Network Working Group Vinton Cerf
Request for Comments: 675 Yogen Dalal
NIC: 2 Carl Sunshine
INWG: 72 December 1974
SPECIFICATION OF INTERNET TRANSMISSION CONTROL PROGRAM
December 1974 Version
1. INTRODUCTION
This document describes the functions to be performed by the
internetwork Transmission Control Program [TCP] and its interface to
programs or users that require its services. Several basic
assumptions are made about process to process communication and these
are listed here without further justification. The interested reader
is referred to [CEKA74, TOML74, BELS74, DALA74, SUNS74] for further
discussion.
The authors would like to acknowledge the contributions of R.
Tomlinson (three way handshake and Initial Sequence Number
Selection), D. Belsnes, J. Burchfiel, M. Galland, R. Kahn, D. Lloyd,
W. Plummer, and J. Postel all of whose good ideas and counsel have
had a beneficial effect (we hope) on this protocol design. In the
early phases of the design work, R. Metcalfe, A. McKenzie, H.
Zimmerman, G. LeLann, and M. Elie were most helpful in eXPlicating
the various issues to be resolved. Of course, we remain responsible
for the remaining errors and misstatements which no douBT lurk in the
nooks and crannies of the text.
Processes are viewed as the active elements of all HOST computers in
a network. Even terminals and files or other I/O media are viewed as
communicating through the use of processes. Thus, all network
communication is viewed as inter-process communication.
Since a process may need to distinguish among several communication
streams between itself and another process [or processes], we imagine
that each process may have a number of PORTs through which it
communicates with the ports of other processes.
Since port names are selected independently by each operating system,
TCP, or user, they may not be unique. To provide for unique names at
each TCP, we concatenate a NETWORK identifier, and a TCP identifier
with a port name to create a SOCKET name which will be unique
throughout all networks connected together.
A pair of sockets form a CONNECTION which can be used to carry data
in either direction [i.e. full duplex]. The connection is uniquely
identified by the <local socket, foreign socket> address pair, and
the same local socket can participate in multiple connections to
different foreign sockets [see Section 2.2].
Processes exchange finite length LETTERS as a way of communicating;
thus, letter boundaries are significant. However, the length of a
letter may be such that it must be broken into FRAGMENTS before it
can be transmitted to its destination. We assume that the fragments
will normally be reassembled into a letter before being passed to the
receiving process. Throughout this document, it is legitimate to
assume that a fragment contains all or a part of a letter, but that a
fragment never contains parts of more than one letter.
We specifically assume that fragments are transmitted from Host to
Host through means of a PACKET SWITCHING NETWORK [PSN] [ROWE70,
POUZ73]. This assumption is probably unnecessary, since a circuit
switched network could also be used, but for concreteness, we
explicitly assume that the hosts are connected to one or more PACKET
SWITCHES [PS] of a PSN [HEKA7O, POUZ74, SCWI71].
Processes make use of the TCP by handing it letters. The TCP breaks
these into fragments, if necessary, and then embeds each fragment in
an INTERNETWORK PACKET. Each internetwork packet is in turn embedded
in a LOCAL PACKET suitable for transmission from the host to one of
its serving PS. The packet switches may perform further formatting or
other operations to achieve the delivery of the local packet to the
destination Host.
The term LOCAL PACKET is used generically here to mean the formatted
bit string exchanged between a host and a packet switch. The format
of bit strings exchanged between the packet switches in a PSN will
generally not be of concern to us. If an internetwork packet is
destined for a TCP in a foreign PSN, the packet is routed to a
GATEWAY which connects the origin PSN with an intermediate or the
destination PSN. Routing of internetwork packets to the GATEWAY may
be the responsibility of the source TCP or the local PSN, depending
upon the PSN Implementation.
One model of TCP operation is to imagine that there is a basic
GATEWAY associated with each TCP which provides an interface to the
local network. This basic GATEWAY performs routing and packet
reformatting or embedding, and may also implement congestion and
error control between the TCP and GATEWAYS at or intermediate to the
destination TCP.
At a GATEWAY between networks, the internetwork packet is unwrapped
from its local packet format and examined to determine through which
network the internetwork packet should travel next. The internetwork
packet is then wrapped in a local packet format suitable to the next
network and passed on to a new packet switch.
A GATEWAY is permitted to break up the fragment carried by an
internetwork packet into smaller fragments if this is necessary for
transmission through the next network. To do this, the GATEWAY
produces a set of internetwork packets, each carrying a new fragment.
The packet format is designed so that the destination TCP may treat
fragments created by the source TCP or by intermediate GATEWAYS
nearly identically.
The TCP is responsible for regulating the flow of internetwork
packets to and from the processes it serves, as a way of preventing
its host from becoming saturated or overloaded with traffic. The TCP
is also responsible for retransmitting unacknowledged packets, and
for detecting duplicates. A consequence of this error
detection/retransmission scheme is that the order of letters received
on a given connection is also maintained [CEKA74, SUNS74]. To perform
these functions, the TCP opens and closes connections between ports
as described in Section 4.3. The TCP performs retransmission,
duplicate detection, sequencing, and flow control on all
communication among the processes it serves.
2. The TCP INTERFACE to the USER
2.1 The TCP as a POST Office
The TCP acts in many ways like a postal service since it provides a
way for processes to exchange letters with each other. It sometimes
happens that a process may offer some service, but not know in
advance what its correspondents' addresses are. The analogy can be
drawn with a mail order house which opens a post office box which can
accept mail from any source. Unlike the post box, however, once a
letter from a particular correspondent arrives, a port becomes
specific to the correspondent until the owner of the port declares
otherwise.
In addition to acting like a postal service, the TCP insures end-to-
end acknowledgment, error correction, duplicate detection,
sequencing, and flow control.
2.2 Sockets and Addressing
We have borrowed the term SOCKET from the ARPANET terminology
[CACR70, MCKE73]. In general, a socket is the concatenation of a
NETWORK identifier, TCP identifier, and PORT identifier. A CONNECTION
is fully specified by the pair of SOCKETS at each end since the same
local socket may participate in many connections to different foreign
sockets.
Once the connections is specified in the OPEN command [see section
2.3.2], the TCP supplies a [short] Local Connection Name by which the
user refers to the connection in subsequent commands. In particular
this facilitates using connections with initially unspecified foreign
sockets.
TCP's are free to associate ports with processes however they choose.
However, several basic concepts seem necessary in an implementation.
There must be well known sockets [WKS] which the TCP associates only
with the "appropriate" processes by some means. We envision that
processes may "own" sockets, and that processes can only initiate
connections on the sockets they own [means for implementing ownership
is a local issue, but we envision a Request Port user call, or a
method of uniquely allocating a group of ports to a given process,
e.g. by associating the high order bits of a port name with a given
process.]
Once initiated, a connection may be passed to another process that
does not own the local socket [e.g. from logger to service process].
Strictly speaking this is a reconnection issue which might be more
elegantly handled by a general reconnection protocol as discussed in
section 3.3. To simplify passing a connection within a single TCP,
such "invisible" switches may be allowed as in TENEX systems.
Of course, each connection is associated with exactly one process,
and any attempt to reference that connection by another process will
be signaled as an error by the TCP. This prevents stealing data from
or inserting data into another process' data stream.
A connection is initiated by the rendezvous of an arriving
internetwork packet and a waiting Transmission Control Block [TCB]
created by a user OPEN, SEND, INTERPUPT, or RECEIVE call [see section
2.3]. The matching of local and foreign socket identifiers determines
when a successful connection has been initiated. The connection
becomes established when sequence numbers have been synchronized in
both directions as described in section 4.3.2.
It is possible to specify a socket only partially by setting the PORT
identifier to zero or setting both the TCP and PORT identifiers to
zero. A socket of all zero is called UNSPECIFIED. The purpose behind
unspecified sockets is to provide a sort of "general delivery"
facility [useful for logger type processes with well known sockets].
There are bounds on the degree of unspecificity of socket
identifiers. TCB's must have fully specified local sockets, although
the foreign socket may be fully or partly unspecified. Arriving
packets must have fully specified sockets.
We employ the following notation:
x.y.z = fully specified socket with x=net, y=TCP, z=port
x.y.u = as above, but unspecified port
x.u.u = as above, but unspecified TCP and port
u.u.u = completely unspecified
with respect to implementation, u = 0 [zero]
We illustrate the principles of matching by giving all cases of
incoming packets which match with existing TCB's. Generally, both
the local (foreign) socket of the TCB and the foreign (local) socket
of the packet must match.
TCB local TCB foreign Packet local Packet foreign
(a) a.b.c e.f.g e.f.g a.b.c
(b) a.b.c e.f.u e.f.g a.b.c
(c) a.b.c e.u.u e.f.g a.b.c
(d) a.b.c u.u.u e.f.g a.b.c
There are no other legal combinations of socket identifiers which
match. Case (d) is typical of the ARPANET well known socket idea in
which the well known socket (a.b.c) LISTENS for a connection from
any (u.u.u) socket. Cases (b) and (c) can be used to restrict
matching to a particular TCP or net.
2.3 TCP USER CALLS
2.3.1 A Note on Style
The following sections functionally define the 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. SVC's, UUO's, EMT's,...].
The user calls described below specify the basic functions the TCP
will perform to support interprocess communication. Individual
implementations should 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, RECEIVE, or INTERRUPT issued by
the user for a given connection.
In providing interprocess communication facilities, the TCP must not
only accept commands, but also return information to the processes
it serves. This communication consists of:
(a) general information about a connection [interrupts, remote
close, binding of unspecified foreign socket].
(b) replies to specific user commands indicating success or various
types of failure.
