unacceptable set of Configuration Options. Configure-Nak packets
are used to refuse a Configuration Option value, and to suggest a
new, acceptable value. Configure-Reject packets are used to
refuse all negotiation about a Configuration Option, typically
because it is not recognized or implemented. The use of
Configure-Nak versus Configure-Reject is more fully described in
the section on LCP Packet Formats.
Send-Terminate-Request (str)
The Send-Terminate-Request action transmits a Terminate-Request
packet. This indicates the desire to close a connection. The
Restart timer is started when the Terminate-Request packet is
transmitted, to guard against packet loss. The Restart counter is
decremented each time a Terminate-Request is sent.
Send-Terminate-Ack (sta)
The Send-Terminate-Ack action transmits a Terminate-Ack packet.
This acknowledges the reception of a Terminate-Request packet or
otherwise serves to synchronize the state machines.
Send-Code-Reject (scj)
The Send-Code-Reject action transmits a Code-Reject packet. This
indicates the reception of an unknown type of packet.
Send-Echo-Reply (ser)
The Send-Echo-Reply action transmits an Echo-Reply packet. This
acknowledges the reception of an Echo-Request packet.
5.6. Loop Avoidance
The protocol makes a reasonable attempt at avoiding Configuration
Option negotiation loops. However, the protocol does NOT guarantee
that loops will not happen. As with any negotiation, it is possible
to configure two PPP implementations with conflicting policies that
will never converge. It is also possible to configure policies which
do converge, but which take significant time to do so. Implementors
should keep this in mind and should implement loop detection
mechanisms or higher level timeouts.
5.7. Counters and Timers
Restart Timer
There is one special timer used by the automaton. The Restart timer
is used to time transmissions of Configure-Request and Terminate-
Request packets. Expiration of the Restart timer causes a Timeout
event, and retransmission of the corresponding Configure-Request or
Terminate-Request packet. The Restart timer MUST be configurable,
but MAY default to three (3) seconds.
Implementation Note:
The Restart timer SHOULD be based on the speed of the link. The
default value is designed for low speed (19,200 bps or less), high
switching latency links (typical telephone lines). Higher speed
links, or links with low switching latency, SHOULD have
correspondingly faster retransmission times.
Max-Terminate
There is one required restart counter for Terminate-Requests. Max-
Terminate indicates the number of Terminate-Request packets sent
without receiving a Terminate-Ack before assuming that the peer is
unable to respond. Max-Terminate MUST be configurable, but should
default to two (2) transmissions.
Max-Configure
A similar counter is recommended for Configure-Requests. Max-
Configure indicates the number of Configure-Request packets sent
without receiving a valid Configure-Ack, Configure-Nak or Configure-
Reject before assuming that the peer is unable to respond. Max-
Configure MUST be configurable, but should default to ten (10)
transmissions.
Max-Failure
A related counter is recommended for Configure-Nak. Max-Failure
indicates the number of Configure-Nak packets sent without sending a
Configure-Ack before assuming that configuration is not converging.
Any further Configure-Nak packets are converted to Configure-Reject
packets. Max-Failure MUST be configurable, but should default to ten
(10) transmissions.
6. LCP Packet Formats
There are three classes of LCP packets:
1. Link Configuration packets used to establish and configure a
link (Configure-Request, Configure-Ack, Configure-Nak and
Configure-Reject).
2. Link Termination packets used to terminate a link (Terminate-
Request and Terminate-Ack).
3. Link Maintenance packets used to manage and debug a link
(Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
Discard-Request).
This document describes Version 1 of the Link Control Protocol. In
the interest of simplicity, there is no version field in the LCP
packet. If a new version of LCP is necessary in the future, the
intention is that a new Data Link Layer Protocol field value will be
used to differentiate Version 1 LCP from all other versions. A
correctly functioning Version 1 LCP implementation will always
respond to unknown Protocols (including other versions) with an
easily recognizable Version 1 packet, thus providing a deterministic
fallback mechanism for implementations of other versions.
