France Telecom France Telecom This document specifies an extension to Multipath TCP (MPTCP) that is meant to assess whether a path used to establish a given subflow is MPTCP-friendly, i.e., intermediate nodes involved in that path do not alter nor strip MPTCP options, which would prevent the establishment of MPTCP communications along that path. A new flag bit, called Probe Flag (P-flag) is defined for this purpose. This document updates RFC6824.