Fascicle VIII.5 - Rec. X.244

                                 Fascicle VIII.5 - Rec. X.244



Recommendation X.244
                                 
   PROCEDURE FOR THE EXCHANGE OF PROTOCOL IDENTIFICATION DURING
VIRTUAL CALL ESTABLISHMENT ON PACKET SWITCHED PUBLIC DATA NETWORKS
                                 
                    (Malaga-Torremolinos, 1984)

     The CCITT,

considering

      (a)    that Recommendation X.25 defines the interface between
data   terminal   equipment  (DTE)  and  data   circuit-terminating
equipment  (DCE)  for terminals operating in the  packet  mode  and
connected to public data networks by dedicated circuit;

      (b)    that Recommendation X.32 defines the interface between
data   terminal   equipment  (DTE)  and  data   circuit-terminating
equipment  (DCE)  for terminals operating in the  packet  mode  and
accessing  a packet switched public data network through  a  public
switched network;

      (c)   that Recommendation X.29 defines the procedures for the
exchange  of  control information and user data  between  a  packet
assembly/disassembly  facility (PAD)  and  a  packet  mode  DTE  or
another PAD;

      (d)    that Recommendation X.224 defines the Transport  Layer
Protocol  for  Open Systems Interconnection for CCITT Applications,
which  includes  provision of a Transport Protocol  Identification;
and

      (e)    that  there is a need to identify and/or negotiate  at
virtual  call establishment the protocol which is to operate  above
the packet level,

unanimously recommends

      that for data terminal equipment operating in the packet mode
the mechanism for identification and/or negotiation of the protocol
which  is  to operate above the packet level should be as specified
below.


1    Introduction

     This Recommendation defines the procedures for the exchange of
protocol identification during virtual call establishment on packet
switched  public data networks. This procedure operates  above  the
packet level of Recommendation X.25. It makes use of bits 8  and  7
of  the  first  octet  of the user data field in  the  call  set-up
packets defined in Recommendation X.25.


2    Call user data field

     The following procedure applies to the call user data field of
Call Request and Incoming Call packets.

      If the call user data field is present, the use and format of
this  field  are determined by bits 8 and 7 of the first  octet  of
this field (see Note below).

     If bits 8 and 7 of the first octet of the call user data field
are  00, a portion of the call user data field is used for protocol
identification  in  accordance with other Recommendations  such  as
Recommendation X.29 and Recommendation X.224.

     If bits 8 and 7 of the first octet of the call user data field
are  01,  a  portion of the call user data field may  be  used  for
protocol  identification  in  accordance  with  specifications   of
Administrations.

     If bits 8 and 7 of the first octet of the call user data field
are  10,  a  portion of the call user data field may  be  used  for
protocol  identification  in  accordance  with  specifications   of
international user bodies.

     If bits 8 and 7 of the first octet of the call user data field
are  11,  no  constraints are placed on the use by the DTE  of  the
remainder of the call user data field.

     Users are cautioned that if bits 8 and 7 of the first octet of
the  call  user data field have any value other than 11, a protocol
may be identified that is implemented within public data networks.

      Note  - When a virtual call is being established between  two
packet mode DTEs, the network does not act on any part of the  call
user data field.


3    Called user data field

      The following procedure applies to the called user data field
of  Call  Accepted and Call Connected packets used  in  conjunction
with the fast select facility.

      If  the called user data field is present, the use and format
of  this field is determined by bits 8 and 7 of the first octet  of
this field (see Note below).

      If  bits  8 and 7 of the first octet of the called user  data
field  are 00, a portion of the called user data field is used  for
protocol identification in accordance with other Recommendations.

      If  bits  8 and 7 of the first octet of the called user  data
field  are 01, a portion of the called user data field may be  used
for  protocol  identification in accordance with specifications  of
Administrations.

      If  bits  8 and 7 of the first octet of the called user  data
field  are 10, a portion of the called user data field may be  used
for  protocol  identification in accordance with specifications  of
international user bodies.

      If  bits  8 and 7 of the first octet of the called user  data
field  are 11, no constraints are placed on the use by the  DTE  of
the remainder of the called user data field.

     Users are cautioned that if bits 8 and 7 of the first octet of
the called user data field have any value other than 11, a protocol
may be identified that is implemented within public data networks.

      Note  - When a virtual call is being established between  two
packet  mode  DTEs, the network does not act on  any  part  of  the
called user data field.