As per Relevance of the word implementation, we have this rfc below:
Network Working Group D.
Request for Comments: 1663
Category: Standards Track July 1994
PPP Reliable
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
The Point-to-Point Protocol (PPP) [1] provides a standard method
transporting multi-protocol datagrams over point-to-point links
This document defines a method for negotiating and using Numbered
Mode, as defined by ISO 7776 [2], to provide a reliable serial link
This document is the product of the Point-to-Point Protocol
Group of the Internet Engineering Task Force (IETF). Comments
be submitted to the ietf-ppp@ucdavis.edu mailing list
Table of
1. Introduction .......................................... 1
2. Physical Layer Requirements ........................... 2
3. The Data Link Layer ................................... 2
3.1 Frame Format ....................................... 2
4. Configuration Option Format ........................... 4
5. Numbered-Mode Operation ............................... 5
5.1 Single Link ........................................ 6
5.2 Inverse Multiplexing ............................... 6
5.3 Using Multi-Link Procedure... ...................... 7
5.4 LAPB Parameter defaults ............................ 8
SECURITY CONSIDERATIONS ...................................... 9
REFERENCES ................................................... 9
ACKNOWLEDGEMENTS ............................................. 9
CHAIR'S ADDRESS .............................................. 10
AUTHOR'S ADDRESS ............................................. 10
Rand [Page 1]
RFC 1663 PPP Reliable Transmission July 1994
1.
By default, PPP packets over HDLC framed links consist
"connectionless" datagrams. If reliable transmission over the
link is desired, the implementation MUST specify the Numbered-
Configuration Option during Link Establishment phase
Generally, serial link reliability is not a major issue.
architecture of protocols used in datagram networking
best-effort non-sequential delivery. When errors are detected
are discarded
However, in certain circumstances, it is advisable to provide
reliable link, at least for a subset of the messages. The
obvious case is when the link is compressed. Since the dictionary
recovered from the compressed data stream, and a lost
corrupts the dictionary, datagrams must not be lost. Not
compression types will require a reliable data stream, since the
to detect and reset a corrupt dictionary is small
The ISO 7776 LAPB can be used guarantee delivery. This is
to in this document as "Numbered Mode" to distinguish it from the
of "Unnumbered Information", which is standard PPP framing practice
Where multiple parallel links are used to emulate a single link
higher speed, Bridged traffic, Source Routed traffic, and
subjected to Van Jacobsen TCP/IP header compression must be
to the higher layer in a certain sequence. However, the fact of
links being relatively asynchronous makes traffic ordering uncertain
The ISO 7776 Multi-Link Procedure MAY be used to restore order
Implementation of the ISO Multi-Link Procedure is deprecated. It
recommended that the PPP multilink procedure [4] be used instead
2. Physical Layer
PPP Reliable Transmission imposes the same requirements that
described in "PPP in HDLC Framing" [3], with the
exceptions
Control
While PPP does not normally require the use of control signals
implementation of Numbered-Mode LAPB or LAPD requires
provision of control signals, which indicate when the link
become connected or disconnected. These in turn provide the
and Down events to the LCP state machine
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RFC 1663 PPP Reliable Transmission July 1994
3. The Data Link
Numbered-Mode affects only the Address and Control fields.
remainder of the frame conforms to the framing in use for PPP
The Address Field of the frame MUST take the value announced in
Numbered-Mode Configuration Option, and the Control Field MAY
any value valid in ISO 7776.
Once the link enters Numbered-Mode, Numbered-Mode MUST be used on
frames, as some implementations do not support the use of
Unnumbered-Information control field or the use of the All-
address intermixed with Numbered-Mode frames
3.1. Frame
The following frame format is valid under Numbered-Mode. The
are transmitted from left to right
Numbered
+----------+----------+----------+
| Flag | Address | Control |
| 01111110 |1-2 octets|1-2 octets
+----------+----------+----------+
+----------+-------------+---------+
| Protocol | Information | Padding |
|1-2 octets| * | * |
+----------+-------------+---------+
+----------+----------+-----------------
| FCS | Flag | Inter-frame
| 16 bits | 01111110 | or next
+----------+----------+-----------------
The Protocol, Information and Padding fields are described in
Point-to-Point Protocol Encapsulation [1]. The FCS and Flag
fields are described in "PPP in HDLC Framing" [3].
