Protocol layering is a common technique to simplify networking designs by dividing them into functional layers, and assigning protocols to perform each layer’s task.
For example, it is common to separate the functions of data delivery and connection management into separate layers, and therefore separate protocols. Thus, one protocol is designed to perform data delivery, and another protocol, layered above the first, performs connection management. The data delivery protocol is fairly simple and knows nothing of connection management. The connection management protocol is also fairly simple, since it doesn’t need to concern itself with data delivery.
Protocol layering produces simple protocols, each with a few well-defined tasks. These protocols can then be assembled into a useful whole. Individual protocols can also be removed or replaced as needed for particular applications.
The most important layered protocol designs are the Internet’s original TCP/IP model, and the OSI Seven Layer Model. The modern Internet represents a fusion of both models.
OSI protocols
The Open Systems Interconnection (OSI) protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T starting in 1977. The OSI model describes seven layers of interconnection: the Physical Layer (Layer 1), Data Link Layer (Layer 2), Network Layer (Layer 3), Transport Layer (Layer 4), Session Layer (Layer 5), Presentation Layer (Layer 6), and the Application Layer (Layer 7).
OSI protocols are used to implement various networks. The OSI protocol stack is split into layers for modularity and orthogonal design. The layers form a hierarchy of functionality starting with the physical hardware components to the user interfaces at the software application level. Each layer receives information from the layer above, processes it and passes it down to the next layer.
Each layer adds its own encapsulation information (header) to the incoming information before it is passed to the lower layer. Headers generally include address of destination and source, check sums (for error control), type of protocol used in the current layer, and other options such as flow control options and sequence numbers (used to ensure data is sent in order).
Not all layers are mandatory, it depends on the protocols that are implemented. The Manufacturing Automation Protocol (MAP) user group, focused on real-time control of manufacturing robots of various types, implements layer 1 (physical), a two-sub-layer layer 2 (data link) with LLC Type 3 on top of the medium access layer, and then the Layer 7 Manufacturing Message System on top. Layers 3 to 6 are not present. This high-performance stack is intended just for the robots themselves; the robot controller would load files with a full 7-layer stack with FTAM file transfer on top. Parts of the Signalling System 7 stack are OSI derivatives.
Routing Protocols
A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a prior knowledge only of networks attached to it directly. A routing protocol shares this information first among immediate neighbours, and then throughout the network. This way, routers gain knowledge of the topology of the network.
The term routing protocol may refer specifically to one operating at layer three of the OSI model, which similarly disseminates topology information between routers.
Although there are many types of routing protocols, three major classes are in widespread use on IP networks:
Interior gateway routing via link-state routing protocols, such as OSPF and IS-IS
Interior gateway routing via path vector or distance vector protocols, such as RIP, IGRP and EIGRP
Exterior gateway routing. Border gateway protocol is the routing protocol used by the public Internet.
Many routing protocols are defined in documents called RFCs.
The specific characteristics of routing protocols include:
the manner in which they either prevent routing loops from forming or break them up if they do
the manner in which they select preferred routes, using information about hop costs
the time they take to converge
how well they scale up
many other factors
In some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP which runs over IP; care is taken in the implementation of such systems not to create a circular dependency between the routing and routed protocols. That a routing protocol runs over particular transport mechanism does not mean that the routing protocol is of layer (N+1) if the transport mechanism is of layer (N).
Routing protocols, according to the OSI Routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:
IS-IS runs over the data link layer
OSPF, IGRP, and EIGRP run directly over IP; OSPF and EIGRP have their own reliable transmission mechanism while IGRP assumed an unreliable transport
RIP runs over UDP
BGP runs over TCP
Interior routing protocols
Interior Gateway Protocols (IGPs) exchange routing information within a single routing domain. A given autonomous system can contain multiple routing domains, or a set of routing domains can be coordinated without being an Internet-participating autonomous system. Common examples include:
IGRP (Interior Gateway Routing Protocol)
EIGRP (Enhanced Interior Gateway Routing Protocol)
OSPF (Open Shortest Path First)
RIP (Routing Information Protocol)
IS-IS (Intermediate System to Intermediate System)
Note that IGRP, a Cisco proprietary routing protocol, is no longer supported. EIGRP accepts IGRP configuration commands, but the internals of IGRP and EIGRP are completely different.
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