Network Working Group J. Forster
Request for Comments: 1613 G. Satz
Category: Informational G. Glick
cisco Systems, Inc.
cisco Systems X.25 over TCP (XOT)
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
3. Relationship Between XOT and X.25...............................2
4. Overall Packet Format...........................................3
4.1 XOT Header....................................................4
5. TCP Connection, Port Number, and Logical Channel Numbers (LCNs).4
6. XOT Packets.....................................................5
6.1 Virtual Circuit Setup and Clearing............................5
6.2 Data and Flow Control.........................................6
6.3 Interrupt, and Reset Packets..................................8
6.4 Restart, DTE Reject, Diagnostics, and Registration............8
6.5 PVC Setup.....................................................8
8. Security Considerations........................................12
10. Authors' Addresses.............................................13
It is sometimes desirable to transport X.25 over IP internets. The
X.25 Packet Level requires a reliable link level below it and
normally uses LAPB. This memo documents a method of sending X.25
packets over IP internets by encapsulating the X.25 Packet Level in
TCP provides a reliable byte stream. X.25 requires that the layer
below it provide message semantics, in particular the boundary
between packets. To provide this, a small (4-byte) XOT header is
used between TCP and X.25. The primary content of this header is a
length field, which is used to separate the X.25 packets within the
In general, the normal X.25 protocol packet formats and state
transition rules apply to the X.25 layer in XOT. Exceptions to this
The following language conventions are used in the items of
specification in this document:
o MUST, SHALL, or MANDATORY -- This item is an absolute
requirement of the specification.
o SHOULD or RECOMMEND -- This item should generally be followed
for all but exceptional circumstances.
o MAY or OPTIONAL -- This item is truly optional and may be
followed or ignored according to the needs of the implementor.
In some places in this document, there is parenthetical material
labeled "DISCUSSION". This material is intended to give
clarification and explanation of the preceding text.
3. Relationship Between XOT and X.25
When a networking device (a host, router, etc.) has an X.25 engine
(i.e., protocol implementation), that engine may be connected to
interface(s) running LAPB, and/or to logical interface(s) running LLC
or XOT/TCP/IP. In general, the XOT layer itself is not at all
sensitive to what kind of packets the X.25 engine passes to it.
However, to improve interoperability between separate
implementations, this document in some cases does specify behavior of
the X.25 engine.
While this document primarily discusses XOT from the perspective of
switching X.25 traffic (i.e., connecting an X.25 Virtual Circuit
between the local X.25 interfaces of two networking devices), this
should not prevent a host from offering X.25 connectivity using XOT.
The various X.25 standards may call a given packet type by a
different name according to the assigned DTE/DCE role of the
interface that originated the packet. XOT is intended to be
insensitive to the DTE/DCE role of the local interfaces at either end
of an XOT TCP connection, so, for this document, the following terms
are interchangeable unless stated otherwise:
o Call, Call Request and Incoming Call
o Call Confirm, Call Accepted and Call Connected
o Clear, Clear Request and Clear Indication
o Clear Confirm, DTE Clear Confirmation and DCE Clear Confirmation
o Data, DTE Data and DCE Data
o Interrupt, DTE Interrupt and DCE Interrupt
o Interrupt Confirm, DTE Interrupt Confirmation and
DCE Interrupt Confirmation
o RR, DTE RR and DCE RR
o RNR, DTE RNR and DCE RNR
o REJ, Reject and DTE REJ
o Reset, Reset Request and Reset Indication
o Reset Confirm, DTE Reset Confirmation and DCE Reset Confirmation
o Restart, Restart Request and Restart Indication
o Restart Confirm, DTE Restart Confirmation and
DCE Restart Confirmation
4. Overall Packet Format
The entire encapsulated packet has the following format:
| IP Header |
| TCP Header |
| XOT Header |
| X.25 Packet |
RFC convention is that a packet format is represented graphically
with the data sent first above the data sent later. This convention
is followed in this document, and therefore, while we refer to X.25
being transported over TCP, we draw the packet format with the X.25
portion of the packet lower on the page than the TCP portion.
4.1 XOT Header
The XOT header has the format:
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
| Version | Length |
Version (2 octets)
The version number of the XOT protocol is encoded in the first
two octets. The version number MUST be 0. Other values of
this field are reserved for future use. If a value other than
0 is received, then the TCP connection MUST be closed.
Length (2 octets)
The length of the X.25 packet is encoded in the second two
octets. Values must be legal X.25 packet lengths. If the
Length field has an illegal value, then the TCP connection MUST
5. TCP Connection, Port Number, and Logical Channel Numbers (LCNs)
A separate TCP connection MUST be used for each X.25 virtual circuit.
