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Network Working Group D. Perkins
Request for Comments: 1172 CMU
R. Hobby
UC Davis
July 1990
The Point-to-Point Protocol (PPP) Initial Configuration Options
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
This proposal is the product of the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). Comments on
this memo should be submitted to the IETF Point-to-Point Protocol
Working Group chair.
Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is composed of
1) a method for encapsulating datagrams over serial links,
2) an extensible Link Control Protocol (LCP), and
3) a family of Network Control Protocols (NCP) for establishing
and configuring different network-layer protocols.
The PPP encapsulating scheme, the basic LCP, and an NCP for
controlling and establishing the Internet Protocol (IP) (called the
IP Control Protocol, IPCP) are defined in The Point-to-Point Protocol
(PPP) [1].
This document defines the intial options used by the LCP and IPCP. It
also defines a method of Link Quality Monitoring and a simple
authentication scheme.
Perkins & Hobby [Page i]
RFC 1172 PPP Initial Options July 1990
Table of Contents
1. Introduction .......................................... 1
2. Link Control Protocol (LCP) Configuration Options ..... 1
2.1 Maximum-Receive-Unit ............................ 2
2.2 Async-Control-Character-Map ..................... 3
2.3 Authentication-Type ............................. 5
2.4 Magic-Number .................................... 7
2.5 Link-Quality-Monitoring ......................... 10
2.6 Protocol-Field-Compression ...................... 11
2.7 Address-and-Control-Field-Compression ........... 13
3. Link Quality Monitoring ............................... 15
3.1 Design Motivation ............................... 15
3.2 Design Overview ................................. 15
3.3 Processes ....................................... 16
3.4 Counters ........................................ 18
3.5 Measurements, Calculations, State Variables ..... 19
3.6 Link-Quality-Report Packet Format ............... 21
3.7 Policy Suggestions .............................. 25
3.8 Example ......................................... 25
4. Password Authentication Protocol ...................... 27
4.1 Packet Format ................................... 27
4.2 Authenticate .................................... 29
4.3 Authenticate-Ack ................................ 31
4.4 Authenticate-Nak ................................ 32
5. IP Control Protocol (IPCP) Configuration Options ...... 33
5.1 IP-Addresses .................................... 34
5.2 Compression-Type ................................ 36
REFERENCES ................................................... 37
SECURITY CONSIDERATIONS ...................................... 37
AUTHOR'S ADDRESS ............................................. 37
Perkins & Hobby [Page ii]
RFC 1172 PPP Initial Options July 1990
1. Introduction
The Point-to-Point Protocol (PPP) [1] proposes a standard method of
encapsulating IP datagrams, and other Network Layer protocol
information, over point-to-point links. PPP also proposes an
extensible Option Negotiation Protocol. [1] specifies only the
protocol itself; the initial set of Configuration Options are
described in this document. These Configuration Options allow MTUs
to be changed, IP addresses to be dynamically assigned, header
compression to be enabled, and much more.
This memo is divided into several sections. Section 2 describes
Configuration Options for the Link Control Protocol. Section 3
specifies the use of the Link Quality Monitoring option. Section 4
defines a simple Password Authentication Protocol. Finally, Section 5
specifies Configuration Options for the IP Control Protocol.
2. Link Control Protocol (LCP) Configuration Options
As described in [1], LCP Configuration Options allow modifications to
the standard characteristics of a point-to-point link to be
negotiated. Negotiable modifications proposed in this document
include such things as the maximum receive unit, async control
character mapping, the link authentication method, etc.
The initial proposed values for the LCP Configuration Option Type
field (see [1]) are assigned as follows:
1 Maximum-Receive-Unit
2 Async-Control-Character-Map
3 Authentication-Type
4 NOT ASSIGNED
5 Magic-Number
6 Link-Quality-Monitoring
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression
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RFC 1172 PPP Initial Options July 1990
2.1. Maximum-Receive-Unit
Description
This Configuration Option provides a way to negotiate the maximum
packet size used across one direction of a link. By default, all
implementations must be able to receive frames with 1500 octets of
Information.
