MPTCPv0.95 can't send package with URG

[lebron89wu]

Hi MPTCP team

We used mptcp to send urgent messages, and no URG flag was found in the captured messages. Check the code, the comment says "MPTCP urgent data is not yet supported", so I want to ask why we can't send urgent data and whether we can make mptcp send urgent data?

Thanks!

kernel/common/net/ipv4/tcp_input.c

/* This is the 'fast' part of urgent handling. */
_static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* MPTCP urgent data is not yet supported */
    if (mptcp(tp))
        return;_
...
}

[matttbe]

Hi @lebron89wu

I think you are reporting the issue in the wrong repository. There is no such code here in MPTCP Upstream project: https://github.com/multipath-tcp/mptcp_net-next/blob/export/net/ipv4/tcp_input.c#L5553

Please see our wiki about the different implementations: https://github.com/multipath-tcp/mptcp_net-next/wiki#upstream-vs-out-of-tree-implementations

[matttbe]

Regarding the issue, it seems the URG flag is not supported at MPTCP level. I cannot be enabled if the checksum feature is enabled. We could have URG flag at the subflow level but it is an optional feature that I guess is not supported by this project.

Please see the RFC:

(...) checksumming relies on the TCP subflow containing contiguous data; therefore, a TCP subflow MUST NOT use the Urgent Pointer to interrupt an existing mapping. Further note, however, that if Urgent data is received on a subflow, it SHOULD be mapped to the data sequence space and delivered to the application, analogous to Urgent data in regular TCP.

https://www.rfc-editor.org/rfc/rfc8684.html#section-3.3.1-9

If there is a need, do not hesitate to share patches!

[lebron89wu]

Thank for matttbe's answer, wish your best!