Regardless of which Configuration Options are enabled, all LCP Link
Configuration, Link Termination, and Code-Reject packets (codes 1
through 7) are always sent in the full, standard form, as if no
Configuration Options were enabled. This ensures that LCP
Configure-Request packets are always recognizable even when one end
of the link mistakenly believes the link to be open.
Exactly one Link Control Protocol packet is encapsulated in the
Information field of PPP Data Link Layer frames where the Protocol
field indicates type hex c021 (Link Control Protocol).
A summary of the Link Control Protocol 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
The Code field is one octet and identifies the kind of LCP packet.
When a packet is received with an invalid Code field, a Code-
Reject packet is transmitted.
The most up-to-date values of the LCP Code field are specified in
the most recent "Assigned Numbers" RFC[11]. Current values are
assigned as follows:
1 Configure-Request
2 Configure-Ack
3 Configure-Nak
4 Configure-Reject
5 Terminate-Request
6 Terminate-Ack
7 Code-Reject
8 Protocol-Reject
9 Echo-Request
10 Echo-Reply
11 Discard-Request
12 RESERVED
Identifier
The Identifier field is one octet and aids in matching requests
and replies. When a packet is received with an invalid Identifier
field, the packet is silently discarded.
Length
The Length field is two octets and indicates the length of the LCP
packet including the Code, Identifier, Length and Data fields.
Octets outside the range of the Length field should be treated as
Data Link Layer padding and should be ignored on reception. When
a packet is received with an invalid Length field, the packet is
silently discarded.
Data
The Data field is zero or more octets as indicated by the Length
field. The format of the Data field is determined by the Code
field.
6.1. Configure-Request
Description
A LCP implementation wishing to open a connection MUST transmit a
LCP packet with the Code field set to 1 (Configure-Request) and
the Options field filled with any desired changes to the default
link Configuration Options.
Upon reception of a Configure-Request, an appropriate reply MUST
be transmitted.
A summary of the Configure-Request 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
1 for Configure-Request.
Identifier
The Identifier field SHOULD be changed on each transmission. On
reception, the Identifier field should be copied into the
Identifier field of the appropriate reply packet.
Options
The options field is variable in length and contains the list of
zero or more Configuration Options that the sender desires to
negotiate. All Configuration Options are always negotiated
simultaneously. The format of Configuration Options is further
described in a later section.
6.2. Configure-Ack
Description
If every Configuration Option received in a Configure-Request is
both recognizable and acceptable, then a LCP implementation should
transmit a LCP packet with the Code field set to 2 (Configure-
Ack), the Identifier field copied from the received Configure-
Request, and the Options field copied from the received
Configure-Request. The acknowledged Configuration Options MUST
NOT be reordered or modified in any way.
On reception of a Configure-Ack, the Identifier field must match
that of the last transmitted Configure-Request. Additionally, the
Configuration Options in a Configure-Ack must exactly match those
of the last transmitted Configure-Request. Invalid packets are
silently discarded.
A summary of the Configure-Ack 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
2 for Configure-Ack.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Ack.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is
acknowledging. All Configuration Options are always acknowledged
simultaneously.
6.3. Configure-Nak
Description
If every element of the received Configuration Options is
recognizable but some are not acceptable, then a LCP
implementation should transmit a LCP packet with the Code field
set to 3 (Configure-Nak), the Identifier field copied from the
received Configure-Request, and the Options field filled with only
the unacceptable Configuration Options from the Configure-Request.
All acceptable Configuration Options are filtered out of the
Configure-Nak, but otherwise the Configuration Options from the
Configure-Request MUST NOT be reordered.
Each of the Nak'd Configuration Options MUST be modified to a
value acceptable to the Configure-Nak sender. Options which have
no value fields (boolean options) use the Configure-Reject reply
instead.
Finally, an implementation may be configured to request the
negotiation of a specific option. If that option is not listed,
then that option may be appended to the list of Nak'd
Configuration Options in order to request the peer to list that
option in its next Configure-Request packet. Any value fields for
the option MUST indicate values acceptable to the Configure-Nak
sender.
On reception of a Configure-Nak, the Identifier field must match
that of the last transmitted Configure-Request. Invalid packets
are silently discarded.
