Minecraft's multiplayer protocol appears to send out packets with an unknown (0x06) DSCP flag. This causes some networking hardware to silently discard the packets as erroneous.
I have attached a Wireshark trace which shows (from 0-5s) the 'nc' utility connecting to a Minecraft server correctly and disconnecting after 5 seconds. From 26-71 seconds the trace shows an up to date Linux Minecraft client attempting to connect to a server. The initial SYN is made and no ACK is never received as the packet is dropped (I am working with an ISP using network equipment which drops these packets; the trace was captured on the machine running nc/MInecraft).
The DSCP field set by nc is 0; the DSCP field set by Minecraft is 6, which is an invalid codepoint that has no meaning in any standard.
The DSCP field on outgoing Minecraft packets should always be 0.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.