All connections MUST be made to TCP port number 1998. This port
number is an IANA Registered Port Number registered by cisco Systems;
cisco has designated it for use by XOT.
The TCP connection MUST be created before the virtual circuit can be
established. The TCP connection MAY be maintained after the virtual
circuit has been cleared. Data MUST NOT be passed along with the TCP
The Logical Channel Number (LCN) field in the X.25 header has no
significance and has arbitrary values. A corollary of this is that
there is no assignment of one side of the connection to be DTE and
another to be DCE.
Consider three devices A, B and C, where A and B both conduct XOT
sessions to C. It's possible that C could receive two calls with
the same LCN and, unless the X.25 engine could tell that they were
received on different logical (XOT) interfaces, here would a
danger of call collision (indeed a valid LCN on one interface may
not even be valid on a different interface). It is therefore
necessary for C's X.25 engine to distinguish between the two
streams, but the LCN field is not sufficient to do this. The XOT
protocol design decision was to expect the XOT layer to
communicate the stream identification to the X.25 layer.
6. XOT Packets
For each X.25 packet received from the TCP connection to be sent out
a local interface, an XOT implementation MUST set the packet's
logical channel number to that used on the outgoing interface. For
the purposes of this RFC, a logical channel number is the 12 bit
field confusingly defined by the X.25 Recommendations as the high-
order 4 bit "logical channel group number" and low-order 8 bit
"logical channel number", where the latter phrase is used to refer to
both the aggregated 12 bits and the low-order 8 bits.
An XOT implementation SHOULD NOT modify the X.25 packet header
information received on a local interface to be transmitted over the
An XOT implementation MUST modify the X.25 packet header information
as required for proper X.25 protocol operation for packets received
on a TCP connection to be transmitted over a local interface.
An XOT implementation MAY support connection between interfaces that
use different flow control modulos. If this feature is supported,
XOT MUST modify the packet General Format Identifier on all packets
received over the TCP connection to set the proper modulus
6.1 Virtual Circuit Setup and Clearing
Once a TCP connection has been established, the X.25 Call packet is
sent by the XOT that initiated the TCP connection. Eventually a Call
Confirm or Clear packet is received, or the X.25 T11/T21 timeout
occurs or the XOT TCP connection is closed. The usual X.25 state
transitions are followed.
Any legal X.25 facilities from the family of X.25 protocols
(including but not limited to the 1980, 1984 and 1988 CCITT X.25
Recommendations) MAY be included in the Call, Call Confirm and Clear
packets. Receipt of an unknown or unsupported X.25 facility received
from the TCP connection SHOULD be ignored (i.e., not presented in the
packet sent out the local interface) or treated as an error as
defined by the X.25 standard implemented.
To simplify end-to-end flow control, the packet size and window size
are always sent explicitly as facilities in the Call packet. The
Call packet MUST contain both Packet Size and Window Size facilities.
The Call Confirm packet MAY contain these facilities. The handling
of a Call received over a TCP connection that does not encode one or
both of the flow control facilities is a local matter--if the XOT
accepts such a Call, it MUST encode the missing flow control facility
values that apply to the connection in the returned Call Confirm
X.25 interfaces normally have a concept of network default values
for packet size and window size. It was thought that when
connecting diverse sites over a TCP/IP network this concept would
be difficult to achieve in practice. If there is no network
default, then each call must state the packet size and window
size. This is the reason for requiring the packet size and window
size facilities. It is expected that this can be achieved either
by the XOT layer itself, or by configuring the X.25 engine such
that there no network default on this interface.
After sending a Clear the TCP connection MAY be closed immediately
without waiting for the Clear Confirm. A Clear Confirm received on
the TCP connection MAY be silently discarded.
A packet with an invalid X.25 Packet Type Identifier (PTI) received
over the TCP connection before a Call has been received (i.e., while
in the P1 state) MUST be silently discarded.
6.2 Data and Flow Control
The implementation of X.25 flow control is a local matter, but
different implementation choices affect XOT behavior.
An XOT implementation may implement either end-to-end flow
control, where DATA, RR and RNR packets are sent over the TCP
connection as received over the local interface, or local flow
control, where flow control packets (RR, RNR and, if supported,
REJ) are sent on a VC according to local criteria, a complete
packet sequence of DATA packets may be fragmented or combined, and
data packet numbering normally has only local DTE-DCE
Existing implementations of XOT perform end-to-end flow control.
