Correctly estimate the initial MTU for legacy packets.

[tinc] / src / net_packet.c

diff --git a/src/net_packet.c b/src/net_packet.c
index 37535c1..8bf399f 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -929,6 +929,25 @@ static length_t choose_initial_maxmtu(node_t *n) {
                mtu -= SPTPS_DATAGRAM_OVERHEAD;
                if((n->options >> 24) >= 4)
                        mtu -= sizeof(node_id_t) + sizeof(node_id_t);
+       } else {
+               mtu -= digest_length(n->outdigest);
+
+               /* Now it's tricky. We use CBC mode, so the length of the
+                  encrypted payload must be a multiple of the blocksize. The
+                  sequence number is also part of the encrypted payload, so we
+                  must account for it after correcting for the blocksize.
+                  Furthermore, the padding in the last block must be at least
+                  1 byte. */
+
+               length_t blocksize = cipher_blocksize(n->outcipher);
+
+               if(blocksize > 1) {
+                       mtu /= blocksize;
+                       mtu *= blocksize;
+                       mtu--;
+               }
+
+               mtu -= 4; // seqno
        }
 
        if (mtu < 512) {