RFC 983

Network Working Group D. E. Cass (NRTC)
Request for Comments: 983 M. T. Rose (NRTC)
April 1986

ISO Transport Services on Top of the TCP

Status of This Memo

This memo describes a proposed protocol standard for the ARPA
Internet community. The intention is that hosts in the ARPA-Internet
that choose to implement ISO TSAP services on top of the TCP be
expected to adopt and implement this standard. Suggestions for
improvement are encouraged. Distribution of this memo is unlimited.

1. Introduction and Philosophy

The ARPA Internet community has a well-developed, mature set of
transport and internetwork protocols (TCP/IP), which are quite
successful in offering network and transport services to end-users.
The CCITT and the ISO have defined various session, presentation, and
application recommendations which have been adopted by the
international community and numerous vendors. To the largest extent
possible, it is desirable to offer these higher level services
directly in the ARPA Internet, without disrupting existing
facilities. This permits users to develop expertise with ISO and
CCITT applications which previously were not available in the ARPA
Internet. It also permits a more graceful transition strategy from
TCP/IP-based networks to ISO-based networks in the medium- and
long-term.

There are two basic approaches which can be taken when "porting" an
ISO or CCITT application to a TCP/IP environment. One approach is to
port each individual application separately, developing local
protocols on top of the TCP. Although this is useful in the
short-term (since special-purpose interfaces to the TCP can be
developed quickly), it lacks generality.

A second approach is based on the observation that both the ARPA
Internet protocol suite and the ISO protocol suite are both layered
systems (though the former uses layering from a more pragmatic
perspective). A key aspect of the layering principle is that of
layer-independence. Although this section is redundant for most
readers, a slight bit of background material is necessary to
introduce this concept.

Externally, a layer is defined by two definitions:

a service-offered definition, which describes the services
provided by the layer and the interfaces it provides to access
those services; and,

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

a service-required definitions, which describes the services used
by the layer and the interfaces it uses to access those services.

Collectively, all of the entities in the network which co-operate to
provide the service are known as the service-provider. Individually,
each of these entities is known as a service-peer.

Internally, a layer is defined by one definition:

a protocol definition, which describes the rules which each
service-peer uses when communicating with other service-peers.

Putting all this together, the service-provider uses the protocol and
services from the layer below to offer the its service to the layer
above. Protocol verification, for instance, deals with proving that
this in fact happens (and is also a fertile field for many Ph.D.
dissertations in computer science).

The concept of layer-independence quite simply is:

IF one preserves the services offered by the service-provider

THEN the service-user is completely naive with respect to the
protocol which the service-peers use

For the purposes of this memo, we will use the layer-independence to
define a Transport Service Access Point (TSAP) which appears to be
identical to the services and interfaces offered by the ISO/CCITT
TSAP (as defined in [ISO-8072]), but we will base the internals of
this TSAP on TCP/IP (as defined in [RFC-793,RFC791]), not on the
ISO/CCITT transport and network protocols. Hence, ISO/CCITT higher
level layers (all session, presentation, and application entities)
can operate fully without knowledge of the fact that they are running
on a TCP/IP internetwork.

The authors hope that the preceding paragraph will not come as a
shock to most readers. However, an ALARMING number of people seem to
think that layering is just a way of cutting up a large problem into
smaller ones, *simply* for the sake of cutting it up. Although
layering tends to introduce modularity into an architecture, and
modularity tends to introduce sanity into implementations (both
conceptual and physical implementations), modularity, per se, is not
the end goal. Flexibility IS.

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ISO Transport Services on Top of the TCP

2. Motivation

In migrating from the use of TCP/IP to the ISO protocols, there are
several strategies that one might undertake. This memo was written
with one particular strategy in mind.

The particular migration strategy which this memo uses is based on
the notion of gatewaying between the TCP/IP and ISO protocol suites
at the transport layer. There are two strong arguments for this
approach:

a. Experience teaches us that it takes just as long to get good
implementations of the lower level protocols as it takes to get
good implementations of the higher level ones. In particular, it
has been observed that there is still a lot of work being done at
the ISO network and transport layers. As a result,
implementations of protocols above these layers are not being
aggressively pursued. Thus, something must be done "now" to
provide a medium in which the higher level protocols can be
developed. Since TCP/IP is mature, and essentially provides
identical functionality, it is an ideal medium to support this
development.

b. Implementation of gateways at the IP and ISO IP layers are
probably not of general use in the long term. In effect, this
would require each Internet host to support both TP4 and TCP. As
such, a better strategy is to implement a graceful migration path
from TCP/IP to ISO protocols for the ARPA Internet when the ISO
protocols have matured sufficiently.