Although the means for signaling user processes and the exact format
of replies will vary from one implementation to another, it would
promote common understanding and testing if a common set of codes
were adopted. Such a set of Event Codes is described in section 2.4.
With respect to error messages, references to "local" and "foreign"
are ambiguous unless it is known whether these refer to the world as
seen by the sender or receiver of the error message. The authors
attempted several different approaches and finally settled on the
convention that these references would be as seen by the receiver of
the message.
2.3.2 OPEN CONNECTION
Format: OPEN(local port, foreign socket [, timeout])
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 source network and TCP identifiers will either be
supplied by the TCP or by the processes that serve it [e.g. the
program which interfaces the TCP to its packet switch or the packet
switch itself]. 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 no foreign socket is specified [i.e. the foreign socket parameter
is 0 or not present], then this constitutes a LISTENING local socket
which can accept communication from any foreign socket. Provision is
also made for partial specification of foreign sockets as described
in section 2.2.
If the specified connection is already OPEN, an error is returned,
otherwise a full-duplex transmission control block [TCB] is created
and partially filled in with data from the OPEN command parameters.
The TCB format is described in more detail in section 4.2.2.
No network traffic is generated by the OPEN command. The first SEND
or INTERRUPT by the local user or the foreign user will cause the TCP
to synchronize the connection.
The timeout, if present, permits the caller to set up a timeout for
all letters transmitted on the connection. If a letter is not
successfully transmitted within the timeout period, the user is
notified and may ignore the condition [TCP will continue trying to
transmit] or direct the TCP to close the connection. The present
global default is 30 seconds, and connections which are set up
without specifying another timeout will retransmit every letter for
at least 30 seconds before notifying the user. The retransmission
rate may vary, and is the responsibility of the TCP and not the user.
Most likely, it will be related to the measured time for responses to
return from letters sent.
Depending on the TCP implementation, either a local connection name
will be returned to the user by the TCP, or the user will specify
this local connection name (in which case another parameter is needed
in the call). The local connection name can then be used as a short
hand term for the connection defined by the <local socket, foreign
socket> pair.
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.3.3 SEND LETTER
Format: SEND(local connection name, buffer address, byte count, EOL
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 EOL flag is set, the data is the End Of a Letter, and the EOL
bit will be set in the last packet created from the buffer. If the
EOL f1ag is not set, subsequent SEND's will appear as part of the
same letter. This extended letter facility should be used sparingly
because some TCP's may delay processing packets until an entire
letter is received.
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 letter arriving for the local port] then the
designated letter is sent to the implied foreign socket. In general,
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 the timeout is specified, then the current default timeout for
this connection is changed to the new one. This can affect not only
all letters sent including and after this one, but also those which
have not yet been sent, since the timeout is kept in the TCB and not
associated with each letter sent. Of course, a time is maintained for
each internetwork packet formed so as to determine how long each of
these has been on the retransmission queue.
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. This simple method is highly subject to
deadlocks and is not recommended. [For example both sides of the
connection try to do SEND's before doing any RECEIVE's.] 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 concurrently. Multiple SENDs
are served in first come, first served order, so the TCP will queue
those it cannot service immediately.
NOTA BENE: In order for the process to distinguish among error or
success indications for different letters, the buffer address should
be returned along with the coded response to the SEND request. We
will offer an example event code format in section 2.4, showing the
information which should be returned to the calling process.
The semantics of the INTERRUPT call are described later, but this
call can have an effect on letters which have been given to the TCP
but not yet sent. In particular, all such letters are flushed by the
source TCP. Thus one of the responses to a SEND may be "flushed due
to interrupt."
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.3.4 RECEIVE LETTER
Format: RECEIVE(local connection name, buffer address, byte count)
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 a letter was received, or some error
occurred, but this scheme is highly subject to deadlocks [see section
2.3.3]. A more sophisticated implementation would permit several
RECEIVE's to be outstanding at once, These would be filled as letters
arrive. This strategy permits increased throughput, at the cost of a
more elaborate scheme [possibly asynchronous] to notify the calling
program that a letter has been received.
If insufficient buffer space is given to reassemble a complete
letter, an indication that the buffer holds a partial letter will be
given; the buffer will be filled with as much data as it can hold.
The remaining parts of a partly delivered letter will be placed in
buffers as they are made available via successive RECEIVES. If a
number of RECEIVES are outstanding, they may be filled with parts of
a single long letter or with at most one letter each. The event codes
associated with each RECEIVE will indicate what is contained in the
buffer.
To distinguish among several outstanding RECEIVES, and to take care
of the case that a letter is smaller than the buffer supplied, the
event code is accompanied by both a buffer pointer and a byte count
indicating the actual length of the letter received.
The semantics of the INTERRUPT system call are discussed later, but
this call can have an effect on outstanding RECEIVES. When the TCP
receives an INTERRUPT, it will flush all data currently queued up
awaiting receipt by the receiving process. If no data is waiting, but
several buffers have been made available by anticipatory RECEIVE
commands, these buffers are returned to the process with an error
indicating that any data that might have been placed in those buffers
has been flushed. This enables the receiving process to synchronize
its RECEIVES with the interrupt. That is, the process can distinguish
between RECEIVES issued before the receipt of the INTERRUPT and these
issued afterwards.
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.3.5 CLOSE CONNECTION
Format: CLOSE(local connection name)
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. Any unfilled receive
buffers or pending send buffers will be returned to the user with
event codes indicating they were aborted due to the CLOSE. Users
should wait for event codes for each SEND before closing the
connection if they wish to be certain that all letters were
successfully delivered.
The user may CLOSE the connection at any time on his own initiative,
or in response to various prompts from the TCP [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 errors.
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.3.6 INTERRUPT
Format: INTERRUPT(local connection name)
A special control signal is sent to the destination indicating an
interrupt condition. This facility can be used to simulate "break"
signals from terminals or error or completion codes from I/O devices,
for example. The semantics of this signal to the receiving process
are unspecified. The receiving TCP will signal the interrupt to the
receiving process immediately upon receipt, and will also flush any
outstanding letters waiting to be delivered. Since it is possib1e to
tell where in the letter stream this command was invoked, it is
possible for the receiving TCP to flush only preceding data. The
sending TCP will flush any letters pending transmission, returning a
special error code to indicate the flush.
If the connection is not open or the calling process is not
authorized to use this connection, an error is returned.
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.3.7 STATUS
Format: STATUS(local connection name)
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 letters awaiting
acknowledgment, number of letters pending receipt [including partial
ones], default transmission timeout
Depending on the state of the connection, 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.
Responses from the TCP which may occur as a result of this call are
detailed in section 2.4.
2.4 TCP TO USER MESSAGES
2.4.1 TYPE CODES
All messages include a type code which identifies the type of user
call to which the message applies. Types are:
0 - General message, does not apply to a particular user call
1 - Applies to OPEN
2 - Applies to CLOSE
3 - Applies to INTERRUPT
10 - Applies to SEND
20 - Applies to RECEIVE
30 - Applies to STATUS
2.4.2 MESSAGE FORMAT [notional]
All messages include the following three fields:
Type code
Local connection name
Event code
For message types 0-3 [General, Open, Close, Interrupt] only these
three fields are necessary.
For message type 10 [Send] one additional field is necessary:
Buffer address
For message type 20 [Receive] three additional fields are necessary:
Buffer address
Byte count
End-of-letter flag
For message type 30 [status] additional data might include;
Local socket, foreign socket
Send window [measures buffer space at foreign TCP]
Receive window [measures buffer space at local TCP]
Connection state [see section 4.3.6]
Number of letters awaiting acknowledgment
Number of letters awaiting receipt
Retransmission timeout
2.4.3 EVENT CODES
The event code specifies the particular event that the TCP wishes to
communicate to the user.
In addition to the event code, three flags may be useful to classify
the event into major categories and facilitate event processing by
the user:
E flag: set if event is an error
L/F flag: indicates whether event was generated by Local TCP, or
Foreign TCP or network
P/T flag: indicates whether the event is Permanent or Temporary
[retry may succeed]
Events are encoded into 8 bits with the high order bits set to
indicate the state of the E, L/F, and P/T flags, respectively.
Events specified so far are listed below with their codes and flag
settings. A * means a flag does not apply or can take both values for
this event. Additional events may be defined in the course of
experimentation.
0 0** general success
1 ELP connection illegal for this process
2 OF* unspecified foreign socket has become bound
3 ELP connection not open
4 ELT no room for TCB
5 ELT foreign socket unspecified
6 ELP connection already open
EFP unacceptable SYN [or SYN/ACK] arrived at foreign
TCP. Note: This is not a misprint, the local meaning is different
from foreign.
7 EFP connection does not exist at foreign TCP
8 EFT foreign TCP inaccessible [may have subcases]
9 ELT retransmission timeout
10 E*P buffer flushed due to interrupt
11 OF* interrupt to user
12 **P connection closing
13 E** general error
14 E*P connection reset
Possible events for each message type are as follows:
Type 0[general]: 2,11,12,14
Type 1[open]: 0,1,4,6,13
Type 2[close]: 0,1,3,13
Type 3[interrupt]: 0,1,3,5,7,8,9,12,13
Type 10[send]: 0,1,3,5,7,8,9,10,11,12,13
Type 20[receive]: 0,1,3,10,12,13
Type 30[status]: 0,1,13
Note that events 6(foreign), 7, 8 are generated at the foreign TCP or
in the network[s], and these same codes are used in the error field
of the internet packet [see section 4.2.1].
3. HIGHER LEVEL PROTOCOLS
3.1 INTRODUCTION
It is envisioned that the TCP will be able to support higher level
protocols efficiently. It should be easy to interface existing
ARPANET protocols like TELNET and FTP to the TCP.
