Internet Engineering Task Force W. Eddy
Internet-Draft MTI Systems
Obsoletes: 793 (if approved) March 20, 2014
Intended status: Standards Track
Expires: September 21, 2014
Transmission Control Protocol Specification
draft-eddy-rfc793bis-01
Abstract
This document specifies the Internet's Transmission Control Protocol
(TCP). TCP is an important transport layer protocol in the Internet
stack, and has continuously evolved over decades of use and growth of
the Internet. Over this time, a number of changes have been made to
TCP as it was specified in RFC 793, though these have only been
documented in a piecemeal fashion. This document collects and brings
those changes together with the protocol specification from RFC 793.
This document obsoletes RFC 793 and several other RFCs (TODO: list
actual RFCs).
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 21, 2014.
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Copyright Notice
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Table of Contents
1. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Functional Specification . . . . . . . . . . . . . . . . . . 4
3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 14
3.4. Establishing a connection . . . . . . . . . . . . . . . . 20
3.5. Closing a Connection . . . . . . . . . . . . . . . . . . 27
3.6. Precedence and Security . . . . . . . . . . . . . . . . . 29
3.7. Data Communication . . . . . . . . . . . . . . . . . . . 30
3.8. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 34
3.8.1. User/TCP Interface . . . . . . . . . . . . . . . . . 34
3.8.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 40
3.9. Event Processing . . . . . . . . . . . . . . . . . . . . 41
3.10. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 64
4. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 69
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70
6. Security Considerations . . . . . . . . . . . . . . . . . . . 71
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 71
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8. References . . . . . . . . . . . . . . . . . . . . . . . . . 71
8.1. Normative References . . . . . . . . . . . . . . . . . . 71
8.2. Informative References . . . . . . . . . . . . . . . . . 71
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 71
1. Purpose and Scope
In 1983, RFC 793 [2] was released, documenting the Transmission
Control Protocol (TCP), and replacing earlier specifications for TCP
that had been published in the past.
Since that time, TCP has been implemented many times, and has been
used as a transport protocol for numerous applications on the
Internet.
For several decades, RFC 793 plus a number of other documents have
combined to serve as the specification for TCP [3]. Over time,
errata have been identified on RFC 793, as well as deficiencies in
security, performance, and other aspects. A number of enhancements
has grown and been documented separately.
The purpose of this document is to bring together all of the IETF
Standards Track changes that have been made to the basic TCP
functional specification and unify them into an update of the RFC 793
protocol specification. Some companion documents are referenced for
important algorithms that TCP uses (e.g. for congestion control), but
have not been attempted to include in this document. This is a
concious choice, as this base specification can be used with multiple
additional algorithms that are developed and incorporated separately,
but all TCP implementations need to implement this specification as a
common basis in order to interoperate. As some additional TCP
features have become quite complicated themselves (e.g. advanced loss
recovery and congestion control), future companion documents may
attempt to similarly bring these together.
In addition to the protocol specification that descibes the TCP
segment format, generation, and processing rules that are to be
implemented in code, RFC 793 and other updates also contain
informative and descriptive text for human readers to understand
aspects of the protocol design and operation. This document does not
attempt to alter or update those parts of RFC 793, and is focused
only on updating the normative protocol specification. We preserve
references to the documentation containing the important explanations
and rationale, where appropriate.
This document is intended to be useful both in checking existing TCP
implementations for conformance, as well as in writing new
implementations.
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2. Introduction
RFC 793 contains a discussion of the TCP design goals and provides
examples of its operation, including examples of connection
establishment, closing connections, and retransmitting packets to
repair losses.
This document describes the functionality expected in modern
implementations of TCP, and replaces the protocol specification in
RFC 793. It does not replicate or attempt to update the examples and
other discussion in RFC 793. Other documents are referenced to
provide explanation of the theory of operation, rationale, and
detailed discussion of design decisions. This document only focuses
on the normative behavior of the protocol.
TEMPORARY EDITOR'S NOTE: This is an early revision in the process of
updating RFC 793. Many planned changes are not yet incorporated.
Please do not use this revision as a basis for any work or reference.
TODO: describe the subsequent structure of the document to-be (e.g.
will it follow the newtcp BSD implementation?), and mention that a
list of changes from RFC 793 will be kept in the final section
TEMPORARY EDITOR'S NOTE: the current revision of this document does
not yet collect all of the changes that will be in the final version.
The set of content changes planned for future revisions is roughly:
-00 was a proposal for the scope of the document and description
of the need for an update to RFC 793
-01 incorporated the RFC 793 section 3 content with no additional
changes into XML2RFC format for easy tracking of the changes
between RFC 793 and future revisions of the document
-02 is planned to incorporate the verified errata on RFC 793
-03 and beyond are intended to incorporate changes from other RFCs
that updated 793
3. Functional Specification
3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2]. A TCP header follows the internet
header, supplying information specific to the TCP protocol. This
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division allows for the existence of host level protocols other than
TCP.
TCP Header Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |U|A|P|R|S|F| |
| Offset| Reserved |R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TCP Header Format
Note that one tick mark represents one bit position.
Figure 1
Source Port: 16 bits
The source port number.
Destination Port: 16 bits
The destination port number.
Sequence Number: 32 bits
The sequence number of the first data octet in this segment (except
when SYN is present). If SYN is present the sequence number is the
initial sequence number (ISN) and the first data octet is ISN+1.
Acknowledgment Number: 32 bits
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If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent.
Data Offset: 4 bits
The number of 32 bit words in the TCP Header. This indicates where
the data begins. The TCP header (even one including options) is an
integral number of 32 bits long.
Reserved: 6 bits
Reserved for future use. Must be zero.
Control Bits: 6 bits (from left to right):
URG: Urgent Pointer field significant
ACK: Acknowledgment field significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender
Window: 16 bits
The number of data octets beginning with the one indicated in the
acknowledgment field which the sender of this segment is willing to
accept.
Checksum: 16 bits
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros to
form a 16 bit word for checksum purposes. The pad is not
transmitted as part of the segment. While computing the checksum,
the checksum field itself is replaced with zeros.
The checksum also covers a 96 bit pseudo header conceptually
prefixed to the TCP header. This pseudo header contains the Source
Address, the Destination Address, the Protocol, and TCP length.
This gives the TCP protection against misrouted segments. This
information is carried in the Internet Protocol and is transferred
across the TCP/Network interface in the arguments or results of
calls by the TCP on the IP.
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+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
The TCP Length is the TCP header length plus the data length in
octets (this is not an explicitly transmitted quantity, but is
computed), and it does not count the 12 octets of the pseudo
header.
Urgent Pointer: 16 bits
This field communicates the current value of the urgent pointer as
a positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following
the urgent data. This field is only be interpreted in segments
with the URG control bit set.
Options: variable
Options may occupy space at the end of the TCP header and are a
multiple of 8 bits in length. All options are included in the
checksum. An option may begin on any octet boundary. There are
two cases for the format of an option:
Case 1: A single octet of option-kind.
Case 2: An octet of option-kind, an octet of option-length, and
the actual option-data octets.
The option-length counts the two octets of option-kind and option-
length as well as the option-data octets.
Note that the list of options may be shorter than the data offset
field might imply. The content of the header beyond the End-of-
Option option must be header padding (i.e., zero).
A TCP must implement all options.
Currently defined options include (kind indicated in octal):
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Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
Specific Option Definitions
End of Option List
+--------+
|00000000|
+--------+
Kind=0
This option code indicates the end of the option list. This
might not coincide with the end of the TCP header according to
the Data Offset field. This is used at the end of all options,
not the end of each option, and need only be used if the end of
the options would not otherwise coincide with the end of the TCP
header.
No-Operation
+--------+
|00000001|
+--------+
Kind=1
This option code may be used between options, for example, to
align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do
not begin on a word boundary.
Maximum Segment Size
+--------+--------+---------+--------+
|00000010|00000100| max seg size |
+--------+--------+---------+--------+
Kind=2 Length=4
Maximum Segment Size Option Data: 16 bits
If this option is present, then it communicates the maximum
receive segment size at the TCP which sends this segment. This
field must only be sent in the initial connection request (i.e.,
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in segments with the SYN control bit set). If this option is
not used, any segment size is allowed.
Padding: variable
The TCP header padding is used to ensure that the TCP header ends
and data begins on a 32 bit boundary. The padding is composed of
zeros.
3.2. Terminology
Before we can discuss very much about the operation of the TCP we
need to introduce some detailed terminology. The maintenance of a
TCP connection requires the remembering of several variables. We
conceive of these variables being stored in a connection record
called a Transmission Control Block or TCB. Among the variables
stored in the TCB are the local and remote socket numbers, the
security and precedence of the connection, pointers to the user's
send and receive buffers, pointers to the retransmit queue and to the
current segment. In addition several variables relating to the send
and receive sequence numbers are stored in the TCB.
Send Sequence Variables
SND.UNA - send unacknowledged
SND.NXT - send next
SND.WND - send window
SND.UP - send urgent pointer
SND.WL1 - segment sequence number used for last window update
SND.WL2 - segment acknowledgment number used for last window
update
ISS - initial send sequence number
Receive Sequence Variables
RCV.NXT - receive next
RCV.WND - receive window
RCV.UP - receive urgent pointer
IRS - initial receive sequence number
The following diagrams may help to relate some of these variables to
the sequence space.