Both of these arguments indicate that gatewaying should occur at or
above the transport layer service access point. Further, the first
argument suggests that the best approach is to perform the gatewaying
exactly AT the transport service access point to maximize the number
of ISO layers which can be developed.

NOTE: This memo does not intend to act as a migration or
intercept document. It is intended ONLY to meet the needs
discussed above. However, it would not be unexpected that the
protocol described in this memo might form part of an overall
transition plan. The description of such a plan however is
COMPLETELY beyond the scope of this memo.

Finally, in general, building gateways between other layers in the
TCP/IP and ISO protocol suites is problematic, at best.

To summarize: the primary motivation for the standard described in

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ISO Transport Services on Top of the TCP

this memo is to facilitate the process of gaining experience with
higher-level ISO protocols (session, presentation, and application).
The stability and maturity of TCP/IP are ideal for providing solid
transport services independent of actual implementation.

3. The Model

The [ISO-8072] standard describes the ISO transport service
definition, henceforth called TP.

ASIDE: This memo references the ISO specifications rather than
the CCITT recommendations. The differences between these parallel
standards are quite small, and can be ignored, with respect to
this memo, without loss of generality. To provide the reader with
the relationships:

Transport service [ISO-8072] [X.214]
Transport protocol [ISO-8073] [X.224]
Session protocol [ISO-8327] [X.225]

The ISO transport service definition describes the services offered
by the TS-provider (transport service) and the interfaces used to
access those services. This memo focuses on how the ARPA
Transmission Control Protocol (TCP) [RFC-793] can be used to offer
the services and provide the interfaces.

+-------------+ +-------------+
| TS-user | | TS-user |
+-------------+ +-------------+
| |
| TSAP interface TSAP interface |
| [ISO-8072] |
| |
+------------+ ISO Transport Services on the TCP +------------+
| client |----------------------------------------| server |
+------------+ (this memo) +------------+
| |
| TCP interface TCP interface |
| [RFC-793] |
| |

For expository purposes, the following abbreviations are used:

TS-peer a process which implements the protocol
described by this memo

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ISO Transport Services on Top of the TCP

TS-user a process talking using the services of a
TS-peer

TS-provider the black-box entity implementing the protocol
described by this memo

For the purposes of this memo, which describes version 1 of the TSAP
protocol, all aspects of [ISO-8072] are supported with one exception:

Quality of Service parameters

In the spirit of CCITT, this is left "for further study". Version 2
of the TSAP protocol will most likely support the QOS parameters for
TP by mapping these onto various TCP parameters.

Since TP supports the notion of a session port (termed a TSAP ID),
but the list of reserved ISO TSAP IDs is not clearly defined at this
time, this memo takes the philosophy of isolating the TCP port space
from the TSAP ID space and uses a single TCP port. This memo
reserves TCP port 102 for this purpose. This protocol manages its
own TSAP ID space independent of the TCP. Appendix A of this memo
lists reserved TSAP IDs for version 1 of this TSAP protocol. It is
expected that future editions of the "Assigned Numbers" document
[RFC-960] will contain updates to this list. (Interested readers are
encouraged to read [ISO-8073] and try to figure out exactly what a
TSAP ID is.)

Finally, the ISO TSAP is fundamentally symmetric in behavior. There
is no underlying client/server model. Instead of a server listening
on a well-known port, when a connection is established, the
TS-provider generates an INDICATION event which, presumably the
TS-user catches and acts upon. Although this might be implemented by
having a server "listen" by hanging on the INDICATION event, from the
perspective of the ISO TSAP, all TS-users just sit around in the IDLE
state until they either generate a REQUEST or accept an INDICATION.

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ISO Transport Services on Top of the TCP

4. The Primitives

The protocol assumes that the TCP [RFC-793] offers the following
service primitives:

Events

connected - open succeeded (either ACTIVE or PASSIVE)

connect fails - ACTIVE open failed

data ready - data can be read from the connection

errored - the connection has errored and is now closed

closed - an orderly disconnection has started

Actions

listen on port - PASSIVE open on the given port

open port - ACTIVE open to the given port

read data - data is read from the connection

send data - data is sent on the connection

close - the connection is closed (pending data is sent)

The protocol offers the following service primitives, as defined in
[ISO-8072], to the TS-user:

Events

T-CONNECT.INDICATION

- a TS-user (server) is notified that connection establishment
is in progress

T-DISCONNECT.INDICATION

- a TS-user is notified that the connection is closed

T-CONNECT.CONFIRMATION

- a TS-user (client) is notified that the connection has been
established

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ISO Transport Services on Top of the TCP

