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+.TH CONS 4P "9 December 1988"
+.ds ]W Wisconsin ARGO 1.0
+.UC 4
+.SH NAME
+CONS \- Connection Oriented Network Service 
+.SH SYNOPSIS
+For use as a network service (CONS):
+.nf
+.sp
+\fB#include <sys/socket.h>\fR
+\fB#include <sys/mbuf.h>\fR
+\fB#include <netargo/iso.h>\fR
+\fB#include <netargo/cons.h>\fR
+\fB#include <netargo/iso_errno.h>\fR
+.sp
+\fBint cons_output(isop, m, len, isdatagram)\fR
+.sp
+or for use as a subnetwork service (COSNS):
+.sp
+\fB#include <sys/socket.h>\fR
+\fB#include <sys/mbuf.h>\fR
+\fB#include <netargo/iso.h>\fR
+\fB#include <net/if.h>\fR
+\fB#include <netargo/cons.h>\fR
+\fB#include <netargo/iso_errno.h>\fR
+.sp
+\fBint cosns_output(ifp, m, dst)
+.fi
+.SH DESCRIPTION
+.PP
+The Connection Oriented Network Service (CONS) implemented for the AOS R2
+at the University of Wisconsin - Madison
+supports transport protocols, acting as a network service,
+and it also supports other network protocols, acting as a subnetwork
+service or link-layer service.
+Several software modules are combined to provide these services.
+.TP 10
+X.25
+The CCITT X.25 packet layer and link layer protocols run on
+a coprocessor (the EICON Network Adapter), which serves as a DTE.
+.TP 10
+Ecn driver
+A device driver manages the interaction between
+the coprocessor and the PC/RT.
+.TP 10
+CONS "glue"
+A software module implements portions of the OSI CONS (ISO 8878),
+which describes a way to use the X.25 protocols to support the
+OSI connection-oriented network service.
+.PP
+The OSI CONS contains several "service elements"
+that ARGO does not use or support.
+Expedited data,
+quality of service  maintenance, 
+call collision resolution,
+permanent virtual circuits,
+user data on connect and release,
+user-level acknowledgement
+("receipt confirmation" in CCITT/ISO argot), and reset/resynchronize
+are not supported.
+Several of the service primitives for connection establishment
+and release are not supported, and 
+numerous parameters to other primitives specified in the OSI CONS
+are not supported.
+The CONS glue does provide all the support necessary to run
+ISO transport classes 0 and 4 over X.25, and ISO CLNP (also called
+ISO IP) over X.25.
+The subnetwork dependent convergence functions implemented in the glue
+permit interoperability with
+OSINET and EAN at this writing.
+Interoperability with other networks will be established in the future.
+.PP
+The coprocessor that implements the X.25 link and packet layers
+is the Eicon Technologies Access/X.25 Stand-Alone Network Adaper 
+(see \fIecn(4)\fR).
+.PP
+The glue module provides two interfaces to higher layers:
+a "subnetwork service" (COSNS) used by network layer protocols, which
+has a typical BSD kernel device driver interface
+and
+a "network service" (CONS) used by transport protocols, which has
+a procedure call interface similar to that of IP and CLNP.
+.PP
+The network service is reliable and sequenced but does not
+provide a graceful close service; it provides only an abort service.
+.PP
+The subnetwork service is neither reliable nor sequenced.
+The subnetwork service implemented by the glue hides the
+connection-oriented aspects of the protocols; nevertheless, 
+we call it the "connection-oriented subnetwork service" (COSNS)
+here, for lack of a better name.
+.SS "LIBRARIES
+No libraries are needed to use the CONS, however,
+the numerous error values returned by X.25 cannot be accommodated
+by the standard \fIperror()\fR in the C library.
+The ISO library 
+.nf
+.sp
+.in +5
+\fC/usr/argo/lib/libisodir.a\fR
+.in -5
+.sp
+.fi
+provides an expanded perror() to handle the additional error return codes.
+.SS "ERROR VALUES
+.PP
+The error codes returned by the CONS are taken from
+the diagnostic code of the X.25 level 3 packets, as
+descibed in figure 14-B of ISO 8208 (the ISO standard which
+is equivalent to CCITT X.25).
+The actual error value returned in 
+\fIerrno\fR
+is the X.25 diagnostic code in the lower 10 bits
+logically "or"ed with the hexadecimal value 0x8400 (bits
+10 and 15 set, counting from zero at the the least significant bit).
+The error values can be found in 
+the file
+.nf
+.in +5
+.sp
+\fC<netargo/iso_errno.h>\fR
+.sp
+.in -5
+.fi
+.SS "PROTOCOL IDENTIFICATION
+.PP
+The purpose of this section is to describe how incoming packets
+are forwarded from the glue to the various higher
+layers (ISO transport, CLNP), how
+routes are chosen from the higher layers to the glue, and
+how NSAP-addresses are related to all this.
+.SS Outgoing path:
+The ARGO transport entity routes packets either to
+the CONS glue, to the CLNP module, or to the DARPA Internet IP
+module, based on the value of the network service parameter
+given to the transport layer by the user.
+The \fInetserv\fR property of records in the ARGO
+directory service database
+can be used to determine the network service to be used by the
+transport layer.  See also \fIisodir(5)\fR and \fIisodir(3)\fR.
+.PP
+The connectionless network layer entity routes packets to the 
+COSNS based on the routing table entries in the connectionless network layer.
+This means that any type of NSAP-address supported by the kernel
+may be used with a CLNP packet
+that is routed over X.25.
+.PP
+When the glue creates an X.25 Call Request packet, it
+places an X.121 address (DTE address)
+in both the Calling and Called DTE address fields.
+The X.121 addresses are extracted from the \fISNPA cache\fR,
+a table that maps NSAP-addresses to SNPA-addresses, and
+is maintained by the ES-IS protocol module of the OSI network layer.
+In addition to placing a DTE address in the X.25 packet,
+the "glue" may 
+uses the 1984 Called Address Extension facility to convey the 
+NSAP-addresses.
+Whether or not this is done depends on the compile-time option -DX25_1984.
+.SS Incoming path:
+The X.25 Call Request User Data field and the
+1984 X.25 Address Extension Facility are used
+to determing the incoming path through the network layer.
+The NSAP addresses passed up along with the packet are taken from the
+Address Extension facility, if present.
+If the facility is absent, the glue creates two type-37 NSAP-addresses,
+filling in the X.121 address from the called and
+calling DTE-addresses on the Call Request packet, if present.
+The glue then requests of the ES-IS module to add an entry to the 
+SNPA cache to associate the calling DTE address with its 
+derived NSAP-address.
+These cache entries have a holding of 5 minutes, and get
+refreshed as long as there is activity on the virtual circuit
+resulting from the call request.
+.PP
+If a Call Request packet contains a protocol identifier
+as described in ISO PTDR 9577, this protocol identifier is used
+to route the packet to the higher layers.
+If there is no protocol identifier, the higher layer is assumed to be ISO
+transport.
+.SH "BUGS
+.PP
+If an incoming X.25 Call Request contains no DTE-addresses and 
+no NSAP-addresses (in the Address Extension facility)
+the kernel panics.
+.SH "SEE ALSO
+.PP
+isodir(3),
+ecn(4),
+clnp(4),
+tp(4),
+isodir(5),
+isoroute(8),
+ifconfig(8),
+netstat(1),
+xstat(8),
+"ARGO 1.0 Kernel Programmer's Manual"