Re: [PATCH net-next v3] mptcp: fix length of MP_PRIO suboption

[Matteo Croce <mcroce@linux.microsoft.com>]

On Mon, Feb 1, 2021 at 2:08 PM Davide Caratti <dcaratti@redhat.com> wrote:
>
> With version 0 of the protocol it was legal to encode the 'Subflow Id' in
> the MP_PRIO suboption, to specify which subflow would change its 'Backup'
> flag. This has been removed from v1 specification: thus, according to RFC
> 8684 §3.3.8, the resulting 'Length' for MP_PRIO changed from 4 to 3 byte.
>
> Current Linux generates / parses MP_PRIO according to the old spec, using
> 'Length' equal to 4, and hardcoding 1 as 'Subflow Id'; RFC compliance can
> improve if we change 'Length' in other to become 3, leaving a 'Nop' after
> the MP_PRIO suboption. In this way the kernel will emit and accept *only*
> MP_PRIO suboptions that are compliant to version 1 of the MPTCP protocol.
>
>  unpatched 5.11-rc kernel:
>  [root@bottarga ~]# tcpdump -tnnr unpatched.pcap | grep prio
>  reading from file unpatched.pcap, link-type LINUX_SLL (Linux cooked v1)
>  dropped privs to tcpdump
>  IP 10.0.3.2.48433 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options [nop,nop,TS val 4032325513 ecr 1876514270,mptcp prio non-backup id 1,mptcp dss ack 14084896651682217737], length 0
>
>  patched 5.11-rc kernel:
>  [root@bottarga ~]# tcpdump -tnnr patched.pcap | grep prio
>  reading from file patched.pcap, link-type LINUX_SLL (Linux cooked v1)
>  dropped privs to tcpdump
>  IP 10.0.3.2.49735 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options [nop,nop,TS val 1276737699 ecr 2686399734,mptcp prio non-backup,nop,mptcp dss ack 18433038869082491686], length 0
>
> Changes since v2:
>  - when accounting for option space, don't increment 'TCPOLEN_MPTCP_PRIO'
>    and use 'TCPOLEN_MPTCP_PRIO_ALIGN' instead, thanks to Matthieu Baerts.
> Changes since v1:
>  - refactor patch to avoid using 'TCPOLEN_MPTCP_PRIO' with its old value,
>    thanks to Geliang Tang.
>
> Fixes: 067065422fcd ("mptcp: add the outgoing MP_PRIO support")
> Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
> Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Davide Caratti <dcaratti@redhat.com>

Reviewed-by: Matteo Croce <mcroce@linux.microsoft.com>

-- 
per aspera ad upstream