net: implement ECN handling, rfc6040 stylefd/propagate-DSCP-bits

To decide whether we should use the compatibility mode or the normal
mode with a peer, we use the handshake messages as a signaling channel.

If we receive the expected ECN bits, it most likely means they're
running a compatible version.

Signed-off-by: Florent Daigniere <nextgens@freenetproject.org>

1 files changed, 59 insertions, 0 deletions

diff --git a/misc.go b/misc.go
index 6786cb5..e5688a5 100644
--- a/misc.go
+++ b/misc.go
@@ -46,3 +46,62 @@ func min(a, b uint) uint {
 	}
 	return a
 }
+
+// called from receive
+func ecn_rfc6040_egress(inner byte, outer byte) (byte, bool) {
+	/*
+	+---------+------------------------------------------------+
+	|Arriving |            Arriving Outer Header               |
+	|   Inner +---------+------------+------------+------------+
+	|  Header | Not-ECT | ECT(0)     | ECT(1)     |     CE     |
+	+---------+---------+------------+------------+------------+
+	| Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)|
+	|  ECT(0) |  ECT(0) | ECT(0)     | ECT(1)     |     CE     |
+	|  ECT(1) |  ECT(1) | ECT(1) (!) | ECT(1)     |     CE     |
+	|    CE   |      CE |     CE     |     CE(!!!)|     CE     |
+	+---------+---------+------------+------------+------------+
+	*/
+	innerECN := CongestionExperienced & inner
+	outerECN := CongestionExperienced & outer
+
+	switch outerECN {
+	case CongestionExperienced:
+		switch innerECN {
+		case NotECNTransport:
+			return 0, true
+		}
+		return (inner  & (CongestionExperienced ^ 255)) | CongestionExperienced, false
+	case ECNTransport1:
+		switch innerECN {
+		case ECNTransport0:
+			return (inner  & (CongestionExperienced ^ 255)) | ECNTransport1, false
+		}
+	}
+	return inner, false
+}
+
+// called from send
+func ecn_rfc6040_ingress(inner byte, useNormalMode bool) byte {
+	/*
+	+-----------------+-------------------------------+
+	| Incoming Header |    Departing Outer Header     |
+	| (also equal to  +---------------+---------------+
+	| departing Inner | Compatibility |    Normal     |
+	|     Header)     |     Mode      |     Mode      |
+	+-----------------+---------------+---------------+
+	|    Not-ECT      |   Not-ECT     |   Not-ECT     |
+	|     ECT(0)      |   Not-ECT     |    ECT(0)     |
+	|     ECT(1)      |   Not-ECT     |    ECT(1)     |
+	|       CE        |   Not-ECT     |      CE       |
+	+-----------------+---------------+---------------+
+	*/
+	if !useNormalMode {
+		inner &= (CongestionExperienced ^ 255)
+	}
+
+	return inner
+}
+
+func ecn_rfc6040_enabled(tos byte) bool {
+	return (CongestionExperienced & tos) == ECNTransport0
+}