Reception of a valid Configure-Nak indicates that a new
Configure-Request MAY be sent with the Configuration Options
modified as specified in the Configure-Nak.
Some Configuration Options have a variable length. Since the
Nak'd Option has been modified by the peer, the implementation
MUST be able to handle an Option length which is different from
the original Configure-Request.
A summary of the Configure-Nak 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
3 for Configure-Nak.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Nak.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is Nak'ing.
All Configuration Options are always Nak'd simultaneously.
6.4. Configure-Reject
Description
If some Configuration Options received in a Configure-Request are
not recognizable or are not acceptable for negotiation (as
configured by a network administrator), then a LCP implementation
should transmit a LCP packet with the Code field set to 4
(Configure-Reject), the Identifier field copied from the received
Configure-Request, and the Options field filled with only the
unacceptable Configuration Options from the Configure-Request.
All recognizable and negotiable Configuration Options are filtered
out of the Configure-Reject, but otherwise the Configuration
Options MUST NOT be reordered or modified in any way.
On reception of a Configure-Reject, the Identifier field must
match that of the last transmitted Configure-Request.
Additionally, the Configuration Options in a Configure-Reject must
be a proper subset of those in the last transmitted Configure-
Request. Invalid packets are silently discarded.
Reception of a valid Configure-Reject indicates that a new
Configure-Request SHOULD be sent which does not include any of the
Configuration Options listed in the Configure-Reject.
A summary of the Configure-Reject 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
4 for Configure-Reject.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Reject.
Options
The Options field is variable in length and contains the list of
zero or more Configuration Options that the sender is rejecting.
All Configuration Options are always rejected simultaneously.
6.5. Terminate-Request and Terminate-Ack
Description
LCP includes Terminate-Request and Terminate-Ack Codes in order to
provide a mechanism for closing a connection.
A LCP implementation wishing to close a connection should transmit
a LCP packet with the Code field set to 5 (Terminate-Request) and
the Data field filled with any desired data. Terminate-Request
packets should continue to be sent until Terminate-Ack is
received, the lower layer indicates that it has gone down, or a
sufficiently large number have been transmitted such that the peer
is down with reasonable certainty.
Upon reception of a Terminate-Request, a LCP packet MUST be
transmitted with the Code field set to 6 (Terminate-Ack), the
Identifier field copied from the Terminate-Request packet, and the
Data field filled with any desired data.
Reception of an unelicited Terminate-Ack indicates that the peer
is in the Closed or Stopped states, or is otherwise in need of
re-negotiation.
A summary of the Terminate-Request and Terminate-Ack packet formats
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
5 for Terminate-Request;
6 for Terminate-Ack.
Identifier
The Identifier field is one octet and aids in matching requests
and replies.
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus four.
6.6. Code-Reject
Description
Reception of a LCP packet with an unknown Code indicates that one
of the communicating LCP implementations is faulty or incomplete.
This error MUST be reported back to the sender of the unknown Code
by transmitting a LCP packet with the Code field set to 7 (Code-
Reject), and the inducing packet copied to the Rejected-
Information field.
Upon reception of a Code-Reject, the implementation SHOULD report
the error, since it is unlikely that the situation can be
rectified automatically.
A summary of the Code-Reject 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
Code
7 for Code-Reject.
Identifier
The Identifier field is one octet and is for use by the
transmitter.
Rejected-Information
The Rejected-Information field contains a copy of the LCP packet
which is being rejected. It begins with the Information field,
and does not include any PPP Data Link Layer headers nor the FCS.
The Rejected-Information MUST be truncated to comply with the
peer's established maximum Information field length.
6.7. Protocol-Reject
Description
Reception of a PPP frame with an unknown Data Link Layer Protocol
indicates that the peer is attempting to use a protocol which is
unsupported. This usually occurs when the peer attempts to
configure a new protocol. If the LCP state machine is in the
Opened state, then this error MUST be reported back to the peer by
transmitting a LCP packet with the Code field set to 8 (Protocol-
Reject), the Rejected-Protocol field set to the received Protocol,
and the inducing packet copied to the Rejected-Information field.
