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INTERNET STANDARD

Network Working Group                                   D.L.  Mills
Request for Comments:  891                              December 1983


                         DCN Local-Network Protocols

This RFC is a description of the protocol used in the DCN local
networks to maintain connectivity, routing, and timekeeping
information.  These procedures may be of interest to designers and
implementers of other networks.

1.  Introduction

     This document describes the local-net architecture and protocols
of the Distributed Computer Network (DCN), a family of local nets
based on Internet technology and an implementation of PDP11-based
software called the Fuzzball.  DCN local nets have been in operation
for about three years and now include clones in the USA, UK, Norway
and West Germany.  They typically include a number of PDP11 or LSI-11
Fuzzballs, one of which is elected a gateway, and often include other
Internet-compatible hosts as well.

     The DCN local-net protocols are intended to provide connectivity,
routing and timekeeping functions for a set of randomly connected
personal computers and service hosts.  The design philosophy guiding
the Fuzzball implementation is to incorporate complete functionality
in every host, which can serve as a packet switch, gateway and service
host all at the same time.  When a set of Fuzzballs are connected
together using a haphazard collection of serial, parallel and
contention-bus interfaces, they organize themselves into a network
with routing based on minimum delay.

     The purpose of this document is to describe the local-net
protocols used by the DCN to maintain connectivity, routing and
timekeeping functions.  The document is an extensive revision and
expansion of Section 4.2 of [1] and is divided into two parts, the
first of which is an informal description of the architecture,
together with explanatory remarks.  The second part consists of a
semi-formal specification of the entities and protocols used to
determine connectivity, establish routing and maintain clock
synchronization and is designed to aid in the implementation of cohort
systems.  The link-level coding is described in the appendix.

2.  Narrative Description

     The DCN architecture is designed for local nets of up to 256
hosts and gateways using the Internet Protocol (IP) and client
protocols.  It provides adaptive routing and clock synchronization
functions in an arbitrary topology including point-to-point links and
multipoint bus systems.  It is intended for use in connecting personal
computers to each other and to service machines, gateways and other
hosts of the Internet community.  However, it is not intended for use
in large, complex networks and does not support the sophisticated
routing and control algorithms of, for example, the ARPANET.

     A brief description of the process and addressing structure used
in the DCN may be useful in the following.  A DCN physical host is a
PDP11-compatible processor which supports a number of cooperating
sequential processes, each of
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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
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neighbors.  Each entry contains the net number, as well as the host ID
of the local-net gateway to that net.  The routing function simply
looks up the net number in the Net Table, finds the host ID of the
gateway and retrieves the port ID of the net-output process from the
Host Table.  Other information is included in the Host Table and Net
Table as described below.

     The delay and offset estimates are updated by HELLO messages
exchanged on the links connecting physical-host neighbors.  The HELLO
messages are exchanged frequently, but not so as to materially degrade
the throughput of the link for ordinary data messages.  A HELLO
message contains a copy of the delay and offset information from the
Host Table of the sender, as well as information to compute the
roundtrip delay and logical-clock offset of the receiver relative to
the sender.

     The routing algorithm is similar to that (formerly) used in the
ARPANET and other places in that the roundtrip (biased) delay estimate
calculated to a neighbor is added to each of the delay estimates given
in its HELLO message and compared with the corresponding delay
estimates in the Host Table.  If a delay computed in this way is less
than the delay already in the Host Table, the routing to the
corresponding virtual host is changed accordingly.  The detailed
operation of this algorithm, which includes provisions for host
up-down logic and loop suppression, is summarized in a later section.

     DCN local nets are self-configuring for all hosts and neighbor
nets; that is, the routing algorithms will find minimum-delay paths
between all hosts and gateways to neighbor nets.  In addition,
timekeeping information can be exchanged using special HELLO messages
between neighboring DCN local nets.  For routing beyond neighbor nets
additional entries can be configured in the Net Tables as required.
In addition, a special entry can be configured in the Net Tables which
specifies the host ID of the gateway to all nets not explicitly
included in the table.

     For routing via the ARPANET and its reachable nets a selected
local-net host is equipped with an IMP interface and configured with a
GGP/EGP Gateway process.  This process maintains the Net Table of the
local host, including ARPANET leaders, dynamically as part of the
GGP/EGP protocol interactions with other ARPANET gateways.  GGP/EGP
protocol interactions are possibly with non-ARPANET gateways as well.

     The portable virtual-host structure used in the DCN encourages a
rather loose interpretation of addressing.  In order to minimize
confusion in the following, the term "host ID" will be applied only to
virtual hosts, while "host number" will be applied to the physical
host, called generically the DCN host.

2.1.  Net and Host Tables

     There are two tables in every DCN host which control routing of
Internet Protocol (IP) datagrams: the Net Table and the Host Table.
The Net Table is used to determine the host ID of the gateway on the
route to a foreign net,
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while the Host Table is used to determine the link, with respect to
the DCN host, on the route to a virtual host.  The Host Table is
maintained dynamically using updates generated by periodic HELLO
messages.  The Net Table is fixed at configuration time for all DCN
hosts except those that support a GGP/EGP Gateway process.  In these
cases the Net Table is updated as part of the gateway operations.  In
addition, entries in either table can be changed by operator commands.

     The Net Table format is shown in Figure 1.

                        1                   0
              5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |           Net Name            |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |    Net(2)     |    Net(1)     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |    Index      |    Net(3)     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |     Hops      |  Gateway ID   |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                               |
             |        Gateway Leader         |
             |                               |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1. Net Table Entry

     The "Net Name" field defines a short (RAD50) name for the net,
while the "Net" fields define the class A/B/C net number.  The
"Gateway ID" field contains the host ID of the first gateway to the
net and the "Hops" field the number of hops to it.  The remaining
fields are used only by the GGP/EGP Gateway process and include the
"Index" field, which contains an index into the routing matrix.  and
the "Gateway Leader" field, which contains the (byte-swapped)
local-net leader for the gateway on a neighbor net.

     The Net Table contains an indefinite number of entries and is
terminated by a special entry with all "Net" fields set to zero.  If
the "Hops" field of this entry is less than 255, the "Gateway ID"
field of this entry is used for all nets not in the table.  If the
"Hops" field is 255 all nets not explicitly mentioned in the table
appear unreachable.

