Network Working Group C. Richards

Network Working Group C. Richards
Request for Comments: 2125 Shiva Corporation
Category: Standards Track K. Smith
Ascend Communications, Inc.
March 1997

The PPP Bandwidth Allocation Protocol (BAP)
The PPP Bandwidth Allocation Control Protocol (BACP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document proposes a method to manage the dynamic bandwidth
allocation of implementations supporting the PPP multilink protocol
[2]. This is done by defining the Bandwidth Allocation Protocol
(BAP), as well as its associated control protocol, the Bandwidth
Allocation Control Protocol (BACP). BAP can be used to manage the
number of links in a multilink bundle. BAP defines datagrams to co-
ordinate adding and removing individual links in a multilink bundle,
as well as specifying which peer is responsible for which decisions
regarding managing bandwidth during a multilink connection.

Table of Contents

1. Introduction .......................................... 2
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. New LCP Configuration Option .......................... 3
2.1 Link Discriminator .............................. 3
3. BACP Operation ........................................ 4
4. BACP Configuration Options ............................ 5
4.1 Favored-Peer .................................... 5
5. BAP Operation ......................................... 7
5.1 Link Management ................................. 7
5.2 Bandwidth Management ............................ 8
5.3 BAP Packets ..................................... 8
5.4 Race Conditions ................................. 9
5.5 BAP Datagram Format ............................. 9
5.5.1 Call-Request .................................... 12
5.5.2 Call-Response ................................... 12

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5.5.3 Callback-Request ................................ 13
5.5.4 Callback-Response ............................... 13
5.5.5 Link-Drop-Query-Request ......................... 13
5.5.6 Link-Drop-Query-Response ........................ 13
5.5.7 Call-Status-Indication .......................... 14
5.5.8 Call-Status-Response ............................ 14
6. BAP Datagram Options .................................. 14
6.1 Link-Type ....................................... 15
6.2 Phone-Delta ..................................... 17
6.2.1 Phone-Delta Sub-Options ......................... 18
6.3 No-Phone-Number-Needed .......................... 19
6.4 Reason .......................................... 20
6.5 Link-Discriminator .............................. 21
6.6 Call-Status ..................................... 21
Appendix - List of BAP datagrams and associated fields ....... 23
ACKNOWLEDEMENTS .............................................. 23
SECURITY CONSIDERATIONS ...................................... 23
REFERENCES ................................................... 24
CHAIR'S ADDRESS .............................................. 24
EDITORS'S ADDRESSES .......................................... 24

1. Introduction

As PPP multilink implementations become increasingly common, there is
a greater need for some conformity in how to manage bandwidth over
such links. BACP and BAP provide a flexible yet robust way of
managing bandwidth between 2 peers. BAP does this by defining Call-
Control packets and a protocol that allows peers to co-ordinate the
actual bandwidth allocation and de-allocation. Phone number deltas
may be passed in the Call-Control packets to minimize the end user's
configuration.

1.1. Specification of Requirements

In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.

MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.

MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.

SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.

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MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.

1.2. Terminology

This document frequently uses the following terms:

peer The other end of the point-to-point link

silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of the
silently discarded packet, and SHOULD record the event in a
statistics counter.

BOD (bandwidth on demand)
BOD refers to the ability of a system to allocate and remove
links in a multilink system to change the bandwidth of a
multilink bundle. This may be done in response to changing
line conditions and it also may be done in response to changing
resource conditions. In either case, changing bandwidth
dynamically during a multilink connection is referred to as
BOD.

2. New LCP Configuration Option

Implementations MUST implement LCP as defined in [1]. LCP MUST be in
the Network-Layer Protocol phase before BACP can be negotiated.

