TCP IP Users Guide
Charles W. Lynn, Jr.
Bolt Beranek and Newman, Inc.
50 Moulton Street
Cambridge MA 02238
May 1982
TCP IP Users Guide May 1982
Table of Contents
Introduction to TCP and IP . . . . . . . . . 2
Transmission Control Protocol . . . . . . . . 3
Creating a TCP Connection . . . . . . . . . 4
Connection Descriptor Block . . . . . . . . 6
Sending and Receiving Data . . . . . . . . 7
Buffer Headers . . . . . . . . . . . . 9
Closing a Connection . . . . . . . . . . 11
Connection Status . . . . . . . . . . . 12
Interrupts . . . . . . . . . . . . . . 13
Error Returns and Codes . . . . . . . . . 14
Options . . . . . . . . . . . . . . . 17
TCP Retransmission Parameters . . . . . . . 19
OPEN JSYS 742 . . . . . . . . . . . . 20
CLOSE JSYS 743 . . . . . . . . . . . . 23
SEND JSYS 740 . . . . . . . . . . . . 25
RECV JSYS 741 . . . . . . . . . . . . 27
ABORT JSYS 747 . . . . . . . . . . . . 29
STAT JSYS 745 . . . . . . . . . . . . 30
CHANL JSYS 746 . . . . . . . . . . . . 32
SCSLV JSYS 744 . . . . . . . . . . . . 33
TELNET Protocol . . . . . . . . . . . . 34
ATNVT JSYS 274 . . . . . . . . . . . . 35
Internet User Queues . . . . . . . . . . . 36
Queue Descriptor Block . . . . . . . . . . 38
Packet Buffer . . . . . . . . . . . . . 39
Ports . . . . . . . . . . . . . . . 41
ASNIQ JSYS 756 . . . . . . . . . . . . 42
RELIQ JSYS 757 . . . . . . . . . . . . 43
SNDIN JSYS 754 . . . . . . . . . . . . 44
RCVIN JSYS 755 . . . . . . . . . . . . 45
Appendix A Internet Packet Header Format . . . . 46
Appendix B Internet Address Formats . . . . . . 47
Appendix C IP Type of Service Field . . . . . . 48
Appendix D Logical Host Facility . . . . . . . 49
Appendix E IP Packet Demultiplexing . . . . . . 50
Appendix F TCP Retransmission Parameters . . . . 51
Appendix G TCP & IP Options . . . . . . . . 55
Appendix H Status Variable Names . . . . . . . 59
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TCP IP Users Guide May 1982
Introduction to TCP and IP
This document describes the TOPS20 user interface to the
DARPA-supported inter-networking protocols: Internet Protocol,
Internet Control Message Protocol, Transmission Control Protocol,
and related network functions. These protocols are available on
several computer systems: HP3000, IBM, MULTICS, PDP-11, TOPS20,
UNIX, and VAX as well as several minicomputers.
The Internet Protocol (IP) is designed for use in
interconnected systems of packet-switched computer communications
networks. It allows the transmission and routing of variable
length blocks of data called datagrams from sources to
destinations. Sources and destinations are hosts identified by
fixed-length addresses. The Internet Protocol is not designed to
be absolutely reliable: some packets may not be delivered and
some may be duplicated. They may also arrive in a different
order than that in which they were sent.
The Internet Control Message Protocol (ICMP) is used to
provide some feedback about problems in the communications
environment, to report errors in datagram processing, or to
supply more recent routing and flow control information. The
user does not interact directly with the ICMP.
The Transmission Control Protocol (TCP) uses the Internet
Protocol to provide users with a highly reliable, multiplexed,
full-duplex, connection-based, host-to-host communications
service. TCP is not, however, intended for applications for
which time is critical, such as digitized speech, or for
communication between more than two processes.
User application programs may interface at either the IP
level, through the use of Internet User Queues, or at the TCP
level. Two common applications which interface at the TCP level
are TELNET, a remote terminal access facility, and FTP, a file
transfer facility. Name and time servers are examples of
applications which typically interface at the IP level.
These protocols and their interfaces have evolved over several
years. This document describes the BBN TOPS20 JSYS interface.
DEC is implementing a TCP: device interface to TCP. It is not
described in this document. Since the interfaces are evolving,
application programs should be written using the symbols defined
in the MONSYM library and used in this document, instead of the
symbols' numeric value, in order to reduce program maintenance if
a value changes in a subsequent software release.
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TCP IP Users Guide May 1982
Introduction to TCP
The TCP allows two processes to create a connection in order
to exchange zero or more octets of data, where an octet consists
of eight data bits. A connection is uniquely specified by the
48-bit identifiers of its local and foreign ends. The 48-bit
identifier is composed of a 32-bit internet address, which
identifies the host, and a 16-bit port number, which identifies a
process at that host. Since connections are full-duplex, each
end may both send and receive data.
TCP data transfer has high reliability: both the headers and all
data octets are checksummed. If a packet is received and its
checksum is not correct, it is immediately discarded.
Each octet of data is conceptually assigned a 32-bit sequence
number. The sequence numbers are used by the receiver to discard
duplicated packets and to detect lost, or out-of-sequence
packets. The receipt of data is acknowledged by returning to the
sender the sequence number of the next octet the receiver wants
to receive; this implies that all lower numbered octets have
been received.
The sender continues to retransmit data until it is
acknowledged. The retransmission rate is a user-specified
parameter. If data unacknowledged for a period of time, the
connection is broken and the user is notified of a Transmission
Timeout error. This time period is also specified by the user.
TCP automatically provides flow control for each connection.
The data receiving TCP periodically notifies the sending TCP of
the number of additional data octets it is willing to accept (the
"window"). This number is based on the amount of buffer space
available at the receiving TCP. The sending TCP is not to exceed
this window.
The TOPS20 monitor has nine TCP related JSYSi which will be
explained in following sections. They are:
OPEN, to specify the ends of, and synchronize, a connection;
SEND, to send data to the foreign end;
RECV, to receive data from the foreign end;
CLOSE, to begin closing a connection;
ABORT, to break a connection;
STAT, to obtain status information about a connection or TCP;
CHANL, to specify interrupt channels;
ATNVT, to translate a connection into an NVT descriptor; and
SCSLV, to specify security levels.
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TCP IP Users Guide May 1982
Creating a TCP Connection
A connection is created by passing the address of a Connection
Descriptor block (which identifies the two ends of the
connection), a Transmission Timeout, and a Retransmission
Parameters word to the OPEN JSYS. A successful OPEN will return
a Job Connection Number or "JCN". The JCN is a small
job-relative number which is given to the other JSYSi to identify
the connection. (The use of a JCN with the TCP JSYSi is
analogous to the use of JFNs with the file system JSYSi. The DEC
user interface uses JFNs and file system JSYSi instead of the
JSYSi documented here.)
Each connection must be unique. This is guaranteed by the
selection of a 16-bit local port value. Unfortunately, there is
no easy way to find which port values are unused other than by
trial and error. Use of local port values less than 256 is
discouraged as they have been reserved for "well known"
functions. A suggested local port value contains the user's job
number in the left-most 8 bits and anything else in the
right-most 8 bits. If the connection is not unique, OPEN will
return a Connection Already Exists error and another local port
value should be selected and tried.
There are three basic ways to specify a connection. They are
a fully specified active open, a fully specified passive open,
and a partially wild passive open.
A fully specified active open is used to initiate a
connection; the identifier of the foreign end of the connection
must be known. In this case, the Connection Descriptor block
contains a non-zero, 16-bit local port; the foreign host's 32-bit
internet address; and the 16-bit foreign port. The TCP%FS flag
must also be set. It causes a SYN packet to be sent to the
foreign host/port specified in the Connection Descriptor block.
If the SYN packet is acknowledged and a SYN is received from the
foreign TCP, the connection is synchronized and OPEN succeeds.
If the foreign end of the connection does not exist, a RESET will
be returned causing the OPEN to fail.
A fully specified passive open is used to wait for a
connection request from a particular foreign TCP. If a
connection with a specific foreign host and port is desired, the
Connection Descriptor block specifies the desired local port,
foreign host, and foreign port. The TCP%FS flag is not set. In
this case, no SYN packet will be sent. TCP will wait
indefinitely until a SYN (or RESET) packet is received from the
foreign TCP. When a SYN is received, an acknowledgement/SYN
packet is returned to the foreign TCP; its acknowledgement of the
SYN successfully opens the connection.
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TCP IP Users Guide May 1982
Creating a TCP Connection
The third case is a partially wild passive open. It is used
to wait for a connection request from any one of a set of
possible foreign hosts and ports. If the foreign port entry in
the Connection Descriptor block is zero, any foreign port will be
accepted. Similarly, if the network field and/or host field in
the foreign internet address entry is zero, a connection request
by any foreign network and/or host will be accepted. A typical
example is the local host's Login responder which accepts
connection requests to local port 23 from any port at any
internet address. In this case, the local port entry is 23; the
foreign net, host, and port entries are zero.
The OPEN JSYS has both blocking and non-blocking forms which
are differentiated by the setting of the TCP%WT flag. The
blocking form (TCP%WT flag set) will not return control to the
user program until either the connection has been successfully
established or an error has occurred. The non-blocking form
(TCP%WT flag not set) will return a success indication
immediately (unless there was a syntax error or the Connection
Already Exists). The user can subsequently check whether the
connection has opened correctly by attempting to send or receive
data or requesting the connection status. A program interrupt
may also be enabled for the connection opening (or closing)
event.
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TCP IP Users Guide May 1982
Connection Descriptor Block
Information describing a connection is passed to TCP in a
Connection Descriptor block via the OPEN JSYS. The block
contains .TCPCS words. Unspecified words and bits should be
zero.
Word
Offset Bits Meaning
.TCPLH B4-35 The Local Host's internet address (a host may
have more than one). If the default (or only)
local host address is to be used, specify zero.
.TCPLP B20-35 A unique non-zero Local Port number. It is
recommended the left-most eight bits contain the
job number in order to avoid conflicts with other
connections. The GJINF JSYS can be used to
determine the job number.
.TCPFH B4-35 The Foreign Host's internet address, or, in a
passive open, zero to accept a connection from
any foreign host. It is possible to specify an
explicit network field and a zero host field to
restrict connections to hosts on a given network.
See Appendix B for a description of internet
addresses.
