The Browser service uses "Mailslots" and is perhaps the only protocol that does. The mailslot "frames" are carried in SMB "transact" packets. The opcode within the Transact SMB packet is Mailslot Write. Within the data portion of the Transact packet is the mailslot frame. The Transact data itself begins with an opcode as shown below:

Table 1. Transact data opcodes

Opcode description
1 HostAnnouncement
2 AnnouncementRequest
8 RequestElection
9 GetBackupListReq
10 GetBackupListResp
11 BecomeBackup
12 DomainAnnouncment
13 MasterAnnouncement
15 LocalMasterAnnouncement