[MPTCP] Questions about DSS checksum

[MPTCP] Questions about DSS checksum

[Geliang Tang]

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In order to solve the issues 134, I'm reading the mptcp.org (branch:
mptcp_v0.95) source code about DSS checksum implementation recently but I
didn't understand it very well.

Here's my first question.

On the outgoing path (net/mptcp/mptcp_output.c), the function
mptcp_write_dss_mapping(), use the following code to generate the checksum
value and write it in the DSS option:

        if (tp->mpcb->dss_csum && data_len) {
                __sum16 *p16 = (__sum16 *)ptr;
                __be32 hdseq = mptcp_get_highorder_sndbits(skb, tp->mpcb);
                __wsum csum;

                *ptr = htonl(((data_len) << 16) |
                             (TCPOPT_EOL << 8) |
                             (TCPOPT_EOL));
                csum = csum_partial(ptr - 2, 12, skb->csum);
                p16++;
                *p16++ = csum_fold(csum_partial(&hdseq, sizeof(hdseq), csum));
        } else {

On the incoming path (net/mptcp/mptcp_input.c), the function
mptcp_parse_options, use the following code to parse MPTCP_SUB_DSS:

                if (mdss->M) {
                        if (mdss->m) {
                                u64 data_seq64 = get_unaligned_be64(ptr);

                                tcb->mptcp_flags |= MPTCPHDR_SEQ64_SET;
                                mopt->data_seq = (u32) data_seq64;

                                ptr += 12; /* 64-bit dseq + subseq */
                        } else {
                                mopt->data_seq = get_unaligned_be32(ptr);
                                ptr += 8; /* 32-bit dseq + subseq */
                        }
                        mopt->data_len = get_unaligned_be16(ptr);

                        tcb->mptcp_flags |= MPTCPHDR_SEQ;

                        /* Is a check-sum present? */
                        if (opsize == mptcp_sub_len_dss(mdss, 1))
                                tcb->mptcp_flags |= MPTCPHDR_DSS_CSUM;

                        /* DATA_FIN only possible with DSS-mapping */
                        if (mdss->F)
                                tcb->mptcp_flags |= MPTCPHDR_FIN;
                }

Here it just sets the MPTCPHDR_DSS_CSUM flag, it doesn't read the checksum
value from the DSS option. So my first question is: How dose it read the value
of the checksum in DSS option on the incoming path?

Please give me some help for it. Thanks.

-Geliang