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Send Sequence Space
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission
4 - future sequence numbers which are not yet allowed
Send Sequence Space
Figure 2
The send window is the portion of the sequence space labeled 3 in
Figure 2.
Receive Sequence Space
1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers allowed for new reception
3 - future sequence numbers which are not yet allowed
Receive Sequence Space
Figure 3
The receive window is the portion of the sequence space labeled 2 in
Figure 3.
There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.
Current Segment Variables
SEG.SEQ - segment sequence number
SEG.ACK - segment acknowledgment number
SEG.LEN - segment length
SEG.WND - segment window
SEG.UP - segment urgent pointer
SEG.PRC - segment precedence value
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A connection progresses through a series of states during its
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional
because it represents the state when there is no TCB, and therefore,
no connection. Briefly the meanings of the states are:
LISTEN - represents waiting for a connection request from any
remote TCP and port.
SYN-SENT - represents waiting for a matching connection request
after having sent a connection request.
SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request.
ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer
phase of the connection.
FIN-WAIT-1 - represents waiting for a connection termination
request from the remote TCP, or an acknowledgment of the
connection termination request previously sent.
FIN-WAIT-2 - represents waiting for a connection termination
request from the remote TCP.
CLOSE-WAIT - represents waiting for a connection termination
request from the local user.
CLOSING - represents waiting for a connection termination request
acknowledgment from the remote TCP.
LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP
(which includes an acknowledgment of its connection termination
request).
TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request.
CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
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ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in Figure 4 illustrates only state changes,
together with the causing events and resulting actions, but addresses
neither error conditions nor actions which are not connected with
state changes. In a later section, more detail is offered with
respect to the reaction of the TCP to events.
NOTE BENE: this diagram is only a summary and must not be taken as
the total specification.
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+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
TCP Connection State Diagram
Figure 4
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3.3. Sequence Numbers
A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is
sequenced, each of them can be acknowledged. The acknowledgment
mechanism employed is cumulative so that an acknowledgment of
sequence number X indicates that all octets up to but not including X
have been received. This mechanism allows for straight-forward
duplicate detection in the presence of retransmission. Numbering of
octets within a segment is that the first data octet immediately
following the header is the lowest numbered, and the following octets
are numbered consecutively.
It is essential to remember that the actual sequence number space is
finite, though very large. This space ranges from 0 to 2**32 - 1.
Since the space is finite, all arithmetic dealing with sequence
numbers must be performed modulo 2**32. This unsigned arithmetic
preserves the relationship of sequence numbers as they cycle from
2**32 - 1 to 0 again. There are some subtleties to computer modulo
arithmetic, so great care should be taken in programming the
comparison of such values. The symbol "=<" means "less than or
equal" (modulo 2**32).
The typical kinds of sequence number comparisons which the TCP must
perform include:
(a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged.
(b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a
retransmission queue).
(c) Determining that an incoming segment contains sequence numbers
which are expected (i.e., that the segment "overlaps" the receive
window).
In response to sending data the TCP will receive acknowledgments.
The following comparisons are needed to process the acknowledgments.
SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP (next sequence
number expected by the receiving TCP)
SEG.SEQ = first sequence number of a segment
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SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:
SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the
sum of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment.
When data is received the following comparisons are needed:
RCV.NXT = next sequence number expected on an incoming segments,
and is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segment
A segment is judged to occupy a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or
RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
The first part of this test checks to see if the beginning of the
segment falls in the window, the second part of the test checks to
see if the end of the segment falls in the window; if the segment
passes either part of the test it contains data in the window.
Actually, it is a little more complicated than this. Due to zero
windows and zero length segments, we have four cases for the
acceptability of an incoming segment:
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Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it is be possible for a TCP to
maintain a zero receive window while transmitting data and receiving
ACKs. However, even when the receive window is zero, a TCP must
process the RST and URG fields of all incoming segments.
We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly
including some control flags in the sequence space so they can be
retransmitted and acknowledged without confusion (i.e., one and only
one copy of the control will be acted upon). Control information is
not physically carried in the segment data space. Consequently, we
must adopt rules for implicitly assigning sequence numbers to
control. The SYN and FIN are the only controls requiring this
protection, and these controls are used only at connection opening
and closing. For sequence number purposes, the SYN is considered to
occur before the first actual data octet of the segment in which it
occurs, while the FIN is considered to occur after the last actual
data octet in a segment in which it occurs. The segment length
(SEG.LEN) includes both data and sequence space occupying controls.
When a SYN is present then SEG.SEQ is the sequence number of the SYN.
Initial Sequence Number Selection
The protocol places no restriction on a particular connection being
used over and over again. A connection is defined by a pair of
sockets. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is
-- "how does the TCP identify duplicate segments from previous
incarnations of the connection?" This problem becomes apparent if
the connection is being opened and closed in quick succession, or if
the connection breaks with loss of memory and is then reestablished.
To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
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be present in the network from an earlier incarnation. We want to
assure this, even if a TCP crashes and loses all knowledge of the
sequence numbers it has been using. When new connections are
created, an initial sequence number (ISN) generator is employed which
selects a new 32 bit ISN. The generator is bound to a (possibly
fictitious) 32 bit clock whose low order bit is incremented roughly
every 4 microseconds. Thus, the ISN cycles approximately every 4.55
hours. Since we assume that segments will stay in the network no
more than the Maximum Segment Lifetime (MSL) and that the MSL is less
than 4.55 hours we can reasonably assume that ISN's will be unique.
For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS)
is learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs must
synchronize on each other's initial sequence numbers. This is done
in an exchange of connection establishing segments carrying a control
bit called "SYN" (for synchronize) and the initial sequence numbers.
As a shorthand, segments carrying the SYN bit are also called "SYNs".
Hence, the solution requires a suitable mechanism for picking an
initial sequence number and a slightly involved handshake to exchange
the ISN's.
The synchronization requires each side to send it's own initial
sequence number and to receive a confirmation of it in acknowledgment
from the other side. Each side must also receive the other side's
initial sequence number and send a confirming acknowledgment.
1) A --> B SYN my sequence number is X
2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Y
Because steps 2 and 3 can be combined in a single message this is
called the three way (or three message) handshake.
A three way handshake is necessary because sequence numbers are not
tied to a global clock in the network, and TCPs may have different
mechanisms for picking the ISN's. The receiver of the first SYN has
no way of knowing whether the segment was an old delayed one or not,
unless it remembers the last sequence number used on the connection
(which is not always possible), and so it must ask the sender to
verify this SYN. The three way handshake and the advantages of a
clock-driven scheme are discussed in [3].
Knowing When to Keep Quiet
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To be sure that a TCP does not create a segment that carries a
sequence number which may be duplicated by an old segment remaining
in the network, the TCP must keep quiet for a maximum segment
lifetime (MSL) before assigning any sequence numbers upon starting up
or recovering from a crash in which memory of sequence numbers in use
was lost. For this specification the MSL is taken to be 2 minutes.
This is an engineering choice, and may be changed if experience
indicates it is desirable to do so. Note that if a TCP is
reinitialized in some sense, yet retains its memory of sequence
numbers in use, then it need not wait at all; it must only be sure to
use sequence numbers larger than those recently used.
The TCP Quiet Time Concept
This specification provides that hosts which "crash" without
retaining any knowledge of the last sequence numbers transmitted on
each active (i.e., not closed) connection shall delay emitting any
TCP segments for at least the agreed Maximum Segment Lifetime (MSL)
in the internet system of which the host is a part. In the
paragraphs below, an explanation for this specification is given.
TCP implementors may violate the "quiet time" restriction, but only
at the risk of causing some old data to be accepted as new or new
data rejected as old duplicated by some receivers in the internet
system.
TCPs consume sequence number space each time a segment is formed and
entered into the network output queue at a source host. The
duplicate detection and sequencing algorithm in the TCP protocol
relies on the unique binding of segment data to sequence space to the
extent that sequence numbers will not cycle through all 2**32 values
before the segment data bound to those sequence numbers has been
delivered and acknowledged by the receiver and all duplicate copies
of the segments have "drained" from the internet. Without such an
assumption, two distinct TCP segments could conceivably be assigned
the same or overlapping sequence numbers, causing confusion at the
receiver as to which data is new and which is old. Remember that
each segment is bound to as many consecutive sequence numbers as
there are octets of data in the segment.
Under normal conditions, TCPs keep track of the next sequence number
to emit and the oldest awaiting acknowledgment so as to avoid
mistakenly using a sequence number over before its first use has been
acknowledged. This alone does not guarantee that old duplicate data
is drained from the net, so the sequence space has been made very
large to reduce the probability that a wandering duplicate will cause
trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours to use
up 2**32 octets of sequence space. Since the maximum segment
lifetime in the net is not likely to exceed a few tens of seconds,
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this is deemed ample protection for foreseeable nets, even if data
rates escalate to l0's of megabits/sec. At 100 megabits/sec, the
cycle time is 5.4 minutes which may be a little short, but still
within reason.