T-DATA.INDICATION

- a TS-user is notified that data can be read from the
connection

T-EXPEDITED DATA.INDICATION

- a TS-user is notified that "expedited" data can be read from
the connection

Actions

T-CONNECT.RESPONSE

- a TS-user (server) indicates that it will honor the request

T-DISCONNECT.REQUEST

- a TS-user indicates that the connection is to be closed

T-CONNECT.REQUEST

- a TS-user (client) indicates that it wants to establish a
connection

T-DATA.REQUEST

- a TS-user sends data

T-EXPEDITED DATA.REQUEST

- a TS-user sends "expedited" data

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ISO Transport Services on Top of the TCP

5. The Protocol

It is the goal of this memo to offer a TP interface on top of the
TCP. Fortunately, the TCP does just about everything that
TS-provider offers to the TS-user, so the hard parts of the transport
layer (e.g., three-way handshakes, choice of ISS, windowing,
multiplexing, ad infinitum) are all taken care of by the TCP.

Despite the symmetry of TP, it is useful to consider the protocol
with the perspective of a client/server model.

The information exchanged between TSAP-peers is in the form of
packets termed "TPKT"s. The format of these packets is described in
the next section. For the purposes of the description below, a TPKT
has a code which is one of:

CR - request connection
CC - confirm connection
DR - request disconnection
DT - data
ED - expedited data

A TSAP server begins by LISTENing on TCP port 102. When a TSAP
client successfully connects to this port, the protocol begins.

A client decides to connect to the port when a TS-user issues a
T-CONNECT.REQUEST action. This action specifies the TSAP ID of the
remote TS-user, whether expedited data is to be supported, and
(optionally) some initial TS-user data. The client consults the TSAP
ID given to ascertain the IP address of the server. If the expedited
data option was requested, the client opens a passive TCP port, in
non-blocking mode, noting the port number. This TCP port is termed
the "expedited port". The client then tries to open a TCP connection
to the server on port 102. If not successful, the client fires
T-DISCONNECT.INDICATION for the TS-user specifying the reason for
failure (and, closes the expedited port, if any). If successful, the
client sends a TPKT with code CR containing:

- the TSAP ID of the TS-user on the client's host (the "caller")
- the TSAP ID of the TS-user that the client wants to talk to
(the "called")
- if the expedited data option was requested, the TSAP ID of the
expedited port for the client's host
- any TS-user data from the T-CONNECT.REQUEST

The client now awaits a response.

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ISO Transport Services on Top of the TCP

The server, upon receipt of the TPKT, validates the contents of the
TPKT (checking the version number, verifying that the code is CR, and
so forth). If the packet is invalid, the server sends a TPKT with
code DR specifying "PROTOCOL ERROR", closes the TCP connection, and
goes back to the LISTEN state.

If the packet is valid, the server examines the TSAP ID that the
remote TS-user wants to communicate with. If the TS-user specified
can be located and started (e.g., the appropriate program which
implements the indicated protocol is present), then the server starts
this TS-user by firing T-CONNECT.INDICATION. Otherwise, the server
sends a TPKT with code DR specifying "SESSION ENTITY NOT ATTACHED TO
TSAP" or "REMOTE TRANSPORT ENTITY CONGESTED AT CONNECT REQUEST TIME"
as appropriate, closes the TCP connection, and goes back to the
LISTEN state.

The server now waits for a T-CONNECT.RESPONSE or T-DISCONNECT.REQUEST
from the TS-user it started. if the latter is given, the server
sends a TPKT with code DR containing the reason for the disconnect as
supplied by the TS-user.

The server then closes the TCP connection and goes back to the LISTEN
state.

Instead, if T-CONNECT.RESPONSE is given, the server sees if an
expedited port was specified in the connection request. If so, the
server opens a second TCP connection and connects to the specified
port. If the connection fails, the server sends a TPKT with code DR
specifying "CONNECTION NEGOTIATION FAILED", closes the TCP
connection, and goes back to the LISTEN state. If the connection
succeeded, the server notes the local port number used to connect to
the expedited port.

If an expedited port was not specified in the TPKT with code CR, and
the server's TS-user indicates that it wants to use expedited data,
then the server sends a TPKT with code DR specifying "CONNECTION
NEGOTIATION FAILED", fires T-DISCONNECT.INDICATION with this error to
the TS-user, closes the TCP connection, and goes back to the LISTEN
state.