3.2 WELL KNOWN SOCKETS
At some point, a set of well known 24 bit port numbers must be
picked. The type of service associated with the well known ports
might include:
(a) Logger
(b) FTP (File transfer protocol)
(c) RJE (Remote job entry)
(d) Host status
(e) TTY Test
(f) HELP - descriptive, interactive system documentation
WE RESERVE WELL KNOWN SOCKET 0 (24 bits of 0) for global messages
destined for a particular TCP but not related to any particular
connection. We imagine that this socket would be used for unusual TCP
synchronization (e.g. RESET ALL) or for testing purposes (e.g.
sending letters to TRASHCAN or ECHO). This does not conflict with the
usage that if a socket is 0, it is unspecified, since no user can
SEND, CLOSE, or INTERRUPT on socket 0.
3.3 RECONNECTION PROTOCOL (RCP)
Port identifiers fall into two categories: permanent and transient.
For example, a Logger process is generally assigned a port identifier
that is fixed and well known. Transient processes will in general
have ID's which are dynamically assigned.
In the distributed processing environment of the network, two
processes that don't have well known port identifiers may often wish
to communicate. This can be achieved with the help of a well known
process using a reconnection protocol. Such a protocol is briefly
outlined using the communication facilities provided by the TCP. It
essentially provides a mechanism by which port identifiers are
exchanged in order to establish a connection between a pair of
sockets.
Such a protoco1 can be used to achieve the dynamic establishment of
new connections in order to have multiple processes solving a problem
cooperatively, or to provide a user process access to a server
process via a logger, when the logger's end of the connection can not
be invisibly passed to the server process.
A paper on this subject by R. Schantz [SCHA74] discusses some of the
issues associated with reconnection, and some of the ideas contained
therein went into the design of the protocol outlined below.
In the ARPANET, a protocol was implemented which would allow a
process to connect to a well known socket, thus making an implicit
request for service, and then be switched to another socket so that
the well known socket could be freed for use by others. Since sockets
in our TCP are permitted to have connections with more than one
foreign socket, this facility may not be explicitly needed (i.e.
connections <A,B> and <A,C> are distinguishable).
However. the well known socket may be in one network and the actual
service socket(s) may be in another network (or at least in another
TCP). Thus, the invisible switching of a connection from one port to
another within a TCP may not be sufficient as an "Initial Connection
Protocol". We imagine that a process wishes to use socket N1.T1.Q to
access well known socket N2.T2.P. However, the process associated
with socket N2.T2.P will actually start up a new process somewhere
which will use N3.T3.S as its server socket. The N(i) and T(i) may be
distinct or the same. The user will send to N2.T2.P the relevant user
information such as user name, passWord, and account. The server will
start up the server process and send to N1.T1.Q the actual service
socket ldentif1er: N3.T3.S. The connection (N1.TI.Q,N2.T2.P) can then
be closed, and the user can do a RECEIVE on (N1.T1.Q,N3.T3.S). The
serving process can SEND on (N3.T3.S,N1.T1.Q). There are many
variations on this scheme, some involving the user process doing a
RECEIVE on a different socket (e.g. (N1.T1.X,U.U.U)) with the server
doing SEND on (N3.T3.S,N1.T1.X). Without showing all the detail of
synchronization of sequence numbers and the like, we can illustrate
the exchange as shown below.
USER SERVER
1. RECEIVE(N2.T2.P,U.U.U)
1. SEND (N1.T1.Q,N2.T2.P)==>
<== 2. SEND(N2.T2.P,N1.T1.Q)
With "N3.T3.S" as data
2. RECEIVE(N1.T1.Q,N2.T2.P)
3. CLOSE(N1.T1.Q,N2.T2.P)==>
<:= 3. CLOSE(N2.T2.P,N1.T1.Q)
4. RECEIVE(N1.T1.Q,N3.T3.S)
<== 4. SEND(N3.T3.S,N1.T1.Q)
At this point, a connection is open between N1.T1.Q and N3.T3.S. A
variation might be to have the user do an extra RECEIVE on
(N1.T1.X,U.U.U) and have the data "N1.T1.X" be sent in the first user
SEND. Then, the server can start up the real serving process and do a
SEND on (N3.T3.S,N1.T1.X) without having to send the "N3.T3.S" data
to the user. Or perhaps both server and receiver exchange this data,
to assure security of the ultimate connection (i.e. some wild process
might try to connect to N1.T1.X if it is merely RECEIVING on foreign
socket U.U.U.).
We do not propose any specific reconnection protocol here, but leave
this to further deliberation, since it is really a user level
protocol issue.
4. TCP IMPLEMENTATION
4.1 INTRODUCTION
Conceptually, the TCP is made up of several processes. Some of these
deal with USER/TCP commands, and others with packets arriving from
the network. The TCP also has an internal measurement facility which
can be activated remotely.
Any particular TCP could be viewed in a number of ways. It could be
implemented as an independent process, servicing many user processes.
It could be viewed as a set of re-entrant library routines which
share a common interface to the local PSN, and common buffer storage.
It could even be viewed as a set of processes, some handling the
user, some the input of packets from the net, and some the output of
packets to the net.
4.2 TCP DATA STRUCTURES
4.2.1 INTERNETWORK PACKET FONMAT
8 bits: Internet information
2 bits: Reserved for local PSN use
2 bits: Header format (11 in binary)
4 bits: Protocol version number
8 bits: Header length in octets (32 is the current value)
16 bits: Length of text in octets
32 bits: Packet sequence number
32 bits: Acknowledgment number (i.e. sequence number of next octet
expected).
16 bits: Window size (in octets)
16 bits: Control Information
Listed from high to low order:
SYN: Request to synchronize sending sequence numbers
ACK: There is a valid acknowledgment in the 32 bit ACK field
FIN: Sender will stop SENDing and RECEIVEing on this connection
DSN: The sender has stopped using sequence numbers and wants to
initiate a new sequence number for sending.
EOS: This packet is the end of a segment and therefore has a
checksum in the 16 bit checksum field. If this bit is not set, the
16 bit checksum field is to be ignored. The bit is usually set,
but if fragmentation at a GATEWAY occurs, the packets preceding
the last one will not have checksums, and the last packet will
have the checksum for the entire original fragment (segment) as it
was calculated by the sending TCP.
EOL: This packet contains the last fragment of a letter. The EOS
bit will always be set in this case.
INT: The sender wants to INTERRUPT on this connection.
XXX: six (6) unused control bits
OD: three (3) bits of control dispatch:
000: Null (the control octet contents should be ignored}
001: Event Code is present in the control octet. These were
defined in section 2.4.3.
010: Special Functions
011: Reject (codes as yet undefined)
1XX: Unused
8 bits: Control Data Octet
If CD is 000 then this octet is to be ignored.
If CD is 001, this octet contains event codes defined in section
2.4.3
If CD is 010, this octet contains a special function code as
defined below:
0: RESET all connections between Source and Destination TCPs
l: RESET the specific connection referenced in this packet
2: ECHO return packet to sender with the special function code
ECHOR (Echo Reply).
3: QUERY Query status of connection referenced in this packet
4: STATUS Reply to QUERY with requested status.
5: ECHOR Echo Reply
6: TRASH Discard packet without acknowledgment
>6: Unused
Note: Special function packets not pertaining to a particular
connection [RESET all, ECHO, ECHOR, and TRASH] are normally
sent using socket zero as described in section 3.2.
If CD is 01l, this octet contains an as yet undefined REJECT code.
If CD is 1XX, this octet is undefined.
4 bits: Length of destination network address in 4 bit units (current
value is 1)
4 bits: Destination network address
1010-1111 are addresses of ARPANET, UCL, CYCLADES, NPL, CADC, and
EPSS respectively.
16 bits: Destination TCP address
8 bits: Padding
4 bits: length of source network address in 4 bit units (current
value is 1)
4 bits: source network address (as for destination address)
16 bits: Source TCP address
24 bits: Destination port address
24 bits: Source port address
16 bits: Checksum (if EOS bit is set)
4.2.2 TRANSMISSION CONTROL BLOCK
It is highly likely that any implementation will include shared data
structures among parts of the TCP and some asynchronous means of
signaling users when letters have been delivered.
One typical data structure is the Transmission Control Block (TCB)
which is created and maintained during the lifetime of a given
connection. The TCB contains the following information (field sizes
are notional only and may vary from one implementation to another):
16 bits: Local connection name
48 bits: Local socket
48 bits: Foreign socket
16 bits: Receive window size in octets
32 bits: Receive left window edge (next sequence number expected)
16 bits: Receive packet buffer size of TCB (may be less than
window)
16 bits: Send window size in octets
32 bits: Send left window edge (earliest unacknowledged octet)
32 bits: Next packet sequence number
16 bits: Send packet buffer size of TCB (may be less than window)
8 bits: Connection state
E/C - 1 if TCP has been synchronized at least once (i.e. has
been established, else O, meaning it is closed; this bit is
reset after FINS are exchanged and the user has done a CLOSE).
The bit is not reset if the connection is only desynchronized
on send or receive or both directions.
SS - SYNCed on send side (if set) else desynchronized
SR - SYNCed on receive side (if set, else desynchronized)
16 bits: Special flags
S1 - SYN sent if set
S2 - SYN verified if set
R - SYN received if set
Y - FIN sent if set
C - CLOSE from local user received if set
U - Foreign socket unspecified if set
SDS - Send side DSN sent if set
SDV - Send side DSN verified if set
RDR - Receive side DSN received if set
Initially, all bits are off [no pun intended] (i.e. SS, SR, E/C, S1,
S2, R, F, C, SDS, SDV, RDR =0). When R is set, so is SR. When S1 and
S2 are both set, so is SS. SR is reset when RDR is set. SS is reset
when both SDS and SDV are set. These bits are used to keep track of
connection state and to aid in arriving packet processing (e.g. Can
sequence number be validated? Only if SR is set.).