     The Host Table format is shown in Figure 2.
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                        1                   0
              5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |             Name              |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |      TTL      |    Port ID    |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |             Delay             |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |             Offset            |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                               |
             +                               +
             |          Local Leader         |
             +                               +
             |                               |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                               |
             +        Update Timestamp       +
             |                               |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2. Host Table Entry

     The ordinal position of each Host Table entry corresponds to its
host ID.  The "Name" field contains a short (RAD50) name for
convenient reference.  The "Port ID" field contains the port ID of the
net-output process on the shortest path to this virtual host and the
"Delay" field contains the measured roundtrip delay to it.  The
"Offset" field contains the difference between the logical clock of
this host and the logical clock of the local host.  The "Local Leader"
field contains information used to construct the local leader of the
outgoing packet, for those nets that require it.  The "Update
Timestamp" field contains the logical clock value when the entry was
last updated and the "TTL" field the time (in seconds) remaining until
the virtual host is declared down.

     All fields except the "Name" field are filled in as part of the
routing update process, which is initiated upon arrival of a HELLO
message from a neighboring DCN host.  This message takes the form of
an IP datagram carrying the reserved protocol number 63 and a data
field as shown in Figure 3.
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                        1                   0
              5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
         --- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fixed        |           Checksum            |
Area         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |             Date              |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                               |
             +              Time             +
             |                               |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |           Timestamp           |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |     Offset    |   Hosts (n)   |
         --- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Host         |          Delay Host 0         |
Area         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |         Offset Host 0         |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...                             ...
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |         Delay Host n-1        |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |         Offset Host n-1       |
         --- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 3. HELLO Message Format

     There are two HELLO message formats, depending on the length of
the message.  One format, sent by a DCN host to another host on the
same local net, includes both the fixed and host areas shown above.
The second format, sent in all other cases, includes only the fixed
area.

     Note that all word fields shown are byte-swapped with respect to
the ordinary PDP11 representation.  The "Checksum" field contains a
checksum covering the fields indicated.  The "Date" and "Time" fields
are filled in with the local date and time of origination.  The
"Timestamp" field is used in the computation of the roundtrip delay
(see below).  The "Offset" field contains the offset of the block af
Internet addresses used by the local net.  The "Delay Host n" and
"Offset Host n" fields represent a copy of the corresponding entries
of the Host Table as they exist at the time of origination.  The
"Hosts (n)" field contains the number of entries in this table.

2.2.  Roundtrip Delay Calculations

     Periodically, each DCN physical host sends a HELLO message to its
neighbor on each of the communication links common to both of them.
For each of these links the sender keeps a set of state variables,
including a copy of the source-address field of the last HELLO message
received.
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When constructing a HELLO message the sender sets the
destination-address field to this state variable and the
source-address field to its own address.  It then fills in the "Date"
and "Time" fields from its logical clock and the "Timestamp" field
from another state variable.  It finally copies the "Delay" and
"Offset" values from its Host Table into the message.

     A host receiving a HELLO message discards it if the format is bad
or the checksum fails.  If valid, it initializes a link state variable
to show that the link is up.  Each time a HELLO message is transmitted
this state variable is decremented.  If it decrements to zero the link
is declared down.

     The host then checks if the source-address field matches the
state variable containing the last address stored.  If not, the link
has been switched to a new host, so the state variables are flushed
and the link forced into a recovery state.  The host then checks if
the destination-address field matches its own address.  If so, the
message has been looped (legal only in the case of a broadcast net)
and the roundtrip delay information is corrected.  The host and net
areas are ignored in this case.  If not, the host and net areas of the
message are processed to update the Host and Net Tables.

     Roundtrip delay calculations are performed in the following way.
The link input/output processes assigned each link maintain an
internal state variable which is updated as each HELLO message is
received and transmitted.  When a HELLO message is received this
variable takes the value of the "Time" field minus the current
time-of-day.  When the next HELLO message is transmitted, the value
assigned the "Timestamp" field is computed as the low-order 16-bits of
this variable plus the current time-of-day.  The value of this
variable is forced to zero if either the link is down of the system
logical clock has been reset since the last HELLO message was
received.

     If a HELLO message is received with zero "Timestamp" field, no
processing other than filling in the internal state variable.
Otherwise, the roundtrip delay is computed as the low-order 16-bits of
the current time-of-day minus the value of this field.  In order to
assure the highest accuracy, the calculation is performed only if the
length of the last transmitted HELLO message (in octets) matches the
length of the received HELLO message.

     The above technique renders the calculation independent of the
clock offsets and intervals between HELLO messages at either host,
protects against errors that might occur due to lost HELLO messages
and works even when a neighbor host simply forwards the HELLO message
back to the originator without modifying it.  The latter behavior,
typical of ARPANET IMPs and gateways, as well as broadcast nets, relies
on the loop-detection mechanism so that correct calculations can be
made and, furthermore, that spurious host updates can be avoided.

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2.3.  Host Updates

     When a HELLO message arrives which results in a valid roundtrip
delay calculation, a host update process is performed.  This consists
of adding the roundtrip delay to each of the "Delay Host n" entries in
the HELLO message in turn and comparing each of these calculated
delays to the "Host Delay" field of the corresponding Host Table
entry.  Each entry is then updated according to the following rules:

1.  If the link connects to another DCN host on the same net and the
    port ID (PID) of the link output process matches the "Port ID"
    field of the entry, then update the entry.

2.  If the link connects to another DCN host on the same net, the PID
    of the link output process does not match the "Port ID" field and the
    calculated delay is less than the "Host Delay" field by at least a
    specified switching threshold (currently 100 milliseconds), then
    update the entry.

3.  If the link connects to a foreign net and is assigned a host ID
    corresponding to the entry, then update the entry.  In this case
    only, use as the calculated delay the roundtrip delay.

4.  If none of the above conditions are met, or if the virtual host
    has been declared down and the "TTL" field contains a nonzero
    value, then no update is performed.

     The update process consists of replacing the "Delay" field with
the calculated delay, the "Port ID" field with the PID of the link
output process, the "Update Timestamp" field with the current time of
day and the "TTL" field by a specified value (currently 120) in
seconds.  If the calculated delay exceeds a specified maximum interval
(currently 30 seconds), the virtual host is declared down by setting
the corresponding "Delay" field to the maximum and the remaining
fields as before.  For the purposes of delay calculations values less
than a specified minimum (currently 100 milliseconds) are rounded up
to that minimum.

