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mptcp: Do TCP fallback on early DSS checksum failure
RFC 8684 section 3.7 describes several opportunities for a MPTCP connection to "fall back" to regular TCP early in the connection process, before it has been confirmed that MPTCP options can be successfully propagated on all SYN, SYN/ACK, and data packets. If a peer acknowledges the first received data packet with a regular TCP header (no MPTCP options), fallback is allowed. If the recipient of that first data packet finds a MPTCP DSS checksum error, this provides an opportunity to fail gracefully with a TCP fallback rather than resetting the connection (as might happen if a checksum failure were detected later). This commit modifies the checksum failure code to attempt fallback on the initial subflow of a MPTCP connection, only if it's a failure in the first data mapping. In cases where the peer initiates the connection, requests checksums, is the first to send data, and the peer is sending incorrect checksums (see #275), this allows the connection to proceed as TCP rather than reset. Fixes: dd8bcd1 ("mptcp: validate the data checksum") Acked-by: Paolo Abeni <pabeni@redhat.com> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
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