The server now sends a TPKT with code CC containing:

- the TSAP ID of the TS-user responding to the connection
(usually the "called")
- if an expedited port was specified in the TPKT with code CR,

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ISO Transport Services on Top of the TCP

the TSAP ID of the port number on the server's host that was
used to connect to the expedited port
- any TS-user data from the T-CONNECT.RESPONSE

After sending the TPKT, the server enters the SYMMETRIC PEER state.

The client, upon receipt of the TPKT, validates the contents of the
TPKT (checking the version number, verifying that the code is CC or
DR, and so forth). If the packet is invalid, the client sends a TPKT
with code DR specifying "PROTOCOL ERROR", fires
T-DISCONNECT.INDICATION with this error to the TS-user, and closes
the TCP connection (and the expedited port, if any).

If the packet's code is DR, the client fires T-DISCONNECT.INDICATION
with the reason given in the TPKT to the TS-user, and closes the TCP
connection (and the expedited port, if any).

If the packet's code is CC, the client checks if an expedited port
was specified and that a connection is waiting on the expedited port.
If not, a protocol error has occurred, a TPKT with code DR is
returned, T-DISCONNECT.INDICATION is fired, and so on. Otherwise,
the client checks the remote address that connected to the expedited
port. If it differs from the port listed in the TPKT with code CC, a
protocol error has occurred. Otherwise, all is well, two TCP
connections have been established, one for all TPKTs except expedited
data, and the second for the exclusive use of expedited data.

The client now fires T-CONNECT.CONFIRMATION, and enters the SYMMETRIC
PEER state.

Once both sides have reached the SYMMETRIC PEER state, the protocol
is completely symmetric, the notion of client/server is lost. Both
TS-peers act in the following fashion:

If the TCP indicates that data can be read, the TS-peer, upon receipt
of the TPKT, validates the contents. If the packet is invalid, the
TS-peer sends a TPKT with code DR specifying "PROTOCOL ERROR", fires
T-DISCONNECT.INDICATION with this error to the TS-user, and closes
the TCP connection (and expedited data connection, if any). If the
TS-peer was the server, it goes back to the LISTEN state.

NOTE: If the expedited data option was requested, then there are
two TCP connections that can supply data for reading. The
dialogue below assumes that only ED TPKTs are read from the
expedited data connection. For simplicity's sake, when reading
from TCP the relation between connections and TPKTs is unimportant
and this memo URGES all implementations to be very lenient in this

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ISO Transport Services on Top of the TCP

regard. When writing to TCP, implementations should use the
expedited data connection only to send TPKTs with code ED.
Section 7 of this memo discusses the handling of expedited data in
greater detail.

If the packet's code is DR, the TS-peer fires T-DISCONNECT.INDICATION
with the reason given in the TPKT to the TS-user, and closes the TCP
connection (and expedited data connection, if any). If the TS-peer
was the server, it goes back to the LISTEN state.

If the packet's code is ED or DT, the TS-peer fires T-DATA.INDICATION
or T-EXPEDITED DATA.INDICATION as appropriate with the enclosed user
data for the TS-user. It then goes back to the SYMMETRIC PEER state.

If the packet is invalid, the TS-peer sends a TPKT with code DR
specifying "PROTOCOL ERROR", fires T-DISCONNECT.INDICATION with this
error to the TS-user, and closes the TCP connection (and expedited
data connection, if any). If the TS-peer was the server, it goes
back to the LISTEN state.

If the TCP indicates that an error has occurred and the connection
has closed, then the TS-peer fires T-DISCONNECT.INDICATION to the
TS-user specifying the reason for the failure. If the expedited data
connection, if any, is still open, it is closed. If the TS-peer was
the server, it goes back to the LISTEN state.

If the TS-user issues a T-DATA.REQUEST or T-EXPEDITED DATA.REQUEST
action, the TS-peer sends a TPKT with code DT or ED containing the
TS-user data. It then goes back to the SYMMETRIC PEER state.

If the TS-user issues a T-DISCONNECT.REQUEST action, the TS-peer
sends a TPKT with code DR containing the reason for the disconnect as
supplied by the TS-user. The TS-peer then closes the TCP connection,
(and expedited data connection, if any). If the TS-peer was the
server, it goes back to the LISTEN state.

In terms of (augmented) state tables, the protocol can be explained
as follows. The server starts in state S0, the client starts in
state C0. "TCP:" refers to an event or action from the TCP service,
"SS:" refers to an event or action from the TS-user (e.g., the ISO
session service [ISO-8327]).

