RFC2687 - PPP in a Real-time Oriented HDLC-like Framing

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　　Network Working Group C. Bormann
Request for Comments: 2687 Universitaet Bremen TZI
Category: Standards Track September 1999

PPP in a Real-time Oriented HDLC-like Framing

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

A companion document describes an architecture for providing
integrated services over low-bitrate links, such as modem lines, ISDN
B-channels, and sub-T1 links [1]. The main components of the
architecture are: a real-time encapsulation format for asynchronous
and synchronous low-bitrate links, a header compression architecture
optimized for real-time flows, elements of negotiation protocols used
between routers (or between hosts and routers), and announcement
protocols used by applications to allow this negotiation to take
place.

This document proposes the suspend/resume-oriented solution for the
real-time encapsulation format part of the architecture. The general
approach is to start from the PPP Multilink fragmentation protocol
[2] and its multi-class extension [5] and add suspend/resume in a way
that is as compatible to existing hard- and firmware as possible.

1. Introduction

As an extension to the "best-effort" services the Internet is well-
known for, additional types of services ("integrated services") that
support the transport of real-time multimedia information are being
developed for, and deployed in the Internet.

The present document defines the suspend/resume-oriented solution for
the real-time encapsulation format part of the architecture. As
described in more detail in the architecture document, a real-time
encapsulation format is required as, e.g., a 1500 byte packet on a

28.8 kbit/s modem link makes this link unavailable for the
transmission of real-time information for about 400 ms. This adds a
worst-case delay that causes real-time applications to operate with
round-trip delays on the order of at least a second -- unacceptable
for real-time conversation.

A true suspend/resume-oriented approach can only be implemented on a
type-1 sender [1], but provides the best possible delay performance
to this type of senders. The format defined in this document may
also be of interest to certain type-2-senders that want to exploit
the better bit-efficiency of this format as compared to [5]. The
format was designed so that it can be implemented by both type-1 and
type-2 receivers.

1.1. Specification Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [8].

2. Requirements

The requirements for this document are similar to those listed in
[5].

A suspend/resume-oriented solution can provide better worst-case
latency than the pre-fragmenting-oriented solution defined in [5].
Also, as this solution requires a new encapsulation scheme, there is
an opportunity to provide a slightly more efficient format.

Predictability, robustness, and cooperation with PPP and existing
hard- and firmware installations are as important with suspend/resume
as with pre-fragmenting. A good suspend/resume solution achieves
good performance even with type-2 receivers [1] and is able to work
with PPP hardware such as async-to-sync converters.

Finally, a partial non-requirement: While the format defined in this
draft is based on the PPP multilink protocol ([2], also abbreviated
as MP), operation over multiple links is in many cases not required.

3. General Approach

As in [5], the general approach is to start out from PPP multilink
and add multiple classes to obtain multiple levels of suspension.
However, in contrast to [5], more significant changes are required to
be able to suspend the transmission of a packet at any point and
inject a higher priority packet.

The applicability of the multilink header for suspend/resume type
implementations is limited, as the "end" bit is in the multilink
header, which is the wrong place for suspend/resume operation. To
make a big packet suspendable, it must be sent with the "end" bit
off, and (unless the packet was suspended a small number of bytes
before its end) an empty fragment has to be sent afterwards to
"close" the packet. The minimum overhead for sending a suspendable
packet thus is twice the multilink header size (six bytes, including
a compressed multilink protocol field) plus one PPP framing (three
bytes). Each suspension costs another six bytes (not counting the
overhead of the framing for the intervening packet).

Also, the existing multi-link header is relatively large; as the
frequency of small high-priority packets increases, the overhead
becomes significant.

The general approach of this document is to start from PPP Multilink
with classes and provide a number of extensions to add functionality
and reduce the overhead of using PPP Multilink for real-time
transmission.

This document introduces two new features:

1) A compact fragment format and header, and

2) a real-time frame format.

4. The Compact Fragment Format

This section describes an optional multilink fragment format that is
more optimized towards single-link operation and frequent suspension
(type 1 senders)/a small fragment size (type 2 senders), with
optional support for multiple links.

When operating over a single link, the Multilink sequence number is
used only for loss detection. Even a 12-bit sequence number clearly
is larger than required for this application on most kinds of links.
We therefore define the following compact multilink header format
option with a three-bit sequence number.

As, with a compact header, there is little need for sending packets
outside the multilink, we can provide an additional compression
mechanism for this format: the MP protocol identifier is not sent
with the compact fragment header. This obviously requires prior
negotiation (similar to the way address and control field compression
are negotiated), as well as a method for avoiding the bit combination

0xFF (the first octet in an LCP frame before any LCP options have
been negotiated), as the start of a new LCP negotiation could
otherwise not be reliably detected.

