RFC 1331
The Point-to-Point Protocol (PPP) for the Transmission of Multi-protocol Datagrams over Point-to-Point Links
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
through 0xff (except 0x5e) in Control Escape format. Since these
octet values are not negotiable, this does not solve the problem
of receivers which cannot handle all non-control characters.
Also, since the technique does not affect the 8th bit, this does
not solve problems for communications links that can send only 7-
bit characters.
A few examples may make this more clear. Packet data is
transmitted on the link as follows:
0x7e is encoded as 0x7d, 0x5e.
0x7d is encoded as 0x7d, 0x5d.
0x01 is encoded as 0x7d, 0x21.
Some modems with software flow control may intercept outgoing DC1
and DC3 ignoring the 8th (parity) bit. This data would be
transmitted on the link as follows:
0x11 is encoded as 0x7d, 0x31.
0x13 is encoded as 0x7d, 0x33.
0x91 is encoded as 0x7d, 0xb1.
0x93 is encoded as 0x7d, 0xb3.
Aborting a Transmission
On asynchronous links, frames may be aborted by transmitting a "0"
stop bit where a "1" bit is expected (framing error) or by
transmitting a Control Escape octet followed immediately by a
closing Flag Sequence.
Time Fill
On asynchronous links, inter-octet and inter-frame time fill MUST
be accomplished by transmitting continuous "1" bits (mark-hold
state).
Note: On asynchronous links, inter-frame time fill can be
viewed as extended inter-octet time fill. Doing so can save
one octet for every frame, decreasing delay and increasing
bandwidth. This is possible since a Flag Sequence may serve as
both a frame close and a frame begin. After having received
any frame, an idle receiver will always be in a frame begin
state.
Robust transmitters should avoid using this trick over-
zealously since the price for decreased delay is decreased
reliability. Noisy links may cause the receiver to receive
garbage characters and interpret them as part of an incoming
frame. If the transmitter does not transmit a new opening Flag
Sequence before sending the next frame, then that frame will be
appended to the noise characters causing an invalid frame (with
high reliability). Transmitters should avoid this by
transmitting an open Flag Sequence whenever "appreciable time"
has elapsed since the prior closing Flag Sequence. It is
suggested that implementations will achieve the best results by
always sending an opening Flag Sequence if the new frame is not
back-to-back with the last. The maximum value for "appreciable
time" is likely to be no greater than the typing rate of a slow
to average typist, say 1 second.
B. Fast Frame Check Sequence (FCS) Implementation B.1. FCS Computation Method The following code provides a table lookup computation for calculating the Frame Check Sequence as data arrives at the interface. This implementation is based on [7], [8], and [9]. The table is created by the code in section B.2. /* * u16 represents an unsigned 16-bit number. Adjust the typedef for * your hardware. */ typedef unsigned short u16; /* * FCS lookup table as calculated by the table generator in section * B.2. */ static u16 fcstab[256] = { 0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf, 0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7, 0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e, 0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876, 0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd, 0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5, 0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c, 0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974, 0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb, 0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3, 0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a, 0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72, 0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9, 0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1, 0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738, 0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70, 0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7, 0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff, 0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036, 0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e, 0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5, 0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd, 0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134, 0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c, 0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3, 0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb, 0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};
#define PPPINITFCS 0xffff /* Initial FCS value */
#define PPPGOODFCS 0xf0b8 /* Good final FCS value */
/*
* Calculate a new fcs given the current fcs and the new data.
*/
u16 pppfcs(fcs, cp, len)
register u16 fcs;
register unsigned char *cp;
register int len;
{
ASSERT(sizeof (u16) == 2);
ASSERT(((u16) -1) > 0);
while (len--)
fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];
return (fcs);
}
B.2. Fast FCS table generator The following code creates the lookup table used to calculate the FCS. /* * Generate a FCS table for the HDLC FCS. * * Drew D. Perkins at Carnegie Mellon University. * * Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier. */ /* * The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408). */ #define P 0x8408 main() { register unsigned int b, v; register int i; printf("typedef unsigned short u16;\n"); printf("static u16 fcstab[256] = {"); for (b = 0; ; ) { if (b % 8 == 0) printf("\n"); v = b; for (i = 8; i--; ) v = v & 1 ? (v >> 1) ^ P : v >> 1; printf("0x%04x", v & 0xFFFF); if (++b == 256) break; printf(","); } printf("\n};\n"); }
C. LCP Recommended Options The following Configurations Options are recommended: SYNC LINES Magic Number Link Quality Monitoring No Address and Control Field Compression No Protocol Field Compression ASYNC LINES Async Control Character Map Magic Number Address and Control Field Compression Protocol Field Compression
Security Considerations Security issues are briefly discussed in sections concerning the Authentication Phase, and the Authentication-Protocol Configuration Option. Further discussion is planned in a separate document entitled PPP Authentication Protocols. References [1] Electronic Industries Association, EIA Standard RS-232-C, "Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange", August 1969. [2] International Organization For Standardization, ISO Standard 3309-1979, "Data communication - High-level data link control procedures - Frame structure", 1979. [3] International Organization For Standardization, ISO Standard 4335-1979, "Data communication - High-level data link control procedures - Elements of procedures", 1979. [4] International Organization For Standardization, ISO Standard 4335-1979/Addendum 1, "Data communication - High-level data link control procedures - Elements of procedures - Addendum 1", 1979. [5] International Organization For Standardization, Proposed Draft International Standard ISO 3309:1983/PDAD1, "Information processing systems - Data communication - High-level data link control procedures - Frame structure - Addendum 1: Start/stop transmission", 1984. [6] International Telecommunication Union, CCITT Recommendation X.25, "Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks", CCITT Red Book, Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984. [7] Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983. [8] Morse, G., "Calculating CRC's by Bits and Bytes", Byte, September 1986. [9] LeVan, J., "A Fast CRC", Byte, November 1987. [10] American National Standards Institute, ANSI X3.4-1977, "American National Standard Code for Information Interchange",
1977.
[11] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
USC/Information Sciences Institute, March 1990.
Acknowledgments
Much of the text in this document is taken from the WG Requirements
(unpublished), and RFCs 1171 & 1172, by Drew Perkins of Carnegie
Mellon University, and by Russ Hobby of the University of California
at Davis.
Many people spent significant time helping to develop the Point-to-
Point Protocol. The complete list of people is too numerous to list,
but the following people deserve special thanks: Rick Adams (UUNET),
Ken Adelman (TGV), Fred Baker (ACC), Mike Ballard (Telebit), Craig
Fox (NSC), Karl Fox (Morning Star Technologies), Phill Gross (NRI),
former WG chair Russ Hobby (UC Davis), David Kaufman (Proteon),
former WG chair Steve Knowles (FTP Software), John LoVerso
(Xylogics), Bill Melohn (Sun Microsystems), Mike Patton (MIT), former
WG chair Drew Perkins (CMU), Greg Satz (cisco systems) and Asher
Waldfogel (Wellfleet).
Chair's Address
The working group can be contacted via the current chair:
Brian Lloyd
Lloyd & Associates
3420 Sudbury Road
Cameron Park, California 95682
Phone: (916) 676-1147
EMail: brian@ray.lloyd.com
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
P O Box 6205
East Lansing, MI 48826-6025
EMail: bsimpson@ray.lloyd.com