Is ICMP a transport layer protocol?

问题:

I was going through a video lecture on networking and there the lecturer mentions ICMP as a transport layer protocol. However, Googling it shows various forums describing it as network layer protocol. This has confused me a lot. Can someone clarify?

回答1:

Transport layer protocols are concerned with send data from end-to-end and ensuring (or explicitly not ensuring) reliability. TCP is used to send data from one computer to another and includes logic necessary to ensure that the data is transported correctly, while UDP is used to send data from one computer to another while hopefully getting reliability.

ICMP really doesn't do this. Its job is to help the routers figure out the shape of the internet and what direction to send packets with different protocols. Consequently, it's considered a network-layer protocol, since its job is to ensure that data is routed to the right place but doesn't actually route the data.

Hope this helps!

回答2:

No doubt it is a network layer protocol. The reasons it is often confused with transport layer is that ICMP works without ANY transport layer. E.g. ping or traceroute do not have well defined ports. Instead each machine's tcp/ip stack is responsible for processing and responding to incoming ICMP messages (rather than looking up a port and passing on to corresponding application, as is the case with all other applications).

Update:

Apparently it sounds confusing to readers, so here is snap shot from TCP/IP Illustrated by Richard Stevens explaining the concept and intent.

Quoting from the book.

"Positioning the protocol boxes labeled ICMP and IGMP is always a challenge. Earlier, we showed them at the same layer as IP, because they really are adjuncts to IP. But here we show them above IP, to reiterate that ICMP messages and IGMP messages are encapsulated in IP datagrams"

Teaching the concept that "some protocols and applications" work differently from the standard concept of each layer being responsible for a different task (transport being end to end, where as network being hop to hop) is tough and takes time to digest.

The next most critical thing to learn is,

Even if you ping using multiple say consoles from the SAME machine to another, it STILL is able to keep track of all individual, so called connections (just relating the message exchange, otherwise there is no connection like tcp).

How the above is done, is OS specific. The Linux stack puts 8 bytes in data field of ICMP, which contains ports from which the original application layer triggered. This is also explained in the same book under the ping program.

The same applies to lower layer protocols such as ARP/RARP.