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PlayStation party chat/Destiny asymmetric communication failures/packet loss

glasshalfempty
Newbie
Posts: 1
Registered: 18-03-2015

PlayStation party chat/Destiny asymmetric communication failures/packet loss

Well, tried logging a service desk call, but apparently problems like this aren't supported by plusnet, so I'm supposed to ask the community. (Never got that response from Zen Internet...!)
The problem: I play Destiny with a number of people, and we use the PlayStation party chat. Up to something like a month ago, no problems.
Then, one of the people (T) said he that either he couldn't hear me, or I was coming through distorted. However, I could hear T perfectly well. In that same session, another player (A) could hear me fine, and I could hear him. And T and A could hear each other.
Fine, leave the party and rejoin (Sony's suggestion): no difference. Try again the following day: the same problem. And every time since. Always the same thing: T can't hear me properly, but everything else is working fine. Both T and A are VirginMedia customers (albeit connected to different hubs - T is on Southampton, A is on Horndean). Both have Asus N66 routers.
I use an Asus N55U router configured with PPPoE, no DMZ (not something I really want to enable), port forwarding configured via uPNP.
The router shows me the uPNP configured port forwarding and it is set up identically (barring port number) for T (which isn't working) and A (which is). I have tried manually setting the port forwarding - no difference.
I have tried forcing the PlayStation to use a reduced MTU (it moaned at 1400, but actually made no difference; also set it to the one the router uses - 1492 - no difference).
T enabled ping response on his router: ICMP pings are going through fine, and I saw no drops either with standard ping packet size, or maximal packet size with "don't fragment" set - even when the chat wasn't working. tracert doesn't show anything particularly surprising - it does show routing to T going via different routes inside the PlusNet network, but that's not unexpected.
If I had to guess, I would say that the UDP packets used for Party Chat are being dropped somewhere between my router and T's router, or that the multiple routes are resulting in out-of-order delivery, but without seeing what is going on inside the PlusNet and VirginMedia networks, that can't be anything more than a guess.
We are also experiencing glitching in the game itself (which looks like loss of game data packets, but that's harder to be certain of.
Fibre modem, router, and PlayStation have all been turned off and restarted multiple times. There is plenty of bandwidth available (in game play, upstream and downstream traffic is less than 60KB/s).
The problem is not unique to T - chat also breaks up with random other people sometimes - but he's the one person it always fails with. Another player (D), also a plusnet user, has repeated problems connecting to A and T, (but usually resetting stuff improves things for a while, and then it goes wrong again), but is always connected to me fine. Another player (B), a Sky customer, works fine. T has no problem communicating with A or B (or, for that matter, a bunch of people in the US - just me who lives 5 miles away).
I originally assumed local router misconfiguration, but I can't see how that can be given that it works with some people and not others.
If anybody in plusnet support wants the IP addresses of A and T I can post them (they are also on the original ticket I raised).
Help?! I can post anything else that is required (and all of us are willing to fiddle around to try things out - within reason - and are all software engineers with an amount of networking knowledge).
Phil