Since we are talking about room stability, I will add my 2c + other facts
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There are two main considerations when hosting. Topology and bandwidth.
There are two topologies in use in GT6, and most other peer to peer games: star and mesh.
Star topology is used when you choose 'fixed room host', and mesh is used otherwise. The advantage of using star is that the host connection is the only critical connection, where a bad peer connection will only cause a problem for that user, and viewers of that car. An example would be cyberstig in R1. However, this means that the host will require NAT1/2 and an excellent upstream connect of at least 150kbps (15 connections x ~10kbps (without voice)). Adding voice to the mix will require an additional ~64kbps per connection, or 1.2Mbps.
Mesh topology is when a new entrant is connected to one or more other entrants (usually not the host). The advantage of this is that the upstream bandwidth is much less critical as it is now balanced through the field, instead of just the host. The main problem with Mesh is the unreliability of some users' upstream, especially if their connection is used for bittorrent, or other peer to peer apps (there are a number of peer to peer video apps as well). If we have a entrant with a bad connection, and the next entrant connects to him, both will be affect, and so on. Resulting in a chain reaction as we have seen many times with a chain of subsequent entrants not seeing all racers.
Mesh is now the standard for peer to peer games as it means we don't need a good host with NAT1 or 2. It just tries until it finds a user in the mesh with a reasonable connection (not necessarily good enough), and it also reduces the peak bandwidth requirements.
On to bandwidth. I was lucky to have ADLS-M and 1.6Mbps upstream. This allows me to easily host. Game data is very small, usually around 10kbps per user. However, if you start adding voice to the mix and fixed host, it quickly adds up and can saturate the link. Switching the mic off does not change the usage, at least in GT6. In most cases, the downstream connection isn't a problem, unless you have other users in your home saturating it. YouTube on a 1Mbps downlink will easily do it.
Anyway, I know what works for me, and it has worked well. PS3, PPPoE, NAT1, fixed host, high quality, no voice. I have a 50/20 3ms connection. Previously I had a 15/2 8ms ADSL2-M connection. Now that I'm on a more stable FTTH (Opticomm crap that was way oversubscribed last year), we can try voice if we really want to (I'm just really annoyed that no one ever stays quiet at the end of the race to wait for the back markers to finish, and of course what disturboed mentioned as well).
PS: NAT1/2/3 is GT game speak.
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