Abstractly, the Flask network server ensures that every network IPC is authorized by the security policy. Of course, a network server cannot independently ensure that a network IPC is authorized by the policy of its node, since it does not have end-to-end control over data delivery to processes on peer nodes. Instead, a network server must extend some level of trust to its peer network servers to enforce its own security policy, in combination with their own security policies, over the peer processes. This requires a reconciliation of security policies, which would be handled by a separate negotiation server. The current negotiation server is limited to negotiating network security protocols and cryptographic mechanisms using the ISAKMP  protocol. The precise form of trust and the precise level of trust extended to peer network servers can vary widely and would be defined within the policy. Extending the concept of policy flexibility to a networked environment will require such support for complex trust relationships.
The principal controlled object type for the network server is the socket. For socket types that maintain message boundaries (e.g., datagram), the network server also binds a separate SID to each message sent or received on a socket. For other socket types, each message is implicitly associated with the SID of its sending socket. Since messages cross the boundary of control of the network server, and may even cross a policy domain boundary, the network server may need to apply cryptographic protections to messages in order to preserve the security requirements of the policy and must bind the security attributes of the message to the message. Our prototype network server uses the IPSEC  protocols for this purpose, with security associations established by the negotiation server. The negotiation server may not pass SIDs across the network, since they are only local identifiers; instead, the negotiation server must pass the actual security attributes to its peer, which can then establish its own SID for the corresponding security context. Although the negotiation server must handle security contexts, it does not interpret them, and thus remains policy-flexible. Attribute translation and interpretation must be performed by the corresponding security servers in accordance with the policy reconciliation.
The network server controls are layered to match the network protocol layering architecture. Hence, the abstract control over the high-level network IPC services consists of a collection of controls over the abstractions at each layer, as shown in Table 9. The layered controls provide the policy with the ability to precisely regulate network operations, using all the information relevant to security decisions, and they allow the policy to take advantage of specific characteristics of the different protocols (e.g., the client/server relationship in TCP). The network server provides another example of the problems with implementing security controls at the server's external interface. This is due to the need to control abstractions and interpose on operations which are not exported by the network server's external interface.
Since the TCP and UDP port spaces are fixed resources, the network server uses the Flask architecture's polyinstantiation support for security union port spaces (SUPs). SUPs are analogous to the SUDs discussed in Section A.1. The polyinstantiation support is used to determine the preferred member port space when a port number is associated with a socket and when an incoming packet has a destination port number which exists in multiple member port spaces. The SUP mechanism provides a unified view of all accessible port spaces within the polyinstantiated port space based on access decisions.
Many of the details of the Flask network server and other servers that support it are beyond the scope of this paper. A much more detailed description of an earlier version of the Flask network server can be found in .