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+.NC "NSAP Addresses & Routing"
+.sh 1 "OSI Address Formats"
+.pp
+ARGO supports an ISO address family, AF_ISO, in addition to the
+DoD Internet address family, AF_INET.
+Addresses in the family AF_ISO 
+take the form described by ISO 8348/DAD2, which is an addendum to the
+OSI network service standard that describes network layer addressing.
+.sh 2 "ISO 8348/DAD2 Tutorial"
+.pp
+.\" FIGURE
+.\".so figs/osi_addr.nr
+.so figs/addrfmt.nr
+.CF
+shows the
+format of an OSI NSAP-address.
+The address has two major parts: the initial domain part
+(IDP) and the domain specific part (DSP). The IDP is further divided into
+two parts: the authority and format identifier (AFI) and the
+initial domain identifier (IDI). The AFI specifies the format of the
+IDI, the authority responsible for allocating values of the IDI, and
+the syntax of the DSP. The IDI specifies the network addressing domain
+from which DSP values are allocated, and the authority responsible for
+allocating DSP values.
+.sh 2 "Supported Formats"
+.pp
+ARGO supports three types of ISO NSAP-addresses:
+one type with AFI 37(hex) and two types with AFI 47(hex).
+.sh 3 "AFI 37"
+.pp
+This value of the AFI defines the IDI to be an X.121 address or DTE
+address. 
+The DTE address is encoded in binary coded decimal.  
+The DSP syntax is binary.
+This form is intended to be used when communicating 
+across a public data network (PDN). 
+The ARGO software and documentation
+refer to this type of NSAP-address as a
+\*(lqtype 37.\*(rq
+address.
+.sh 3 "AFI 47"
+.pp
+The value of 47 for the AFI defines the IDI to be a 4 digit International
+Code Designator (ICD) allocated according to ISO 6523. 
+ARGO support two
+ICD values.
+.sh 4 "ICD 0004"
+.pp
+The ICD value of 0004 is assigned to OSINET,
+an experimental OSI network overseen by 
+National Institute of Science and Technology.\**
+.(f
+\** formerly the National Bureau of Standards
+.)f
+When this style of NSAP-address
+is used, 
+the DSP is divided into four parts: an organization identifier (2 bytes),
+a subnet identifier (2 bytes), 
+an SNPA-part (4-8 bytes), and
+an NSAP selector (1 byte). 
+The use of these fields is defined by the OSINET steering committee.
+This type of address is known as an
+\*(lqOSINET\*(rq
+address.
+.sh 4 "ICD 0006"
+.pp
+The ICD value of 0006 is assigned to the Department of Defense (DoD).
+In ARGO, NSAP-addresses with an ICD value of 0006 
+are of the format defined in RFC 986, a proposal for embedding DARPA Internet
+addresses within an OSI NSAP-address. 
+In this case, the DSP takes the form:
+version (1 byte), DARPA Internet Address (4 bytes), upper layer protocol
+identifier (1 byte).
+This is called an
+\*(lqrfc986\*(rq
+address.
+.sh 1 "Internal Representation"
+.pp
+Internally, an NSAP address takes the form
+.(b
+\fC
+.TS
+tab(+);
+l s s s s.
+struct iso_addr {
+.T&
+l l l l l.
++u_char+isoa_afi;+/* authority &
++++format id */
++union+{+ 
+++struct addr_37+addr_37;+/* x.121 */
+++struct addr_osinet+addr_osinet;+/* OSINET */
+++struct addr_rfc986+addr_rfc986;+/* Internet*/
++}+isoa_u;
++u_char+isoa_len;+/* length */
+}
+.TE
+\fR
+.)b
+The field \fIisoa_afi\fR contains the AFI for the address. 
+The union
+\fIisoa_u\fR contains the remainder of the address. 
+The length of the entire address (the AFI, IDI, and DSP) is 
+stored in \fIisoa_len\fR.
+.sh 1 "Network Layer Routing"
+.pp
+Routing at the network layer is performed by the 
+routing procedure \fIrtalloc()\fR as described in Chapter 5. 
+\fIRtalloc()\fR was designed for used in the DoD Internet
+domain.
+An unfortunate
+effect of this is that routing decisions are based upon either the
+entire NSAP address or the network portion of the address. 
+The problem is defining the network portion of an NSAP address. 
+The location and extent of the 
+network portion of an NSAP address depends on the
+style of NSAP address. 
+This decision is made by the function \fIiso_netof()\fR.
+.sh 2 "Network Portion of Type 37 Addresses"
+.pp
+There is no network portion of an X.121 address. 
+In ARGO, the network portion of a type 37 address 
+is defined to be just the AFI. 
+The obvious consequence of this is that all type 37 addresses will 
+match all other type 37 addresses
+in a network-portion comparison.
+.sh 2 "Network Portion of OSINET Addresses"
+.pp
+The network portion of an OSINET address is the organization identifier and
+the subnet identifier.
+.sh 2 "Network Portion of RFC986 Addresses"
+.pp
+The network portion of an RFC986 address is the network portion of the
+embedded DARPA Internet address. 
+ARGO does not support subnetting, a method of subdividing Internet addresses.
+.sh 1 "NSAP Address / Subnetwork Point of Attachment Mapping"
+.pp
+In order to transmit a packet on a real subnetwork, the destination
+NSAP address
+must be mapped to an SNPA address. 
+An SNPA address is the real, "hardware" address
+of a system on a network. 
+This address corresponds to the 6 byte Ethernet or Token Ring
+Adapter address, 
+or to the DTE address, which may be up to 7 bytes
+long (14 decimal digits).
+.pp
+A table, \fIsnpa_cache\fR is kept in the kernel which contains the 
+translation between NSAP-addresses and SNPA-addresses. 
+This table is used by \fIiso_snparesolve()\fR whenever a 
+datagram must be dispatched.
+The table is maintained by the the ISO ES-IS protocol entity.
+Entries can be added and deleted
+by the user program \fIclnlutil(8)\fR and
+by the CONS entity.