Middlebox

A middlebox or network appliance is a computer networking device that transforms, inspects, filters, or otherwise manipulates traffic for purposes other than packet forwarding. Common examples of middleboxes include firewalls, which filter unwanted or malicious traffic, and network address translators, which modify packets' source and destination addresses. Dedicated middlebox hardware is widely deployed in enterprise networks to improve network security and performance, however, even home network routers often have integrated firewall, NAT, or other middlebox functionality. The widespread deployment of middleboxes and other network appliances has resulted in some challenges and criticism due to poor interaction with higher layer protocols.

Lixia Zhang, the Jonathan B. Postel Professor of Computer Science at the University of California, Los Angeles, coined the term "middlebox" in 1999.

The following are examples of commonly deployed middleboxes:

Although widely deployed, middleboxes have generated some technical challenges for application development and some controversy regarding their impact.

Some middleboxes interfere with application functionality, restricting or preventing end host applications from performing properly.

Network Address Translators present a challenge in that NAT devices divide traffic destined to a public IP address across several receivers. When connections between a host on the Internet and a host behind the NAT are initiated by the host behind the NAT, the NAT learns that traffic for that connection belongs to the local host. Thus, when traffic coming from the Internet is destined to the public (shared) address on a particular port, the NAT can direct the traffic to the appropriate host. However, connections initiated by a host on the Internet do not present the NAT any opportunity to "learn" which internal host the connection belongs to. Moreover, the internal host itself may not even know its own public IP address to announce to potential clients what address to connect to. To resolve this issue, several new protocols have been proposed.

Other common middlebox-induced application challenges include web proxies serving "stale" or out of date content, and firewalls rejecting traffic on desired ports.

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