2.1. Link Discriminator

Description

This LCP Configuration Option is used to declare a unique
discriminator for the link that the option is sent over. This
option MUST be negotiated by LCP on every link. BAP uses the link
discriminator to differentiate the various links in a multilink
bundle. Each link in a multilink bundle MUST have a unique
discriminator. The discriminator is independent for each peer, so
each link may have 2 different LCP Link Discriminator values, one
for each peer. When the Link Discriminator is sent in a BAP
packet, the transmitter sends the Link Discriminator Option value
received from its peer in the peer's LCP Configure Request packet.

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A summary of the Link Discriminator LCP Option format is shown below.
The fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link Discriminator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

23 for Link Discriminator option.

Length

4

Link Discriminator

The Link Discriminator field is 2 octets in Length, and it
contains a unique identifier used to indicate a particular link in
a multilink bundle. The Link Discriminator for a link MUST be
unique among the Link Discriminators assigned by this endpoint for
this bundle. The Link Discriminator MAY be assigned in a
sequential, monotonically increasing manner.

3. BACP Operation

BACP uses the same packet exchange mechanism as the Link Control
Protocol defined in [1]. BACP packets MUST NOT be exchanged until
PPP has reached the Network-Layer Protocol phase. BACP packets
received before this phase is reached should be silently discarded.

BACP is negotiated once per multilink bundle. If BACP is negotiated
on any of the links in a multilink bundle, it is opened for all of
the links in the bundle.

The Bandwidth Allocation Control Protocol is exactly the same as the
Link Control Protocol [1] with the following exceptions:

Data Link Layer Protocol Field

Exactly one BACP packet is encapsulated in the Information
field of PPP Data Link Layer frames where the Protocol field
indicates Type hex c02b (Bandwidth Allocation Control
Protocol).

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Code field

Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
Ack and Code-Reject) are used. Other Codes should be treated
as unrecognized and should result in Code-Rejects.

Configuration Option Types

BACP has a distinct set of Configuration Options, which are
defined in the next section.

4. BACP Configuration Options

BACP Configuration Options allow negotiation of desirable BACP
parameters. These options are used in Config-Request, Config-Ack,
Config-Nak, and Config-Reject packets. BACP uses the same
Configuration Option format defined for LCP [1], with a seperate set
of Options.

Current values of BACP Configuration Options are assigned as follows:

1 Favored-Peer

4.1. Favored-Peer

Description

This Configuration Option is used to determine which peer is
favored in the event of a race condition in which 2 peers
simultaneously transmit the same BAP request. Each peer
negotiates a 4 octet magic number, which is successfully
negotiated when the 2 Magic-Numbers are different. The favored
peer is the peer that transmits the lowest Magic-Number in its
Favored-Peer Configuration Option.

The Favored-Peer Configuration Option MUST be implemented.

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BACP will usually be negotiated after only one link of a multilink
bundle has reached the Network-Layer Protocol phase. In this
situation, it is acceptable for the peer that initiated the
connection to use a Magic-Number of 1, and the peer that responded
to the connection to use a Magic-Number of 0xFFFFFFFF. If a
multilink bundle has been established with links that were
originated by each peer, or if it is not clear which peer has
initiated a link (on a leased line, for example), then a random
number MUST be used for the Magic-Number. Refer to the
description of the LCP Magic-Number Configuration Option in [1]
for an explanation of how to create a useful random number.

When a Configure-Request is received with a Favored-Peer
Configuration Option, the received Magic-Number is compared with
the Magic-Number of the last Configure-Request sent to the peer.
If the two Magic-Numbers are different, then the Favored-Peer
negotiation has been successful, and the Favored-Peer Option
SHOULD be acknowledged. If the two Magic-Numbers are equal, a
Configure-Nak MUST be sent specifying a different Magic-Number
value. A new Configure-Request SHOULD NOT be sent to the peer
until normal processing would cause it to be sent (that is, until
a Configure-Nak is received or the Restart timer runs out).

A summary of the Favored-Peer Option format is shown below. The
fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Magic-Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

1 for Favored-Peer

Length

6

Magic-Number

The Magic-Number field is four octets, and indicates a number
which is very likely to be unique to one end of the link. A
Magic-Number of zero is illegal and MUST always be Nak'd.

