21 Jan
2021
21 Jan
'21
12:11 a.m.
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:
--
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
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