As you read this section, consider the following questions: What is a Middlebox? Why do we need them?
When the TCP/IP architecture and the IP protocol were defined, two type of devices were considered in the network layer: endhosts and routers. Endhosts are the sources and destinations of IP packets while routers forward packets. When a router forwards an IP packet, it consults its forwarding table, updates the packet’s TTL, recomputes its checksum and forwards it to the next hop. A router does not need to read or change the contents of the packet’s payload.
However, in today’s Internet, there exist devices that are not strictly routers but which process, sometimes modify, and forward IP packets. These devices are often called middleboxes RFC 3234. Some middleboxes only operate in the network layer, but most middleboxes are able to analyse the payload of the received packets and extract the transport header, and in some cases the application layer protocols.
In this section, we briefly describe two type of middleboxes: firewalls and network address translation (NAT) devices. A discussion of the different types of middleboxes with references may be found in RFC 3234.
Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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