[Courses] Re: Firewall theory -- ICMP

[Jenn Vesperman]

On Wed, 2002-03-20 at 05:37, No Body wrote:
> 
> Can someone illustrate for me the difference between
> network layer and transport layer protocols? (I'm a
> very visual person, so pictures are my friends:)

I'm NOT a visual person, and I'm lousy at ASCII images, but in the 
interests of someone else being able to help, here's the results of 
a quick google-search to remind myself of the 7 layers:


   1. The Physical Layer describes the physical properties of the
various communications media, as well as the electrical properties and
interpretation of the exchanged signals. Ex: this layer defines the size
of Ethernet coaxial cable, the type of BNC connector used, and the
termination method.
[Jenn: if you kick it and it breaks, it was the physical layer.]


   2. The Data Link Layer describes the logical organization of data
bits transmitted on a particular medium. Ex: this layer defines the
framing, addressing and checksumming of Ethernet packets.
[Jenn: the details of this layer are usually determined by the firmware
on the stuff you can kick.]

   3. The Network Layer describes how a series of exchanges over various
data links can deliver data between any two nodes in a network. Ex: this
layer defines the addressing and routing structure of the Internet.
[Jenn: This is covered in the IP part of TCP/IP. TCP or UDP - it all
uses the same layer-3 network.]

   4. The Transport Layer describes the quality and nature of the data
delivery. Ex: this layer defines if and how retransmissions will be used
to ensure data delivery.
[Jenn: TCP or UDP? If there are packets that don't get through, do you
slow down? speed up? ignore them?]

   5. The Session Layer describes the organization of data sequences
larger than the packets handled by lower layers. Ex: this layer
describes how request and reply packets are paired in a remote procedure
call.
[Jenn: Ping? Ack.]

   6. The Presentation Layer describes the syntax of data being
transferred. Ex: this layer describes how floating point numbers can be
exchanged between hosts with different math formats.
[Jenn: The RFC describing legal internet date formats would be an
example of this.]

   7. The Application Layer describes how real work actually gets done.
Ex: this layer would implement file system operations.
[Jenn: What type of responses is an HTTP server allowed to give for a
GET request?]


Network layer:

Host1 ------------------ Host2 ------------------ Host3
                           |
                           |
                           \--------------------- Host4

defines how a packet gets from Host1 to Host4



Transport layer:

Host1 ------------------ Host2
                           |
                           |
                           \--------------------- Host4

already knows it's going to Host4 - that was determined last time.
It's now concerned with what to do if Host2 is choked and slow to 
respond... does it flood Host2 with requests, or does it back off and
wait patiently?



Jenn V.
-- 
    "Do you ever wonder if there's a whole section of geek culture 
        	you miss out on by being a geek?" - Dancer.

jenn at anthill.echidna.id.au     http://anthill.echidna.id.au/~jenn/