Re: [PATCH 1/1] Add 'Transport Interface' (triface) option. This can be used to specify the IP interface to use for the connection. The driver uses that to set SO_BINDTODEVICE on the socket before connecting.

[Hannes Reinecke <hare@suse.de>]

On 5/11/21 7:13 PM, Sagi Grimberg wrote:
> 
>> I've been thinking about what you said regarding the need to repeat 
>> the -w option when two interfaces share the same IP address. I think 
>> we're looking at the problem from a different point of view. The 
>> current implementation uses an IP address to identify an interface. I, 
>> on the other hand, believe that the best way to identify an interface 
>> is by its "interface name or index". In previous emails, I provided 
>> examples of the problems that may occur when using an IP address to 
>> identify an interface. For example, one can assign the same IP address 
>> to different interfaces making it impossible to distinguish interfaces 
>> by their IP address alone. Another example is that the low level APIs 
>> (e.g. setsockopt(SO_BINDTODEVICE) don’t even require the source IP 
>> address. They only need the interface name/index. So, why go through 
>> the trouble of performing a reverse address lookup to retrieve the 
>> interface name/index when the address is not used at all?
>>
>> By the way, if nvme-cli/linux-nvme allowed specifying interfaces by 
>> name/index, then we would not really need to repeat the -w option 
>> unless we also wanted to set the source address at the same time. 
>> Setting the source address is a completely different thing from 
>> setting the interface. One should be allowed to set one independently 
>> from the other, or both, or none.
>>
>> If you look at how ping is implemented, they do not infer the 
>> interface from the IP address. If one wants to force ping to go over 
>> an interface, then one must provide the interface by name/index using 
>> the -I option. If one wants to change the source IP address (without 
>> forcing a specific interface), then one provides the IP address to the 
>> -I option. It's simple and intuitive. And ping also supports appending 
>> the interface to the Destination IP using the '%' delimiter for 
>> IPv6-only as per RFC4007.
>>
>> I think that nvme-cli/linux-nvme should follow the ping approach. 
>> Interfaces should never be inferred from source IP addresses, but 
>> instead be clearly identified by their name or index. And setting the 
>> source address should be independent from setting the interface.
> 
> I'm starting to think that we are going in circles, I'm getting to
> the point that having host_iface is the right way to go.
> 
> We can have nvme-cli convert <addr>%iface notation to
> "..,host_traddr=<addr>,host_iface=<iface>,.." when creating the
> controller string...

That was my thinking, too; the only _other_ way would be to have a 
host_traddr _imply_ a binding to the underlying interface.
We could define that, but having a single option doing _two_ different 
things is not good software design.
So let's do the 'host_iface' thingie.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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