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ICMP tunneling works by injecting arbitrary data into an echo packet sent to a remote computer. The remote computer replies in the same manner, injecting an answer into another ICMP packet and sending it back. The client performs all communication using ICMP echo request packets, while the proxy uses echo reply packets.
The related ping utility is implemented using the ICMP echo request and echo reply messages. ICMP uses the basic support of IP as if it were a higher-level protocol, however, ICMP is actually an integral part of IP. Although ICMP messages are contained within standard IP packets, ICMP messages are usually processed as a special case ...
The process repeats until a packet is delivered to the destination host, or earlier along the route, when a router has no default route available and cannot route the packet otherwise. In the latter case, the packet is dropped and an ICMP Destination Unreachable message may be returned. [1]
With ECN, the congestion is indicated by setting the ECN field within an IP packet to CE (Congestion Experienced) and is echoed back by the receiver to the transmitter by setting proper bits in the header of the transport protocol. For example, when using TCP, the congestion indication is echoed back by setting the ECE bit.
A functionality which lets a computer A find out whether a computer B is reachable and responding is built into the Internet Control Message Protocol (ICMP). Through an "Echo Request" Computer A asks B to send back an "Echo Reply". [2] These two messages are also sometimes called "ping" and "pong" for historical purposes. [citation needed]
Proceeding in this way, traceroute uses the returned ICMP Time Exceeded messages to build a list of routers that packets traverse, until the destination is reached and returns an ICMP Destination Unreachable message if UDP packets are being used or an ICMP Echo Reply message if ICMP Echo messages are being used. [10]
However, experienced sniffers can prevent this (e.g., using carefully designed firewall settings). An example is sending a ping (ICMP echo request) with the wrong MAC address but the right IP address. If an adapter is operating in normal mode, it will drop this frame, and the IP stack never sees or responds to it.
The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It appeared in 1999, in the context of the boom of DSL as the solution for tunneling packets over the DSL connection to the ISP's IP network, and from there to the rest of the Internet.