Stream: udp streams.

Stream: udp streams.

Previously, only one client packet could be processed in a udp stream session
even though multiple response packets were supported.  Now multiple packets
coming from the same client address and port are delivered to the same stream
session.

If it's required to maintain a single stream of data, nginx should be
configured in a way that all packets from a client are delivered to the same
worker.  On Linux and DragonFly BSD the "reuseport" parameter should be
specified for this.  Other systems do not currently provide appropriate
mechanisms.  For these systems a single stream of udp packets is only
guaranteed in single-worker configurations.

The proxy_response directive now specifies how many packets are expected in
response to a single client packet.