[HECnet] Thousands of DECnet errors on Tops-20

[Paul Koning]

> On Jan 17, 2021, at 10:45 PM, Thomas DeBellis <tommytimesharing at gmail.com> wrote:
> 
> I think I may have finally gotten to the bottom of this.  It's a level 1 routing message that I'm getting from 2.1023 (A2RTR) that does not appear to be respecting lengths, viz:
> 
> 22:04:30.749823 aa:00:04:00:ff:0b > ab:00:00:03:00:00, ethertype DN (0x6003), length 1478: lev-1-routing src 2.1023 {ids 0-726 cost 0 hops 0
> 
> This is two (2) bytes over the maximum that Tops-20 can accept.
> NCP>SHOW LINE NI-0 CHARACTERISTICS 
> NCP>
> 22:16:04     NCP
> 
> Request # 23; Show Line Characteristics Completed
> 
> Line = NI-0
> 
>   Receive Buffers = 6
>   Controller = Normal
>   Protocol = Ethernet
>   Hardware Address = 00 1F 16 EC CE 47
>   Receive buffer size = 1476
> It would appear that the 20's are advertising this length in their layer 1 hello messages:
> 
> 22:04:21.018507 aa:00:04:00:0a:0a > ab:00:00:03:00:00, ethertype DN (0x6003), length 60: router-hello l1rout vers 2 eco 0 ueco 0 src 2.522 blksize 1476 pri 5 hello 15
> 22:04:21.082680 aa:00:04:00:08:0a > ab:00:00:03:00:00, ethertype DN (0x6003), length 60: router-hello l1rout vers 2 eco 0 ueco 0 src 2.520 blksize 1476 pri 5 hello 15 

The buffer size in the routing message and in the NCP characteristics (1476) is defined as the size of the routing layer message; it does NOT include the data link layer overhead.  So the 14 byte Ethernet header is not part of that count; neither is the 2 byte DEC Ethernet length field -- which DNA considers a data link field.

I assume the tcpdump report includes everything after the Ethernet header in the reported length, so 1478 is correct, given that it includes the Ethernet length field.  If TOPS is not expecting that, this would be a bug on its part.

That said, I didn't really intend to send packets that big.  The code was supposed to use the minimum of the neighbor buffer sizes or my own, but in one of the two places where the calculation was done the part "my own" was missing.  Fixed in rev 577, so with that you should be seeing 590 byte messages.

You should still look at why TOPS-20 is apparently getting the buffer length rules wrong.

	paul
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