Re: Multicasting ion about cu-seeme

Bob Quinn (
Fri, 1 Dec 95 14:10:06 EST

> The reflector, since it runs on a UNIX platform, is able to take advantage of
> the one to many functionality of MULTICASTING on those UNIX platforms that
> have MULTICAST capability. Unfortunately, the PC's and MAC's don't yet have
> this capability in ther TCP/IP stacks, so the reflector can only use
> MULTICASTING with NV and VAT, and to connect to other reflectors.
> As MULTICASTING becomes available on PC and MAC platforms, rest assured that
> we will have versions of CU-SeeME that will take advantage of it.

Actually, there are three PC TCP/IP stack vendors that support
multicast now, with WinSock APIs that are compatible with Steve
Deering's original (UNIX) multicast API (the setsockopt() options):

- FTP Software, Inc (for Win 3.1 and Win95)
- Microsoft (for WFWG, Win95 and WinNT)
- TGV (sorry, I don't have details)

There is one problematic difference between Microsoft's impele-
mentation, and FTP Software's: the values for the multicast
socket options differ. Microsoft uses Steve Deering's original
values (e.g. IP_MULTICAST_IF=2), and we use the Berkeley values
(e.g. IP_MULTICAST_IF=9). We did so, because we also support
the Berkeley IP_TTL socket option (value=4), which conflicts
with Deering's IP_MULTICAST_LOOP option. From the code sample
it looks like TGV supports the Deering values also.

WinSock 2 supports both the Deering multicast API, and a new
protocol independent API. The WinSock 2 Protocol Specific
Annex specifies the Berkeley values for IP multicast.

For more details on multicast see no excuses for not having a server for WinSock... :-)


 Bob Quinn                                   
 FTP Software, Inc.                                No. Andover, MA