.TCPFP B20-35 Either a non-zero Foreign Port number, or, in a
passive open, zero to allow a connection from any
foreign port.
.TCPOP B0-17 The address of an IP Option block, or zero.
B18-35 The address of a TCP Option block, or zero. See
the section on Options and Appendix G for further
details about Option blocks.
.TCPIP IP datagram parameters:
1B1 Do-not-Fragment this datagram. If set, IP is not
allowed to fragment a datagram. Packets which
are too large for an intermediate network will be
discarded, not delivered (in fragments).
377B17 Maximum Time-to-Live, in seconds. Datagrams
which have not been delivered in the specified
number of seconds will be discarded. The default
value of 60 seconds is used if the field is zero.
377B35 IP Type-of-Service field; See Appendix C.
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TCP IP Users Guide May 1982
Sending and Receiving Data
Data is transferred between the user and TCP in buffers.
Associated with each buffer is a Buffer Header which contains
control information. Data to be sent to the foreign end is
placed into one or more buffers, the associated headers are
filled in, and the address of each Buffer Header is passed to TCP
using the SEND JSYS.
The TCP collects data from the user buffers and decides, based
on the current window, etc., when to create a packet and transmit
it to the foreign end. The user can force TCP to transmit data
in a buffer by setting the PUSH flag, TCP%PU, in the Buffer
Header before doing the SEND.
When TCP has finished extracting the data from a SEND buffer
it sets the DONE flag, TCP%DN, in the Buffer Header and
optionally generates an interrupt. It is also possible to cause
a process to block until a buffer has been processed by setting
the TCP%WT flag in AC1; the TCP%PU flag should also be set in the
Buffer Header. Beware of the trap of setting TCP%WT without
TCP%PU--the process may be blocked forever. The SEND will not
return control to the user program until the data has been placed
into a packet and, due to the presence of TCP%PU, sent.
Data is received from the foreign end by first passing TCP
(one or more) empty buffers using the RECV JSYS. The TCP%WT flag
may be specified if control should not be returned to the user
program until the buffer has been filled. When data arrives, TCP
places it into the buffers.
RECV buffers are returned to the user either when full or when
a packet arrives which has the PUSH flag set. Before returning a
buffer, TCP sets the TCP%DN flag and places an error code in the
Buffer Header. The TCP%PU flag will be set if the buffer is
being returned because the received packet had the PUSH flag set.
(The buffer might not be full in this case.)
It is only possible to send and receive whole octets. If an
application requires transfer of some number of bits which is not
a multiple of eight, then control information must be placed into
the data stream. Note the PUSH function cannot be used to signal
the other end of "record boundaries"; the number of (RECV)
buffers with TCP%PU set may be less than the number of PUSHes
received which may be less than the number of (SEND) buffers
which had the TCP%PU flag set.
Several buffers may be passed to TCP using SEND and/or RECV
if the non-blocking form of the calls is used. (TCP%WT was not
set in AC1.)
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TCP IP Users Guide May 1982
Sending and Receiving Data
TCP also provides an asynchronous signal mechanism called
URGENT. If a (SEND) Buffer Header has the TCP%UR flag set, an
urgent signal is associated with the last data octet in the
buffer. Its sequence number is communicated to the receiving
TCP. The receiving user will then find the TCP%UR flag set in
all RECV Buffer Headers whose associated data buffer contains
lower sequence numbered octets. An interrupt may be enabled for
the Urgent data has arrived event. If the sender signals another
URGENT, the new sequence number is used. The receiver may only
"see" one URGENT if data is being processed slowly or the system
is heavily loaded.
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TCP IP Users Guide May 1982
Buffer Headers
Data is transferred between the user program and TCP in
buffers. Each Data Buffer contains zero or more octets of data,
packed four octets per word, in bits 0-31. Data is transferred
between TCPs beginning with the left-most bit of the left-most
octet. There is no padding between individual buffers.
Associated with each Data Buffer is a Buffer Header containing
control information describing the data. A Buffer Header is
.TCPBS words long. The address of the Buffer Header is passed to
TCP in AC2 by the SEND and RECV JSYSi.
The format of a Buffer Header is:
Word
Offset Bits Meaning
.TCPBF B0-17 Flags (note that the following flags have full-
word values, not half-word).
TCP%ER This buffer has an error associated with
it.
TCP%LE The error occurred at the local end, not
the foreign end.
TCP%PE The error is permanent, not temporary.
TCP%EC Field containing the error code. See the
Error Codes section for a listing of
error codes.
TCP%DN Asynchronous flag, cleared by TCP when it
receives the buffer, set by TCP when
processing associated with the buffer has
been completed.
TCP%UR SEND: The last octet in the buffer should
be communicated to the receiver as Urgent
Data.
RECV: There is more Urgent Data.
TCP%PU SEND: Promptly send (any buffered data
and) data in this buffer to the receiving
user (do not wait for more data to fill a
packet).
RECV: Sender requested that data in this
buffer be delivered promptly, even if the
buffer is not full.
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TCP IP Users Guide May 1982
Buffer Headers
TCP%WM The Data buffer is formatted as 36-bit
bytes. Off if the Data buffer has four
left-justified 8-bit bytes per word.
Note: 36-bit mode is not yet
implemented.
B18-35 Reserved for user program, not used by TCP.
.TCPBA B18-35 Address of first data octet (note that the Data
Buffer need not follow the Buffer Header). Each
word of the Data Buffer (except the last)
contains four left-justified eight-bit data
octets (the last word may contain from one to
four octets). Data is transmitted beginning with
the left-most bit of the first word.
.TCPBC B0-35 Data Buffer count.
SEND: The number of octets of data in the buffer.
Updated when TCP has finished with the buffer to
the number of octets not sent (usually 0).
RECV: The number octets available for data.
Updated when TCP has finished with the buffer to
the number of unused data octets.
.TCPBO B0-17 The address of an IP Option block, or zero.
B18-35 The address of a TCP Option block, or zero.
SEND: If the address is not zero, merge the
options from the Option block with those which
have been received from the foreign end for
inclusion in future packets.
RECV: If the address is not zero, return any
options from the most recently received packet in
the Option block. See the section on Options and
Appendix G for further details about option
formats.
Any unspecified bits in the Buffer Header should be zero.
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TCP IP Users Guide May 1982
Closing a Connection
When a user has no more data to be sent, the connection should
be "closed". The CLOSE JSYS is used to communicate this fact to
the foreign end of the connection. Since CLOSE implies a PUSH,
any data which TCP has internally buffered will first be sent;
then a FIN packet will be sent. The FIN causes the foreign user
to be notified that the connection is "Closing". After doing a
CLOSE, no more SENDs are allowed.
CLOSE, by itself, does not fully "close" the full-duplex
connection since there may still be data from the other end to
process (and the other end may even have more data to be sent).
There are two recommended procedures for closing a connection.
If the user is notified (either by a RECV error code or an
interrupt) that the connection is "Closing", no more RECVs should
be attempted. Any data for the foreign end should be sent using
SEND, and then a CLOSE with the TCP%WT flag set should be issued.
The CLOSE will block until the connection is fully "Closed". If
CLOSE returns without error, it has also released the JCN. If an
error is returned, the JCN has not been released; it should be
released using the ABORT JSYS.
If a CLOSE is issued before being notified that the connection
is "Closing", the TCP%WT flag should not be set. (Setting the
TCP%WT flag in this case is a frequent cause of "connections
failing to close".) Control will be returned to the user program
to allow RECVs to be issued until the foreign end (CLOSEs and
consequently) sends a FIN packet, which will cause a "Closed"
error to be returned. (Failure to RECV this data (or more
frequently "unexpected data") is another common cause of
"connections failing to close".) The JCN should then be released
using the ABORT JSYS.
A connection may be abnormally terminated at any time by
issuing the ABORT JSYS. ABORT also releases the JCN associated
with the connection.
After a connection has been fully CLOSEd or ABORTed, the
internal connection block (TCB) is retained by TCP for several
seconds to allow any packets which might still be in the network
(e.g., old retransmissions) to arrive and be discarded. During
this period, an attempt to reuse the same connection (internet
addresses and port numbers) will fail.
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TCP IP Users Guide May 1982
Connection Status
The status of a particular connection (or the values of TCP's
internal counters) may be examined using the STAT JSYS. The most
frequent use of STAT is to determine the addresses/ports
associated with a connection or the state of its send and receive
sides. Use of the symbolic name form of STAT call is strongly
recommended to reduce site and software release dependencies.
The following table lists the names of the most interesting
variables. Appendix H lists all the quantities which are
available. See the description of the STAT JSYS for details.
ASCII Name Contents
TFH Foreign Host internet Address
TLH Local Host internet Address
TFP Foreign Port
TLP Local Port
TRSYN Receive Side State (see below)
TSSYN Send Side State (see below)
Receive Send
(TRSYN) (TSSYN)
Value Symbol Symbol Meaning
4 SYNABL SYNABL Waiting for SYN packet to arrive/be sent
5 SYNRCV SYNSNT SYN packet received or sent
7 SYNCED SYNCED Connection synchronized, data may be sent
2 FINRCV FINSNT FIN packet received or sent
0 NOTSYN NOTSYN FIN acknowledged, connection closed
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TCP IP Users Guide May 1982
Interrupts
TCP allows the user to specify software interrupt channels for
five TCP related events: Connection open/closing/closed, SEND
buffer done, RECV buffer done, presence of URGENT data, and
error. They may each be left assigned to no channel, or assigned
to separate channels, or they may share a common channel. An
interrupt may be reassigned to another channel at any time; but
once assigned, the interrupt cannot be assigned to "no channel"
(although it may be assigned to a channel whose interrupts are
"ignored").
Since the TCP typically runs at a higher priority than most
user programs, successive interrupts may occur more quickly than
the user program can process them. One can thus not expect, for
example, that the number of RECV buffer full interrupts be the
same as the number of RECVs issued; it may be less if two buffers
were filled without the user program being run in between.
The open/closing/closed interrupt occurs when the connection
has become fully synchronized ("open"), when a FIN is received
from the other end ("closing"), and when the connection is fully
closed ("closed"). Note that these interrupts will NOT occur if
the connection is being ABORTed.
The error interrupt occurs when a reset is received
(Connection Rejected) or when a transmission timeout has occurred
(Transmission Timeout). Note that these interrupts will NOT
occur if the connection is being ABORTed.
