Network Working Group K. Smith
Request For Comments: 1934 Ascend Communications
Category: Informational April 1996
Ascend"s Multilink Protocol Plus (MP+)
Status of This Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document proposes an extension to the PPP Multilink Protocol
(MP) [1]. Multilink Protocol Plus (MP+) is a new control protocol for
managing multiple data links that are bundled by MP.
Table Of Contents
1. IntrodUCtion.................................................2
1.1 Functional Description...............................2
1.2 Conventions..........................................3
2. General Overview.............................................3
2.1 Operation............................................4
3. MP+ Frame Formats............................................4
3.1 Error Control (EC) Layer.............................6
3.1.1 Error Control State Machine..................7
3.2 Multilink Plus Control Messages......................9
3.3 Multilink Plus Message Formats......................10
3.3.1 VERSION_EXCHANGE_REQ Message Format.........10
3.3.2 VERSION_EXCHANGE_RSP Message Format.........12
3.3.3 ADD_REQ Message Format......................13
3.3.4 ADD_RSP Message Format......................15
3.3.5 ADD_COMPLETE Message Format.................16
3.3.6 REMOVE_REQ Message Format...................17
3.3.7 REMOVE_RSP Message Format...................17
3.3.8 REMOVE_COMPLETE Message Format..............18
3.3.9 CLOSE_REQ Message Format....................19
3.3.10 CLOSE_RSP Message Format....................19
3.3.11 REMOTE_MGMT_REQ Message Format..............20
3.3.12 REMOTE_MGMT_RSP Message Format..............20
3.3.13 REMOTE_MGMT_RX_REQ Message Format...........21
3.3.14 REMOTE_MGMT_TX_REQ Message Format...........22
3.3.15 REMOTE_MGMT_TX_RSP Message Format...........22
3.3.16 CLEAR_REQ Message Format....................23
3.4 Events..............................................23
3.5 State Machine.......................................25
3.5.1 States......................................25
3.5.2 Common Actions..............................26
3.5.3 MP+STATE_INITIAL state machine..............32
3.5.4 MP+STATE_IDLE state machine.................35
3.5.5 MP+STATE_ADD state machine..................37
3.5.6 MP+STATE_REMOVE state machine...............41
3.5.7 MP+STATE_CLOSE state machine................44
4. PPP LCP Extensions..........................................46
5. Security Considerations.....................................47
6. References..................................................47
7. Author"s Address............................................47
1. Introduction
The PPP Multilink Protocol (MP), is a set of features that provide
inverse multiplexing at the packet/fragment level by bundling
multiple independent links between a fixed pair of systems, providing
a virtual link with greater bandwidth than any of the constituent
members.
Once multiple channels have been established MP is responsible for
managing channel use to insure in-sequence delivery of user packets.
MP+ is an extension to MP that adds an inband control channel to
provide a new level of session management and control.
MP+ also allows remote device management of (unconfigured) systems.
This feature allows a network operations center to dial into an
unconfigured system and remotely manage it, before ethernet
interface, IP address, and other LCP and system configuration
information is entered. (This does require local configuration of
the WAN interfaces to the extent required to answer an incoming
call).
1.1 Functional Description
The features of MP+ include:
* Ability to negotiate to add and suBTract channels when bandwidth
needs change.
* Phone number management so calling stations need not know every
possible number; answering stations can manage their own resources.
* A simple remote management interface.
To perform the above functions MP+ is split into a call management
layer and a reliable delivery layer. The call management layer is
the source and sink of MP+ control messages. The reliable delivery
layer adds a simple acknowledge and retry mechanism.
MP+ only takes network bandwidth when in the process of performing a
user request, e.g. adding and subtracting bandwidth.
NOTE: Neither MP, or MP+ define the process that makes the bandwidth
requirement determination. That is outside the scope of either of
these protocols and will likely be implementation dependent.
1.2 Conventions
The following language conventions are used in the items of
specification in this document:
MUST, SHALL or MANDATORY -- the item is an absolute requirement
of the specification.
SHOULD or RECOMMENDED -- the item should generally be followed
for all but exceptional circumstances.
MAY or OPTIONAL -- the item is truly optional and may be followed
or ignored according to the needs of the implementor.
2. General Overview
PPP
In order to establish communications over a point-to-point link,
each end of the PPP [2] link must first send LCP packets to
configure the data link during link establishment phase. After
the link has been established, PPP provides for an authentication
phase.
MP The goal of multilink operation is to bundle multiple
independent links between a fixed pair of systems, providing a
virtual link with greater bandwidth than any of the constituent
members.
MP+ MP+ is also negotiated during initial LCP option negotiation. A
system indicates to its peer that it is willing to do MP+ by
sending the MP+ option as part of the initial LCP option
negotiation. The MP+ option MUST NOT be negotiated unless MP is
also negotiated. When used, MP+ adds a virtual unit-to-unit
control channel.
A peer may elect to:
Acknowledge both the MP and MP+ options, indicating that both MP and
MP+ will be used.
Acknowledge the MP option and reject the MP+ option. Operation will
fall back to MP.
Reject both options. Standard PPP will be used for this connection.
2.1. Operation
Standard PPP
In standard PPP the LCP negotiation phase is followed by an
optional authentication phase, and then one or more NCPs are
initiated.
PPP with MP The LCP negotiation phase and authentication phase are
identical to standard PPP. The ability to initiate an MP
aggregate data link is indicated by sending an MP option - as
described in [1].
PPP with MP and MP+ When MP+ is negotiated at LCP startup, the same
procedures are followed as when MP is negotiated alone. The MP+
LCP option is negotiated to indicate the ability to use the MP+
feature.The first connection between endpoints causes the MP+
process to be started for the connection.
3. MP+ Frame Formats
+---------------+---------------+
PPP Header: Address 0xff Control 0x03
+---------------+---------------+
PID(H) 0x00 PID(L) 0x73
+-+-+-+-+-------+---------------+
MP Header: 11000001 seq # high
+-+-+-+-+-------+---------------+
sequence number low bits
+---------------+---------------+
control data
.