16 bits: Retransmission timeout (in eighths of a second#]
16 bits: Head of Send buffer queue [buffers SENT from user to TCP,
but not packetized]
16 bits: Tail of Send buffer queue
16 bits: Pointer to last octet packetized in partially packetized
buffer (refers to the buffer at the head of the queue)
16 bits: Head of Send packet queue
16 bits: Tail of Send packet queue
16 bits: Head of Packetized buffer Queue
16 bits: Tail of Packetized buffer queue
16 bits: Head of Retransmit packet queue
16 bits: Tail of Retransmit packet queue
16 bits: Head of Receive buffer queue [queue of buffers given by user
to RECEIVE letters, but unfilled]
16 bits: Tail of Receive buffer queue
16 bits: Head of Receive packet queue
16 bits: Tail of receive packet queue
16 bits: Pointer to last contiguous receive packet
16 bits: Pointer to last octet filled in partly filled buffer
16 bits: Pointer to next octet to read from partly emptied packet
[Note: The above two pointers refer to the head of the receive
buffer and receive packet queues respectively]
16 bits: Forward TCB pointer
16 bits: Backward TCB pointer
4.3 CONNECTION MANAGEMENT
4.3.1 INITIAL SEQUENCE NUMBER SELECTION
The protocol places no restriction on a particular connection being
used over and over again. New instances of a connection will be
referred to as incarnations of the connection. The problem that
arises owing to this is, "how does the TCP identify duplicate packets
from previous incarnations of the connection?". This problem becomes
harmfully 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.
The essence of the solution [TOML74] is that the initial sequence
number [ISN] must be chosen so that a particular sequence number can
never refer to an "o1d" octet, Once the connection is established the
sequencing mechanism provided by the TCP filters out duplicates.
For an association to be established or initialized, the two TCP's
must synchronize on each other's initial sequence numbers. Hence the
solution requires a suitable mechanism for picking an initial
sequence number [ISN], and a slightly involved handshake to exchange
the ISN's. A "three way handshake" is necessary because sequence
numbers are not tied to a global clock in the network, and TCP's may
have different mechanisms for picking the ISN's. The receiver of the
first SYN has no way of knowing whether the packet 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 [TOML74]. More on the subject, and algorithms
for implementing the clock-driven scheme can be found in [DALA74].
4.3.2 ESTABLISHING A CONNECTION
The "three way handshake" is essentially a unidirectional attempt to
establish the connection, i.e. there is an initiator and a responder.
The TCP's should however be able to establish the connection even if
a simultaneous attempt is made by both TCP's to establish the
connection. Simultaneous attempts are treated like "collisions" in
"Aloha" systems and these conflicts are resolved into unidirectional
attempts to establish the connection. This scheme was adopted because
(i) Connections will normally have a passive and an active end,
and so the mechanism should in most cases be as simple as
possible.
(ii) It is easy to implement as special cases do not have to be
accounted for.
The example below indicates what a three way handshake between TCP's
A and B looks like
A B
--> <SEQ x><SYN> -->
<-- <SEQ y><SYN, ACK x+l> <--
--> <SEQ x+1><ACK y+l><DATA BYTES> -->
The receiver of a "SYN" is able to determine whether the "SYN" was
real (and not an old duplicate) when a positive "ACK" is returned for
the receiver's "SYN,ACK" in response to the "SYN". The sender of a
"SYN" gets verification on receipt of a "SYN,ACK" whose "ACK" part
references the sequence number proposed in the original "SYN" [pun
intended]. If the TCP is in the state where it is waiting for a
response to its SYN, but gets a SYN instead, then it always thinks
this is a collision and goes into the state prior to having sent the
SYN, i.e. it forgets that it had sent a SYN. The TCP will try to
establish the connection again after some time, unless it has to
respond to an arriving SYN. Even if the wait times in the two TCPs
are the same, the varying delays in network transmission will usually
be adequate to avoid a collision on the next cycle of attempts to
send SYN.
When establishing a connection, the state of the TCP is represented
by 3 bits --
S1 S2 R
S1 = 1 -- SYN sent
S2 = 1 -- My SYN verified
R = 1 -- SYN received
Some examples of attempts to establish the connection are now shown.
The state of the connection is indicated when a change occurs. We
specifically do not show the cases in which connection
synchronization is carried out with packets containing both SYN and
data. We do this to simplify the explanation, but we do not rule out
an implementation which is capable of dealing with data arriving in
the first packet (it has to be stored temporarily without
acknowledgment or delivery to the user until the arriving SYN has
been verified).
The "three way handshake" now looks like --
A B
------------ ------------
S1 S2 R S1 S2 R
0 0 0 0 0 0
--> <SEQ x><SYN> -->
1 0 0 0 0 1
<-- <SEQ y><SYN, ACK x+l> <--
1 1 1 1 0 1
--> <SEQ x+1><ACK y+1>(DATA OCTETS) -->
1 1 1 1 1 1
The scenario for a simultaneous attempt to establish the connection
without the arrival of any delayed duplicates is --
A B
------------ ------------
S1 S2 R S1 S2 R
0 0 0 0 0 0
(M1) 1 0 0 --> <SEQ x><SYN> ...
(M2) 0 0 0 <-- <SEQ y><SYN) <-- 1 0 0
(M1) B returns no SYN sent --> 0 0 0
(M1) 1 0 0 --> <SEQ z><SYN> * --> 0 0 1
(M3) 1 1 1 <-- <SEQ y+1><SYN,ACK z+1> <-- 1 0 1
(M4) 1 1 1 --> <SEQ z+1><ACK y+1><DATA> --> 1 1 1
Note: "..." means that a message does not arrive, but is delayed
in the network. State changes are upon arrival or upon departure
of a given message, as the case may be. Packets containing the SYN
or INT or DSN bits implicitly contain a "dummy" data octet which
is never delivered to the user, but which causes the packet
sequence numbers to be incremented by 1 even if no real data is
sent. This permits the acknowledgment of these controls without
acknowledging receipt of any data which might also have been
carried in the packet. A packet containing a FIN bit has a dummy
octet following the last octet of data (if any) in the packet.
* Once in state 000 sender selects new ISN z when attempting to
establish the connection again.
4.3.3 HALF-OPEN CONNECTIONS
An established connection is said to be a "half-open" connection if
one of the TCP's has closed 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
somewhat involved.
If one end of the connection no longer exists, then any attempt by
the other user to send any data on it will result in the sender
receiving the event code "Connection does not exist at foreign TCP".
Such an error message should indicate to the user process that
something is wrong and it is expected to CLOSE the connection.
Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to B's TCP.
Depending on the operating system supporting B's TCP, it is likely
that some error recovery mechanism exists. When the TCP is up again B
is likely to start again from the beginning or from a recovery point.
As a result B will probably try to OPEN the connection again or try
to SEND on the connection it believes open. In the latter case 1t
receives the error message "connection not open" from the local TCP.
In an attempt to establish the connection B's TCP will send a packet
containing SYN. A's TCP thinks that the connection is already
established and so will respond with the error "unacceptable SYN (or
SYN/ACK) arrived at foreign TCP". B's TCP knows that this refers to
the SYN it just sent out, and so should reset the connection and
inform the user process of this fact.
It may happen that B is passive and only wants to receive data. In
this case A's data will not reach B because the TCP at B thinks the
connection is not established. As a result A'S TCP will timeout and
send a QRY to B's TCP. B's TCP will send STATUS saying the connection
is not synched. A's TCP will treat this as if an implicit CLOSE had
occurred and tell the user process, A, that the connection is
closing. A is expected to respond with a CLOSE command to his TCP.
However, A's TCP does not send a FIN to B's TCP, since it would not
be accepted anyway on the unsynced connection. Eventually A will try
to reopen the connection or B will give up and CLOSE. If B CLOSES,
B's TCP will simply delete the connection since it was not
established as far as B's TCP is concerned. No message will be sent
to A'S TCP as a result.
4.3.4 RESYNCHRONIZING A CONNECTION
Details of resynchronization have not yet been specified since the
need for this should be infrequent in the initial testing stages.
4.3.5 CLOSING A CONNECTION
There are essentially three cases:
a) The user initiates by telling the TCP to CLOSE the connection
b) The remote TCP initiates by sending a FIN control signal
c) Both users CLOSE simultaneously
Two bits are used to maintain control over the closing of a
connection: these are called the "FIN sent" bit [F] and the "USER
Closed" bit, [C] respectively. The control procedure uses these two
bits to assure that the connection is properly closed.
Case 1: Local user initiates the close
In this case, both the F and C bits are initially zero, but the C
bit is set immediately upon receipt of the user call "CLOSE." When
the FIN is sent out by the TCP, the F bit is set. All pending
RECEIVES are terminated and the user is told that they have been
prematurely terminated ("connection closing"} without data.
Similarly, any pending SENDS are terminated with the same
response, "connection closing."
Several responses may arrive as the result of sending a FIN. The
one which is generally expected is a matching FIN. When this is
received, the TCB CAN BE ELIMINATED. If a "connection does not
exist at foreign TCP" message comes in response to the FIN, then
the TCB can likewise be eliminated. If no response is forthcoming,
or if "Foreign TCP inaccessible" arrives then the resolution is
moot. One might simply timeout and discard the TCB. Since the
local user wants to CLOSE anyway, this is probably satisfactory,
although it will leave a potential "half-open" connection at the
other side. We deal with half open connections in section 4.3.3.
When the acknowledging FIN arrives after the connection state bits
are set (F=1, C=1), then the TCB can be deleted.