This Configuration Option may be sent to inform the remote end
that you can receive larger frames, or to request that the remote
end send you 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 but not the header, trailer or
any transparency bits or bytes.
Default
1500
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RFC 1172 PPP Initial Options July 1990
2.2. 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 times it is unnecessary to map any
characters. A PPP implementation may use this Configuration
Option to inform the remote end which control characters must
remain mapped and which control characters need not remain mapped
when the remote end sends them. The remote end may still send
these control characters in mapped format if it is necessary
because of constraints at its (the remote) end. This option does
not solve problems for communications links that can send only 7-
bit characters or that can not send all non-control characters.
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 implemention 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
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RFC 1172 PPP Initial Options July 1990
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.
Default
All ones (0xffffffff).
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RFC 1172 PPP Initial Options July 1990
2.3. Authentication-Type
Description
On some links it may be desirable to require a peer to
authenticate itself before allowing Network Layer protocol data to
be exchanged. This Configuration Option provides a way to
negotiate the use of a specific authentication protocol. By
default, authentication is not necessary. If an implementation
requires that the remote end authenticate with some specific
authentication protocol, then it should negotiate the use of that
authentication protocol with this Configuration Option.
Successful negotiation of the Authentication-Type option adds an
additional Authentication phase to the Link Control Protocol.
This phase is after the Link Quality Determination phase, and
before the Network Layer Protocol Configuration Negotiation phase.
Advancement from the Authentication phase to the Network Layer
Protocol Configuration Negotiation phase may not occur until the
peer is successfully authenticated using the negotiated
authentication protocol.
An implementation may allow the remote end to pick from more than
one authentication protocol. To achieve this, it may include
multiple Authentication-Type Configuration Options in its
Configure-Request packets. An implementation receiving a
Configure-Request specifying multiple Authentication-Types may
accept at most one of the negotiable authentication protocols and
should send a Configure-Reject specifying all of the other
specified authentication protocols.
It is recommended that each PPP implementation support
configuration of authentication parameters at least on a per-
interface basis, if not a per peer entity basis. The parameters
should specify which authetication techniques are minimally
required as a prerequisite to establishment of a PPP connection,
either for the specified interface or for the specified peer
entity. Such configuration facilities are necessary to prevent an
attacker from negotiating a reduced security authentication
protocol, or no authentication at all, in an attempt to circumvent
this authentication facility.
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 remote end to authenticate
with the acknowledged protocol.
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RFC 1172 PPP Initial Options July 1990
There is no requirement that authentication be full duplex or that
the same authentication protocol be used in both directions. It
is perfectly acceptable for different authentication protocols to
be used in each direction. This will, of course, depend on the
specific authentication protocols negotiated.
This document defines a simple Password Authentication Protocol in
Section 4. Development of other more secure protocols is
encouraged.
A summary of the Authentication-Type 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-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
3
Length
>= 4
Authentication-Type
The Authentication-Type field is two octets and indicates the type
of authentication protocol desired. Values for the
Authentication-Type 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-Type field are
specified in "Assigned Numbers" [2]. Initial values are assigned
as follows:
Value (in hex) Protocol
c023 Password Authentication Protocol
Data
The Data field is zero or more octets and contains additional data
as determined by the particular authentication protocol.
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Default
No authentication protocol necessary.
2.4. 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 Link-Quality-Monitoring 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 should be 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 loop-back 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.
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RFC 1172 PPP Initial Options July 1990
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 may 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 may 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 Echo-Request, Echo-Reply, Discard-Request, and
Link-Quality-Report LCP packets have a Magic-Number field which
MUST normally be transmitted as zero, and MUST normally be ignored
on reception. However, once a Magic-Number has been successfully
negotiated, an LCP implementation MUST begin transmitting these
packets with the Magic-Number field set to its negotiated Magic-
Number. Additionally, the Magic-Number field of these packets may
be inspected on reception. All received Magic-Number fields should
be equal to either zero or the peer's unique Magic-Number,
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RFC 1172 PPP Initial Options July 1990
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 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 an LCP Physical-Layer-Down
event and make an immediate transition to the Closed state. A
further Active-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 not be sent.