     The "Offset" field is also replaced during the update process.
When the HELLO message arrives, The value of the current logical clock
is subtracted from the "Time" field and the difference added to
one-half the roundtrip delay.  The resulting sum, which represents the
offset of the local clock to the clock of the sender, is added to the
corresponding "Offset" field of the HELLO message and the sum replaces
the "Offset" field of the Host Table.  Thus, the "Offset" field in the
Host Table for a particular virtual host is replaced only if that host
is up and on the minimum-delay path to the DCN host.

     The purpose of the switching threshold in (2) above and the
minimum delay specification in the update process is to avoid
unnecessary switching between links and transient loops which can
occur due to normal variations in propagation delays.  The purpose of
the "TTL" field test in (4) above is to ensure consistency by purging
all paths to a virtual host when that virtual host goes down.
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     In addition to the updates performed as HELLO messages arrive, each
virtual host in a DCN host also performs a periodic update of its own
Host Table entry.  The update procedure is identical to the above,
except that the calculated delay and offset are taken as zero.  At
least one of the virtual hosts in a DCN host must have the same host
ID as the host number assigned the DCN host itself and all must be
assigned the same net number.  Other than these, there are no
restrictions on the number or addresses of internet processes resident
in a single DCN host.

     It should be appreciated that virtual hosts are truly portable
and can migrate about the net, should such a requirement arise.  The
host update protocols described here insure that the routing
procedures always converge to the minimum-delay paths via operational
links and DCN hosts.  In the case of broadcast nets such as Ethernets,
the procedures are modified slightly as described below.  In this case
the HELLO messages are used to determine routing from the various
Ethernet hosts to destinations off the cable, as well as to provide
time synchronization.

2.4.  Timeouts

     The "TTL" field in every Host Table entry is decremented once a
second in normal operation.  Thus, if following a host update another
update is not received within an interval corresponding to the value
initialized in that field, it decrements to zero, at which point the
virtual host is declared down and the Host Table entry set as
described above.  The 120-second interval used currently provides for
at least four HELLO messages to be generated by every neighbor on
every link during that interval, since the maximum delay between HELLO
messages is 30 seconds on the lowest-speed link (1200 bps).  Thus, if
no HELLO messages are lost, the maximum number of links between any
virtual host and any other is four.

     The "TTL" field is initialized at 120 seconds when an update
occurs and when the virtual host is declared down.  During the
interval this field decrements to zero immediately after being
declared down, updates are ignored.  This provides a decent interval
for the bad news to propagate throughout the net and for the Host
Tables in all DCN hosts to reflect the fact.  Thus, the formation of
routing loops is prevented.

     The IP datagram forwarding procedures call for decrementing the
"time-to-live" field in the IP header once per second or at each point
where it is forwarded, whichever comes first.  The value used
currently for this purpose is 30, so that an IP datagram can live in
the net no longer than that number of seconds.  This is thus the
maximum delay allowed on any path between two virtual hosts.  If this
maximum delay is exceeded in calculating the roundtrip delay for a
Host Table entry, the corresponding virtual host will be declared
down.

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     The interval between HELLO messages on any link depends on the
data rate supported by the link.  As a general rule, this interval is
set at 16 times the expected roundtrip time for the longest packet to
be sent on that link.  For 1200-bps asynchronous transmission and
packet lengths to 256 octets, this corresponds to a maximum HELLO
message interval of about 30 seconds.

     Although the roundtrip delay calculation, upon which the routing
process depends, is relatively insensitive to net traffic and
congestion, stochastic variations in the calculated values ordinarily
occur due to coding (bit or character stuffing) and medium
perturbations.  In order to suppress loops and needless path changes a
minimum switching threshold is incorporated into the routing mechanism
(see above).  The interval used for this threshold, as well as for the
minimum delay on any path, is 100 milliseconds.

3.  Formal Specification

     The following sections provide a formal framework which describe
the DCN HELLO protocol.  This protocol is run between neighboring DCN
hosts that share a common point-to-point link and provides automatic
connectivity determination, routing and timekeeping functions.

     The descriptions to follow are organized as follows: First a
summary of data structures describes the global variables and packet
formats.  Then three processes which implement the protocol are
described: the CLOCK, HELLO and HOST processes.  The description of
these processes is organized into sections that describe (1) the local
variables used by that process, (2) the parameters and constants and
(3) the events that initiate processing together with the procedures
they evoke.  In the case of variables a distinction is made between
state variables, which retain their contents between procedure calls,
and temporaries, which have a lifetime extending only while the
process is running.  Except as noted below, the initial contents of
state variables are unimportant.

3.1.  Data Structures

3.1.1.  Global Variables

ADDRESS
    This is a 32-bit bit-string temporary variable used to contain an
    Internet address.


CLOCK-HID
    This is an eight-bit integer state variable used to contain the
    host ID of the local-net host to be used as the master clock.  It
    is initialized to the appropriate value depending upon the net
    configuration.

DATE
    This is a 16-bit bit-string state variable used to contain the
    date in RT-11 format.  Bits 0-4 contain the year, with zero
    corresponding to 1972, bits 5-9 contain the day of the month and
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    bits 10-14 contain the month, starting with one for January.

DATE-VALID
    This is a one-bit state variable used to indicate whether the
    local date and time are synchronized with the master clock.  A
    value of one indicates the local clock is not synchronized with
    the master clock.  This variable is set to one initially and when
    the local time-of-day rolls over past midnight.  It is set to zero
    each time a valid date and time update has been received from the
    master clock.

DELAY
    This is a 16-bit integer temporary variable which represents the
    roundtrip delay in milliseconds to a host.

HID
    This is an eight-bit integer temporary variable containing the
    host ID of some host on the local net.

    There is a one-to-one correspondence between the Internet
    addresses of local hosts and their HIDs.  The mapping between them
    is selected on the basis of the octet number of the Internet
    address.  For DCN hosts it is the fourth octet, while for hosts
    directly connected to a class-A ARPANET IMP or gateway, it is the
    third octet (logical-host field).  The contents of this octet are
    to be added to ADDRESS-OFFSET to form the HID associated
    with the address.

HOLD
    This is an eight-bit counter state variable indicating whether
    timestamps are valid or not.  While HOLD is nonzero, timestamps
    should be considered invalid.  When set to some nonzero value, the
    counter decrements to zero at a 1-Hz rate.  Its initial value is
    zero.

HOST-TABLE
    This is a table of NHOSTS entries indexed by host ID (HID).  There
    is one entry for each host in the local net.  Each entry has the
    following format:

    HOST-TABLE.DELAY
        This is a 16-bit field containing the computed roundtrip delay
        in milliseconds to host HID.

