RFC1963 - PPP Serial Data Transport Protocol (SDTP)

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　　Network Working Group K. Schneider
Request for Comments: 1963 S. Venters
Category: Informational ADTRAN, Inc.
August 1996

PPP Serial Data Transport Protocol (SDTP)

Status of This Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different
network-layer protocols.

This document describes a new Network level protocol (from the PPP
point of view), PPP Serial Data Transport Protocol, that provides
encapsulation and an associated control protocol for transporting
serial data streams over a PPP link. This protocol was developed for
the purpose of using PPP's many features to provide a standard method
for synchronous data compression. The encapsulation uses a header
structure based on that of the ITU-T Recommendation V.120 [2].

Table of Contents

1. Introduction .......................................... 2
2. SDTP Packets .......................................... 3
2.1 Padding ......................................... 4
2.2 Packet Formats .................................. 4
3. Serial Data Control Protocol .......................... 11
4. SDCP Configuration Option Format ...................... 12
4.1 Packet-Format ................................... 13
4.2 Header-Type ..................................... 13
4.3 Length-Field-Present ............................ 14
4.4 Multi-Port ...................................... 14
4.5 Transport-Mode .................................. 15
4.6 Maximum-Frame-Size .............................. 16
4.7 Allow-Odd-Frames ................................ 16
4.8 FCS-Type ........................................ 17
4.9 Flow-Expiration-Time ............................ 18
SECURITY CONSIDERATIONS ...................................... 19

REFERENCES ................................................... 19
CHAIR'S ADDRESS .............................................. 20
AUTHORS' ADDRESSES ........................................... 20

1. Introduction

This document is a product of the TR30.1 ad hoc committee on
compression of synchronous data. It represents a component of a
proposal to use PPP to provide compression of synchronous data in
DSU/CSUs.

In addition to providing support for multi-protocol datagrams, the
Point-to-Point Protocol (PPP) [1] has defined an effective and robust
negotiating mechanism that can be used on point to point links. When
used in conjunction with the PPP Compression Control Protocol [3] and
one of the PPP Compression Protocols [4-10], PPP provides an
interoperable method of employing data compression on a point-to-
point link.

This document provides a PPP encapsulation for serial data,
specifying a transport protocol, PPP Serial Data Transport Protocol
(PPP-SDTP), and an associated control protocol, PPP Serial Data
Control Protocol (PPP-SDCP). When these protocols are added to above
mentioned PPP protocols, PPP can be used to provide compression of
serial data on a point-to-point link.

This first edition of PPP-SDTP/SDCP covers HDLC-like synchronous
serial data and asynchronous serial data. It does this by using a
terminal adaption header based on that of ITU-T Recommendation V.120
[2]. Support may be added in the future for other synchronous
protocols as the marketplace demands.

The V.120 terminal adaption header allows transported data frames to
be split over several packets, supports the transport of DTE port
idle and error information, and optionally supports the transport of
DTE control state information.

In addition to the V.120 Header, fields can be added to the packet
format through negotiation to provide support for features not
included in the V.120 header. The extra fields are: a Length Field,
which is used to distinguish packets in compound frames, and a Port
field, which is used to provide multi-port multiplexing capability.
The protocol also allows reserved bits in the V.120 header to be used
to transport non-octet aligned frames and to provide a flow control
mechanism.

To provide these features, PPP-SDTP permits a single frame format to
be selected from several possible formats by using PPP-SDCP
negotiation. The terminal adaption header can be either fixed length
or variable length, to allow either simplicity or flexibility.

The default frame format places the terminal adaption header at the
end of the packet. This permits optimal transmitter timelines when
user frames are segmented and compression is also used in conjunction
with this protocol.

2. SDTP Packets

Before any SDTP packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the SDTP Control Protocol must
reach the Opened state.

By default, exactly one SDTP packet is encapsulated in the PPP
Information field, where the PPP Protocol field indicates type hex
0049 (PPP-SDTP). If the Length-Field-Present Configuration Option
and the LCP Compound-Frames Configuration Option are successfully
negotiated, multiple SDTP packets may be placed in the PPP
Information field, and they are distinguished by the presence of
Length fields in each packet.

The maximum length of the SDTP datagram transmitted over a PPP link
is limited only by the negotiated Maximum-Frame-Size and the maximum
length of the Information field of a PPP encapsulated packet. Note
that if compression is used on the PPP link, this the maximum length
of the SDTP datagram may be larger or smaller than the maximum
length of the Information field of a PPP encapsulated packet,
depending on the particular compression algorithm and protocol used.