Default
None.
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RFC 1172 PPP Initial Options July 1990
2.5. Link-Quality-Monitoring
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 and is implemented by periodically transmitting
Link-Quality-Report packets as described in Section 3. The Link-
Quality-Monitoring Configuration Option provides a way to enable
the use of Link-Quality-Report packets, and also to negotiate the
rate at which they are transmitted. By default, Link Quality
Monitoring and the use of Link-Quality-Report packets is disabled.
A summary of the Link-Quality-Monitoring 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 | Reporting-Period
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reporting-Period (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
6
Length
6
Reporting-Period
The Reporting-Period field is four octets and indicates the
maximum time in micro-seconds that the remote end should wait
between transmission of LCP Link-Quality-Report packets. A value
of zero is illegal and should always be nak'd or rejected. An LCP
implementation is always free to transmit LCP Link-Quality-Report
packets at a faster rate than that which was requested by, and
acknowledged to, the remote end.
Default
None
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RFC 1172 PPP Initial Options July 1990
2.6. 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 may be sent to inform the remote end
that you can receive compressed single octet Protocol fields.
Compressed Protocol fields may not be transmitted unless this
Configuration Option has been received.
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.
To guarantee unambiguous recognition of LCP packets, the Protocol
field must never be compressed when sending any LCP packet. In
addition, PPP implementations must continue to be robust and MUST
accept PPP frames with double-octet, as well as single-octet,
Protocol fields, and MUST NOT distinguish between them.
When a Protocol field is compressed, the Data Link Layer FCS field
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RFC 1172 PPP Initial Options July 1990
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.
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RFC 1172 PPP Initial Options July 1990
2.7. 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 may be used to
inform the remote end that you can receive compressed Address and
Control fields.
Compressed Address and Control fields are formed by simply
omitting them in all non-ambiguous cases. Ambiguous frames may
not be compressed. Ambiguous cases result when the two octets
following the Address and Control fields have values that could be
interpreted as valid Address and Control fields (i.e., 0xff,
0x03). This can happen when Protocol-Field-Compression is enabled
and the Protocol field is compressed to one octet. If the
Protocol value is 0xff, and the first octet of the Information
field is 0x03, the result is ambiguous and the Address and Control
fields must not be compressed on transmission.
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.
One additional case in which the Address and Control fields must
never be compressed is 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 1172 PPP Initial Options July 1990
Type
8
Length
2
Default
Not compressed.
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RFC 1172 PPP Initial Options July 1990
3. Link Quality Monitoring
Data communications links are rarely perfect. Packets can be dropped
or corrupted for various reasons (line noise, equipment failure,
buffer overruns, etc.). Sometimes, it is desirable to determine
when, and how often, the link is dropping data. Routers, for
example, may want to temporarily allow another route to take
precedence. An implementation may also have the option of
disconnecting and switching to an alternate link. The process of
determining data loss is called "Link Quality Monitoring".
3.1. Design Motivation
There are many different ways to measure link quality, and even more
ways to react to it. Rather than specifying a single scheme, Link
Quality Monitoring is divided into a "mechanism" and a "policy". PPP
fully specifies the "mechanism" for Link Quality Monitoring by
defining the LCP Link-Quality-Report (LQR) packet and specifying a
procedure for its use. PPP does NOT specify a Link Quality
Monitoring "policy" -- how to judge link quality or what to do when
it is inadequate. That is left as an implementation decision, and
can be different at each end of the link. Implementations are
allowed, and even encouraged, to experiment with various link quality
policies. The Link Quality Monitoring mechanism specification
insures that two implementations with different policies may
communicate and interoperate.
To allow flexible policies to be implemented, the PPP Link Quality
Monitoring mechanism measures data loss in units of packets, octets,
and Link-Quality-Reports. Each measurement is made separately for
each half of the link, both inbound and outbound. All measurements
are communicated to both ends of the link so that each end of the
link can implement its own link quality policy for both its outbound
and inbound links.
