feat: dcutr (#279)

content/concepts/nat/dcutr.md

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+---
+title: "DCUtR"
+description: "DCUtR is a protocol for establishing direct connections between nodes behind NATs."
+weight: 210
+---
+
+## Background
+
+Relays are used to traverse NATs by acting as proxies, but this can be expensive
+to scale and maintain, and may result in low-bandwidth, high-latency
+connections. [Hole punching](/concepts/nat/hole-punching) is another technique
+that enables NAT traversal by enabling two nodes behind NATs to communicate
+directly.  However, in addition to relay nodes, it requires another piece of
+infrastructure called signaling servers.
+
+> A signaling server is a server or service that facilitates communication
+> between nodes in a P2P network, specifically in context of setting up,
+> maintaining and terminating a direct communication channel between two peers
+> which are behind NATs. It helps in discovering the external IP address and
+> port of the peers and also in NAT traversal by relaying messages between the
+> peers.
+
+The good news is that libp2p offers a hole punching solution which eliminates
+the need for centralized signaling servers and allows the use of distributed
+relay nodes.
+
+## What is Direct Connection Upgrade through Relay?
+
+The libp2p DCUtR (Direct Connection Upgrade through Relay) is a protocol for
+establishing direct connections between nodes through hole punching, without a
+signaling server. DCUtR involves synchronizing and opening connections to each
+peer's predicted external addresses.
+
+The DCUtR protocol uses the protocol ID `/libp2p/dcutr` and involves the
+exchange of `Connect` and `Sync` messages.
+
+The DCUtR protocol supports different types of connections, such as TCP and
+[QUIC](../transports/quic.md), the process of establishing a connection is
+different for each type.
+
+@Dennis-tra has a [great talk](https://www.youtube.com/watch?v=fyhZWlDbcyM) on
+dctur and its holepunching success rates.
+
+<!-- ADD DIAGRAMS -->
+
+A helpful resource for understanding how NAT traversal works is [this blog post](https://tailscale.com/blog/how-nat-traversal-works/) by Tailscale.
+
+{{< alert icon="💡" context="note" text="See the DCUtR <a class=\"text-muted\" href=\"https://github.com/libp2p/specs/blob/master/relay/DCUtR.md\">technical specification</a> for more details." />}}