RFC1859 - ISO Transport Class 2 Non-use of Explicit Flow Control over TCP RFC100

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　　Network Working Group Y. Pouffary
Request For Comments: 1859 Digital Equipment Corporation
Category: Informational October 1995

ISO Transport Class 2 Non-use of Explicit Flow Control over TCP
RFC1006 extension

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.

Table of Contents

1. Introduction - General recommendations.......................2
2. The protocol.................................................3
2.1 TCP service as a Network Service - The Primitives...........3
2.2 Connection Establishment....................................4
2.3 Data Transfer...............................................5
2.4 Connection Release..........................................6
3. Packet Format................................................6
4. DIGITAL DECnet over TCP/IP...................................8
Acknowledgements................................................9
References......................................................9
Author's Address................................................9

1. Introduction - General recommendations

This document is an extension to STD35, RFC1006, a standard for the
Internet community. The document does not duplicate the protocol
definitions contained in RFC1006 and in International Standard ISO
8073. It supplements that information with the description of how to
implement ISO Transport Class 2 Non-use of Explicit Flow Control on
top of TCP.

The document should be used in conjunction with the RFC1006 and ISO
8073.

The RFC1006 standard defines how to implement ISO 8073 Transport
Class 0 on top of TCP. This memo defines how to implement ISO 8073
Transport Class 2 Non-use of Explicit Flow Control on top of TCP.
Like ISO Transport Class 0, Class 2 Non-use of Explicit Flow Control
provides basic connection with minimal overhead.

A Transport protocol class is selected for a particular Transport
connection based upon the characteristics of the lower layers and the

requirements of the upper layer. Use of class 2 Non-use of Explicit
Flow Control is suitable when the use of separate virtual data
channels for normal and expedited Data are desirable or when an
explicit disconnection of the Transport connection is desirable.

Hosts which choose to implement this extension are expected to listen
on the well-known TCP port number 399.

It is recommended that the well-known RFC1006 TCP port 102 not be
used. This recommendation is done to minimise impact to an existing
RFC1006 implementation.

The memo also describes the use of this extension within the DIGITAL
Network Architecture (DNA).

2. The protocol

The protocol specified by this memo is fundamentally equivalent to
the protocol ISO 8073 Transport Class 2 Non-use of Explicit Flow
Control, with the following extensions:

- Expedited Data service is supported.

- Splitting and Recombining may be used for Expedited Data
transmission.

- The Network Service used is provided by TCP.

The ISO 8073 Transport protocol Class 2 allows Multiplexing. It is
recommended that this capability not be use for performance reasons.

The ISO 8073 Transport protocol exchanges information between peers
in discrete units of information called transport protocol data units
(TPDUs). The protocol defined in this memo encapsulates these TPDUs
in discrete units called TPKTs. The structure of these TPKTs and
their relationship to TPDUs are discussed in the next sections.

2.1 TCP service as a Network Service - The Primitives

The mapping between the TCP service primitives and the service
primitives expected by ISO 8073 Transport when operation over
Connection-oriented network service is straightforward.

Note: The following description of the mapping is a repeat from the
RFC1006 standard.

network service TCP
--------------- ---
CONNECTION ESTABLISHMENT

N-CONNECT.REQUEST open completes

N-CONNECT.INDICATION listen (PASSIVE open) finishes

N-CONNECT.RESPONSE listen completes

N-CONNECT.CONFIRMATION open (ACTIVE open) finishes

DATA TRANSFER

N-DATA.REQUEST send data

N-DATA.INDICATION data ready followed by read data

CONNECTION RELEASE

N-DISCONNECT.REQUEST close

N-DISCONNECT.INDICATION connection closes or errors

Mapping parameters between the TCP service and the network service is
also straightforward:

network service TCP
--------------- ---
CONNECTION ESTABLISHMENT

Called address server's IP address (4 octets)

Calling address client's IP address (4 octets)

all others ignored

DATA TRANSFER

NS-user data (NSDU) data

CONNECTION RELEASE

all parameters ignored

2.2 Connection Establishment

The principles used in connection establishment are based upon those
described in ISO 8073, with the following extensions.

- Connection Request and Connection Confirmation TPDUs may negotiate
the use of Expedited Data transfer using the negotiation mechanism
specified in ISO 8073.

- Connection Request and Connection Confirmation TPDUs must not
negotiate the Use of Explicit Flow Control.

To perform an N-CONNECT.REQUEST action, the TS-peer performs an
active open to the desired IP address using the well know TCP port
399. When the TCP signals either success or failure, this results in
an N-CONNECT.INDICATION action.

To await an N-CONNECT.INDICATION event, a server listens on the well
know TCP port 399. When a client successfully connects to this port,
the event occurs and an implicit N-CONNECT.RESPONSE action is
performed.

2.3 Data Transfer

The elements of procedure used during transfer are based upon those
presented in ISO 8073, with the two following extensions.

- Expedited Data may be supported (if negotiated during connection
establishment).

In Non-Use of Explicit Flow Control Expedited Data requires no
Expedited Data Acknowledgement.

- Splitting and Recombining may be used for Expedited Data
transmission.

