Propagation delay

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Once a bit is pushed into the link, it needs to propagate to router B. The time

required to propagate from the beginning of the link to router B is the propagation

delay. The bit propagates at the propagation speed of the link. The propagation

speed depends on the physical medium of the link (that is, fiber optics, twisted-pair

copper wire, and so on) and is in the range of

2 108 meters/sec to 3 108 meters/sec

which is equal to, or a little less than, the speed of light. The propagation delay is

the distance between two routers divided by the propagation speed. That is, the

propagation delay is d/s, where d is the distance between router A and router B and s

is the propagation speed of the link. Once the last bit of the packet propagates to

node B, it and all the preceding bits of the packet are stored in router B.

 

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Protocol layering
To
provide structure to the design of network protocols, network designers organize

protocols—and the network hardware and software that implement the protocols—

in layers. Each protocol belongs to one of the layers, just as each function in the

airline architecture in Figure 1.22 belonged to a layer. We are again interested in

the services that a layer offers to the layer above—the so-called service model of

a layer. Just as in the case of our airline example, each layer provides its service

by (1) performing certain actions within that layer and by (2) using the services of

the layer directly below it. For example, the services provided by layer n may

include reliable delivery of messages from one edge of the network to the other.

This might be implemented by using an unreliable edge-to-edge message delivery

service of layer n 1, and adding layer n functionality to detect and retransmit

lost messages.

 

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Application layer

The application layer is where network applications and their application-layer proto￾cols reside. The Internet’s application layer includes many protocols, such as the HTTP

protocol (which provides for Web document request and transfer), SMTP (which pro￾vides for the transfer of e-mail messages), and FTP (which provides for the transfer of

files between two end systems). We’ll see that certain network functions, such as the

translation of human-friendly names for Internet end systems like www.ietf.org to a

32-bit network address, are also done with the help of a specific application-layer pro￾tocol, namely, the domain name system (DNS). We’ll see in Chapter 2 that it is very

easy to create and deploy our own new application-layer protocols.

An application-layer protocol is distributed over multiple end systems, with the

application in one end system using the protocol to exchange packets of information

with the application in another end system. We’ll refer to this packet of information

at the application layer as a message.

 

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Transport layer

The Internet’s transport layer transports application-layer messages between

application endpoints. In the Internet there are two transport protocols, TCP and

UDP, either of which can transport application-layer messages. TCP provides a

connection-oriented service to its applications. This service includes guaranteed

delivery of application-layer messages to the destination and flow control (that is,

sender/receiver speed matching). TCP also breaks long messages into shorter seg￾ments and provides a congestion-control mechanism, so that a source throttles its

transmission rate when the network is congested. The UDP protocol provides a con￾nectionless service to its applications. This is a no-frills service that provides no

reliability, no flow control, and no congestion control. In this book, we’ll refer to a

transport-layer packet as a segment.

 

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Network layer

The Internet’s network layer is responsible for moving network-layer packets

known as datagrams from one host to another. The Internet transport-layer proto￾col (TCP or UDP) in a source host passes a transport-layer segment and a destina￾tion address to the network layer, just as you would give the postal service a letter

with a destination address. The network layer then provides the service of deliver￾ing the segment to the transport layer in the destination host.

The Internet’s network layer includes the celebrated IP Protocol, which defines

the fields in the datagram as well as how the end systems and routers act on these

fields. There is only one IP protocol, and all Internet components that have a net￾work layer must run the IP protocol. The Internet’s network layer also contains rout￾ing protocols that determine the routes that datagram

 

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Link Layer

The Internet’s network layer routes a datagram through a series of routers between

the source and destination. To move a packet from one node (host or router) to the

next node in the route, the network layer relies on the services of the link layer. In

particular, at each node, the network layer passes the datagram down to the link

layer, which delivers the datagram to the next node along the route. At this next

node, the link layer passes the datagram up to the network layer.

The services provided by the link layer depend on the specific link-layer proto￾col that is employed over the link. For example, some link-layer protocols provide

reliable delivery, from transmitting node, over one link, to receiving node. Note that

this reliable delivery service is different from the reliable delivery service of TCP,

which provides reliable delivery from one end system to another. Examples of link￾layer protocols include Ethernet, WiFi, and the cable access network’s DOCSIS pro￾tocol. As datagrams typically need to traverse several links to travel from source to

destination, a datagram may be handled by different link-layer protocols at different

links along its route. For example, a datagram may be handled by Ethernet on one

link and by PPP on the next link. The network layer will receive a different service

from each of the different link-layer protocols. In this book, we’ll refer to the link￾layer packets as frames.

 

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Physical Layer

While the job of the link layer is to move entire frames from one network element

to an adjacent network element, the job of the physical layer is to move the individ￾ual bits within the frame from one node to the next. The protocols in this layer are

again link dependent and further depend on the actual transmission medium of the

link (for example, twisted-pair copper wire, single-mode fiber optics). For example,

Ethernet has many physical-layer protocols: one for twisted-pair copper wire,

another for coaxial cable, another for fiber, and so on. In each case, a bit is moved

across the link in a different way