RFC 891 (rfc891) - Page 2 of 25

DCN local-network protocols

D.L. Mills

which is given a unique 8-bit identifier called its port ID.  Every
DCN physical host contains one or more internet processes, each of
which supports a virtual host given a unique 8-bit identifier called
its host ID.

     Each virtual host can support multiple internet protocols,
connections and, in addition, a virtual clock.  Each physical host
contains a physical clock which can operate at an arbitrary rate and,
in addition, a 32-bit logical clock which operates at 1000 Hz and is
assumed to be reset each day at 0000 hours UT.  Not all physical hosts
implement the full 32-bit precision; however, in such cases the
resolution of the logical clock may be somewhat less.

     There is a one-to-one correspondence between Internet addresses
and host IDs.  The host ID is formed from a specified octet of the
Internet address to which is added a specified offset.  The octet
number and offset are selected at configuration time and must be the
same for all DCN hosts sharing the local net.  For class-B and class-C
nets normally the fourth octet is used in this way for routing within
the local net.  In the case of class-B nets, the third octet is
considered part of the net number by DCN hosts; therefore, this octet
can be used for routing between DCN local nets.  For class-A nets
normally the third octet (ARPANET logical-host field) is used for
routing where necessary.

     Each DCN physical host is identified by a host ID for the purpose
of detecting loops in routing updates, which establish the
minimum-delay paths between the virtual hosts.  By convention, the
physical host ID is assigned as the host ID of one of its virtual
hosts.  A link to a neighbor net is associated with a special virtual
host, called a gateway, which is assigned a unique host ID.

     The links connecting the various physical hosts together and to
foreign nets can be distributed in arbitrary ways, so long as the net
remains fully connected.  If full connectivity is lost, due to a link
or host fault, the virtual hosts in each of the surviving segments can
continue to operate with each other and, once connectivity is
restored, with all of them.

     Datagram routing is determined entirely by internet address -
there is no local leader as in the ARPANET.  Each physical host
contains two tables, the Host Table, which is used to determine the
outgoing link to each other local-net host, and the Net Table, which
is used to determine the outgoing host (gateway) to each other net.
The Host Table contains estimates of roundtrip delay and logical-clock
offset for all virtual hosts in the net and is indexed by host ID.
For the purpose of computing these estimates the delay and offset of
each virtual host relative to the physical host in which it resides is
assumed zero.  The single exception to this is a special virtual host
associated with an NBS radio time-code receiver, where the offset is
computed relative to the broadcast time.

     The Net Table contains an entry for every neighbor net that may
be connected to the local net and, in addition, certain other nets
that are not

DCN Local-Network Protocols