Section 4. Glossary

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.
   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 network-layer address of the endpoint intended to receive
           a segment.
   FIN    
           A control bit (finis) occupying one sequence number, which
           indicates that the sender will send no more data or control
           occupying sequence space.
   flush  
           To remove all of the contents (data or segments) from a store
           (buffer or queue).
   fragment
           A portion of a logical unit of data.  In particular, an
           internet fragment is a portion of an internet datagram.
   header 
           Control information at the beginning of a message, segment,
           fragment, packet, or block of data.
   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.
   internet address
           A network-layer address.
   internet datagram
           A unit of data exchanged between internet hosts, together
           with the internet header that allows the datagram to be
           routed from source to destination.
   internet fragment
           A portion of the data of an internet datagram with an
           internet header.
   IP     
           Internet Protocol.  See [1] and [13].
   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 in a way that
           is unique within a given period of time and is unpredictable
           to attackers.
   ISS    
           The Initial Send Sequence number.  The first sequence number
           used by the sender on a connection.
   left sequence
           This is the next sequence number to be acknowledged by the
           data-receiving TCP endpoint (or the lowest currently
           unacknowledged sequence number) and is sometimes referred to
           as the left edge of the send window.
   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.
   packet 
           A package of data with a header that may or may not be
           logically complete.  More often a physical packaging than a
           logical packaging of data.
   port   
           The portion of a connection identifier used for
           demultiplexing connections at an endpoint.
   process
           A program in execution.  A source or destination of data from
           the point of view of the TCP endpoint 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
   RCV.WND
           receive window
   receive next sequence number
           This is the next sequence number the local TCP endpoint is
           expecting to receive.
   receive window
           This represents the sequence numbers the local (receiving)
           TCP endpoint is willing to receive.  Thus, the local TCP
           endpoint 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 this range are considered duplicates or injection
           attacks 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.
   SEG.ACK
           segment acknowledgment
   SEG.LEN
           segment length
   SEG.SEQ
           segment sequence
   SEG.UP 
           segment urgent pointer field
   SEG.WND
           segment window field
   segment
           A logical unit of data.  In particular, a TCP segment is the
           unit of data transferred 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 that 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
           endpoint 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 that the remote
           (receiving) TCP endpoint is willing to receive.  It is the
           value of the window field specified in segments from the
           remote (data-receiving) TCP endpoint.  The range of new
           sequence numbers that may be emitted by a TCP implementation
           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 (or socket number, or socket address, or socket identifier)
           An address that specifically includes a port identifier, that
           is, the concatenation of an Internet Address with a TCP port.
   Source Address
           The network-layer address of the sending endpoint.
   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.
   TCP    
           Transmission Control Protocol: a host-to-host protocol for
           reliable communication in internetwork environments.
   TOS    
           Type of Service, an obsoleted IPv4 field.  The same header
           bits currently are used for the Differentiated Services field
           [4] containing the Differentiated Services Codepoint (DSCP)
           value and the 2-bit ECN codepoint [6].
   Type of Service
           See "TOS".
   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 by 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 that
           indicates the data octet associated with the sending user's
           urgent call.