ITU-T Recommendation V.120 [2] defines an adaption header that is
used with its asynchronous and synchronous modes of operation. SDTP
packets include this header as a Header field to provide the protocol
adaption function. Using negotiation, additional fields can be added
to the packet to provide sequencing and multiplexing capability
within SDTP. SDTP also has an option of using the reserved bits of
the header to provide a flow control mechanism and support for
transporting non-octet aligned data frames.

The default SDTP packet format is designed to allow the efficient use
of the protocol's segmentation feature when combined with a PPP
Compression Protocol [4-10]. This format is a little different from
other PPP NCP's in that data is read from both ends of the packet.
The Header field is placed at the end of the SDTP packet, with the
order of the octets reversed. This somewhat unique format has been
selected to allow optimal transmitter timelines when compression is

used and transported data frames are split into multiple SDTP
packets. In such a situation, the Header field contains the
information about whether the data is split into multiple packets or
not, so if it is located at the end of a packet, the decision can be
made after observing the compressed size of the packet. The Header
field can then simply be run through the compressor after the
decision has been made.

When the Header field is placed before the data, as in the optional
packet format, the transmitter must make the decision about whether
to split a frame over multiple packets without knowing about the
compressibility of the frame. Therefore the optional format is
designed to be used when transported frames are not split into
multiple SDTP packets or where SDTP is not coupled with compression.
It is believed that this format may be useful for some hardware
implementations.

2.1. Padding

If padding is used, SDTP packets require the use of the Length Field
or the previous negotiation of the LCP Self-Describing-Padding
Configuration Option [11].

2.2. Packet Formats

The default SDTP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP Protocol ID | Transported Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header - H |
+-+-+-+-+-+-+-+-+

The two complete frame formats are shown below: Header-Last and
Header-First. Header-Last is the default packet format. The
additional fields provided support for: Control State Information
(CS), multiple packets and multi-port multiplexing. Again, the
fields are transmitted from left to right. Descriptions of the
fields follow the packet formats.

Header-Last

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP Protocol ID | (Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (Port) | Transported Data / (Odd-Pad) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header - (CS) : H |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Header-First

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP Protocol ID | (Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (Port) | Header - H : (CS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transported Data / (Odd-Pad) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PPP Protocol ID

The PPP Protocol ID field is described in the Point-to-Point
Protocol Encapsulation [1].

When the SDTP Protocol is successfully negotiated by the SDTP
Control Protocol (SDCP), the value is 0049 hex. This value may be
compressed to one octet when Protocol-Field-Compression is
negotiated, or if one of the PPP compression protocols [4-10] is
used.

Length

The optional Length field is present in every SDTP packet upon
successful negotiation of the Length-Field-Present Configuration
Option.

The value of the Length field is the combined lengths of the
Length, Port (if present), Header, Transmitted Data, and Odd-Pad
(if present) fields in octets.

The length of the Length field defaults to one octet. Valid
lengths are from 2 to 255 octets, since each packet must include

at least a one octet Header field.

If desired, the length field can be negotiated to be two octets in
length. In that case, valid lengths are from 2 to 65535 octets,
and the field is transmitted most significant octet first.

In either case, a length of 0 means that the combined length is
the same as the length of the remainder of the PPP Information
Field.

Port

The optional Port field is present in every SDTP packet upon
successful negotiation of the Multi-Port Option.

The length of the Port field is one octet. Valid Port numbers are
0 to 254. Port number 255 is reserved for control purposes (see
section on flow control).

Header

The Header field is the terminal adaption header from ITU-T
Recommendation V.120. As specified in that document, it contains
up to two octets: The terminal adaption header octet (H), and the
optional header extension for control state information (CS).
SDTP only supports the protocol sensitive operation of V.120; bit
transparent operation is not supported. The descriptions of the
header bits provided below are derived from the descriptions
provided in Recommendation V.120. In addition to the bit
definitions of V.120, SDTP optionally permits the use of reserved
bits to be used for flow control and to provide support for non-
octet aligned frames.

The length of the Header field is either one or two octets, and is
determined by the value of the E bit in the first octet. By
default, the E-bit must be set in the H octet and the CS octet is
not present. A Configuration Option may be negotiated to allow
the use of the CS octet, or even to require its presence in every
packet.

H (V.120 Terminal Adaption Header)

The format of the first octet of the Header field is shown
below:

0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| E | BR | Res | FC | C2 | C1 | B | F |
+-----+-----+-----+-----+-----+-----+-----+-----+

E - Extension Bit

The E bit is the extension bit. If set to 0, it indicates
that the Control-2 field is present.

BR - Break / HDLC Idle Bit

In asynchronous mode, the BR bit indicates the invocation of
the BREAK function by the DTE. A value of 1 indicates
BREAK.