Finally, the Link Quality Monitoring protocol is designed to be
implementable on many different kinds of systems. Although it may be
common to implement PPP (and especially Link Quality Monitoring) as a
single software process, multi-process implementations with hardware
support are also envisioned. The PPP Link Quality Monitoring
mechanism provides for this by careful definition of the Link-
Quality-Report packet format, and by specifiying reference points for
all data transmission and reception measurements.
3.2. Design Overview
Each Link Quality Monitoring implementation maintains counts of the
number of packets and octets transmitted and successfully received,
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RFC 1172 PPP Initial Options July 1990
and periodically transmits this information to its peer in a Link-
Quality-Report packet. These packets contain three sections: a
Header section, a Counters section, and a Measurements section.
The Header section of the packet consists of the normal LCP Link
Maintenance packet header including Code, Identifier, Length and
Magic-Number fields.
The Counters section of the packet consists of four counters, and
provides the information necessary to measure the quality of the
link. The LQR transmitter fills in two of these counters: Out-Tx-
Packets-Ctr and Out-Tx-Octets-Ctr (described later). The LQR
receiver fills in the two remaining counters: In-Rx-Packets-Ctr and
In-Rx-Octets-Ctr (described later). These counters are similar to
sequence numbers; they are constantly increasing to give a "relative"
indication of the number of packets and octets communicated across
the outbound link. By comparing the values in successive Link-
Quality-Reports, an LQR receiver can compute the "absolute" number of
packets and octets communicated across its inbound link. Comparing
these absolute numbers then gives an indication of an inbound link's
quality. Relative numbers, rather than absolute, are transmitted
because they greatly simplify link synchronization; an implementation
merely waits to receive two LQR packets.
The Measurements section of the packet consists of six state
variables: In-Tx-LQRs, Last-In-Id, In-Tx-Packets, In-Tx-Octets, In-
Rx-Packets, and In-Rx-Octets (described later). This section allows
an implementation to report inbound link quality measurements to its
peer (for which the report will instead indicate outbound link
quality) by transmitting the absolute, rather than relative, number
of LQRs, packets, and octets communicated across the inbound link.
These values are calculated by observing the Counters section of the
Link-Quality-Report packets received on the inbound link. Absolute
numbers may be used in this section without synchronization problems
because it is necessary to receive only one LQR packet to have valid
information.
Link Quality Monitoring is described in more detail in the following
sections. First, a description of the processes comprising the Link
Quality Monitoring mechanism is presented. This is followed by the
packet and byte counters maintained; the measurements, calculations,
and state variables used; the format of the Link-Quality-Report
packet; some policy suggestions; and, finally, an example link
quality calculation.
3.3. Processes
The PPP Link Quality Monitoring mechanism is described using a
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RFC 1172 PPP Initial Options July 1990
"logical process" model. As shown below, there are five logical
processes duplicated at each end of the duplex link.
+---------+ +-------+ +----+ Outbound
| |-->| Mux |-->| Tx |=========>
| Link- | +-------+ +----+
| Manager |
| | +-------+ +----+ Inbound
| |<--| Demux |<--| Rx |<=========
+---------+ +-------+ +----+
Link-Manager
The Link-Manager process transmits and receives Link-Quality-
Reports, and implements the desired link quality policy. LQR
packets are transmitted at a constant rate, which is negotiated by
the LCP Link-Quality-Monitoring Configuration Option. The Link-
Manager process fills in only the Header and Measurements sections
of the packet; the Counters section of the packet is filled in by
the Tx and Rx processes.
Mux
The Mux process multiplexes packets from the various protocols
(e.g., LCP, IP, XNS, etc.) into a single, sequential, and
prioritized stream of packets. Link-Quality-Report packets MUST
be given the highest possible priority to insure that link quality
information is communicated in a timely manner.
Tx
The Tx process maintains the counters Out-Tx-Packets-Ctr and Out-
Tx-Octets-Ctr which are used to measure the amount of data which
is transmitted on the outbound link. When Tx processes a Link-
Quality-Report packet, it inserts the values of these counters
into the Counters section of the packet. Because these counters
represent relative, rather than absolute, values, the question of
when to update the counters, before or after they are inserted
into a Link-Quality-Report packet, is left as an implementation
decision. However, an implementation MUST make this decision the
same way every time. The Tx process MUST follow the Mux process
so that packets are counted in the order transmitted to the link.