The basic duplicate detection and sequencing algorithm in TCP can be
defeated, however, if a source TCP does not have any memory of the
sequence numbers it last used on a given connection. For example, if
the TCP were to start all connections with sequence number 0, then
upon crashing and restarting, a TCP might re-form an earlier
connection (possibly after half-open connection resolution) and emit
packets with sequence numbers identical to or overlapping with
packets still in the network which were emitted on an earlier
incarnation of the same connection. In the absence of knowledge
about the sequence numbers used on a particular connection, the TCP
specification recommends that the source delay for MSL seconds before
emitting segments on the connection, to allow time for segments from
the earlier connection incarnation to drain from the system.
Even hosts which can remember the time of day and used it to select
initial sequence number values are not immune from this problem
(i.e., even if time of day is used to select an initial sequence
number for each new connection incarnation).
Suppose, for example, that a connection is opened starting with
sequence number S. Suppose that this connection is not used much and
that eventually the initial sequence number function (ISN(t)) takes
on a value equal to the sequence number, say S1, of the last segment
sent by this TCP on a particular connection. Now suppose, at this
instant, the host crashes, recovers, and establishes a new
incarnation of the connection. The initial sequence number chosen is
S1 = ISN(t) -- last used sequence number on old incarnation of
connection! If the recovery occurs quickly enough, any old
duplicates in the net bearing sequence numbers in the neighborhood of
S1 may arrive and be treated as new packets by the receiver of the
new incarnation of the connection.
The problem is that the recovering host may not know for how long it
crashed nor does it know whether there are still old duplicates in
the system from earlier connection incarnations.
One way to deal with this problem is to deliberately delay emitting
segments for one MSL after recovery from a crash- this is the "quite
time" specification. Hosts which prefer to avoid waiting are willing
to risk possible confusion of old and new packets at a given
destination may choose not to wait for the "quite time".
Implementors may provide TCP users with the ability to select on a
connection by connection basis whether to wait after a crash, or may
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informally implement the "quite time" for all connections.
Obviously, even where a user selects to "wait," this is not necessary
after the host has been "up" for at least MSL seconds.
To summarize: every segment emitted occupies one or more sequence
numbers in the sequence space, the numbers occupied by a segment are
"busy" or "in use" until MSL seconds have passed, upon crashing a
block of space-time is occupied by the octets of the last emitted
segment, if a new connection is started too soon and uses any of the
sequence numbers in the space-time footprint of the last segment of
the previous connection incarnation, there is a potential sequence
number overlap area which could cause confusion at the receiver.
3.4. Establishing a connection
The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP and
responded to by another TCP. The procedure also works if two TCP
simultaneously initiate the procedure. When simultaneous attempt
occurs, each TCP receives a "SYN" segment which carries no
acknowledgment after it has sent a "SYN". Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the
recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases.
Several examples of connection initiation follow. Although these
examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP
doesn't deliver the data to the user until it is clear the data is
valid (i.e., the data must be buffered at the receiver until the
connection reaches the ESTABLISHED state). The three-way handshake
reduces the possibility of false connections. It is the
implementation of a trade-off between memory and messages to provide
information for this checking.
The simplest three-way handshake is shown in Figure 5 below. The
figures should be interpreted in the following way. Each line is
numbered for reference purposes. Right arrows (-->) indicate
departure of a TCP segment from TCP A to TCP B, or arrival of a
segment at B from A. Left arrows (<--), indicate the reverse.
Ellipsis (...) indicates a segment which is still in the network
(delayed). An "XXX" indicates a segment which is lost or rejected.
Comments appear in parentheses. TCP states represent the state AFTER
the departure or arrival of the segment (whose contents are shown in
the center of each line). Segment contents are shown in abbreviated
form, with sequence number, control flags, and ACK field. Other
fields such as window, addresses, lengths, and text have been left
out in the interest of clarity.
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TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Basic 3-Way Handshake for Connection Synchronization
Figure 5
In line 2 of Figure 5, TCP A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it
received from TCP A. Note that the acknowledgment field indicates
TCP B is now expecting to hear sequence 101, acknowledging the SYN
which occupied sequence 100.
At line 4, TCP A responds with an empty segment containing an ACK for
TCP B's SYN; and in line 5, TCP A sends some data. Note that the
sequence number of the segment in line 5 is the same as in line 4
because the ACK does not occupy sequence number space (if it did, we
would wind up ACKing ACK's!).
Simultaneous initiation is only slightly more complex, as is shown in
Figure 6. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
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TCP A TCP B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
Simultaneous Connection Synchronization
Figure 6
The principle reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion. To deal
with this, a special control message, reset, has been devised. If
the receiving TCP is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED, FIN-
WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user. We discuss this latter
case under "half-open" connections below.
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TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Recovery from Old Duplicate SYN
Figure 7
As a simple example of recovery from old duplicates, consider
Figure 7. At line 3, an old duplicate SYN arrives at TCP B. TCP B
cannot tell that this is an old duplicate, so it responds normally
(line 4). TCP A detects that the ACK field is incorrect and returns
a RST (reset) with its SEQ field selected to make the segment
believable. TCP B, on receiving the RST, returns to the LISTEN
state. When the original SYN (pun intended) finally arrives at line
6, the synchronization proceeds normally. If the SYN at line 6 had
arrived before the RST, a more complex exchange might have occurred
with RST's sent in both directions.
Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the
TCPs has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a crash that resulted in loss of
memory. Such connections will automatically become reset if an
attempt is made to send data in either direction. However, half-open
connections are expected to be unusual, and the recovery procedure is
mildly involved.
If at site A the connection no longer exists, then an attempt by the
user at site B to send any data on it will result in the site B TCP
receiving a reset control message. Such a message indicates to the
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site B TCP that something is wrong, and it is expected to abort the
connection.
Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP.
Depending on the operating system supporting A's TCP, it is likely
that some error recovery mechanism exists. When the TCP is up again,
A is likely to start again from the beginning or from a recovery
point. As a result, A will probably try to OPEN the connection again
or try to SEND on the connection it believes open. In the latter
case, it receives the error message "connection not open" from the
local (A's) TCP. In an attempt to establish the connection, A's TCP
will send a segment containing SYN. This scenario leads to the
example shown in Figure 8. After TCP A crashes, the user attempts to
re-open the connection. TCP B, in the meantime, thinks the
connection is open.
TCP A TCP B
1. (CRASH) (send 300,receive 100)
2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Half-Open Connection Discovery
Figure 8
When the SYN arrives at line 3, TCP B, being in a synchronized state,
and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK
100). TCP A sees that this segment does not acknowledge anything it
sent and, being unsynchronized, sends a reset (RST) because it has
detected a half-open connection. TCP B aborts at line 5. TCP A will
continue to try to establish the connection; the problem is now
reduced to the basic 3-way handshake of Figure 5.
An interesting alternative case occurs when TCP A crashes and TCP B
tries to send data on what it thinks is a synchronized connection.
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This is illustrated in Figure 9. In this case, the data arriving at
TCP A from TCP B (line 2) is unacceptable because no such connection
exists, so TCP A sends a RST. The RST is acceptable so TCP B
processes it and aborts the connection.
TCP A TCP B
1. (CRASH) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Active Side Causes Half-Open Connection Discovery
Figure 9
In Figure 10, we find the two TCPs A and B with passive connections
waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B
into action. A SYN-ACK is returned (line 3) and causes TCP A to
generate a RST (the ACK in line 3 is not acceptable). TCP B accepts
the reset and returns to its passive LISTEN state.
TCP A TCP B
1. LISTEN LISTEN
2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN
Old Duplicate SYN Initiates a Reset on two Passive Sockets
Figure 10
A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing.
Reset Generation
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As a general rule, reset (RST) must be sent whenever a segment
arrives which apparently is not intended for the current connection.
A reset must not be sent if it is not clear that this is the case.
There are three groups of states:
1. If the connection does not exist (CLOSED) then a reset is sent
in response to any incoming segment except another reset. In
particular, SYNs addressed to a non-existent connection are
rejected by this means.
If the incoming segment has an ACK field, the reset takes its
sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment.
The connection remains in the CLOSED state.
2. If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
something not yet sent (the segment carries an unacceptable ACK),
or if an incoming segment has a security level or compartment
which does not exactly match the level and compartment requested
for the connection, a reset is sent.
If our SYN has not been acknowledged and the precedence level of
the incoming segment is higher than the precedence level requested
then either raise the local precedence level (if allowed by the
user and the system) or send a reset; or if the precedence level
of the incoming segment is lower than the precedence level
requested then continue as if the precedence matched exactly (if
the remote TCP cannot raise the precedence level to match ours
this will be detected in the next segment it sends, and the
connection will be terminated then). If our SYN has been
acknowledged (perhaps in this incoming segment) the precedence
level of the incoming segment must match the local precedence
level exactly, if it does not a reset must be sent.
If the incoming segment has an ACK field, the reset takes its
sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment.
The connection remains in the same state.
3. If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
any unacceptable segment (out of window sequence number or
unacceptible acknowledgment number) must elicit only an empty
acknowledgment segment containing the current send-sequence number
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and an acknowledgment indicating the next sequence number expected
to be received, and the connection remains in the same state.