.
.
+---------------+---------------+
PPP FCS: FCS
+---------------+---------------+
Figure 1: Multilink Plus Frame Format (long sequence number format)
+---------------+---------------+
PPP Header: Address 0xff Control 0x03
+---------------+---------------+
PID(H) 0x00 PID(L) 0x73
+-+-+-+-+-------+---------------+
MP Header: 1101 sequence number
+-+-+-+-+-------+---------------+
control data
.
.
.
+---------------+---------------+
PPP FCS: FCS
+---------------+---------------+
Figure 2: Multilink Plus Frame Format (short sequence number format)
MP+ frames use a similar structure to MP fragments.
The MP+ assigned PID is designated 00 73.
MP+ control uses the following two rules:
- MP+ control frames have their own sequence number space,
controlled by MP+.
- MP+ control frames MUST NOT be fragmented.
NOTE: Implementations of this protocol prior to the date of submission
of this specification to the IETF use the same PID as MP, but
sets the LSB of the reserved bits in the MP header to 1 - this
is how the MP+ packets are discriminated from MP fragments.
So the header of the MP+ packet looks like:
00 3d c1 ......
As compared to an MP packet that looks like:
00 3d c0 ...... or
00 3d 80 ...... or
00 3d 40 ......
3.1. Error Control (EC) Layer (MP+ control only)
The error control layer that runs over the virtual inband channel is
as simple as it can get, while handling the possibility of errors on
the line.
An assumption is made that errors are infrequent, and that at the
same time messages are rarely, if ever, dropped on the floor. The
implication of this is that "timing out" on retransmission of
messages does no harm. If a message cannot get through, then it
simply is retried some number of times. After giving up, the only
recourse is to notify the call management layer (of MP) that the
session has died.
+---------------+---------------+
PPP Header: Address 0xff Control 0x03
+---------------+---------------+
PID(H) 0x00 PID(L) 0x73
+-+-+-+-+-------+---------------+
MP+ Header: 11000001 seq # high
+-+-+-+-+-------+---------------+
sequence number low bits
+---------------+---------------+
EC Header: Error Control Message Type
32 bits reserved
+---------------+---------------+
MP+ Data: MP+ Message May not be
present.
Figure 3: MP+ control message format (shown long sequence number
format)
Error Control Message Type:
1 DATA_MSG: This message contains MP+ data transferred
between the peers.
2 ACK_MSG: An acknowledgement of a previous data message.
When set to DATA_MSG, the remainder of the frame contains an MP+
Control message.
When set to ACK_MSG, the remainder of the frame consists only of the
PPP Frame Check Sum (FCS).
3.1.1. Error Control State Machine
This layer is controlled by a simple state machine. There are three
states:
Stopped There is no connection between peers.
Idle There is a connection between peers;
no unacknowledged messages pending.
Pending There is a connection between peers;
awaiting an acknowledgement to the
last message sent.
Messages from the call management layer are queued for transmission
whenever the link is in the pending state. For simplicity, only
one outstanding message may be in the link at any given time. The
entire procedure is defined in table 1.
Event State
______________________________________________________________________
Stopped Idle Pending
======================================================================
Start 1,Idle -,* -,*
______________________________________________________________________
Received ACK_MSG ** 2,Start 5,IdlePending
current tx sequence number
______________________________________________________________________
Received ACK_MSG ** -,* -,*
last tx sequence number
______________________________________________________________________
Received ACK_MSG ** 2,Start 2,Start
other tx sequence number
______________________________________________________________________
Received DATA_MSG ** 6,* 6,*
current rx sequence number
______________________________________________________________________
Received DATA_MSG ** 7,* 7,*
previous rx sequence number
______________________________________________________________________
Received DATA_MSG ** 2,Start 2,Start
other rx sequence number
______________________________________________________________________
Receive Invalid Frame ** 2,Start 2,Start
______________________________________________________________________
Retransmit Timer EXPire ** ** 4,Start*
______________________________________________________________________
______________________________________________________________________
Transmit Request from call -,* 3,Pending 8,*
management layer
______________________________________________________________________
Stop 9,Start 9,Start 9,Start
______________________________________________________________________
Table 1: Error Control State Machine
Legend:
- No action
* Stay in same state
** Invalid or meaningless event for state, ignored.
Notes:
[1] Data from the call management layer will always be copied before
being queued for transmission. The call management layer is
responsible for its own buffers.
[2] MP always copies data for transmission and returns immediately.
Any buffers allocated to build control messages MUST be released
immediately upon return from MP transmission requests.
Actions:
1 Reset rx sequence number
Reset tx sequence number
Reset tx retransmit count
Stop retransmit timer
2 Report error to user
Stop retransmit timer
Stop frame transmit timer
Free buffers
3 Save call management message in pending transmit queue
Build DATA_MSG from first message in pending transmit
queue using current tx sequence number.
Send message to MP for transmission.
Reset tx retransmit count
4 Increment tx retransmit count
If tx retransmit count >= RETRANSMIT_COUNT
Action 2 (followed by state change to the Start state)
else
Build DATA_MSG from first message in pending
transmit queue using current tx sequence number.
Send message to MP for transmission.
5 Dequeue first element on pending transmit queue and release
its buffer
Increment the tx sequence number
Stop the retransmit timer
if pending transmit queue not empty
Build DATA_MSG from first message in pending
transmit queue using current tx sequence number.
Send message to MP for transmission.
Reset tx retransmit count
6 Build ACK_MSG using the current rx sequence number
Send ack message to MP for transmission
Pass message to call managment layer
Increment rx sequence number
7 Build ACK_MSG using the previous rx sequence number
Send the ack message to MP for transmission
8 Add the message to the end of the pending transmit queue
9 Stop retransmit timer
Free buffers
3.2. Multilink Plus Control Messages
Message Type Value
VERSION_EXCHANGE_REQ 1
VERSION_EXCHANGE_RSP 2
ADD_REQ 3
ADD_RSP 4
ADD_COMPLETE 5
REMOVE_REQ 6
REMOVE_RSP 7
REMOVE_COMPLETE 8
CLOSE_REQ 9
CLOSE_RSP 10
REMOTE_MGMT_REQ 11
REMOTE_MGMT_RSP 12
REMOTE_MGMT_RX_REQ 13
REMOTE_MGMT_TX_REQ 14
REMOTE_MGMT_TX_RSP 15
CLEAR_REQ 16
3.3. Multilink Plus Message Formats
The fields of all messages defined here MUST be encoded/decoded in
Network Byte Order (big endian).