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ISO Transport Services on Top of the TCP

S E R V E R S T A T E S

state event action goto
----- ----- ------ ----
S0 TCP: listen on port 102 S1

S1 TCP: connected TCP: read TPKT
parse, on error
TCP: send DR, close S0
code is CR
start session server
SS: T-CONNECT S2
.INDICATION
otherwise,
TCP: send DR, close S0

S2 SS: T-CONNECT.RESPONSE if expedited option,
TCP: open port EXPD
TCP: send CC P0

S2 SS: T-DISCONNECT TCP: send DR, close S0
.REQUEST

Any event occuring for a state not listed above is considered an
error, and handled thusly:

state event action goto
----- ----- ------ ----
S* TCP: other if TCP is open, TCP: close S0
otherwise ignore S0
S* SS: other SS: T-DISCONNECT
.INDICATION
if TCP is open, close S0

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ISO Transport Services on Top of the TCP

C L I E N T S T A T E S

state event action goto
----- ----- ------ ----
C0 SS: T-CONNECT.REQUEST if expedited option,
TCP: non-blocking
listen on port EXPD
TCP: open port 102 C1

C1 TCP: connected TCP: send CR C2

C1 TCP: connect fails TCP: close
SS: T-DISCONNECT C0
.INDICATION

C2 TCP: data ready TCP: read TPKT
parse, on error
TCP: send DR, close
SS: T-DISCONNECT C0
.INDICATION
code is CC
if expedited option,
verify port EXPD
connected correctly,
if not, treat as error
SS: T-CONNECT P0
.CONFIRMATION
code is DR
TCP: close
SS: T-DISCONNECT C0
.INDICATION
otherwise
TCP: send DR, close
SS: T-DISCONNECT C0
.INDICATION

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ISO Transport Services on Top of the TCP

Any event occuring for a state not listed above is considered an
error, and handled thusly:

state event action goto
----- ----- ------ ----
C* TCP: other if TCP is open, close C0
otherwise ignore C0

C* SS: other SS: T-DISCONNECT
.INDICATION
if TCP is open, close C0

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ISO Transport Services on Top of the TCP

P E E R S T A T E S

state event action goto
----- ----- ------ ----
P0 TCP: data ready TCP: read TPKT
parse, on error
TCP: send DR, close
SS: T-DISCONNECT end
.INDICATION
code is DT
SS: T-DATA.INDICATION P0
code is ED
SS: T-EXPEDITED DATA P0
.INDICATION
code is DR
TCP: close
SS: T-DISCONNECT end
.INDICATION
otherwise
TCP: send DR, close
SS: T-DISCONNECT end
.INDICATION

P0 TCP: other TCP: close
SS: T-DISCONNECT end
.INDICATION

P0 SS: T-DATA.REQUEST TCP: send DT P0

P0 SS: T-EXPEDITED DATA TCP: send ED P0
.REQUEST

P0 SS: T-DISCONNECT TCP: send DR, close end
.REQUEST

P0 SS: other TCP: send DR, close
SS: T-DISCONNECT end
.INDICATION

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ISO Transport Services on Top of the TCP

6. Packet Format

Two TS-peers exchange information over a TCP connection by
encapsulating information in well-defined packets. A packet, denoted
as "TPKT" in the previous sections, is viewed as an object composed
of an integral number of octets, of variable length.

NOTE: For the purposes of presentation, these objects are shown
as being 4 octets (32 bits wide). This representation is an
artifact of the style of this memo and should not be interpreted
as requiring that a TPDU be a multiple of 4 octets in length.

A packet consists of two parts: a packet-header and a pseudo-TPDU.
The format of the header is constant regardless of the type of
packet. The format of the pseudo-TPDU follows the [ISO-8073]
recommendation very closely with the exceptions listed below. As per
[ISO-8073], each TPDU consists of two parts: header and data.

It is EXTREMELY important to observe that TPKTs represent
"indivisible" units of data to the TS-user. That is, a
T-DATA.REQUEST initiated by the TS-user at the sending end of a
connection should result in exactly one T-DATA.INDICATION being
generated (with exactly that data) for the TS-user at the receiving
end. To ensure this behavior, it is critical that any INDICATION
event resulting from a TPKT be initiated ONLY after the entire TPKT
is fully received. Furthermore, exactly one such INDICATION event
should be generated by the TS-peer. The packet length field, as
described below, can accommodate on the order of 65K octets of user
data. This should be well above the requirements of the size of any
SPDU (Session Protocol Data Unit) for any real implementation. As a
result, version 1 of this protocol has no need to either fragment or
re-assemble TS-user data. If an application arises which requires an
SPDU of size greater than 65K octets, this memo will be revised.