If an incoming segment has a security level, or compartment, or
precedence which does not exactly match the level, and
compartment, and precedence requested for the connection,a reset
is sent and connection goes to the CLOSED state. The reset takes
its sequence number from the ACK field of the incoming segment.
Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated
by checking their SEQ-fields. A reset is valid if its sequence
number is in the window. In the SYN-SENT state (a RST received in
response to an initial SYN), the RST is acceptable if the ACK field
acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the
receiver was in the LISTEN state, it ignores it. If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state. If the receiver
was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state.
3.5. Closing a Connection
CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection. We have chosen to treat CLOSE
in a simplex fashion. The user who CLOSEs may continue to RECEIVE
until he is told that the other side has CLOSED also. Thus, a
program could initiate several SENDs followed by a CLOSE, and then
continue to RECEIVE until signaled that a RECEIVE failed because the
other side has CLOSED. We assume that the TCP will signal a user,
even if no RECEIVEs are outstanding, that the other side has closed,
so the user can terminate his side gracefully. A TCP will reliably
deliver all buffers SENT before the connection was CLOSED so a user
who expects no data in return need only wait to hear the connection
was CLOSED successfully to know that all his data was received at the
destination TCP. Users must keep reading connections they close for
sending until the TCP says no more data.
There are essentially three cases:
1) The user initiates by telling the TCP to CLOSE the connection
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2) The remote TCP initiates by sending a FIN control signal
3) Both users CLOSE simultaneously
Case 1: Local user initiates the close
In this case, a FIN segment can be constructed and placed on the
outgoing segment queue. No further SENDs from the user will be
accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs
are allowed in this state. All segments preceding and including
FIN will be retransmitted until acknowledged. When the other TCP
has both acknowledged the FIN and sent a FIN of its own, the first
TCP can ACK this FIN. Note that a TCP receiving a FIN will ACK
but not send its own FIN until its user has CLOSED the connection
also.
Case 2: TCP receives a FIN from the network
If an unsolicited FIN arrives from the network, the receiving TCP
can ACK it and tell the user that the connection is closing. The
user will respond with a CLOSE, upon which the TCP can send a FIN
to the other TCP after sending any remaining data. The TCP then
waits until its own FIN is acknowledged whereupon it deletes the
connection. If an ACK is not forthcoming, after the user timeout
the connection is aborted and the user is told.
Case 3: both users close simultaneously
A simultaneous CLOSE by users at both ends of a connection causes
FIN segments to be exchanged. When all segments preceding the
FINs have been processed and acknowledged, each TCP can ACK the
FIN it has received. Both will, upon receiving these ACKs, delete
the connection.
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TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL)
CLOSED
Normal Close Sequence
Figure 11
TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL)
CLOSED CLOSED
Simultaneous Close Sequence
Figure 12
3.6. Precedence and Security
The intent is that connection be allowed only between ports operating
with exactly the same security and compartment values and at the
higher of the precedence level requested by the two ports.
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The precedence and security parameters used in TCP are exactly those
defined in the Internet Protocol (IP) [2]. Throughout this TCP
specification the term "security/compartment" is intended to indicate
the security parameters used in IP including security, compartment,
user group, and handling restriction.
A connection attempt with mismatched security/compartment values or a
lower precedence value must be rejected by sending a reset.
Rejecting a connection due to too low a precedence only occurs after
an acknowledgment of the SYN has been received.
Note that TCP modules which operate only at the default value of
precedence will still have to check the precedence of incoming
segments and possibly raise the precedence level they use on the
connection.
The security paramaters may be used even in a non-secure environment
(the values would indicate unclassified data), thus hosts in non-
secure environments must be prepared to receive the security
parameters, though they need not send them.
3.7. Data Communication
Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses
retransmission (after a timeout) to ensure delivery of every segment.
Duplicate segments may arrive due to network or TCP retransmission.
As discussed in the section on sequence numbers the TCP performs
certain tests on the sequence and acknowledgment numbers in the
segments to verify their acceptability.
The sender of data keeps track of the next sequence number to use in
the variable SND.NXT. The receiver of data keeps track of the next
sequence number to expect in the variable RCV.NXT. The sender of
data keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA. If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal.
When the sender creates a segment and transmits it the sender
advances SND.NXT. When the receiver accepts a segment it advances
RCV.NXT and sends an acknowledgment. When the data sender receives
an acknowledgment it advances SND.UNA. The extent to which the
values of these variables differ is a measure of the delay in the
communication. The amount by which the variables are advanced is the
length of the data in the segment. Note that once in the ESTABLISHED
state all segments must carry current acknowledgment information.
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The CLOSE user call implies a push function, as does the FIN control
flag in an incoming segment.
Retransmission Timeout
Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the
retransmission timeout must be dynamically determined. One procedure
for determining a retransmission time out is given here as an
illustration.
An Example Retransmission Timeout Procedure
Measure the elapsed time between sending a data octet with a
particular sequence number and receiving an acknowledgment that
covers that sequence number (segments sent do not have to match
segments received). This measured elapsed time is the Round Trip
Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as:
SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
and based on this, compute the retransmission timeout (RTO) as:
RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
where UBOUND is an upper bound on the timeout (e.g., 1 minute),
LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
factor (e.g., 1.3 to 2.0).
The Communication of Urgent Information
The objective of the TCP urgent mechanism is to allow the sending
user to stimulate the receiving user to accept some urgent data and
to permit the receiving TCP to indicate to the receiving user when
all the currently known urgent data has been received by the user.
This mechanism permits a point in the data stream to be designated as
the end of urgent information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
must tell the user to go into "urgent mode"; when the receive
sequence number catches up to the urgent pointer, the TCP must tell
user to go into "normal mode". If the urgent pointer is updated
while the user is in "urgent mode", the update will be invisible to
the user.
The method employs a urgent field which is carried in all segments
transmitted. The URG control flag indicates that the urgent field is
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meaningful and must be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that there is
no urgent data outstanding.
To send an urgent indication the user must also send at least one
data octet. If the sending user also indicates a push, timely
delivery of the urgent information to the destination process is
enhanced.
Managing the Window
The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently
prepared to accept. There is an assumption that this is related to
the currently available data buffer space available for this
connection.
Indicating a large window encourages transmissions. If more data
arrives than can be accepted, it will be discarded. This will result
in excessive retransmissions, adding unnecessarily to the load on the
network and the TCPs. Indicating a small window may restrict the
transmission of data to the point of introducing a round trip delay
between each new segment transmitted.
The mechanisms provided allow a TCP to advertise a large window and
to subsequently advertise a much smaller window without having
accepted that much data. This, so called "shrinking the window," is
strongly discouraged. The robustness principle dictates that TCPs
will not shrink the window themselves, but will be prepared for such
behavior on the part of other TCPs.
The sending TCP must be prepared to accept from the user and send at
least one octet of new data even if the send window is zero. The
sending TCP must regularly retransmit to the receiving TCP even when
the window is zero. Two minutes is recommended for the
retransmission interval when the window is zero. This retransmission
is essential to guarantee that when either TCP has a zero window the
re-opening of the window will be reliably reported to the other.
When the receiving TCP has a zero window and a segment arrives it
must still send an acknowledgment showing its next expected sequence
number and current window (zero).
The sending TCP packages the data to be transmitted into segments
which fit the current window, and may repackage segments on the
retransmission queue. Such repackaging is not required, but may be
helpful.
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In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence
number so there will be no way to reorder them if they arrive out of
order. This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from
the data receiver. A refinement to avoid this problem is to act on
the window information from segments that carry the highest
acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received).
The window management procedure has significant influence on the
communication performance. The following comments are suggestions to
implementers.
Window Management Suggestions
Allocating a very small window causes data to be transmitted in
many small segments when better performance is achieved using
fewer large segments.
One suggestion for avoiding small windows is for the receiver to
defer updating a window until the additional allocation is at
least X percent of the maximum allocation possible for the
connection (where X might be 20 to 40).
Another suggestion is for the sender to avoid sending small
segments by waiting until the window is large enough before
sending data. If the the user signals a push function then the
data must be sent even if it is a small segment.
Note that the acknowledgments should not be delayed or unnecessary
retransmissions will result. One strategy would be to send an
acknowledgment when a small segment arrives (with out updating the
window information), and then to send another acknowledgment with
new window information when the window is larger.
The segment sent to probe a zero window may also begin a break up
of transmitted data into smaller and smaller segments. If a
segment containing a single data octet sent to probe a zero window
is accepted, it consumes one octet of the window now available.
If the sending TCP simply sends as much as it can whenever the
window is non zero, the transmitted data will be broken into
alternating big and small segments. As time goes on, occasional
pauses in the receiver making window allocation available will
result in breaking the big segments into a small and not quite so
big pair. And after a while the data transmission will be in
mostly small segments.
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The suggestion here is that the TCP implementations need to
actively attempt to combine small window allocations into larger
windows, since the mechanisms for managing the window tend to lead
to many small windows in the simplest minded implementations.
3.8. Interfaces
There are of course two interfaces of concern: the user/TCP interface
and the TCP/lower-level interface. We have a fairly elaborate model
of the user/TCP interface, but the interface to the lower level
protocol module is left unspecified here, since it will be specified
in detail by the specification of the lowel level protocol. For the
case that the lower level is IP we note some of the parameter values
that TCPs might use.
