Named Pipes vs. Mail Slots for Broadcast-Based Programs

ID: Q61797

The information in this article applies to:

Microsoft LAN Manager, versions 1.1, 2.0, 2.1, 2.1a, 2.2

SUMMARY

Question:

Are mail slots the preferred method to be used with broadcast-based communication applications on an OS/2 LAN-Manager-based LAN?

Also, does the mail slot API use NetBIOS broadcasts to implement its broadcast call?

Response:

Many application-specific requirements must be considered when making a decision as to whether to use named pipes or mail slots for a broadcast-based OS/2 LAN communications program.

To help answer the questions above, let's review how named pipes and mail slots work.

Named Pipes

When a workstation connects to a named pipe that has been established on a server (or peer service workstation), a session (NetBIOS virtual connection) is established between the two network nodes. This virtual connection (VC) is maintained until either node explicitly closes the pipe. Therefore, named pipes provide an efficient platform for frequent data transfer as the underlying protocols do not have the extra overhead of creating and dropping a session each time an instance of data is transferred. Moreover, data written to the named pipe is guaranteed to reach its destination.

The down side to using named pipes is that a session is taken up for each internodal VC, which tends to deplete the session pool. Please note that if a session has been established with a workstation (for example, by a NET USE done earlier), the named pipe will use this same session. Therefore, if A and B already have a session established between them, the creation of a named pipe will not add another session. Also, please note that OS/2 LAN Manager is designed to minimize session pool depletion owing to its transparent auto-disconnect/reconnect feature and its efficient exploitation of the network interface unit session handling capability.

Remember that a named pipe can be created only by a server or by an OS/2 workstation that is configured as a peer server. An OS/2 workstation (no server service running) or a DOS workstation can read from and write to a named pipe but can never create one.

Mail Slots

OS/2 LAN Manager offers two types of mail slots: first class and second class.

First class mail slots work like a temporary named-pipe session. When mail is sent, a session is established, the instance data is transferred, and then, after a time-out period, the session is dropped. Like named pipes, first class mail slots provide secure, guaranteed delivery. First class mail slots are useful where guaranteed delivery is essential, but where instances of data transfer to any given node are expected to be sporadic or seldom.

First class mail slots can ONLY be used to send data from a DOS or OS/2 workstation (or OS/2 server) to an OS/2 server, and cannot be used for peer data transfer between DOS workstations and/or OS/2 workstations (without the server service).

Unlike first class mail slots, second class mail slots use the NetBIOS broadcast function to send data. Therefore, a VC is never established. Second class mail cannot be guaranteed. Thus, the sender will never know if the message reached its destination unless within the message the sender asks the receiver to return confirmation of receipt. This (if required) could potentially add significant overhead as the sender can spend a lot of time resending the message until a confirmation of receipt is finally obtained.

However, if guaranteed delivery is not required, second class mail is a good platform for high-speed messaging owing to low overhead (no need to create/drop sessions to guarantee delivery). As an added bonus, second class mail slots can be used between peer DOS workstations as well as OS/2 workstations and servers. >

Additional query words: 1.00 2.00 2.10 2.10a 2.20

Keywords :
Version : :1.1,2.0,2.1,2.1a,2.2
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