The urgent data interrupt occurs when a packet containing
urgent data arrives. If another packet arrives with more urgent
data, a second interrupt will NOT be given unless the program has
already RECVed all of the urgent data from the first packet.
Note that this interrupt will NOT occur if the connection is
being ABORTed.
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TCP IP Users Guide May 1982
Error Returns and Codes
All of the TCP JSYSi are called with arguments in one or more
of ACs 1 through 4. If the call is successful the JSYS will skip
an instruction when control is returned to the user program. The
contents of ACs 1 through 4 may be changed depending on the
particular JSYS involved. If the call was not successful,
control is returned to the instruction in the user's program
immediately following the JSYS. In this case AC1 contains an
error code which identifies the cause of the problem. The error
codes returned (with the exception of ATNVT) are not in the
format of other TOPS20 errors, but are instead in the form of the
Buffer Header error field.
The error byte consists of three flag bits and a 5-bit error
code, right-justified in AC1. The AC1 bits and the corresponding
Buffer Header symbols are listed in the table below.
Buffer AC1
Header Bit Meaning
TCP%ER 1B28 Set if an error was detected ("E"), zero
otherwise ("X").
TCP%LE 1B29 Set if the error was detected locally ("L"), zero
if the error was detected by the foreign TCP
("F").
TCP%PE 1B30 Set if the error is permanent ("P"), zero if the
error is temporary ("T").
TCP%EC 37B35 Error Code.
The Error flag indicates an error occurred.
The Local flag indicates whether the problem was detected at
the local host or the foreign host.
The Permanent flag indicates whether the problem is permanent
or temporary. Temporary errors are usually associated with
resource shortages; repeating the request later is encouraged.
Permanent errors should not be retried.
Some error codes are only associated with a JSYS, some may
only be returned by a JSYS if the TCP%WT flag was set in AC1, and
some are only returned in a Buffer Header.
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TCP IP Users Guide May 1982
Error Returns and Codes
The Error Codes and their meanings are:
Key
j Error can be returned in AC1 by a JSYS.
w Error can be returned in AC1 by a JSYS which set TCP%WT.
b Error code can be returned in a Buffer Header.
Codes
0 b No Error. The buffer was processed without error.
1 j Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
2 j Invalid Options specified. The routing option does not
parse or an option length exceeds the amount of option
space available.
3 j Connection Not Open. Attempt to CLOSE a connection which
is not open.
4 j Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
5 Wild foreign host/port only allowed if listening.
6 j Connection Already Exists. Attempt to OPEN a connection
which has already been OPENed.
7 wb Connection Rejected or Reset. A RESET was received for
the connection.
9 w Transmission Timeout.
12 wb Connection Closed or Closing. Closed locally or
remotely; cannot (re)activate the connection because it
was not fully closed. Note: this code, without the
"error" flag set, is returned in RECV Buffer Headers
after a FIN has been received (the foreign end is
closing).
13 j Wild local port is illegal. A zero local port is not
allowed.
14 wb Connection Reset. A Transmission Timeout has occurred or
the connection is being aborted.
15 j Bad Buffer Argument. The Buffer Header count (.TCPBC) is
negative or the end of the buffer is beyond the end of
memory section 0.
16 j Insufficient resources to process data buffer. Either
insufficient free storage or TCP%WT was specified and
there are no free "wait flags".
17 j Invalid interrupt channel. A channel number is not 0-5
or 23-35.
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TCP IP Users Guide May 1982
Error Returns and Codes
20 j Bad STAT offset. Offset to too large for TCB or
Statistics area.
21 j Bad STAT count. Count is zero, positive, or forms an
offset which is too large.
22 j Bad STAT name. An ASCII name is not in STAT's table.
29 j Changing security levels is not allowed.
30 j Use of TCP%VT not allowed. Only Job0 (internet fork) can
create TVTs (Use ATNVT).
31 j TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
Options
Both TCP and IP provide the means to include a selection of
standard options in packets. Options are only required in
special situations. Most options are automatically processed by
TCP. In particular, the routing options are automatically
inverted without user intervention. Received options are
automatically merged with those specified by the user. If an
option is both specified by the user and received in a packet,
the received version is ignored.
Options may be specified with OPEN or SEND. Options are
specified using the .TCPOP word in the Connection Descriptor
block and the .TCPBO word in the Buffer Header block. The
left-half contains the address of a block for IP options, and the
right-half contains the address of a block for TCP options. Each
block is .TCPOW words long and contains one to forty option
octets, packed four left-justified octets per word.
Once specified, the options are sent in each packet generated.
If an Option block address is zero, the corresponding options
remain unchanged. The current options may be cancelled by
specifying an address pointing to an Option block which begins
with an End-of-Options octet (zero).
Options contained in packets which are received may be
obtained by specifying non-zero address(es) in the .TCPBO word
associated with a RECV.
Options have the form of a string of one to forty octets.
Except for NOP and End-of-options which are each one octet long,
an individual option is a string of two or more octets. The
first octet identifies the type of option; the second is its
length in octets (counting the type and length octets), followed
by additional octets relevant to the particular option.
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TCP IP Users Guide May 1982
Options
The currently defined TCP options are:
Option
Code Option (Length) Meaning
0 End-of-Options (single octet). Terminates the option
string.
1 NOP (single octet). Ignored.
2 Maximum Segment Size (four octets). The receiver of this
option should not generate packets whose packet length
exceeds the number of octets specified by the 16-bit
number contained in the third and fourth octets. This
option should be specified with OPEN.
The format of the IP options may be found in Appendix G. The
processing which TCP automatically provides for each IP option is
listed in the following table. Note that the most significant
bit of IP options indicates, if set, that the option must be
copied into each fragment of a fragmented datagram.
Option
Code Option Action taken by TCP
0 End-of-Options. None.
1 NOP. None.
7 Record Route. The recorded route is inverted and changed
into a Strict Source Route for inclusion in subsequently
generated packets.
68 Internet Timestamp. If the space reserved for timestamps
is not yet full, the option is copied and returned to the
originator.
130 Security. None.
131 Loose Source Routing. The source route is automatically
inverted for inclusion in subsequently generated packets.
136 Stream Identifier. Copied for inclusion in subsequently
generated packets.
137 Strict Source Routing. The source route is automatically
inverted for inclusion in subsequently generated packets.
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TCP IP Users Guide May 1982
TCP Retransmission Parameters
Both OPEN and SEND require a Transmission Timeout and
Retransmission Parameters. The Transmission Timeout is the
number of seconds for which retransmissions of unacknowledged
octets should be attempted before returning a Transmission
Timeout error to the user program. Permissible values are 1 to
300 seconds, or zero. A zero value means that there is no
timeout and retransmissions should continue "forever" (until the
other end sends a RESET or the program is ^C'd and the connection
ABORTed).
If you are not interested in the details of retransmissions,
use zero for the Retransmission Parameters. The standard
retransmission algorithm, based on measured round trip delay,
will be used. For a detailed description of the available
retransmission algorithms see Appendix F.
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TCP IP Users Guide May 1982
OPEN
OPEN JSYS 742
Specifies a TCP connection and returns its JCN.
ACCEPTS IN AC1: Flags ! Address-of-Connection-Descriptor-Block
TCP%FS Set to initiate synchronization (active
open); cleared to await foreign
connection attempt (passive open).
TCP%WT Block until either the connection has
been synchronized or an error has
occurred.
TCP%VT Create a TCP virtual terminal connection.
Note: Only Job 0 may use this flag. User
programs should use the ATNVT JSYS.
TCP%PS If set and the connection attempt results
in a failure, (wait a couple seconds and)
restart the connection procedure. Note:
Use of this flag is not recommended.
TCP%HP Connection should be high priority.
OBSOLETE, use the Type-of-Service byte in
the .TCPIP word of the Connection
Descriptor block (see Appendix C).
TCP%SC Secure connection. Unsupported, see the
IP Security option in Appendix G.
TCP%ET Reserved, must be 0.
TCP%PT Reserved, must be 0.
AC2: Transmission Timeout, in seconds (5 minute max)
AC3: Retransmission Parameters Word (See Appendix F)
B0-9 Backoff Numerator
B10-17 Backoff Denominator
B18-35 Initial Retransmission Interval
RETURNS +1: Failure, TCP error byte in B28-35 of AC1. If a
JCN had been assigned before the error was
detected, it is in the left-half of AC1 (it
should be ABORTed).
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TCP IP Users Guide May 1982
OPEN
+2: Success, AC1 contains the JCN in the right-half
with the TCP%JS flag set.
The format of the .TCPCS-word Connection Descriptor block,
whose address is contained in the right-half of AC1, was
described in the Connection Descriptor block section. The JCN
returned in AC1 should be used to specify this connection to the
other TCP JSYSi.
If the TCP%WT flag is not set, control will be returned to the
user program as soon as the connection parameters have been
validated, The internal control block (TCB) created, and, if the
TCP%FS flag was set, the connection synchronization process
begun. The connection will (probably) not yet be fully
synchronized. An interrupt may be enabled for the connection
fully synchronized event, or the state of the internal TSOPN flag
may be checked using the STAT JSYS.
Note that SENDs and RECVs may be issued before a connection is
fully synchronized.
OPEN ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELP+2 Invalid Options specified. The routing option does not
parse or an option length exceeds the amount of option
space available.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
ELP+6 Connection Already Exists. Attempt to OPEN a connection
which has already been OPENed.
EFP+7 Connection Rejected or Reset. A RESET was received for
the connection.
ELP+9 Transmission Timeout.
ELP+12 Connection Closed or Closing. Closed locally or
remotely; cannot (re)activate the connection because it
was not fully closed.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+14 Connection Reset. A Transmission Timeout has occurred or
the connection is being aborted.
ELP+30 Use of TCP%VT not allowed. Only Job0 (internet fork) can
create TVTs (Use ATNVT).
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TCP IP Users Guide May 1982
OPEN
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
CLOSE
CLOSE JSYS 743
Close a TCP connection.
ACCEPTS IN AC1: TCP%JS ! Flags ! JCN
TCP%JS Must be set since right-half has a JCN.
TCP%WT Wait for close to happen in both
directions or for an error to occur.
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success.