3.8.1. User/TCP Interface
The following functional description of user commands to the TCP is,
at best, fictional, since every operating system will have different
facilities. Consequently, we must warn readers that different TCP
implementations may have different user interfaces. However, all
TCPs must provide a certain minimum set of services to guarantee that
all TCP implementations can support the same protocol hierarchy.
This section specifies the functional interfaces required of all TCP
implementations.
TCP User Commands
The following sections functionally characterize a USER/TCP
interface. The notation used is similar to most procedure or
function calls in high level languages, but this usage is not
meant to rule out trap type service calls (e.g., SVCs, UUOs,
EMTs).
The user commands described below specify the basic functions the
TCP must perform to support interprocess communication.
Individual implementations must define their own exact format, and
may provide combinations or subsets of the basic functions in
single calls. In particular, some implementations may wish to
automatically OPEN a connection on the first SEND or RECEIVE
issued by the user for a given connection.
In providing interprocess communication facilities, the TCP must
not only accept commands, but must also return information to the
processes it serves. The latter consists of:
(a) general information about a connection (e.g., interrupts,
remote close, binding of unspecified foreign socket).
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(b) replies to specific user commands indicating success or
various types of failure.
Open
Format: OPEN (local port, foreign socket, active/passive [,
timeout] [, precedence] [, security/compartment] [, options])
-> local connection name
We assume that the local TCP is aware of the identity of the
processes it serves and will check the authority of the process
to use the connection specified. Depending upon the
implementation of the TCP, the local network and TCP
identifiers for the source address will either be supplied by
the TCP or the lower level protocol (e.g., IP). These
considerations are the result of concern about security, to the
extent that no TCP be able to masquerade as another one, and so
on. Similarly, no process can masquerade as another without
the collusion of the TCP.
If the active/passive flag is set to passive, then this is a
call to LISTEN for an incoming connection. A passive open may
have either a fully specified foreign socket to wait for a
particular connection or an unspecified foreign socket to wait
for any call. A fully specified passive call can be made
active by the subsequent execution of a SEND.
A transmission control block (TCB) is created and partially
filled in with data from the OPEN command parameters.
On an active OPEN command, the TCP will begin the procedure to
synchronize (i.e., establish) the connection at once.
The timeout, if present, permits the caller to set up a timeout
for all data submitted to TCP. If data is not successfully
delivered to the destination within the timeout period, the TCP
will abort the connection. The present global default is five
minutes.
The TCP or some component of the operating system will verify
the users authority to open a connection with the specified
precedence or security/compartment. The absence of precedence
or security/compartment specification in the OPEN call
indicates the default values must be used.
TCP will accept incoming requests as matching only if the
security/compartment information is exactly the same and only
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if the precedence is equal to or higher than the precedence
requested in the OPEN call.
The precedence for the connection is the higher of the values
requested in the OPEN call and received from the incoming
request, and fixed at that value for the life of the
connection.Implementers may want to give the user control of
this precedence negotiation. For example, the user might be
allowed to specify that the precedence must be exactly matched,
or that any attempt to raise the precedence be confirmed by the
user.
A local connection name will be returned to the user by the
TCP. The local connection name can then be used as a short
hand term for the connection defined by the <local socket,
foreign socket> pair.
Send
Format: SEND (local connection name, buffer address, byte
count, PUSH flag, URGENT flag [,timeout])
This call causes the data contained in the indicated user
buffer to be sent on the indicated connection. If the
connection has not been opened, the SEND is considered an
error. Some implementations may allow users to SEND first; in
which case, an automatic OPEN would be done. If the calling
process is not authorized to use this connection, an error is
returned.
If the PUSH flag is set, the data must be transmitted promptly
to the receiver, and the PUSH bit will be set in the last TCP
segment created from the buffer. If the PUSH flag is not set,
the data may be combined with data from subsequent SENDs for
transmission efficiency.
If the URGENT flag is set, segments sent to the destination TCP
will have the urgent pointer set. The receiving TCP will
signal the urgent condition to the receiving process if the
urgent pointer indicates that data preceding the urgent pointer
has not been consumed by the receiving process. The purpose of
urgent is to stimulate the receiver to process the urgent data
and to indicate to the receiver when all the currently known
urgent data has been received. The number of times the sending
user's TCP signals urgent will not necessarily be equal to the
number of times the receiving user will be notified of the
presence of urgent data.
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If no foreign socket was specified in the OPEN, but the
connection is established (e.g., because a LISTENing connection
has become specific due to a foreign segment arriving for the
local socket), then the designated buffer is sent to the
implied foreign socket. Users who make use of OPEN with an
unspecified foreign socket can make use of SEND without ever
explicitly knowing the foreign socket address.
However, if a SEND is attempted before the foreign socket
becomes specified, an error will be returned. Users can use
the STATUS call to determine the status of the connection. In
some implementations the TCP may notify the user when an
unspecified socket is bound.
If a timeout is specified, the current user timeout for this
connection is changed to the new one.
In the simplest implementation, SEND would not return control
to the sending process until either the transmission was
complete or the timeout had been exceeded. However, this
simple method is both subject to deadlocks (for example, both
sides of the connection might try to do SENDs before doing any
RECEIVEs) and offers poor performance, so it is not
recommended. A more sophisticated implementation would return
immediately to allow the process to run concurrently with
network I/O, and, furthermore, to allow multiple SENDs to be in
progress. Multiple SENDs are served in first come, first
served order, so the TCP will queue those it cannot service
immediately.
We have implicitly assumed an asynchronous user interface in
which a SEND later elicits some kind of SIGNAL or pseudo-
interrupt from the serving TCP. An alternative is to return a
response immediately. For instance, SENDs might return
immediate local acknowledgment, even if the segment sent had
not been acknowledged by the distant TCP. We could
optimistically assume eventual success. If we are wrong, the
connection will close anyway due to the timeout. In
implementations of this kind (synchronous), there will still be
some asynchronous signals, but these will deal with the
connection itself, and not with specific segments or buffers.
In order for the process to distinguish among error or success
indications for different SENDs, it might be appropriate for
the buffer address to be returned along with the coded response
to the SEND request. TCP-to-user signals are discussed below,
indicating the information which should be returned to the
calling process.
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Receive
Format: RECEIVE (local connection name, buffer address, byte
count) -> byte count, urgent flag, push flag
This command allocates a receiving buffer associated with the
specified connection. If no OPEN precedes this command or the
calling process is not authorized to use this connection, an
error is returned.
In the simplest implementation, control would not return to the
calling program until either the buffer was filled, or some
error occurred, but this scheme is highly subject to deadlocks.
A more sophisticated implementation would permit several
RECEIVEs to be outstanding at once. These would be filled as
segments arrive. This strategy permits increased throughput at
the cost of a more elaborate scheme (possibly asynchronous) to
notify the calling program that a PUSH has been seen or a
buffer filled.
If enough data arrive to fill the buffer before a PUSH is seen,
the PUSH flag will not be set in the response to the RECEIVE.
The buffer will be filled with as much data as it can hold. If
a PUSH is seen before the buffer is filled the buffer will be
returned partially filled and PUSH indicated.
If there is urgent data the user will have been informed as
soon as it arrived via a TCP-to-user signal. The receiving
user should thus be in "urgent mode". If the URGENT flag is
on, additional urgent data remains. If the URGENT flag is off,
this call to RECEIVE has returned all the urgent data, and the
user may now leave "urgent mode". Note that data following the
urgent pointer (non-urgent data) cannot be delivered to the
user in the same buffer with preceeding urgent data unless the
boundary is clearly marked for the user.
To distinguish among several outstanding RECEIVEs and to take
care of the case that a buffer is not completely filled, the
return code is accompanied by both a buffer pointer and a byte
count indicating the actual length of the data received.
Alternative implementations of RECEIVE might have the TCP
allocate buffer storage, or the TCP might share a ring buffer
with the user.
Close
Format: CLOSE (local connection name)
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This command causes the connection specified to be closed. If
the connection is not open or the calling process is not
authorized to use this connection, an error is returned.
Closing connections is intended to be a graceful operation in
the sense that outstanding SENDs will be transmitted (and
retransmitted), as flow control permits, until all have been
serviced. Thus, it should be acceptable to make several SEND
calls, followed by a CLOSE, and expect all the data to be sent
to the destination. It should also be clear that users should
continue to RECEIVE on CLOSING connections, since the other
side may be trying to transmit the last of its data. Thus,
CLOSE means "I have no more to send" but does not mean "I will
not receive any more." It may happen (if the user level
protocol is not well thought out) that the closing side is
unable to get rid of all its data before timing out. In this
event, CLOSE turns into ABORT, and the closing TCP gives up.
The user may CLOSE the connection at any time on his own
initiative, or in response to various prompts from the TCP
(e.g., remote close executed, transmission timeout exceeded,
destination inaccessible).
Because closing a connection requires communication with the
foreign TCP, connections may remain in the closing state for a
short time. Attempts to reopen the connection before the TCP
replies to the CLOSE command will result in error responses.
