A middlebox is a computer networking device that transforms, inspects, filters, and manipulates traffic for purposes other than packet forwarding.
[1] Examples of middleboxes include firewalls, network address translators (NATs), load balancers, and deep packet inspection (DPI) devices.
For example, one carrier set an "aggressive timeout value to quickly recycle the resources held by inactive TCP connections in the firewall, unexpectedly causing frequent disruptions to long-lived and occasionally idle connections maintained by applications such as push-based email and instant messaging".
[8] Other common middlebox-induced application challenges include web proxies serving "stale" or out-of-date content,[15] and firewalls rejecting traffic on desired ports.
For example, IPv6 can be deployed on public endpoints such as load balancers, proxies, or other forms of NAT, with backend traffic routed over IPv4 or IPv6.