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5. BAP Operation

5.1. Link Management

BAP defines packets, parameters and negotiation procedures to allow
two endpoints to negotiate gracefully adding and dropping links from
a multilink bundle. An implementation can:

o Request permission to add a Link to a bundle (Call-Request)

o Request that the peer add a link to a bundle via a callback
(Callback-Request)

o Negotiate with the peer to drop a link from a bundle (this
implies that the peer can refuse) (Link-Drop-Query-Request)

After BACP reaches the opened state, either peer MAY request that
another link be added to the bundle by sending a BAP Call- or
Callback-Request packet. A Call-Request packet is sent if the
implementation wishes to originate the call for the new link, and a
Callback-Request packet is sent if the implementation wishes its peer
to originate the call for the new link. The implementation receiving
a Call- or Callback-Request MUST respond with a Call- or Callback-
Response with a valid Response Code.

After BACP reaches the opened state, either peer MAY request that a
link be dropped from the bundle. A BAP Link-Drop-Query-Request
packet is sent to the peer to negotiate dropping a link. The peer
MUST respond with a Link-Drop-Query-Response. If the peer is
agreeable to dropping the link the implementation MUST issue an LCP
Terminate-Request to initiate dropping the link.

If an implementation wishes to force dropping a link without
negotiation, it should simply send an LCP Terminate-Request packet on
the link (without sending any BAP Link-Drop-Query-Request).

After an LCP Terminate-Request is sent an implementation SHOULD stop
transmitting data packets on that link, but still continue to receive
and process data packets normally until receipt of a Terminate-Ack
from the peer. The receiver of an LCP Terminate-Request SHOULD stop
transmitting packets before issuing the Terminate-Ack. This
procedure will insure that no data is lost in either direction.

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5.2. Bandwidth Management

BAP allows two peer implementations to manage the bandwidth available
to the protocols using the multilink bundle by negotiating when to
add and drop links (See Link Management). Use of the negotiation
features of BAP makes it unnecessary to require a 'common' algorithm
for determining when to add and remove links in a multilink bundle.

BOD decisions can be based on link utilization. A BAP implementation
may monitor its transmit traffic, both transmit and receive traffic,
or choose not to monitor traffic in either direction. If a server
system implements bi-directional monitoring, it will allow BOD
operation with a client that does not monitor traffic in either
direction, which will minimize the end-user's configuration. When an
implementation decides that it is time to remove a link due to
traffic monitoring, it MUST transmit a Link-Drop-Query-Request to
inquire if the peer agrees to drop a link from the current multilink
bundle. When an implementation receives a Link-Drop-Query-Request,
it SHOULD base its response on the traffic it is monitoring. It MUST
NOT base its response solely on its receive data heuristics.

The operation of the Link-Drop-Query-Request and -Response datagrams
causes a link in a multilink bundle to be left up as long as either
implementation that is monitoring link utilization determines that it
is necessary.

BOD decisions can also be based on the resources (e.g., physical
port, B-channel, etc.) available to an implementation. For example,
an implementation might remove a link from a multilink bundle to
answer an incoming voice call, or might add a link when a line
becomes free due to the termination of a separate PPP call on another
port. An implementation MUST use an LCP Terminate-Request to remove
a link due to a resource condition.

5.3. BAP Packets

All of the BAP Request and Indication packets require a Response
packet in response before taking any action.

An implementation MUST set a timer when sending a Request or
Indication packet. The value of this timer SHOULD depend on the type
and speed of the link or links in use. Upon expiration of this
timer, the implementation MUST retransmit the request or indication,
with an identical identification number. This procedure will insure
that the peer receives the proper request or indication even if a
packet is lost during transmission. If a response packet is lost the
peer will realize that this is not a new request or indication
packet.

