Avian carriers can provide high delay, low throughput, and
low altitude service. The connection topology is limited to a single point-to-point
path for each carrier, used with standard carriers, but many carriers can
be used without significant interference with each other, outside of early
spring.
—D. Weitzmann
One policy, one system, and universal service.
—Theodore Vail
Well if I called the wrong number, why did you answer the
phone?
—James Thurber
In designing internetworks, the usual emphasis is on logical addressing.
But logical addresses are abstractions that travel over lower-level transmission
systems, and these transmission systems often have their own systems of addressing.
This chapter deals with those lower-level addresses and their relationships
to logical addresses.
The bottom is always a fine place to start. Underlying this chapter
is the little-known but useful architectural extension of the OSI Reference
Model called the Internal
Organization of the Network Layer, which provides a basis for systematic
mapping between logical and transmission system addresses.
Regardless of what one thinks of OSI as a solution, it does contain useful
architectural methods. These methods did not freeze with the original OSI
Reference Model.
Remember that OSI evolved in formal international standards bodies,
whose roots were in telecommunications. These roots were largely in governmental
national monopolies. Telecommunications providers classically interface to
subscribers at the edge of the provider's “cloud,” the details
of which are hidden from the subscriber. Subscribers use signaling protocols
to set up and receive connections from the cloud. Subscriber endpoints are
identified by telephone numbers.
Do not become overly concerned because this information is hidden. Simply assume, for the initial analysis, that
the provider network is magical and provides appropriate connectivity. The
endpoint addresses in the wide area network (WAN) are the conventions by which
user networks access it.
Also understand
the underlying protocol model. In a local area network (LAN), the lower-layer
protocols often are end-to-end, or at least map to real media. In a WAN, the
lower-layer protocols are consciously not end-to-end, but intended to manage
access between a provider and customer.
Taking a slightly tongue-in-cheek view of OSI development in the late
1970s, these organizations viewed all communications media as belonging to
them, and they would tell subscribers how to connect to them. LAN development
in the Institute for Electrical and Electronic Engineers (IEEE) was separate
and largely did not affect the original OSI Reference Model published in 1984.
Serious coordination between the two cultures was more of a mid-1980s matter.
IEEE LAN standards work is IEEE Project 802.
A result of that coordination was the Internal Organization
of the Network Layer, a document that relaxed the rigid boundaries
between Layer 2 and Layer 3 of the OSI Reference Model and came up with a
much more useful picture.
Ignoring the rather obscure OSI terminology for these new layers, the model,
shown in Figure 4.1,
is split into three parts:
Logical addressing is what we usually think of as Layer 3: largely transmission
system independent such as IP. The transmission medium address is the next
level down.
As shown in Figure
4.2, IEEE further
subdivided the transmission system for the LAN environment into Media Access
Control (MAC) and Logical Link Control (LLC) at the data link layer.
IEEE also divided the physical layer into media-independent (MDI) and
physical-media dependent (PMD) sublayers. General physical layer and
PMD functions tend to be implemented in different hardware chip sets.
WAN services, on
the other hand, were usefully split into persistent and transient
levels. Think of a five-button telephone to which is assigned a five-line
hunt group, a group of five telephone lines with a single telephone number.
The telephone number is the persistent identifier while the button that lights
for a specific call is the transient identifier.
The distinction between LLC and MAC has allowed LLC to be a general
buffer management mechanism that hides the timing-critical aspects of the
MAC sublayer. LLC also provided a place for more general mechanisms such as
upper layer protocol identification and optional retransmission.
Transmission systems have their own addressing, distinct from the network
layer. Their addressing models tend to fall into one of two categories, LAN
or WAN. This categorization is not really based on the geographic scope of
the transmission system, but on the underlying administrative model.
Figure 4.3 illustrates
a typical WAN model, as might be seen with Frame Relay, X.25, or ATM. User
organizations use access mechanisms to interact with a service provider cloud,
the details of which are usually not visible to the calling or called user.
Users have virtual circuits across the cloud.
Even if the WAN service is of physical rather than virtual circuits,
there still is a clear demarcation point between carrier/provider and user
responsibility.
This demarcation point is logically inside the data circuit
terminating equipment (DCE). DCE is often assumed to stand for data
communications equipment, but this is the formal standard meaning
of the acronym. Special contractual arrangements are needed, in many cases,
for the user organization to know the path taken through the provider network.
If, for reliability reasons, the user wants to have multiple circuits
with no common point of failure, this takes specific engineering from the
carrier, called facility diversity. This can be quite
expensive. Some users have made the extremely dangerous assumption that leasing
circuits from multiple providers inherently gives diversity. In practice,
providers lease transmission facilities from each other and from “wholesale”
providers. Diversity can be engineered when a specific organization is responsible
for assuring it, but if the different carriers are not aware of each other,
they may wind up leasing a common facility from a third party—a facility
that can be affected by a single failure.
In contrast, all components of a LAN are operated by the same organization.
Internetworking has its roots in the WAN culture, and it is worth reviewing
the WAN concepts that shape communications.
Residential
Broadband, Second Edition){kind=link}
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