The CLOSE JSYS causes any data in TCP's buffers to be sent to
the receiver. A FIN is then sent to notify the receiver that no
more data will be sent to it. After a CLOSE, SENDs are illegal.
Note that if the TCP%WT flag is not set, the JCN will remain
valid, allowing more RECVs. RECVs should be repeated until a
Connection Closed error is returned. ABORT is then used to
release the JCN.
If the TCP%WT flag is set, the CLOSE JSYS will block until
FINs have been exchanged to fully close the connection, or an
error is detected. Note that a connection cannot be fully closed
if there is data which has not been RECVed. If no error was
detected, the JCN is also released.
CLOSE ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELP+3 Connection Not Open. Attempt to CLOSE a connection which
is not open.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
EFP+7 Connection Rejected or Reset. A RESET was received for
the connection.
ELP+7 Connection does not exist. There is no TCB for the
specified connection.
ELP+9 Transmission Timeout.
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TCP IP Users Guide May 1982
CLOSE
ELP+14 Connection Reset. A Transmission Timeout has occurred or
the connection is being aborted.
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TCP IP Users Guide May 1982
SEND
SEND JSYS 740
Send a buffer of data over a TCP connection.
ACCEPTS IN AC1: TCP%JS ! Flags ! JCN
TCP%JS Must be set since right-half has a JCN.
TCP%WT Wait for data to be copied into a packet,
or for an error. Note the packet may not
be sent if the TCP%PU flag was not set in
the Buffer Header.
TCP%HP Connection should be high priority.
OBSOLETE, use the Type-of-Service byte in
the .TCPIP word of the Connection
Descriptor block (see Appendix C).
TCP%SC Secure connection. Unsupported, see the
IP Security option in Appendix G.
AC2: Buffer Header Address
AC3: Transmission Timeout, in seconds (5 minute max)
AC4: Retransmission Parameters Word (See Appendix F)
B0-9 Backoff Numerator
B10-17 Backoff Denominator
B18-35 Initial Retransmission Interval
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success.
The format of the .TCPBS-word Buffer Header block whose
address is contained in AC2 was described in the Buffer Header
section. Unused words and bits should be zero.
When TCP accepts the Buffer Header, the Error field and TCP%DN
flag are cleared. Retransmissions will occur as specified by the
Retransmission Parameters word, if necessary, until the data has
been acknowledged by the receiver, the Transmission Timeout has
passed, or an error is detected. When processing of the data has
been completed, an error code is placed into the Error field, the
count word is changed to the number of octets not sent (normally
0), and the TCP%DN flag is set.
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TCP IP Users Guide May 1982
SEND
If the TCP%WT flag is not specified, control will return
immediately to the user program. The buffer will remain in use
until the TCP%DN flag is set. An interrupt may be enabled for
this event.
SEND ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELP+2 Invalid Options specified. The routing option does not
parse or an option length exceeds the amount of option
space available.
ELP+3 Connection Not Open. Attempt to CLOSE a connection which
is not open.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
EFP+7 Connection Rejected or Reset. A RESET was received for
the connection.
ELP+9 Transmission Timeout.
ELP+12 Connection Closed or Closing. Closed locally or
remotely; cannot (re)activate the connection because it
was not fully closed.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+14 Connection Reset. A Transmission Timeout has occurred or
the connection is being aborted.
ELP+15 Bad Buffer Argument. The Buffer Header count (.TCPBC) is
negative or the end of the buffer is beyond the end of
memory section 0.
ELT+16 Insufficient resources to process data buffer. Either
insufficient free storage or TCP%WT was specified and
there are no free "wait flags".
ELP+30 Use of TCP%VT not allowed. Only Job0 (internet fork) can
create TVTs (Use ATNVT).
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
RECV
RECV JSYS 741
Receive a buffer of data from a TCP connection.
ACCEPTS IN AC1: TCP%JS ! Flags ! JCN
TCP%JS Must be set since right-half has a JCN.
TCP%WT Wait for data buffer to be filled, or for
an error.
AC2: Buffer Header Address
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success.
The format of the .TCPBS-word Buffer Header block whose
address is contained in AC2 was described in the Buffer Header
section. Unused words and bits should be zero.
The count word in the Buffer Header should contain the
(maximum) number of octets the Data Buffer will hold. When TCP
accepts the Buffer Header, the Error field and TCP%UR, TCP%PU,
and TCP%DN flags are cleared. Data received will be placed into
the buffer until either it is full or a PUSH is received. When
TCP is finished with the buffer, an error code will be placed
into the Error field and the TCP%DN flag will be set. The count
word will be set to the number of unused octets remaining in the
buffer. If there is more Urgent data, the TCP%UR flag will be
set. If the buffer is being returned due to the presence of a
PUSH, the TCP%PU flag will be set.
If the TCP%WT flag is not specified, control will return
immediately to the user program. The buffer will remain in use
until the TCP%DN flag is set. An interrupt may be enabled for
this event.
RECV ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
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TCP IP Users Guide May 1982
RECV
EFP+7 Connection Rejected or Reset. A RESET was received for
the connection.
ELP+7 Connection does not exist. There is no TCB for the
specified connection.
ELP+9 Transmission Timeout.
ELP+12 Connection Closed or Closing. Closed locally or
remotely; cannot (re)activate the connection because it
was not fully closed. Note: this code, without the
"error" flag set, is returned in RECV Buffer Headers
after a FIN has been received (the foreign end is
closing).
XLP+12 Connection Closing. Closed remotely, no more data to be
RECVed.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+14 Connection Reset. A Transmission Timeout has occurred or
the connection is being aborted.
ELP+15 Bad Buffer Argument. The Buffer Header count (.TCPBC) is
negative or the end of the buffer is beyond the end of
memory section 0.
ELT+16 Insufficient resources to process data buffer. Either
insufficient free storage or TCP%WT was specified and
there are no free "wait flags".
ELP+30 Use of TCP%VT not allowed. Only Job0 (internet fork) can
create TVTs (Use ATNVT).
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
ABORT
ABORT JSYS 747
Abort a TCP connection.
ACCEPTS IN AC1: TCP%JS ! JCN
TCP%JS Must be set since right-half has a JCN.
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success, Connection broken, JCN released.
The local end of the connection is forgotten: partially
processed packets are deleted, queued SEND and RECV buffers are
returned, the retransmission queue is purged.
An attempt is made to send a RESET packet to the remote end.
Should this not be delivered, the other end will discover its
half-open connection the next time it attempts to use it.
Note that TCP's internal connection block (TCB) will remain
for several seconds to allow any packets which were in transit to
arrive and be discarded. An attempt to reuse the same connection
(host addresses and ports) will fail until the waiting time has
passed.
ABORT ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
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TCP IP Users Guide May 1982
STAT
STAT JSYS 745
Obtain information about either a connection or TCP's internal
counters.
ACCEPTS IN AC1: Flags ! (TVT- or connection-index, or JCN, or 0)
TCP%JS Return information about the connection
whose JCN is in the right-half of AC1.
TCP%ST Return TCP statistics information instead
of information about a connection. The
right-half of AC1 is ignored. The names
of the entries in the statistics area are
listed in Appendix H (see TCP30SY).
TCP%NI Return in AC2 an AOBJN counter for the
currently active connections. (See
TCP%IX.)
TCP%IX Return information about the connection
specified by the index in the right-half
of AC1. The index is a system-wide
identifier whose binding may change from
call to call. (See TCP%NI.)
TCP%NT Return in AC2 an AOBJN counter for the
currently active TVTs. (See TCP%TV.)
TCP%TV Return information about the TVT
specified by the index in the right-half
of AC1. The index is a system-wide
identifier whose binding may change from
call to call. (See TCP%NT.)
TCP%SY The right-half of AC2 contains the
address of a list of ASCII variable names
associated with either a connection (TCB)
or TCP Statistics for which information
should be returned.
TCP%SD An LDB pointer should be returned for
each of the specified symbols instead of
the value. (The pointers assume AC14
(octal) is pointing to a TCB image and
AC13 (octal) is pointing at STAT0 (offset
0 of the statistics area).) Requires
TCP%SY to be set and a valid connection
identifier (e.g. TCP%IX!1).
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TCP IP Users Guide May 1982
STAT
AC2: -N,,Offset into TCB or Statistics area, N
successive words are desired beginning at the
specified; or
AC2: -N,,Address of symbol names, N symbols are given
beginning at the specified address.
AC3: -M,,Address in user's space to which information
should be returned; the area contains M words.
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success, Data copied to user specified area,
counts and addresses in AC2 and AC3 updated.
The TCB offset identifies the transfer starting point and the
Address in user space identifies the starting point of the
destination area. If successful, min(M,N) words have been
transferred from the TCB (or statistics area) to the caller's
space.
Beware: the order/entries may be site dependent.
STAT ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+20 Bad STAT offset. Offset to too large for TCB or
Statistics area.
ELP+21 Bad STAT count. Count is zero, positive, or forms an
offset which is too large.
ELP+22 Bad STAT name. An ASCII name is not in STAT's table.
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
CHANL
CHANL JSYS 746
Specify PSI channels for TCP interrupts.
ACCEPTS IN AC1: TCP%JS ! JCN
TCP%JS Must be set since right-half has a JCN.
AC2: Six channel numbers.
B0-5 Urgent data available
B6-11 RECV buffer done
B12-17 SEND buffer done
B18-23 Error
B24-29 Connection change (open/closing/closed)
B30-35 Reserved, must be 77 (octal)
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success. This fork will receive TCP PSIs for the
specified events.
Each of the 6-bit bytes may be either a decimal channel number
(0-5 or 24-35) or 77 (octal) if no change is desired for the
corresponding event. PSIs may be dropped or may be tardy on
heavily loaded systems. Some defensive programming is required
to guard against these problems.
CHANL ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+17 Invalid interrupt channel. A channel number is not 0-5
or 23-35.
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
SCSLV
SCSLV JSYS 744
Set the security level for a TCP connection. Note: SCSLV has not
been tested in the current release.
ACCEPTS IN AC1: TCP%JS ! JCN
TCP%JS Must be set since right-half has a JCN.
AC2: 36-bit security value
RETURNS +1: Failure, TCP error byte in B28-35 of AC1.
+2: Success, The security value has been associated
with the connection.
The security value is not interpreted by the TCP (except to
see that it matches). The number of bits of the security value
actually used varies depending on the actual security procedure
in use. In all cases, the right-most bits of the word are used.