Case 2: TCP receives a FIN from the network
First of all, a FIN must have a sequence number which lies in the
valid receive window. If not, it is discarded and the left window
edge is sent as acknowledgment. If the FIN can be processed, it is
handled (possibly out of order, since it is taken as an imperative
to shut down the connection). All pending RECEIVES and SENDS are
responded to by showing that they were terminated by the other
side's close request (i.e. "connection closing"). The user is also
told by an unsolicited event or signal that the connection has
been closed (in some systems, the user might have to request
STATUS to get this information). Finally, the TCP sends FIN in
response.
Thus, because a FIN arrived, a FIN is sent back, so the F bit is
set. However, the TCB stays around until the local user does a
CLOSE in acknowledgment of the unsolicited signal that the
connection has been closed by the other side. Thus, the C bit
remains unset until this happens. If the C and F bits go from (F=1
C=O) to (F=l, C=1), then the connection is closed and the TCB can
be removed.
Case 3: both users close simultaneously
If this happens, both connections will be in the (F=1, C=1) state.
When the FINs arrive, the connections w11i be shut down. If one
FIN fails to arrive, we have two choices. One is to insist on
acknowledgments for FINs, in which case the missing one will be
retransmitted. Another is merely to permit the half-open
connection to remain (we prefer this solution}. It can timeout
independently and go away after a while. If an attempt is made to
reestablish the connection, the initiator will discover the
existence of the open connection since an "inappropriate SYN
received" message will be sent by the TCP which holds the "half-
open" connection. The receiver of this message can tell the other
TCP to reset the connection. We cannot permit the holder of the
half-open connection to reset automatically on receipt of the SYN
since its receipt is not necessarily prima facie evidence of a
half open connection. (The SYN could be a delayed duplicate.)
4.3.6. CONNECTION STATE and its relation to USER and INCOMING CONTROL
REQUESTS
In order to formalize the action taken by the TCP when it receives
commands from the User, or Control information from the network, we
define a connection to be in one of 7 states at any instant. These
are known as the TCB Major States. Each Major State is simply a
convenient name for a particular setting or group of settings of the
state bits, as follows:
S1 S2 R U F C # name
- - - - - - 0 no TCB
0 0 0 0/1 0 0 1 unsync
1 0 0 0 0 0 2 SYN sent
1 0 1 0/1 0 0 3 SYN received
1 1 1 0 0 0 4 established
1 0/1 1 0/1 1 1 5 FIN wait
1 1 1 0 1 0 6 FIN received
The connection moves from state to state as shown below. The
transition from one state to another will be represented as
[X, Y]<cause><action>
which means that there is a transition from state X to state Y owing
to <cause>. The action taken by the TCP is specified as <action>. We
use this notation to give the important state transitions, often
simplifying the cause and action fields to take into account a number
of situations. Figure 1 illustrates these transitions in traditional
state diagram form. Section 4.4.6 and section 4.4.7 fully specify the
effect of all User commands and Control information arriving from the
network.
[0,l] <OPEN> <create TCB>
[1,2] <SEND,INTERRUPT, or collision timeout> <send SYN>
[1,3] <SYN arrives> <send SYN,ACK>
[1,0] <CLOSE> <remove TCB>
[2,1] <SYN arrives (collision)> <set timeout, forget SYNs>
[2,0] <CLOSE> <remove TCB>
[2,4] <appropriate SYN,ACK arrives> <send ACK>
[3,4] <appropriate ACK arrives> <none>
[3,1] <error arrives or timeout> <(forget SYN)>
[3,5] <CLOSE> <send FIN>
[4,5] <CLOSE> <send FIN>
[4,6] <appropriate FIN arrives> <send FIN, inform user>
[5,0] <FIN or error arrives, or timeout> <remove TCB>
[6,0] <CLOSE> <remove TCB>
4.4 STRUCTURE 0F THE TCP
4.4.l INTRODUCTION [See figure 2.1]
There are many possible implementations of the TCP. We offer one
conceptual framework in which to view the various algorithms that
make up the TCP design. In our concept, the TCP is written in two
parts, an interrupt or signal driven part (consisting of four
processes), and a reentrant library of subroutines or system calls
which interface the user process to the TCP. The subroutines
communicate with the interrupt part through shared data structures
(TCB's, shared buffer queues etc.). The four processes are the Output
Packet Handler which sends packets to the packet switch; the
Packetizer which formats letters into internet packets; the Input
Packet Handler which processes incoming packets; and the Reassembler
which builds letters for users.
The ultimate bottleneck is the pipe through which arriving and
departing packets must travel. This is the Host/Packet Switch
interface. The interrupt driven TCP shares among all TCB's its
limited packet buffer resources for sending and receiving packets.
From the standpoint of controlling buffer congestion, it appears
better to TREAT INCOMING PACKETS WITH HIGHER PRIORITY THAN OUTGOING
PACKETS. That is, packet buffers which can be released by copying
their contents into user buffers clearly help to reduce congestion.
Neither the packetizer nor the input packet handler should be allowed
to take up all available packet buffer space; an analogous problem
arises in the IMP in the allocation of store and forward, and
reassembly buffer space. One policy is to permit neither contender
more than, say, two-thirds of the space. The buffer allocation
routines can enforce these limits and reject buffer requests as
needed. Conceptually, the scheduler can monitor the amounts of
storage dedicated to the input and output routines, and can force
either to sleep if its buffer allocation exceeds the limit.
As an example, we can consider what happens when a user executes a
SEND call to the TCP service routines. The buffer containing the
letter is placed on a SEND buffer queue associated with the user's
TCB. A 'packetizer' process is awakened to look through all the TCB's
for 'packetizing' work. The packetizer will keep a roving pointer
through the TCB list which enables it to pick up new buffers from the
TCB queue and packetize them into output buffers. The packetizer
takes no more than one letter at a time from any single TCB. The
packetizer attempts to maintain a non-empty queue of output packets
so that the output handler will not fall idle waiting for the
packetizing operation. However, since arriving packets compete with
departing packets, care must be taken to prevent either class from
occupying all of the shared packet buffer space. Similarly since the
TCB's all compete for space in service to their connections, neither
input nor output packet space should be dominated by any one TCB.
When a packet is created, it is placed on a FIFO SEND packet queue
associated with its origin TCB. The packetizer wakes the output
handler and then continues to packetize a few more buffers, perhaps,
before going to sleep. The output handler is awakened either by a
'hungry' packet switch or by the packetizer; in either case, it uses
a roving TCB pointer to select the next TCB for service. The send
packet queue can be used as a 'work queue' for the output handler.
After a packet has been sent, but usually before an ACK is returned,
the output handler moves the packet to a retransmission queue
associated with each TCB.
Retransmission timeouts can refer to specific packets and the
retransmission list can be searched for the specific packet. If an
ACK is received, the retransmission entry can be removed from the
retransmit queue. The send packet queue contains only packets waiting
to be sent for the first time. INTERRUPT requests can remove entries
in both the send packet queue and the retransmit packet queue.
Since packets are never in more than one queue at a time, it appears
possible for INT, FIN or RESET commands to remove packets from the
receive, send, or retransmit packet queues with the assurance that an
already issued signal to enter the reassembler, the packetizer or the
output handler will not be confusing.
Handling the INTERRUPT and CLOSE functions can however require some
care to avoid confusing the scheduler, and the various processes. The
scheduler must maintain status information for the processes. This
information includes the current TCB being serviced. When an
INTERRUPT is issued by a local process, the output queue of letters
associated with the local port reference is to be deleted. The
packetizer, for example, may however be working at that time on the
same queue. As usual, simultaneous reading and writing of the TCB
queue pointers must be inhibited through some sort of semaphore or
lockout mechanism. When the packetizer wants to serve the next send
buffer queue, it must lock out all other access to the queue, remove
the head of the queue (assuming of course that there are enough
buffers for packetization), advance the head of the queue, and then
unlock access to the queue.
If the packetizer keeps only a TCB pointer in a global place called
CPTCB (current packetizer TCB address), and always uses the address
in CPTCB to find the TCB in which to examine the send buffer queue,
then removal of the output buffer queue does not require changes to
any working storage belonging to the packetizer. Even more important,
the arrival and processing of a RESET or CLOSE, which clears the
system of a given TCB, can update the CPTCB pointer, as long as the
removal does not occur while the packetizer is still working on the
TCB.
Incoming packets are examined by the input packet handler. Here they
are checked for valid connection sockets, and acknowledgments are
processed, causing packets to be removed, possibly, from the SEND or
RETRANSMIT packet queues as needed. As an example, consider the
receipt of a valid FIN request on a particular TCB. If a FIN had not
been sent before (i.e. F bit not set), then a FIN packet is
constructed and sent after having cleared out the SEND buffer and
SEND packet queues as well as the RETRANSMIT queue. Otherwise, if the
F and C bits are both set, all queues are emptied and the TCB is
returned to free storage.
Packets which should be reassembled into letters and sent to users
are queued by the input packet handler, on the receive packet queue,
for processing by the reassembly process. The reassembler looks at
its FIFO work queue and tries to move packets into user buffers which
are queued up in an input buffer queue on each TCB. If a packet has
arrived out of order, it can be queued for processing in the correct
sequence. Each time a packet is moved into a user buffer, the left
window edge of the receiving TCB is moved to the right so that
outgoing packets can carry the correct ACK information. If the SEND
buffer queue is empty, then the reassembler creates a packet to carry
the ACK.
As packets are moved 1nto buffers and they are filled, the buffers
are dequeued from the RECEIVE buffer queue and passed to the user.
The reassembler can also be awakened by the RECEIVE user call should
it have a non-empty receive packet queue with an empty RECEIVE buffer
queue. The awakened reassembler goes to work on each TCB, keeping a
roving pointer, and sleeping if a cycle is made of all TCB's without
finding any work.
