[HECnet] Thousands of DECnet errors on Tops-20

[Johnny Billquist]

Hey, speaking of this...

I decided to go and check how RSX actually works. And that have led to 
some interesting observations on my side.

1. RSX actually do respect the maximum data layer block size the other 
end sends in its init message. So even if I have a large size defined in 
RSX, it should be working correct against another end with a smaller 
block size.

2. With a block size of 576 (either local or remote), the actual packets 
being sent are 578 bytes. Seems there is a 2 byte checksum that is in 
addition to the actual payload. I found this a bit surprising, but it 
seems this might be very intentional. So I'm trying to see if this also 
is true for other systems. If someone else can do some checking that 
would also be interesting.

3. VMS V5.4 seem to not be happy at all if the other end declares a 
largee maximum data layer block size that the local one. This is the 
problem I had when talking to a VAX at Peter, which is running V5.4. I 
have no idea if that problems is still there in V7...

It would be really interesting to have a Multinet VMS machine setup in 
the same area as me, with a link to RSX, so I could do some more testing...

   Johnny

On 2021-01-20 21:06, Paul Koning wrote:
> 
> 
>> On Jan 17, 2021, at 10:45 PM, Thomas DeBellis 
>> <tommytimesharing at gmail.com <mailto: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

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol