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Home > Archive > CCNA > May 2002 > Which layer?
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| I ran into a test question that stumped me.
Which layer in the TCP/IP model is ICMP on? What about PING? I understood it to be on the Internet layer. The answer it gave was on the application layer. How is this true. Is it on the application layer if it is utilized through DOS?
Some insight please. | |
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| ping itself is a utility. It relies on ICMP at layer 3 to send ICMP packets 
Check out www.mcsefreak.com/interosi.htm for an easy, interactive way to learn the OSI model... | |
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| definately layer 3
i think you had better get used the world of wrong questions | |
| marathoner 2002-05-22, 5:42 pm |
| yah welcome to the world of boneheaded questions. The problem is that Cisco has them too. 
As for this question, ICMP is on layer 3 of OSI definitely; however, that's layer 2 of the 4-layer DoD model because they lump physical & datalink together. But then again other books (notably Scott Mann) say the TCP/IP network model is a FIVE layer model. And did they just want one answer for both ping and ICMP or what. Because it could be argued that ping is an application (although I would call it a utility) however, the source code for ping drills right down to packets probably for efficiency reasons.
Furthermore ping's purpose is to TEST OSI layer three connectivity.
So your best hope is that you don't get very many questions like this because you will either miss the trick or you will overthink it. Either way you're probably hosed. It's just so Cisco can make more money. | |
| YoungOne 2002-05-22, 5:54 pm |
| Hi!
ICMP is a protocol that is used to check the arrival of packets, ping is a utility used to troubleshoot connections. Also, there is no internet layer in the OSI model, but a Network layer. I believe ICMP works on the transport layer, because it checks the correct arrival of packets.
There are various sites on the internet where you can check this out. | |
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| i don't see how this question can be turned into a trick question, then later as some sort of justification for failing an exam and the greed of cisco... | |
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| quote:
Originally posted by YoungOne
Hi!
I believe ICMP works on the transport layer, because it checks the correct arrival of packets.
once again, ICMP is a layer 3 protocol... not 4. | |
| YoungOne 2002-05-22, 6:48 pm |
| freak is correct. Thanks for reminding me. | |
| SureshHomepage 2002-05-22, 7:43 pm |
| Hi,
The TCP/IP destination host will communicate with a source host to report an error in datagram processing. For this kind of purposes the Internet Control Message Protocol (ICMP) is used. ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by every IP module. No doubt IP and and its associate ICMP falls on Layer 3.
ICMP is discussed in rfc 792. | |
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| thanks for teh link to the rfc | |
| Detour 2002-05-23, 10:54 am |
| This is question that has been argued many times(i remember a discussion from a year ago on this same forum). The accepted answer is that ICMP is on OSI layer 3, alongside IP. The reason for this is that ICMP was defined within the DOD model as being on the Internet layer with IP. Also, ICMP is handled within the "internet protocol module" along with ip. Translating DOD to OSI is a whole different animal, subject to interpretations and individual perspectives.
Within the OSI model, it's possible to argue it's a layer 4 protocol. Actual "clear-cut" layer 3 protocols, such as ARP or IPX, have their own ethertype (0x806 and 0x8137 respectively.) ICMP does not. It uses IP's ethertype(0x800).
A standard icmp packet follows a standard ip header, just as tcp or udp. within the ip header, ICMP has it's own protocol value(0x01) just as TCP(0x06) and UDP(0x11).
So when you look at an icmp packet, it's "encapsulated" within an ip header, just as tcp or udp.
But again, this is all just an academic exercise. There are quite a few protocols that are vague in their true layer. Go to deja.com and type in "OSI DOD" there are tons of arguments based on various protocols. I could also argue all day about why we even have layer 5+6+7 vs. having one layer above 4(with pros and cons). All protocol to layer translations can be extremely vague. Sure, books will give you "exact" layers for various protocols. YMMV.
So that solves nothing I'm glad i could help  | |
| nigguru 2002-05-23, 3:25 pm |
| ICMP is a management protocol and messaging service for IP.
ICMP along with IP, ARP, RARP, and BOOTP exist at the Network layer (3). | |
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| that settles that | |
| The Reamer 2002-05-24, 2:35 pm |
| ICMP, as others have stated so well, is on layer 3. PING is an application that relies on ICMP. As an application, Ping, is on the application layer.
Reamer |
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