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Tunneling protocol

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The five-layer TCP/IP model
5. Application layer
DHCP · DNS · FTP · Gopher · HTTP · IMAP4 · IRC · NNTP · XMPP · POP3 · SIP · SMTP · SNMP · SSH · TELNET · RPC · RTCP · RTSP · TLS · SDP · SOAP · GTP · STUN · NTP · (more)
4. Transport layer
TCP · UDP · DCCP · SCTP · RTP · RSVP · IGMP · (more)
3. Network/Internet layer
IP (IPv4 · IPv6) · OSPF · IS-IS · BGP · IPsec · ARP · RARP · RIP · ICMP · ICMPv6 · (more)
2. Data link layer
802.11 (WLAN) · 802.16 · Wi-Fi · WiMAX · ATM · DTM · Token ring · Ethernet · FDDI · Frame Relay · GPRS · EVDO · HSPA · HDLC · PPP · PPTP · L2TP · ISDN · ARCnet · (more)
1. Physical layer
Ethernet physical layer · Modems · PLC · SONET/SDH · G.709 · Optical fiber · Coaxial cable · Twisted pair · (more)
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A tunneling protocol is a network protocol which encapsulates a payload protocol, acting as a payload protocol. Reasons to tunnel include carrying a payload over an incompatible delivery network, or to provide a secure path through an untrusted network. Tunneling does not always fit a layered protocol model such as those of OSI or TCP/IP. To understand a particular protocol stack, both the payload and delivery protocol sets must be understood. Protocol encapsulation that is carried out by conventional layered protocols, in accordance with the OSI model or TCP/IP model, for example HTTP over TCP over IP over PPP over a V.92 modem, should not be considered as tunneling. As an example of network layer over network layer, Generic Routing Encapsulation (GRE), which is a protocol running over IP ( IP Protocol Number 47), often is used to carry IP packets, with RFC 1918 private addresses, over the Internet using delivery packets with public IP addresses. In this case, the delivery and payload protocols are compatible, but the payload addresses are incompatible with those of the delivery network. In contrast, an IP payload might believe it sees a data link layer delivery when it is carried inside the Layer 2 Tunneling Protocol, which appears to the payload mechanism as a protocol of the data link layer. L2TP, however, actually runs over the transport layer using User Datagram Protocol (UDP) over IP. The IP in the delivery protocol could run over any data link protocol from IEEE 802.2 over IEEE 802.3 (i.e., standards-based Ethernet) to the Point-to-Point Protocol (PPP) over a dialup modem link. Tunneling protocols may use data encryption to transport insecure payload protocols over a public network such as the Internet thereby providing VPN functionality. IPSec has an end-to-end Transport Mode, but also can be operate in a Tunneling Mode through a trusted security gateway.

1 Common tunneling protocols
2 SSH tunneling
3 Tunneling to circumvent firewall policy
4 See also
5 References
6 External links

Common tunneling protocols

Examples of tunneling protocols include: Datagram-based:

IPsec
GRE (Generic Routing Encapsulation) supports multiple protocols and multiplexing
IP in IP Tunneling ^[1] Lower overhead than GRE and used when only 1 IP stream is to be tunneled
L2TP (Layer 2 Tunneling Protocol) ^[2]
MPLS (Multi-Protocol Label Switching)
GTP (GPRS Tunnelling Protocol)
PPTP (Point-to-Point Tunneling Protocol) ^[3]
PPPoE (point-to-point protocol over Ethernet)
PPPoA (point-to-point protocol over ATM)
IEEE 802.1Q (Ethernet VLANs)
DLSw (SNA over IP)
XOT (X.25 datagrams over TCP)
IPv6 tunneling: 6to4; 6in4; Teredo
Anything In Anything (AYIYA; e.g. IPv6 over UDP over IPv4, IPv4 over IPv6, IPv6 over TCP IPv4, etc.)

Stream-based:

TLS
SSH
SOCKS
HTTP CONNECT command
Various circuit-level proxy protocols, such as Microsoft Proxy Server's Winsock Redirection Protocol, or WinGate Winsock Redirection Service.

SSH tunneling

SSH is frequently used to tunnel insecure traffic over the Internet in a secure way. For example, Windows machines can share files using the SMB protocol, which is not encrypted. If you were to mount a Windows filesystem remotely through the Internet, someone snooping on the connection could see your files. So to mount an SMB file system securely, one can establish an SSH tunnel that routes all SMB traffic to the fileserver inside an SSH-encrypted connection. Even though the SMB traffic itself is insecure, because it travels within an encrypted connection it becomes secure.

Tunneling to circumvent firewall policy

Tunneling can also be used to traverse a firewall (firewall policy permitting). In this case, protocols that are normally blocked by the firewall are encapsulated inside a commonly allowed protocol such as HTTP. If the policy on the firewall does not exercise enough control over HTTP requests, this can sometimes be used to circumvent the intended firewall policy. Another HTTP-based tunneling method uses the HTTP CONNECT method/command. This command tells an HTTP proxy to make a TCP connection to the specified server:port, and relay data back and forth between that connection and the client connection. Therefore, for security reasons, CONNECT-capable HTTP proxies commonly restrict access to the CONNECT method to accessing TLS/SSL-based HTTPS services only.