Networking Essay Help

nezzy456

Hi Guys,

I have been given an essay to discuss how the layers of the TCP/IP stack work together in regards to connection.

I have come to the conclusion that;

The Application layer can be - Connection Oriented (FTP) or Connectionless (TFTP)
The Transport layer can also be - Connection Oriented (TCP) or Connectionless (UDP)
The Network layer is only - Connectionless (IP)

I have having a problem with the data link layer though;
Does anyone have any ideas?

Thanks

nel

If i were you i would read up on the tcp/ip model as its not just about connection or connectionless orientation.

http://www.cisco.com/univercd/cc/td/doc/product/iaabu/centri4/user/scf4ap1.htm

Google it, there are loads of sites

EDIT: oops i just re-read what you were speaking about. sorry! check out the link though it maybe usesful.

Met44

Here are some things to think about which might help you with your essay:

What about FTP and TFTP (or any other Layer 7 protocol) makes these protocols connection-oriented or connectionless?

What Layer 4 protocol does each of them use?

Why would a packet that is created with information from only Layer 3 and below never have a chance to be connection-oriented (such as ICMP and ARP)?

There is only one tool (protocol) in the TCP/IP stack which provides for connection-oriented service. Every other protocol that wishes to be connection-oriented must use it. Based on the above, what is that protocol?

nezzy456

Getting the cogs in my head turning, i like it.

Okay so FTP uses TCP and TFTP uses UDP? Therefore is it the transport layer which defines the connection/connectionless properties?

And ICMP and ARP do not have the luxury of the Transport layer (TCP) so therefore cannot become connection orientated?

Have I made the correct assumption?


Also though im still confused with the data link layer as when a frame is corrupted or does not arrive it is normally resent, but is this due to Transport layer or The Data Link Layer?

For example if I send a packet from A to C

A

---

B

---

C

But it gets lost between B and C, will the packet be resent from B to C or will it be resent from A to C.

If it is resent from B to C that would imply the Data Link layer is connection orientated, but if it comes from A it implies that its the transport layer doing the recovery.

Cheers Guys

Met44

For the purposes of frame forwarding, up to what OSI layer does a switch inspect a datagram? According to the principles of the OSI model, should the switch know anything about the layers above that?

If your scenario was a WAN, what about a router, for the purposes of packet forwarding?

Given the answers to the above, try to answer your own question.

And you are right on track.

nezzy456

Okay so a switch will go up to layer 2 and a router to layer 3.

And neither should be aware of anything above them. Which includes the TCP protocol - the connection orientated one.

But the TCP ensure delivery from source device to end device it does not concern itself with anything in between, if it recieves all the datagrams it is happy if it doesnt not it will request them to be sent again.

So i suppose if a frame is lost between point B(Router/Switch) and C(Destination)

A----B----C

The TCP protocol can step in at device C and request device A to resend the datagram, in essence making the Data Link layer connectionless as all it does is fire frames from point to point with no real concern with the data arriving to the next point.

If I was to introduce B as a hub this strengthens this point because the Hub just repeats the message and therefore it would be up to the TCP layer to sort these problems out.

Also am I correct in thinking that CRC and CSMA/CD have no role in connection orientation as all they do is ensure the data is undamaged and the media is free, if there is a problem the frame is dropped with no attempt to recover it, they just wait for the TCP layer to resend and if UDP is in place this doesnt even happen.

So would I be correct to go with the theory that the Data Link layer is therefore connectionless?

So for important data an E-mail

Application - Conectionless
Transport - Connection Orientated
Internet - Conectionless
Data Link - Conectionless

And for "faster" data eg VOIP

Application - Conectionless
Transport - Conectionless
Internet - Conectionless
Data Link - Conectionless

Met44

Right, a Layer 2 device will let the end device worry about the fact that it did not receive a particular frame - after all, the switch would have no concept of whether the source would desire a notification that the packet was dropped, since it does not know whether TCP or UDP is being used. For that matter, the Transport Layer protocol may not necessarily be either TCP or UDP. If it were SPX (used in Novell's IPX/SPX stack), a TCP retransmission request would be rather unexpected. Not only would the switch not know what IP address to send the request to, it wouldn't know even what protocol to be requesting.

The data link layer would be considered connectionless; it has no concept of connection-oriented service. Like you said, the main purpose of layer 2 is to determine which interface to switch frames to based on MAC addresses, and do so only when the media is free. A switch will simply drop frames which do not pass the cyclic redundancy check.

Regarding E-mail traffic, I tend to think of SMTP (and POP3, etc.) as connection-oriented because it will always ride on top of TCP, as per their respective RFCs (faqs.org/rfcs/rfc821.html for SMTP). It "inherits" the service provided by TCP. However, technically speaking there is no connection oriented service built into the actual SMTP protocol; it relies on TCP to establish the connection, so that is probably the correct answer where your essay is concerned. Just realize that if an SMTP datagram gets dropped, it will be retransmitted since it uses TCP.

It looks like you're getting this. Do you have any more questions?

nezzy456

Cheers Met,

Im all cleared up now and feel I can have a good go on my essay.

Many thanks

Met44

Good deal!