4.4.2 INPUT PACKET HANDLER [See figure 2.2]
The Input Packet Handler is awakened when a packet arrives from the
network. It first verifies that the packet is for an existing TCB
(i.e. the local and foreign socket numbers are matched with those of
existing TCB's). If this fails, an error message is constructed and
queued on the send packet queue of a dummy TCB. A signal is also sent
to the output packet handler. Generally, things to be transmitted
from the dummy TCB have a default retransmission timeout of zero, and
will not be retransmitted. (We use the idea of a dummy TCB so that
all packets containing errors, or RESET can be sent by the output
packet handler, instead of having the originator of them interface to
the net. These packets, it will be noticed, do not belong to any
TCB).
The input packet handler looks out for control or error information
and acts appropriately. Section 4.4.7 discusses this in greater
detail, but as an example, if the incoming packet is a RESET request
of any kind (i.e. all connections from designated TCP or given
connection), and is believable, then the input packet handler clears
out the related TCB(s), empties the send and receive packet queues,
and prepares error returns for outstanding user SEND(s) and
RECEIVE(s) on each reset TCB. The TCB's are marked unused and
returned to storage. If the RESET refers to an unknown connection, it
is ignored.
Any ACK's contained in incoming packets are used to update the send
left window edge, and to remove the ACK'ed packets from the TCB
retransmit packet queue. If the packet being removed was the end of a
user buffer, then the buffer must be dequeued from the packetized
buffer queue, and the User informed. The packetizer is also signaled.
Only one signal, or one for each packet, will have to be sent,
depending on the scheduling scheme for the processes. See section
4.4.7 for a detailed discussion.
The packet sequence number, the current receive window size, and the
receive left window edge determine whether the packet lies within the
window or outside of it.
Let W = window size
S = size of sequence number space
L = left window edge
R = L+W-1 = right window edge
x = sequence number to be tested
For any sequence number, x, if
(R-x) mod S <= W
then x is within the window.
A packet should be rejected only if all of it lies outside the
window. This is easily tested by letting x be, first the packet
sequence number, and then the sum of packet sequence number and
packet text length, less one. If the packet lies outside the window,
and there are no packets waiting to be sent, then the input packet
handler should construct a dummy ACK and queue it for output on the
send packet queue, and signal the output packet handler. Successfully
received packets are placed on the receive packet queue in the
appropriate sequence order, and the reassembler signaled.
The packet window check can not be made if the associated TCB is not
in the 'established' state, so care must be taken to check for
control and TCB state before doing the window check.
4.4.3 REASSEMBLER [See figure 2.3]
The Reassembler process is activated by both the Input Packet Handler
and the RECEIVE user call. While the reassembler is asleep, if
multiple signals arrive, all but one can be discarded. This is
important as the reassembler does not know the source of the signal.
This is so in order that "dangling" signals from work in TCB's that
have subsequently been removed don't confuse it. Each signal simply
means that there may be work to be done. If the reassembler is awake
when a signal arrives, it may be necessary to put 1t in a
"hyperawake" state so that even if the reassembler tries to quit, the
scheduler will run it one more time.
When the reassembler is awakened it looks at the receive packet queue
for each TCB. If there are some packets there then it sees whether
the RECEIVE buffer queue is empty. If it is then the reassembler
gives up on this TCB and goes on to the next one, otherwise if the
first packet matches the left window edge, then the packet can be
moved into the User's buffer. The reassembler keeps transferring
packets into the User's buffer until the letter is completely
transferred, or something causes it to stop. Note that a buffer may
be partly filled and then a sequence 'hole' is encountered in the
receive packet queue. The reassembler must mark progress so that the
buffer can be filled up starting at the right place when the 'hole'
is filled. Similarly a packet might be only partially emptied when a
buffer is filled, so progress in the packet must be marked.
If a letter was successfully transferred to a User buffer then the
reassembler signals the User that a letter has arrived and dequeues
the buffer associated with it from the TCB RECEIVE buffer queue. If
the buffer is filled then the User is signaled and the buffer
dequeued as before. The event code indicates whether the buffer
contains all or part of a letter, as described in section 2.4.
In every case when a packet is delivered to a buffer, the receive
left window edge is updated, and the packetizer is signaled. This
updating must take account of the extra octet included in the
sequencing for certain control functions [SYN, INT, FIN, DSN]. If the
send packet queue is empty then the reassembler must create a packet
to carry the ACK, and place it on the send packet queue.
Note that the reassembler never works on a TCB for more than one User
buffer's worth of time, in order to give all TCB's equal service.
Scheduling of the reassembler is a big issue, but perhaps running to
completion will be satisfactory, or else it can be time sliced. In
the latter case it will continue from where it left off, but a new
signal may have arrived producing some possible work. This work will
be processed as part of the old incomplete signal, and so some
wasteful processing may occur when the reassembler wakes up again.
This is the general problem of trying to implement a protocol that is
fundamentally asynchronous, but at least it is immune to harmful
race-conditions. E.g. if we were to have the reassembler 'remove' the
signal that caused it to wake up, just before it went to sleep (in
order that new arriving ones were discarded) then a new signal may
arrive at a critical time causing 1t not to be recognized; thus
leaving some work pending, and this may result in a deadlock [see
previous comments on "hyperawake" state].
4.4.4 PACKETIZER [See figure 2.4]
The Packetizer process gets work from both the Input Packet Handler
and the SEND user call. The signal from the SEND user call indicates
that there is something new to send, while the one from the input
packet handler indicates that more TCP buffers may be available from
delivered packets. This latter signal is to prevent deadlocks in
certain kind of scheduling schemes. We assume the same treatment of
signals as discussed in section 4.4.3.
When the packetizer is awakened it looks at the SEND buffer queue for
each TCB. If there is a new or partial letter awaiting packetization,
it tries to packetize the letter, TCB buffer and window permitting.
It packetizes no more than one letter for a TCB before servicing
another TCB. For every packet produced it signals the output packet
handler (to prevent deadlock in a time sliced scheduling scheme). If
a 'run till completion' scheme is used then one signal only need be
produced, the first time a packet is produced since awakening. If
packetization is not possible the packetizer goes on to the next TCB.
If a partial buffer was transferred then the packetizer must mark
progress in the SEND buffer queue. Completely packetized buffers are
dequeued from the SEND buffer queue, and placed on a Packetized
buffer queue, so that the buffer can be returned to the user when an
ACK for the last bit is received.
When the packetizer packetizes a letter it must see whether it is the
first piece of data being sent on the connection, in which case it
must include the SYN bit. Some implementations may not permit data to
be sent with SYN and others may discard any data received with SYN.
The Packetizer goes to sleep if it finds no more work at any TCB.
4.4.5 OUTPUT PACKET HANDLER [see figure 2.5]
When activated by the packetizer, or the input packet handler, or
some of the user call routines, the Output Packet Handler attempts to
transmit packets on the net (may involve going through some other
network interface program). It looks at the TCB's in turn,
transmitting some packets from the send packet queue. These are
dequeued and put on the retransmit queue along with the time when
they should be retransmitted.
All data packets that are transmitted have the latest receive left
window edge in the ACK field. Error and control messages may have no
ACK [ACK bit off], or set the ACK field to refer to a received
packet's sequence number.
The RETRANSMIT PROCESS:
This process can either be viewed as a separate process, or as part
of the output packet handler. Its implementation can vary; it could
either perform its function, by being woken up at regular intervals,
or when the retransmission time occurs for every packet put on the
retransmit queue. In the first case the retransmit queue for each TCB
is examined to see if there is anything to retransmit. If there is, a
packet is placed on the send packet queue of the corresponding TCB.
The output packet handler is also signaled.
Another "demon" process monitors all user Send buffers and
retransmittable control messages sent on each connection, but not yet
acknowledged. If the global retransmission timeout is exceeded for
any of these, the User is notified and he may choose to continue or
close the connection. A QUERY packet may also be sent to ascertain
the state of the connection [this facilitates recovery from half open
connections as described in section 4.3.3].
4.4.6 USER CALL PROCESSING
OPEN [See figure 3.1]
1. If the process calling does not own the specified local socket,
return with <type 1><ELP 1 "connection illegal for this process">.
2. If no foreign socket is specified, construct a new TCB and add
it to the list of existing TCB's. Select a new local connection
name and return it along with <type 1><OLP 0 "success">. If there
is no room for the TCB, respond with <type 1><ELT 4 "No room for
TCB">.
3. If a foreign socket is specified, verify that there is no
existing TCB with the same <local socket, foreign socket> pair
(i.e. same connection), otherwise return <type l><ELP 6
"connection already open">. If there is no TCB space, return as in
(2), otherwise, create the TCB and link it with the others,
returning a local connection name with the success event code.
Note: if a TCB is created, be sure to copy the timeout parameter
into it, and set the "U" bit to 0 if a foreign socket is
specified, else set U to 1 (to show unspecified foreign socket).
SEND [see figure 3.2]
1. Search for TCB with local connection name specified. If none
found, return <type 10><ELP 3 "connection not open">
2. If TCB is found, check foreign socket specification. If not set
(i.e. U = 1 in TCB), return <type 10><ELT 5 "foreign socket
unspecified">. If the connection is in the "closing" state (i.e.
state 5 or 6), return <type 3><ELP 12 "connection closing"> and do
not process the buffer.
3. Put the buffer on the Send buffer queue and signal the
packetizer that there is work to do.
INTERRUPT [see figure 3.3]
1. Validate existence of the referenced connection, sending out
error messages of the form <type 3><ELP 3 "connection not open">
or <type 3><ELT 5 "foreign socket unspecified"> as appropriate. If
the local connection refers to a connection not accessible to the
process interrupting, send <type 3><ELP 1 "connection illegal for
this process">.