Figure 1: Compact Fragment Format

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| R | sequence | class | 1 |
+---+---+---+---+---+---+---+---+
| data |
: :
+---+---+---+---+---+---+---+---+

Having the least significant bit always be 1 helps with HDLC chips
that operate specially on least significant bits in HDLC addresses.
(Initial bytes with the least significant bit set to zero are used
for the extended compact fragment format, see next section.)

The R bit is the inverted equivalent of the B bit in the other
multilink fragment formats, i.e. R = 1 means that this fragment
resumes a packet previous fragments of which have been sent already.

The following trick avoids the case of a header byte of 0xFF (which
would mean R=1, sequence=7, and class=7): If the class field is set
to 7, the R bit MUST never be set to one. I.e., class 7 frames by
design cannot be suspended/resumed. (This is also the reason the
sense of the B bit is inverted to an R bit in the compact fragment
format -- class 7 would be useless otherwise, as a new packet could
never be begun.)

As the sequence number is not particularly useful with the class
field set to 7, it is used to distinguish eight more classes -- for
some minor additional complexity, the applicability of prefix elision
is significantly increased by providing more classes with possibly
different elided prefixes.

For purposes of prefix elision, the actual class number of a fragment
is computed as follows:

- If the class field is 0 to 6, the class number is 0 to 6,

- if the class field is 7 and the sequence field is 0 to 7, the
class number is 7 to 14.

As a result of this scheme, the classes 0 to 6 can be used for
suspendable packets, and classes 7 to 14 (where the class field is 7
and the R bit must always be off) can be used for non-suspendable
high-priority classes, e.g., eight highly compressed voice streams.

5. The Extended Compact Fragment Format

For operation over multiple links, a three-bit sequence number will
rarely be sufficient. Therefore, we define an optional extended
compact fragment format. The option, when negotiated, allows both
the basic compact fragment format and the extended compact fragment
format to be used; each fragment indicates which format it is in.

Figure 1: Extended Compact Fragment Format

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| R | seq LSB | class | 0 |
+---+---+---+---+---+---+---+---+
| sequence -- MSB | 1 |
+---+---+---+---+---+---+---+---+
| data |
: :
+---+---+---+---+---+---+---+---+

In the extended compact fragment format, the sequence number is
composed of three least significant bits from the first octet of the
fragment header and seven most significant bits from the second
octet. (Again, the least significant bit of the second octet is
always set to one for compatibility with certain HDLC chips.)

For prefix elision purposes, fragments with a class field of 7 can
use the basic format to indicate classes 7 to 14 and the extended
format to indicate classes 7 to 1030. Different classes may use
different formats concurrently without problems. (This allows some
classes to be spread over a multi-link and other classes to be
confined to a single link with greater efficiency.) For class fields
0 to 6, i.e. suspendable classes, one of the two compact fragment
formats SHOULD be used consistently within each class.

If the use of the extended compact fragment format has been
negotiated, receivers MAY keep 10-bit sequence numbers for all
classes to facilitate senders switching formats in a class. When a
sender starts sending basic format fragments in a class that was
using extended format fragments, the 3-bit sequence number can be
taken as a modulo-8 version of the 10-bit sequence number, and no
discontinuity need result. In the inverse case, if a 10-bit sequence
number has been kept throughout by the receiver (and no major slips

of the sequence number have occurred), no discontinuity will result,
although this cannot be guaranteed in the presence of errors.
(Discontinuity, in this context, means that a receiver has to
resynchronize sequence numbers by discarding fragments until a
fragment with R=0 has been seen.)

6. Real-Time Frame Format

This section defines how fragments with compact fragment headers are
mapped into real-time frames. This format has been designed to
retain the overall HDLC based format of frames, so that existing
synchronous HDLC chips and async to sync converters can be used on
the link. Note that if the design could be optimized for async only
operation, more design alternatives would be available [4]; with the
advent of V.80 style modems, asynchronous communications is likely to
decrease in importance, though.

The compact fragment format provides a compact rendition of the PPP
multilink header with classes and a reduced sequence number space.
However, it does not encode the E-bit of the PPP multilink header,
which indicates whether the fragment at hand is the last fragment of
a packet.

For a solution where packets can be suspended at any point in time,
the E-bit needs to be encoded near the end of each fragment. The
real-time frame format, to ensure maximum compatibility with type 2
receivers, encodes the E-bit in the following way: Any normal frame
ending also ends the current fragment with E implicitly set to one.
This ensures that packets that are ready for delivery to the upper
layers immediately trigger a receive interrupt even at type-2
receivers.

