cisco Systems X.25 over TCP (XOT)

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
   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:

      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
         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
      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.

   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
      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.

   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.

   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.

      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
      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.

   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.

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