3.3.1. VERSION_EXCHANGE_REQ Message Format
The version exchange message is sent by the call originator to inform
the answerer the version of the MP+ protocol being used as well as
any other information that may need to be conveyed outside of the
normal PPP parameter negotiation.
+---------------+---------------+
Message type
0x00000001
+---------------+---------------+
Protocol Version
+---------------+---------------+
Protocol Revision
+---------------+---------------+
Session Identifier
+---------------+---------------+
Hardware Type
+---------------+---------------+
Nailed Mode
+---------------+---------------+
Use Multiple Trunk Groups
+---------------+---------------+
Descriptor Length
+---------------+---------------+
Descriptor
+---------------+---------------+
Figure 4: Version Exchange Request
A message sent from call originator to call answerer specifying the
callers protocol version and other state info and requesting the
answerer to respond with its version info.
Protocol Version:
caller MP+ protocol version number.
2 octets fixed length (initially 1)
Protocol Revision:
caller MP+ protocol revision number.
2 octets fixed length (initially 4)
Session Identifier:
A non-zero identifier unique to the caller.
2 octets fixed length.
Hardware Type:
caller hardware type (can be vendor defined).
2 octets fixed length.
Nailed Mode:
caller nailed mode from the session profile.
2 octets fixed length.
Use Multiple Trunk Groups:
non-zero if the call may use channels from multiple trunk
groups.
2 octets fixed length
Descriptor Length:
length of the end point descriptor.
2 octets fixed length
Descriptor:
the end point descriptor. This field allows for vendor
specific identification of the peer.
Variable length as defined above.
3.3.2. VERSION_EXCHANGE_RSP Message Format
The version exchange response message is sent by the call answerer in
response to a version exchange request message. The answerer uses
the message to inform the caller the version of the MP+ protocol
being used as well as any other information that needs to be conveyed
outside of the normal PPP parameter negotiation.
+---------------+---------------+
Message type
0x00000002
+---------------+---------------+
Protocol Version
+---------------+---------------+
Protocol Revision
+---------------+---------------+
Session Identifier
+---------------+---------------+
Hardware Type
+---------------+---------------+
Nailed Mode
+---------------+---------------+
Use Multiple Trunk Groups
+---------------+---------------+
Descriptor Length
+---------------+---------------+
Descriptor
+---------------+---------------+
Figure 5: Version Exchange Response
A message sent from call answerer to the call originator specifying
the answerers protocol version and other state info. Sent in
response to receiving a version exchange request.
Protocol Version:
caller MP+ protocol version number.
2 octets fixed length (initially 1)
Protocol Revision:
caller MP+ protocol revision number.
2 octets fixed length (initially 4)
Session Identifier:
A non-zero identifier unique to the answerer.
2 octets fixed length.
Hardware Type:
caller hardware type (can be vendor defined).
2 octets fixed length.
Nailed Mode:
caller nailed mode from the session profile.
2 octets fixed length.
Use Multiple Trunk Groups:
non-zero if call may use channels from multiple trunk groups.
2 octets fixed length
Descriptor Length:
length of the remote descriptor in 4-octet units.
2 octets fixed length
Descriptor:
the remote unit descriptor. This field allows for vendor
specific identification of the peer.
Variable length Nx4 octets long - total length defined above.
3.3.3. ADD_REQ Message Format
A message of this type is sent by either caller or answerer to
initiate an increase of bandwidth. When sent by the caller the
request is aSKINg for permission to dial a certain number of
channels; the response will contain permission and the phone numbers
of the channels to dial. When sent by the answerer, this message
contains the phone numbers to dial. The message looks like:
+---------------+---------------+
Message type
0x00000003
+---------------+---------------+
Number of channels requested
+---------------+---------------+
Number of phone numbers
+---------------+---------------+
A phone number list for
each phone number
.
.
.
+---------------+---------------+
Figure 6: Add Request
A message sent by either caller or answerer to request that additional
bandwidth be added to a session.
Number of channels requested:
The maximum number of channels to add.
2 octets fixed length.
Number of phone numbers:
The number of phone numbers provided. This
value will always be zero when the caller
initiates an add and will be at least
Number of channels requested when the answerer
initiates the add.
2 octets fixed length.
Phone number list:
A list of up to 32 phone number lists
containing the phone numbers to call.
Each description is of fixed length as described below:
Each phone number is represented by a phone number list.
The format of a phone number list is:
+---------------+---------------+
in use flag
+---------------+---------------+
call service type
+---------------+---------------+
Phone number
20 octets
plus null terminator
(21 octet total)






+---------------+
Must be 0
+---------------+---------------+
must be 0
+---------------+---------------+
Figure 7: Phone number list
A structure containing information about a connection within the
system.
in use flag:
non-zero if the phone number indicated
in this descriptor is currently in use.
2 octets fixed length
call service type:
Defines the type of service, switched, nailed,
or other, associated with a phone number.
1 Nailed
2 Switched
>=3 Undefined
Phone number:
The null terminated phone number of this channel.
Fixed length 21 octets. Each octet contains IA5 character
representation of a digit (or #, *).
Must be 0:
Filler to force alignment to 32-bit boundary.
3.3.4 ADD_RSP Message Format
A message of this type gives permission to dial some number of
channels and, when sent by the answerer of the original call, gives
the phone numbers of channels to dial.
+---------------+---------------+
Message type
0x00000004
+---------------+---------------+
Number of channels allowed
+---------------+---------------+
Number of phone numbers
+---------------+---------------+
A phone number list for
each phone number
.
