The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Feature Protocols

PPP Multilink Protocol (MP/MLP/MLPPP) 1 2 PPP Protocol Frame Formats

BACP Operation: Configuring the Use of BAP

Let's start with BACP, since it is the protocol used for initial setup of the feature. BACP is very similar in general concept to all those other PPP protocols with “Control” in their names, such as LCP, the NCP family, CCP and ECP, but is actually even simpler. It is used only during link configuration to set up BAP. This is done using Configure-Request, Configure-Ack, Configure-Nak and Configure-Reject messages just as described in the LCP topic. The only configuration option that is negotiated in BACP is one called Favored-Peer, which is used to ensure that the two devices on the link don't get “stuck” if each tries to send the same request at the same time.

If both devices support BAP then the BACP negotiation will succeed and BAP will be activated.

BAP defines a special set of messages that can be sent between devices to add or drop links to/from the current PPP bundle. What's particularly interesting about BAP is that it includes the tools necessary to have a device actually initiate different types of physical layer connections (such as dialing a modem for bundled analog links or enabling an extra ISDN channel) when more bandwidth is required, and then shut them down when no longer needed.

A brief description of the BAP message types:

• Call-Request and Call-Response: When one device on the link wants to add a link to the bundle and initiate the new physical layer link itself, it sends a Call-Request frame to tell the other device, which replies with a Call-Response.
  
  
• Callback-Request and Callback-Response: These are just like the two message types above, but used when a device wants its peer (the other device on the link) to initiate the call to add a new link. So, if device A says “I need more bandwidth but I want you to call me, instead of me calling you”, it sends device B a Callback-Request.
  
  
• Call-Status-Indication and Call-Status-Response: After a device attempts to add a new link to the bundle (after sending a Call-Request or receiving a Callback-Request) it reports the status of the new link using the Call-Status-Indication frame. The other device then replies with a Call-Status-Response.
  
  
• Link-Drop-Query-Request and Link-Drop-Query-Response: These messages are used by one device to request that a link be dropped and the other to respond to that request.

I should also point out that the decision of when to add or remove links is not made by these protocols. It is left up to the particular implementation.

PPP Multilink Protocol (MP/MLP/MLPPP) 1 2 PPP Protocol Frame Formats