Applications Messaging Standard

[Tony Linde]

What if the user is running some other software which already makes use of
that port?

T.

> -----Original Message-----
> From: owner-apps at eso.org [mailto:owner-apps at eso.org] On 
> Behalf Of Mike Fitzpatrick
> Sent: 17 February 2007 05:15
> To: Patrick Dowler
> Cc: apps at ivoa.net
> Subject: Re: Applications Messaging Standard
> 
> On 2/16/07, Patrick Dowler <patrick.dowler at nrc-cnrc.gc.ca> wrote:
> 
> > On a multi-user machine, the initial hand-shake could 
> exchange the user-name
> > to make sure client and hub are running under the same user 
> (spoofable, but
> > we aren't trying to make this overly secure, just simple for users).
> >
> > If there is a collision on the port in a multi-user 
> environment, just
> > increment the port number and try again. So clients and 
> hubs start by
> > connecting to port N and checking the user; if they connect 
> and find the same
> > user is running the hub, the client proceeds and the (2nd) 
> hub terminates
> > quietly (or not). If the new hub doesn't find anything, it 
> tries to take the
> > port (starts listening). If the client or hub finds a hub 
> but user doesn't
> > match, they try again on port N+1. If the client doesn't 
> find a hub, it
> > probably has to check higher port numbers (in case that 
> users hub was not the
> > first to start and hence is on port N+something). For 
> sanity and quick
> > failure, one would want relatively short socket connection 
> timeouts and to
> > limit the number of times one would increment the port 
> number and try again.
> 
> Admittedly it will be a rare case, but I think I see a hole in this
> scheme:  Assume
> two users start msg-enabled apps at about the same time, so 
> the first users
> get the well-know port (say 2000) and the second user gets 
> (N+1, 2001).  If
> the first user shuts down then port 2000 is now free, if the 
> second user then
> starts another msg-enabled app in their session it will try to connect
> to port 2000
> and/or possibly start a second hub since the port is free and it has
> no reason to
> look for N+1.
> 
> How about a scheme where the address is some agreed base value (call
> it 2000) but apps/hubs use the userid as an offset?  This guarantees a
> unique
> port for each user, can be overridden by an env var to start 
> a separate session,
> and avoids "port scanning" since a user will only every try a single
> port number.
> I dont know if Windows has a concept of 'userid' but they 
> could just fallback to
> the base address anyway.  Assuming the userid is the same 
> across a LAN all
> we need is a host name to connect to "our" hub on a remote 
> machine since
> we'll already know what the port will be.
> 
> -Mike
> 

http://www.Taglocity.com Tags: IVOA, apps