Messages, Segments, Datagram's, and Frames - The physical path that data takes down a sending end system's protocol stack, up and down the protocol stacks of an intervening link-layer switch and router, and then up the protocol stack at the receiving end system. As we discuss later in this book, routers and link-layer switches are both packet switches. |
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The physical path that data takes down a sending end system's protocol stack, up and down the protocol stacks of an intervening link-layer switch and router, and then up the protocol stack at the receiving end system. As we discuss later in this book, routers and link-layer switches are both packet switches.
Author: Imran Rashid
Date: Dec 6, 2009 - 10:44:54 AM
The
physical path that data takes down a sending end system's protocol
stack, up and down the protocol stacks of an intervening link-layer
switch and router, and then up the protocol stack at the receiving end
system. As we discuss later in this book, routers
and link-layer switches are both packet switches. Similar to end
systems, routers and link-layer switches organize their networking
hardware and software into layers. But routers and link-layer switches
do not implement all of the layers in the protocol stack; they
typically implement only the bottom layers. Link-layer switches
implement layers I and 2; routers implement layers I 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, We'll see later that while link-layer
switches do not recognize IP addresses, they are capable of recognizing
layer 2 addresses, such as Ethernet addresses. Note that hosts
implement all five layers; this is consistent with the view that the
Internet architecture puts much of its complexity at the edges of the
network. At the sending host, an application-layer message 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) that will he used by the
receiver-side transport layer.
The application-layer message and the transport-layer header information together constitute the transport-layer segment. The transport-layer segment
thus encapsulates the application-layer message. The added information
might include information allowing the receiver-side transport layer to
deliver the message up to the appropriate application, and error-
detection bits that allow the receiver to determine whether bits in the
message have been changed in route. The transport layer then passes the
segment to the network layer, which adds network-layer header
information 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. Thus, we see that 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. A useful analogy here is the sending of an
interoffice memo from one corporate branch office to another via the
public postal service.
Suppose Alice, who is in one branch office, wants to send a memo to
Bob, who is in another branch office. The memo is analogous to the
applications-layer message. Alice puts the memo in an interoffice
envelope with Rob's name and department written on the front of the
envelope. The interoffice envelope is analogous to a transport-layer
segment--it contains header information (Bob's name and department
number) and it encapsulates the application-layer message (the memo).
When the sending branch-office mailroom receives the interoffice
envelope, it puts the interoffice envelope inside yet another envelope,
which is suitable for sending through the public postal service. The
sending mailroom also writes the postal address of the sending and
receiving branch offices on the postal envelope. Here, the postal
envelope is analogous to the datagram--it encapsulates the
transport-layer segment (the interoffice envelope), which encapsulates
the original message (the memo). The postal service delivers the postal
envelope to the receiving branch-office mailroom. There, the process of
de-encapsulation is begun. The mailroom extracts the interoffice memo
and forwards it to Bob. Finally, Bob opens the envelope and removes the
memo.
The process of encapsulation can be more complex than that described
above. For example, a large message may be divided into multiple
transport-layer segments (which might themselves each be divided into
multiple network-layer datagrams). At the receiving end, such a segment
must then be reconstructed from its constituent datagrams.
Imran Rashid has working knowledge about windows operating systems.For more information visit
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