What is tunneling?

In the physical world, tunneling is a way to cross terrain or boundaries that could not normally be crossed. Similarly, in networking, tunnels are a method for transporting data across a network using protocols that are not supported by that network. Tunneling works by encapsulating packets: wrapping packets inside of other packets. (Packets are small pieces of data that can be re-assembled at their destination into a larger file.)

Tunneling is often used in virtual private networks (VPNs). It can also set up efficient and secure connections between networks, enable the usage of unsupported network protocols, and in some cases allow users to bypass firewalls.

How does packet encapsulation work?

Data traveling over a network is divided into packets. A typical packet has two parts: the header, which indicates the packet’s destination and which protocol it uses, and the payload, which is the packet’s actual contents.

An encapsulated packet is essentially a packet inside another packet. In an encapsulated packet, the header and payload of the first packet goes inside the payload section of the surrounding packet. The original packet itself becomes the payload.

Why is encapsulation useful?

All packets use networking protocols — standardized ways of formatting data — to get to their destinations. However, not all networks support all protocols. Imagine a company wants to set up a wide area network (WAN) connecting Office A and Office B. The company uses the IPv6 protocol, which is the latest version of the Internet Protocol (IP), but there is a network between Office A and Office B that only supports IPv4. By encapsulating their IPv6 packets inside IPv4 packets, the company can continue to use IPv6 while still sending data directly between the offices.

Encapsulation is also useful for encrypted network connections. Encryption is the process of scrambling data in such a way that it can only be unscrambled using a secret encryption key; the process of undoing encryption is called decryption. If a packet is completely encrypted, including the header, then network routers will not be able to forward the packet to its destination since they do not have the key and cannot see its header. By wrapping the encrypted packet inside another unencrypted packet, the packet can travel across networks like normal.

What is a VPN tunnel?

A VPN is a secure, encrypted connection over a publicly shared network. Tunneling is the process by which VPN packets reach their intended destination, which is typically a private network.

Many VPNs use the IPsec protocol suite. IPsec is a group of protocols that run directly on top of IP at the network layer. Network traffic in an IPsec tunnel is fully encrypted, but it is decrypted once it reaches either the network or the user device. (IPsec also has a mode called “transport mode” that does not create a tunnel.)

Another protocol in common use for VPNs is Transport Layer Security (TLS). This protocol operates at either layer 6 or layer 7 of the OSI model depending on how the model is interpreted. TLS is sometimes called SSL (Secure Sockets Layer), although SSL refers to an older protocol that is no longer in use.

What is split tunneling?

Usually, when a user connects their device to a VPN, all their network traffic goes through the VPN tunnel. Split tunneling allows some traffic to go outside of the VPN tunnel. In essence, split tunneling lets user devices connect to two networks simultaneously: one public and one private.

What is GRE tunneling?

Generic Routing Encapsulation (GRE) is one of several tunneling protocols. GRE encapsulates data packets that use one routing protocol inside the packets of another protocol. GRE is one way to set up a direct point-to-point connection across a network, for the purpose of simplifying connections between separate networks.

GRE adds two headers to each packet: the GRE header and an IP header. The GRE header indicates the protocol type used by the encapsulated packet. The IP header encapsulates the original packet’s IP header and payload. Only the routers at each end of the GRE tunnel will reference the original, non-GRE IP header.

What is IP-in-IP?

IP-in-IP is a tunneling protocol for encapsulating IP packets inside other IP packets. IP-in-IP does not encrypt packets and is not used for VPNs. Its main use is setting up network routes that would not normally be available.

What is SSH tunneling?

The Secure Shell (SSH) protocol sets up encrypted connections between client and server, and can also be used to set up a secure tunnel. SSH operates at layer 7 of the OSI model, the application layer. By contrast, IPsec, IP-in-IP, and GRE operate at the network layer.

What are some other tunneling protocols?

In addition to GRE, IPsec, IP-in-IP, and SSH, other tunneling protocols include:

Point-to-Point Tunneling Protocol (PPTP)
Secure Socket Tunneling Protocol (SSTP)
Layer 2 Tunneling Protocol (L2TP)
Virtual Extensible Local Area Network (VXLAN)