Theory Question (osi model)

[PeterMichael]

If the main point of the transport layer is to establish flow control I can understand the sequencing process that occurs there, but then if the network layer's main purpose is to route data then why does the Network layer perform "segmentation?" I would think this would be a function of the Transport layer.(ie flow control) It is kind of confusing to me as I would perceive segmentation of data to be similar to sequencing of data.

Also a more practical question. If I am on a dial up, and I am getting a file from a friend using a cable modem(much faster), through a server such as audio galaxy or napster, where is flow control established? And is that flow control a function of the Transport layer or the Data Link Layer, or both.

Thanks for any hints any gurus may have for me. I am almost there.

 

[ScottMac]

"Segments" (especially as a key word on a test) happens at layer four (transport).

Layer three adds a logical address, and "packetizes" the segments.

Layer two stuffs the packets into a Frame and assigns a "physical" address ( MAC address).

There may be some "segmentation" (as in "segmentation and re-assembly" or SAR) which may also occur below layer four to accommodate PDU (MTU?) sizes for IP. It's IP's job to create and address packets that conform to the low-level protocol (like Ethernet, Token ring, FDDI) specifications. FDDI and Token-Ring , by specification, allow larger frame sizes than Ethernet, so it's likely that less SAR is happening during a session.....TCP/UDP whacks the data into "session-sized" chunks (for the tests: "Segments") that (in the case of TCP) are serialized and tracked to insure reception (the destination host "ACKs" the reception of segment XXXXX). UDP doesn't "do" acknowledgement, it relies on some other process up the stack to verify reception (or assumes it's happening...in many cases it isn't).

I might be a little off, it's early and the drugs haven't kicked in, but that's the way I remember it.

FWIW

Scott

 

[PeterMichael]

Ahh the confusion continues. The first reply is way over my lowly head. The second basically reinstates how I figure it to be. Yet my book on Net+ specifies that segmentation occurs in the Network layer and Sequencing in the Transport layer.

Why couldn't there be more simplified explaination? Geese.

But thanks for trying to explain, gents.

I must be dense on this one.

 

[usmc666]

Here we learn how vendors have different answers to the same question. Also "segmentation" can mean many things in different networking situations. VLAN's can segment a broadcast domain, but is kinda what compTIA is talking about here. They mean that layer 3 "segments" by logically making segments (ip subnets).

 

[FFC]

It's fragmentation for IP. Segmentation as in SAR is an ATM thing where you take lovely variable length packets and break them up into horrible fixed length cells, (Segmentation) which have to be put back together at the boundary of the evil ATM network (re-assembly).