.
.
+---------------+---------------+
Figure 8: Add Response
A message sent by either caller or answerer to indicate the number of
channels that may be added to a session.
Number of channels allowed:
The actual number of channels to add. This
may be less than the number requested.
2 octets fixed length.
Number of phone numbers:
The number of phone numbers provided. This
value will always be zero when sent by the
caller and will be at least channelCount
when sent by the answerer.
2 octets fixed length.
Phone number list:
A list of up to 32 phone number lists
containing the phone numbers to call.
Each description is of fixed length as described above.
3.3.5. ADD_COMPLETE Message Format
This message is sent by the caller to the answerer after all calls
have been placed. The message is used to notify the answerer that
the add transaction is complete and it may return to the idle state.
+---------------+---------------+
Message type
0x00000005
+---------------+---------------+
Channels added
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 9: Add Complete
A message sent by caller to indicate the number of channels that were
added successfully. This message was added in MP+ Version 1.1.
Channels added:
The actual number of channels added.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.6. REMOVE_REQ Message Format
A message of this type is sent when a peer decides, for any reason,
to remove channels from use. The purpose of the message is to tell
the remote end of the remove and give it a chance to adjust the
number of channels to remove.
+---------------+---------------+
Message type
0x00000006
+---------------+---------------+
Number of channels to remove
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 10: Remove Request
A message sent by either caller or answerer to request that bandwidth
be removed from a session.
Number of channels to remove:
The maximum number of channels to remove.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.7. REMOVE_RSP Message Format
This message is sent in response to a remove request. The responder
specifies the number of channels that can be removed. If the
response specifies 0 channels the remove is cancelled.
+---------------+---------------+
Message type
0x00000007
+---------------+---------------+
Number of channels to remove
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 11: Remove Response
A message sent in response to a remove request specifying the number
of channels that the peer agrees can be removed.
Number of channels to remove:
The maximum number of channels to remove.
May be zero, in which case the remove is
cancelled.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.8. REMOVE_COMPLETE Message Format
This message is sent by the initiator of a remove transaction when
the agreed upon number of channels have been removed. The message is
used to notify the peer that the remove transaction is complete and
it may return to the idle state.
+---------------+---------------+
Message type
0x00000008
+---------------+---------------+
Number of channels removed
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 12: Remove Complete
A message sent by the caller or answerer to indicate how many channels
were actually removed. This message was added in MP+ CM version 1.1.
Number of channels removed:
The number of channels that were removed.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.9. CLOSE_REQ Message Format
This message is sent when the peer requests to close the whole
session. This is typically due to a configuration option that
indicates when a system should request to close the session (an
example being, a link has been idle for greater than a preconfigured
time period).
+---------------+---------------+
Message type
0x00000009
+---------------+---------------+
Figure 13: MP+ close request.
There are no data fields associated with this message.
3.3.10. CLOSE_RSP Message Format
If the peer agrees that closing the session is acceptable based on
it"s own configuration (an example reject reason would be that the
peer is configured with a *minimum* number of channels to keep
active).
+---------------+---------------+
Message type
0x0000000a
+---------------+---------------+
OK To Close
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 14: MP+ close response
The response to a close request. May be sent by caller or answerer.
OK To Close:
If non-zero, peer said I can close all channels.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.11. REMOTE_MGMT_REQ Message Format
This message is sent from a master station to a slave station when
the master wishes to manage the remote station. The message is also
used to cancel remote management once it"s been started.
+---------------+---------------+
Message type
0x0000000b
+---------------+---------------+
Mode
+---------------+---------------+
Must be zero
+---------------+---------------+
Figure 15: Remote Management Request
A message sent from master to slave to initiate or clear a remote
management session.
Mode:
One to start session. Zero to stop session.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.12. REMOTE_MGMT_RSP Message Format
The slave side of a remote management session has the opportunity to
reject remote management. The master side is informed of accept/deny
status via the remote management response.
+---------------+---------------+
Message type
0x0000000c
+---------------+---------------+
Mode
+---------------+---------------+
Figure 16: Remote Management Response
A message sent from slave to master to allow or deny initiation of a
remote management session.
Mode:
One to accept session. Zero to deny session.
2 octets fixed length
Must be zero:
Padding to 32-bit boundary.
2 octets fixed length
3.3.13. REMOTE_MGMT_RX_REQ Message Format
This message type is used to convey keyboard input from the
management master to be processed by the management slave. The
message format consists of an octet count (in network byte order) and
then an array of octets to be processed. It looks like:
+---------------+---------------+
Message type
0x0000000d
+---------------+---------------+
character count
+---------------+---------------+
array of characters
.
.
.
+---------------+---------------+
Figure 17: Remote Management Receive Request
A message sent from master to slave, conveying keystrokes typed on the
masters keyboard that will be processed by the slave.
character count:
Number of characters to process.
2 octets fixed length
array of characters:
Array of characters to process.
3.3.14. REMOTE_MGMT_TX_REQ Message Format
The remote management slave conveys output to be displayed on the
masters terminal with a remote management transmit request message.
Only one message may be outstanding. The next transmit request may
not be sent until the previous has been acknowledged.
+---------------+---------------+
Message type
0x0000000e
+---------------+---------------+
character count
+---------------+---------------+
array of characters
.
.
.
+---------------+---------------+
Figure 18: Remote Management Transmit Request
A message sent from slave to master, conveying output to be output on
the master"s display.
Character count:
Number of characters to process.
2 octets fixed length
array of characters:
Array of characters to process.
3.3.15. REMOTE_MGMT_TX_RSP Message Format
This message is used to acknowledge remote management transmit
requests. The slave may send the next transmit request once this
message has been received.
+---------------+---------------+
Message type
0x0000000f
+---------------+---------------+
Figure 19: Remote Management Transmit Response
There are no data fields associated with this message.
