For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected IP network routing region and bounded by encapsulating border nodes. These virtual topologies are manifested by tunnels that may span multiple IP and/or sub-IP layer forwarding hops, where they may incur packet duplication, packet reordering, source address spoofing and traversal of links with diverse Maximum Transmission Units (MTUs). This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that addresses these issues.