Close also implies push function.
Status
Format: STATUS (local connection name) -> status data
This is an implementation dependent user command and could be
excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection.
This command returns a data block containing the following
information:
local socket,
foreign socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting acknowledgment,
number of buffers pending receipt,
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urgent state,
precedence,
security/compartment,
and transmission timeout.
Depending on the state of the connection, or on the
implementation itself, some of this information may not be
available or meaningful. If the calling process is not
authorized to use this connection, an error is returned. This
prevents unauthorized processes from gaining information about
a connection.
Abort
Format: ABORT (local connection name)
This command causes all pending SENDs and RECEIVES to be
aborted, the TCB to be removed, and a special RESET message to
be sent to the TCP on the other side of the connection.
Depending on the implementation, users may receive abort
indications for each outstanding SEND or RECEIVE, or may simply
receive an ABORT-acknowledgment.
TCP-to-User Messages
It is assumed that the operating system environment provides a
means for the TCP to asynchronously signal the user program.
When the TCP does signal a user program, certain information is
passed to the user. Often in the specification the information
will be an error message. In other cases there will be
information relating to the completion of processing a SEND or
RECEIVE or other user call.
The following information is provided:
Local Connection Name Always
Response String Always
Buffer Address Send & Receive
Byte count (counts bytes received) Receive
Push flag Receive
Urgent flag Receive
3.8.2. TCP/Lower-Level Interface
The TCP calls on a lower level protocol module to actually send and
receive information over a network. One case is that of the ARPA
internetwork system where the lower level module is the Internet
Protocol (IP) [2].
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If the lower level protocol is IP it provides arguments for a type of
service and for a time to live. TCP uses the following settings for
these parameters:
Type of Service = Precedence: routine, Delay: normal, Throughput:
normal, Reliability: normal; or 00000000.
Time to Live = one minute, or 00111100.
Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it cannot
be delivered by the internet system within one minute.
If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP checksum
be the originating source and ultimate destination. It is also
important to preserve the return route to answer connection requests.
Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivlent service of
IP and to be used in the TCP checksum.
3.9. Event Processing
The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different
processing sequences, but they should differ from those in this
section only in detail, not in substance.
The activity of the TCP can be characterized as responding to events.
The events that occur can be cast into three categories: user calls,
arriving segments, and timeouts. This section describes the
processing the TCP does in response to each of the events. In many
cases the processing required depends on the state of the connection.
Events that occur:
User Calls
OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
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Arriving Segments
SEGMENT ARRIVES
Timeouts
USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response.
Error responses are given as character strings. For example, user
commands referencing connections that do not exist receive "error:
connection not open".
Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space. Also note that "=<" means less than or
equal to (modulo 2**32).
A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.
When a segment overlaps other already received segments we
reconstruct the segment to contain just the new data, and adjust the
header fields to be consistent.
Note that if no state change is mentioned the TCP stays in the same
state.
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OPEN Call
CLOSED STATE (i.e., TCB does not exist)
Create a new transmission control block (TCB) to hold
connection state information. Fill in local socket identifier,
foreign socket, precedence, security/compartment, and user
timeout information. Note that some parts of the foreign
socket may be unspecified in a passive OPEN and are to be
filled in by the parameters of the incoming SYN segment.
Verify the security and precedence requested are allowed for
this user, if not return "error: precedence not allowed" or
"error: security/compartment not allowed." If passive enter
the LISTEN state and return. If active and the foreign socket
is unspecified, return "error: foreign socket unspecified"; if
active and the foreign socket is specified, issue a SYN
segment. An initial send sequence number (ISS) is selected. A
SYN segment of the form <SEQ=ISS><CTL=SYN> is sent. Set
SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT state, and
return.
If the caller does not have access to the local socket
specified, return "error: connection illegal for this process".
If there is no room to create a new connection, return "error:
insufficient resources".
LISTEN STATE
If active and the foreign socket is specified, then change the
connection from passive to active, select an ISS. Send a SYN
segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT
state. Data associated with SEND may be sent with SYN segment
or queued for transmission after entering ESTABLISHED state.
The urgent bit if requested in the command must be sent with
the data segments sent as a result of this command. If there
is no room to queue the request, respond with "error:
insufficient resources". If Foreign socket was not specified,
then return "error: foreign socket unspecified".
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SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection already exists".
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SEND Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, then
return "error: connection illegal for this process".
Otherwise, return "error: connection does not exist".
LISTEN STATE
If the foreign socket is specified, then change the connection
from passive to active, select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit
if requested in the command must be sent with the data segments
sent as a result of this command. If there is no room to queue
the request, respond with "error: insufficient resources". If
Foreign socket was not specified, then return "error: foreign
socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
Queue the data for transmission after entering ESTABLISHED
state. If no space to queue, respond with "error: insufficient
resources".
ESTABLISHED STATE
CLOSE-WAIT STATE
Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). If there is
insufficient space to remember this buffer, simply return
"error: insufficient resources".
If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
urgent pointer in the outgoing segments.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection closing" and do not service request.
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RECEIVE Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
Queue for processing after entering ESTABLISHED state. If
there is no room to queue this request, respond with "error:
insufficient resources".
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient
resources".
Reassemble queued incoming segments into receive buffer and
return to user. Mark "push seen" (PUSH) if this is the case.
If RCV.UP is in advance of the data currently being passed to
the user notify the user of the presence of urgent data.
When the TCP takes responsibility for delivering data to the
user that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming
segment.
CLOSE-WAIT STATE
Since the remote side has already sent FIN, RECEIVEs must be
satisfied by text already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get a
"error: connection closing" response. Otherwise, any remaining
text can be used to satisfy the RECEIVE.
CLOSING STATE
LAST-ACK STATE
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TIME-WAIT STATE
Return "error: connection closing".
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CLOSE Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return
"error: connection illegal for this process".
Otherwise, return "error: connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs are returned with "error: closing"
responses. Delete TCB, enter CLOSED state, and return.
SYN-SENT STATE
Delete the TCB and return "error: closing" responses to any
queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE
If no SENDs have been issued and there is no pending data to
send, then form a FIN segment and send it, and enter FIN-WAIT-1
state; otherwise queue for processing after entering
ESTABLISHED state.
ESTABLISHED STATE
Queue this until all preceding SENDs have been segmentized,
then form a FIN segment and send it. In any case, enter FIN-
WAIT-1 state.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Strictly speaking, this is an error and should receive a
"error: connection closing" response. An "ok" response would
be acceptable, too, as long as a second FIN is not emitted (the
first FIN may be retransmitted though).
CLOSE-WAIT STATE
Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter CLOSING state.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
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Respond with "error: connection closing".
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ABORT Call
CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs should be returned with "error:
connection reset" responses. Delete TCB, enter CLOSED state,
and return.
SYN-SENT STATE
All queued SENDs and RECEIVEs should be given "connection
reset" notification, delete the TCB, enter CLOSED state, and
return.
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
Send a reset segment:
<SEQ=SND.NXT><CTL=RST>
All queued SENDs and RECEIVEs should be given "connection
reset" notification; all segments queued for transmission
(except for the RST formed above) or retransmission should be
flushed, delete the TCB, enter CLOSED state, and return.
CLOSING STATE LAST-ACK STATE TIME-WAIT STATE
Respond with "ok" and delete the TCB, enter CLOSED state, and
return.
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STATUS Call
CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
Return "state = LISTEN", and the TCB pointer.
SYN-SENT STATE
Return "state = SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE
Return "state = SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE
Return "state = ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE
Return "state = FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE
Return "state = FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE
Return "state = CLOSE-WAIT", and the TCB pointer.
CLOSING STATE
Return "state = CLOSING", and the TCB pointer.
LAST-ACK STATE
Return "state = LAST-ACK", and the TCB pointer.
TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer.
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SEGMENT ARRIVES
If the state is CLOSED (i.e., TCB does not exist) then
all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make
the reset sequence acceptable to the TCP that sent the
offending segment.
If the ACK bit is off, sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST>
Return.
If the state is LISTEN then
first check for an RST
An incoming RST should be ignored. Return.
second check for an ACK
Any acknowledgment is bad if it arrives on a connection
still in the LISTEN state. An acceptable reset segment
should be formed for any arriving ACK-bearing segment. The
RST should be formatted as follows:
<SEQ=SEG.ACK><CTL=RST>
Return.
third check for a SYN
If the SYN bit is set, check the security. If the security/
compartment on the incoming segment does not exactly match
the security/compartment in the TCB then send a reset and
return.
<SEQ=SEG.ACK><CTL=RST>
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If the SEG.PRC is greater than the TCB.PRC then if allowed
by the user and the system set TCB.PRC<-SEG.PRC, if not
allowed send a reset and return.
<SEQ=SEG.ACK><CTL=RST>
If the SEG.PRC is less than the TCB.PRC then continue.
Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any
other control or text should be queued for processing later.
ISS should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any
other incoming control or data (combined with SYN) will be
processed in the SYN-RECEIVED state, but processing of SYN
and ACK should not be repeated. If the listen was not fully
specified (i.e., the foreign socket was not fully
specified), then the unspecified fields should be filled in
now.
fourth other text or control
Any other control or text-bearing segment (not containing
SYN) must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid,
since it could not have been sent in response to anything
sent by this incarnation of the connection. So you are
unlikely to get here, but if you do, drop the segment, and
return.