SCSLV ERROR CODES:
ELP+1 Argument Error in JSYS. An AC1 flag (TCP%xx) was set
which is inappropriate for the JSYS; the TVT or
connection index or JCN is invalid.
ELT+4 Temporarily Out of Resources. A new connection cannot be
created due to a lack of JCNs (for the job), free storage
for the TCB, an available TVT, or because the system
limit on simultaneous connections would be exceeded.
ELP+13 Wild local port is illegal. A zero local port is not
allowed.
ELP+29 Changing security levels is not allowed.
ELT+31 TCP not available. TCP has not been initialized or has
been turned off.
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TCP IP Users Guide May 1982
TELNET Protocol
The TELNET Protocol is a general 8-bit byte oriented
communications facility intended for use in remote terminal
access situations. It uses TCP as the data transfer protocol.
An open TCP connection which has not been used for SENDs or
RECVs may be converted by the ATNVT JSYS into a Network Virtual
Terminal using the TELNET protocol. ATNVT is given the JCN of
the connection (which it releases). It returns a TOPS20 TTY
designator. The designator can then be used to obtain input and
output JFNs and the connection can be used as any other TOPS20
TTY.
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TCP IP Users Guide May 1982
ATNVT
ATNVT JSYS 274
Attach a Network Virtual Terminal to a TCP connection.
ACCEPTS IN AC1: AN%TCP ! AN%NTP ! JCN
AN%TCP This flag is set to indicate that the
right-half of AC1 has a JCN (instead of a
JFN).
AN%NTP This flag is ASSUMED since TCP only
supports the "new" TELNET protocol.
RETURNS +1: Failure, TOPS20 error code in AC1, the JCN is
still valid.
+2: Success, The terminal designator for the Network
Virtual Terminal is in AC1, the JCN has been
released.
The caller must first OPEN a TCP connection, as described
above. The connection must be synchronized with no intervening
SENDs or RECVs before the ATNVT call. If successful, the JCN is
released and the connection is transferred to the Network Virtual
Terminal whose TTY designator is returned in AC1. If
unsuccessful, one of the error codes listed below is returned and
the JCN remains valid. (It should probably be ABORTed.)
ATNVT ERROR CODES:
ATNX1 Invalid JCN
ATNX2 Receive side not SYNCED
ATNX3 User CLOSEd/ABORTed connection
ATNX5 Receive side has been used (RECVs)
ATNX6 Connection has been closed or has errors
ATNX8 Send side not SYNCED
ATNX11 Send side has been used (SENDs)
ATNX13 No free TVTs
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TCP IP Users Guide May 1982
Internet User Queues
The Internet User Queue facility allows user programs to send
and receive internet datagrams. In order to use this facility a
program must first assign an Internet User Queue using ASNIQ.
ASNIQ's argument is the address of a Queue Descriptor block
containing a set of mask and value words that determine what
messages may be sent from this queue and which of the incoming
messages will be delivered to this queue. A successful ASNIQ
returns an Internet Queue Handle (a small system-wide number
identifying this queue).
Datagrams may be sent by creating an IP packet image and
calling SNDIN with the Internet Queue Handle and the address of
the packet image. Various fields of the header are checked for
legality. The source host internet address is validated if it
was specified, or it is filled in with the default host address
if it was not specified (i.e. was zero). The internet header
checksum is computed and placed into the packet's internet
header. The routing algorithms select a network interface, and a
local leader for that network is constructed in the packet. The
packet is then queued for output on the selected interface.
User programs have no control over which network or interface
will be selected for transmission of a datagram. This decision
will be made by the routing algorithms based on routing
information supplied by internet gateways. This means that all
network interfaces to which a host is connected must go down
before internet communications will be completely stifled; and
even then, processes within a host will be able to communicate
due to the local delivery mechanism. User programs also have no
knowledge of which interface received an incoming packet.
There is an exception when a system is equipped with a "Raw
Packet Interface" (RPI, which is used to connect to a remote
gateway or an encryption device such as a BCR). Flag AQ%SCR will
mark the queue as using the RPI rather than the normal
interfaces.
Datagrams are received by giving RCVIN an Internet Queue
Handle and the address of an empty buffer into which an IP packet
should be copied. RCVIN normally blocks if there are no
datagrams waiting in the queue. The non-blocking form is
specified by setting the RIQ%NW flag. If it is set and no
datagrams are waiting, an error code of -1 is returned in the
right-half of AC1.
RELIQ is used to release an Internet Queue Handle when the
user program has finished with it. The argument is either a
specific Internet Queue Handle, or a job or (multiple) fork
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TCP IP Users Guide May 1982
Internet User Queues
handle if all Internet User Queues associated with the job/fork
are to be released.
An Internet User Queue which contains a received datagram is
flushed if the datagram is not RCVINed within 30 seconds. The
number of messages which will be placed into a queue is limited
to eight. Flooding the input queue of a slow receiver will thus
result in dropped messages. (Note that the stated limits are
monitor assembly parameters and may vary from site to site.)
Datagrams sent from the local host to itself will ordinarily
be delivered without sending them over any network and are
reasonably fast.
Programs using Internet User Queues must handle the cases of
datagrams not being delivered in the order in which they were
sent, being dropped, or being duplicated. Some may traverse a
broadcast network and may be clobbered by other packets,
lightning, etc. Thus, some higher level protocol is needed in
most cases. TCP is one example, but there are others (the
"Datagram Protocol", XNET, etc.).
The local host may receive ICMP error messages relating to
datagrams for an Internet User Queue. Normally these messages
are discarded. The user may, however, request that these
datagrams be placed into the input queue so that they may be
processed. To enable this option, the AQ%ICM flag must be
specified when the queue is assigned. RCVIN may then be used to
read received ICMP error messages, and SNDIN may be used to send
ICMP error messages.
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TCP IP Users Guide May 1982
Queue Descriptor Block
The Queue Descriptor block is used to identify datagrams which
may be sent from or delivered to an Internet User Queue. The
packets are (de)multiplexed based on four fields: the source and
destination internet addresses, the protocol number, and the
source and destination port numbers. For each field the Queue
Descriptor block contains a value entry and a mask entry. A one
bit in the mask entry means that the corresponding bit in the
value entry and the datagram must match.
ASNIQ will return an error unless the current Queue Descriptor
block differs in the masked bits from all other Internet User
Queues which are assigned.
The format of the Queue Descriptor block is shown in the
following table.
Word
Offset Bits Usage
.IQPRV B24-31 Internet protocol number.
.IQFHV B0-31 Foreign host internet address.
.IQSHV B0-31 Local host internet address.
.IQPTV B0-15 Local port.
B16-31 Foreign port (ignored if AQ%SPT is set).
.IQPRM B24-31 Mask for internet protocol number.
.IQFHM B0-31 Mask for foreign host internet address. (Zero
for any foreign host.)
.IQSHM B0-31 Mask for local host internet address. (Zero for
any of this host's addresses.)
.IQPTM B0-15 Mask for local port.
B16-31 Mask for foreign port (zero if AQ%SPT is set).
Note B0-31 should be zero for portless protocols.
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TCP IP Users Guide May 1982
Packet Buffer
The Packet Buffer begins with one word containing the length
of the buffer followed immediately by an IP header of five or
more words and the contents of the IP datagram (the "data").
The right-half of the length word contains, for SNDIN, the
length in words of the IP packet to be sent, plus one for the
length word. For RCVIN the right-half should contain the maximum
number of words, including the length word, available for a
received IP packet; the left-half will be set to the number of
words actually received plus one for the length word.
The IP header and data use the left-most 32 bits of each word.
Word
Offset Bits Contents
0 B0-17 RCVIN: Number of valid words actually used by the
IP packet, plus one (this word).
0 B18-35 RCVIN: Maximum number of words in the buffer,
including this word.
SNDIN: Number of words in the IP packet to be
sent, plus one (this word).
Beginning of IP Header (See Appendix A)
1 B0-3 IP Version number. This field must contain 4.
1 B4-7 Data Offset. The length of the IP header, in
words (5 to 15).
1 B8-15 Type of Service byte. See Appendix C.
1 B16-31 Datagram Length, in octets, beginning with this
word (20 to 2**16-1).
2 B0-15 Segment ID. This field should be unique for each
datagram sent; it is used to identify which
fragments should be combined to reconstruct a
fragmented datagram.
2 B16 Reserved, must be zero.
2 B17 Do-not-Fragment flag. If set, a packet may not
be fragmented by an internet module. If the
packet is too large to process, it will be
discarded.
2 B18 More-Fragments flag. Set if this is NOT the last
fragment of a packet. Since user programs do not
send fragments, it should be zero.
2 B19-31 Fragment-Offset. Since user programs do not send
fragments, it should be zero.
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TCP IP Users Guide May 1982
Packet Buffer
3 B0-7 Time-to-Live, in seconds. If the packet cannot
be delivered in the time specified, the packet
should be discarded.
3 B8-15 Protocol number. Protocol numbers are assigned
by Jon Postel@ISI.
3 B16-31 IP header checksum. This field is filled in by
SNDIN; the user program does not have to compute
it.
4 B0-31 Source host internet address.
5 B0-31 Destination host internet address.
6-15 B0-31 IP options, if desired. If necessary, the last
word of options is padded with zero
(End-of-Options) octets to fill the word.
End of IP Header
H+1 B0-15 Port number, for single-port protocols, or source
port number for two-port protocols, or
nonexistent for portless protocols.
H+1 B16-31 Destination port number for two-port protocols,
or nonexistent for single-port or portless
protocols.
... B0-31 Data.
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TCP IP Users Guide May 1982
Ports
Not all internet protocols have ports. If an application does
not have ports, then .IQPTM in the Queue Descriptor block should
contain a zero. If the protocol uses both local and foreign
ports, they are expected to be in the first two 16-bit bytes
following the internet header (offset H+1 in the table above),
source port in left-most 16 bits, and destination port in the
next 16 bits. The location of this word is found by adding the
contents of the Data Offset field to the address of the first
word of the internet header.
Provision has been made for protocols (such as XNET) which
have only a local port. Turning on the AQ%SPT control flag for
ASNIQ selects single-port mode. The first 16-bit byte following
the internet header is expected to contain the port. This port
is used both for demultiplexing incoming packets and for checking
access when packets are being sent.