2. If the connection is in the "closing" state (i.e. states 5 or
6), return <type 3><ELT 12 "connection closing"> and do not send
an INT packet to the destination.
3. Any pending SEND buffers should be returned with <type 10><ELP
10 "buffer flushed due to interrupt">. An INT packet should be
created and placed on the output packet queue, and the output
packet handler should be signaled.
RECEIVE [See figure 3.4]
1. If the caller does not have access to the referenced local
connection name, return <type 20><ELP 1 "connection illegal for
this process">. And if the connection is not open, return <type
20><ELP 3 "connection not open"). If the connection is in the
closing state (e.g. a FIN has been received or a user CLOSE is
being processed), return <type 20><ELP 12 "connection closing">.
2. Otherwise, put the buffer on the receive buffer queue and
signal the reassembler that buffer space is available.
CLOSE [See figure 3.5]
1. If the connection is not accessible to the caller, return <type
2><ELP 1 "connection illegal for this process">. If there is no
such connection respond with <type 2><ELP 3 "connection not
open">.
2. If the R bit is 0 (i.e. connection is in state 1 or 2), simply
remove the TCB.
3. If the R bit is set and the F bit is set, then remove the TCB.
4. Otherwise, if the R bit is set, but F is 0 (i.e. states 3 or
4), return all buffers to the User with <type x><ELP 12
"connection closing">, clear all output and input packet queues
for this connection, create a FIN packet, and signal the output
packet handler. Set the C and F bits to show this action.
STATUS [See figure 3.6]
1. If the connection is illegal for the caller to access, send
<type 30><ELP 1 "connection illegal for this process">.
2. If the connection does not exist, return <type 30><ELP 3
"connection not open">.
3. Otherwise set status information from the TCB and return it via
<type 30><O-T 0 "status data...">.
4.4.7 NETWORK CONTROL PROCESSING
The Input Packet Handler examines the header to see if there is any
control information or error codes present. We do not discuss the
action taken for various special function codes, as it is often
implementation dependent, but we describe those that affect the state
of the connection. After initial screening by the IPC [see section
4.4.2 and figure 2.2], control and error packets are processed as
shown in figures 4.l-4.7. [ACK and data processing is done within the
IPC.]
4.4.8 TCP ERROR HANDLING
Error messages have CD=001 and do not carry user data. Depending on
the error, zero or more octets of error information will be carried
in the packet text field. We explicitly assume that this data is
restricted in length so as to fall below the GATEWAY fragmentation
threshold (probably 512 bits of data and header). Errors generally
refer to specific connections, so the source and destination socket
identifiers are relevant here. The ACK field of an error packet
contains the sequence number of the packet that caused the error, and
the ACK bit is off. [RESET and STATUS special functions may use the
ACK field in the same way.] This allows the receiver of an error
message to determine which packet caused the error. Error packets are
not ACK'ed or retransmitted.
4.5. BUFFER AND WINDOW ALLOCATION
4.5.1 INTRODUCTION
The TCP manages buffer and window allocation on connections for two
main purposes: equitably sharing limited TCP buffer space among all
connections (multiplexing function), and limiting attempts to send
packets, so that the receiver is not swamped (flow control function).
For further details on the operation and advantages of the window
mechanism see CEKA74.
Good allocation schemes are one of the hardest problems of TCP
design, and much experimentation must be done to develop efficient
and effective algorithms. Hence the following suggestions are merely
initial thoughts. Different implementations are encouraged with the
hope that results can be compared and better schemes developed.
Several of the measurements discussed in a later section are aimed at
providing information on the performance of allocation mechanisms.
This should aid in determining significant parameters and evaluating
alternate schemes.
4.5.2 The SEND Side
The window is determined by the receiver. Currently the sender has no
control over the SEND window size, and never transmits beyond the
right window edge. There exists the possibility of specifying two
more special function codes so that the sender can request the
receiver to INCREASE or DECREASE the window size, without specifying
by how much. The receiver, of course, needn't satisfy this request.
Buffers must be allocated for outgoing packets from a TCP buffer
pool. The TCP may not be willing to allocate a full window's worth of
buffers, so buffer space for a connection may be less than what the
window would permit. No deadlocks are possible even if there is
insufficient buffer or window space for one letter, since the
receiver will ACK parts of letters as they are put into the user's
buffer, thus advancing the window and freeing buffers for the
remainder of the letter.
It is not mandatory that the TCP buffer outgoing packets until
acknowledgments for them are received, since it is possible to
reconstruct them from the actual letters sent by the user.
However, for purposes of retransmission and processing efficiency it
is very convenient to do.
4.5.3 The RECEIVE Side
At the receiving side there are two requirements for buffering:
(l) Rate Discrepancy:
If the sender produces data much faster or much slower than the
receiver consumes it, little buffering is needed to maintain the
receiver at near maximum rate of operation. Simple queuing
analysis indicates that when the production and consumption
(arrival and service) rates are similar in magnitude, more
buffering is needed to reduce the effect of stochastic or bursty
arrivals and to keep the receiver busy.
(2) Disorderly Arrivals:
When packets arrive out of order, they must be buffered until the
missing packets arrive so that packets (or letters) are delivered
in sequence. We do not advocate the philosophy that they be
discarded, unless they have to be, otherwise a poor effective
bandwidth may be observed. Path length, packet size, traffic
level, routing, timeouts, window size, and other factors affect
the amount by which packets come out of order. This is expected to
be a major area of investigation.
The considerations for choosing an appropriate window are as follows:
Suppose that the receiver knows the sender's retransmission timeout,
also, that the receiver's acceptance rate is 'U' bits/sec, and the
window size is 'W' bits. Ignoring line errors and other traffic, the
sender transmits at a rate between W/K and the maximum line rate (the
sender can send a window's worth of data each timeout period).
If W/K is greater than U, the difference must be retransmissions
which is undesirable, so the window should be reduced to W', such
that W'/K is approximately equal to U. This may mean that the entire
bandwidth of the transmission channel is not being used, but it is
the fastest rate at which the receiver is accepting data, and the
line capacity is free for other users. This is exactly the same case
where the rates of the sender and receiver were almost equal, and so
more buffering is needed. Thus we see that line utilization and
retransmissions can be traded off against buffering.
If the receiver does not accept data fast enough (by not performing
sufficient RECEIVES) the sender may continue retransmitting since
unaccepted data will not be ACK'ed. In this case the receiver should
reduce the window size to "throttle" the sender and inhibit useless
retransmissions.
Receiver window control:
If the user at the receiving side is not accepting data, the
window should be reduced to zero. In particular, if all TCP
incoming packet buffers for a connection are filled with received
packets, the window must go to zero to prevent retransmissions
until the user accepts some packets.
Short term flow control:
Let F = the number of user receive buffers filled
B = the total user receive buffers
W = the long-term or nominal window size
W' = the window size returned to the sender
then a possible value for W' is
W' = W*[1-F/B]**a
The value of 'a' should be greater than one, in order to shut the
window faster as buffers run out. The values of W' and F actually
used could be averages of recent values, in order to get smooth
control. Note that W' is constantly being recomputed, while the
value of W, which sets the upper limit of W', only changes slowly
in response to other factors.
The value of W can be large (up to half the sequence number space)
to allow for good throughput on high delay channels. The sender
needn't allocate W worth of buffer space anyway. The long-term
variation of W to match flow requirements may be a separate
question
This short-term mechanism for flow control allows some buffering in
the two TCP's at either end, (as much as they are willing), and the
rest in the user process at the send side where the data is being
created. Hence the cost of buffering to smooth out bursty traffic is
borne partly by the TCP's, and partly by the user at the send side.
None of it is borne by the communication subnet.
5. NETWORK MEASUREMENT PLANS FOR TCP
5.1 USERLEVEL DIAGNOSTICS
We have in mind a program which will exercise a given TCP, causing it
to cycle through a number of states; opening, closing, and
transmitting on a variety of connections. This program will collect
statistics and will generally try to detect deviation from TCP
functional specifications. Clearly there will have to be a copy of
this program both at the local site being tested and some site which
has a certified TCP. So we will have to produce a specification for
this user level diagnostic program also.
There needs to be a master and a slave side to all this so the master
can tell the slave what's going wrong with the test.
5.2 SINGLE CONNECTION MEASUREMENTS
Round trip delay times
Time from moment the packet is sent by the TCP to the time that
the ACK is received by the TCP.
Time from the moment the USER issues the SEND to the time that the
USER gets the successful return code.
Note: packet size should be used to distinguish from one set of
round trip times and another.
Network destination, and current configuration and traffic load
may also be issues of importance that must be taken into
account.
What if the destination TCP decides to queue up ACKs and send a
single ACK after a while? How does this affect round trip
statistics?
What about out of order arrivals and the bunched ACK for all of
them?
The histogram of round trip times include retransmission times
and these must be taken into account in the analysis and
evaluation of the collected data.
Packet size statistics
Histogram of packet length in both directions on the full duplex
connection.
Histogram of letter size in both directions.
Measure of disorderly arrival
Distance from the first octet of arriving packet to the left
window edge. A histogram of this measure gives an idea of the out
of order nature of packet arrivals. It will be 0 for packets
arriving in order.
Retransmission Histogram
Effective throughput
This is the effective rate at which the left edge of the window
advances. The time interval over which the measure is made is a
parameter of the measurement experiment. The shorter the interval,
the more bursty we would expect the measure to be.
It is possible to measure effective data throughput in both
directions from one TCP by observing the rate at which the left
window edge is moving on ACK sent and received for the two
windows.
