8 Nov
2021
8 Nov
'21
11:18 p.m.
On 2021-11-08 16:32, Paul Koning wrote:
Well. The question here becomes:
Is there any relevance you want to attach to the packets as such, or do
you actually intend it to be stream based and then using CR+LF as the
termination of lines? Which also means you need to implement another
layer in all the software to reformat the data into that stream, and
process a line at the time from that.
And of course, for protocols implementing the network virtual terminal,
you then want to escape 0xff, in order to implement all the other
functions that might be implied. And padding of standalone CRs.
And how much should you buffer before you actually should send it?
And for how long?
On Nov 8, 2021, at 10:13 AM, Johnny Billquist
<bqt at softjar.se> wrote:
On 2021-11-08 15:27, Paul Koning wrote:
Streaming mode DECnet sockets would be interesting to learn something about.
Because otherwise the problem is really that DECnet is packet based, and not byte based
like TCP. So there is potentially some problems with adapting TCP protocols for DECnet.
Not really. Stream based protocols are protocols that do not rely on message boundaries
-- more precisely, do not rely on boundaries being marked by lower layers. If you have a
transport that does report message boundaries, and you want to carry a stream protocol,
the simple answer is to ignore those boundaries. On Nov 7,
2021, at 9:54 PM, Peter Lothberg <roll at stupi.com> wrote:
In the "old days" we did SMTP over DECnet, has anyone considered doing Telnet,
FTP, HTTP etc over
DECnet transport?
I've thought about http over DECnet. That would be really
easy. The obvious way to deal with any TCP-based protocol is just to send the byte
streams over DECnet messages, in the same fashion as DECnet/Ultrix streaming mode DECnet
sockets do. (Precisely how that works I don't remember.) In the case of HTTP, a
natural simplification would be to send the entire header, in both directions, as a single
message, with any data following in one or more additional messages. The reason I'm interested in how Ultrix did
streaming sockets is because of the question "when do you send the message".
That's actually a question with TCP as well, which is why it has a "no
delay" option. The trivial answer is "for each send() call to the socket, send
that data as a complete NSP message". That would obviously work; the only question
is whether something fancier is a useful optimization.
You might want to definitely do something smart here sometime. But it's
not trivial how you want to do that.
Actually, there might be one other question. Consider
HTTP: if the server wants to send a file (say, a .jpg image) can it send the entire file
as a single NSP message? From the DECnet architecture point of view, sure. Do DECnet
implementations put some limit on the length of an NSP message? I don't know, except
for RSTS which doesn't simply because it leaves segmentation and reassembly to the
application.
There are definitely limits here.
The problem is that in DECnet, these limits are not global. Consider
RSX. You can send any size you want (well, within limits - DECnet I/O in
RSX is limited to max 8128 bytes). However, the receiver must setup a
receive that is large enough to receive the whole message, or else parts
are lost. (Remember the term "data overrun"?)
Remember - DECnet is packet based. Not stream based. It have more
implications.
Data not received in one call, because the packet was too big are not
received in the next call. They are lost.
Very analogue to magtape.
The mismatch that's harder to deal with is an
application that needs message boundaries, running over a transport that doesn't have
these. The TCP/IP world is full of these, and in every instance the application protocol
concocts an ad-hoc solution to this issue. Consider iSCSI and NFS as two examples, with
different solutions (or multiple solutions, if you take the "markers" hack in
iSCSI seriously).
It's not that easy. See above...
Johnny
--
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