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If the number of retransmissions exceeds the number supported by the
implementation for this packet, the implementation MAY take
appropriate recovery action. For example, if no response to a Link-
Drop-Query-Request is received after 2 retransmissions, an
implementation MAY initiate dropping the link by sending an LCP
Terminate-Request for that link.

Since BAP packets help determine the amount of bandwidth available to
an implementation, PPP SHOULD give them priority over other data
packets when transmitting. This will help insure the prompt addition
and removal of links in a multilink bundle. This is especially
important when adding links to a bundle due to bandwidth constraints.

5.4. Race Conditions

In order to resolve race conditions, an implementation MUST implement
the BACP Favored-Peer Configuration Option.

A race condition can occur if both implementations send a Call-
Request, Callback-Request or Link-Drop-Query-Request at the same
time. These race conditions should be solved as follows:

If each implementation sends a Call-Request or Callback-Request at
the same time, the implementation with the lowest BACP Favored-
Peer Magic-Number value SHOULD be favored.

If each implementation sends a Link-Drop-Query-Request at the same
time, the same scheme SHOULD be used as for Call-Requests.

5.5. BAP Datagram Format

Description

Before any BAP packets may be communicated, PPP MUST reach the
Network-Layer Protocol phase, and BACP MUST reach the opened
state.

Exactly one BAP packet is encapsulated in the Information field of
PPP Data Link Layer frames where the Protocol field indicates type
hex c02d (Bandwidth Allocation Protocol).

Because ISDN Terminal Adapters sometimes are used to do multilink
with a non-multilink aware client, BAP datagrams MUST NOT be
compressed or encrypted. Otherwise, the ISDN TA may not be able
to properly intercept BAP datagrams needed to control the
multilink connection. This refers to compression of the whole
datagram; Address-and-Control-Field-Compression and Protocol-
Field-Compression are allowed if properly negotiated.

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The maximum length of a BAP packet transmitted over a PPP link is
the same as the maximum length of the Information field of a PPP
data link layer frame.

Bandwidth Allocation Protocol datagrams can be catagorized as
either Request, Indication or Response packets. Every Request and
Indication datagram has a corresponding Response packet. Request
and Indication datagrams have a slightly different format from
Response datagrams, as the Response datagrams include a Response
Code octet.

All of the BAP datagrams MUST be supported by an implementation.
However, that does not mean an implementation must support all BAP
datagram actions. An implementation MAY send a Request-Rej to a
Request that it does not implement.

A summary of the Bandwidth Allocation Protocol datagram Request and
Indication packet format is shown below. The fields are transmitted
from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-+-+-+-+

A summary of the Bandwidth Allocation Protocol datagram Response
packet format is shown below. The fields are transmitted from left
to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response Code | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

The Type field is one octet and identifies the type of BAP
datagram packet. Datagram types are defined as follows. This
field is coded in binary coded hexadecimal.

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01 Call-Request
02 Call-Response
03 Callback-Request
04 Callback-Response
05 Link-Drop-Query-Request
06 Link-Drop-Query-Response
07 Call-Status-Indication
08 Call-Status-Response

The various types of BAP datagrams are explained in the following
sections.

Identifier

The Identifier field is one octet and is binary coded. It aids in
matching Requests and Indications with Responses. Call-Status-
Indication packets MUST use the same Identifier as was used by the
original Call-Request or Callback-Request that was used to
initiate the call. All other Request or Indication packets MUST
use a unique Identifier for each new Request or Indication. All
Response packets MUST use the same Identifier as the Identifier in
the Request or Indication packet being responded to. When re-
transmitting a request or indication, the Identifier MUST be the
same as the Identifier used on the previous transmission of the
request or indication.

Length

The Length field is two octets and indicates the length of the
packet including the Type, Identifier, Length and Options fields.
It is binary encoded. Octets outside the range of the Length field
should be treated as Data Link Layer padding and should be ignored
on reception.