Fragments of packets that are to be suspended are terminated within
the HDLC frame by a special "fragment suspend escape" byte (FSE).
The overall structure of the HDLC frame does not change; the
detection and handling of FSE bytes is done at a layer above HDLC
framing.

The suspend/resume format with FSE detection is an alternative to
address/control field compression (ACFC, LCP option 8). It does not
apply to frames that start with 0xFF, the standard PPP-in-HDLC
address field; these frames are handled as defined in [6] and [7].
(This provision ensures that attempts to renegotiate LCP do not cause
ambiguities.)

For frames that do not start with 0xFF, suspend/resume processing
performs a scan of every HDLC frame received. The FCS of the HDLC
frame is checked and stripped. Compact fragment format headers (both
basic and extended) are handled without further FSE processing.
(Note that, as the FSE byte was chosen such that it never occurs in
compact fragment format headers, this does not require any specific
code.)

Within the remaining bytes of the HDLC frame ("data part"), an FSE
byte is used to indicate the end of the current fragment, with an E
bit implicitly cleared. All fragments up to the last FSE are
considered suspended (E = 0); the final fragment is terminated (E =
1), or, if it is empty, ignored (i.e., the data part of an HDLC frame
can end in an FSE to indicate that the last fragment has E = 0).

Each fragment begins with a normal header, so the structure of a
frame could be:

Figure 2: Example frame with FSE delimiter

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| R | sequence | class | 1 |
+---+---+---+---+---+---+---+---+
| data |
: :
+---+---+---+---+---+---+---+---+
+ FSE + previous fragment implicitly E = 0
+---+---+---+---+---+---+---+---+
| R | sequence | class | 1 |
+---+---+---+---+---+---+---+---+
| data |
: :
+---+---+---+---+---+---+---+---+
| Frame | previous fragment implicitly E = 1
| CRC |
+---+---+---+---+---+---+---+---+

The value chosen for FSE is 0xDE (this is a relatively unlikely byte
to occur in today's data streams, it does not trigger octet stuffing
and triggers bit stuffing only for 1/8 of the possible preceding
bytes).

The remaining problem is that of data transparency. In the scheme
described so far, an FSE is always followed by a compact fragment
header. In these headers, the combination of a class field set to 7

with R=1 is reserved. Data transparency is achieved by making the
occurrence of an FSE byte followed by one of 0x8F, 0x9F, ... to 0xFF
special.

Figure 3: Data transparency with FSE bytes present

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| R | sequence | class | 1 |
+---+---+---+---+---+---+---+---+
| data |
: :
+---+---+---+---+---+---+---+---+
+ FSE + fragment NOT terminated
+---+---+---+---+---+---+---+---+
| R | S | T | U | 1 | 1 | 1 | 1 | R always is 1
+---+---+---+---+---+---+---+---+
| data | fragment continues
: :

In a combination of FSE/0xnF (where n is the first four-bit field in
the second byte, RSTU in Figure 3), the n field gives a sequence of
four bits indicating where in the received data stream FSE bytes,
which cannot simply be transmitted in the data stream, are to be
added by the receiver:

0x8F: insert one FSE, back to data
0x9F: insert one FSE, copy two data bytes, insert one FSE, back to data
0xAF: insert one FSE, copy one data byte, insert one FSE, back to data
0xBF: insert one FSE, copy one data byte, insert two FSE bytes, back
to data
0xCF: insert two FSE bytes, back to data
0xDF: insert two FSE bytes, copy one data byte, insert one FSE, back
to data
0xEF: insert three FSE bytes, back to data
0xFF: insert four FSE bytes, back to data

The data bytes following the FSE/0xnF combinations and corresponding
to the zero bits in the N field may not be FSE bytes.

This scheme limits the worst case expansion factor by FSE processing
to about 25 %. Also, it is designed such that a single data stream
can either trigger worst-case expansion by octet stuffing (or by bit
stuffing) or worst-case FSE processing, but never both. Figure 4
illustrates the scheme in a few examples; FSE/0xnF pairs are written
in lower case.

Figure 4: Data transparency examples

Data stream FSE-stuffed stream

DD DE DF E0 DD de 8f DF E0
01 DE 02 DE 03 01 de af 02 03
DE DA DE DE DB de bf DA DB
DE DE DE DE DE DA de ff de 8f DA

In summary, the real-time frame format is a HDLC-like frame delimited
by flags and containing a final FCS as defined in [7], but without
address and control fields, containing as data a sequence of FSE-
stuffed fragments in compact fragment format, delimited by FSE bytes.
As a special case, the final FSE may occur as the last byte of the
data content (i.e. immediately before the FCS bytes) of the HDLC-like
frame, to indicate that the last fragment in the frame is suspended
and no final fragment is in the frame (e.g., because the desirable
maximum size of the frame has been reached).