Upon reception of a Protocol-Reject, a LCP implementation SHOULD
stop transmitting frames of the indicated protocol.
Protocol-Reject packets may only be sent in the LCP Opened state.
Protocol-Reject packets received in any state other than the LCP
Opened state SHOULD be silently discarded.
A summary of the Protocol-Reject 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Protocol | Rejected-Information ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
8 for Protocol-Reject.
Identifier
The Identifier field is one octet and is for use by the
transmitter.
Rejected-Protocol
The Rejected-Protocol field is two octets and contains the
Protocol of the Data Link Layer frame which is being rejected.
Rejected-Information
The Rejected-Information field contains a copy from the frame
which is being rejected. It begins with the Information field,
and does not include any PPP Data Link Layer headers nor the FCS.
The Rejected-Information MUST be truncated to comply with the
peer's established maximum Information field length.
6.8. Echo-Request and Echo-Reply
Description
LCP includes Echo-Request and Echo-Reply Codes in order to provide
a Data Link Layer loopback mechanism for use in exercising both
directions of the link. This is useful as an aid in debugging,
link quality determination, performance testing, and for numerous
other functions.
An Echo-Request sender transmits a LCP packet with the Code field
set to 9 (Echo-Request), the Identifier field set, the local
Magic-Number inserted, and the Data field filled with any desired
data, up to but not exceeding the peer's established maximum
Information field length minus eight.
Upon reception of an Echo-Request, a LCP packet MUST be
transmitted with the Code field set to 10 (Echo-Reply), the
Identifier field copied from the received Echo-Request, the local
Magic-Number inserted, and the Data field copied from the Echo-
Request, truncating as necessary to avoid exceeding the peer's
established maximum Information field length.
Echo-Request and Echo-Reply packets may only be sent in the LCP
Opened state. Echo-Request and Echo-Reply packets received in any
state other than the LCP Opened state SHOULD be silently
discarded.
A summary of the Echo-Request and Echo-Reply packet formats 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
9 for Echo-Request;
10 for Echo-Reply.
Identifier
The Identifier field is one octet and aids in matching Echo-
Requests and Echo-Replies.
Magic-Number
The Magic-Number field is four octets and aids in detecting links
which are in the looped-back condition. Unless modified by a
Configuration Option, the Magic-Number MUST be transmitted as zero
and MUST be ignored on reception. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus eight.
6.9. Discard-Request
Description
LCP includes a Discard-Request Code in order to provide a Data
Link Layer data sink mechanism for use in exercising the local to
remote direction of the link. This is useful as an aid in
debugging, performance testing, and for numerous other functions.
A discard sender transmits a LCP packet with the Code field set to
11 (Discard-Request) the Identifier field set, the local Magic-
Number inserted, and the Data field filled with any desired data,
up to but not exceeding the peer's established maximum Information
field length minus eight.
A discard receiver MUST simply throw away an Discard-Request that
it receives.
Discard-Request packets may only be sent in the LCP Opened state.
A summary of the Discard-Request packet formats 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
11 for Discard-Request.
Identifier
The Identifier field is one octet and is for use by the Discard-
Request transmitter.
Magic-Number
The Magic-Number field is four octets and aids in detecting links
which are in the looped-back condition. Unless modified by a
configuration option, the Magic-Number MUST be transmitted as zero
and MUST be ignored on reception. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
maximum Information field length minus four.
7. LCP Configuration Options
LCP Configuration Options allow modifications to the standard
characteristics of a point-to-point link to be negotiated.
Negotiable modifications include such things as the maximum receive
unit, async control character mapping, the link authentication
method, etc. If a Configuration Option is not included in a
Configure-Request packet, the default value for that Configuration
Option is assumed.
The end of the list of Configuration Options is indicated by the
length of the LCP packet.
Unless otherwise specified, each Configuration Option is not listed
more than once in a Configuration Options list. Some Configuration
Options MAY be listed more than once. The effect of this is
Configuration Option specific and is specified by each such
Configuration Option.