If the state is SYN-SENT then
first check the ACK bit
If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset
(unless the RST bit is set, if so drop the segment and
return)
<SEQ=SEG.ACK><CTL=RST>
and discard the segment. Return.
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If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is
acceptable.
second check the RST bit
If the RST bit is set
If the ACK was acceptable then signal the user "error:
connection reset", drop the segment, enter CLOSED state,
delete TCB, and return. Otherwise (no ACK) drop the
segment and return.
third check the security and precedence
If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset
If there is an ACK
<SEQ=SEG.ACK><CTL=RST>
Otherwise
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If there is an ACK
The precedence in the segment must match the precedence
in the TCB, if not, send a reset
<SEQ=SEG.ACK><CTL=RST>
If there is no ACK
If the precedence in the segment is higher than the
precedence in the TCB then if allowed by the user and the
system raise the precedence in the TCB to that in the
segment, if not allowed to raise the prec then send a
reset.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the precedence in the segment is lower than the
precedence in the TCB continue.
If a reset was sent, discard the segment and return.
fourth check the SYN bit
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This step should be reached only if the ACK is ok, or there
is no ACK, and it the segment did not contain a RST.
If the SYN bit is on and the security/compartment and
precedence are acceptable then, RCV.NXT is set to SEG.SEQ+1,
IRS is set to SEG.SEQ. SND.UNA should be advanced to equal
SEG.ACK (if there is an ACK), and any segments on the
retransmission queue which are thereby acknowledged should
be removed.
If SND.UNA > ISS (our SYN has been ACKed), change the
connection state to ESTABLISHED, form an ACK segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls which were queued for
transmission may be included. If there are other controls
or text in the segment then continue processing at the sixth
step below where the URG bit is checked, otherwise return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. If there are other controls or text in the
segment, queue them for processing after the ESTABLISHED
state has been reached, return.
fifth, if neither of the SYN or RST bits is set then drop the
segment and return.
Otherwise,
first check sequence number
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Segments are processed in sequence. Initial tests on
arrival are used to discard old duplicates, but further
processing is done in SEG.SEQ order. If a segment's
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contents straddle the boundary between old and new, only the
new parts should be processed.
There are four cases for the acceptability test for an
incoming segment:
Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
If the RCV.WND is zero, no segments will be acceptable, but
special allowance should be made to accept valid ACKs, URGs
and RSTs.
If an incoming segment is not acceptable, an acknowledgment
should be sent in reply (unless the RST bit is set, if so
drop the segment and return):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable
segment and return.
In the following it is assumed that the segment is the
idealized segment that begins at RCV.NXT and does not exceed
the window. One could tailor actual segments to fit this
assumption by trimming off any portions that lie outside the
window (including SYN and FIN), and only processing further
if the segment then begins at RCV.NXT. Segments with higher
begining sequence numbers may be held for later processing.
second check the RST bit,
SYN-RECEIVED STATE
If the RST bit is set
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If this connection was initiated with a passive OPEN
(i.e., came from the LISTEN state), then return this
connection to LISTEN state and return. The user need
not be informed. If this connection was initiated
with an active OPEN (i.e., came from SYN-SENT state)
then the connection was refused, signal the user
"connection refused". In either case, all segments on
the retransmission queue should be removed. And in
the active OPEN case, enter the CLOSED state and
delete the TCB, and return.
ESTABLISHED
FIN-WAIT-1
FIN-WAIT-2
CLOSE-WAIT
If the RST bit is set then, any outstanding RECEIVEs and
SEND should receive "reset" responses. All segment
queues should be flushed. Users should also receive an
unsolicited general "connection reset" signal. Enter the
CLOSED state, delete the TCB, and return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
If the RST bit is set then, enter the CLOSED state,
delete the TCB, and return.
third check security and precedence
SYN-RECEIVED
If the security/compartment and precedence in the segment
do not exactly match the security/compartment and
precedence in the TCB then send a reset, and return.
ESTABLISHED STATE
If the security/compartment and precedence in the segment
do not exactly match the security/compartment and
precedence in the TCB then send a reset, any outstanding
RECEIVEs and SEND should receive "reset" responses. All
segment queues should be flushed. Users should also
receive an unsolicited general "connection reset" signal.
Enter the CLOSED state, delete the TCB, and return.
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Note this check is placed following the sequence check to
prevent a segment from an old connection between these ports
with a different security or precedence from causing an
abort of the current connection.
fourth, check the SYN bit,
SYN-RECEIVED
ESTABLISHED STATE
FIN-WAIT STATE-1
FIN-WAIT STATE-2
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
If the SYN is in the window it is an error, send a reset,
any outstanding RECEIVEs and SEND should receive "reset"
responses, all segment queues should be flushed, the user
should also receive an unsolicited general "connection
reset" signal, enter the CLOSED state, delete the TCB,
and return.
If the SYN is not in the window this step would not be
reached and an ack would have been sent in the first step
(sequence number check).
fifth check the ACK field,
if the ACK bit is off drop the segment and return
if the ACK bit is on
SYN-RECEIVED STATE
If SND.UNA =< SEG.ACK =< SND.NXT then enter
ESTABLISHED state and continue processing.
If the segment acknowledgment is not acceptable,
form a reset segment,
<SEQ=SEG.ACK><CTL=RST>
and send it.
ESTABLISHED STATE
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If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <-
SEG.ACK. Any segments on the retransmission queue
which are thereby entirely acknowledged are removed.
Users should receive positive acknowledgments for
buffers which have been SENT and fully acknowledged
(i.e., SEND buffer should be returned with "ok"
response). If the ACK is a duplicate (SEG.ACK <
SND.UNA), it can be ignored. If the ACK acks
something not yet sent (SEG.ACK > SND.NXT) then send
an ACK, drop the segment, and return.
If SND.UNA < SEG.ACK =< SND.NXT, the send window
should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1
= SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-
SEG.ACK.
Note that SND.WND is an offset from SND.UNA, that
SND.WL1 records the sequence number of the last
segment used to update SND.WND, and that SND.WL2
records the acknowledgment number of the last segment
used to update SND.WND. The check here prevents using
old segments to update the window.
FIN-WAIT-1 STATE
In addition to the processing for the ESTABLISHED
state, if our FIN is now acknowledged then enter FIN-
WAIT-2 and continue processing in that state.
FIN-WAIT-2 STATE
In addition to the processing for the ESTABLISHED
state, if the retransmission queue is empty, the
user's CLOSE can be acknowledged ("ok") but do not
delete the TCB.
CLOSE-WAIT STATE
Do the same processing as for the ESTABLISHED state.
CLOSING STATE
In addition to the processing for the ESTABLISHED
state, if the ACK acknowledges our FIN then enter the
TIME-WAIT state, otherwise ignore the segment.
LAST-ACK STATE
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The only thing that can arrive in this state is an
acknowledgment of our FIN. If our FIN is now
acknowledged, delete the TCB, enter the CLOSED state,
and return.
TIME-WAIT STATE
The only thing that can arrive in this state is a
retransmission of the remote FIN. Acknowledge it, and
restart the 2 MSL timeout.
sixth, check the URG bit,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and
signal the user that the remote side has urgent data if
the urgent pointer (RCV.UP) is in advance of the data
consumed. If the user has already been signaled (or is
still in the "urgent mode") for this continuous sequence
of urgent data, do not signal the user again.
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
This should not occur, since a FIN has been received from
the remote side. Ignore the URG.
seventh, process the segment text,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Once in the ESTABLISHED state, it is possible to deliver
segment text to user RECEIVE buffers. Text from segments
can be moved into buffers until either the buffer is full
or the segment is empty. If the segment empties and
carries an PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received.
When the TCP takes responsibility for delivering the data
to the user it must also acknowledge the receipt of the
data.
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Once the TCP takes responsibility for the data it
advances RCV.NXT over the data accepted, and adjusts
RCV.WND as apporopriate to the current buffer
availability. The total of RCV.NXT and RCV.WND should
not be reduced.
Please note the window management suggestions in section
3.7.
Send an acknowledgment of the form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
This acknowledgment should be piggybacked on a segment
being transmitted if possible without incurring undue
delay.
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
This should not occur, since a FIN has been received from
the remote side. Ignore the segment text.
eighth, check the FIN bit,
Do not process the FIN if the state is CLOSED, LISTEN or
SYN-SENT since the SEG.SEQ cannot be validated; drop the
segment and return.
If the FIN bit is set, signal the user "connection closing"
and return any pending RECEIVEs with same message, advance
RCV.NXT over the FIN, and send an acknowledgment for the
FIN. Note that FIN implies PUSH for any segment text not
yet delivered to the user.
SYN-RECEIVED STATE
ESTABLISHED STATE
Enter the CLOSE-WAIT state.
FIN-WAIT-1 STATE
If our FIN has been ACKed (perhaps in this segment),
then enter TIME-WAIT, start the time-wait timer, turn
off the other timers; otherwise enter the CLOSING
state.
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FIN-WAIT-2 STATE
Enter the TIME-WAIT state. Start the time-wait timer,
turn off the other timers.
