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more work on SLIP encoding/decoding for packet radio interfaces.

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This commit is contained in:
gardners 2013-02-05 07:38:50 +10:30 committed by Jeremy Lakeman
parent 37ed7694bd
commit 7f0a22408a
1 changed files with 53 additions and 26 deletions
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79
overlay_packetradio.c
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@ -5,7 +5,6 @@
/* interface decoder states. broadly based on RFC1055 */
#define DC_NORMAL 0
#define DC_ESC 1
#define DC_END 2
/* SLIP-style escape characters used for serial packet radio interfaces */
#define SLIP_END 0300
@ -115,9 +114,8 @@ void overlay_packetradio_poll(struct sched_ent *alarm)
The challenge is that we need to make sure that the packet encapsulation
is self-synchronising in the event that a data error occurs (including
failure to receive an arbitrary number of bytes).
For now we will reuse the antiquated and sub-optimal method described in
RFC1055 for SLIP, but with a couple of tweaks to reduce byte wastage when
encountering ESC and END characters in packets.
For now we will reuse the functional but sub-optimal method described in
RFC1055 for SLIP.
*/
int i;
for(i=0;i<nread;i++)
@ -127,28 +125,15 @@ void overlay_packetradio_poll(struct sched_ent *alarm)
// escaped character
switch(buffer[i]) {
case SLIP_ESC_END: // escaped END byte
overlay_rx_packet_append_byte(interface,SLIP_END); break;
case SLIP_ESC_ESC: // escaped escape character
overlay_rx_packet_append_byte(interface,SLIP_ESC); break;
default: /* unknown escape character
This is where the inefficiency comes, because
we don't use the spare bits.
We can reduce the inefficiency by making ESC <otherbyte>
mean literally that. */
overlay_rx_packet_append_byte(interface,SLIP_ESC);
overlay_rx_packet_append_byte(interface,buffer[i]);
break;
}
break;
case DC_END:
// character preceeded by END character
switch(buffer[i]) {
case SLIP_ESC_END:
overlay_rx_packet_complete(interface);
break;
default:
overlay_rx_packet_append_byte(interface,SLIP_END);
overlay_rx_packet_append_byte(interface,buffer[i]);
break;
case SLIP_ESC_ESC: // escaped escape character
overlay_rx_packet_append_byte(interface,SLIP_ESC);
break;
default: /* Unknown escape character. This is an error. */
if (config.debug.packetradio)
WARN("Packet radio stream contained illegal escape sequence -- ignoring packet.");
interface->recv_offset=0;
break;
}
break;
@ -158,7 +143,7 @@ void overlay_packetradio_poll(struct sched_ent *alarm)
case SLIP_ESC:
interface->decoder_state=DC_ESC; break;
case SLIP_END:
interface->decoder_state=DC_END; break;
overlay_rx_packet_complete(interface);
default:
overlay_rx_packet_append_byte(interface,buffer[i]);
}
@ -180,6 +165,48 @@ int overlay_packetradio_tx_packet(int interface_number,
unsigned char *bytes,int len)
{
if (config.debug.packetradio) DEBUGF("Sending packet of %d bytes",len);
/*
This is a bit interesting, because we have to deal with RTS/CTS potentially
blocking our writing of the packet.
For now, just try to write it, and if we only write part of it, then so be it.
We will surround each packet with SLIP END characters, so we should be able to
deal with such truncation in a fairly sane manner.
*/
if (len>OVERLAY_INTERFACE_RX_BUFFER_SIZE) {
if (config.debug.packetradio) WHYF("Not sending over-size packet");
return -1;
}
/* Encode packet with SLIP escaping.
XXX - Add error correction here also */
char buffer[len*2+4];
int out_len=0;
int i;
buffer[out_len++]=SLIP_END;
for(i=0;i<len;i++)
{
switch(bytes[i]) {
case SLIP_END:
buffer[out_len++]=SLIP_ESC;
buffer[out_len++]=SLIP_ESC_END;
break;
case SLIP_ESC:
buffer[out_len++]=SLIP_ESC;
buffer[out_len++]=SLIP_ESC_ESC;
break;
default:
buffer[out_len++]=bytes[i];
}
}
buffer[out_len++]=SLIP_END;
DEBUGF("Encoded length is %d",out_len);
return 0;
}