3.3.16. CLEAR_REQ Message Format
A message sent to initiate a friendly shutdown of an MP+ link. The
sender will stop sending data immediately. The receiver of the
message will also stop sending user data and start a clean shutdown
of all NCPs and the LCP of each member link of the bundle. When the
last member link terminates, the session is completely closed.
+---------------+---------------+
Message type
0x00000010
+---------------+---------------+
Figure 20: Clear Request
There are no data fields associated with this message.
3.4. Events
The MP+ state machine is event driven. Reception of an event triggers
an action and possibly a state change. The events processed by the
MP+ state machine can be roughly classed into two types:
Events that originate within the unit, e.g. notification that a
call has cleared, an MP+ session may be started, etc.
Events that originate with the reception of an MP+ control
message from the peer unit.
Both types are processed by the state machine in the sequence they
occurred. The events processed are:
MP+START_SESSION: Notification from PPP/MP that an
MP+ session is starting.
MP+SESSION_DOWN: Notification from the error-control
layer that end-to-end connectivity
has been lost and control messages can
not be delivered.
MP+SESSION_TERM: Session termination notification from
PPP/MP. This event is not sent until
the last channel of a multi-channel
session is cleared.
MP+TIMER_EXPIRED: Timers are used in various states and
sub-states. This event is signaled whenever
a timer expires.
MP+CALL_COMPLETE: A call placed during an add request has
completed. The call may have succeeded or
failed.
MP+UTILIZATION: Notification from MP/PPP that link
utilization has crossed a threshold and that
channels may need to be added/removed.
(The number of channels to add/remove will be
passed with the notification).
MP+RX_VERSION_REQ: A Version Exchange request message has
been received from the peer.
MP+RX_VERSION_RSP: A Version Exchange response message has
been received from the peer.
MP+ADD_REQ: An Add request message has been received
from the peer.
MP+ADD_RSP: An Add response message has been received
from the peer.
MP+ADD_COMP: An Add Complete message has been received
from the peer.
MP+REMOVE_REQ: A Remove request message has been received
from the peer.
MP+REMOVE_RSP: A Remove response message has been
received from the peer.
MP+REMOVE_COMP: A Remove Complete message has been received
from the peer.
MP+RX_RM_REQ: A Remote Management request has been received
from the peer.
MP+RX_RM_RSP: A Remote Management response has been received
from the peer.
MP+RX_RM_RX_REQ: A Remote Management Receive Request has been
received from the far end.
MP+RX_RM_TX_REQ: A Remote Management Transmit Request has
been received from the peer.
MP+RX_RM_TX_RSP: A Remote Management Transmit Response has
been received from the peer.
MP+RX_CLEAR: A request to shut down the session has been
received from the peer.
MP+CLOSE_REQ: A Close Request message has been received
from the peer.
MP+CLOSE_RSP: A Close Response message has been received
from the peer.
MP+START_RM Request to start a remote management session
with this station being the master.
MP+SEND_RMS: Request to send data to a remote management
master from a slave.
MP+SEND_RMM: Request to send data to a remote management
slave from a master.
MP+RECV_RMM: Request to send an ack to a remote management
slave for data received from the slave.
MP+STOP_RM: Request to stop a remote management session.
3.5. State Machine
3.5.1 States
To ease readability and understanding the major states are considered
as separate state machines, each having two to four sub-states. The
sub-states are named by the letters, A, B, C, and D. State
information is maintained for every interface.
The major states are:
MP+STATE_INITIAL: The state of an unused session. Session table
entries are intialized to this state at startup
and return to this state when sessions are
terminated.
MP+STATE_IDLE: The state of an active session that is not
performing any MP+ function.
MP+STATE_ADD: The state of a session when an add transaction is
in progress.
MP+STATE_REMOVE: The state of a session when a remove transaction is
in progress.
MP+STATE_CLOSE: The state of a session that is in the process of
being closed.
State transitions are triggered by the reception of an event. Tables
2 through 6 contain the state tables for the major states. All state
tables use the following symbols.
- No action
* Stay in same state
+ Target state is defined by the action taken
** An error has occurred, log an error message but no
state change.
States and sub-state transitions are noted as state:sub-state, e.g.,
initial:A. Alternative transitions are listed on separate lines.
3.5.2 Common Actions
Some actions are common to all states, they are defined here.
Error Close Action
Called to close a session when an error occurs. Actions are:
[1] Stop timer if running.
[2] Log an error message.
[3] Close the MP+EC layer for this session.
[4] Close MP for this session
[5] Clean up, restore state variables to their initial state.
Term Action
Processed when a MP+SESSION_TERM event occurs in most states.
Actions are:
[1] Stop timer if running.
[2] Close the MP+EC layer for this session.
[3] Call the passed termination callback function if not null.
[4] Clean up, restore state variables to their initial state.
Ignore Action
We don"t care about this event in this state. Do nothing.
Timer Action
This action is called when a timer expires in one of the on-line
states. The timer is used to implement add and remove locks. A
lock is set when an add or remove fails and is not cleared until
a bandwidth change or the timer expires. This keeps us from
retrying add"s and subtracts until there is a likelyhood that it
will succeed.
[1] Check add lock flag.
[1] If set an add lock occured last timeout period so
triple the timeout value (to a max of 81 minutes).
[2] If not set restore the timeout value to its initial
value of one minute.
[2] Clear the add lock flag.
[3] Clear the remove lock flag.
[4] Restart the retry timer.
Enter Remove [local] Action
The local unit is initiating a remove transaction. The desired
bandwidth is given.
[1] Restart the idle timer.
[2] Calculate number of channels to remove (difference between
number in use and number in desired).
[3] Build and send a remove request and send to remote.
[4] Go to REMOVE:A.
Enter Remove [remote] Action
The remote unit is initiating a remove transaction. The incoming
message contains the number of channels to remove.
[1] Restart the idle timer.
[2] Request the number of channels required. If greater than the
number available after removing the number of channels indicated
in the incoming message reduce the number of channels to remove
and set a remove lock.
[3] Build a remove response message indicating the number of
channels we will allow the requester to remove and send to the
remote.
[4] Go to REMOVE:B.
