[HECnet] Thousands of DECnet errors on Tops-20

[Johnny Billquist]

Another interesting observation:

All systems I can observe over ethernet (various modern VMS, Ultrix, 
whatever) seem to like to give a maximum data layer block size of 1498.

I suspect that this might be related to the two additional bytes I see 
added in actual communication. Which leads to total packet size of 1500.

I'm going to change my systems to use 1498 based on this.

   Johnny

On 2021-01-20 23:54, Johnny Billquist wrote:
> 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