mirror of
https://github.com/tailscale/tailscale.git
synced 2025-10-31 16:22:03 +01:00
61 lines
3.1 KiB
Markdown
61 lines
3.1 KiB
Markdown
# DERP
|
|
|
|
This directory (and subdirectories) contain the DERP code. The server itself is
|
|
in `../cmd/derper`.
|
|
|
|
DERP is a packet relay system (client and servers) where peers are addressed
|
|
using WireGuard public keys instead of IP addresses.
|
|
|
|
It relays two types of packets:
|
|
|
|
* "Disco" discovery messages (see `../disco`) as the a side channel during [NAT
|
|
traversal](https://tailscale.com/blog/how-nat-traversal-works/).
|
|
|
|
* Encrypted WireGuard packets as the fallback of last resort when UDP is blocked
|
|
or NAT traversal fails.
|
|
|
|
## DERP Map
|
|
|
|
Each client receives a "[DERP
|
|
Map](https://pkg.go.dev/tailscale.com/tailcfg#DERPMap)" from the coordination
|
|
server describing the DERP servers the client should try to use.
|
|
|
|
The client picks its home "DERP home" based on latency. This is done to keep
|
|
costs low by avoid using cloud load balancers (pricey) or anycast, which would
|
|
necessarily require server-side routing between DERP regions.
|
|
|
|
Clients pick their DERP home and report it to the coordination server which
|
|
shares it to all the peers in the tailnet. When a peer wants to send a packet
|
|
and it doesn't already have a WireGuard session open, it sends disco messages
|
|
(some direct, and some over DERP), trying to do the NAT traversal. The client
|
|
will make connections to multiple DERP regions as needed. Only the DERP home
|
|
region connection needs to be alive forever.
|
|
|
|
## DERP Regions
|
|
|
|
Tailscale runs 1 or more DERP nodes (instances of `cmd/derper`) in various
|
|
geographic regions to make sure users have low latency to their DERP home.
|
|
|
|
Regions generally have multiple nodes per region "meshed" (routing to each
|
|
other) together for redundancy: it allows for cloud failures or upgrades without
|
|
kicking users out to a higher latency region. Instead, clients will reconnect to
|
|
the next node in the region. Each node in the region is required to to be meshed
|
|
with every other node in the region and forward packets to the other nodes in
|
|
the region. Packets are forwarded only one hop within the region. There is no
|
|
routing between regions. The assumption is that the mesh TCP connections are
|
|
over a VPC that's very fast, low latency, and not charged per byte. The
|
|
coordination server assigns the list of nodes in a region as a function of the
|
|
tailnet, so all nodes within a tailnet should generally be on the same node and
|
|
not require forwarding. Only after a failure do clients of a particular tailnet
|
|
get split between nodes in a region and require inter-node forwarding. But over
|
|
time it balances back out. There's also an admin-only DERP frame type to force
|
|
close the TCP connection of a particular client to force them to reconnect to
|
|
their primary if the operator wants to force things to balance out sooner.
|
|
(Using the `(*derphttp.Client).ClosePeer` method, as used by Tailscale's
|
|
internal rarely-used `cmd/derpprune` maintenance tool)
|
|
|
|
We generally run a minimum of three nodes in a region not for quorum reasons
|
|
(there's no voting) but just because two is too uncomfortably few for cascading
|
|
failure reasons: if you're running two nodes at 51% load (CPU, memory, etc) and
|
|
then one fails, that makes the second one fail. With three or more nodes, you
|
|
can run each node a bit hotter. |