Response Code

The Response Code is only present in Response datagrams. It is
binary coded and can have the following values:

00000000 Request-Ack
00000001 Request-Nak
00000010 Request-Rej
00000011 Request-Full-Nak

The Request-Ack Response Code is sent to indicate that the Request
or Indication command is valid and was successfully received by an
implementation. The Request-Nak Response Code is sent to indicate
that the Request command was received, but an implementation does

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not want the requested action performed at this time. If a
Response containing a Request-Nak Response Code is received, the
original Request MAY be retried after an implementation determines
that sufficient time has elapsed. The Request-Rej Response Code
is sent to indicate that the Request command received by an
implementation is not implemented (i.e., if reception of a
particular request type is not supported by the peer.) The
Request-Full-Nak Response Code is sent to indicate that the
Request command was received, but an implementation does not want
the requested action performed. The Request-Full-Nak is used to
indicate that an implementation has reached the maximum (for a
Call- or Callback-Request) or the minimum (for a Link-Drop-Query-
Request) bandwidth configured or available for this multilink
bundle. If a Response containing a Request-Full-Nak Response Code
is received, the original Request SHOULD NOT be retried until the
total bandwidth of the multilink bundle has changed.

Data

The Data field is variable in length, and will usually contain the
list of zero or more BAP Options that the sender desires to
transmit. The format of BAP Options is described in a later
chapter.

5.5.1. Call-Request

Before originating a call to add another link to a multilink bundle,
an implementation MUST transmit a Call-Request packet. This will
inform the receiver of the request to add another link to the bundle
and give the receiver a chance to inform the implementation of the
phone number of a free port that can be called.

The options field MUST include the Link-Type option. The options
field MAY include the No-Phone-Number and/or the Reason options.

Upon reception of a Call-Request, a Call-Response datagram MUST be
transmitted.

5.5.2. Call-Response

An implementation MUST transmit a Call-Response datagram in response
to a received Call-Request datagram. If the Call-Request is
acceptable, the Call-Response MUST have a Response Code of Request-
Ack. The Phone-Delta option MUST be included in a Call-Response
packet with a Response Code of Request-Ack unless the Call-Request
included the No-Phone-Number option. The options field MAY include
the Reason and/or Link-Type options.

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5.5.3. Callback-Request

An implementation that wants its peer to originate another link to
add to the multilink bundle MUST transmit a Callback-Request packet
to its peer. This will inform the receiver of the request to add
another link to the bundle along with the number to be called.

The options field MUST include the Link-Type and Phone-Delta options.
The Reason option MAY also be included.

Upon reception of a Callback-Request, a Callback-Response datagram
MUST be transmitted.

5.5.4. Callback-Response

An implementation MUST transmit a Callback-Response datagram in
response to a received Callback-Request datagram. If the Callback-
Request is acceptable, the Callback-Response MUST have a Response
Code of Request-Ack. A Callback-Response packet MAY include the
Link-Type option.

5.5.5. Link-Drop-Query-Request

An implementation that determines that a link is no longer needed and
wishes to negotiate dropping it (e.g., based on a throughput BOD
decision), MUST transmit a Link-Drop-Query-Request packet. The
options field MUST include the Link-Discriminator option (containing
the receiver's Link-Discriminator), and MAY include the Reason
option.

Upon reception of a Link-Drop-Query-Request, an implementation MUST
transmit a Link-Drop-Query-Response datagram. The Response-Code will
be Request-Ack if it agrees to drop the link; if it does not agree to
drop the link the Response-Code will be Request-Nak or Request-Full-
Nak. After the receipt of a Link-Drop-Query-Response with a Response
Code of Request-Ack, the transmitter of the Link-Drop-Query-Request
MUST initiate tear down of the indicated link by sending an LCP
Terminate-Request packet on the designated link.

5.5.6. Link-Drop-Query-Response

An implementation transmits a Link-Drop-Query-Response datagram in
response to a received Link-Drop-Query-Request datagram. If the
implementation agrees (e.g., based on its throughput BOD algorithm)
to reduce the bandwidth of the multilink bundle, then the Response
Code MUST be set to Request-Ack.