    HOST-TABLE.OFFSET
        This is a 16-bit field containing the computed signed offset
        in milliseconds which must be added to the local apparent
        clock to agree with the apparent clock of host HID.

    HOST-TABLE.PID
        This is an eight-bit field containing the PID of the net-output
        process selected by the routing algorithm to forward packets
        to host HID.

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 HOST-TABLE.TTL
     This is an eight-bit field used as a time-to-live indicator.
     It is decremented by the HOST process once each second and
     initialized to a chosen value when a HELLO message is
     received. The table is initialized with the HOST-TABLE.DELAY
     field set to  MAXDELAY for all entries.  The contents of the
     other fields are unimportant.

LOCAL-ADDRESS
    This is a 32-bit bit-string state variable used to contain the
    local host Internet address.

NET-TABLE
    This is a table of NNETS entries with the following format:

    NET-TABLE.HID
        This is an eight-bit field containing the host ID of the
        pseudo-process to forward packets to the NET-TABLE.NET net.

    NET-TABLE.NET
        This is a 24-bit field containing an Internet class-A (eight
        bits), class-B (16 bits) or class-C (24 bits) net number.
        Note that the actual field width for class-B net numbers is 24
        bits in order to provide a subnet capability, in which the
        high-order eight bits of the 16-bit host address is
        interpreted as the subnet number.

    The table is constructed at configuration time and must include an
    entry for every net that is a potential neighbor.  A neighbor net
    is defined as a net containing a host that can be directly
    connected to a host on the local net.  The entry for such a net is
    initialized with NET-TABLE.NET set to the neighbor net number and
    NET-TABLE.HID set to an arbitrary vitual-host ID not assigned any
    other local-net virtual host.

    The remaining entries in NET-TABLE are initialized at initial-boot
    time with the NET-TABLE.NET fields set to zero and the
    NET-TABLE.HID fields set to a configuration-selected host ID to be
    used to forward packets to all nets other than neighbor nets.  In
    the case where a gateway module is included in the local host
    configuration, the GGP and/or EGP protocols will be used to
    maintain these entries;  while, in the case where no gateway
    module is included, only one such entry is required.

OFFSET
    This is a 16-bit signed integer temporary variable which
    represents the offset in milliseconds to be added to the apparent
    clock time to yield the apparent clock time of the neighbor host.

3.1.2.  Parameters

ADDRESS-OFFSET
    This is an integer which represents the value of the Internet
    address field corresponding to the first host in HOST-TABLE.
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NHOSTS
    This is an integer which defines the number of entries in HOST-TABLE.

NNETS
    This is an integer which defines the number of entries in MET-TABLE.

3.1.3.  HELLO Packet Fields

PKT.ADDRESS-OFFSET
    This eight-bit is copied from ADDRESS-OFFSET by the sender.

PKT.DATESTAMP
    Bits 0-14 of this 16-bit field are copied from DATE by the sender,
    while bit 15 is copied from DATE-VALID.

PKT.DATE-VALID
    This one-bit field is bit 15 of PKT.DATESTAMP.

PKT.DESTINATION
    This 32-bit field is part of the IP header.  It is copied from
    HLO.NEIGHBOR-ADDRESS by the sender.

PKT.HOST-TABLE
    This is a table of PKT.NHOSTS entries, each entry of which
    consists of two fields.  The entries are indexed by host ID and
    have the following format:

    PKT.HOST-TABLE.DELAY
        This 16-bit field is copied from the corresponding HOST-TABLE.DELAY
        field by the sender.

    PKT.HOST-TABLE.OFFSET
        This 16-bit field is copied from the corresponding HOST-TABLE.OFFSET
        field by the sender.

PKT.LENGTH
    This 16-bit field is part of the IP header.  It is set by the sender to
    the number of octets in the packet.

PKT.NHOSTS
    This eight-bit field is copied from NHOST by the sender.

PKT.SOURCE
    This 16-bit field is part of the IP header.  It is copied from
    LOCAL-ADDRESS by the sender.

PKT.TIMESTAMP
    This 32-bit field contains the apparent time the packet was transmitted
    in milliseconds past midnight UT.

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PKT.TSP
    This 16-bit field contains a variable used in roundtrip delay
    calculations.

3.2 CLOCK Process (CLK)

     The timekeeping system maintains three clocks: (1) the physical
clock, which is determined by a hardware oscillator/counter; (2) the
apparent clock, which maintains the time-of-day used by client
processes and (3) the actual clock, which represents the time-of-day
provided by an outside reference.  The apparent and actual clocks are
maintained as 48-bit quantities with 32 bits of significance available
to client processes.  These clocks run at a rate of 1000 Hz and are
reset at midnight UT.

     The CLOCK process consists of a set of state variables along with
a set of procedures that are called as the result of hardware
interrupts and client requests.  An interval timer is assumed
logically separate from the local clock mechanism, although both could
be derived from the same timing source.

3.2.1.  Local Variables

CLK.CLOCK
    This is a 48-bit fixed-point state variable used to represent the
    apparent time-of-day.  The decimal point is to the right of bit 16
    (numbering from the right at bit 0).  Bit 16 increments at a rate
    equivalent to 1000 Hz independent of the hardware clock.  (In the
    case of programmable-clock hardware the value of CLK.CLOCK must be
    corrected as described below.)

CLK.COUNT
    This is a hardware register that increments at rate R.  It can be
    represented by a simple line clock, which causes interrupts at the
    line-frequency rate, or by a programmable clock, which contains a 16-bit
    register that is programmed to count at a 1000-Hz rate and causes an
    interrupt on overflow.  The register is considered a fixed-point variable
    with decimal point to the right of bit 0.

CLK.DELTA
    This is a 48-bit signed fixed-point state variable used to represent the
    increment to be added to CLK.CLOCK to yield the actual time-of-day.  The
    decimal point is to the right of bit 16.

3.2.3.  Parameters

ADJUST-FRACTION
    This is an integer which defines the shift count used to compute a
    fraction that is used as a multiplier of CLK.DELTA to correct CLK.CLOCK
    once each clock-adjust interval.  A value of seven is suggested.

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ADJUST-INTERVAL
    This is an integer which defines the clock-adjust interval in
    milliseconds.  A value of 500 (one-half second) is suggested for
    the line clock and 4000 (four seconds) for the 1000-Hz clock.

