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Tanenbaum's Computer Networks says:

5.6 THE NETWORK LAYER IN THE INTERNET

...

5.6.4 Internet Control Protocols

In addition to IP, which is used for data transfer, the Internet has several companion control protocols that aIn re used in the network layer. They include ICMP, ARP, and DHCP. In this section, we will look at each of these in turn, describing the versions that correspond to IPv4 because they are the protocols that are in common use. ICMP and DHCP have similar versions for IPv6; the equivalent of ARP is called NDP (Neighbor Discovery Protocol) for IPv6.

Does it mean that the companion control protocols of the Internet, e.g. ICMP, ARP/NDP, and DHCP, are in the network layer?

https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol lists ICMP and NDP in the Internet layer (i.e. the network layer?), and DHCP in the application layer.

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Is DHCP in the application layer or the network layer? If the former, why are ICMP and NDP in the Internet layer (i.e. the network layer?)?

Thanks.

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DHCP, like other application-layer protocols, uses a transport protocol, which is above the network layer. DHCP uses UDP as its transport protocol.

In the case of DHCP, the application using the protocol is the host OS.

"Does it mean that the companion control protocols of the Internet, e.g. ICMP, ARP/NDP, and DHCP, are in the network layer?"

ICMP, by definition, is an integral part of IP, which is a network-layer protocol, is in the network layer, although it rides IP as if it were a transport-layer protocol.

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.

ARP is used by some data-link protocols to find a data-link address for a network address, and it is generally considered a data-link protocol, much as ICMP is a network-layer protocol. Neither IP, IPv4 nor IPv6, knows anything about ARP, and IP is the network-layer protocol, so ARP is not a network-layer protocol. A network protocol is used to forward network packets between different networks, but ARP is for building data-link frames, not network packets.

NDP is part of IPv6, using ICMPv6, not separate from IPv6 as ARP is from IPv4. NDP does much more than the ARP function.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Ron Maupin ♦
    May 30, 2022 at 19:03
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It is key to understand that Internet Protocol itself is software implemented virtual networking on top of dissimilar hardware. ICMP is considered a network layer protocol because it is used by communicating nodes for control purposes. RFC 792 states:

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.

DHCP is considered an application layer protocol because it is optional and requires a server be explicitly enabled by a network administrator. RFC 2131 states:

The Dynamic Host Configuration Protocol (DHCP) provides configuration parameters to Internet hosts. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host and a mechanism for allocation of network addresses to hosts.

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  • Thanks. How is a protocol decided which layer it belongs to?
    – Tim
    May 30, 2022 at 16:07
  • based on it's functionality in the overall Internet Protocol paradigm.
    – Ronnie Royston
    May 30, 2022 at 16:16
  • @Tim, "How is a protocol decided which layer it belongs to?" We have tried to explain that the layers are defined by models, but the models are just models. The real world does not strictly follow the models, which were created to give you a general way in which protocols work. There are protocols that do not fit the models. You use the models to understand abstraction and encapsulation, but you will go insane trying to fit every protocol into a model because the real world does not care about the models.
    – Ron Maupin ♦
    May 30, 2022 at 16:45

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