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PPP Multilink Protocol (MP/MLP/MLPPP)
(Page 3 of 3)
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
- 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.
PPP Multilink Protocol Operation
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
- 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.
Related Information: I also recommend reading the next topic, which describes two protocols defined after MP to better control how it works: BAP and BACP.
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Version 3.0 - Version Date: September 20, 2005
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