Re: [PATCH 1/4] net: if_arp: add ARPHRD_PUREIP type

[Rocco Yue <rocco.yue@mediatek.com>]

On Thu, 2021-06-24 at 07:29 +0200, Greg KH wrote:
> 
> Thanks for the explaination, why is this hardware somehow "special" in
> this way that this has never been needed before?
> 
> thanks,
> 
> greg k-h
> 

Before kernel-4.18, RAWIP was the same as PUREIP, neither of them
automatically generates an IPv6 link-local address, and the way to
generate an IPv6 global address is the same.

After kernel-4.18 (include 4.18 version), the behavior of RAWIP had
changed due to the following patch:
@@  static int ipv6_generate_eui64(u8 *eui, struct net_device *dev)
+	case ARPHRD_RAWIP:
+		return addrconf_ifid_rawip(eui, dev);
 	}
 	return -1;
}

the reason why the kernel doesn't need to generate the link-local
address automatically is as follows:

In the 3GPP 29.061, here is some description as follows:
"in order to avoid any conflict between the link-local address of
MS and that of the GGSN, the Interface-Identifier used by the MS to
build its link-local address shall be assigned by the GGSN. The GGSN
ensures the uniqueness of this Interface-Identifier. Then MT shall
then enforce the use of this Interface-Identifier by the TE"

In other words, in the cellular network, GGSN determines whether to
reply to the Router Solicitation message of UE by identifying the
low 64bits of UE interface's ipv6 link-local address.

When using a new kernel and RAWIP, kernel will generate an EUI64
format ipv6 link-local address, and if the device uses this address
to send RS, GGSN will not reply RA message.

Therefore, in that background, we came up with PUREIP to make kernel
doesn't generate a ipv6 link-local address in any address generate
mode.

Thanks,
Rocco