Also unless otherwise specified, all Configuration Options apply in a
half-duplex fashion. When negotiated, they apply to only one
direction of the link, typically in the receive direction when
interpreted from the point of view of the Configure-Request sender.
7.1. Format
A summary of the Configuration 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
Configuration Option. The most up-to-date values of the LCP
Option Type field are specified in the most recent "Assigned
Numbers" RFC[11]. Current values are assigned as follows:
1 Maximum-Receive-Unit
2 Async-Control-Character-Map
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
6 RESERVED
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression
Length
The Length field is one octet and indicates the length of this
Configuration Option including the Type, Length and Data fields.
If a negotiable Configuration Option is received in a Configure-
Request but with an invalid Length, a Configure-Nak SHOULD be
transmitted which includes the desired Configuration Option with
an appropriate Length and Data.
Data
The Data field is zero or more octets and indicates the value or
other information for this Configuration Option. The format and
length of the Data field is determined by the Type and Length
fields.
7.2. Maximum-Receive-Unit
Description
This Configuration Option may be sent to inform the peer that the
implementation can receive larger frames, or to request that the
peer send smaller frames. If smaller frames are requested, an
implementation MUST still be able to receive 1500 octet frames in
case link synchronization is lost.
A summary of the Maximum-Receive-Unit Configuration 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 | Maximum-Receive-Unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
4
Maximum-Receive-Unit
The Maximum-Receive-Unit field is two octets and indicates the new
maximum receive unit. The Maximum-Receive-Unit covers only the
Data Link Layer Information field. It does not include the
header, padding, FCS, nor any transparency bits or bytes.
Default
1500
7.3. Async-Control-Character-Map
Description
This Configuration Option provides a way to negotiate the use of
control character mapping on asynchronous links. By default, PPP
maps all control characters into an appropriate two character
sequence. However, it is rarely necessary to map all control
characters and often it is unnecessary to map any characters. A
PPP implementation may use this Configuration Option to inform the
peer which control characters must remain mapped and which control
characters need not remain mapped when the peer sends them. The
peer may still send these control characters in mapped format if
it is necessary because of constraints at the peer.
There may be some use of synchronous-to-asynchronous converters
(some built into modems) in Point-to-Point links resulting in a
synchronous PPP implementation on one end of a link and an
asynchronous implementation on the other. It is the
responsibility of the converter to do all mapping conversions
during operation. To enable this functionality, synchronous PPP
implementations MUST always accept a Async-Control-Character-Map
Configuration Option (it MUST always respond to an LCP Configure-
Request specifying this Configuration Option with an LCP
Configure-Ack). However, acceptance of this Configuration Option
does not imply that the synchronous implementation will do any
character mapping, since synchronous PPP uses bit-stuffing rather
than character-stuffing. Instead, all such character mapping will
be performed by the asynchronous-to-synchronous converter.
A summary of the Async-Control-Character-Map Configuration 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 | Async-Control-Character-Map
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ACCM (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
6
Async-Control-Character-Map
The Async-Control-Character-Map field is four octets and indicates
the new async control character map. The map is encoded in big-
endian fashion where each numbered bit corresponds to the ASCII
control character of the same value. If the bit is cleared to
zero, then that ASCII control character need not be mapped. If
the bit is set to one, then that ASCII control character must
remain mapped. E.g., if bit 19 is set to zero, then the ASCII
control character 19 (DC3, Control-S) may be sent in the clear.
Note: The bit ordering of the map is as described in section
3.1, Most Significant Bit to Least Significant Bit. The least
significant bit of the least significant octet (the final octet
transmitted) is numbered bit 0, and would map to the ASCII
control character NUL.
Default
All ones (0xffffffff).
7.4. Authentication-Protocol
Description
On some links it may be desirable to require a peer to
authenticate itself before allowing network-layer protocol packets
to be exchanged. This Configuration Option provides a way to
negotiate the use of a specific authentication protocol. By
default, authentication is not necessary.
An implementation SHOULD NOT include multiple Authentication-
Protocol Configuration Options in its Configure-Request packets.
Instead, it SHOULD attempt to configure the most desirable
protocol first. If that protocol is Rejected, then the
implementation could attempt the next most desirable protocol in
the next Configure-Request.