Data and flow control packets are simply transferred between the
two local interfaces via the TCP connection, adjusting the X.25
header data as necessary for mixed modulo operation. This does
not preclude an XOT implementation that performs local flow
control, but interoperability requires that a local flow control
implementation conduct the XOT session such that a connecting
end-to-end flow control implementation receives Data packets of
the proper size and flow control fields with appropriate P(S) and
An X.25 implementation that performs local flow control similarly
may set up a Call between two local interfaces where each logical
channel has its own packet and window sizes and Data packets must
be fragmented or collected between the interfaces and each
interface manages distinct packet sequence numbers; XOT operation
is simply an extension to this operation as a VC is connected
between the local interface and an XOT/TCP virtual interface, each
of which have distinct window and packet sizes.
An XOT that implements local flow control MUST send data packet
acknowledgements across the TCP connection for the DATA packets it
receives from the TCP connection, using the received packet numbers,
and MUST observe the maximum packet sizes agreed to across the TCP
An XOT implementation MUST NOT assume that an RNR sent across the TCP
connection will stop the flow of DATA packets in the other direction.
An RNR packet received from the TCP connection MAY cause an RNR
packet to be sent across the local interface; end-to-end flow control
implementations MAY communicate the P(R) in an RNR packet received
from the TCP connection by sending an RR packet on the local
An XOT implementation that allows mixed-modulo connections and
implements end-to-end flow control MUST intervene in the window size
negotiation process when a modulo 128 Call Request proposes a window
size of 8 or larger to an XOT connection that serves a modulo 8
interface. The intervention MUST either refuse the connection or
lower the too-large window size(s) to a value valid for the interface
and indicate the final result of the window size negotiation process
in the Call Confirm packet returned over the TCP connection.
For any type of flow control implementation that supports mixed
modulo connections, both cooperating XOTs MUST interpret the the P(S)
and P(R) values received from the TCP connection and perform any flow
control operation appropriate for correct X.25 operation of the local
interface. End-to-end flow control implementations MUST translate
between the two modulos and construct the analogous X.25 header P(S)
and P(R) fields for DATA, RR and RNR packets.
An XOT implementation MAY support connecting two XOT TCP sessions to
each other. If this feature is supported, XOT MUST simply connect
the two TCP sessions without modifying the data passed.
6.3 Interrupt, and Reset Packets
Interrupt, Interrupt Confirm, Reset and Reset Confirm packets are
sent over the TCP connection using the normal X.25 packet formats and
state transitions. The end-to-end nature of both the Interrupt and
Reset services MUST be maintained for correct X.25 operation.
6.4 Restart, DTE Reject, Diagnostics, and Registration
X.25 packets that have only a local DTE/DCE interface significance
(Restart, Restart Confirm, DTE Reject, Diagnostic, Registration
Request and Registration Confirmation) MUST NOT be sent over the TCP
connection. If one of these packets is received, then it MUST be
6.5 PVC Setup
An XOT implementation MAY support connecting a PVC via XOT.
X.25 PVCs are Virtual Circuits that are presumed to be available
when the X.25 service is available (i.e., in the R1 state).
Connecting a PVC via XOT is complicated because no Call, Call
Confirm, Clear or Clear Confirm packets are transferred (or
allowed) across the X.25 interface--PVCs are simply available
because they have been provisioned by the network provider as
contracted for by the network users.
Supporting a PVC using XOT requires a data exchange between the
XOT entities that is outside the scope of the X.25 standards, and
must provide for a number of error conditions.
The setup of a PVC between two XOT entities is performed by
exchanging a non-standard X.25 packet type (encapsulated in an XOT
Header); the PVC setup exchange takes place immediately after a new
TCP XOT connection has been established. The XOT implementation that
initiated the TCP connection is the initiator; the other XOT is the
The PVC Setup packet includes the X.25 General Format Identifier, LCN
and Packet Type Identifier fields followed by additional data. This
non-standard packet type takes the form:
| X.25 GFI | X.25 LCN |
| X.25 PTI | PVC setup PTI (= 0xF5)
| | version (= 0x81)
| | status
| | initiator interface name length (N)
| | initiator LCN (high octet)
| | initiator LCN (low octet)
| | responder interface name length (M)
| | responder LCN (high octet)
| | responder LCN (low octet)
| | sender incoming window
| | sender outgoing window
| | sender incoming max. packet size
| | sender outgoing max. packet size
| | initiator interface name (N octets)
| | responder interface name (M octets)
The PVC setup packet was designed so that the responder could
simply modify a few fields of the received packet and send it back
to the initiator.
The Packet Type Identifier was chosen from the unused X.25 PTI
values so it is distinct from the standard X.25 Packet Type
The PVC setup version value was chosen to prevent connections with
prior experimental implementations.
