[86] in 6.033 discussion
Question on Glassman paper
Saltzer@ATHENA.MIT.EDU (Saltzer@ATHENA.MIT.EDU)
Tue Mar 19 10:47:35 1996
At 9:41 AM 3/19/96, Simon Lau wrote:
> Hi there! What is a caching relay? Is it a server of some sort?
>Glassman kept using the term relay without defining it.
Simon,
Glassman's paper is about a new kind of system component which he calls a
caching relay, so the whole paper is in effect a definition of the term.
He is using the word "relay" in the traditional sense, in which a message
from A to B goes first from A to an intermediary (the relay) who sends it
on to B. Perhaps part of the confusion is that the relay Glassman
discusses is an End-to-End level relay, while the standard network router
can be described as a network level relay.
>Is the relay the
>only link between DEC and the rest of the world? What I mean is that,
does
>anything else besides WWW page requests go thru the caching relay as well?
Your question gets into details of how DEC has designed their firewall to
avoid accidental leakage of internal network traffic to the outside world.
The Overview section says that Gopher requests go through this relay, but
that FTP requests do not. The question doesn't have a crisp answer,
because "Web" requests can be HTTP, FTP, or many other protocols.
But we hoped that the firewall aspects at DEC weren't terribly relevant to
the project we set you to do. Have you found some way in which it matters?
>Or does the caching relay only handles Web requests and routes some of the
>requests to a gateway that connects DEC with the outside world?
Perhaps this will help: The firewall is created by placing a single
network-level relay (called variously a router, a gateway, or a firewall)
between DEC's internal network and the rest of the Internet. That
network-level relay is programmed to discard all packets that aren't coming
from or going to a very short list of computers on the DEC-internal
network. The end-to-end level relays (which are attached to the
DEC-internal network) are on that short list, but most other DEC computers
are not. So the only way a desktop computer can get a web page from
outside DEC is to send the request for the page to the WWW relay.
Jerry Saltzer
