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Frequently Asked Questions (FAQ)

Other FAQ sections

  • FAQ Item
    • Features
    • Configuration
    • Special ICMPv6 replies from sixxsd
    • sixxsd IPv6 address for ICMPv6 replies

What is sixxsd?

"sixxsd" is the SixXS Daemon, it is the software that runs on the PoPs and that handles the server-side of proto-41, heartbeat and AYIYA tunnels. In effect it is SixXS's own routing platform as the complete process of en/decapsulation of tunneled packets and passing it to the proper location is handled by it.

sixxsd also takes care of the latency tests and traffic statistic collection. Various statistics can be seen, when logged in, in real-time from the user home under tunnel details, eg the current location of an endpoint of a tunnel.


A short summary of features of sixxsd:

  • Handles the full IPv6 Routing process
  • En/Decapsulation of protocol-41 (6in4) and AYIYA (IPv4 in IPv6-UDP and IPv6 in IPv4-UDP)
  • Very high performance (during tests forwarded 4 Gbit/s of mixed AYIYA/proto-41 traffic)
  • Heartbeat support
  • Latency testing of active endpoints
  • Per-tunnel remote debugging option showing per-packet decisions being made
  • Per-tunnel statistics and error information
  • Per-tunnel default routed /64 towards tunnel endpoint


Except for receiving configuration instructions from our central database, and that that actually presents the statistics through the website, it effectively runs completely on it's own. Configuration instructions are pushed to the PoPs every 10 minutes.

Special ICMPv6 replies from sixxsd

sixxsd has a few 'special' ICMP replies that it replies with when a tunnel is unreachable. This allows one to easily differentiate between a tunnel that is not configured, one that is disabled or one that is not sending active heartbeats (AYIYA and heartbeat tunnels only).


This along of course with all the neccesary things like ICMPv6 Packet Too Big to indicate packets that are too large to be routed to the next hop etc.

sixxsd IPv6 address for ICMPv6 replies

When a response can be associated with a tunnel, the gateway address for that tunnel is used, this makes debugging clear as it indicates the tunnel it is related to.

When a response cannot be associated to a tunnel sixxsd uses <tunnelprefix>ffff::1 as it's source address. This address has a forward and reverse in DNS as; For example for the sixxsd's DNS label is with IPv6 address 2001:610:600:ffff::1. As sixxsd does not implement TCP/UDP etc that IP always responds with ICMPv6 unreachables even though it responds from that address.

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