Sandvine Sandvine Orange Orange At IETF97, at a meeting regarding the Path Layer UDP Substrate (PLUS) protocol, a request was made for documentation about the benefits that might be provided by permitting middleboxes to have some visibility to transport-layer information. This document summarizes benefits provided to the Internet by intermediary devices that provide functions apart from normal IP forwarding. Such intermediary devices are often called "middleboxes". RFC3234 defines a taxonomy of middleboxes and issues in the Internet. Most of those middleboxes utilize or modify application-layer data. This document primarily focuses on devices that observe and act on information carried in the transport layer, and especially information carried in TCP packets. A primary goal of this document is to provide information to working groups developing new transport protocols, in particular the PLUS and QUIC working groups, to aid understanding of what might be gained or lost by design decisions that may affect (or be affected by) middlebox operation.