In synchronous HDLC mode, the BR bit is used to indicate
that DTE port is receiving HDLC idle condition. A value of
1 indicates this idle condition.

Res - Reserved

This bit is reserved and MUST be set to 0. (This is a
reserved bit in V.120.)

FC - Flow Control

This bit can be used for flow control of SDTP traffic on the
network, for applications which require it. When SDTP is
used in conjunction with data compression, flow control may
be needed. Reasons for this could be that the DTE port uses
an X.21 interface (and therefore does not have independent
control of DTE transmit and receive clocks), or simply that
the underlying link layer (such as PPP in HDLC-like Framing)
does not include a mechanism for network flow control, so
some flow control mechanism is needed.

This bit set to a value of 0 indicates that the receiver is
ready to receive data (Flow-On). A value of 1 indicates that
the receiver does not wish to receive data and the
transmitting peer should stop sending it (Flow-Off). Flow

control operates on a per port basis. Flow control messages
on Port 255 affect all ports.

To ensure that a missed Flow-On message cannot cause a
hangup condition, a Flow-Off is defined to expire after a
time of T1 seconds. If a unit desires to keep its peer in
the Flow-Off state for more than T1 seconds, it MUST
transmit another Flow-Off message after every period of T1
seconds. A unit that receives a Flow-Off message may resume
transmitting T1 seconds after the last Flow-Off was
received. The value of T1 is controlled by the Flow-
Expiration-Time Configuration Option. The default value is
10 seconds. There is not a separate value for T1 for each
port; all ports use the same T1 value.

(This bit is a reserved bit in V.120, which requires the bit
to be set to a value of zero. The above definition of flow
control provides compatibility with this definition when
flow control is not used.)

C1, C2 - Error Control Bits

The C1 and C2 bits are used for DTE port Error detection and
transmission. Their meaning is defined in the following
table:

+----+----+--------------+--------------+
| | Meaning |
+----+----+--------------+--------------+
| C1 | C2 | Synchronous | Asynchronous |
+----+----+--------------+--------------+
| 0 | 0 | No Error | No Error |
| | | Detected | Detected |
+----+----+--------------+--------------+
| 0 | 1 | FCS Error | Stop-bit |
| | | (DTE) | Error |
+----+----+--------------+--------------+
| 1 | 0 | Abort | Parity Error |
| | | | on the Last |
| | | | Character in |
| | | | Frame |
+----+----+--------------+--------------+
| 1 | 1 | DTE Overrun* | Stop-bit and |
| | | | Parity Error |
+----+----+--------------+--------------+

Appropriate responses to these bits are provided in Sections
2.2.1 and 2.2.2 of the V.120 standard (where R reference
point is translated to mean DTE port.)

B, F - Segmentation Bits

The B and F bits are used for segmenting and reassembly of
the transported frames in synchronous HDLC mode. Setting
the B bit to 1 indicates that the packet contains the
beginning of a transported frame or a Begin Frame. Setting
the F bit indicates that the packet contains the final
portion of a transported frame, or a Final Frame. A packet
that contains neither the beginning of a frame nor the end
is said to contain a Middle Frame. For asynchronous mode
and bit transparent mode operation both bits MUST be set to
1. The following table summarizes the use of these bits:

+---+---+--------------+----------------+
| | Application |
+---+---+--------------+----------------+
| B | F | Synchronous | Asynchronous |
+---+---+--------------+----------------+
| 1 | 0 | Begin Frame | Not Applicable |
+---+---+--------------+----------------+
| 0 | 0 | Middle Frame | Not Applicable |
+---+---+--------------+----------------+
| 1 | 0 | Final Frame | Not Applicable |
+---+---+--------------+----------------+
| 1 | 1 | Single Frame | Required |
+---+---+--------------+----------------+

CS (V.120 optional Header Extension for Control State Information)

The format of the second Header octet (CS) is shown below:
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| E | DR | SR | RR | Res |(Odd-Pad Length) |
+-----+-----+-----+-----+-----+-----+-----+-----+

E - Extension Bit

The E bit is the extension bit, and allows further extension
of the Header field. It is set to 1, to indicate no further
extension of the Header field.

DR - Data Ready

This bit set to 1 indicates that the DTE port is activated.

SR - Send Ready

This bit set to 1 indicates that the DTE is ready to send
data.

RR - Receive Ready

This bit set to 1 indicates that the DTE is ready to receive
data. It can be used for DTE flow control in half-duplex
transmissions.

Res - Reserved

This bit is reserved and set to 0. (This is a V.120 reserved
bit.)