7. Implementation notes

7.1. MRU Issues

The LCP parameter MRU defines the maximum size of the packets sent on
the link. Async-to-sync converters that are monitoring the LCP
negotiations on the link may interpret the MRU value as the maximum
HDLC frame size to be expected.

Implementations of this specification should preferably negotiate a
sufficiently large MRU to cover the worst-case 25 % increase in frame
size plus the increase caused by suspended fragments. If that is not
possible, the HDLC frame size should be limited by monitoring the
HDLC frame sizes and possibly suspending the current fragment by
sending an FSE with an empty final fragment (FSE immediately followed
by the end of the information field, i.e. by CRC bytes and a flag) to
be able to continue in a new HDLC frame. This strategy also helps
minimizing the impact of lengthening the HDLC frame on the safety of
the 16-bit FCS at the end of the HDLC frame.

7.2. Implementing octet-stuffing and FSE processing in one automaton

The simplest way to add real-time framing to an implementation may be
to perform HDLC processing as usual and then, on the result, to
perform FSE processing. A more advanced implementation may want to
combine the two levels of escape character processing. Note,
however, that FSE processing needs to wait until two bytes from the
HDLC frame are available and followed by a third to ensure that the
bytes are not the final HDLC FCS bytes, which are not subject to FSE

processing. I.e., on the reception of normal data byte, look for an
FSE in the second-to-previous byte, and, on the reception of a
frame-end, look for an FSE as the last data byte.

8. Negotiable options

The following options are already defined by MP [2]:

o Multilink Maximum Received Reconstructed Unit

o Multilink Short Sequence Number Header Format

o Endpoint Discriminator

The following options are already defined by MCML [5]:

o Multilink Header Format

o Prefix Elision

This document defines two new code points for the Multilink Header
Format option.

8.1. Multilink header format option

The multilink header format option is defined in [5]. A summary of
the Multilink Header Format Option format is shown below. The fields
are transmitted from left to right.

Figure 5: Multilink header format option

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 = 27 | Length = 4 | Code | # Susp Clses |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

As defined in [5], this LCP option advises the peer that the
implementation wishes to receive fragments with a format given by
the code number, with the maximum number of suspendable classes (see
below) given. This specification defines two additional values for
Code, in addition to those defined in [5]:

- Code = 11: basic and extended compact real-time fragment format
with classes, in FSE-encoded HDLC frame

- Code = 15: basic compact real-time fragment format with classes,
in FSE-encoded HDLC frame

An implementation MUST NOT request a combination of both LCP
Address-and-Control-Field-Compression (ACFC) and the code values 11
or 15 for this option.

The number of suspendable classes negotiated for the compact real-
time fragment format only limits the use of class numbers that allow
suspending. As class numbers of 7 and higher do not require
additional reassembly space, they are not subject to the class number
limit negotiated.

9. Security Considerations

Operation of this protocol is believed to be no more and no less
secure than operation of the PPP multilink protocol [2]. Operation
with a small sequence number range increases the likelihood that
fragments from different packets could be incorrectly reassembled
into one packet. While most such packets will be discarded by the
receiver because of higher-layer checksum failures or other
inconsistencies, there is an increase in likelihood that contents of
packets destined for one host could be delivered to another host.
Links that carry packets where this raises security considerations
SHOULD use the extended sequence number range for multi-fragment
packets.

10. References

[1] Bormann, C., "Providing Integrated Services over Low-bitrate
Links", RFC2689, September 1999.

[2] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
Coradetti, "The PPP Multilink Protocol (MP)", RFC1990, August
1996.

[3] Simpson, W., "PPP in Frame Relay", RFC1973, June 1996.

[4] Andrades, R. and F. Burg, "QOSPPP Framing Extensions to PPP",
Work in Progress.

[5] Bormann, C., "The Multi-Class Extension to Multi-Link PPP", RFC
2686, September 1999.

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

[7] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC
1662, July 1994.

[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

11. Author's Address

Carsten Bormann
Universitaet Bremen FB3 TZI
Postfach 330440
D-28334 Bremen, GERMANY

Phone: +49.421.218-7024
Fax: +49.421.218-7000
EMail: cabo@tzi.org

Acknowledgements

The participants in a lunch BOF at the Montreal IETF 1996 gave useful
input on the design tradeoffs in various environments. Richard
Andrades, Fred Burg, and Murali Aravamudan insisted that there should
be a suspend/resume solution in addition to the pre-fragmenting one
defined in [5]. The members of the ISSLL subgroup on low bitrate
links (ISSLOW) have helped in coming up with a set of requirements
that shaped this solution.

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