Rx
The Rx process maintains the counters In-Rx-Packets-Ctr and In-
Rx-Octets-Ctr which are used to measure the amount of data which
is received by the inbound link. When Rx processes a Link-
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RFC 1172 PPP Initial Options July 1990
Quality-Report packet, it inserts the values of these counters
into the Counters section of the packet. Again, the question of
when to update the counters, before or after they are inserted
into a Link-Quality-Report packet, is left as an implementation
decision which MUST be made consistently the same way.
Demux
The Demux process demultiplexes packets for the various protocols.
The Demux process MUST follow the Rx process so that packets are
counted in the order received from the link.
3.4. Counters
In order to fill in the Counters section of a Link-Quality-Report
packet, Link Quality Monitoring requires the implementation of one
8-bit unsigned, and four 32-bit unsigned, monotonically increasing
counters. These counters may be reset to any initial value before
the first Link-Quality-Report is transmitted, but MUST NOT be reset
again until LCP has left the Open state. Counters wrap to zero when
their maximum value is reached (for 32 bit counters: 0xffffffff + 1 =
0).
Out-Identifier-Ctr
Out-Identifier-Ctr is an 8-bit counter maintained by the Link-
Manager process which increases by one for each transmitted Link-
Quality-Report packet.
Out-Tx-Packets-Ctr
Out-Tx-Packets-Ctr is a 32-bit counter maintained by the Tx
process which increases by one for each transmitted Data Link
Layer packet.
Out-Tx-Octets-Ctr
Out-Tx-Octets-Ctr is a 32-bit counter maintained by the Tx process
which increases by one for each octet in a transmitted Data Link
Layer packet. All octets which are included in the FCS
calculation MUST be counted, as should the FCS octets themselves.
All other octets MUST NOT be counted.
In-Rx-Packets-Ctr
In-Rx-Packets-Ctr is a 32-bit counter maintained by the Rx process
which increases by one for each successfully received Data Link
Layer packet. Packets with incorrect FCS fields or other problems
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RFC 1172 PPP Initial Options July 1990
MUST not be counted.
In-Rx-Octets-Ctr
In-Rx-Octets-Ctr is a 32-bit counter maintained by the Rx process
which increases by one for each octet in a successfully received
Data Link Layer packet. All octets which are included in an FCS
calculation MUST be counted, as should the FCS octets themselves.
All other octets MUST NOT be counted.
3.5. Measurements, Calculations, State Variables
In order to fill in the Measurements section of a Link-Quality-Report
packet, Link Quality Monitoring requires the Link-Manager process to
make a number of calculations and keep a number of state variables.
These calculations are made, and these state variables updated, each
time a Link-Quality-Report packet is received from the inbound link.
In-Tx-LQRs
In-Tx-LQRs is an 8-bit state variable which indicates the number
of Link-Quality-Report packets which the peer had to transmit in
order for the local end to receive exactly one LQR. In-Tx-LQRs
defines the length of the "period" over which In-Tx-Packets, In-
Tx-Octets, In-Rx-Packets, and In-Rx-Octets were measured. In-Tx-
LQRs is calculated by subtracting Last-In-Id from the received
Identifier. If more than 255 LQRs in a row are lost, In-Tx-LQRs
will be ambiguous since the Identifier field and all state
variables based on it are only 8 bits. It is assumed that the
Link Quality Monitoring policy will be robust enough to handle
this case (it should probably close down the link long before this
happens).
Last-In-Id
Last-In-Id is an 8-bit state variable which stores the value of
the last received Identifier. Last-In-Id should be updated after
In-Tx-LQRs has been calculated.
In-Tx-Packets
In-Tx-Packets is a 32-bit state variable which indicates the
number of packets which were transmitted on the inbound link
during the last period. In-Tx-Packets is calculated by
subtracting Last-Out-Tx-Packets-Ctr from the received Out-Tx-
Packets-Ctr.