Note that for a given set of addresses and protocol numbers,
etc., a single-port protocol queue will not be assigned if a
dual-port version already exists, and vice versa. This rule
permits an incoming packet to specify a unique queue. Without
the rule, either the first or second 16-bit field following the
internet header would have to be used for demultiplexing; but we
cannot know which unless there is something else to tell which
queue is being addressed.
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TCP IP Users Guide May 1982
ASNIQ
ASNIQ JSYS 756
Assign an Internet User Queue (NET WIZARD capability must be
enabled)
ACCEPTS IN AC1: Flags ! Address of Queue Descriptor block
AQ%SPT Set for single-port protocols, cleared
for two-port or portless protocols.
AQ%ICM Deliver ICMP error datagrams to this
queue and allow them to be sent from it.
AQ%SCR Use Raw Packet Interface.
AC2: Reserved, must be 0
AC3: Reserved, must be 0
RETURNS +1: Failure, TOPS20 error code in AC1, conflicting
job number in AC2.
+2: Success, Internet Queue Handle returned in AC1
and maximum IP packet size (before fragmentation
will occur) in AC2.
The mask words specify those bit positions where a match is
required. Thus, one can make .IQFHM contain zero in order to
communicate with any internet host. By making, for example, the
low 3 bits of the local port mask word zero, the queue owns eight
ports. Note that an error will occur unless the Queue Descriptor
block differs in the masked bits from all other Internet User
Queues assigned at the instant the ASNIQ is issued. The job
number of the job which owns a conflicting queue is returned in
AC2. If the queue is assigned, the Internet Queue Handle is
returned in AC1; the maximum size packet (before fragmentation)
is returned in AC2.
ASNIQ ERROR CODES:
NTWZX1 NET WIZARD capability required
ASNSX1 All Internet User Queues in use (or IP not initialized)
ASNSX2 Conflict with some other job (# in AC2)
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TCP IP Users Guide May 1982
RELIQ
RELIQ JSYS 757
Release an Internet User Queue.
ACCEPTS IN AC1: An Internet Queue Handle, or -1 for all queues
owned by this job, or a job or fork-relative
process handle.
AC2: Reserved, must be 0
AC3: Reserved, must be 0
RETURNS +1: Failure, TOPS20 error code in AC1.
+2: Success.
RELIQ releases ownership of an Internet User Queue so that
other jobs can assign it. See the TOPS20 Monitor Calls Reference
Manual for a description of the various process handles.
RELIQ ERROR CODES:
SQX1 Internet Queue Handle out of range
SQX2 Internet Queue Handle not assigned
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TCP IP Users Guide May 1982
SNDIN
SNDIN JSYS 754
Send an internet datagram.
ACCEPTS IN AC1: Internet Queue Handle
AC2: Address of Packet Buffer containing the IP packet
AC3: Reserved, must be 0
RETURNS +1: Failure, TOPS20 error code in AC1.
+2: Success.
The format of the Packet Buffer was described in the Packet
Buffer section.
If port filtering is being used (.IQPTM was non-zero for
ASNIQ), the port(s) must be located in the word following the
internet header. The address of this word is found by adding the
address of word-1 in the Packet Buffer to the number in the Data
Offset field.
IP validates the source host field in the packet and fills in
the internet header checksum. The rest of the header is supplied
by the user.
SNDIN ERROR CODES:
SQX1 Internet Queue Handle out of range
SQX2 Internet Queue Handle not assigned
SNDIX1 Invalid message size
SNDIX2 Insufficient resources (no buffers available)
SNDIX4 Invalid header value for this queue (includes Datagram
Length too big, Data Offset too small, filtering on ports
but Datagram Length is too small for packet to contain a
port word, or header does not fit the ASNIQ arguments)
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TCP IP Users Guide May 1982
RCVIN
RCVIN JSYS 755
Receive internet datagram.
ACCEPTS IN AC1: Flags ! Internet Queue Handle
RIQ%NW Set to cause fail-return instead of
blocking when no datagram is waiting.
AC2: Address of a Packet Buffer to hold the IP packet
AC3: Reserved, must be 0
RETURNS +1: Failure, TOPS20 error code in AC1.
+2: Success
Each RCVIN gets one datagram from the specified queue. These
datagrams match the values in the Queue Descriptor block when
masked by the mask words in the block. The maximum size of the
Packet Buffer (including the count word) is expected in the
right-half of word-0 of the Packet Buffer when the RCVIN is
executed.
When a datagram is available, it is placed into the user
supplied Packet Buffer. The number of words filled plus one (for
the count word) is set into the left-half of word-0 (the count
word) of the Packet Buffer. Ordinarily this can be ignored, but
if the datagram was too big for the Packet Buffer, this tells how
much space would have been required. If the datagram was too big
(determined by the Datagram Length field), as much of the
datagram as will fit into the Packet Buffer is transferred and an
error is returned. No retry for the same datagram is possible.
RCVIN ERROR CODES:
SQX1 Internet Queue Handle out of range
SQX2 Internet Queue Handle not assigned
SNDIX1 Invalid datagram size
777777 RIQ%NW was on and no datagrams were waiting
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TCP IP Users Guide May 1982
Appendix A Internet Packet Header Format
The following structure definitions have been excerpted from
the file INPAR.MAC which is used in building TOPS20 and TENEX
monitors with internet code. Those who are unfamiliar with the
structure facility should refer to SYS:MACSYM.MAC. Briefly,
DEFSTR(Name,Location,Bit,Size)
defines a field called Name. This field is held in the word
(possibly indexed) at Location. The right-most bit number is Bit
and the field is Size bits wide. A typical reference to the data
offset field might be:
LOAD 1,PIDO,(PKT)
where PKT is an index register which points at the first word of
the packet structure.
Word offsets:
.IPKVR==0 ; Word with version, type of service, etc.
.IPKSG==1 ; Word with segmentation information
.IPKPR==2 ; Word with time to live, checksum, protocol
.IPKSH==3 ; Word with source host
.IPKDH==4 ; Word with destination host
DEFSTR(PIVER,.IPKVR,3,4) ; PACKET.INTERNET.VERSION
DEFSTR(PIDO,.IPKVR,7,4) ; PACKET.INTERNET.DATA-OFFSET
DEFSTR(PITOS,.IPKVR,15,8) ; PACKET.INTERNET.TYPE-OF-SERVICE
; Overlays for the above:
DEFSTR(PIPRC,.IPKVR,10,3) ; PACKET.IP.PRECEDENCE
DEFSTR(PILDY,.IPKVR,11,1) ; PACKET.IP.LOW-DELAY
DEFSTR(PIHTR,.IPKVR,12,1) ; PACKET.IP.HIGH-THROUGHPUT
DEFSTR(PIHRL,.IPKVR,13,1) ; PACKET.IP.HIGH-RELIABILITY
; 2 Unused bits
DEFSTR(PIPL,.IPKVR,31,16) ; PACKET.INTERNET.PACKET-LENGTH
DEFSTR(PISID,.IPKSG,15,16) ; PACKET.INTERNET.SEGMENT-ID
DEFSTR(PIFLG,.IPKSG,18,3) ; PACKET.INTERNET.FLAGS
; Overlays for the above:
; Unused bit
DEFSTR(PIDF,.IPKSG,17,1) ; PACKET.INTERNET.DONT-FRAGMENT
DEFSTR(PIMF,.IPKSG,18,1) ; PACKET.INTERNET.MULTIFRAGMENT
DEFSTR(PIFO,.IPKSG,31,13) ; PACKET.INTERNET.FRAGMENT-OFFSET
DEFSTR(PITTL,.IPKPR,7,8) ; PACKET.INTERNET.TIME-TO-LIVE
DEFSTR(PIPRO,.IPKPR,15,8) ; PACKET.INTERNET.PROTOCOL
DEFSTR(PICKS,.IPKPR,31,16) ; PACKET.INTERNET.HEADER-CHECKSUM
DEFSTR(PISH,.IPKSH,31,32) ; PACKET.INTERNET.SOURCE-HOST
DEFSTR(PIDH,.IPKDH,31,32) ; PACKET.INTERNET.DESTINATION-HOST
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TCP IP Users Guide September 1981
Appendix B Internet Address Formats
An internet address is a 32-bit quantity, in which the
left-most one, two, or three octets specify the network number
and the remaining octets identify a host on that network.
Class A addresses begin with a zero bit, followed by a 7-bit
network number and a 24-bit local address.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Network | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class B addresses begin with bits 10, followed by a 14-bit
network number and a 16-bit local address.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0| Network | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class C addresses begin with bits 110, followed by a 21-bit
network number and an 8-bit local address.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0| Network | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class D addresses begin with bits 111. The interpretation of
the remaining bits is currently unspecified.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1| Unspecified |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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TCP IP Users Guide September 1981
Appendix C IP Type of Service
The IP Type-of-Service field is composed of six sub-fields, two
of which are currently undefined.
+------+------+------+------+------+------+------+------+
| | Low | High | High | | |
| Precedence |Delay |Throu-|Relia-| 0 | 0 |
| | |ghput |bility| | |
+------+------+------+------+------+------+------+------+
The left-most three bits form the Precedence field. Its eight
possible values are:
7 - Network Control
6 - Internetwork Control
5 - CRITIC/ECP
4 - Flash Override
3 - Flash
2 - Immediate
1 - Priority
0 - Routine
The next bit is the Delay flag. If the flag is set, the
packet will be processed in a manner which tends to minimize
"delay" through the internet. If the flag is not set, the packet
will be treated in the "standard" manner with respect to delay.
The next bit is the Throughput flag. If the flag is set the
packet will be processed in a manner which tends to maximize
"throughput" through the internet. If the flag is not set, the
packet will be treated in the "standard" manner with respect to
throughput.
The next bit is the Reliability flag. If the flag is set, the
packet will be processed in a manner tending to maximize
"reliability" through the internet. If the flag is not set, the
packet will be treated in the "standard" manner with respect to
reliability.
The remaining two bits are reserved and should be zero.
Note that setting any of the Delay, Throughput, or Reliability
flags may increase user "costs" in some sense; setting all three
flags is illegal.
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TCP IP Users Guide May 1982
Appendix D Logical Host Facility
The internet Logical Host facility allows several internet
addresses to share a single (physical) network interface. This
is implemented by having IP modules which make routing decisions
"ignore" certain (network dependent) bits in the local address
field (see Appendix D). The destination IP module, however,
includes those bits when it demultiplexes packets.
