1.5 Protocol Layers and Their Service Models

transport-layer segment

At the sending host, an application-layer message (M in Figure 1.24) is passed to the transport layer. In the simplest case, the transport layer takes the message and appends additional information (so-called transport-layer header information, Ht in Figure 1.24) that will be used by the receiver-side transport layer. The applica- tion-layer message and the transport-layer header information together constitute the transport-layer segment. The transport-layer segment thus encapsulates the application-layer message.

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.

benefits of transport layer segment

The added information might include information allowing the receiver-side transport layer to deliver the message up to the appro- priate application, and error-detection bits that allow the receiver to determine whether bits in the message have been changed in route.

translation of human-friendly names for Internet end systems like www.ietf.org to a 32-bit network address,

done with the help of a specific application-layer pro- tocol, namely, the domain name system (DNS)

link- layer packets

frames

at each layer, a packet has two types of fields:

header fields and a payload field. The payload is typically a packet from the layer above.

examples of what layers link layer and router switches can implement

link-layer switches implement layers 1 and 2; routers imple- ment layers 1 through 3. This means, for example, that Internet routers are capable of implementing the IP protocol (a layer 3 protocol), while link-layer switches are not. Link layers can recognize ethernet addresses. hosts implement all five layers. the Internet architecture puts much of its complexity at the edges of the network.

packet of information at the application layer

message.

physical layer

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.

UDP

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.

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.

network layer

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. The network layer then provides the service of deliver- ing the segment to the transport layer in the destination host.

do routers and link layer switches implement all of a layers in a protocol stack?

routers and link-layer switches do not implement all of the layers in the protocol stack; they typically implement only the bottom layers.

transport-layer packet

segment.

A protocol layer can be implemented in

software, in hardware, or in a combina- tion of the two

service model

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

transport layer

transports application-layer messages between application endpoints.

application layer

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).

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.

Seven-layer ISO OSI reference model

Application Presentation Session Transport Network Link Physical

Five-layer Internet protocol stack

Application Transport Network Link Physical

Examples of link- layer protocols include

Ethernet, WiFi, and the cable access network's DOCSIS pro- tocol.

2 drawbacks of layering

One potential drawback of layering is that one layer may dupli- cate lower-layer functionality. For example, many protocol stacks provide error recovery on both a per-link basis and an end-to-end basis. A second potential draw- back is that functionality at one layer may need information (for example, a time- stamp value) that is present only in another layer; this violates the goal of separation of layers.

In the Internet there are two transport protocols

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

routing protocols

The Internet's network layer also contains rout- ing protocols that determine the routes that datagrams take between sources and destinations. The Internet has many routing protocols.

IP Protocol

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.

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.

what happens after the transport layer segment?

The transport layer then passes the segment to the network layer, which adds network-layer header infor- mation (Hn in Figure 1.24) such as source and destination end system addresses, creating a network-layer datagram. The datagram is then passed to the link layer, which (of course!) will add its own link-layer header information and create a link-layer frame.

layers

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.