The format of the packet-header is as follows:

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:

1. vrsn 8 bits

This field is always 1 for this memo.

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ISO Transport Services on Top of the TCP

2. packet length 16 bits (min=8, max=65535)

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

The format of the TPDU (to re-phrase from [ISO-8073]) depends on the
type of a TPDU. All TPDUs start with a fixed-part header length and
the code. The information following after the code varies, depending
on the value of the code. In the context of this memo, the following
codes are valid:

CR: connect request
CC: connect confirm
DR: disconnect request
DT: data
ED: expedited data

The format of a CR or CC TPDU is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header length | code | credit| destination reference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source reference | class |options| variable data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | ... | ... | ... |
| ... | ... | ... | ... |
| ... | user data | ... | ... |
| ... | ... | ... | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

3. header length 8 bits (min=6, max=min(254,packet
length-6))

The TPDU-header length in octets including parameters but
excluding the header length field and user data (if any).

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ISO Transport Services on Top of the TCP

4. code 4 bits

The type of TPDU. Values, in the context of this memo, are:

value meaning
----- -------
14 CR: connection request (binary 1110)
13 CC: connection confirm (binary 1101)
8 DR: disconnect request (binary 1000)
15 DT: data (binary 1111)
1 ED: expedited data (binary 0001)
all other reserved

5. credit 4 bits

This field is always ZERO on output and ignored on input.

6. destination reference 16 bits

This field is always ZERO on output and ignored on input.

7. source reference 16 bits

This field is always ZERO on output and ignored on input.

8. class 4 bits

This field is always 4 (binary 0100) on output and ignored on
input. It is anticipated that future versions of this protocol
will make use of this field.

9. options 4 bits

This field is always ZERO on output and ignored on input.

10. variable data (header length - 6) octets

This portion of the TPDU is of variable length. For most
TPDUs, this portion is empty (the header length field of the
TPDU is exactly 6). The contents of the variable data consist
of zero or more "parameters". Each parameter has the following
format:

parameter code 1 octet in size
parameter length 1 octet in size, value is the number
of octets in parameter value
parameter value parameter data

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ISO Transport Services on Top of the TCP

Normally, the parameter length is 1 octet. Any implementation
conforming to this version of the protocol must recognize all
parameter types listed in [ISO-8073]. With the exception of
the parameters listed below, these parameters are simply
ignored.

o Parameter name: Transport service access point
identifier (TSAP-ID) of the client
TSAP

Parameter code: 193 (binary 1100 0001)
Parameter length: variable
Parameter value: TSAP-ID attributes

Each TSAP-ID consists of 1 or more attributes. Each
attribute has this format:

Attribute code 1 octet in size
Attribute length 1 octet in size, value is the number
of octets in attribute value
Attribute value attribute data

In version 1 of this protocol, only two attributes are
defined. All others are reserved.

Attribute name: Internet Address

Attribute code: 1
Attribute length: 6
Attribute value: IP address (4 octets)
session port (2 octets, unsigned
integer)

This attribute is ALWAYS required. Values for session
port can be found in Appendix A of this memo.

Attribute name: Internet Address for Expedited Data

Attribute code: 2
Attribute length: 6
Attribute value: IP address (4 octets)
TCP port (2 octets, unsigned integer)

This attribute is required ONLY if expedited data is
to be exchanged. The attribute value is an pair designated by the TS-peer for
use with expedited data.

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

o Parameter name: TSAP-ID of the server TSAP

Parameter code: 194 (binary 1100 0010)
Parameter length: variable
Parameter value: TSAP-ID attributes

o Parameter name: Additional option selection

Parameter code: 198 (binary 1100 0110)
Parameter length: 1
Parameter value: additional flags

The additional flags octet consists of 8-bits of optional
flags. Only one bit is of interest to this memo, the
remaining bits should be ZERO on output and ignored on
input. This bit indicates if the transport expedited data
service is to be used. If this bit is set (bit mask 0000
0001) or this parameter does not appear in the TPDU, then
the expedited data service is requested. If this parameter
appears in the TPDU and the bit is not set then the service
is disabled. If the service is requested, then the TSAP-ID
of the sender of the TPDU must include an attribute
indicating the internet address to use for expedited data.

o Parameter name: Alternative protocol classes

Parameter code: 199 (binary 1100 0111)

Parameter length: variable
Parameter value: 64 (binary 0100 0000) in each octet

This is used as a NOOP in the variable data. Its use is
HIGHLY discouraged, but for those implementors who wish
to align the user data portion of the TPDU on word (or
page) boundaries, use of this parameter for filling is
recommended.