4. Configuration Option
The LCP Numbered-Mode Configuration Option negotiates the use
Numbered-Mode on the link. By default or ultimate disagreement
Unnumbered-Mode is used
A summary of the Numbered-Mode Configuration Option format is
below. The fields are transmitted from left to right
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RFC 1663 PPP Reliable Transmission July 1994
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 | Window | Address...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
11
>= 4
A value between 1 and 127. This indicates the number of
the receiver will buffer, which is the maximum number that
sender should send without receiving an acknowledgement.
window < 8, then modulo 8 sequencing is used on the link
Otherwise, modulo 128 sequencing is used
It is conceivable and legal that differing window values might
announced. However, it is not permitted for one system to
modulo 8 sequencing and the other to use modulo 128. Therefore
the rule is: a Configure-Nak may reduce the window but may
increase it
An HDLC Address as specified in ISO 3309. ISO 7776 specifies
of the possible values: 1 and 3 for single link operation, 7
15 for the Multi-Link Procedure. Other values consistent with
3309 are considered legal
Implementation of the Multi-Link Procedure is optional;
Configure-Nak may therefore force a change from MLP to single
mode, but not the reverse
Should the address be zero upon receipt, the receiver
Configure-Nak with an appropriate address. If both peers
address zero, the system advertising the numerically
window will select the smaller address. If both windows are
same size, a random choice MUST be made; when good sources
randomness are used, the link will converge in a reasonable time
Rand [Page 4]
RFC 1663 PPP Reliable Transmission July 1994
If magic numbers have been negotiated on the link, the system
the numerically smaller magic number SHOULD specify the
address
5. Numbered-Mode
When using the Numbered-Mode, each link is established in the
manner for the type of link. The Numbered-Mode Configuration
is negotiated, the Magic-Number Configuration Option MUST also
negotiated, and the Address-and-Control-Field-
Configuration Option MUST NOT be negotiated
Following the successful negotiation of the Numbered-
Configuration Option during LCP Link Establishment phase, the
with the numerically smaller Magic-Number will send a SABM
SABM(E), and the other will respond with a UA. In the event
either the SABM or UA is lost, this exchange may be
according to the same parameters as the configuration
itself, using the Restart Timer and counter values. Authentication
Link Quality Determination, and NCP Configuration follow this step
Once the link has been established with Numbered-Mode, when re
negotiation of link configuration occurs, the entire re-
MUST be conducted in Numbered-Mode. If the Numbered-
Configuration Option is not successfully re-negotiated, the
reverts to Unnumbered-Information operation prior to Authentication
Link Quality Determination, and NCP Configuration
When an implementation which is capable of Numbered-Mode, and is
currently configured for Numbered-Mode operation, detects a
which has a correct FCS but does not have a UI Control octet,
implementation MUST send a DM message, immediately followed by a
Configure-Request
When an implementation which is currently configured for Numbered
Mode operation receives a DM message, it MUST revert to Unnumbered
Information operation, and immediately send a LCP Configure-Request
5.1. Single
When Network-Layer packets are sent over a single link, the
are encapsulated in the following order
+----------+ +----------+ +----------+
| | | | | Numbered |
| Header |-->| Data |-->| Mode |-->
| Compress | | Compress | | Header |
+----------+ +----------+ +----------+
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RFC 1663 PPP Reliable Transmission July 1994
5.2. Inverse
Since sending several connections over a single link is often
"multiplexing", sending packets from a single connection
multiple parallel links is sometimes called "inverse-multiplexing".
