Glossary
This glossary defines key terms and abbreviations used in RFC 793.
A
ACK (Acknowledgment)
A control bit that occupies no sequence space, used to acknowledge the receipt of data. Also refers to a segment carrying such acknowledgment.
ARPANET
A research network sponsored by the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense.
C
CLOSED
Represents no connection state at all (fictional state, as it represents the state when no TCB exists).
Connection
A logical communication path identified by a pair of sockets, used for inter-process communication.
D
Datagram
An independent, self-contained data packet whose transmission is based on best-effort network service.
Destination
The process for which data is to be received.
Destination Address
The destination address, typically a network and host identifier.
Destination Port
The destination process port identifier for demultiplexing to the destination process.
E
ESTABLISHED
Represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
F
FIN (Finish)
A control bit that occupies one sequence number, used to indicate the sender has no more data to send.
Fragment
A portion of a datagram or segment.
H
Header
Control information at the beginning of a message, segment, fragment, packet, or block of data.
Host
A computer. From a communication network point of view, particularly the source or destination of messages.
I
Identification
An Internet Protocol field. This identification value assigned by the sender aids in assembling the fragments of a datagram.
IMP (Interface Message Processor)
The packet switches 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)
The Internet Protocol.
IRS (Initial Receive Sequence Number)
The first sequence number used by the receiver on a connection.
ISN (Initial Sequence Number)
The first sequence number used on a connection (ISS or IRS). Selected by a clock-based procedure.
ISS (Initial Send Sequence Number)
The first sequence number used by the sender on a connection.
L
Leader
Control information at the beginning of a message or block of data. 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 sequence number), sometimes called the left edge of the send window.
Local Packet
The unit of transmission within a local network.
M
Module
An implementation of a protocol or other procedure, typically a software implementation.
MSL (Maximum Segment Lifetime)
The time a TCP segment can exist in the Internet system. Arbitrarily defined to be 2 minutes.
O
Octet
An eight-bit byte.
Options
The options 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.
P
Packet
A package of data with a header which may or may not be logically complete. More often a physical packaging of data than a logical packaging.
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.
R
RCV.NXT (Receive Next Sequence Number)
This is the next sequence number the local TCP is expecting to receive.
RCV.UP (Receive Urgent Pointer)
The receive urgent pointer.
RCV.WND (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 (Reset)
A control bit occupying no sequence space, indicating 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 to 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 time-critical information communication.
S
SEG.ACK (Segment Acknowledgment)
The sequence number in the acknowledgment field of the arriving segment.
SEG.LEN (Segment Length)
The amount of sequence number space occupied by a segment, including any controls which occupy sequence space.
SEG.PRC (Segment Precedence Value)
The segment precedence value.
SEG.SEQ (Segment Sequence)
The number in the sequence field of the arriving segment.
SEG.UP (Segment Urgent Pointer Field)
The segment urgent pointer field.
SEG.WND (Segment Window Field)
The segment window field.
Segment
A logical unit of data, particularly a TCP segment is the unit of data transferred between a pair of TCP modules.
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 sequencing control transmitted.
Send Window
This represents the sequence numbers 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)
Send sequence.
SND.UNA (Left Sequence)
Left sequence.
SND.UP (Send Urgent Pointer)
Send urgent pointer.
SND.WL1 (Segment Sequence Number of Last Window Update)
Segment sequence number of last window update.
SND.WL2 (Segment Acknowledgment Number of Last Window Update)
Segment acknowledgment number of last window update.
SND.WND (Send Window)
Send window.
Socket
An address which specifically includes a port identifier, that is, the concatenation of an Internet address with a TCP port.
Format: IP address:port number
Example: 192.168.1.1:80
Source
The process which is producing data.
Source Address
The source address, typically a network and host identifier.
Source Port
The source process port identifier for demultiplexing data from the source process.
SYN (Synchronize)
A control bit in an incoming segment that occupies one sequence number, used at the initiation of a connection to indicate where the sequence numbering will start.
T
TCB (Transmission Control Block)
A 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. Indicates the type of service for this Internet fragment.
Type of Service
An Internet Protocol field indicating the type of service for this Internet fragment.
U
URG (Urgent)
A control bit 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 set. This field communicates the value of the urgent pointer which indicates the data octet associated with the sending user's urgent call.
Common Abbreviations Reference
| Abbreviation | Full Name | Description |
|---|---|---|
| ACK | Acknowledgment | Acknowledgment |
| FIN | Finish | Finish |
| IP | Internet Protocol | Internet Protocol |
| IRS | Initial Receive Sequence | Initial Receive Sequence Number |
| ISN | Initial Sequence Number | Initial Sequence Number |
| ISS | Initial Send Sequence | Initial Send Sequence Number |
| MSL | Maximum Segment Lifetime | Maximum Segment Lifetime |
| PSH | Push | Push |
| RCV | Receive | Receive |
| RST | Reset | Reset |
| RTP | Real Time Protocol | Real Time Protocol |
| SEG | Segment | Segment |
| SND | Send | Send |
| SYN | Synchronize | Synchronize |
| TCB | Transmission Control Block | Transmission Control Block |
| TCP | Transmission Control Protocol | Transmission Control Protocol |
| TOS | Type of Service | Type of Service |
| URG | Urgent | Urgent |
State Names Reference
| State | Description |
|---|---|
| CLOSED | No connection state |
| LISTEN | Waiting for connection request |
| SYN-SENT | Connection request sent |
| SYN-RECEIVED | Connection request received |
| ESTABLISHED | Connection established, data transfer possible |
| FIN-WAIT-1 | Waiting for connection termination request or acknowledgment |
| FIN-WAIT-2 | Waiting for connection termination request |
| CLOSE-WAIT | Waiting for local user to close connection |
| CLOSING | Waiting for connection termination acknowledgment |
| LAST-ACK | Waiting for final acknowledgment |
| TIME-WAIT | Waiting for enough time to ensure remote TCP received acknowledgment |
Key Concept Comparisons
Datagram vs Segment vs Packet
| Term | Layer | Description |
|---|---|---|
| Packet | Generic | Data package with header, physical packaging |
| Datagram | IP Layer | Internet Protocol data unit |
| Segment | TCP Layer | TCP data unit |
Window vs Buffer
| Term | Description |
|---|---|
| Window | Sequence number range, flow control mechanism |
| Buffer | Physical memory space, stores data |
Sequence vs Acknowledgment
| Term | Direction | Description |
|---|---|---|
| Sequence Number | Send | Identifies sent data bytes |
| Acknowledgment Number | Receive | Identifies next expected byte |
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