The TOPS20 IP is given a Logical-Host-Mask (for each network
to which it has an interface) which identifies the particular
bits to be so treated (if any). In the case of the ARPANET,
which is Class A network 10, the logical host field is the third
octet in the internet address.
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TCP IP Users Guide May 1982
Appendix E IP Packet Demultiplexing
When the destination IP receives a packet, it must pass it to
the appropriate protocol module for further processing. This
decision is based on the values in three fields. The first is
the internet Version Number; if it is not 4, the packet is
discarded. The other two fields are the internet Protocol field
and the Logical Host field in the destination host internet
address (see Appendix B).
If the value in the logical host field is less than or equal
to (an installation-dependent constant whose default value is) 3,
the packet is passed to the internal routine for the protocol
identified by the internet Protocol field if one is implemented,
or to the Internet User Queue dispatcher if it isn't implemented.
If the value in the logical host field is greater than the
installation dependent constant, an attempt is first made to pass
the packet to an internal routine for the protocol identified by
the Logical Host field value, times 256., plus the value from the
packet's internet Protocol field if one is implemented, or
secondly, to the Internet User Queue dispatcher if such a
protocol is not implemented.
A packet for logical host 2 protocol 6 (TCP) would thus be
passed to the standard TOPS20 TCP routines since protocol 6 is
implemented. A packet for logical host 5 protocol 6 would be
passed to an Internet User Queue if one has been assigned (which
might be an experimental TCP implementation) since protocol 1286
(5*256+6) is not an internally implemented protocol.
This means that no messages of a particular protocol type
implemented internally will be passed to users via Internet User
Queues. Assigning such a queue will still be possible, but no
traffic will reach the user unless the internal protocol module
has been disabled.
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TCP IP Users Guide May 1982
Appendix F TCP Retransmission Parameters
Both OPEN and SEND require a Transmission Timeout and a
Retransmission Parameters word. The Transmission Timeout is the
number of seconds for which retransmissions of unacknowledged
octets should be attempted before returning a Transmission
Timeout error to the user program. Permissible values are 1 to
300 seconds, or zero. A zero value means that there is no
timeout, and retransmissions should continue "forever" (until the
other end sends a RESET or the program is ^C'd and the connection
ABORTed).
The Retransmission Interval is the time between successive
transmissions of a packet. The user's choice of which of two
algorithms to use in its computation is specified by the value of
the Retransmission Parameters word. Both algorithms are based on
an estimate of the round trip delay associated with the
connection. The default algorithm is used as long as the value
of the Retransmission Parameters word is zero; if a non-zero
value is ever specified, the second algorithm is activated for
the duration of the connection.
The default algorithm maintains an estimate of the round trip
delay based on the equation:
ERTT = ERTT' * alpha + MRTT * (1.-alpha)
where
MRTT is the measured round trip delay of a packet,
ERTT' is the prior estimate of the round trip delay,
(initially 3 seconds), alpha is a smoothing
factor equal to 0.8125, and
ERTT is the new estimate.
The Initial Retransmission Interval for a packet is then computed
from:
IRXI = ERTT * beta
where
ERTT is the current round trip delay estimate,
beta is a variance factor equal to 1.5, and
IRXI is the Initial Retransmission Interval.
The retransmission interval is essentially doubled for successive
retransmissions of the same packet.
NRXI = CT - TG
where
CT is the current time,
TG is the time at which the packet was generated,
and
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TCP IP Users Guide May 1982
Appendix F TCP Retransmission Parameters
NRXI is the Next Retransmission Interval.
A second algorithm, which allows the user specify its factors,
is activated if the Retransmission Parameters word is ever
non-zero. The right-half, if not zero, is the Initial
Retransmission Interval, in seconds. If the right-half is zero,
the Initial Retransmission Interval will be computed based on the
estimated round trip delay.
As each packet is acknowledged, this algorithm updates a
minimum and maximum round trip delay variable.
MNRTT = MIN(MNRTT',MRTT)
MXRTT = MAX(MXRTT',MRTT)
where
MRTT is the measured round trip delay of a packet,
MNRTT' is the shortest delay encountered during the
30-second interval (initially 3 seconds if the
right-half word was 0),
MNRTT is the new minimum,
MXRTT' is the longest delay encountered during the
30-second interval (initially 3 seconds if the
right-half word was 0),
MXRTT is the new maximum.
Every thirty seconds the minimum and maximum delay times are
reset, and a new Initial Retransmission Interval, IRXI, is
computed according to the equations:
MNRTT = MNRTT + (MXRTT-MNRTT)/10.
MXRTT = MXRTT - (MXRTT-MNRTT)/2.
IRXI = (MNRTT+MXRTT)/2 + 1/2 second.
The left-half of the Retransmission Parameters word contains
two 9-bit quantities, called the Backoff Numerator (in B0-8) and
the Backoff Denominator (in B9-17). In computing the Next
Retransmission Interval for a packet, the last interval is
multiplied by the Numerator and then divided by the Denominator
(using integer arithmetic).
NRXI = ( NRXI' * BN ) / BD
where
NRXI' is the last Next Retransmission Interval
(initially IRXI),
BN is the Backoff Numerator,
BD is the Backoff Denominator, and
NRXI is the Next Retransmission Interval.
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TCP IP Users Guide May 1982
Appendix F TCP Retransmission Parameters
The Backoff Numerator must be greater than or equal to the
Backoff Denominator. Default values are used if a factor is
zero:
Initial Interval 3 seconds
Backoff Numerator 3
Backoff Denominator 2
Both algorithms clamp the retransmission interval between a
minimum value of 1 second and a maximum value of 60 seconds.
There are several details which might be of interest. One
involves the computation of the measured round trip delay. This
delay is the time between TCP's filling a packet (and passing it
to IP) and the time that its last octet is acknowledged. The
delay thus includes the time the packet spent in IP waiting to
enter the network and the time the receiving TCP required to send
an acknowledgement in addition to actual "network" delays. Note
also that if nothing has been received (e.g., before the first
SYN or ACK arrives), then the round trip estimate cannot be
computed (and the initial guess of 3 seconds is used).
Packets are transmitted and placed into the retransmission
queue when they are generated (filled). A timer is set for the
Initial Retransmission Interval, and the Transmission Timeout
time is recorded. If the retransmission timer goes off, the
packet with the lowest unacknowledged sequence number is checked
to see if it should be retransmitted. If the packet is still in
IP's (or the network's) output queue, the timer is just reset.
(Since it hasn't yet been transmitted, the network might be off
or be blocking.)
Otherwise, if the packet has never been retransmitted, its
Transmission Timeout time and Initial Retransmission Interval are
recomputed (there may be better estimates now), the packet is
retransmitted, and a timer for the interval is started.
If a packet has already been retransmitted, then the
transmission timeout is checked; if it has passed, a Transmission
Timeout error is returned to the user and the connection is
broken. If the timeout has not yet passed, the Next
Retransmission Interval (the elapsed time since the packet was
generated, or the last interval times the backoff numerator
divided by the backoff denominator) is computed, the packet is
retransmitted, and a timer for the interval is started.
There is also interaction between the measured round trip
delay and retransmissions since TCP cannot tell which of several
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TCP IP Users Guide May 1982
Appendix F TCP Retransmission Parameters
possible retransmissions of a packet is being acknowledged. The
prior TCP release did not include retransmitted packets in its
delay measurements. This omission had the undesirable effect of
never updating the initial three-second guess if the actual delay
was much greater than three seconds. The current TCP release
includes such measurements (timed from the initial transmission).
This method seems more reasonable since it is correct if there is
just a large delay. On the other hand, if a retransmitted packet
is being acknowledged, then a packet was lost (probably due to
congestion somewhere). Thus, slowing things down is reasonable.
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TCP IP Users Guide May 1982
Appendix G TCP & IP Options
Both TCP and IP provide the means to include a selection of
standard options in packets. Options are only required in
special situations. See the section on Options for a description
of using options with TCP; the section on the Packet Buffer
should be consulted to use options with Internet User Queues.
A TCP or IP option has the form of a string of one to forty
octets. Except for NOP and End-of-Options, which are each one
octet long, an individual option is a string of two or more
octets. The first octet identifies the type of option, the
second is its length in octets (counting the type and length
octets), followed by additional octets relevant to the particular
option.
The currently defined TCP options are:
Option
Code Option (Length) Meaning
0 End-of-Options (single octet). Terminates the option
string.
1 NOP (single octet). Ignored.
2 Maximum Segment Size (four octets). The receiver of this
option should not generate packets whose packet length
exceeds the number of octets specified by the 16-bit
number contained in the third and fourth octets. This
option should be specified with OPEN.
The most significant bit of IP options indicates, if set, that
the option must be copied into each fragment of a fragmented
datagram. The currently defined TCP options are:
Option
Code Option (Length) Meaning
0 End-of-Options (single octet). Terminates the option
string.
1 NOP (single octet). Ignored.
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TCP IP Users Guide May 1982
Appendix G TCP & IP Options
7 Record Route (seven to thirty-nine octets). The
originator of this option provides space for each IP
module which processes this datagram to insert its
(sending) internet address into the Route Data area at
the position specified by the Pointer (whose minimum
value is four). As each address is added, the value of
the pointer is increased by four. If the value of the
pointer exceeds that of the Length, the Route Data area
is full and some IP modules may not have been able to
insert their addresses.
+-------------+-------------+-------------+
| 7 | Length | Pointer |
+-------------+-------------+-------------+-------------+
| Route Data (internet addresses) |
+-------------+-------------+-------------+-------------+
...
+-------------+-------------+-------------+-------------+
68 Internet Timestamp (eight to forty octets). The
originator of this option provides space for intermediate
IP modules to record the time when the packet was
processed and, optionally, their internet address. The
option begins with the Type and Length octets which are
followed by Pointer and Control octets and a data area.
Each entry in the data area contains four octets.
+-------------+-------------+-------------+-------------+
| 68 | Length | Pointer | Ovfl | Frmt |
+-------------+-------------+-------------+-------------+
| Internet Address (Frmt 1 or 3 only) |
+-------------+-------------+-------------+-------------+
| Timestamp |
+-------------+-------------+-------------+-------------+
...