By default, PPP performs no special processing for such links.
link is established and terminated independently, negotiates its
configuration options, and may have different combinations of
options as ACCM, Protocol Field Compression and IP-Address.
facilitates using the links simultaneously over dissimilar media
such as 56K sync with async backup
Every link in a single machine MUST have different Magic Numbers,
each end of every link between two peers SHOULD have Magic
which are unique to those peers. This protects against patch-
errors in addition to looped-back links
The distribution to each link is controlled by higher level
mechanisms. When Network-Layer specific compression techniques (
as Van Jacobsen Compression) rely on sequential delivery,
Multi-Link Procedure support such compression MUST be applied on
link by link basis
+----------+ +----------+ +----------+
| | | | | Numbered |
+--->| Header |-->| Data |-->| Mode |--> link 1
| | Compress | | Compress | | Header |
+--------------+ +----------+ +----------+ +----------+
| Distribution |
+--------------+ +----------+ +----------+ +----------+
| | | | | | Numbered |
+--->| Header |-->| Data |-->| Mode |--> link 2
| Compress | | Compress | | Header |
+----------+ +----------+ +----------+
5.3. Using Multi-Link
This document does not offer a standard for ISO Multi-Link, but
offer a method for agreeing on the addressing scheme usable
Multi-Link. A sample implementation is shown below.
of Multi-Link is not required
When using the ISO 7776 Multi-Link Procedure, each link
established as described above. In addition, the Numbered-
Configuration Option is negotiated with appropriate addresses for
Multi-Link Procedure. The distribution to each link is controlled
the Multi-Link Procedure, as is the recovery of sequence in
receiving system
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RFC 1663 PPP Reliable Transmission July 1994
+---> link 1
+----------+ +----------+ +----------+ |
| | | | | Multi | +--------------+
| Header |-->| Data |-->| Link |-->| Distribution |
| Compress | | Compress | | Procedure| +--------------+
+----------+ +----------+ +----------+ |
+---> link 2
5.4. LAPB Parameter
The following guidelines specify the default values of
configurable parameters
Timer T
Timer T1 is the maximum time permitted before a
is started, as a result of no response to a transmitted
frame. This value must be greater than the time required for
maximum sized frame to be received by the other side of
link, and for a response to be generated for the frame.
SHOULD be determined dynamically, based on the measured
trip time delay of the link at the LAPB level. In the
that the system cannot determine the round trip time of
link, this value SHOULD be set to twice the bit rate of
link, divided by the maximum number of bits per frame, plus 100
milliseconds processing time. For example, on a 14,400
link, with a maximum frame size of 8000 bits (1000 octects),
the T1 value would be set to 3.7 seconds
Timer T
Timer T3 gives an indication of the idle state of the link
Its value must be greater than the T1 value
Maximum number of attempts to complete a transmission, N
Parameter N2 gives the maximum number of
attempts for a given frame. If this value is exceeded,
link SHOULD be terminated. The default value for parameter N
SHOULD be 3.
Security
Security issues are not discussed in this memo
Rand [Page 7]
RFC 1663 PPP Reliable Transmission July 1994
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, Daydreamer, July 1994.
[2] ISO 7776, Information Processing Systems - Data Communication -
High Level Data Link Control Procedures - Description of the X.25
LAPB-Compatible DTE Data Link
[3] Simpson, W., Editor, "PPP in HDLC Framing", STD 51, RFC 1662,
Daydreamer, July 1994.
[4] Sklower, K., "PPP MultiLink Procedure", Work in Progress
Fred Baker was the original author of this document
Bill Simpson contributed materially to the document
Chair's
The working group can be contacted via the current chair
Fred
Advanced Computer
315 Bollay
Santa Barbara, California 93117
EMail: fbaker@acc.
Author's
Questions about this memo can also be directed to
Dave
2180 Fortune
San Jose, CA 95131
Phone: +1 408 321-1259
EMail: dave_rand@novell.
Rand [Page 8]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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