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List:       linux-net
Subject:    Re: Ethernet MTU and CRC questions.
From:       Ben Greear <greearb () candelatech ! com>
Date:       2000-07-30 0:56:27
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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
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