example/6lowpan: expand readme, do not use monitor interface.

Using a raw socket on `monitor0` causes weird results: packets we receive
include FCS, packets we send are parsed as if they didn't have FCS, except
by wireshark which always expects a FCS??

Turns out the sane way is to use raw sockets on normal `wpanX` interfaces,
in which case all packets we send/receive are without FCS.

examples/sixlowpan.rs

@@ -1,27 +1,44 @@
-/*
-
-# Setup
-
-    modprobe mac802154_hwsim
-
-    ip link set wpan0 down
-    iwpan dev wpan0 set pan_id 0xbeef
-    ip link add link wpan0 name lowpan0 type lowpan
-    ip link set wpan0 up
-    ip link set lowpan0 up
-
-    iwpan dev wpan1 del
-    iwpan phy phy1 interface add monitor%d type monitor
-
-# Running
-
-    sudo ./target/debug/examples/sixlowpan
-
-# Teardown
-
-    rmmod mac802154_hwsim
-
- */
+//! 6lowpan exmaple
+//!
+//! This example is designed to run using the Linux ieee802154/6lowpan support,
+//! using mac802154_hwsim.
+//!
+//! mac802154_hwsim allows you to create multiple "virtual" radios and specify
+//! which is in range with which. This is very useful for testing without
+//! needing real hardware. By default it creates two interfaces `wpan0` and
+//! `wpan1` that are in range with each other. You can customize this with
+//! the `wpan-hwsim` tool.
+//!
+//! We'll configure Linux to speak 6lowpan on `wpan0`, and leave `wpan1`
+//! unconfigured so smoltcp can use it with a raw socket.
+//!
+//! # Setup
+//!
+//!     modprobe mac802154_hwsim
+//!
+//!     ip link set wpan0 down
+//!     ip link set wpan1 down
+//!     iwpan dev wpan0 set pan_id 0xbeef
+//!     iwpan dev wpan1 set pan_id 0xbeef
+//!     ip link add link wpan0 name lowpan0 type lowpan
+//!     ip link set wpan0 up
+//!     ip link set wpan1 up
+//!     ip link set lowpan0 up
+//!
+//! # Running
+//!
+//! Run it with `sudo ./target/debug/examples/sixlowpan`.
+//!
+//! You can set wireshark to sniff on interface `wpan0` to see the packets.
+//!
+//! Ping it with `ping fe80::180b:4242:4242:4242%lowpan0`.
+//!
+//! Speak UDP with `nc -uv fe80::180b:4242:4242:4242%lowpan0 6969`.
+//!
+//! # Teardown
+//!
+//!     rmmod mac802154_hwsim
+//!
 
 mod utils;
 
@@ -35,7 +52,7 @@ use smoltcp::phy::{wait as phy_wait, Medium, RawSocket};
 use smoltcp::socket::SocketSet;
 use smoltcp::socket::{UdpPacketMetadata, UdpSocket, UdpSocketBuffer};
 use smoltcp::time::Instant;
-use smoltcp::wire::{IpAddress, IpCidr};
+use smoltcp::wire::{Ieee802154Pan, IpAddress, IpCidr};
 
 fn main() {
     utils::setup_logging("");
@@ -45,7 +62,7 @@ fn main() {
 
     let mut matches = utils::parse_options(&opts, free);
 
-    let device = RawSocket::new("monitor0", Medium::Ieee802154).unwrap();
+    let device = RawSocket::new("wpan1", Medium::Ieee802154).unwrap();
 
     let fd = device.as_raw_fd();
     let device = utils::parse_middleware_options(&mut matches, device, /*loopback=*/ false);
@@ -64,7 +81,10 @@ fn main() {
         64,
     )];
 
-    let mut builder = InterfaceBuilder::new(device).ip_addrs(ip_addrs);
+    let mut builder = InterfaceBuilder::new(device)
+        .ip_addrs(ip_addrs)
+        .dst_pan_id(Ieee802154Pan(0xbeef))
+        .src_pan_id(Ieee802154Pan(0xbeef));
     builder = builder
         .hardware_addr(ieee802154_addr.into())
         .neighbor_cache(neighbor_cache);