+-------------+-------------+-------------+-------------+
The four-bit Frmt field specifies the format of the the
data portion of the packet. Frmt 0 means that only
Timestamps appear; there are no Internet Addresses. For
Frmts 1 and 3, each Timestamp is preceded by an Internet
Address. Each IP module fills in its address if Frmt is
1. In Frmt 3, the Internet Addresses are pre-specified;
the IP module only enters its Timestamp if (one of) its
address(es) is next.
The Pointer specifies the current position in the data
area for the next (Internet Address and) Timestamp. The
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TCP IP Users Guide May 1982
Appendix G TCP & IP Options
minimum value of 5 indicates the first entry (the octet
following the Ovfl/Frmt octet). It is incremented by
four (Frmt 0) or eight (Frmts 1 and 3). If it points
beyond the last data entry (i.e., Pointer exceeds
Length), no further entries can be made. The Ovfl field
is incremented in this case (Frmts 0 and 1).
The format of the Timestamps is the number of
milliseconds since midnight Universal Time. If the time
is not known in that format, any local time may be used
with the left-most bit set.
130 Security (eleven octets). Security, Compartmentation,
Handling Restrictions, and TCC parameters are specified.
The type and length octets are followed by two octets
which specify one of sixteen Security levels. The valid
values are:
00000000 00000000 - Unclassified
11110001 00110101 - Confidential
01111000 10011010 - EFTO
10111100 01001101 - MMMM
01011110 00100110 - PROG
10101111 00010011 - Restricted
11010111 10001000 - Secret
01101011 11000101 - Top Secret
00110101 11100010 - (Reserved for future use)
10011010 11110001 - (Reserved for future use)
01001101 01111000 - (Reserved for future use)
00100100 10111101 - (Reserved for future use)
00010011 01011110 - (Reserved for future use)
10001001 10101111 - (Reserved for future use)
11000100 11010110 - (Reserved for future use)
11100010 01101011 - (Reserved for future use)
Two Compartment field octets follow the Security octets.
They may be obtained from the Defense Intelligence
Agency.
Next are two Handling Restriction octets. The control
and release markings are given in the "Standard Security
Markings" section of Defense Intelligence Agency Manual
DIAM 65-19.
The last three octets are used to segregate Traffic and
define Controlled Communities of subscribers. Their
values may be obtained from HQ DCA Code 530.
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TCP IP Users Guide May 1982
Appendix G TCP & IP Options
131 Loose Source Routing (eleven to thirty-nine octets). The
originator of this option specifies the addresses of IP
modules through which the datagram must pass on its way
to the destination. The Pointer indicates the Next
Address in the Route Data area; its initial value is
four. The last address in the Route Data area must be
the destination address. As each entry is processed, the
IP module's out-going internet address replaces the Next
Address, and the Pointer is increased by four. If the
value of Pointer exceeds that of Length, the source route
is exhausted and the option is ignored (sent to the
destination in the IP header). The route is Loose
because any number of intermediate gateways may be used
between successive Route Data entries.
+-------------+-------------+-------------+
| 131 | Length | Pointer |
+-------------+-------------+-------------+-------------+
| Route Data (internet addresses) |
+-------------+-------------+-------------+-------------+
...
+-------------+-------------+-------------+-------------+
136 Stream Identifier (four octets). The two-octet SATNET
stream ID is passed through the internet in this option.
137 Strict Source Routing (eleven to thirty-nine octets).
The originator of this option specifies the addresses of
IP modules through which the datagram must pass on its
way to the destination. The Pointer indicates the Next
Address in the Route Data area; its initial value is
four. The last address in the Route Data area must be
the destination address. As each entry is processed, the
IP module's out-going internet address replaces the Next
Address, and the Pointer is increased by four. If the
value of Pointer exceeds that of Length, the source route
is exhausted and the option is ignored (sent to the
destination in the IP header). The route is Strict
because only those gateways specifically listed may be
used.
+-------------+-------------+-------------+
| 137 | Length | Pointer |
+-------------+-------------+-------------+-------------+
| Route Data (internet addresses) |
+-------------+-------------+-------------+-------------+
...
+-------------+-------------+-------------+-------------+
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TCP IP Users Guide May 1982
Appendix H Status Variable Names
The names of the the variables in a connection's internal
control block (TCB) and the TCP statistics area which may be
given to the STAT JSYS are listed below. They are defined in
STG.MAC (TOPS20) or STORAG.MAC (TENEX) between STAT0 and STATZZ.
The following TCP statistics are always collected.
Statistic
Variable Contents
BGRNC Number of times the TCP background process (BG) has run
BYTRC Total number of data octets received
BYTSC Total number of data octets sent
DGRNC Number of times the TCP delayed action process (DG) has
run
DUPKC Total number of duplicate packets received
FINRC Total number of FIN packets received
FINSC Total number of FIN packets sent
INTBY -1 if the internal bypass (local delivery mechanism) is
enabled, 0 if not
IPPKC Total number of packets received
IPRNC Number of times the TCP input process (IP) has run
OPPKC Total number of packets (instances) sent (includes
retransmissions)
OPRNC Number of times the TCP output process (OP) has run
PZPKC Total number of packets generated
PZRNC Number of times the TCP packetizer process (PZ) has run
RAPKC Total number packets reassembled
RARNC Number of times the TCP reassembler process (RA) has run
RSTRC Total number of RESET packets received
RSTSC Total number of RESET packets sent
RXPKC Total number of packet (instances) retransmitted
RXRNC Number of times the TCP retransmitter process (RX) has
run
SYNRC Total number of SYN packets received
SYNSC Total number of SYN packets sent
TASKC Total number of times a TCP process has been run
The following TCP statistics are not usually collected and
will probably be zero.
Statistic
Variable Length Contents
ACDLA 17. Activation delay histogram
BGUSE Total time charged to background process
DGUSE Total time charged to delayed action process
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TCP IP Users Guide May 1982
Appendix H Status Variable Names
IPDLA 17. Input delay histogram
IPUSE Total time charged to input process
OHUSE Total time charged to TCP overhead
OPDLA 17. Output delay histogram
OPUSE Total time charged to output process
PZDLA 17. Packetizer delay histogram
PZUSE Total time charged to packetizer process
RADLA 17. Reassembler delay histogram
RAUSE Total time charged to reassembler process
RXDLA 17. Retransmitter delay histogram
RXUSE Total time charged to retransmitter process
The internal connection block (TCB) for each connection
contains:
TCB
Variable Length Contents
TABTF ForkX of fork ABORTing this connection
TCBIO 10. IP options to be sent
TCBIR 10. IP options received in last packet
TCBIU 10. IP options specified by user program
TCBTO 10. TCP options to be sent
TCBTR 10. TCP options received in last packet
TCBTU 10. TCP options specified by user program
TCTBS (unimplemented)
TCTSQ (unimplemented)
TERBF (unimplemented)
TERJN (unimplemented)
TERR Error field
TERRF Error wait bit index
TERRT (unimplemented)
TFH Foreign host internet address
TFP Foreign port number
TIFDF IP Do-not-Fragment flag
TIPDO Send IP Data Offset (words)
TIPOR Number of received IP option bytes
TIPOU Number of user specified IP option bytes
TJCN Connection's JCN
TLH Local host internet address
TLP Local port number
TMNRT Retransmission factor "MNRTT"
TMXRT Retransmission factor "MXRTT"
TOFRK ForkN number of fork which owns this connection
TOPFH Foreign host internet address from the Connection
Descriptor block
TOPFP Foreign port number from the Connection
Descriptor block
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TCP IP Users Guide May 1982
Appendix H Status Variable Names
TOPLH Local host internet address from the Connection
Descriptor block
TOPNF OPEN wait bit index
TOWNR Job number of job owning this connection
TPICA Channel to receive "reserved" interrupt
TPICE Channel to receive error interrupt
TPICR Channel to receive receive buffer full interrupt
TPICS Channel to receive send buffer empty interrupt
TPICU Channel to receive urgent data interrupt
TPICX Channel to receive open/closing/closed interrupt
TPIFA Fork to receive "reserved" interrupt
TPIFE Fork to receive error interrupt
TPIFR Fork to receive receive buffer full interrupt
TPIFS Fork to receive send buffer empty interrupt
TPIFU Fork to receive urgent data interrupt
TPIFX Fork to receive open/closing/closed interrupt
TRBS Amount of user receive buffer space available
TRCBY Byte number of next octet to be reassembled
TRIS Initial receive sequence number
TRLAK Sequence number of last acknowledgement sent
TRLFT Sequence number of next octet to be reassembled
TRLWN Sequence number of last receive window sent
TRPP Have a partially reassembled packet
TRSYN State of receive side of connection
TRURG Have more urgent data to be reassembled
TRURP Sequence number of last (+1) receive urgent octet
TRWND Receive window
TRXI Retransmission factor "IRXI"
TRXPD Retransmission factor "BD"
TRXPI Retransmission factor Initial Interval
TRXPN Retransmission factor "BN"
TSABT Connection is being ABORTed (RESET/CLZFF JSYSi)
TSBYT Number of octets available to be sent
TSCB Current send (internal) buffer header
TSCR Secure connection
TSEP Packetizer is being encouraged to generate a
packet
TSFP Packetizer is being forced to generate a packet
TSLFT Sequence number of oldest unacknowledged octet
TSLVC Current security level
TSLVN Next security level
TSMRT Retransmission factor "ERTT"
TSMXB (unimplemented)
TSMXP Maximum send packet size
TSOPN User has been notified connection is open
TSPRS TCP%PS was set at OPEN
TSSEQ Current send sequence number
TSSV TSSEQ is valid
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TCP IP Users Guide May 1982
Appendix H Status Variable Names
TSSYN State of send side of connection
TSTO Retransmission timeout
TSUOP User program opened connection
TSURG Urgent send mode
TSURP Sequence number of current send urgent octet
TSWND Current send window
TTOS IP Type-of-Service field
TTPDO Send TCP Data Offset (words)
TTPOR Number of received TCP option bytes
TTPOU Number of user specified TCP option bytes
TTTL Initial IP Time-to-Live
TTVT Connection is a TVT
TVTL Associated TVT line number
TWLDN Connection was opened with wild net field
TWLDP Connection was opened with wild port field
TWLDT Connection was opened with wild host field
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