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The Reason option MAY be included in the Link-Drop-Query-Response
packet.

The Link-Drop-Query-Request datagram MUST be supported, as well as
the underlying implementation to respond to it. This means that a
Link-Drop-Query-Response with a Response Code of Request-Rej MUST NOT
be transmitted in response to a Link-Drop-Query-Request.

5.5.7. Call-Status-Indication

After an implementation attempts to add a link to a bundle as the
result of a Call-Request or a Callback-Request, it MUST send a Call-
Status-Indication packet to its peer to indicate if the attempt to
add the link succeeded or failed. One Indication MUST be sent for
each attempt made. For each Call-Status-Indication packet transmitted
with the Call-Status Option Action octet set to Retry, a subsequent
Call-Status-Indication packet MUST be sent to indicate the success or
failure of the retry. The Call-Status option MUST be included to
inform the receiver of the status of the attempt to add a link and
the action the implementation will take in case of failure. The
reason option MAY also be included in the Call-Status-Indication
packet.

Upon reception of a Call-Status-Indication packet which indicates a
failure, an implementation may log the failure and reason code. Upon
reception of any Call-Status-Indication packet, a Call-Status-
Response datagram MUST be transmitted.

5.5.8. Call-Status-Response

An implementation transmits a Call-Status-Response datagram in
response to a received Call-Status-Indication datagram. The Response
Code field MUST be set to Request-Ack in this packet. The Reason
option MAY be included in this packet.

6. BAP Datagram Options

BAP Datagram Options are used in various BAP packets. Their use in
various packets is as defined below. The format of these options
loosely follows the formatting conventions of LCP Configuration
Options. When there are multiple BAP Options in one BAP packet, the
options MAY be transmitted in any order.

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A summary of the BAP Option format is shown below. The fields are
transmitted from left to right.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

The type field is one octet, and indicates the type of the BAP
Datagram Option. This field is binary coded Hexadecimal. The
following options are currently defined:

01 Link-Type
02 Phone-Delta
03 No-Phone-Number-Needed
04 Reason
05 Link-Discriminator
06 Call-Status

Length

The Length field is one octet, and indicates the length of this
BAP Option including the Type, Length, and Data fields.

Data

The Data field is zero or more octets, and contains information
specific to the BAP Option. The format and length of the Data
field is determined by the Type and Length fields.

6.1. Link-Type

Description

This option indicates the general type of link indicated for the
operation being performed. This option does not indicate a
specific link type, rather it gives some general characteristics
of the desired link type. This option MAY be used along with
other knowledge (i.e., the type of the other link(s) in the bundle
or user configuration) to determine the type of link desired to be
used in the operation. It MUST be included in a Call- or
Callback-Request, and MAY be included in a Call- or Callback-
Response.

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A summary of the Link-Type BAP Option format is shown below. The
fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link Speed (kbps) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type |
+-+-+-+-+-+-+-+-+

Type

01 for Link-Type.

Length

The Length field is one octet, and indicates the length of this
BAP Option including the Type, Length and Link Type fields.

Link Speed

The Link Speed field is 2 octets, and indicates the requested
speed of the desired link in kilobits per second. This field is
coded as 2 binary coded hexadecimal octets, with the most
significant octet sent first.

Link Type

The Link Type field is a bit mask. It is 1 octet in length. Bit
0 of the Link Type field corresponds to bit 39 of the Link-Type
BAP Option as described above. If a bit is set, it indicates
support of the corresponding link type. If the link indicated is
different than the supported link types, no bit will be set.
Otherwise, at least one bit MUST be set. If an implementation
supports more than one link type, more than one bit MAY be set.

Bit Link type
--- -------------
0 ISDN
1 X.25
2 analog
3 switched digital (non-ISDN)
4 ISDN data over voice
5-7 reserved

If the Length field contains more bits than are defined by this
specification, then any bits that are not defined should be

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ignored. In order to allow for future expansion of this field, it
is important to properly support receiving a Link Type field
longer than what is defined by this specification. If the Length
field is shorter than the number of bits defined, then the
implementation should set all bits not received to 0.

