Encapsulation (Linktionary term)

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Encapsulation

Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Encapsulation is the technique of putting information within a packet for delivery to some process or entity. In the networking world, encapsulation takes place in two primary ways:

Packetization When a network application or network process needs to communicate with an application or process on another system, data is sent down through the protocol stack. At each layer, the data from the upper layer is encapsulated within a new packet, and the protocol at that layer adds its own header information to the new packet. At the bottom layer (the physical layer), the packet is encapsulated into frames and transmitted across the network. Refer to "Network Architecture" for more information.

Tunneling A tunnel describes a way to transmit packets of one protocol across a network that uses a different protocol. Think of a ferry that carries cars across a river. Figure E-5 (see the book) illustrates how an IPX packet is encapsulated in an IP packet and transported across a TCP/IP network. Another example is the use of tunnels to deliver nonroutable protocols like SNA and NetBEUI across a routed network (such as a TCP/IP intranets and the Internet). The SNA or NetBEUI packets are encapsulated within IP packets and delivered to an SNA or NetBEUI network on the other side of the IP network. DLSw (Data-Link Switching) is an IBM protocol designed to deliver SNA traffic across multiprotocol networks such as IP and IPX networks. See "Tunnels" for more information.

See Figure 5 in the book

Actually, encapsulation takes place almost everywhere in the network environment. Data from host systems is encapsulated into the frames or cells of the underlying network. In the WAN, IP packets are encapsulated into ATM cells that are, in turn, encapsulated into SONET frames. In the campus environment, Ethernet frames may be encapsulated into the frames of an FDDI (Fiber Distributed Data Interface) backbone network for delivery to an Ethernet network in another building.

Most computers that are connected to the Internet with modems use the PPP (Point-to-Point Protocol), which is a serial point-to-point link protocol. The user's IP datagrams are encapsulated into the PPP frames and delivered across the modem link to the ISP access device. At that point, the datagrams are removed from the PPP frames and forwarded on to the Internet.

VPNs (virtual private networks) illustrate another form of encapsulation, in which many different types of network traffic are delivered across the Internet (or an internal TCP/IP network) in a private and secure way. Packet contents are encrypted to maintain security, but the packet headers remain readable so that packets may be routed as normal.

GRE (Generic Routing Encapsulation) is a protocol developed by Cisco for encapsulating just about any protocol inside IP. The result is a tunnel for delivering non-IP protocols across an IP network. Microsoft uses a modified version of GRE to encapsulate PPP frames in its PPTP (Point-to-Point Tunneling Protocol) solution.