Odd-Pad Length (Optional)

The Odd-Pad Length field is used when non-octet aligned HDLC
frames are allowed. It is a 3-bit field, that can take on
the values of 0 through 7. Its value is the length of the
Odd-Pad field in bits. This value is determined as the
number of bits necessary to have the combined length of the
Transported Data Field and the Odd-Pad Field be aligned with
an octet boundary.

If non-octet aligned frames are not allowed, this field is
not used and all bits are set to the value of 0. (These
bits are reserved in V.120.)

Transported Data

The transported data field contains the transported serial data.

When the serial data type has been negotiated to be HDLC-like
synchronous, this field will contain all or part of a transported
HDLC-like frame.

A sample transported HDLC frame is shown below. The figure does
not show bits inserted for transparency.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag:01111110 | (Address, Control and Information Fields) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (FCS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - - - - - - - -+
| Flag:01111110 |
+-+-+-+-+-+-+-+-+

Only the data between the flags is transported. The flags are not
transported. The FCS is tranported unless the FCS-Mode
Configuration Option has been successfully negotiated otherwise.

Odd-Pad

The optional Odd-Pad (Odd Frame Pad) field is used when the
transported data frame is non-octet aligned, and the Allow-Odd-
Frames Option has been successfully negotiated. It contains the
bits that are required to pad the Transported Data field out to an
octet boundary. The Odd-Pad field is in the high order bits of
the last octet of the Transported Data field. The values of these
bits are all zero.

3. Serial Data Control Protocol

The Serial Data Control Protocol (SDCP) is responsible for
configuring, enabling and disabling the SDTP modules on both ends of
the point-to-point link. SDCP uses the same packet exchange
mechanism and state machine as the Link Control Protocol. SDCP
packets may not be exchanged until PPP has reached the Network-Layer
Protocol phase. SDCP packets received before this phase is reached
SHOULD be silently discarded.

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

Frame Modifications

The packet may utilize any modifications to the basic frame format
which have been negotiated during the Link Establishment phase.

Data Link Layer Protocol Field

Exactly one SDCP packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates type hex 8049 (PPP-
SDCP).

Code Field

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

Timeouts

SDCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation SHOULD be
prepared to wait for Authentication and Link Quality Determination
to finish before timing out waiting for a Configure-Ack or other
response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time.

Configuration Option Types

SDCP has a distinct set of Configuration Options which are defined
in this document.

4. SDCP Configuration Option Format

SDCP Configuration Options allow modifications to the default SDCP
characteristics to be negotiated. If a Configuration Option is not
included in a Configure-Request packet, the default value for that
Configuration Option is assumed.

SDCP uses the same Configuration Option format defined in LCP [1],
with a separate set of Options.

The Option Types are:

1 Packet-Format
2 Header-Type
3 Length-Field-Present
4 Multi-Port
5 Transport-Mode
6 Maximum-Frame-Size
7 Allow-Odd-Frames
8 FCS-Type
9 Flow-Expiration-Time

Note that Option Types 5-8 are specific to a single port and require
port numbers in their format. Option Types 6-8 are specific to the
HDLC-Synchronous Transport-Mode.

4.1. Packet-Format

This option selects whether the Header field precedes or follows the
data field. When the Header field follows the data field, the order
of its octets are reversed.

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

Type

1

Length

3

Format

0 Header-Last (default)
1 Header-First

4.2. Header-Type

This option selects the type of the Header field. The Header-Type of
H-and-CS means that the CS octet will be present if indicated by the
E-bit in the H-octet. The Header-Type of H-and-CS-Always signifies
that both the H and CS octets are present in every packet.

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

Type

2

Length

3

Header-Type

0 H-Only (default)
1 H-and-CS
2 H-and-CS-Always

4.3. Length-Field-Present

By default, a PPP Information Field contains only a single SDTP
packet, and an SDTP Packet does not contain a length field.
Successful negotiation of this option causes all SDTP packets to
contain the length field, and allows SDTP packets to be contained in
compound frames (see LCP Compound-Frames Configuration Option [11]).

This option is required if the LCP Length-Field-Present Configuration
option has been negotiated.

The size of the Length field is negotiated via the Length-Size
parameter.

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

Type

3

Length

3

Length-Size

0 No Length Field (default)
1 Length field of 1 octet
2 Length field of 2 octets

4.4. Multi-Port

By default, packets do not contain a port number and all packets are
sent to the default port, Port 0. The Successful negotiation of the
Multi-Port configuration option means that every packet will contain
a port number. The maximum port number, and hence the number of
ports, is negotiated by using the Max-Port-Num field. A value of 0
specifies that a single port is to be used and no port field will be

present in an SDTP packet. (This is the same as not negotiating or
rejecting this option.) Port numbers begin with 0 and range to 254.
Port number 255 is reserved for control purposes (see section on flow
control).