Enter Add [local] Action
The local unit is initiating an add transaction. We are given
the number of channels desired. The steps are:
[1] Restart the idle timer.
[2] Calculate number of channels to add (difference between
number desired and number in use).
[3] Reserve number of channels, retrieving their phone numbers.
[4] If number of channels reserved less than the number desired
set an add lock.
[5] If number of channels reserved greater than zero.
[1] Build an add request. If the answerer the request
includes the phone numbers for the caller to dial.
[2] If caller, go to ADD:A.
[3] If answerer, go to ADD:C.
[6] Go to IDLE state.
Enter Remove [remote] Action
The remote unit is initiating a remove transaction. The
incoming message contains the number of channels to remove.
[1] Clear the remove lock.
[2] Restart the idle timer.
[3] Request the number of channels required. If greater
than the number available after removing the number of
channels indicated in the incoming message reduce the
number of channels to remove.
[4] Build a remove response message indicating the number
of channels we will allow the requester to remove and
send to the remote.
[5] Go to REMOVE, sub-state B.
Enter Add [remote answerer] Action
We"ve received a message from the remote requesting that
bandwidth be added. The message contains the number of
channels to add. Since the remote is the answerer, the mes-
sage also contains the phone nubmers to dial. We may dial
less than the number requested.
[1] Restart idle timer.
[2] If the number of channels requested will put us over
the maximum number of channels allowed for the session
reduce the channel count.
[3] For each channel to add,
[1] Integrate the phone number returned from the
answerer with the original phone number dialed.
[2] Request that a session be extended by dialing
the integrated phone number. A callback is
passed with the request so call success or fail-
ure can be reported back to MP+.
[4] Go to ADD:B. Note: This change must actu-
ally occur before requesting the first outgoing call.
If not, the callback could be called (and ignored)
because the session is not in the correct state.
Enter Add [remote caller] Action
We"ve received a message from the remote requesting that
bandwidth be added. The message contains the number of
channels to add. Since the remote is the caller, it needs
us to send the phone numbers to dial. We may send fewer
phone numbers than requested
[1] Restart idle timer.
[2] If the number of channels requested will put us over
the maximum number of channels allowed for the session
reduce the channel count.
[3] Reserve the adjusted number of channels, retrieving
their phone numbers.
[4] If the number of channels reserved is less than the
adjusted number requested.
[5] Build an add response message, including the phone
numbers for the channels we will let the caller add
and send it to the far end.
[6] Go to ADD:C.
Enter Idle Action
The IDLE state is entered at the end of normal transactions.
At entry the current status of the connection should be
checked and new transactions initiated if necessary. To be
safe, we can also use this state as a catch all place to
release any bandwidth reserved for adds. The functions to
perform are:
[1] Restart the idle timer using the current retry value.
[2] Release any reserved bandwidth not actually in use.
[3] Check if bandwidth change reqested during last trans-
action. If change indicated:
[1] Query channel counts.
[2] If current bandwidth less than suggested band-
width and removes are not locked, store the
requested bandwidth and initiate a remove trans-
action (Enter Remove Action).
[3] If current bandwidth greater than suggested
bandwidth and adds are not locked:
[1] Store the requested bandwidth.
[2] Intiate an add transaction (Enter Add
[local] Action).
[4] Go to the IDLE state.
Remote Management Request Action
We received a request to start/stop remote management.
If this is a start request
If we can/allow remote management:
Build and send a Remote management response Allow
message.
Else
Build and send a Remote management response Deny
message.
Else (this is a stop)
Notify the remote management slave process to terminate.
Remote Management Response Action
We received a response to our remote management start request.
If the response was an Allow response
Notify the remote management master process, we can
start sending keystrokes/commands
Else
The peer denied the request, so notify the remote
management master process of failure.
Remote Management Receive Data Action
We (the slave) received data from the remote management master.
Pass the received data to the remote management slave process.
This is typically keystroke data received from the remote user
interface.
Remote Management Transmit Data Action
We (the master) received data from the remote management slave.
Pass the received data to the remote management master process.
This is typically screen-updates to be passed to the user
interface.
Remote Management Transmit Data Response Action
We (the slave) received an ack to data we previously sent to the
master. Notify the remote management slave process so that it
can queue further transmissions.
Remote management (Master) start Action
Build a REMOTE_MGMT_REQ start message and send to the far end.
Send a proceeding message to the RM master process.
Remote management (Slave) data Action
Build a REMOTE_MGMT_TX_REQ message with the data passed from
the remote management slave process, send it to the far end.
Remote management (Master) data Action
Build a REMOTE_MGMT_RX_REQ message with data passed from the
remote management master process, send it to the far end.
Remote management data acknowledgement Action
Build a REMOTE_MGMT_TX_RSP message and send it so the slave can
send the next block. There is no data associated with this
message.
Remote management (Master) stop Action
Build a REMOTE_MGMT_REQ stop message and send to the far end.
3.5.3. MP+STATE_INITIAL state machine
All sessions start from this state, sub-state A. The state is not
exited until version exchange succeeds.
The sub-states are:
A Initial state
B Sent version request, waiting for version response.
C Waiting for version request.
Event Sub-state
______________________________________________________________________
A B C
======================================================================
MP+START_SESSION 1,+ ** **
______________________________________________________________________
MP+SESSION_DOWN ** 2,Initial:A 2,Initial:A
______________________________________________________________________
MP+SESSION_TERM ** 3,Initial:A 3,Initial:A
______________________________________________________________________
MP+TIMER_EXPIRED ** 4,Initial:A 7,Initial:B
______________________________________________________________________
MP+RX_VERSION_REQ ** 8,Initial:A 5,+
______________________________________________________________________
MP+RX_VERSION_RSP ** 6,+ **
______________________________________________________________________
MP+START_RM 9,* 9,* 9,*
______________________________________________________________________
All other events ** ** **
______________________________________________________________________
Table 2: Initial State Machine
Actions:
1 Start timer, 60 seconds if originator, 30 seconds if answerer.
Start MP+
If originator
Build and send version exchange request
Go to INITIAL, sub-state B.