An implementation receiving a Configure-Request specifying
Authentication-Protocols MAY choose at most one of the negotiable
authentication protocols and MUST send a Configure-Reject
including the other specified authentication protocols. The
implementation MAY reject all of the proposed authentication
protocols.
If an implementation sends a Configure-Ack with this Configuration
Option, then it is agreeing to authenticate with the specified
protocol. An implementation receiving a Configure-Ack with this
Configuration Option SHOULD expect the peer to authenticate with
the acknowledged protocol.
There is no requirement that authentication be full duplex or that
the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.
A summary of the Authentication-Protocol Configuration 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 | Authentication-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
3
Length
>= 4
Authentication-Protocol
The Authentication-Protocol field is two octets and indicates the
authentication protocol desired. Values for this field are always
the same as the PPP Data Link Layer Protocol field values for that
same authentication protocol.
The most up-to-date values of the Authentication-Protocol field
are specified in the most recent "Assigned Numbers" RFC[11].
Current values are assigned as follows:
Value (in hex) Protocol
c023 Password Authentication Protocol
c223 Challenge Handshake Authentication
Protocol
Data
The Data field is zero or more octets and contains additional data
as determined by the particular protocol.
Default
No authentication protocol necessary.
7.5. Quality-Protocol
Description
On some links it may be desirable to determine when, and how
often, the link is dropping data. This process is called link
quality monitoring.
This Configuration Option provides a way to negotiate the use of a
specific protocol for link quality monitoring. By default, link
quality monitoring is disabled.
There is no requirement that quality monitoring be full duplex or
that the same protocol be used in both directions. It is
perfectly acceptable for different protocols to be used in each
direction. This will, of course, depend on the specific protocols
negotiated.
A summary of the Quality-Protocol Configuration 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 | Quality-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
4
Length
>= 4
Quality-Protocol
The Quality-Protocol field is two octets and indicates the link
quality monitoring protocol desired. Values for this field are
always the same as the PPP Data Link Layer Protocol field values
for that same monitoring protocol.
The most up-to-date values of the Quality-Protocol field are
specified in the most recent "Assigned Numbers" RFC[11]. Current
values are assigned as follows:
Value (in hex) Protocol
c025 Link Quality Report
Data
The Data field is zero or more octets and contains additional data
as determined by the particular protocol.
Default
None
7.6. Magic-Number
Description
This Configuration Option provides a way to detect looped-back
links and other Data Link Layer anomalies. This Configuration
Option MAY be required by some other Configuration Options such as
the Monitoring-Protocol Configuration Option.
Before this Configuration Option is requested, an implementation
must choose its Magic-Number. It is recommended that the Magic-
Number be chosen in the most random manner possible in order to
guarantee with very high probability that an implementation will
arrive at a unique number. A good way to choose a unique random
number is to start with an unique seed. Suggested sources of
uniqueness include machine serial numbers, other network hardware
addresses, time-of-day clocks, etc. Particularly good random
number seeds are precise measurements of the inter-arrival time of
physical events such as packet reception on other connected
networks, server response time, or the typing rate of a human
user. It is also suggested that as many sources as possible be
used simultaneously.
When a Configure-Request is received with a Magic-Number
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 link is not
looped-back, and the Magic-Number should be acknowledged. If the
two Magic-Numbers are equal, then it is possible, but not certain,
that the link is looped-back and that this Configure-Request is
actually the one last sent. To determine this, a Configure-Nak
should 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 (i.e., until a Configure-Nak
is received or the Restart timer runs out).
Reception of a Configure-Nak with a Magic-Number different from
that of the last Configure-Nak sent to the peer proves that a link
is not looped-back, and indicates a unique Magic-Number. If the
Magic-Number is equal to the one sent in the last Configure-Nak,
the possibility of a looped-back link is increased, and a new
Magic-Number should be chosen. In either case, a new Configure-
Request should be sent with the new Magic-Number.
