21 Jan
2021
21 Jan
'21
12:06 p.m.
Oh, right...
And TOPS-20 do seem to be getting something wrong.
If they announce a receive buffer size of 1476, the actual payload they
need to be able to receive is 1478, on the ethernet level.
This appears to be totally according to spec.
Thomas might need to go and dig really deep into the TOPS-20 DECnet code
here. Because announcing a blksize of 1476 means that you really should
expect and accept 1478 byte packets (payload). The first two bytes being
a length field for the rest, and thus does not count to the actual
DECnet packet.
RSX announces whatever you configure. The actual receive buffers set up
on the ethernet controller will have 36 additional bytes added, to cover
for potential overhead (which means the ethernet overhead of 16 bytes,
plus the 2 for the length field). Why 36 then, you might ask. Well, just
because there is a standard overhead allocation factor independent of
the link layer, so it's the same everywhere. It should be big enough to
cover the overhead of any link technology. It's just that for ethernet,
the actual overhead is just 18 bytes. But it does mean that it can deal
with packets that are even slightly larger than what has been announced
as acceptable.
Johnny
On 2021-01-21 11:58, Johnny Billquist wrote:
On 2021-01-20 21:06, Paul Koning wrote:
No. tcpdump length information is just the payload length.
Which here is consistent with the receive buffer size.
The receive buffer size is 1476. DECnet adds the two bytes for length
information, which leads to a total payload of 1478, which is also the
size of the packet that tcpdump reports.
The ethernet headers are not included here.
--
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.
My understanding was that even the "broken" code would not be sending
anything close to 1500 bytes. Did I get that wrong?
You should still look at why TOPS-20 is
apparently getting the buffer
length rules wrong.
I would be more interested in why TOPS-20 wants 1476, while most other
things goes for 1498, if we're not going for the 576. Not sure TOPS-20
actually does anything wrong in any other way...
? Johnny