Network Working Group Marshall T. Rose, Dwight E. Cass
Request for Comments: RFC 1006 Northrop Research and Technology Center
Obsoletes: RFC 983 May 1987
ISO Transport Service on top of the TCP
Version: 3
Status of this Memo
This memo specifies a standard for the Internet community. Hosts
on the Internet that choose to implement ISO transport services
on top of the TCP are expected to adopt and implement this
standard. TCP port 102 is reserved for hosts which implement this
standard. Distribution of this memo is unlimited.
This memo specifies version 3 of the protocol and supersedes
[RFC983]. Changes between the protocol as described in Request for
Comments 983 and this memo are minor, but are unfortunately
incompatible.
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1. Introduction and Philosophy
The 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 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 convergence and 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,
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.
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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 [ISO8072]), but we will in fact
implement the ISO TP0 protocol on top of TCP/IP (as defined in
[RFC793,RFC791]), not on top of the the ISO/CCITT network
protocol. Since the transport class 0 protocol is used over the
TCP/IP connection, it achieves identical functionality as
transport class 4. 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.
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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:
1. Experience teaches us that it takes just as long to get good
implementations of the lower level protocols as it takes to get
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.
2. 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
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
M. Rose & D. Cass [Page 4]
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providing solid transport services independent of actual
implementation.
M. Rose & D. Cass [Page 5]
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3. The Model
The [ISO8072] 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 [ISO8072] [X.214]
Transport protocol [ISO8073] [X.224]
Session protocol [ISO8327] [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) [RFC793] can be used to offer
the services and provide the interfaces.
For expository purposes, the following abbreviations are used:
TS-peer a process which implements the protocol described
by this memo
TS-user a process talking using the services of a TS-peer
M. Rose & D. Cass [Page 6]
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TS-provider the black-box entity implementing the protocol
described by this memo
For the purposes of this memo, which describes version 2 of the
TSAP protocol, all aspects of [ISO8072] are supported with one
exception:
Quality of Service parameters
In the spirit of CCITT, this is left "for further study". A
future version of the protocol will most likely support the QOS
parameters for TP by mapping these onto various TCP parameters.
The ISO standards do not specify the format of a session port
(termed a TSAP ID). This memo mandates the use of the GOSIP
specification [GOSIP86] for the interpretation of this field.
(Please refer to Section 5.2, entitled "UPPER LAYERS ADDRESSING".)
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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4. The Primitives
The protocol assumes that the TCP[RFC793] 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)
This memo describes how to use these services to emulate the following
service primitives, which are required by [ISO8073]:
Events
N-CONNECT.INDICATION
- An NS-user (responder) is notified that
connection establishment is in progress
N-CONNECT.CONFIRMATION
- An NS-user (responder) is notified that
the connection has been established
N-DATA.INDICATION
- An NS-user is notified that data can be
read from the connection
M. Rose & D. Cass [Page 8]
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N-DISCONNECT.INDICATION
- An NS-user is notified that the connection
is closed
Actions
N-CONNECT.REQUEST
- An NS-user (initiator) indicates that it
wants to establish a connection
N-CONNECT.RESPONSE
- An NS-user (responder) indicates that it
will honor the request
N-DATA.REQUEST - An NS-user sends data
N-DISCONNECT.REQUEST
- An NS-user indicates that the connection
is to be closed
The protocol offers the following service primitives, as defined
in [ISO8072], to the TS-user:
Events
T-CONNECT.INDICATION
- a TS-user (responder) is notified that
connection establishment is in progress
T-CONNECT.CONFIRMATION
- a TS-user (initiator) is notified that the
connection has been established
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
T-DISCONNECT.INDICATION
- a TS-user is notified that the connection
is closed
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Actions
T-CONNECT.REQUEST
- a TS-user (initiator) indicates that it
wants to establish a connection
T-CONNECT.RESPONSE
- a TS-user (responder) indicates that it
will honor the request
T-DATA.REQUEST - a TS-user sends data
T-EXPEDITED DATA.REQUEST
- a TS-user sends "expedited" data
T-DISCONNECT.REQUEST
- a TS-user indicates that the connection
is to be closed
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5. The Protocol
The protocol specified by this memo is identical to the protocol
for ISO transport class 0, with the following exceptions:
- for testing purposes, initial data may be exchanged
during connection establishment
- for testing purposes, an expedited data service is
supported
- for performance reasons, a much larger TSDU size is
supported
- the network service used by the protocol is provided
by the TCP
The ISO 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
section.
PRIMITIVES
The mapping between the TCP service primitives and the service
primitives expected by transport class 0 are quite straight-
forward:
network service TCP
--------------- ---
CONNECTION ESTABLISHMENT
N-CONNECT.CONFIRMATION open (ACTIVE open)
finishes
DATA TRANSFER
N-DATA.REQUEST send data
N-DATA.INDICATION data ready followed by
M. Rose & D. Cass [Page 11]
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read data
CONNECTION RELEASE
N-DISCONNECT.REQUEST close
N-DISCONNECT.INDICATION connection closes or
errors
Mapping parameters is also straight-forward:
network service TCP
--------------- ---
CONNECTION RELEASE
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
CONNECTION ESTABLISHMENT
The elements of procedure used during connection establishment
are identical to those presented in [ISO8073], with three
exceptions.
In order to facilitate testing, the connection request and
connection confirmation TPDUs may exchange initial user data,
using the user data fields of these TPDUs.
