I’ve been running an IPv6 tunnel for a long time, but occasionally I’ve been seeing traffic hang on it. It looks like it was the MTU, defaulting at 1500 bytes, causing issues when large amounts of data were being shuffled OUT from my Linux box, back to the ‘net’.
The fix is easy: /etc/network/interfaces should have an “up” line for the interface definition saying: up ip link set mtu 1280 dev henet, where henet is the name of your tunnel interface.
Easy enough to skip this line if your tunnel appears to be working OK, but interesting to track down.