CLOCK-TICK
    This is a fixed-point integer which defines the increment in
    milliseconds to be added to CLK.CLOCK as the result of a clock
    tick.  The decimal point is to the right of bit 16.  In the case
    of a line-clock interrupt, the value of CLOCK-TICK should be
    16.66666 (60 Hz) or 20.00000 (50 Hz).  In the case of a 1000-Hz
    programmable-clock overflow, the value should be 65536.00000.

HOLD-INTERVAL
    This is an integer which defines the number of seconds that HOLD will
    count down after CLK.CLOCK has been reset.  The resulting interval must be
    at least as long as the maximum HELLO-INTERVAL used by any HELLO process.

3.2.3.  Events and Procedures

INCREMENT-CLOCK Event
    This event is evoked as the result of a tick interrupt, in the case of a
    line clock, or a counter overflow, in the case of the 1000-Hz clock.  It
    causes the logical clock to be incremented by the value of CLOCK-TICK.

    1.  Add the value of CLOCK-TICK to CLK.CLOCK.

ADJUST-CLOCK Event
    This event is evoked once every ADJUST-INTERVAL milliseconds to slew the
    apparent clock time to the actual clock time as set by the SET-CLOCK
    procedure.  This is done by subtracting a fraction of the correction
    factor CLK.DELTA from the value of CLK.DELTA and adding the same fraction
    to CLK.CLOCK.  This continues until either the next SET-CLOCK call or
    CLK.DELTA has been reduced to zero.

    The suggested values for ADJUST-INTERVAL and ADJUST-FRACTION
    represent a maximum slew rate of less than +-2 milliseconds per
    second, in the case of 1000-Hz clock.  The action is to smooth
    noisy clock corrections received from neighboring systems to
    obtain a high-quality local reference, while insuring the apparent
    clock time is always monotonically increasing.

    1.  Shift the 48-bit value of CLK.DELTA arithmetically ADJUST-FRACTION
        bits to the right, discarding bits from the right and saving the
        result in a temporary variable F.  Assuming the decimal point of F to
        be positioned to the right of bit 16 and sign-extending as necessary,
        subtract F from CLK.DELTA and add F to CLK.CLOCK.

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DECREMENT-HOLD Event
    This event is evoked once per second to decrement the value of HOLD.

    1.  If the value of HOLD is zero, do nothing;  otherwise, decrement its
        value.

READ-CLOCK Procedure

    This procedure is called by a client process.  It returns the apparent
    time-of-day computed as the integer part of the sum CLK.CLOCK plus
    CLK.COUNT.  Note that the precision of the value returned is limited to
    +-1 millisecond, so that client processes must expect the apparent
    time to "run backward" occasionally due to drift corrections.  When
    this happens the backward step will never be greater than one
    millisecond and will never occur more often than twice per second.

    1.  In the case of line clocks CLK.COUNT is always zero, while in
        the case of programmable clocks the hardware must be
        interrogated to extract the value of CLK.COUNT.  If following
        interrogation a counter-overflow condition is evident, add
        CLOCK-TICK to CLK.CLOCK and interrogate the hardware again.

    2.  When the value of CLK.COUNT has been determined compute the sum
        CLK.COUNT + CLK.CLOCK.  If this sum exceeds the number of
        milliseconds in 24 hours (86,400,000), reduce CLK.CLOCK by
        86,400,000, set HOLD-INTERVAL -> HOLD, set CLOCK-VALID (bit 15
        of DATE) to one, roll over DATE to the next calendar day and
        start over.  If not, return the integer part of the sum as the
        apparent time-of-day.

        The CLOCK-VALID bit is set to insure that a master-clock update is
        received at least once per day.  Note that, in the case of
        uncompensated crystal oscillators of the type commonly used as the
        1000-Hz time base, a drift of several parts per million can be
        expected, which would result in a time drift of several tenths of a
        second per day, if not corrected.

SET-CLOCK Procedure
    This procedure is called by a client process.  It sets a time-of-day
    correction factor in milliseconds.  The argument represents a 32-bit
    signed fixed-point quantity with decimal point to the right of bit
    0 that is to be added to CLK.CLOCK so that READ-CLOCK subsequently
    returns the actual time-of-day.

    1.  If the correction factor is in the range -2**(16-ADJUST-FRACTION) to
        +2**(16-ADJUST-FRACTION) - 1 (about +-128 milliseconds with the
        suggested value of ADJUST-FRACTION), the value of the argument
        replaces CLK.DELTA and the procedure is complete.  If not, add the
        value of the sign-extended argument to CLK.CLOCK and set CLK.DELTA to
        zero.  In addition, set HOLD-INTERVAL -> HOLD, since this
        represents a relatively large step-change in apparent time.
        The value of HOLD represents the remaining number of seconds
        in which timestamps should be considered invalid and is used
        by the HELLO process to suppress roundtrip delay calculations
        which might involve invalid timestamps.
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D.L. Mills



3.3.  HELLO Process

     The HELLO process maintains clock synchronization with a neighbor
HELLO process using the HELLO protocol.  It also participates in the
routing algorithm.  There is one HELLO process and one set of local
state variables for each link connecting the host to one of its
neighbors.

3.3.1.  Local variables

HLO.BROADCAST
    This is a one-bit switch state variable.  When set to zero a
    point-to-point link is assumed.  When set to one a broadcast (e.g.
    Ethernet) link is assumed.

HLO.KEEP-ALIVE
    This is an eight-bit counter state variable used to indicate whether the
    link is up.  It is initialized with a value of zero.

HLO.LENGTH
    This is a 16-bit integer state variable used to record the length in
    octets of the last HELLO message sent.

HLO.NEIGHBOR-ADDRESS
    This is a 32-bit integer state variable used to contain the neighbor host
    Internet address.

HLO.PID
    This is an eight-bit integer state variable used to identify the
    net-output process associated with this HELLO process.  It is initialized
    by the kernel when the process is created and remains unchanged
    thereafter.

HLO.POLL
    This is a one-bit switch state variable.  When set the HELLO process
    spontaneously sends HELLO messages.  When not set the HELLO process
    responds to HELLO messages, but does not send them spontaneously.

HLO.TIMESTAMP
    This is a 32-bit integer temporary variable used to record the time of
    arrival of a HELLO message.

HLO.TSP
    This is a 16-bit signed integer state variable used in roundtrip delay
    calculations.

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3.3.2.  Parameters

HELLO-INTERVAL
    This is an integer which defines the interval in seconds between HELLO
    messages.  It ranges from about eight to a maximum of 30 seconds,
    depending on line speed.

HOLD-DOWN-INTERVAL
    This is an integer which defines the interval in seconds a host will be
    considered up following receipt of a HELLO message indicating that
    host is up.  A value of 120 is suggested.

KEEP-ALIVE-INTERVAL
    This is an integer which defines the interval, in units of
    HELLO-INTERVAL, that a HELLO process will consider the link up.  A
    value of four is suggested.

MAXDELAY
    This is an integer which defines the maximum roundtrip delay in
    seconds on a path to any reachable host.  A value of 30 is suggested.

MINDELAY
    This is an integer which defines the minimum switching threshold in
    milliseconds below which routes will not be changed.  A value of 100 is
    suggested.

3.3.3.  Events and Procedures

INPUT-PACKET Event
    When a packet arrives the net-input process first sets HLO.TIMESTAMP to
    the value returned by the READ-CLOCK procedure, then checks the
    packet for valid local leader, IP header format and checksum.  If
    the protocol field in the IP header indicates a HELLO message, the
    packet is passed to the HELLO process.  If any errors are found
    the packet is dropped.

    The HELLO process first checks the packet for valid HELLO header format
    and checksum.  If any errors are found the packet is dropped.  Otherwise,
    it proceeds as follows:

    1.  If PKT.SOURCE is equal to LOCAL-ADDRESS, then the line to the
        neighbor host is looped.  If this is a broadcast link
        (HLO.BROADCAST is set to one), then ignore this nicety;  if
        not, this is considered an error and further processing is
        abandoned.  Note that, in special configurations involving
        other systems (e.g.  ARPANET IMPs and gateways) it may be
        useful to use looped HELLO to monitor connectivity.  The DCN
        implementation provides this feature, but is not described here.

    2.  Set KEEP-ALIVE-INTERVAL -> HLO.KEEP-ALIVE.  This indicates the
        maximum number of HELLO intervals the HLO.TSP field is
        considered valid.

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    3.  Set PKT.TIMESTAMP - HLO.TIMESTAMP -> HLO.TSP.  This is part of the
        roundtrip delay calculation.  The value of HLO.TSP will be
        updated and returned to the neighbor in the next HELLO message
        transmitted.  Next, compute the raw roundtrip delay and offset:
        HLO.TIMESTAMP - PKT.TSP -> DELAY and HLO.TSP + DELAY/2 -> OFFSET.
        Note:  in the case of a broadcast link (HLO.BROADCAST set to one) set
        DELAY to zero.

    4.  Perform this step only in the case of non-broadcast links
        (HLO.BROADCAST set to zero).  If PKT.SOURCE is not equal to
        HLO.NEIGHBOR-ADDRESS, then a new neighbor has appeared on this
        link. Set PKT.SOURCE -> HLO.NEIGHBOR ADDRESS, MAXDELAY ->
        DELAY and proceed to the next step.  This will force the line
        to be declared down and result in a hold-down cycle.
        Otherwise, if either PKT.TSP is zero or HOLD is nonzero, then
        the DELAY calculation is invalid and further processing is
        abandoned.  Note that a hold-down cycle is forced in any
        case if a new neighbor is recognized.

    5.  If processing reaches this point the DELAY and OFFSET
        variables can be assumed valid as well as the remaining data
        in the packet.  First, if DELAY < MINDELAY, set MINDELAY ->
        DELAY.  This avoids needless path switching when the
        difference in delays is not significant and has the effect
        that on low-delay links the routing algorithm degenerates to
        min-hop rather than min-delay.  Then set HLO.PID -> PID.  There are
        two cases:

        Case 1:  PKT.NHOSTS is zero.
            This will be the case when the neighbor host has just come up or
            is on a different net or subnet.  Set NEIGHBOR-ADDRESS -> ADDRESS
            and call the ROUTE procedure, which will return the host
            ID.  Then call the UPDATE procedure.  In the case of
            errors, do nothing but return.

        Case 2:  PKT.NHOSTS is nonzero.
            This is the case when the neighbor host is on the same net or
            subnet.  First, save the values of DELAY and OFFSET in temporary
            variables F and G.  Then, for each value of HID from zero to
            NHOSTS-1 consider the corresponding PKT.HOSTS-TABLE entry and do
            the following:  Set F + PKT.HOST-TABLE.DELAY -> DELAY and
            G + PKT.HOST-TABLE.OFFSET -> OFFSET and call the UPDATE procedure.
            This completes processing.

        ROUTE Procedure
            This procedure returns the host ID in HID of the host represented
            by the global variable ADDRESS.

    1.  First, determine if the host represented by ADDRESS is on the same
        local net as LOCAL-ADDRESS.  For the purposes of this
        comparison bits 0-7 and 16-31 are compared for class-A nets
        and bits 8-31 are compared for class-B and class-C nets.  This
        provides for a subnet capability, where the bits 0-7 and 16-23
        (class-A) or 8-15 (class-B) are used as a subnet number.
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        Case 1:  The host is on the same net or subnet.
            Extract the address field of ADDRESS, subtract ADDRESS-OFFSET and
            store the result in HID.  If 0 <= HID < NHOSTS, the procedure
            completes normally;  otherwise it terminates in an error
            condition.

        Case 2:  The host is not on the same net or subnet.
            Search the NET-TABLE for a match of the net fields of
            LOCAL-ADDRESS and NET-TABLE.NET.  If found set
            NET-TABLE.HID -> HID and return normally.  If the NET-TABLE.NET
            field is zero, indicating the last entry in the table, set
            HET-TABLE.HID -> HID and return normally.  Note that, in the case
            of hosts including GGP/EGP gateway modules, if no match is found
            the procedure terminates in an error condition.

UPDATE Procedure
    This procedure updates the entry of HOST-TABLE indicated by HID using
    three global variables:  DELAY, OFFSET and PID.  Its purpose is to update
    the HOST-TABLE entry corresponding to host ID HID.  In the following all
    references are to this entry.

    1.  If PID is not equal to HOST-TABLE.PID, the route to host HID is not
        via the net-output process associated with this HELLO process.  In
        this case, if DELAY + MINDELAY > HOST-TABLE.DELAY, the path is longer
        than one already in HOST-TABLE, so the procedure does nothing.

    2.  This step is reached only if either the route to host HID is via the
        net-output process associated with this HELLO process or the newly
        reported path to this host is shorter by at least MINDELAY.
        There are two cases:

        Case 1:  HOST-TABLE.DELAY < MAXDELAY.
            The existing path to host HID is up and this is a point-to-point
            link (HLO.BROADCAST is set to zero).  If DELAY < MAXDELAY the
            newly reported path is also up.  Proceed to the next step.
            Otherwise, initiate a hold-down cycle by setting
            MAXDELAY -> HOST-TABLE.DELAY and
            HOLD-DOWN-INTERVAL -> HOST-TABLE.TTL and return.

        Case 2:  HOST-TABLE.DELAY >= MAXDELAY.
            The existing path to host HID is down.  If DELAY < MAXDELAY and
            HOST-TABLE.TTL is zero, the hold-down period has expired and the
            newly reported path has just come up.  Proceed to the next step.
            Otherwise simply return.

    3.  In this step the HOST-DELAY entry is updated.  Set
        DELAY -> HOST-TABLE.DELAY, HOLD-DOWN-INTERVAL -> HOST-TABLE.TTL and
        HLO.PID -> HOST-TABLE.PID.

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    4.  For precise timekeeping, the offset can be considered valid only if
        the length of the last HELLO packet transmitted is equal to
        the length of the last one received.  Thus, if HLO.LENGTH
        equal to PKT.LENGTH, set OFFSET -> HOST-TABLE.OFFSET;
        otherwise, leave this field alone. Finally, if HID is equal to
        CLOCK-HID and bit 15 (the DATE-VALID bit)
        of DATE is zero, set PKT.DATESTAMP -> DATE and call the SET-CLOCK
        procedure of the CLOCK process with argument HLO.TIMESTAMP.

OUTPUT-PACKET Event
    This event is evoked once every HELLO-INTERVAL seconds.  It determines if
    a HELLO message is to be transmitted, transmits it and updates state
    variables.

    1.  If HLO.KEEP-ALIVE is nonzero decrement its value.

    2.  If HLO.POLL is zero and HLO.KEEP-ALIVE is zero, do not send a HELLO
        message.  If either is nonzero initialize the packet fields as
        follows:  LOCAL-ADDRESS -> PKT.SOURCE,
        HLO.NEIGHBOR-ADDRESS -> PKT.DESTINATION and DATE -> PKT.DATESTAMP.
        Note:  PKT.DESTINATION is set to zero if this is a broadcast link
        (HLO.BROADCAST set to one).  Also, note that bit 15 of DATE is the
        DATE-VALID bit.  If this bit is one the receiver will not update its
        master clock from the information in the transmitted packet.
        This is significant only if the sending host is on the
        least-delay path to the master clock.  Set PKT.TIMESTAMP to
        the value returned from the READ-CLOCK procedure.  If
        HLO.KEEP-ALIVE is zero or HOLD is nonzero, set PKT.TSP to
        zero;  otherwise, set PKT.TIMESTAMP + HLO.TSP -> PKT.TSP.

    3.  Determine if the neighbor is on the same net or subnet.  If the
        neighbor is on a different net set PKT.NHOSTS to zero and
        proceed with the next step.  Otherwise, set NHOSTS ->
        PKT.NHOSTS and for each value of HID from zero to PKT.HOSTS-1
        copy the HOST-TABLE.DELAY and HOST-TABLE.OFFSET fields of the
        corresponding HOST-TABLE entry in order into the packet.  For
        each entry copied test if the HOST-TABLE.PID field matches the
        HLO.PID of the HELLO process.  If so, a potential routing loop
        is possible.  In this case use MAXDELAY for the delay field in
        the packet instead.

    4.  Finally, set HLO.LENGTH to the number of octets in the packet
        and send the packet.

3.4.  HOST Process (HOS)

     This process maintains the routing tables.  It is activated once per
second to scan HOST-TABLE and decrement the HOST-TABLE.TTL field of each
entry.  It also performs housekeeping functions.

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3.4.1.  Local variables

HOS.PID
    This is an eight-bit integer used to identify the HOST process.  It is
    initialized by the kernel when the process is created and remains
    unchanged thereafter.

HOS.HID
    This is an eight-bit temporary variable.

3.4.2.  Events and Procedures

SCAN Event
    This event is evoked once each second to scan the HOST-TABLE and perform
    housekeeping functions.

    1.  For each value of a temporary variable F from zero to NHOSTS-1 do the
        following:  Set LOCAL-ADDRESS -> ADDRESS and call the ROUTE
        procedure, which will return the host ID HID.  If F is equal
        to HID, then set both DELAY and OFFSET to zero, HOS.PID -> PID
        and call the UPDATE procedure.  This will cause all packets
        received with the local address to be routed to this process.

        If HOST-TABLE.TTL is zero skip this step.  Otherwise, decrement
        HOST-TABLE.TTL by one.  If the result is nonzero skip the
        remainder of this step.  Otherwise, If HOST-TABLE.DELAY <MAXDELAY set
        HOLDOFF-INTERVAL -> HOST-TABLE.TTL and MAXDELAY -> HOST-TABLE.DELAY.
        The effect of this step is to declare a hold-down cycle when a host
        goes down.

4.  References

1.  Mills, D.L.  Final Report on Internet Research, ARPA Packet Switching
    Program.  Technical Report TSLAB 82-7, COMSAT Laboratories,
    December 1982.
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D.L. Mills


Appendix A.  Link-Level Packet Formats

A.1.  Serial Links Using Program-Interrupt Interfaces

     Following is a description of the frame format used on
asynchronous and synchronous serial links with program-interrupt
interfaces such as the DEC DLV11 and DPV11.  This format provides
transparency coding for all messages, including HELLO messages, but
does not provide error detection or retransmission functions.  It is
designed to be easily implemented and compatible as far as possible
with standard industry protocols.

     The protocol is serial-by-bit, with the same interpretation on
the order of transmission as standard asynchronous and synchronous
interface devices; that is, the low-order bit of each octet is
transmitted first.  The data portion of the frame consists of one
Internet datagram encoded according to a "character-stuffing"
transparency convention:

1.  The frame begins with the two-octet sequence DLE-STX, in the case of
    asynchronous links, or the four-octet sequence SYN-SYN-DLE-STX, in the
    case of synchronous links.  The data portion is transmitted next,
    encoded as described below, followed by the two-octet sequence
    DLE-ETX.  No checksum is transmitted or expected.  If it is
    necessary for any reason to transmit time-fill other than in the
    data portion, the DEL (all ones) is used.

2.  Within the data portion of the frame the transmit buffer is
    scanned for a DLE.  Each DLE found causes the sequence DLE-DLE to
    be transmitted.  If it is necessary for some reason for the
    transmitter to insert time-fill within the data portion, the
    sequence DLE-DEL is used.

3.  While scanning the data stream within the data portion of the
    frame the sequence DLE-DLE is found, a single DLE is inserted in
    the receive buffer.  If the sequence DLE-ETX is found, the buffer
    is passed on for processing. The sequence DLE-DEL is discarded.
    Any other two-octet sequence beginning with DLE and ending with
    other than DLE, ETX or DEL is considered a protocol error
    (see note below).

     Note: In the case of synchronous links using program-interrupt
interfaces such as the DPV11, for example, a slightly modified
protocol is suggested when both ends of the link concur.  These
interfaces typically provide a parameter register which can be loaded
with a code used both to detect the receiver synchronizing pattern and
for time-fill when the transmit buffer register cannot be serviced in
time for the next character.

     The parameter register must be loaded with the SYN code for this
protocol to work properly.  However, should it be necessary to
transmit time-fill, a single SYN will be transmitted, rather than the
DLE-DEL sequence specified.  Disruptions due to these events can be
minimized by use of the following rules:
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1.  If the transmitter senses a time-fill condition (usually by a
    control bit assigned for this purpose) between frames or
    immediately following transmission of a DLE, the condition is ignored.

2.  If the transmitter senses a time-fill condition at other times it sends
    the sequence DLE-CAN.

3.  If the receiver finds a SYN either between frames or immediately
    following DLE, the SYN is discarded without affecting sequence
    decoding.

4.  If the receiver finds the sequence DLE-CAN in the data portion, it
    discards the sequence and the immediately preceding octet.

     These rules will work in cases where a single SYN has been
inserted by the transmitter and even when a SYN has been inserted in
the DLE-CAN sequence.  If an overrun (lost data) condition is sensed
at the receiver, the appropriate action is to return to the
initial-synchronization state.  This should also be the action if any
code other than STX is found following the initial DLE.  or if any
code other than DLE, ETX, DEL or CAN is found following a DLE in the
data portion.

A.2.  Serial Links Using DDCMP Devices

     Following is a description of the frame format used on DEC DDCMP links
with DMA interfaces such as the DEC DMV11 and DMR11.  These interfaces
implement the DEC DDCMP protocol, which includes error detection and
retransmission capabilities.  The DDCMP frame format is as follows:

+-------------+-----+-----+-----+-----+-----+------+------+------+
| SYN SYN SOH |Count|Flag |Resp | Seq | Adr | CRC1 | Data | CRC2 |
+-------------+-----+-----+-----+-----+-----+------+------+------+
bits   24       14     2     8     8     8     16     ...    16

With respect to this diagram, each octet is transmitted starting from the
leftmost octet, with the bits of each octet transmitted low-order bit first.
The contents of all fields except the "Data" field are managed by the
interface.  The Internet datagram is placed in this field as-is, with no
character or bit stuffing (the extent of this field is indicated by the
interface in the "Count" field.

A.3.  Serial Links Using HDLC Devices

     Following is a description of the frame format used on HDLC links with
program-interrupt interfaces such as the DEC DPV11.

        +--------+--------+--------+--------+--------+--------+
        |  Flag  |  Addr  |  Ctrl  |  Data  |  CRC   |  Flag  |
        +--------+--------+--------+--------+--------+--------+
coding   01111110 00000000 00000000 xxxxxxxx cccccccc 01111110

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With respect to this diagram, each octet is transmitted starting from
the leftmost octet, with the bits of each octet transmitted low-order
bit first.  The code xxxxxxxx represents the data portion and cccccccc
represents the checksum.  The bits between the "Flag" fields are
encoded with a bit-stuffing convention in which a zero bit is stuffed
following a string of five one bits.  The "Addr" and "Ctrl" fields are
not used and the checksum is ignored.  The Internet datagram is placed
in the "Data" field, which must be a multiple of eight bits in length.

A.4.  ARPANET 1822 Links Using Local or Distant Host Interfaces

     Following is a description of the frame format used with ARPANET
1822 Local or Distant Host interfaces.  These interfaces can be used
to connect a DCN host to an ARPANET IMP, Gateway or Port Expander or
to connect two DCN hosts together.  When used to connect a DCN host to
an ARPANET IMP, Gateway or Port Expander, a 96-bit 1822 leader is
prepended ahead of the Internet datagram.  The coding of this leader
is as described in BBN Report 1822.  When used to connect two DCN
hosts together, no leader is used and the frame contains only the
Internet datagram.

A.5.  ARPANET 1822 Links Using HDH Interfaces

     Following is a description of the frame format used with ARPANET
1822 HDH interfaces.  These interfaces can be used to connect a DCN
host to an ARPANET IMP or Gateway or to connect two DCN hosts
together.  In either case, the frame format is as described in
Appendix J of BBN Report 1822.

A.6.  X.25 LAPB Links Using RSRE Interfaces

     Following is a description of the frame format used on X.25 LAPB
links with the Royal Signals and Radar Establishment interfaces.
These interfaces implement the X.25 Link Access Protocol - Balanced
(LAPB), also known as the frame-level protocol, using a frame format
similar to that described under A.3 above.  Internet datagrams are
placed in the data portion of I frames and encoded with the
bit-stuffing procedure described in A.3.  There is no packet-level
format used with these interfaces.

A.7.  Ethernet Links

     Following is a description of the frame format used on Ethernet links.

        +-----------+-----------+------+------+-----+
        | Dest Addr | Srce Addr | Type | Data | CRC |
        +-----------+-----------+------+------+-----+
bits          48          48       16     ...   32

With respect to this diagram, each field is transmitted starting from
the leftmost field, with the bits of each field transmitted low-order
bit first.  The "Dest Addr" and "Srce Addr" contain 48-bit Ethernet
addresses, while the "Type" field contains the assigned value for IP
datagrams (0800 hex) or for
DCN Local-Network Protocols                                        Page 26
D.L. Mills


ARP datagrams (0806 hex).  The Internet datagram is placed in the
"Data" field and followed by the 32-bit checksum.  The Address
Resolution Protocol (ARP) is used to establish the mapping between
Ethernet address and Internet addresses.


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