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mptcp: Use 32-bit DATA_ACK when possible

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RFC8684 allows to send 32-bit DATA_ACKs as long as the peer is not
sending 64-bit data-sequence numbers. The 64-bit DSN is only there for
extreme scenarios when a very high throughput subflow is combined with a
long-RTT subflow such that the high-throughput subflow wraps around the
32-bit sequence number space within an RTT of the high-RTT subflow.

It is thus a rare scenario and we should try to use the 32-bit DATA_ACK
instead as long as possible. It allows to reduce the TCP-option overhead
by 4 bytes, thus makes space for an additional SACK-block. It also makes
tcpdumps much easier to read when the DSN and DATA_ACK are both either
32 or 64-bit.

Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Christoph Paasch
2020-05-14 08:53:03 -07:00
committed by David S. Miller
parent 9efd6a3cec
commit a0c1d0eafd
4 changed files with 31 additions and 10 deletions
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2
net/mptcp/subflow.c
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@@ -667,9 +667,11 @@ static enum mapping_status get_mapping_status(struct sock *ssk)
if (!mpext->dsn64) {
map_seq = expand_seq(subflow->map_seq, subflow->map_data_len,
mpext->data_seq);
subflow->use_64bit_ack = 0;
pr_debug("expanded seq=%llu", subflow->map_seq);
} else {
map_seq = mpext->data_seq;
subflow->use_64bit_ack = 1;
}
if (subflow->map_valid) {