Go to INITIAL, sub-state C.
2 Do Error Close Action, go to INITIAL, sub-state A.
3 Do Term Action, go to INITIAL, sub-state A.
4 Do Error Close Action, go to INITIAL, sub-state A.
5 Stop the idle timer.
Compare protocol versions, if they do not match Do Error Close
Action, go to INITIAL, sub-state A.
Store off info received from remote.
Build a version exchange response and send to remote end.
Do Enter Idle Action which causes a state change.
6 Stop the retry timer.
Compare protocol versions, if they do not match Do Error Close
Action, go to INITIAL, sub-state A.
Store off info received from remote.
Check the base channel count in the callers profile.
If greater than 1
Set the requested bandwidth to the base channel count.
Do Enter Add Caller action which causes a state change.
Do Enter Idle Action which causes a state change.
7 Both sides think they are the answerer. This is possible if both
dial each other at the same time and the first channel that
completed PPP negotiation happened to be the channel associated
with the incoming call on both units. We resolve this by trying
to become the originator.
If both sides try to become the originator the one with the
largest endpoint discriminator will fall back to being the
answerer.
Restart Idle timer at 60 seconds
Build and send Version Exchange Request message
Go to Initial:B
8 Both sides think they are the originator. This can happen if
both dial each other at the same time and the first channel
that completed PPP negotiation happened to be the channel
associated with the originating call on both units. MP+
determines which will be the caller and which the answerer by
comparing the endpoind discriminator in the version exchange
request with the local endpoint discriminator. The unit with
the smaller endpoint is arbitrarily called the originator. The
actions are:
Compare local endpoint discriminator with endpoint discrimator
in message.
If local endpoint discriminator is less than the remote value
we are the caller, ignore the incoming message.
Otherwise, if local endpoint discriminator is greater than the
remote value we are the answerer:
Compare protocol versions, if they do not match
Do Error Close Action, go to INITIAL, sub-state A.
Store off info received from remote.
Build a version exchange response and send to remote end.
Do Enter Idle Action which causes a state change.
If the two values match, there is a problem, Do Error Close
Action,go to INITIAL, sub-state A.
9 Log an error message.
Notify the user interface of remote management failure.
3.5.4. MP+STATE_IDLE state machine
The Idle state is the state of an active session with no call
management activity in progress.
There are no sub-states.
Event State
_____________________________________________
A
=============================================
MP+SESSION_DOWN 1,Initial:A
_____________________________________________
MP+SESSION_TERM 2,Initial:A
_____________________________________________
MP+TIMER_EXPIRED 3,*
_____________________________________________
MP+UTILIZATION 4,+
_____________________________________________
MP+RX_ADD_REQ 5,+
_____________________________________________
MP+RX_REMOVE_REQ 6,Remove:B
_____________________________________________
MP+RX_RM_REQ 7,+
_____________________________________________
MP+RX_RM_RSP 8,+
_____________________________________________
MP+RX_RM_RX_REQ 9,+
_____________________________________________
MP+RX_RM_TX_REQ 10,+
_____________________________________________
MP+RX_RM_TX_RSP 11,+
_____________________________________________
MP+RX_CLOSE_REQ 12,+
_____________________________________________
MP+START_RM 13,*
_____________________________________________
MP+SEND_RMS 14,*
_____________________________________________
MP+SEND_RMM 15,*
_____________________________________________
MP+RECV_RMM 16,*
_____________________________________________
MP+STOP_RM 17,*
_____________________________________________
All other events **
_____________________________________________
Table 3: Idle State Machine
Actions:
1 Do Error Close Action, go to INITIAL, sub-state A.
2 Do Term Action, go to INITIAL, sub-state A.
3 Do Timer Action.
4 Note that a bandwidth change has been reqested.
Do Enter Idle Action which may cause a state change.
5 If we are the caller:
Do Enter Add [remote answerer] Action.
Else
Do Enter Add [remote caller] Action.
6 Do Enter Remove [remote] Action
7 Do Remote Management Request Action
8 Do Remote Management Response Action
9 Do Remote Management Receive Data Action
10 Do Remote Management Transmit Data Action
11 Do Remote Management Transmit Data Response Action
12 Clear remove lock.
If local recommended channels == 0, then:
send a Close Response message with OK To Close
set to TRUE.
Else
send a Close Response message with OK To Close
set to FALSE.
Do Enter Idle Action.
13 Do Remote management (Master) start Action
14 Do Remote management (Slave) data Action
15 Do Remote management (Master) data Action
16 Do Remote management data acknowledgement Action
17 Do Remote management (Master) stop Action
3.5.5. MP+STATE_ADD state machine
The add state is used by both caller and answerer when an add
transaction is in progress.
The sub-states are:
A Add request sent to answerer, waiting for add response
from the answerer.
B Caller waiting for call complete notification for calls
placed.
C Answerer waiting for add complete message from caller.
Event Sub-state
______________________________________________________________________
A B C
======================================================================
MP+SESSION_DOWN 1,Initial:A 7,Closing:A 1,Initial:A
______________________________________________________________________
MP+SESSION_TERM 2,Initial:A 7,Closing:B 2,Initial:A
______________________________________________________________________
MP+TIMER_EXPIRED 3,+ 3,+ 3,+
______________________________________________________________________
MP+UTILIZATION 4,* 4,* 4,*
______________________________________________________________________
MP+CALL_COMPLETE ** 8,Idle:A **
______________________________________________________________________
MP+RX_VERSION_REQ -,* ** **
______________________________________________________________________
MP+ADD_REQ 5,Add:B ** **
______________________________________________________________________
MP+ADD_RSP 6,+ ** **
______________________________________________________________________
MP+ADD_COMP ** ** 9,Idle:A
______________________________________________________________________
MP+RX_RM_REQ 10,+ 10,+ 10,+
______________________________________________________________________
MP+RX_RM_RSP 11,+ 11,+ 11,+
______________________________________________________________________
MP+RX_RM_RX_REQ 12,+ 12,+ 12,+
______________________________________________________________________
MP+RX_RM_TX_REQ 13,+ 13,+ 13,+
______________________________________________________________________
MP+RX_RM_TX_RSP 14,+ 14,+ 14,+
______________________________________________________________________
MP+RX_REMOVE_REQ -,* ** **
______________________________________________________________________
MP+START_RM 15,* 15,* 15,*
______________________________________________________________________
MP+SEND_RMS 16,* 16,* 16,*
______________________________________________________________________
MP+SEND_RMM 17,* 17,* 17,*
______________________________________________________________________
MP+RECV_RMM 18,* 18,* 18,*
______________________________________________________________________
MP+STOP_RM 19,* 19,* 19,*
______________________________________________________________________
All other events ** ** **
______________________________________________________________________
Table 4: Add State Machine
Actions:
1 Phone numbers (may) have been reserved, they must be released
before the normal error processing occurs.
