Thanks for the info. Tcpdump is now reporting a pkt of 1518 leaving, but only 1514 received. Could it be that it's looking at a checksum? 17:59:38.231988 > 0:0:0:0:0:0 0:48:54:85:2f:b8 1111 1514: 17:59:38.232058 > 0:0:0:0:0:0 0:48:54:85:2f:b8 1111 1514: 17:59:38.356113 < 0:20:78:e0:24:fe 0:0:0:0:0:1 1111 1518: 17:59:38.356357 < 0:48:54:66:68:68 0:0:0:0:0:1 1111 1518: NOTE: I only see this on RTL1839 cards, the other card reports TX and RX to be 1514. I'm definately attempting to TX 1514, not counting the CRC. Matti Aarnio wrote: > > On Sat, Jul 29, 2000 at 04:39:59PM -0700, Ben Greear wrote: > > I'm using a PF_PACKET, SOCK_RAW socket to send ethernet packets. > > > > I write the dst MAC address, src MAC address, and proto into a buffer, > > then append my payload. I send the whole thing to the socket, using > > a length that includes the ethernet header and my payload. > > > > But, I think I've been using MTU wrong the whole time. > > > > Assuming a 'normal' MTU of 1500 on an ethernet NIC. Does that mean > > I have 14 bytes of ethernet payload, 1500 bytes of payload, and 4 bytes of > > ethernet CRC (I don't specify the CRC, I assume the driver appends that?), or > > does it mean I have 14 bytes of header, 1486 bytes of payload and a CRC? > > 1500 bytes of IP level payload. The MACs, type field and CRC > are extra. For transmit you need 6+6+2+1500 bytes for max > size IP frame. > > > Am I correct in assuming that the CRC is taken care of for me? > > For transmit it is generated by the hardware, for receive > it is checked by the hardware, and if found invalid, usually > the packet is discarded adding "frame" errors counter. > > > Can I see the CRC in user-space? > > Propably yes. The last 4 bytes of the received packet at the > ethernet drivers typically carry the CRC. Not always, though! > > > Thanks, > > Ben > > > > -- > > Ben Greear (greearb@candelatech.com) http://www.candelatech.com > > Author of ScryMUD: scry.wanfear.com 4444 (Released under GPL) > > http://scry.wanfear.com http://scry.wanfear.com/~greear > > /Matti Aarnio <matti.aarnio@zmailer.org> -- Ben Greear (greearb@candelatech.com) http://www.candelatech.com Author of ScryMUD: scry.wanfear.com 4444 (Released under GPL) http://scry.wanfear.com http://scry.wanfear.com/~greear - : send the line "unsubscribe linux-net" in the body of a message to majordomo@vger.rutgers.edu