11. user data (packet length - header length - 5)
octets

This portion of the TPDU is actual user data, most probably one
or more SPDUs (session protocol data units).

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

The format of a DR TPDU is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header length | code | credit| destination reference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source reference | reason | variable data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | ... | ... | ... |
| ... | ... | ... | ... |
| ... | user data | ... | ... |
| ... | ... | ... | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the fields is identical to those of a CR or CC TPDU,
with the following exceptions:

where:

8. reason 8 bits

This replaces the class/option fields of the CR or CC TPDU. Any
value, as specified in [ISO-8073], may be used in this field.
This memo makes use of several:

value meaning
----- -------
1 Congestion at TSAP
2 Session entity not attached to TSAP
3 Address unknown (at TCP connect time)
128+0 Normal disconnect initiated by the session
entity
128+1 Remote transport entity congestion at connect
request time
128+3 Connection negotiation failed
128+5 Protocol Error
128+8 Connection request refused on this network
connection

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

The format of a DT or ED TPDU is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+
| header length | code | credit| TPDU-NR and EOT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| user data | ... | ... | ... |
| ... | ... | ... | ... |
| ... | ... | ... | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

After the credit field (which is always ZERO on output and ignored
on input), there is one additional field prior to the user data:

6. TPDU-NR and EOT 16 bits

This field is always ZERO on output and ignored on input.

7. Expedited Data

This memo utilizes a second TCP connection to handle expedited data
and does not make use of the TCP URGENT mechanism. The primary
disadvantage of this decision is that single-threaded implementations
of TCP may have some difficulty in supporting two simultaneous
connections. There are however several advantages to this approach:

a. Use of a single connection to implement the semantics of
expedited data implies that the TSAP peer manage a set of buffers
independent from TCP. The peer would, upon indication of TCP
urgent information, have to buffer all preceeding TPKTs until the
TCP buffer was empty. Expedited data would then be given to the
TS-user. When the expedited data was flushed, then the buffered
(non-expedited) data could be passed up to the receiving user.

b. It assumes that implementations support TCP urgency correctly.
This is perhaps an untrue assumption, particular in the case of
TCP urgency occuring when the send window is zero-sized. Further,
it assumes that the implementations can signal this event to the
TCP-user in a meaningful fashion. In a single-threaded
implementation, this is not likely.

Given a reasonable TCP implementation, the TS-peer need listen on two

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

TCP sockets in either polling or interrupt mode. When the TS-peer is
given data, the TCP must indicate which connection should be read
from.

The only tricky part of the protocol is that the client must be able
to start a passive OPEN for the expedited port, and then wait to read
from another connection. In between the passive OPEN and the other
connection supplying data, the server will connect to the expedited
port, prior to sending data on the other connection. To summarize
from Section 5, the sequence of events, with respect to TCP, is:

time client Server
---- ------ ------
0. passive OPEN of port 102

1. T-CONNECT.REQUEST from user
passive OPEN of expedited
port (non-blocking)

2. active OPEN of port 102

3. send CC TPKT

4. port 102 connected

5. receive CC TPKT
T-CONNECT.INDICATION to
user
T-CONNECT.RESPONSE from
user

6. active OPEN to expedited
port

7. expedited port connected

8. send CR TPKT

9. receive CR TPKT
verify expedited port
connected correctly

Multi-threaded implementations of TCP should be able to support this
sequence of events without any great difficulty.

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

8. Conclusions

There are two design decisions which should be considered. The first
deals with particular packet format used. It should be obvious to
the reader that the TP packet format was adopted for use in this
memo. Although this results in a few fields being ignored (e.g.,
source reference), it does not introduce an unacceptable amount of
overhead. For example, on a connection request packet (the worst
case) there are 6 bytes of "zero on output, ignore on input" fields.
Considering that the packet overhead processing is fixed, requiring
that implementations allocate an additional 1.5 words is not
unreasonable! Of course, it should be noted that some of these
fields (i.e., class) may be used in future versions of the protocol
as experience is gained.

