Local Area Networks (LANs) in Aircraft - FTP Directory Listing - FAA

Deployments that need to support multiple protocol families usually identify a target protocol
system and target infrastructure for the deployment to standardize upon. That target system
becomes the network core and the other protocol systems “hang off” of that core. Almost
without exception, the target protocol is the IP family, which is also known as TCP/IP. The
nontarget protocol deployments are generically referred to as being legacy systems. To
communicate with IP systems, legacy protocol systems either must be gatewayed into the core
target system via protocol translators or converted over time to become part of the target system.
For example, Bob Stephens [48] describes a possible approach where the NAS’ ATN protocol
and infrastructure can be modified to replace their current OSI connectionless network protocol
(CLNP) protocol by IPv6, 12 thereby making ATN become an IP protocol system. 13
ATN is currently an OSI protocol system. The OSI Reference Model defines the high-level
unifying networking constructs that guided the creation of the OSI protocols. The OSI protocol
deployments are divided into two major protocol variants:
• One major variant uses a connection-oriented network service (CONS) layer protocol and
a less complicated transport layer protocol (TP) variant (i.e., CONS/TP0). This variant is
patterned after historic X.25 protocol systems.
• The other major OSI variant that existed in the early 1990s uses a CLNP and a morecomplicated
TP variant (i.e., CLNP/TP4). This system is closely patterned after TCP/IP,
whose definition preceded it by about a decade. This is the OSI variant that was selected
for ATN.
The right-hand side of figure 11 shows the TCP/IP family protocol stack. The middle material
between the two stack charts in figure 11 is an elaboration of the distinct sublayers defined by
OSI for their data link and network layers. One should observe that although the IP Family’s IP
Layer is often referred to as being TCP/IP’s network layer, it primarily corresponds to OSI Layer
3c (i.e., OSI’s internetwork sublayer of the network layer).
This nit is being explicitly mentioned to introduce a very important point: Even though the OSI
CLNP/TP4 variant was closely patterned after TCP/IP, the two protocol systems differ from each
other in numerous significant ways. These differences ensure that their protocol deployments
cannot naturally communicate together. For example, the state machines that underlie their
various protocols are sufficiently different from each other to cause opportunities for the
translation gateway devices connecting the two systems to potentially experience problems.
12
13
The IP has two major variants: IPv4 (version 4) is the historic variant that currently populates the majority of
the worldwide Internet infrastructure today. IPv6 (version 6) improves upon IPv4’s scaling properties and is
gradually replacing IPv4. The IETF has created several transition technologies to ease the migration from IPv4
to IPv6 and to enable networks to simultaneously support both protocols—a topic that is outside of the scope of
this document.
There is a significant “bug” in reference 48: The presentation contains stack charts that show both OSI’s CLNP
and TCP/IP’s protocols being hosted over the data link limited liability corporation (LLC) protocol (i.e., OSI
Layer shown in reference 48 could not interoperate with existing IP systems if IP is conveyed over an LLC
protocol as 2b—See figure 11). This representation is true for OSI but it is false for IP. Specifically, the IP
stacks shown (i.e., IP interoperability requires that a LLC protocol not be present).
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