Theoretical Networking

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Matty2d

Weaksauce
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Mar 30, 2003
Messages
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I’m taking a network course and right now we are studying the different layers. What I’am confused on is each layer is sending something to the next, being a, packet,segment…. Frame, I’m not seeing the overall picture of what is actually being sent over the transmission line. Like a Packet contains it’s attributes, blah blah frame contains the MAC (physical) address and other attributes, is there any diagrams or any visual aid which could help me on this. ( I need to see from the frames point of view, where each layer comes into play somewhat visually).

Am I making any sense here?

Thanks
 
Well I think I know where you seem to be confused....

there are not different packets from different layers. Every packet goes from the top (Application Layer) to the bottom (Physical Layer) before it's sent along the wire. Each layer wraps itself AROUND the previous layer, meaning the previous layer's stuff is still there plus some. And when data is being read, it "unwraps" the data and if it needs to resend the data, as is the case with a router for example, it "re-wraps" the data in order to send it.

For example, every packet that's sent contains the MAC address from Layer 2 as well as the IP from Layer 3, etc. There are no specific IP or MAC packets, as information from all layers is used by different equipment to get the packet to where it needs to go.

For example, most switches only look up to Layer 2. They dont care about an IP address, nor do they even know what it is. They care onlyabout the MAC so they can get the packet to the right switch port. Routers, on the other hand, need to have that IP address so they look at all layers up to 3.

Make sense?
 
i believe it goes something like this...
<MAC address> | <IP address> | <Transport Protocols (TCP etc)> | <Session Protocols (RDP, SQL etc.)> | <Multimedia Protocols (MPEG, etc.)> | <App data (port and socket number, etc.)>
 
Think of it as envelopes inside of envelopes.

So the application layer IE has an http request
Which is passed down through the other layers, each adding their own information outside of the existing layer. As it goes down, IP information is added, then the TCP info, then the MAC info on and on.

So the final result is a datagram which looks like what SlickJesus wrote.

The process in encapsulation.
 
. . . . and once the higher layers have added their envelopes w/ relevant "postage information" the physical layer turns the package into an electrical signal and puts it on the wire.
 
The book TCP/IP Unleashed puts it like this....

you have data from a program that needs to be sent. anything at the program (or Application) layer is at Layer 7, or at the top. Before data can be sent along the wire, which is layer 1, it must go throuh 5 other layers to get there. Each layer add's its own data.

the book shows us starting with

Layer 7 Header + Layer 7 Data
Click to expand...
at the top.... then
Layer 6 Header + Layer 7 Header + Layer 7 Data
Click to expand...
then
Layer 5 Header + Layer 6 Header + Layer 7 Header + Layer 7 Data
Click to expand...
all the way to the bottom when you have
Layer 1 Header + Layer 2 Header + Layer 3 Header + Layer 4 Header + Layer 5 Header + Layer 6 Header + Layer 7 Header + Layer 7 Data.
Click to expand...
Now in order for this data to be used, each layer takes the contents out of the outermost envelope, to use TrueBukeye's analogy.

So the layers work like this:

7 - Application - High level protocols like HTTP, Oscar (AIM's protocol), etc. Everyone and their mom can made a layer 7 protocol. Hell, even I had a primitive one when I made a simple IM app in VB a while back.
6 - Presentation - Converts data between several different data-encoding schemes. The book uses ASCII and EBCDIC as an exmaple, as well as Encryption and Decryption.
5 - Session - Not used much. The book says most protocols include the session layers functionality in the transport layers.
4 - Transport - TCP, guarantee's delivery of all data.
3 - Network - IP addressing/routing
2 - Data Link - Creates packets and frames. Verifies integrity of recieved frames. MAC addresses are here.
1 - Physical - All 1's and 0's
 
I keep getting confused so I took some note on the basic functions of each layer.

Application Layer (7)
Telnet
Ftp
tftp
nfs
snmp
smtp
http
(and all other applications)

Presentation Layer (6)
Jpeg
gif
mpeg
flash
wav
mp3
ascii
(data formats)

Session Layer (5)
Establishes, manages, terminates & synchronises Sessions
Nfs
SQL

Transport Layer (4)
Segments Data
Deals with Reliability
Establishes, manages, terminates & synchronises Virtual Circuits
SPX - Sequenced Package Exchange
TCP
UDP
NetBeui

Network Layer (3)
Connectivity
Path selection
Logical Addressing
Routing AND Routed Protocols
ARP
Not NetBeui/Decnet

Data Link Layer (2)
Frames
CSMA/CD
Physical Addressing
Network topology
Ordered delivery
Flow control
LLC
MAC

Physical Layer (1)
Binary
Media
Physical characteristics
 
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