If the link is indeed looped-back, this sequence (transmit
Configure-Request, receive Configure-Request, transmit Configure-
Nak, receive Configure-Nak) will repeat over and over again. If
the link is not looped-back, this sequence might occur a few
times, but it is extremely unlikely to occur repeatedly. More
likely, the Magic-Numbers chosen at either end will quickly
diverge, terminating the sequence. The following table shows the
probability of collisions assuming that both ends of the link
select Magic-Numbers with a perfectly uniform distribution:
Number of Collisions Probability
-------------------- ---------------------
1 1/2**32 = 2.3 E-10
2 1/2**32**2 = 5.4 E-20
3 1/2**32**3 = 1.3 E-29
Good sources of uniqueness or randomness are required for this
divergence to occur. If a good source of uniqueness cannot be
found, it is recommended that this Configuration Option not be
enabled; Configure-Requests with the option SHOULD NOT be
transmitted and any Magic-Number Configuration Options which the
peer sends SHOULD be either acknowledged or rejected. In this
case, loop-backs cannot be reliably detected by the
implementation, although they may still be detectable by the peer.
If an implementation does transmit a Configure-Request with a
Magic-Number Configuration Option, then it MUST NOT respond with a
Configure-Reject if its peer also transmits a Configure-Request
with a Magic-Number Configuration Option. That is, if an
implementation desires to use Magic Numbers, then it MUST also
allow its peer to do so. If an implementation does receive a
Configure-Reject in response to a Configure-Request, it can only
mean that the link is not looped-back, and that its peer will not
be using Magic-Numbers. In this case, an implementation should
act as if the negotiation had been successful (as if it had
instead received a Configure-Ack).
The Magic-Number also may be used to detect looped-back links
during normal operation as well as during Configuration Option
negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
Request packets have a Magic-Number field which MUST normally be
zero, and MUST normally be ignored on reception. If Magic-Number
has been successfully negotiated, an implementation MUST transmit
these packets with the Magic-Number field set to its negotiated
Magic-Number.
The Magic-Number field of these packets SHOULD be inspected on
reception. All received Magic-Number fields MUST be equal to
either zero or the peer's unique Magic-Number, depending on
whether or not the peer negotiated one.
Reception of a Magic-Number field equal to the negotiated local
Magic-Number indicates a looped-back link. Reception of a Magic-
Number other than the negotiated local Magic-Number or the peer's
negotiated Magic-Number, or zero if the peer didn't negotiate one,
indicates a link which has been (mis)configured for communications
with a different peer.
Procedures for recovery from either case are unspecified and may
vary from implementation to implementation. A somewhat
pessimistic procedure is to assume a LCP Down event. A further
Open event will begin the process of re-establishing the link,
which can't complete until the loop-back condition is terminated
and Magic-Numbers are successfully negotiated. A more optimistic
procedure (in the case of a loop-back) is to begin transmitting
LCP Echo-Request packets until an appropriate Echo-Reply is
received, indicating a termination of the loop-back condition.
A summary of the Magic-Number Configuration 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
5
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, if it is not
Rejected outright.
Default
None.
7.7. Protocol-Field-Compression
Description
This Configuration Option provides a way to negotiate the
compression of the Data Link Layer Protocol field. By default,
all implementations MUST transmit standard PPP frames with two
octet Protocol fields. However, PPP Protocol field numbers are
chosen such that some values may be compressed into a single octet
form which is clearly distinguishable from the two octet form.
This Configuration Option is sent to inform the peer that the
implementation can receive such single octet Protocol fields.
Compressed Protocol fields MUST NOT be transmitted unless this
Configuration Option has been negotiated.
As previously mentioned, the Protocol field uses an extension
mechanism consistent with the ISO 3309 extension mechanism for the
Address field; the Least Significant Bit (LSB) of each octet is
used to indicate extension of the Protocol field. A binary "0" as
the LSB indicates that the Protocol field continues with the
following octet. The presence of a binary "1" as the LSB marks
the last octet of the Protocol field. Notice that any number of
"0" octets may be prepended to the field, and will still indicate
the same value (consider the two representations for 3, 00000011
and 00000000 00000011).
