The same argument exists for many routing protocols: for example, BGP is used to route at Layer 3, but BGP itself is carried by TCP (and IP). This shows how arbitrary the bucket game is: a packet capture shows the ICMP header at Layer 4, so many network engineers will want to answer “Layer 4:” never argue with a packet. We call this the “bucket game.” For example, which bucket does ICMP go into: Layer 3 or Layer 4? ICMP headers are at Layer 4, just like TCP and UDP, so many will answer “Layer 4.” Others argue ICMP is a Layer 3 protocol, since it assists IP (a Layer 3 protocol), and has no ports. “Which protocol runs at which layer” is often a subject of fierce debate.
#Firewall udp unreplied code
ICMP header fields that are relevant for enforcing security include the Type field, which identifies the type of ICMP message, and the Code field, which reports the error code. Although ICMP messages are carried by IP, ICMP is regarded to be at the same level as IP, the network layer level.
ICMP packets occupy the data portion of an IP packet while being transported over the Internet. To slow down transmission at the source, a Source Quench message is generated by the router and sent to the source. The ability to reach an IP address is checked using an Echo Request message that is used in the Ping utility. When a route is to be changed a Redirect message is used. For example, when an IP packet cannot be delivered due to a destination not being reachable a Destination Unreachable ICMP message is used. ICMP provides error and control messages that are used in a variety of situations.
ICMP is used for status reporting over the Internet. Kaushal Chari, in Encyclopedia of Information Systems, 2003 II.A.2. By invalid, we mean zero, loopback, multicast, or broadcast. ICMP messages can not be sent in response to multicast or broadcast traffic, nor can they be sent for traffic that is from an invalid address. Whenever there is fragmented traffic, ICMP messages are only sent for errors on the first fragment. See what we mean? ICMP also has a method in place to prevent multiple messages for the same error in the way that error messages are sent in response to fragmented traffic. This is another good design concept since you could have a situation where one error message creates another, and another, and another. Also, ICMP messages cannot be sent in response to other ICMP messages. Some devices might even see them as interruptions, so they might end up lost or discarded. To make sure that ICMP messages won’t flood the network, they are given no special priority, and their messages are treated as normal traffic. These are the ground rules as to how ICMP operates. For ICMP to work efficiently in a networked environment, there has to be some basic ground rules as to how ICMP messages should be handled. Any network device that is using TCP/IP has the capability to send, receive, or process ICMP messages. ICMP gives TCP/IP a way to handle errors.
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