Editing Transmission Control Protocol (section)

== Wire image and ossification ==
The [[wire data]] of TCP provides significant information-gathering and modification opportunities to on-path observers, as the protocol metadata is transmitted in [[cleartext]].{{sfn|RFC 8546|p=6}}{{sfn|RFC 8558|p=3}} While this transparency is useful to network operators{{sfn|RFC 9065|loc=2. Current Uses of Transport Headers within the Network}} and researchers,{{sfn|RFC 9065|loc=3. Research, Development, and Deployment}} information gathered from protocol metadata may reduce the end-user's privacy.{{sfn|RFC 8558|p=8}} This visibility and malleability of metadata has led to TCP being difficult to extend&mdash;a case of [[protocol ossification]]&mdash;as any intermediate node (a '[[middlebox]]') can make decisions based on that metadata or even modify it,{{sfn|RFC 9170|loc=2.3. Multi-party Interactions and Middleboxes}}{{sfn|RFC 9170|loc=A.5. TCP}} breaking the [[end-to-end principle]].{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=620}} One measurement found that a third of paths across the Internet encounter at least one intermediary that modifies TCP metadata, and 6.5% of paths encounter harmful ossifying effects from intermediaries.{{sfn|Edeline|Donnet|2019|p=175-176}} Avoiding extensibility hazards from intermediaries placed significant constraints on the design of [[MPTCP]],{{sfn|Raiciu|Paasch|Barre|Ford|2012|p=1}}{{sfn|Hesmans|Duchene|Paasch|Detal|2013|p=1}} and difficulties caused by intermediaries have hindered the deployment of TCP Fast Open in [[web browsers]].{{sfn|Rybczyńska|2020}} Another source of ossification is the difficulty of modification of TCP functions at the endpoints, typically in the [[operating system kernel]]{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=621}} or in hardware with a [[TCP offload engine]].{{sfn|Corbet|2015}}