CLOSE-WAIT STATE
Remain in the CLOSE-WAIT state.
CLOSING STATE
Remain in the CLOSING state.
LAST-ACK STATE
Remain in the LAST-ACK state.
TIME-WAIT STATE
Remain in the TIME-WAIT state. Restart the 2 MSL
time-wait timeout.
and return.
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USER TIMEOUT
USER TIMEOUT
For any state if the user timeout expires, flush all queues,
signal the user "error: connection aborted due to user timeout"
in general and for any outstanding calls, delete the TCB, enter
the CLOSED state and return.
RETRANSMISSION TIMEOUT
For any state if the retransmission timeout expires on a
segment in the retransmission queue, send the segment at the
front of the retransmission queue again, reinitialize the
retransmission timer, and return.
TIME-WAIT TIMEOUT
If the time-wait timeout expires on a connection delete the
TCB, enter the CLOSED state and return.
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3.10. Glossary
1822 BBN Report 1822, "The Specification of the Interconnection of
a Host and an IMP". The specification of interface between a
host and the ARPANET.
ACK
A control bit (acknowledge) occupying no sequence space,
which indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all
previous sequence numbers.
ARPANET message
The unit of transmission between a host and an IMP in the
ARPANET. The maximum size is about 1012 octets (8096 bits).
ARPANET packet
A unit of transmission used internally in the ARPANET between
IMPs. The maximum size is about 126 octets (1008 bits).
connection
A logical communication path identified by a pair of sockets.
datagram
A message sent in a packet switched computer communications
network.
Destination Address
The destination address, usually the network and host
identifiers.
FIN
A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control
occupying sequence space.
fragment
A portion of a logical unit of data, in particular an
internet fragment is a portion of an internet datagram.
FTP
A file transfer protocol.
header
Control information at the beginning of a message, segment,
fragment, packet or block of data.
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host
A computer. In particular a source or destination of
messages from the point of view of the communication network.
Identification
An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram.
IMP
The Interface Message Processor, the packet switch of the
ARPANET.
internet address
A source or destination address specific to the host level.
internet datagram
The unit of data exchanged between an internet module and the
higher level protocol together with the internet header.
internet fragment
A portion of the data of an internet datagram with an
internet header.
IP
Internet Protocol.
IRS
The Initial Receive Sequence number. The first sequence
number used by the sender on a connection.
ISN
The Initial Sequence Number. The first sequence number used
on a connection, (either ISS or IRS). Selected on a clock
based procedure.
ISS
The Initial Send Sequence number. The first sequence number
used by the sender on a connection.
leader
Control information at the beginning of a message or block of
data. In particular, in the ARPANET, the control information
on an ARPANET message at the host-IMP interface.
left sequence
This is the next sequence number to be acknowledged by the
data receiving TCP (or the lowest currently unacknowledged
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sequence number) and is sometimes referred to as the left
edge of the send window.
local packet
The unit of transmission within a local network.
module
An implementation, usually in software, of a protocol or
other procedure.
MSL
Maximum Segment Lifetime, the time a TCP segment can exist in
the internetwork system. Arbitrarily defined to be 2
minutes.
octet
An eight bit byte.
Options
An Option field may contain several options, and each option
may be several octets in length. The options are used
primarily in testing situations; for example, to carry
timestamps. Both the Internet Protocol and TCP provide for
options fields.
packet
A package of data with a header which may or may not be
logically complete. More often a physical packaging than a
logical packaging of data.
port
The portion of a socket that specifies which logical input or
output channel of a process is associated with the data.
process
A program in execution. A source or destination of data from
the point of view of the TCP or other host-to-host protocol.
PUSH
A control bit occupying no sequence space, indicating that
this segment contains data that must be pushed through to the
receiving user.
RCV.NXT
receive next sequence number
RCV.UP
receive urgent pointer
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RCV.WND
receive window
receive next sequence number
This is the next sequence number the local TCP is expecting
to receive.
receive window
This represents the sequence numbers the local (receiving)
TCP is willing to receive. Thus, the local TCP considers
that segments overlapping the range RCV.NXT to RCV.NXT +
RCV.WND - 1 carry acceptable data or control. Segments
containing sequence numbers entirely outside of this range
are considered duplicates and discarded.
RST
A control bit (reset), occupying no sequence space,
indicating that the receiver should delete the connection
without further interaction. The receiver can determine,
based on the sequence number and acknowledgment fields of the
incoming segment, whether it should honor the reset command
or ignore it. In no case does receipt of a segment
containing RST give rise to a RST in response.
RTP
Real Time Protocol: A host-to-host protocol for communication
of time critical information.
SEG.ACK
segment acknowledgment
SEG.LEN
segment length
SEG.PRC
segment precedence value
SEG.SEQ
segment sequence
SEG.UP
segment urgent pointer field
SEG.WND
segment window field
segment
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A logical unit of data, in particular a TCP segment is the
unit of data transfered between a pair of TCP modules.
segment acknowledgment
The sequence number in the acknowledgment field of the
arriving segment.
segment length
The amount of sequence number space occupied by a segment,
including any controls which occupy sequence space.
segment sequence
The number in the sequence field of the arriving segment.
send sequence
This is the next sequence number the local (sending) TCP will
use on the connection. It is initially selected from an
initial sequence number curve (ISN) and is incremented for
each octet of data or sequenced control transmitted.
send window
This represents the sequence numbers which the remote
(receiving) TCP is willing to receive. It is the value of
the window field specified in segments from the remote (data
receiving) TCP. The range of new sequence numbers which may
be emitted by a TCP lies between SND.NXT and SND.UNA +
SND.WND - 1. (Retransmissions of sequence numbers between
SND.UNA and SND.NXT are expected, of course.)
SND.NXT
send sequence
SND.UNA
left sequence
SND.UP
send urgent pointer
SND.WL1
segment sequence number at last window update
SND.WL2
segment acknowledgment number at last window update
SND.WND
send window
socket
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An address which specifically includes a port identifier,
that is, the concatenation of an Internet Address with a TCP
port.
Source Address
The source address, usually the network and host identifiers.
SYN
A control bit in the incoming segment, occupying one sequence
number, used at the initiation of a connection, to indicate
where the sequence numbering will start.
TCB
Transmission control block, the data structure that records
the state of a connection.
TCB.PRC
The precedence of the connection.
TCP
Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments.
TOS
Type of Service, an Internet Protocol field.
Type of Service
An Internet Protocol field which indicates the type of
service for this internet fragment.
URG
A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed
with sequence numbers less than the value indicated in the
urgent pointer.
urgent pointer
A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer which
indicates the data octet associated with the sending user's
urgent call.
4. Changes from RFC 793
TODO: this entire section will need to be edited and condensed before
the document is finalized. It currently represents a plan for future
updates.
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The -00 version of this document was merely a proposal and rough plan
for updating RFC 793.
The -01 revision of this document incorporates the content of RFC 793
Section 3 titled "FUNCTIONAL SPECIFICATION". Other content from RFC
793 has not been incorporated. The -01 revision of this document
makes some minor formatting changes to the RFC 793 content in order
to convert the content into XML2RFC format and account for left-out
parts of RFC 793. For instance, figure numbering differs and some
indentation is not exactly the same.
TODO: Incomplete list of changes - these need to be added to and made
more specific, as the document proceeds:
1. incorporate the accepted errata
2. incorporate 1122 additions
3. point to major additional docs like 1323bis and 5681
4. incorporate relevant parts of 3168 (ECN)
5. incorporate 6093 (urgent pointer)
6. incorporate 6528 (sequence number)
7. incorporate Fernando's new number-checking fixes (if past the
IESG in time)
8. point to PMTUD?
9. point to 5461 (soft errors)
10. mention 5961 state machine option
11. mention 6161 (reducing TIME-WAIT)
12. incorporate 6429 (ZWP/persist)
13. incorporate 6691 (MSS)
5. IANA Considerations
This memo includes no request to IANA. Existing IANA registries for
TCP parameters are sufficient.
TODO: check whether entries pointing to 793 and other documents
obsoleted by this one should be updated to point to this one instead.
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6. Security Considerations
TODO
7. Acknowledgements
This document is largely a revision of RFC 793, which Jon Postel was
the editor of. Due to his excellent work, it was able to last for
three decades before we felt the need to revise it.
Andre Oppermann was a contributor and helped to edit the first
revision of this document.
We are thankful for the assistance of the IETF TCPM working group
chairs:
Michael Scharf
Yoshifumi Nishida
Pasi Sarolahti
On the TCPM mailing list, and at the IETF 88 meeting in Vancouver,
helpful comments, critiques, and reviews were received from (listed
alphebetically): David Borman, Yuchung Cheng, Martin Duke, Kevin
Lahey, Kevin Mason, Matt Mathis, Hagen Paul Pfeifer, Anthony
Sabatini, Joe Touch, Reji Varghese, Lloyd Wood, and Alex Zimmermann.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[3] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", draft-ietf-tcpm-tcp-
rfc4614bis-00 (work in progress), August 2013.
Author's Address
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Wesley M. Eddy
MTI Systems
US
Email: wes@mti-systems.com
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