The procedure of Splitting and Recombining allows a transport
connection to make use of multiple TCP connections.
TCP connections created for Splitting purposes should also use
the primitives described in 2.1.

It is recommended to only create a second TCP connection for
Expedited Data when transmission of Expedited Data is requested.

Expedited Data must only be sent over an outgoing TCP connection.
This second TCP connection must not be shared among transport
connections and must remain established until the transport
connection is terminated, at which time it must be closed.

Implementors note: The procedure of Splitting and Recombining for
Expedited Data transmission guaranties that a congested Normal Data
TCP connection cannot block an Expedited Data TCP connection. It also
ensures independence of the Normal Data TCP connection from the
Expedited Data TCP connection.

To perform an N-DATA.REQUEST action, the TS-peer constructs the
desired TPKT and uses the TCP send data primitive.

To trigger an N-DATA.INDICATION action, the TCP indicates that data
is ready and a TPKT is read using the TCP read data primitive.

2.4 Connection Release

The elements of procedure used during a connection release are
identical to those presented in ISO 8073.

A connection can be terminated by the user in one of two ways:

- Abort Disconnect specifies that all messages at the source are not
required to be sent to the destination before the connection is
disconnected.

- Synchronous Disconnect specifies that all messages at the source
must be sent to the destination, and that all messages at the
destination must be delivered, before the connection is
disconnected.

Disconnect Request and Disconnect Confirmation TPDUs are exchanged in
both cases. The Disconnect Request TPDU carries a code indicating the
reason for the disconnection.

In the case of a Synchronous Disconnect the Disconnect Request reason
code is normal (80 hex). For an Abort Disconnect the Disconnect
Request reason code is normal with additional information parameter
value set to (c0 hex).

Upon receipt of a Disconnect Confirmation TPDU a N-DISCONNECT.REQUEST
action is performed to close the TCP connection.

If the TCP connection fails for some other reason, this generates an
N-DISCONNECT.INDICATION event.

3. Packet Format

A fundamental difference between TCP and the network service expected
by ISO transport is that TCP manages a continuous stream of octets,
with no explicit boundaries.

The protocol described in RFC1006 uses a simple packetization scheme
in order to delimit TPDUs. Each packet, termed a TPKT, consists of
two parts: a packet-header and a TPDU.

We use the same scheme described in RFC1006 for this extension.
There is no need to change the version number. The ISO transport TPDU
sufficiently describes the transport protocol class being used.

The format of the packet-header described below is a repeat from
RFC1006.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| vrsn | reserved | packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

vrsn 8 bits

This field is always 3 for the version of the protocol
described in this memo.

packet length 16 bits (min=7, max=65535)

The packet length is the length of the entire packet in
octets, including packet-header.

The format of the ISO transport TPDU is defined in ISO 8073.

4. DIGITAL DECnet over TCP/IP

DECnet over TCP/IP is implemented using the DECnet Session Control
layer over this RFC1006 extension protocol.

The informational RFCdefined in this document provides the Transport
Service functionality required by DECnet Applications while operating
over TCP/IP.

The next paragraph is a brief summary of the role of the DECnet
Session Control Layer. For further details, refer to the DIGITAL DNA
Session Control Layer Specification.

The DECnet Session Control Layer makes a Transport Service available
to End Users of a network. This layer is concerned with system-
dependent functions related to creating, maintaining, and destroying
Transport Connections. Separate virtual data channels, known as

"Normal" and "Expedited", are provided to End Users. DECnet
Session Control must be guaranteed independence of these channels by
the Transport Layer. Expedited Data transmission cannot be blocked by
a congested normal data channel. DECnet Session Control requires that
all data in transit be delivered before initiating the release of the
Transport Connection.

DECnet, DNA, and the DIGITAL logo are trademarks of Digital Equipment
Corporation.

Acknowledgements

Bill Duane, Jim Bound, David Sullivan, Mike Dyer, Matt Thomas, Dan
Harrington and many other members of the DECnet engineering team.

References

[ISO8072] ISO. "International Standard 8072. Information
Processing Systems -- Open Systems Interconnection:
Transport Service Definition."

[ISO8073] ISO. "International Standard 8073. Information
Processing Systems -- Open Systems Interconnection:
Transport Protocol Specification."

[ISO8327] ISO. "International Standard 8327. Information
Processing Systems -- Open Systems Interconnection:
Session Protocol Specification."

[RFC791] Postel, J., "Internet Protocol - DARPA Internet Program
Protocol Specification", STD 5, RFC791,
USC/Information Sciences Institute, September 1981.

[RFC793] Postel, J., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", STD 7, RFC
793, USC/Information Sciences Institute, September 1981.

[RFC1006] Rose, M., and D. Cass, "ISO Transport Services on Top of
the TCP - Version: 3", STD 35, RFC1006, Northrop
Research and Technology Center, May 1987.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

Yanick Pouffary
End Systems Networking
Digital Equipment Corporation
Centre Technique (Europe)
B.P. 027
950 Routes des colles
06901 Sophia antipolis, France

Phone: +33 92-95-62-85
Fax: +33 92-95-62-32

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