OSI Model Data Transfer Services and LAN Manager Requirements

DATA TRANSFER SERVICES

A connection oriented service requires that before any data transfer can take place a connection must be established from the source station to a single destination station. A connection functions like a tube: the sender pushes objects in at one end, and the receiver takes them out in proper sequence at the other end. Since sequencing is provided, a message that is larger than the maximum transmit frame size can be broken down into multiple frames, sent across the network, and properly reassembled at the receiving station.

A connectionless service requires no initial connection, and offers no guarantee that data units sent will arrive in sequence. Since no connection is required, messages can be sent to one or multiple destination stations. However, since no sequencing is provided, only single-frame messages can be sent.

A "reliable" service never loses data because it requires that the the receiver acknowledge receipt of all data sent. If the sender does not receive acknowledgment, a reliable service re-sends. An "unreliable" service requires no acknowledgments.

With message mode, data is sent, received, sequenced and acknowledged as discrete messages. Message boundaries are preserved from the sending entity to the receiving entity. With byte mode, data is sent, received, sequenced and acknowledged as a stream of bytes, with no message boundaries: when a 2K stream is received, there is no way to tell if it was sent as one 2K message, two 1K messages, or even 2048 1-byte messages.

A connectionless message (single-frame size) can be unacknowledged (datagram); or it can be acknowledged either with a simple ack (acked datagram) or an ack with data (request-reply).

LAN MANAGER REQUIREMENTS

LAN Manager requires that the underlying network drivers provide both reliable connection oriented, and unreliable connectionless datagram data transfer services.

Reliable Connection Oriented

• Network logon (verified)
• User password modification
• Messages sent/received (computername or logon name)
• Network file access
• Network printer access
• Network comm I/O device access
• Remote server administration
• Remote command execution
• Reliable inter-process communications (Named Pipes,

       First-class Mailslots)
  

• Domain controller synchronization
• Dynamic file replication

Unreliable Connectionless Datagram

• Network logon (verified)
• User password modification
• Messages sent/received (domain or network)
• Unreliable inter-process communications (Second-class Mailslots)
• Server announcements
• Domain controller synchronization
• Dynamic file replication
• Domain logon list maintenance