[MUD-Dev] UDP vs TCP/IP

[J C Lawrence]

On Fri, 22 Jun 2001 17:48:38 +0100 
Daniel Harman <Daniel.Harman at barclayscapital.com> wrote:

> Anyone got a take on UDP vs TCP for graphical MMORPGs?

I suspect you are going to find that there are multiple packet
streams running over multiple parallel connections; some TCP, some
UDP.  Incremental state change messages are likely to be over UDP as
you can interpolate across drops and you already have periodic or
on-request current-state frames.  Dropped packets however on certain
classes of player commands (eg SAY) are conversely not tolerable and
thus will likely run over TCP.

Note: There is nothing to say you can't implement your own overlay
protocol atop IP with customised error handling etc such that
different packet streams have different levels and fallback patterns
in error conditions (ignore errors, minimal only-if-easy correction,
aggressive correction and interpolation, full guarantees, etc).
I've spoke to a number of content distribution companies that are
doing precisely this for their products/services.  It makes sense to
extend the same basic ideas to large game servers.

> I always assumed UDP was the obvious choice for the majority of
> the data (if not all), seeing as old missed packets don't really
> need resending for a lot of the information (do you care where
> player y was standing 3 updates ago?).

Some player commands are sensitive to being dropped.  Other's are
not as sensitive (arguably they're all sensitive to being discarded,
but are not all sensitive to correct sequence processing).

> In addition, I imagine TCP might increase ones bandwidth costs as
> you don't have control of the resends.

You can also build protocol stacks that dynamically shift between
UDP and TCP as the packet loss rates vary.

--
J C Lawrence                                       claw at kanga.nu
---------(*)                          http://www.kanga.nu/~claw/
The pressure to survive and rhetoric may make strange bedfellows
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