Basic tunnel exercise fixes (#625)

* Support negative-valued action parameters
by converting them in the Python helper code into a positive value
whose bit pattern is the 2's complement representation of the negative
value.

* Change send.py for exercise basic_tunnel so its README is correct
The README said to expect a packet with a TCP header at the receiver,
but send.py was not adding a TCP header to the packet when the
`--dst_id` command line option was specified.

---------

Co-authored-by: jafinger <jafinger@cisco.com>

Author: jafingerhut

exercises/basic_tunnel/README.md

@@ -112,7 +112,7 @@ to hosts. For this step you will need to add your forwarding rules to the
   ./send.py 10.0.2.2 "P4 is cool"
   ```
   The packet should be received at `h2`. If you examine the received packet
-  you should see that is consists of an Ethernet header, an IP header, a TCP
+  you should see that it consists of an Ethernet header, an IP header, a TCP
   header, and the message. If you change the destination IP address (e.g. try
   to send to `10.0.3.3`) then the message should not be received by `h2`, and
   will instead be received by `h3`.

exercises/basic_tunnel/send.py

@@ -33,7 +33,7 @@ def main():
     if (dst_id is not None):
         print("sending on interface {} to dst_id {}".format(iface, str(dst_id)))
         pkt =  Ether(src=get_if_hwaddr(iface), dst='ff:ff:ff:ff:ff:ff')
-        pkt = pkt / MyTunnel(dst_id=dst_id) / IP(dst=addr) / args.message
+        pkt = pkt / MyTunnel(dst_id=dst_id) / IP(dst=addr) / TCP(dport=1234, sport=random.randint(49152,65535)) / args.message
     else:
         print("sending on interface {} to IP addr {}".format(iface, str(addr)))
         pkt =  Ether(src=get_if_hwaddr(iface), dst='ff:ff:ff:ff:ff:ff')