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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.
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