In order to experiment with expedited data services, the
connection request and connection confirmation TPDUs may
negotiate the use of expedited data transfer using the
negotiation mechanism specified in [ISO8073] is used (e.g.,
setting the "use of transport expedited data transfer service"
bit in the "Additional Option Selection" variable part). The
default is not to use the transport expedited data transfer
service.
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In order to achieve good performance, the default TPDU size is
65531 octets, instead of 128 octets. In order to negotiate a
smaller (standard) TPDU size, the negotiation mechanism
specified in [ISO8073] is used (e.g., setting the desired bit
in the "TPDU Size" variable part).
To perform an N-CONNECT.REQUEST action, the TS-peer performs
an active open to the desired IP address using TCP port 102.
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
TCP port 102. When a client successfully connects to this
port, the event occurs, and an implicit N-CONNECT.RESPONSE
action is performed.
NOTE: In most implementations, a single server will
perpetually LISTEN on port 102, handing off
connections as they are made
DATA TRANSFER
The elements of procedure used during data transfer are identical
to those presented in [ISO8073], with one exception: expedited
data may be supported (if so negotiated during connection
establishment) by sending a modified ED TPDU (described below).
The TPDU is sent on the same TCP connection as all of the other
TPDUs. This method, while not faithful to the spirit of [ISO8072],
is true to the letter of the specification.
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.
CONNECTION RELEASE
To perform an N-DISCONNECT.REQUEST action, the TS-peer simply closes
the TCP connection.
If the TCP informs the TS-peer that the connection has been closed or
has errored, this indicates an N-DISCONNECT.INDICATION event.
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6. Packet Format
A fundamental difference between the TCP and the network service
expected by TP0 is that the TCP manages a continuous stream of
octets, with no explicit boundaries. The TP0 expects information
to be sent and delivered in discrete objects termed network
service data units (NSDUs). Although other classes of transport
may combine more than one TPDU inside a single NSDU, transport
class 0 does not use this facility. Hence, an NSDU is identical
to a TPDU for the purposes of our discussion.
The protocol described by this memo uses a simple packetization
scheme in order to delimit TPDUs. Each packet, termed a TPKT, 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 TPKT be a multiple of 4 octets in length.
A TPKT consists of two parts: a packet-header and a TPDU. The
format of the header is constant regardless of the type of packet.
The format of the packet-header is as follows:
This field is always 3 for the version of the protocol described in
this memo.
packet length 16 bits (min=7, max=65535)
This field contains the length of entire packet in octets,
including packet-header. This permits a maximum TPDU size of
65531 octets. Based on the size of the data transfer (DT) TPDU,
this permits a maximum TSDU size of 65524 octets.
The format of the TPDU is defined in [ISO8073]. Note that only
TPDUs formatted for transport class 0 are exchanged (different
transport classes may use slightly different formats).
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To support expedited data, a non-standard TPDU, for expedited data
is permitted. The format used for the ED TPDU is nearly identical
to the format for the normal data, DT, TPDU. The only difference
is that the value used for the TPDU's code is ED, not DT:
After the credit field (which is always ZERO on output and ignored
on input), there is one additional field prior to the user data.
TPDU-NR and EOT 8 bits
Bit 7 (the high-order bit, bit mask 1000 0000) indicates the end
of a TSDU. All other bits should be ZERO on output and ignored on
input.
Note that the TP specification limits the size of an expedited
transport service data unit (XSDU) to 16 octets.
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7. Comments
Since the release of RFC983 in April of 1986, we have gained much
experience in using ISO transport services on top of the TCP. In
September of 1986, we introduced the use of version 2 of the
protocol, based mostly on comments from the community.
In January of 1987, we observed that the differences between
version 2 of the protocol and the actual transport class 0
definition were actually quite small. In retrospect, this
realization took much longer than it should have: TP0 is is meant
to run over a reliable network service, e.g., X.25. The TCP can be
used to provide a service of this type, and, if no one complains
too loudly, one could state that this memo really just describes a
method for encapsulating TPO inside of TCP!
The changes in going from version 1 of the protocol to version 2
and then to version 3 are all relatively small. Initially, in
describing version 1, we decided to use the TPDU formats from the
ISO transport protocol. This naturally led to the evolution
described above.
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8. References
[GOSIP86] The U.S. Government OSI User's Committee.
"Government Open Systems Interconnection Procurement
(GOSIP) Specification for Fiscal years 1987 and
1988." (December, 1986) [draft status]
[ISO8072] ISO.
"International Standard 8072. Information Processing
Systems -- Open Systems Interconnection: Transport
Service Definition."
(June, 1984)
[ISO8073] ISO.
"International Standard 8073. Information Processing
Systems -- Open Systems Interconnection: Transport
Protocol Specification."
(June, 1984)
[ISO8327] ISO.
"International Standard 8327. Information Processing
Systems -- Open Systems Interconnection: Session
Protocol Specification."
(June, 1984)
[RFC791] Internet Protocol.
Request for Comments 791 (MILSTD 1777)
(September, 1981)
[RFC793] Transmission Control Protocol.
Request for Comments 793 (MILSTD 1778)
(September, 1981)
[RFC983] ISO Transport Services on Top of the TCP.
Request for Comments 983
(April, 1986)
[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)
M. Rose & D. Cass [Page 17]
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[X.225] CCITT.
"Recommendation X.225. Session Protocol Specification
for Open Systems Interconnection (OSI) for CCITT
Applications."
(October, 1984)