6.2. Phone-Delta

Description

The BAP Phone-Delta Option is used by an implementation to give
its peer the information needed to make a call. Due to the
difficulty of determining which dialing prefixes (if any) are
necessary to dial a given phone number/national destination
code/country code combination, the phone number to be dialed will
be based on a previously known number. This MAY be the original
number used to establish the first link of the multilink bundle, a
number configured by the user, the phone number used to make a
callback connection, or a number determined in some other way.
The Phone-Delta Option will consist of a Subscriber-Number Sub-
Option along with a Unique-Digits Sub-Option that indicates how
many of the digits of the Subscriber-Number are unique among the
ports in use, previously used, and to be used in the multilink
bundle. There is also an optional Phone-Number-Sub-Address Sub-
Option.

An implementation MAY include more than one Phone-Delta option in
a response. This indicates that there is more than one phone
number that can be used for the requested operation. The Phone-
Delta option MUST appear in a Callback-Request. It also MUST
appear in a Call-Response with a Response Code set to Request-Ack
if the Call-Request did not contain the No-Phone-Number option.
It MAY be included in the Call-Status-Indication packet.

A summary of the Phone-Delta BAP Option format is shown below. The
fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Sub-Option Type| Sub-Option Len|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Option...
+-+-+-+-+-+-+-+-+

Type

02 for Phone-Delta.

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Length

The Length field is one octet, and indicates the length of this
BAP Option including the Type, Length, and Sub-Option fields.

Sub-Option Type

The following Sub-Option Types are defined for the Phone-Delta
option.

01 Unique-Digits
02 Subscriber-Number
03 Phone-Number-Sub-Address

Sub-Option Length

The Sub-Option Length field is one octet, and indicates the length
of this BAP Sub-Option including the Sub-Option Type, Sub-Option
Length, and Sub-Option fields.

6.2.1. Phone-Delta Sub-Options

Unique-Digits

The Unique-Digits Sub-Option field consists of one octet that is a
count of the number of rightmost digits of the Subscriber-Number
that are different from the set of phone numbers of the ports used
in this multilink connection. (For example, if the first port of
a multilink bundle has a phone number of 123456789, and an
implementation wanted its peer to call a port with a phone number
of 123456888, the Unique-Digits octet would be 3.) If the Phone-
Number-Sub-Address Sub-Option is present, the Unique-Digits Sub-
Option MUST NOT include any of the Sub Address digits in its count
of different rightmost digits.

This field is required.

Subscriber-Number

This field is the phone number of the port that should be called
by the peer. Any digits that precede the rightmost unique digits
of the Subscriber-Number are provided for informational purposes
only, and do not need to be included in this field. This field is
an ASCII string and MUST contain only ASCII characters indicating
valid phone number digits. This field is required.

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Phone-Number-Sub-Address

This field is the sub address of the port to be called by the
peer. This sub-option SHOULD only be used for an ISDN call. This
field is an ASCII string and only contains valid phone number
digits. This field is optional.

6.3. No-Phone-Number-Needed

Description

The No-Phone-Number option indicates that the calling
implementation is already configured with the phone number of its
multilink peer and the answering implementation MUST NOT include
the Phone Number option in the response. This may be for security
reasons, for configuration reasons, or for any other reason.

This option MAY be used in a Call-Request packet.

A summary of the No-Phone-Number BAP Option format is shown below.
The fields are transmitted from left to right.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

03 for No-Phone-Number.

Length

2

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6.4. Reason

Description

This option is used to indicate a reason for the Request or
Response. It is meant to be used for informational purposes only.
This option MAY be used in any BAP packet.

A summary of the Reason BAP Option format is shown below. The fields
are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reason String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

04 for Reason.