Release all reserved phone numbers
Do Error Close Action.
2 Phone nubmers (may) have been reserved, they must be released
before the normal close processing occurs.
Release all reserved phone numbers
Do Term Action.
3 Do Timer Action
4 Note that a bandwidth change has been reqested. This will be
processed the next time IDLE state is entered.
5 An add collision has occured. Since the answerer has sent phone
numbers we will try to use what he as sent, within the limits of
the local system.
Compare local channels to add with current channels to
add.
If the local channels to add is less than the remote
channels to add
If the remote number of channels requested will
put us over the maximum number of channels
allowed for the session reduce the channel count
and set an add lock.
Re-reserve the channels. If the number reserved
are less than the number of phone numbers
provided by the far end, set an add lock and
reduce the number of channels to add to what we
could reserve.
Now treat the remote add request as if it were an add
response and process by:
Integrate the phone number returned from the
answerer with the original phone number dialed.
Request that a session be extended by dialing
the integrated phone number. A callback is
passed with the request so call success or
failure can be reported back to MP+.
Go to ADD:B. Note: This change must actually
occur before requesting the first outgoing call.
If not, the callback could be called (and
ignored) because the session is not in the
correct state.
6 If the answerer provided fewer phone numbers than requested set
an add lock.
If the number of channels is zero send an add complete message
(there"s nothing to do) and go to the IDLE state.
For each phone number returned
Integrate the phone number returned from the answerer
with the original phone number dialed.
Request that a session be extended by dialing the
integrated phone number. A callback is passed with the
request so call success or failure can be reported back
to MP+.
Go to ADD:B. Note: This change must actually occur before
requesting the first outgoing call. If not, the callback could
be called (and ignored) because the session is not in the correct
state.
7 Restart idle timer for abort.
8 Increment the count of calls completed.
If the call succeeded, increment the count of calls that
succeeded.
If the count of calls completed equals the number of calls placed
If number of calls completed is not the same as the
nubmer that succeeded set an add lock.
Build an add complete message and send it to the far end.
If at least one channel was added clear any remove lock.
Go to the IDLE state.
9 If number of channels requested not equal to number connected
set add lock.
If at least one channel was added clear any remove lock.
Go to the IDLE state.
10 Do Remote Management Request Action
11 Do Remote Management Response Action
12 Do Remote Management Receive Data Action
13 Do Remote Management Transmit Data Action
14 Do Remote Management Transmit Data Response Action
15 Do Remote management (Master) start Action
16 Do Remote management (Slave) data Action
17 Do Remote management (Master) data Action
18 Do Remote management data acknowledgement Action
19 Do Remote management (Master) stop Action
3.5.6. MP+STATE_REMOVE state machine
The state of a session while processing a remove transaction.
The sub-states are:
A Remove request sent, waiting for remove response
B Remove response sent, waiting for remove complete
Event Sub-state
_________________________________________________________
A B
=========================================================
MP+SESSION_DOWN 1,Initial:A 1,Initial:A
_________________________________________________________
MP+SESSION_TERM 2,Initial:A 2,Initial:A
_________________________________________________________
MP+TIMER_EXPIRED 3,+ 3,+
_________________________________________________________
MP+UTILIZATION 4,* 4,*
_________________________________________________________
MP+RX_ADD_REQ 5,+ **
_________________________________________________________
MP+RX_REMOVE_REQ 6,+ **
_________________________________________________________
MP+RX_REMOVE_RSP 7,Idle:A **
_________________________________________________________
MP+RX_REMOVE_COMP ** 8,Idle:A
_________________________________________________________
MP+RX_CLOSE_REQ -,* **
_________________________________________________________
MP+RX_RM_REQ 9,* 9,*
_________________________________________________________
MP+RX_RM_RSP 10,* 10,*
_________________________________________________________
MP+RX_RM_RX_REQ 11,* 11,*
_________________________________________________________
MP+RX_RM_TX_REQ 12,* 12,*
_________________________________________________________
MP+RX_RM_TX_RSP 13,* 13,*
_________________________________________________________
MP+START_RM 14,* 14,*
_________________________________________________________
MP+SEND_RMS 15,* 15,*
_________________________________________________________
MP+SEND_RMM 16,* 16,*
_________________________________________________________
MP+RECV_RMM 17,* 17,*
_________________________________________________________
MP+STOP_RM 18,* 18,*
_________________________________________________________
All other events ** **
_________________________________________________________
Table 5: Remove State Machine
Actions:
1 Do Error Close Action
2 Do Term Action
3 Do Timer Action
4 Note that a bandwidth change has been reqested. This will be
processed the next time IDLE state is entered.
5 Our remove conflicted with the remote end Add. The add takes
preference.
Set a remove lock.
If we are the caller Do Enter Add [remote answerer] Action .
Otherwise Do Enter Add [remote caller] Action .
6 Two remove requests collided. We give preference to the caller
(an arbitrary decision).
If caller, ignore message.
Else
Check maximum number of channels needed by the local end.
Reduce the requested remove count and set a remove lock if
necessary.
Build and send a remove response to the remote.
Go to Remove:B.
7 Compare the number of channels requested