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Home > Archive > CCNA > June 2003 > upper and lower layers
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upper and lower layers
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| jason892 2003-06-18, 6:32 pm |
| I got a QOD in email today that asked what the seperation of the layers was. The answer said that the lower layers are 1, 2, and 3. Upper was 4 5 6 7. The transport layer according to the cisco books is a lower layer. Are my books wrong? Since 4 has tcp and that is a routed protocol, wouldn't it be a lower layer? | |
| anchor40 2003-06-19, 10:34 am |
| Well, I don't remember "exactly" what the textbook answer is, but I would say the boundary is between 3 and 4. I base it on the Routed vs routing description (anything that is routed is upper layer), and the difference between the TCP/IP stack and the OSI stack (TCP/IP is 1,2,3,4 and OSI is true 7 layer).
Also consider the names, Internetworking Protocol vs Transport Control Protocol. The TCP name describes its administrative function, like the jockey of a racehorse. He just controls and directs the horse, he doesn't carry it.
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| hmm strange. From everything I know about the OSI model I'd have said 1234 were lower and 567 were upper. Infact just looking at my CCNA book it confirms that is the case.
I think the QoD might have a mistake in it. | |
| jason892 2003-06-19, 5:05 pm |
| quote:
Originally posted by anchor40
I base it on the Routed vs routing description (anything that is routed is upper layer), and the difference between the TCP/IP stack and the OSI stack (TCP/IP is 1,2,3,4 and OSI is true 7 layer).
I kinda see what you're saying, except that I thought that IP, IPX, etc... were routed protocols also. I thought that a routing protocol would be like cdp, dhcp, stuff that aids routers and hosts build tables with address for the routed protocol. I thought TCP/IP, IPX/SPX, APPLETALK, etc... were all routed protocols, and therefore located on the lower layers. Since tcp and spx are layer 4, that would put layer 4 on the lower layer. I may be way off base, and if so please clear me up.
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| ciscoteacher 2003-06-21, 5:43 pm |
| Routing protocols build routing tables. Examples are RIP, OSPF, BGP, IGRP, EIGRP, etc. Routed protocols direct traffic. Examples are IP, IPX, and AppleTalk.
The upper-layer IP protocols like DHCP and SMTP aren't classified as routing protocols, but rather are considered application layer protocols. (Whoa, now that i've typed that, i'm not positive that DHCP works at the application layer!??)
As always, there is great info available on cisco's Web site...
http://www.cisco.com/univercd/cc/td.../ito_doc/ip.htm | |
| jason892 2003-06-22, 9:16 am |
| quote:
Originally posted by ciscoteacher
The upper-layer IP protocols like DHCP and SMTP aren't classified as routing protocols, but rather are considered application layer protocols. (Whoa, now that i've typed that, i'm not positive that DHCP works at the application layer!??)
I think DHCP is a layer 3 protocol. Since it's main purpose in life is for client ip address acquistion. In the Cisco press book, it is described in the layer 3 chapter, but the packets that are exchanged are udp packets. So what determines the layer a protocol is assigned to? Is it the layer that the protocol serves? If that is the case, and each layer only provides a service to the next higher layer, how can a layer 3 protocol use a layer 4 protocol to obtain information? If layer 3 is requesting a service from Layer 4 then Layer 3 would no longer be transparent to layer 4, which violates the definition of the tcp/ip stack. Now I'm really confused. Help please | |
| anchor40 2003-06-23, 2:11 pm |
| This is fairly lengthy with several links, so sorry in advance.
Look at it this way, all of your "applications" like FTP, SNMP, HTTP, etc. have port numbers assigned to them, so that when data is passed to upper layers, it gets sent to the right application.
Here's a fairly good definition of TCP/IP http://makeashorterlink.com/?V66A64505
and a very good explanation of TCP/IP and its layers and protocols at each layer(http://makeashorterlink.com/?T1BA51505).
Here's a great chart that matches the OSI and TCP/IP layers side-by-side (http://makeashorterlink.com/?W2AB12505). When I first learned to spell TCP/IP, I was taught that there was the Physical, Data Link, Network, and Transport layers (the application layer was blended into the Transport layer) thus my 4-layer comment. Could be Dmitry (foundewr of Examnotes) and I are old school, and the QOD reflects our "old ways." :P
Back to the first paragraph, DHCP uses the Bootp port numbers 67 and 68 for the server/client relationship(http://www.iana.org/assignments/port-numbers), because it was developed from the BootP application architecture. But the actual request uses UDP port 17 to broadcast the request to the network. Check out http://makeashorterlink.com/?N2FA32505 for a good overview of DHCP.
But it still is an upper layer application (it runs as a service on Windoze servers, right?!?!) that has to request services from lower layer protocols like UDP. Specifically, take this request packet and get it to the DHCP server.
I just did a Sniffer Trace of a release/renew process on my Windoze laptop, and DHCP is upper layer (DLC header for layer 2, IP Header for Layer 3, UDP header for layer 4, and DHCP Header for the upper layer application. It gathers the required information (MAC Address, last IP Address, workstation name, etc) for a DHCP request and passes it down to the Transport Layer for UDP to bundle it and blast it out. I'm attaching a screenprint of the trace (in a zipped Word document)
HTH...
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| donaldmc 2003-06-26, 4:06 pm |
| what sniffer trace do you use, I like ethereal |
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