Length

The Length field is one octet, and indicates the length of this
BAP Option including the Type, Length and Reason String fields.

Reason String

This is an ASCII string. The content of the field is
implementation dependent. An implementation MAY ignore the Reason
String field.

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6.5. Link-Discriminator

Description

The Link-Discriminator option MUST be used in a Link-Drop-Query-
Request datagram. This option is used to inform the receiver of a
Link-Drop-Query-Request of which link will be dropped.

A summary of the Link-Discriminator BAP Option format is shown below.
The fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link Discriminator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

05 for Link-Discriminator

Length

4

Link Discriminator

The Link Discriminator field is 2 octets in length. It contains
the Link Discriminator that was contained in the LCP Link-
Discriminator Configuration Option sent by the receiver of the
packet containing the Link Discriminator.

6.6. Call-Status

Description

The Call-Status option MUST be used in a Call-Status-Indication
datagram. This option is used to inform the receiver of the
Call-Status-Indication datagram of the status of the completed
call attempt, as well as a possible action that will be taken (if
the call failed).

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A summary of the Call-Status BAP Option format is shown below. The
fields are transmitted from left to right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Status | Action |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

06 for Call-Status.

Length

4

Status

The Status field is 1 octet in length. If the call was
successful, the value MUST be set to 0. A non-zero value
indicates a call failure. A value of 255 indicates a non-specific
failure, and a more specific call status MAY be indicated by using
the same number as the Q.931 cause value (i.e., 1 is unassigned
number, 17 is user busy, etc.)

Action

The Action octet indicates what action the calling implementation is
taking after a failed call. If the call was sucessful, the Action
octet MUST be set to 0.

The Action octet can have the following values:

0 - No retry
1 - Retry

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Appendix

List of BAP datagrams and associated fields.

datagram mandatory fields allowed options
-------- ----------------- ---------------
Call-Request Link-Type No-Phone-Number
Call-Response Phone-Delta
Link-Type
Callback-Request Link-Type
Phone-Delta
Callback-Response Link-Type
Link-Drop-Query-Request Link-Discriminator
Link-Drop-Query-Response
Call-Status-Indication Call-Status Phone-Delta
Call-Status-Response

The Reason option is allowed to be included with any BAP datagram.

History of BACP

The first version of BACP was written by Craig Richards of Shiva
Corporation. This version was enhanced and improved by the MPCP
Working Group, a collaborative effort of 3Com, Ascend, Bay Networks,
Cisco, Microsoft, Shiva, US Robotics and Xylogics.

Acknowledgements

Kevin Smith of Ascend for his contributions based on his work on the
MP+ Specification. Gerry Meyer and Robert Myhill of Shiva for their
early comments and improvements. Andy Nicholson of Microsoft for his
improvements to the bandwidth management scheme. Dana Blair and Andy
Valencia of Cisco, Cheng Chen and Dan Brennan of 3Com for their good
ideas as part of the MPCP Working Group. All of the members of the
MPCP working group for their ability to work with their competitors
with enthusiasm to produce a better protocol for the industry.

Security Considerations

Security issues are not discussed in this memo.

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References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, Daydreamer, July 1994.

[2] Sklower, Lloyd, McGregor, Carr & Coradetti, "The PPP Multilink
Protocol", RFC 1990, University of California, Berkeley, Lloyd
Internetworking, Newbridge Networks Corporation, Sidewalk
Software, August 1996.

Chair's Address

The working group can be contacted via the current chair:

Karl Fox
Ascend Communications
3518 Riverside Drive, Suite 101
Columbus, Ohio 43221

(614)451-1883

EMail: [email protected]

Editors' Addresses

Craig Richards
Shiva Corporation
28 Crosby Drive
Bedford, MA 01730
VOICE +1 617 270 8419
FAX +1 617 270 8599

EMail: [email protected]

Kevin Smith
Ascend Communications, Inc.
1275 Harbor Bay Parkway
Alameda, CA 94501
CA

EMail: [email protected]

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