Cryptographic Protocols - Codes: The Guide to Secrecy from Ancient to Modern Times - page 218

5.7 Protocol Layers and SSL

Don't express your ideas too clearly. Most people think little of what they
understand, and venerate what they do not.
Baltasar Gracian (1601-1658), Spanish philosopher

To become acquainted with the notion of a “protocol layer”, we must un-
derstand its (formal) inception, which began with the following organization.
The International Organization for Standardization (ISO), embodying members
from 148 countries, is a world federation of national standards organizations.
ISO is a nongovernmental body, created in 1947 to promote the development of
standardization and related activities. The reader alreadywill have noticed that
ISO is not an acronym. Its roots are from the Greek isos meaning equal , which
will be recognized as the prefix iso- , such as in isometric . It happened that
equal devolved to standard , and the ISO name was adopted. Additionally, this
provides the feature of not requiring translation in each country, as would an
acronym. ISO develops precise criteria for such applications as the development
of a framework of international standards in computer networks, for instance.
(A network is a hardware and software communications system.) In 1978, ISO
developed a model of network protocols, called a protocol stack , which is a lay-
ered set of protocols working together to render a set of network functions. The
ISO model divides the architecture among seven layers, where we understand a
layer to be the environment of two or more communications devices in which
a particular network protocol operates. The ISO model is called the Open Sys-
tems Interconnection Reference Model (OSI-RM). OSI is the umbrella name for
a set of nonproprietaryprotocols and specifications, which includes the OSI-RM,
having the following seven layers, from the bottom to the top.

OSI-RM Seven Layer Protocol Stack
1. Physical Layer : This bottom layer deals with electrical and mechanical
connections to the network.

2. Data Link Layer : This layer splits data into frames , which are data packets
containing the header and trailer information required bythe phsical
layer. The data link layer executes error checking and retransmits correct
frames for anycorrupted frames it receives, therebyproviding an error-free
connection to the next layer up to which it sends the frames.

3. The Network Layer : This is the communications subnet layer, which
decides the routing of packets received from the data link layer to be used
bythe next laer up. Most commonl, IP is used (see page 199).

4. The Transport Layer : This middle layer is essentially the communications
system component of a given protocol. For instance, the TCP protocol
discussed on page 199 is one such communications system. Although TCP
itself is not cryptographically secure, mechanisms can be used to make it