<P> The systems both make use of the same protocol suite . The vertical flows (and protocols) are in system and the horizontal message flows (and protocols) are between systems . The message flows are governed by rules, and data formats specified by protocols . The blue lines therefore mark the boundaries of the (horizontal) protocol layers . </P> <P> The vertical protocols are not layered because they don't obey the protocol layering principle which states that a layered protocol is designed so that layer n at the destination receives exactly the same object sent by layer n at the source . The horizontal protocols are layered protocols and all belong to the protocol suite . Layered protocols allow the protocol designer to concentrate on one layer at a time, without worrying about how other layers perform . </P> <P> The vertical protocols need not be the same protocols on both systems, but they have to satisfy some minimal assumptions to ensure the protocol layering principle holds for the layered protocols . This can be achieved using a technique called Encapsulation . </P> <P> Usually, a message or a stream of data is divided into small pieces, called messages or streams, packets, IP datagrams or network frames depending on the layer in which the pieces are to be transmitted . The pieces contain a header area and a data area . The data in the header area identifies the source and the destination on the network of the packet, the protocol, and other data meaningful to the protocol like CRC's of the data to be sent, data length, and a timestamp . </P>

Difference between protocol and service interface in computer networks