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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 Encryption Control Protocol (ECP) and Encryption Algorithms 1 2 3 PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP)

PPP Multilink Protocol Setup and Configuration

To use MP, both devices must have it implemented as part of their PPP software and must negotiate its use. This is done by LCP as part of the negotiation of basic link parameters in the Link Establishment phase (just like Link Quality Reporting). Three new configuration options are defined to be negotiated to enable MP:

• Multilink Maximum Received Reconstructed Unit: Provides the basic indication that the device starting the negotiation supports MP and wants to use it. The option contains a value specifying the maximum size of PPP frame it supports. If the device receiving this option does not support MP it must respond with a Configure-Reject LCP message.
  
  
• Multilink Short Sequence Number Header Format: Allows devices to negotiate use of a shorter sequence number field for MP frames, for efficiency. (See the topic on MP frames for more.)
  
  
• Endpoint Discriminator: Uniquely identifies the system; used to allow devices to determine which links go to which other devices.

Before MP can be used, a successful negotiation of at least the Multilink Maximum Received Reconstructed Unit option must be performed on each of the links between the two devices. Once this is done and an LCP link exists for each of the physical links, a virtual bundle is made of the LCP links and MP is enabled.

As mentioned above, MP basically sits between the network layer and the regular PPP links and acts as a “middleman”. Here is what it does for each “direction” of communication:

• Transmission: MP accepts datagrams received from any of the network layer protocols configured using appropriate NCPs. It first encapsulates them into a modified version of the regular PPP frame. It then takes that frame and decides how to transmit it over the multiple physical links. Typically, this is done by dividing the frame into fragments that are evenly spread out over the set of links. These are then encapsulated and sent over the physical links. However, an alternate strategy can also be implemented as well, such as alternating full-sized frames between the links. Also, smaller frames are typically not fragmented, nor are control frames such as those used for link configuration.
  
  
• Reception: MP takes the fragments received from all physical links and reassembles them into the original PPP frame. That frame is then processed like any PPP frame, by looking at its Protocol field and passing it to the appropriate network layer protocol.

The fragments used in MP are similar in concept to IP fragments, but of course these are different protocols running at different layers. To PPP or MP, an IP fragment is just an IP datagram like any other.

The fragmenting of data in MP introduces a number of complexities that the protocol must handle. For example, since fragments are being sent roughly concurrently, we need to identify them with a sequence number to facilitate reassembly. We also need some control information to identify the first and last fragments. A special frame format is used for MP fragments to carry this extra information, which I describe in the section on PPP frame formats. That topic also contains more information about how fragmenting is accomplished, as well as an illustration that demonstrates how it works.

PPP Encryption Control Protocol (ECP) and Encryption Algorithms 1 2 3 PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP)