rfc823.txt
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RFC 823
5 GATEWAY SOFTWARE
The DARPA Internet Gateway runs under the MOS operating
system [9] which provides facilities for:
o Multiple processes
o Interprocess communication
o Buffer management
o Asynchronous input/output
o Shareable real-time clock
There is a MOS process for each network that the gateway is
directly connected to. A data structure called a NETBLOCK
contains variables of interest for each network and pointers to
local network routines. Network processes run common gateway
code while network-specific functions are dispatched to the
routines pointed to in the NETBLOCK. There are also processes
for gateway functions which require their own timing, such as GGP
and HMP.
5.1 Software Structure
The gateway software can be divided conceptually into three
parts: MOS Device Drivers, Network software, and Shared Gateway
software.
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5.1.1 Device Drivers
The gateway has a set of routines to handle sending and
receiving data for each type of hardware interface. There are
routines for initialization, initiation, and interruption for
both the transmit and receive sides of a device. The gateway
supports the following types of devices:
a) ACC LSI-11 1822
b) DEC IMP11a 1822
c) ACC LHDH 1822
d) ACC VDH11E
e) ACC VDH11C
f) Proteon Ring Network
g) RSRE HDLC
h) Interlan Ethernet
i) BBN Fibernet
j) ACC XQ/CP X.25 **
k) ACC XQ/CP HDH **
5.1.2 Network Software
For each connected network, the gateway has a set of eight
routines which handle local network functions. The network
routines and their functions are described briefly below.
_______________
** Planned, not yet supported.
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Up.net Perform local network initialization such as
flapping the 1822 ready line.
Sg.net Handle specific local network timing functions
such as timing out 1822 Destination Deads.
Rc.net A message has been received from the network
interface. Check for any input errors.
Wc.net A message has been transmitted to the network
interface. Check for any output errors.
Rs.net Set up a buffer (or buffers) to receive messages
on the network interface.
Ws.net Transmit a message to the network interface.
Hc.net Check the local network header of the received
message. Perform any local network protocol
tasks.
Hb.net Rebuild the local network header.
There are network routines for the following types of
networks:
o Arpanet (a,b,c,k)
o Satnet (d,e,k)
o Proteon Ring Network (f)
o Packet Radio Network (a,b,c)
o Rsre HDLC Null Network (g)
o Ethernet (h)
o Fibernet (i)
o Telenet X.25 (j) **
Note: The letters in parentheses refer to the device drivers used
_______________
** Planned, not yet supported.
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for each type of network as described in the previous section.
5.1.3 Shared Gateway Software
The internet processing of a datagram is performed by a body
of code which is shared by the network processes. This code
includes routines to check the IP header, perform IP
fragmentation, calculate the IP checksum, forward a datagram, and
implement the routing, monitoring, and error reporting protocols.
5.2 Gateway Processes
5.2.1 Network Processes
When the gateway starts up, each network process calls its
local network initialization routine and read start routine. The
read start routine sets up two maximum network size buffers for
receiving datagrams. The network process then waits for an input
complete signal from the network device driver.
When a message has been received, the MOS Operating System
signals the appropriate network process with an input complete
signal. The network process wakes up and executes the net read
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complete routine. After the message has been processed, the
network process waits for more input.
The net read complete routine is the major message
processing loop in the gateway. The following actions are
performed when a message has been received:
o Call Local Network Read Complete Routine
o Start more reads
o Check local Network Header
o Check Internet header
o Check if datagram is for the gateway
o Forward the datagram if necessary
o Send ICMP error message if necessary.
5.2.2 GGP Process
The GGP process periodically sends GGP echos to each of the
gateway's neighbors to determine neighbor connectivity, and sends
interface status messages addressed to itself to determine
network connectivity. The GGP process also sends out routing
updates when necessary. The details of the algorithms currently
implemented by the GGP process are given in Section 4.4,
Gateway-to-Gateway Protocol, and in Appendix C.
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5.2.3 HMP Process
The HMP process handles timer-based gateway statistics
collection and the periodic transmission of traps.
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APPENDIX A. GGP Message Formats
Note that the GGP protocol is currently undergoing extensive
changes to introduce the Exterior Gateway Protocol facility; this
is the vehicle needed to permit gateways in other systems to
exchange routing information with the gateways described in this
document.
Each GGP message consists of an Internet header followed by
one of the messages explained below. The values (in decimal) in
the Internet header used in a GGP message are as follows.
Version 4.
IHL Internet header length in 32-bit words.
Type of Service 0.
Total Length Length of Internet header and data in
octets.
ID, Flags,
Fragment Offset 0.
Time to Live Time to live in seconds. This field is
decremented at least once by each
machine that processes the datagram.
Protocol Gateway Protocol = 3.
Header Checksum The 16 bit one's complement of the one's
complement sum of all 16-bit words in
the header. For computing the checksum,
the checksum field should be zero.
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Source Address The address of the gateway's interface
from which the message is sent.
Destination Address The address of the gateway to which the
message is sent.
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ROUTING UPDATE
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!Gateway Type ! unused (0) ! ; 2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence Number ! ; 2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! need-update ! n-distances ! ; 2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! distance 1 ! n1-dist ! ; 2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net11 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; bytes
! net12 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; bytes
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net1n1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; n1 nets at
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; dist 1
. ...
. ; ndist groups
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; of nets
! distance n ! nn-dist ! ; 2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! netn1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; bytes
! netn2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; bytes
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! netnnn !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; nn nets at
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; dist n
Gateway Type 12 (decimal)
Sequence Number The 16-bit sequence number used to
identify routing updates.
need-update An 8-bit field. This byte is set to 1
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