Protocol Specific negotiations which are on a per port basis, require
the port number to be specified as part of the configuration
negotiation.

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

Type

4

Length

3

Max-Port-Num

The maximum port number that can be used. The number of ports
present is Max-Port-Num + 1. The value can range from 0 to 254.

4.5. Transport-Mode

This parameter selects the mode of transport for the specified port.

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 | Port | Mode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

5

Length

4

Port

The port for which this option applies.

Mode

The transport mode to be used for this port.

0 HDLC Synchronous (default)
1 Asynchronous

4.6. Maximum-Frame-Size

This parameter specifies the maximum number of octets allowed in a
transported data frame.

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 | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum-Frame-Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

6

Length

7

Port

The port for which this option applies.

Maximum-Frame-Size

The maximum allowed length of a transported data frame in octets.
Default is 10,000. Negotiable range is 1 to 2**31 - 1. The value
0 is reserved to mean no limit. This field is transmitted most
significant octet first.

4.7. Allow-Odd-Frames

By default, only octet-aligned data frames are allowed for transport.
Successful negotiation of this option allows the transport of non-
octet aligned frames. The size of the padding required to align the

frames is carried in the CS Header octet.

Use of Header-Type H-Only is not permitted in conjunction with this
option.

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

Type

7

Length

3

Port

The port for which this option applies.

4.8. FCS-Type

By default, the transported data frame FCS is transported. This
option allows the FCS to be removed by the transmitter and
regenerated by the receiver.

It is important that implementations not use regeneration unless they
are using PPP Reliable Transmission [12] or operating over some other
layer that will provide reliable notification of a dropped packet.
Implementations are not permitted to send a incomplete or bad frame
to the user with a good (regenerated) FCS.

This option also selects the type of user FCS that will be
regenerated.

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 | Port | FCS-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

8

Length

4

Port

The port for which this option applies.

FCS-Type

0 Transparent-Transport (Default)
1 16-bit ITU-T CRC
2 32-bit ITU-T CRC

4.9. Flow-Expiration-Time

As described in section 2.2, Flow-Off messages expire after T1
seconds. By default, T1 is 10 seconds. This configuration option
allows the value of T1 to be changed.

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

Type

9

Length

5

Flow-Expiration-Time

The Flow-Expiration-Time field contains a 16 bit unsigned integer
which is used to specify the value to be assigned to T1 as
follows: T1 = Flow-Expiration-Time / 10 seconds. Therefore this
value is in units of 1/10 of a second, with allowable values from
1 to 2^16-1 (0.1 to 6553.5 seconds). It is transmitted most
significant octet first. The default value is 100 (10 seconds),
which all must support.

Security Considerations

Security issues are not discussed in this memo.

References

[1] Simpson, W., ed., "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, July 1994.

[2] CCITT Recommendation V.120 (09/92), "Support by an ISDN of
Data Terminal Equipment with V-Series Type Interfaces with
Provision for Statistical Multiplexing", 1993.

[3] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
1962, June 1996.

[4] Friend, R., and W. Simpson, "PPP Stac LZS Compression
Protocol", RFC1974, August 1996.

[5] Rand, D., "PPP Predictor Compression Protocol", RFC1978,
August 1996.

[6] Petty, J., "PPP Hewlett-Packard Packet-by-Packet Compression
(HP PPC) Protocol", Work in Progress.

[7] Carr, D., "PPP Gandalf FZA Compression Protocol", Work in
Progress.

[8] Schryver, V., "PPP BSD Compression Protocol", RFC1977,
August 1996.

[9] Schremp, et. al., "PPP Magnalink Variable Resource
Compression", RFC1975, August 1996.

[10] Schneider, K., "PPP Stacker LZS Compression Protocol using a
DCP Header (LZS-DCP)", RFC1967, August 1996.

[11] Simpson, W.A., "PPP LCP Extensions", RFC1570, January 1994.

[12] Rand, D., "PPP Reliable Transmission", RFC1663, July 1994.

Chair's Address

The working group can be contacted via the current chair:

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

EMail: karl@ascend.com

Authors' Addresses

Questions about this memo should be directed to:

Kevin Schneider
Adtran, Inc.
901 Explorer Blvd.
Huntsville, AL 35806-2807

Phone: (205) 971-8000
EMail: kevin@adtran.com

Stuart Venters
Adtran, Inc.
901 Explorer Blvd.
Huntsville, AL 35806-2807

Phone: (205) 971-8000
EMail: sventers@adtran.com

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