2. The MUD Model and Semantic Meaning
A MUD file consists of a YANG model instance that has been serialized in JSON [RFC7951]. For purposes of MUD, the nodes that can be modified are access lists as augmented by this model. The MUD file is limited to the serialization of only the following YANG schema:
- ietf-access-control-list [RFC8519]
- ietf-mud (RFC 8520)
- ietf-acldns (RFC 8520)
Extensions may be used to add additional schema. This is described further on.
To provide the widest possible deployment, publishers of MUD files SHOULD make use of the abstractions in this memo and avoid the use of IP addresses. A MUD manager SHOULD NOT automatically implement any MUD file that contains IP addresses, especially those that might have local significance. The addressing of one side of an access list is implicit, based on whether it is applied as to-device-policy or from-device-policy.
With the exceptions of the "name" of the ACL, "type", "name" of the Access Control Entry (ACE), and TCP and UDP source and destination port information, publishers of MUD files SHOULD limit the use of ACL model leaf nodes expressed to those found in this specification. Absent any extensions, MUD files are assumed to implement only the following ACL model features:
- match-on-ipv4, match-on-ipv6, match-on-tcp, match-on-udp, match-on-icmp
Furthermore, only "accept" or "drop" actions SHOULD be included. A MUD manager MAY choose to interpret "reject" as "drop". A MUD manager SHOULD ignore all other actions. This is because manufacturers do not have sufficient context within a local deployment to know whether reject is appropriate. That is a decision that should be left to a network administrator.
Given that MUD does not deal with interfaces, the support of the "ietf-interfaces" module [RFC8343] is not required. Specifically, the support of interface-related features and branches (e.g., interface-attachment and interface-stats) of the ACL YANG module is not required.
In fact, MUD managers MAY ignore any particular component of a description or MAY ignore the description in its entirety, and they SHOULD carefully inspect all MUD descriptions. Publishers of MUD files MUST NOT include other nodes except as described in Section 3.9. See that section for more information.