The second decision deals with how the TCP and TSAP port space is
administered. It is probably a very bad idea to take any
responsibility, whatsoever, for managing this addressing space, even
after ISO has stabilized. There are two issues involved. First, at
what level do the TCP and TSAP port spaces interact; second, who
defines what this interaction looks like. With respect to the first,
it wholly undesirable to require that each TSAP port map to a unique
TCP port. The administrative problems for the TCP "numbers czar (and
czarina)" would be non-trivial. Therefore, it is desirable to
allocate a single TCP port, namely port 102, as the port where the
"ISO Transport Services" live in the TCP domain. Second, the
interaction defined in Appendix A of this memo is embryonic at best.
It will no doubt be replaced as soon as the ISO world reaches
convergence on how services are addressed in ISO TP. Therefore
readers (and implementors) are asked to keep in mind that this aspect
of the memo is guaranteed to change. Unfortunately, the authors are
not permitted the luxury of waiting for a consensus in ISO. As a
result, the minimal effort approach outlined in the appendix below
was adopted.

A prototype implementation of the protocol described by this memo is
available for 4.2BSD UNIX. Interested parties should contact the
authors for a copy. To briefly mention its implementation, a given
ISO service is implemented as a separate program. A daemon listens
on TCP port 102, consults a database when a connection request packet
is received, and fires the appropriate program for the ISO service
requested. Of course, given the nature of the BSD implementation of
TCP, as the child fires, responsibility of that particular connection
is delegated to the child; the daemon returns to listening for new
connection requests. The prototype implementation consists of both
the daemon program and subroutine libraries which are loaded with
programs providing ISO services.

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

9. References

[ISO-8072] ISO.
"International Standard 8072. Information Processing
Systems -- Open Systems Interconnection: Transport
Service Definition."
(June, 1984)

[ISO-8073] ISO.
"International Standard 8073. Information Processing
Systems -- Open Systems Interconnection: Transport
Protocol Specification."
(June, 1984)

[ISO-8327] ISO.
"International Standard 8327. Information Processing
Systems -- Open Systems Interconnection: Session
Protocol Specification."
(June, 1984)

[RFC-791] Internet Protocol.
Request for Comments 791
(September, 1981)
(See also: MIL-STD-1777)

[RFC-793] Transmission Control Protocol.
Request for Comments 793
(September, 1981)
(See also: MIL-STD-1778)

[RFC-960] Assigned Numbers.
Request for Comments 960
(December, 1985)

[X.214] CCITT.
"Recommendation X.214. Transport Service Definitions
for Open Systems Interconnection (OSI) for CCITT
Applications."
(October, 1984)

[X.224] CCITT.
"Recommendation X.224. Transport Protocol Specification
for Open Systems Interconnection (OSI) for CCITT
Applications."
(October, 1984)

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

[X.225] CCITT.
"Recommendation X.225. Session Protocol Specification
for Open Systems Interconnection (OSI) for CCITT
Applications."
(October, 1984)

[X.410] CCITT.
"Recommendation X.410. Message Handling Systems: Remote
Operations and Reliable Transfer Server."
(October, 1984)

Appendix A: Reserved TSAP IDs

Version 1 of this protocol uses a relatively simple encoding scheme
for TSAP IDs. A TSAP ID is an attribute list containing two
parameters, a 32-bit IP address, and a 16-bit port number. This is
used to identify both the client TSAP and the server TSAP. When it
appears in a TPKT with code CR or CC, the TSAP ID is encoded in the
variable data part for the client TSAP as:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 193 | 8 | 1 | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| a | b | c | d |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

and for the server TSAP as:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 194 | 8 | 1 | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| a | b | c | d |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

(Neither of these examples include an attribute for a TCP connection
for expedited data. If one were present, the length of the TSAP ID
attribute would be 16 instead of 8, and the 8 bytes following the
Internet address would be "2" for the attribute code, "6" for the

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RFC 983 April 1986
ISO Transport Services on Top of the TCP

attribute length, and then 6 octets for the Internet address to use
for expedited data, 4 octets for IP address, and 2 octets for TCP
port.)

Where [a.b.c.d] is the IP address of the host where the respective
TSAP peer resides, and port is a 16-bit unsigned integer describing
where in the TSAP port space the TS-user lives.

Port value Designation
---------- -----------
0 illegal
1-4096 privileged
4097-65535 user

The following table contains the list of the "official" TSAP ID port
numbers as of the first release of this memo. It is expected that
future editions of the "Assigned Numbers" document[RFC-960] will
contain updates to this list.

Port name ISO service
---- ---- -----------
1 echo unofficial echo
2 sink unofficial data sink
3 FTAM File Transfer, Access, and Management
4 VTS ISO-8571 Virtual Terminal Service
5 MHS Message Handling System [X.411]
CCITT X.400
6 JTM Job Transfer and Manipulation
ISO 8831/8832
7 CASE Common Application Service Elements
Kernel ISO-8650/2

If anyone knows of a list of "official" ISO services, the authors
would be very interested.

Cass & Rose [Page 27]