netdev - Re: [PATCH net-next v2] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

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Date:   Mon, 2 Aug 2021 07:37:08 -0600
From:   David Ahern <dsahern@...il.com>
To:     Rocco Yue <rocco.yue@...iatek.com>,
        David Ahern <dsahern@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, rocco.yue@...il.com,
        chao.song@...iatek.com, zhuoliang.zhang@...iatek.com
Subject: Re: [PATCH net-next v2] ipv6: add IFLA_INET6_RA_MTU to expose mtu
 value in the RA message

On 8/1/21 9:19 PM, Rocco Yue wrote:
> On Sat, 2021-07-31 at 11:17 -0600, David Ahern wrote:
> On 7/30/21 7:52 PM, Rocco Yue wrote:
>>> In this way, if the MTU values that the device receives from
>>> the network in the PCO IPv4 and the RA IPv6 procedures are
>>> different, the user space process can read ra_mtu to get
>>> the mtu value carried in the RA message without worrying
>>> about the issue of ipv4 being stuck due to the late arrival
>>> of RA message. After comparing the value of ra_mtu and ipv4
>>> mtu, then the device can use the lower MTU value for both
>>> IPv4 and IPv6.
>>
>> you are storing the value and sending to userspace but never using it
>> when sending a message. What's the pointing of processing the MTU in the
>> RA if you are not going to use it to control message size?
> 
> Hi David,
> 
> In the requirement of mobile operator at&t in 2021:
> AT&T <CDR-CDS-116> Prioritize Lower MTU value:
> If the MTU values that the device receives from the network in the PCO
> IPv4 <CDR-CDS-110> and the RA IPv6 <CDR-CDS-112> procedures are different,
> then the device shall use the lower MTU value for both IPv4 and IPv6.
> 
> And in the 3GPP 23.060:
> The PDP PDUs shall be routed and transferred between the MS and the GGSN
> or P-GW as N-PDUs. In order to avoid IP layer fragmentation between the
> MS and the GGSN or P-GW, the link MTU size in the MS should be set to the
> value provided by the network as a part of the IP configuration. This
> applies to both IPv6 and IPv4.
> 
> That means user needs to be able to correctly read the mtu value carried
> in the RA message so that user can correctly compare PCO ipv4 mtu and
> RA ipv6 mtu.
> 

Then userspace should get a link notification when ra_mtu is set so it
does not have to poll.