The PVC status field has the following values defined:
0x00 Waiting to connect
0x08 Destination disconnected
0x09 PVC/TCP connection refused
0x0A PVC/TCP routing error
0x0B PVC/TCP connect timed out
0x10 Trying to connect via TCP
0x11 Awaiting PVC-SETUP reply
0x13 No such destination interface
0x14 Destination interface is not up
0x15 Non-X.25 destination interface
0x16 No such destination PVC
0x17 Destination PVC configuration mismatch
0x18 Mismatched flow control values
0x19 Can't support flow control values
0x1A PVC setup protocol error
Not all of the PVC status values are appropriate for a PVC setup
packet; these values represent a particular implementation that
chose to assign values in three groups that correspond to a short
timer for a connect attempt (0x00 through 0x07), a long timer for
a connect attempt (0x08 through 0x0F) and no attempt to connect
(greater than 0x0F). The values that are appropriate for a PVC
setup packet are 0x00 and those values greater than 0x12.
Most of the PVC status error values that may be found in a setup
message are self-explanatory, with a few exceptions. The value
0x17, "Destination PVC configuration mismatch" may returned in the
case that the targeted PVC already has an XOT PVC connection
active. The value 0x19, "Can't support flow control values", may
be returned when the flow control values match but, for instance,
a modulo 8 interface is requested to set up a PVC with a window
size greater than 7 or an interface is requested to set up a PVC
with a maximum packet size that is too large for its data link
layer to transfer.
An XOT MAY retry a failed PVC setup; if implemented the XOT SHOULD
wait between attempts (5 minutes is suggested).
Each XOT PVC is configured with the identity of the other XOT (i.e.,
IP address), the name of the interface to connect to, the Logical
Channel Number on that interface and the flow control values to use.
These data are present in the PVC setup packets and the responding
XOT verifies the configurations are compatible.
The interface name fields are the ASCII names of the two interfaces
involved. These names SHOULD be case-insensitive. There MUST NOT be
any padding or trailing zero octets between or after the interface
The flow control values are the values from the perspective of the
local interface of the XOT implementation that sent the PVC setup
packet. The maximum packet size values are encoded as they are in
the packet size facility, (i.e., the base-2 log of the size in
octets, so 7 represents a maximum packet size of 128 octets). If the
responding XOT implements end-to-end flow control, it will require
that the configured flow control values be complimentary, so a
returned status of 0x18 will indicate the values required by the
responding XOT (note that the incoming value of one local interface
corresponds to the outgoing value of the connecting local interface,
After establishing the TCP connection the initiator sends a PVC setup
packet, the status value MUST be 0x00; the responder will reply with
its own PVC setup packet or by closing the TCP connection. An XOT
PVC setup is successful if the responder returns a status of 0x00.
Once the XOT PVC connection is successfully established, each XOT
MUST complete a Reset procedure on the local interface, so if each
local interface LCI is in state D1, a Reset packet would be generated
both to the local interface and the XOT TCP connection.
An XOT PVC connection is broken by simply closing the TCP connection;
X.25 packets that are not legal for PVCs MUST NOT be transferred
across an XOT PVC connection. When a local interface undergoes the
Restart procedure, the XOT PVC connections MUST be either perform a
Reset (which is appropriate if the interface remains in state R1) or
close the XOT PVC connection.
An XOT implementation SHOULD also consider how a PVC setup
collision will be handled. Receipt of an XOT PVC setup for a PVC
that is itself attempting to setup an XOT connection could either
accept a (valid) setup attempt and, if two TCP XOT connections
result, simply use one connection to send XOT data (XOT MUST NOT
send traffic over both) and accept XOT data on either, or it can
close the incoming attempt and, if no connections result, retry
the connection after waiting for a random interval. If two
connections are allowed for a PVC, closure of one SHOULD result in
the closure of the other.
Greg Satz is the original designer and implementor of X.25 over TCP.
Aviva Garrett of cisco Systems reviewed the specification and made
many editorial corrections.
8. Security Considerations
Security issues are not discussed in this memo.
 Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
 CCITT, Blue Book Volume VIII--Fascicle VIII.2, "Data
Communication Networks: Services and Facilities, Interfaces";
Recommendation X.25, "Interface Between Data Circuit-Terminating
Equipment (DCE) for Terminals Operating in the Packet Mode and
Connected to Public Data Networks by Dedicated Circuit", 1989,
10. Authors' Addresses
James R. Forster
1525 O'Brien Dr.
Menlo Park. CA. 94025
1525 O'Brien Dr.
Menlo Park. CA. 94025
1525 O'Brien Dr.
Menlo Park. CA. 94025
Joint Network Team
c/o Rutherford Appleton Laboratory
Oxfordshire OX11 0QX