it reasonable to REQUIRE implementations
to always use GID index 0 (maybe this depends on whether existing
implementations all use GID0 only)? The reason this might be 
interesting is that when a node receives a packet without a GRH 
(whereby it already knows the sender's LID and QPN), it can possibly 
reverse lookup the sender's LID to identify the sender's GID index 
0 and concatenate the sender's QPN to figure out the sender's link 
address. Thus, a receiver can always figure out the sender's address, 
even when a GRH is not present. This would be similar to ethernet, 
where a receiver can always determine a sender's address, and thus
is more DLPIv2 (unitdata_indication) friendly.

Regarding comment #1, what does "unambiguously identify an endpoint"
mean? GIDs are unique in the subnet; and if an IP link contains
members in multiple IBA subnets, the subnet prefixes will disambiguate
which endport is being referred. 

Thanks.

Kanoj

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