In the interest of simplicity, the standard PPP frame uses this
fact and always sends Protocol fields with a two octet
representation. Protocol field values less than 256 (decimal) are
prepended with a single zero octet even though transmission of
this, the zero and most significant octet, is unnecessary.
However, when using low speed links, it is desirable to conserve
bandwidth by sending as little redundant data as possible. The
Protocol Compression Configuration Option allows a trade-off
between implementation simplicity and bandwidth efficiency. If
successfully negotiated, the ISO 3309 extension mechanism may be
used to compress the Protocol field to one octet instead of two.
The large majority of frames are compressible since data protocols
are typically assigned with Protocol field values less than 256.
In addition, PPP implementations must continue to be robust and
MUST accept PPP frames with either double-octet or single-octet
Protocol fields, and MUST NOT distinguish between them.
The Protocol field is never compressed when sending any LCP
packet. This rule guarantees unambiguous recognition of LCP
packets.
When a Protocol field is compressed, the Data Link Layer FCS field
is calculated on the compressed frame, not the original
uncompressed frame.
A summary of the Protocol-Field-Compression Configuration 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
7
Length
2
Default
Disabled.
7.8. Address-and-Control-Field-Compression
Description
This Configuration Option provides a way to negotiate the
compression of the Data Link Layer Address and Control fields. By
default, all implementations MUST transmit frames with Address and
Control fields and MUST use the hexadecimal values 0xff and 0x03
respectively. Since these fields have constant values, they are
easily compressed. This Configuration Option is sent to inform
the peer that the implementation can receive compressed Address
and Control fields.
Compressed Address and Control fields are formed by simply
omitting them. By definition the first octet of a two octet
Protocol field will never be 0xff, and the Protocol field value
0x00ff is not allowed (reserved) to avoid ambiguity.
On reception, the Address and Control fields are decompressed by
examining the first two octets. If they contain the values 0xff
and 0x03, they are assumed to be the Address and Control fields.
If not, it is assumed that the fields were compressed and were not
transmitted.
If a compressed frame is received when Address-and-Control-Field-
Compression has not been negotiated, the implementation MAY
silently discard the frame.
The Address and Control fields MUST NOT be compressed when sending
any LCP packet. This rule guarantees unambiguous recognition of
LCP packets.
When the Address and Control fields are compressed, the Data Link
Layer FCS field is calculated on the compressed frame, not the
original uncompressed frame.
A summary of the Address-and-Control-Field-Compression configuration
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
8
Length
2
Default
Not compressed.
A. Asynchronous HDLC
This appendix summarizes the modifications to ISO 3309-1979 proposed
in ISO 3309:1984/PDAD1, as applied in the Point-to-Point Protocol.
These modifications allow HDLC to be used with 8-bit asynchronous
links.
Transmission Considerations
All octets are transmitted with one start bit, eight bits of data,
and one stop bit. There is no provision in either PPP or ISO
3309:1984/PDAD1 for seven bit asynchronous links.
Flag Sequence
The Flag Sequence is a single octet and indicates the beginning or
end of a frame. The Flag Sequence consists of the binary sequence
01111110 (hexadecimal 0x7e).
Transparency
On asynchronous links, a character stuffing procedure is used.
The Control Escape octet is defined as binary 01111101
(hexadecimal 0x7d) where the bit positions are numbered 87654321
(not 76543210, BEWARE).
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control
Escape octet and octet with value less than hexadecimal 0x20 which
is flagged in the Remote Async-Control-Character-Map is replaced
by a two octet sequence consisting of the Control Escape octet and
the original octet with bit 6 complemented (i.e., exclusive-or'd
with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire frame
between the two Flag Sequences. Each octet with value less than
hexadecimal 0x20 is checked. If it is flagged in the Local
Async-Control-Character-Map, it is simply removed (it may have
been inserted by intervening data communications equipment). For
each Control Escape octet, that octet is also removed, but bit 6
of the following octet is complemented. A Control Escape octet
immediately preceding the closing Flag Sequence indicates an
invalid frame.
Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [10] excluding DEL (Delete)
to be transparently communicated through almost all known data
communications equipment.
The transmitter may also send octets with value in the range 0x40