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rfc:rfc1613

Network Working Group J. Forster Request for Comments: 1613 G. Satz Category: Informational G. Glick

                                                   cisco Systems, Inc.
                                                                R. Day
                                                                 JANET
                                                              May 1994
                 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

 1.  Introduction....................................................1
 2.  Conventions.....................................................2
 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
 7.  Acknowledgments................................................12
 8.  Security Considerations........................................12
 9.  References.....................................................12
10.  Authors' Addresses.............................................13

1. Introduction

 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 packets.
 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

Forster, Satz, Glick & Day [Page 1] RFC 1613 X.25 Over TCP (XOT) May 1994

 length field, which is used to separate the X.25 packets within the
 TCP stream.
 In general, the normal X.25 protocol packet formats and state
 transition rules apply to the X.25 layer in XOT.  Exceptions to this
 are noted.

2. Conventions

 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:

Forster, Satz, Glick & Day [Page 2] RFC 1613 X.25 Over TCP (XOT) May 1994

    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:
  1. ——————————–

| |

                |       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.

Forster, Satz, Glick & Day [Page 3] RFC 1613 X.25 Over TCP (XOT) May 1994

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
       be closed.

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
 SYN packet.
 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.
 DISCUSSION
    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

Forster, Satz, Glick & Day [Page 4] RFC 1613 X.25 Over TCP (XOT) May 1994

    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
 TCP connection.
 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
 identifier.

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.

Forster, Satz, Glick & Day [Page 5] RFC 1613 X.25 Over TCP (XOT) May 1994

 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
 packet.
 DISCUSSION
    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

 DISCUSSION
    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
    significance.
    Existing implementations of XOT perform end-to-end flow control.
    Data and flow control packets are simply transferred between the

Forster, Satz, Glick & Day [Page 6] RFC 1613 X.25 Over TCP (XOT) May 1994

    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
    P(R) values.
    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
 connection.
 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
 interface.
 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.

Forster, Satz, Glick & Day [Page 7] RFC 1613 X.25 Over TCP (XOT) May 1994

 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
 silently discarded.

6.5 PVC Setup

 An XOT implementation MAY support connecting a PVC via XOT.
    DISCUSSION
    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
 responder.

Forster, Satz, Glick & Day [Page 8] RFC 1613 X.25 Over TCP (XOT) May 1994

 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)
 |                          |
 +--+--+--+--+--+--+--+--+--+
 DISCUSSION
    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.

Forster, Satz, Glick & Day [Page 9] RFC 1613 X.25 Over TCP (XOT) May 1994

    The Packet Type Identifier was chosen from the unused X.25 PTI
    values so it is distinct from the standard X.25 Packet Type
    Identifiers.
    The PVC setup version value was chosen to prevent connections with
    prior experimental implementations.
 The PVC status field has the following values defined:
 Status    Meaning
 ------    --------------------------------------
  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
  0x12     Connected
  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
 DISCUSSION
    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

Forster, Satz, Glick & Day [Page 10] RFC 1613 X.25 Over TCP (XOT) May 1994

    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
 names.
 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,
 and vice-versa).
 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.

Forster, Satz, Glick & Day [Page 11] RFC 1613 X.25 Over TCP (XOT) May 1994

 DISCUSSION
    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.

7. Acknowledgments

 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.

9. References

 [1] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
     USC/Information Sciences Institute, July 1992.
 [2] 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,
     Geneva.

Forster, Satz, Glick & Day [Page 12] RFC 1613 X.25 Over TCP (XOT) May 1994

10. Authors' Addresses

     James R. Forster
     Engineering Dept.
     cisco Systems
     1525 O'Brien Dr.
     Menlo Park. CA. 94025
     Phone: 1.415.688.8245
     Fax:   1.415.688.8282
     EMail: forster@cisco.com
     Greg Satz
     Engineering Dept.
     cisco Systems
     1525 O'Brien Dr.
     Menlo Park. CA. 94025
     Phone: 1.415.688.8245
     Fax:   1.415.688.8282
     EMail: satz@cisco.com
     Gilbert Glick
     Engineering Dept.
     cisco Systems
     1525 O'Brien Dr.
     Menlo Park. CA. 94025
     Phone: 1.415.688.8245
     Fax:   1.415.688.8282
     EMail: gglick@cisco.com
     Bob Day
     Joint Network Team
     c/o Rutherford Appleton Laboratory
     Chilton
     Didcot
     Oxfordshire OX11 0QX
     United Kingdom
     Phone: 44.235.44.5163
     Fax:   44.235.44.6251
     EMail: R.Day@jnt.ac.uk

Forster, Satz, Glick & Day [Page 13]

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