selftests: mptcp: capture pcap on both sides

When investigating performance issues that involve latency / loss /
reordering it is useful to have the pcap from the sender-side as it
allows to easier infer the state of the sender's congestion-control,
loss-recovery, etc.

Allow the selftests to capture a pcap on both sender and receiver so
that this information is not lost when reproducing.

This patch also improves the file names. Instead of:

  ns4-5ee79a56-X4O6gS-ns3-5ee79a56-X4O6gS-MPTCP-MPTCP-10.0.3.1.pcap

We now have something like for the same test:

  5ee79a56-X4O6gS-ns3-ns4-MPTCP-MPTCP-10.0.3.1-10030-connector.pcap
  5ee79a56-X4O6gS-ns3-ns4-MPTCP-MPTCP-10.0.3.1-10030-listener.pcap

It was a connection from ns3 to ns4, better to start with ns3 then. The
port is also added, easier to find the trace we want.
Co-developed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -395,10 +395,14 @@ do_transfer()
 			capuser="-Z $SUDO_USER"
 		fi
 
-		local capfile="${listener_ns}-${connector_ns}-${cl_proto}-${srv_proto}-${connect_addr}.pcap"
+		local capfile="${rndh}-${connector_ns:0:3}-${listener_ns:0:3}-${cl_proto}-${srv_proto}-${connect_addr}-${port}"
+		local capopt="-i any -s 65535 -B 32768 ${capuser}"
 
-		ip netns exec ${listener_ns} tcpdump -i any -s 65535 -B 32768 $capuser -w $capfile > "$capout" 2>&1 &
-		local cappid=$!
+		ip netns exec ${listener_ns}  tcpdump ${capopt} -w "${capfile}-listener.pcap"  >> "${capout}" 2>&1 &
+		local cappid_listener=$!
+
+		ip netns exec ${connector_ns} tcpdump ${capopt} -w "${capfile}-connector.pcap" >> "${capout}" 2>&1 &
+		local cappid_connector=$!
 
 		sleep 1
 	fi
@@ -423,7 +427,8 @@ do_transfer()
 
 	if $capture; then
 		sleep 1
-		kill $cappid
+		kill ${cappid_listener}
+		kill ${cappid_connector}
 	fi
 
 	local duration