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RFC 1172 PPP Initial Options July 1990
Last-Out-Tx-Packets-Ctr
Last-Out-Tx-Packets-Ctr is a 32-bit state variable which stores
the value of the last received Out-Tx-Packets-Ctr. Last-Out-Tx-
Packets-Ctr should be updated after In-Tx-Packets has been
calculated.
In-Tx-Octets
In-Tx-Octets is a 32-bit state variable which indicates the number
of octets which were transmitted on the inbound link during the
last period. In-Tx-Octets is calculated by subtracting Last-Out-
Tx-Octets-Ctr from the received Out-Tx-Octets-Ctr.
Last-Out-Tx-Octets-Ctr
Last-Out-Tx-Octets-Ctr is a 32-bit state variable which stores the
value of the last received Out-Tx-Octets-Ctr. Last-Out-Tx-
Octets-Ctr should be updated after In-Tx-Octets has been
calculated.
In-Rx-Packets
In-Rx-Packets is a 32-bit state variable which indicates the
number of packets which were received on the inbound link during
the last period. In-Rx-Packets is calculated by subtracting
Last-In-Rx-Packets-Ctr from the received In-Rx-Packets-Ctr.
Last-In-Rx-Packets-Ctr
Last-In-Rx-Packets-Ctr is a 32-bit state variable which stores the
value of the last received In-Rx-Packets-Ctr. Last-In-Rx-
Packets-Ctr should be updated after In-Rx-Packets has been
calculated.
In-Rx-Octets
In-Rx-Octets is a 32-bit state variable which indicates the number
of octets which were received on the inbound link during the last
period. In-Rx-Octets is calculated by subtracting Last-In-Rx-
Octets-Ctr from the received In-Rx-Octets-Ctr.
Last-In-Rx-Octets-Ctr
Last-In-Rx-Octets-Ctr is a 32-bit state variable which stores the
value of the last received In-Rx-Octets-Ctr. Last-In-Rx-Octets-
Ctr should be updated after In-Rx-Octets has been calculated.
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RFC 1172 PPP Initial Options July 1990
Measurements-Valid
Measurements-Valid is a 1-bit boolean state variable which
indicates whether or not the In-Tx-Packets, In-Tx-Octets, In-Rx-
Packets, and In-Rx-Octets state variables contain valid
measurements. These measurements cannot be considered valid until
two or more Link-Quality-Report packets have been received on the
inbound link. This bit should be reset when LCP reaches the Open
state and should be set after the receipt of exactly two LQRs.
3.6. Link-Quality-Report Packet Format
A Summary of the Link-Quality-Report packet format is shown below.
The fields are transmitted from left to right. The Code, Identifier,
Length, and Magic-Number fields make up the normal LCP Link
Maintenance packet header; the In-Tx-LQRS, Last-In-Id, V, In-Tx-
Packets, In-Tx-Octets, In-Rx-Packets, In-Rx-Octets fields contain
digested absolute measurements; and the Out-Tx-Packets-Ctr, Out-Tx-
Octets-Ctr, In-Rx-Packets-Ctr, and In-Rx-Octets-Ctr fields contain
raw relative counts. Note that as transmitted over the link, this
packet format does not include the In-Rx-Packets-Ctr and In-Rx-
Octets-Ctr fields which are logically appended to the packet by the
Rx process after reception on the inbound link.
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RFC 1172 PPP Initial Options July 1990
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Tx-LQRs | Last-In-Id | Reserved |V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Tx-Packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Tx-Octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Rx-Packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Rx-Octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Out-Tx-Packets-Ctr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Out-Tx-Octets-Ctr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/
/
/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Rx-Packets-Ctr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| In-Rx-Octets-Ctr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
12 for Link-Quality-Report.
Identifier
The Identifier field is one octet and indicates the sequence
number for this Link-Quality-Report. The Identifier field is
copied from the Out-Identifier-Ctr counter on transmission. On
reception, the Identifier field is used to calculate In-Tx-LQRs
and is then stored in Last-In-Id.
The Link-Quality-Report Identifier sequence number space MUST be
separate from that of all other LCP packets; for example,
transmission of an LCP Echo-Request must not cause the Out-
Identifier-Ctr count
