A client in SEAM is identified by its principal. A principal is a unique identity to which the KDC can assign tickets. A principal can be a user, such as joe, or a service, such as nfs.
By convention, a principal name is divided into three parts: the primary, the instance, and the realm. A typical SEAM principal would be, for example, joe/admin@ENG.ACME.COM, where:
joe is the primary. This can be a username, as shown here, or a service, such as nfs. It can also be the word host, signifying that this is a service principal set up to provide various network services.
admin is the instance. An instance is optional in the case of user principals, but it is required for service principals. For example: if the user joe sometimes acts as a system administrator, he can use joe/admin to distinguish himself from his usual user identity. Likewise, if joe has accounts on two different hosts, he can use two principal names with different instances (for example, joe/denver.acme.com and joe/boston.acme.com). Notice that SEAM treats joe and joe/admin as two completely different principals.
In the case of a service principal, the instance is the fully qualified hostname. bigmachine.eng.acme.com is an example of such an instance, so that the primary/instance might be, for example, nfs/bigmachine.eng.acme.com or host/bigmachine.eng.acme.com.
ENG.ACME.COM is the SEAM realm. Realms are discussed in the next section.
The following are all valid principal names:
joe
joe/admin
joe/admin@ENG.ACME.COM
nfs/host.eng.acme.com@ENG.ACME.COM
host/eng.acme.com@ENG.ACME.COM