21-Oct-81 18:50:05-PDT,3037;000000000001
Mail-from: ARPANET host SRI-NIC rcvd at 21-Oct-81 1849-PDT
Date: 21 Oct 1981 1337-PDT
From: Francine Perillo <PERILLO at SRI-NIC>
Subject: ANEWS-9
To: anews1: ;


                  The ARPANET HDH Host Interface
 
There is a new alternative host interface for the ARPANET C/30 IMPs
called HDH (HDLC Host).  This interface method is similar to the
existing VDH (Very Distant Host) interface in that it provides for
reliable transmission of messages between the host and its IMP over a
communication circuit of arbitrary length.  As with VDH, the HDH
interface can be used with communication circuits that range in speed
from 9.6KB to 56KB. 
 
HDH is superior to the VDH interface in that it uses as a reliable
transmission protocol the HDLC protocol which is the link level control
procedure of the CCITT international standard X.25.  HDLC is supported
by a much wider range of vendor equipment than the special ARPANET VDH
protocol.  It is also technically superior to VDH in that it provides
for a window of up to seven outstanding frames instead of the two
allowed by VDH, thus increasing the potential throughput.  The HDH
interface is also capable of accepting a full-length ARPANET host/IMP
message in a single frame, where VDH always requires fragmentation into
buffer-sized frames.
 
The HDH protocol is composed of three layers: the reliable transmission
protocol layer (HDLC), an encapsulation layer, and the host/IMP layer
(1822).  The HDLC layer is a full implementation of the CCITT X.25 Level
2 only, it does NOT extend to the X.25 Level 3 protocol. The HDH level 2
is capable of operating in either LAPB or LAP mode.  In addition, the
framing and data transparency scheme can be either bit-oriented (HDLC)
or byte-oriented (Bisynch).  The encapsulation layer provides some
additional link-level functions not provided by the standard HDLC layer.
These include line quality monitoring and loopback mode.  This layer
operates in one of two modes, packet or message.  In packet mode,
host/IMP messages are segmented into a series of frames containing a
separate host/IMP leader and data packets no larger than 128 data bytes,
requiring up to nine frames per host/IMP message.  In message mode,
complete host/IMP messages can be sent in a single frame.  The host/IMP
layer is the standard ARPANET host/IMP protocol (1822) with 96-bit
leaders. 
 
The HDH interface will be available only on C/30 IMPs equipped with a
specialized I/O board that can accomodate up to 16 HDH ports.  Although
the VDH protocol will continue to be supported for some time, new hosts
are encouraged to adopt the HDH interface in order to gain increased
performance and to take advantage of standard vendor host hardware and
software.  The entire HDH protocol is documented in BBN Report 1822, the
host/IMP layer in Section 3, the encapsulation layer in Appendix J, and
the HDLC layer in Appendix K.  For more information contact Nancy Mimno
(Mimno@BBN-UNIX or 617-497-3623). 
 
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