[ipv4] Include network device metadata in packet traces

(Ab)use the "ident" field in transmitted IPv4 packets to convey
metadata about the network device.  In particular:

    bits 0-3 represent the low bits of the "RX" good packet counter
    bits 4-7 represent the low bits of the "RXE" bad packet counter
    bits 8-15 represent the transmitted packet sequence number

This allows some relevant information about the internal state of the
network device to be read out from a packet trace from a non-debug
build of iPXE.  In particular, it allows a packet trace containing
packets transmitted by iPXE to indicate whether or not any packets
have been received by iPXE.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv4.c

@@ -23,8 +23,8 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
-/* Unique IP datagram identification number */
-static uint16_t next_ident = 0;
+/* Unique IP datagram identification number (high byte) */
+static uint8_t next_ident_high = 0;
 
 /** List of IPv4 miniroutes */
 struct list_head ipv4_miniroutes = LIST_HEAD_INIT ( ipv4_miniroutes );
@@ -314,7 +314,6 @@ static int ipv4_tx ( struct io_buffer *iobuf,
 	iphdr->verhdrlen = ( IP_VER | ( sizeof ( *iphdr ) / 4 ) );
 	iphdr->service = IP_TOS;
 	iphdr->len = htons ( iob_len ( iobuf ) );	
-	iphdr->ident = htons ( ++next_ident );
 	iphdr->ttl = IP_TTL;
 	iphdr->protocol = tcpip_protocol->tcpip_proto;
 	iphdr->dest = sin_dest->sin_addr;
@@ -335,6 +334,14 @@ static int ipv4_tx ( struct io_buffer *iobuf,
 		goto err;
 	}
 
+	/* (Ab)use the "ident" field to convey metadata about the
+	 * network device statistics into packet traces.  Useful for
+	 * extracting debug information from non-debug builds.
+	 */
+	iphdr->ident = htons ( ( (++next_ident_high) << 8 ) |
+			       ( ( netdev->rx_stats.bad & 0xf ) << 4 ) |
+			       ( ( netdev->rx_stats.good & 0xf ) << 0 ) );
+
 	/* Determine link-layer destination address */
 	if ( ( rc = ipv4_ll_addr ( next_hop, iphdr->src, netdev,
 				   ll_dest ) ) != 0 ) {