Since throughput is largely dependent upon buffer allocation and
window size, we must record these values also. Varying window for
a fixed file transmission might be a good way to discover the
sensitivity of throughput to window size.
Output measurement
The throughput measurement is for data only, but includes
retransmission. The output rate should include all octets
transmitted and will give a measure of retransmission overhead.
Output rate also includes packet format overhead octets as well as
data.
Utilization
The effective throughput divided by the output rate gives a
measure of utilization of the communication connection.
Window and buffer allocation measurements
Histogram of letters outstanding, measured at the instant of SEND
receipt by TCP from user or at instant of arrival of a letter for
a receiving user.
Buffers in use on the SEND side upon packet departure into the
net; buffers in use on the RECEIVE side upon delivery of packet
into a USER Buffer.
5.3 MULTICONNECTION MEASUREMENTS
Statistics on User Commands sent to the local TCP
Statistics of error or success codes returned [histogram of each type
of error or return response]
Statistics of control bit use
Counter for each control bit over all packets emitted by the TCP
and another for packets accepted
Count data carrying packets
Count ACK packets with no data
Error packets distribution by error type code received from the net
and sent out into the net
5.4 MEASUREMENT IMPLEMENTATION PHILOSOPHY
We view the measurement process as something which occurs internal to
the TCP but which is controllable from outside. A well known socket
owned by the TCP can be used to accept control which will select one
or more measurement classes to be collected. The data would be
periodically sent to a designated foreign socket which would absorb
the data for later processing, in the manner currently used in the
ARPANET IMPs. Each measurement class has its own data packet format
to make the job of parsing and analyzing the data easier.
We would restrict access to TCP measurement control to a few
designated sites [e.g. NMC, SU-DSL, BBN]. This is easily done by
setting up listening control connections on partially specified
foreign sockets.
6. SCHEDULE OF IMPLEMENTATION
7. REFERENCES
1. CEKA74
V. Cerf and R. Kahn, "A Protocol For Packet Network
Intercommunication," IEEE Transactions on Communication, vol. C-
2O, No. 5. May 1974, pp. 637-648.
2. CERF74
V. Cerf, "An Assessment of ARPANET Protocols," in Proceedings of
the Jerusalem Conference on Information Technology, July l974
[RFC#635, INWG Note # ***].
3.CESU74
V. Cerf and C. Sunshine, "Protocols and Gateways for the
Interconnection of Packet Switching Networks," Proc. of the
Subconference on Computer Nets, Seventh Hawaii International
Conference on Systems Science, January 1974.
4. HEKA70
F. Heart, R.E. Kahn, et al, "The Interface Message Processor for
the ARPA Computer Network," AFIPS 1970 SJCC Proceedings, vol. 36,
Atlantic City, AFIPS Press, New Jersey, pp. 551-567.
5. POUZ74
L. Pouzin, "CIGALE, the packet switching machine of the CYCLADES
computer network," Proceedings of the IFIP74 Congress, Stockholm,
Sweden.
6. ROWE74
L. Roberts and B. Wessler, "Computer Network Development to
achieve resource sharing," AFIPS 1970, SJCC Proceedings, vol. 36,
Atlantic City, AFIPS Press, New Jersey, pp. 543-549.
7. POUZ73
L. Pouzin, "Presentation and major design ASPects of the CYCLADES
Computer Network," Data Networks: Analysis and Design, Third Data
Communications Symposium, St. Petersburg, Florida, November 1973,
pp. 80-87.
8. SCWI71
R. Scantlebury and P.T. Wilkinson, "The Design of a Switching
System to allow remote Access to Computer Services by other
computers and Terminal Devices," Second Symposium on Problems in
the Optimization of Data Communication Systems Proceedings, Palo
Alto, California, 0ctober 1971, pp. 160-167.
9. POST72
J. Postel, "Official Initial Connection Protocol," Current Network
Protocols, Network Information Center, Stanford Research
Institute, Menlo Park, California. January 1972 (NIC 7101).
10. CACR70
C.S. Carr, S.D. Crocker, and V.G. Cerf, "Host-Host Communication
Protocol in the ARPA Network," AFIPS Conference Proceedings, vol.
36, 1970 SJCC, AFIPS Press, Montvale, N.J.
11. ZIEL74
H. Zimmerman and M. Elie, "Transport Protocol. Standard Host-Host
Protocol for heterogeneous computer networks," INWG#61, April
1974.
12. CRHE72
S. D. Crocker, J. F. Heafner, R. M. Metcalfe and J. B. Postel,
"Function-oriented protocols for the ARPA Computer Network," AFIPS
Conference Proceedings, vol. 41, 1972 FJCC, AFIPS Press, Montvale,
N.J.
13. DALA74
Y. Dalal, "More on selecting sequence numbers," INWG Protocol Note
#4, October 1974.
14. SUNS74
C. Sunshine, "Issues in communication protocol design -- formal
correctness." INWG Protocol Note #5, October 1974
BELS74
D. Belsnes, "Note on single message communication," INWG Protocol
Note #3. September 1974.
16. TOML74
R. Tomlinson, "Selecting sequence numbers," INWG Protocol Note #2,
September 1974.
17. SCHA74
R. Schantz, "Reconnection Protocol", private communication;
available from Schantz at BBN.
18. POUZ74A
L. Pouzin, "A proposal for interconnecting packet switching
networks, INWG Note #60, March 1974 [also submitted to EUROCOMP
74].
19. DLMG74
D. Lloyd, M. Galland, and P. T. Kirstein, "Aims and objectives of
internetwork experiments," to be published as an INWG Experiments
Note.
20. MCKE73
A. McKenzie, "Host-Host Protocol for the ARPANET," NIC # 8246,
Stanford Research Institute [also in ARPANET Protocols Notebook
NIC 7104].
21. BELS74A
D. Belsnes, "Flow control in packet switching networks," INWG Note
#63, October 1974.
FIGURE 1: TCB Major States
0-no TCB
\____________________________________________________________/
OPEN A CLOSE CLOSE A
---------- ---------- ----------
set up TCB remove TCB remove TCB
collision retry,
SYN arrives __V______ SEND, INTER
------------- / S1=0 \ ----------------
send SYN, ACK S2=0 F=0 send SYN
______________________ R=0 C=0 _____________________
U=0/1
SYN arrives
error,timeout 1-OPEN -----------
------------- \__________/ collision;
clear TCB A A set timeout
_____________________ _____________________
__V______ ____V___
/ S1=1 \ / S1=1 S2=0 F=0 S2=0 F=0
R=1 C=0 SYN, ACK arrives R=0 C=0
U=0/1 ACK arrives ---------------- U=0
----------- send ACK
3-SYN rcvd _________________ _________________ 2-SYN sent
\__________/ \__________/
__V_____V__
/ S1=1 CLOSE S2=1 F=0
-------- R=1 C=0 FIN arrives
send FIN U=0 -------------------
tell user, send FIN
________________4-established______________________
CLOSE \___________/
-------
__V_____V_ send FIN _______V__
/ S1=1 \ / S1=1 S2=0/1 F=1 timeout or S2=1 F=1
R=1 C=1 FIN, error, arrives CLOSE R=1 C=0
U=0/1 ------------------- ---------- U=0
remove TCB remove TCB
5-FIN wait _____________________ _____________ 6-FIN rcvd
\__________/ \__________/
____________________________V_____V_______________________
/ 0-no TCB
FIGURE 2.1: Structure of the TCP
_____________ _______________
INPUT PACKET <---->
REASSEMBLER HANDLER
_____________ _______________
_______________
_________
__V_________V____ NETWORK
<===== SYSTEM or
CALLS <======== TCB's <======== some
USERS =====> or and NETWORK
USER ========>ASSOCIATED QUEUES========> INTERFACE
<---->INTERFACE _________________ PROGRAM
_________ A A
______________
____________ ______________
PACKETIZER OUTPUT PACKET
HANDLER <---->
_____________ _______________
=======> Logical or physical flow of data (packets/letters)
-------> "Interaction"
NOTE: The signalling of processes by others is not shown
FIGURE 2.2a: ________
Address Check / Begin \________/
_V_
.' '.
.' packet '.
.' foreign '.
___________________.' socket matches '.
no '. a TCB local .'
'. socket .'
'. ? .'
'.___.'
yes
_V_
.' '.
.' packet '. ___
.'local socket '. / .' matches fully '.____\ YES
'. specified TCB .' / \___/
'.fgn socket .'
'. ? .'
_V_ '.___.'
.' '. no
.' SYN, '. _V_
.'FIN,INT,DSN, '. .' '.
_____.'or text length>0 './_____ .' matches '.
no '. or QUERY .' \ .'partly spec. '.
'. .' ___.' or unspec. TCB '.
'. ? .' no '. foreign .'
'.___.' '. socket .'
yes '. ? .'
__________V_________ '.___.'
yes
Create error 7 _V_
packet. Signal OPH .' '.
____________________ .' packet '.
______.' has SYN set '.
____V____ no '. .'
'. ? .'
_________\ discard /________ '.___.'
/_________\
_V_
_V_ / / \ YES
NO \___/
\___/
FIGURE 2.2b-1: _______
Input Packet Handler / Begin \_______/
________________________________________\/_________________________
A /\
_V_
.' '. _______
.' input '. go to
.' packet '.____\ sleep
'.available.' no /_______
'.__?__.'
yes
_V_
.' '.
.->SPECIAL FUNCT. Fig 4.7 .'address'.
.->ERR Fig 4.5,4.6 ___.' check OK '.
.->SYN Fig 4.1,4.2 no '. ? .'
.->INT Fig 4.3 '._____.'
.->FIN Fig 4.4 yes _________
_V_ discard
________________ .' '. (or queue)
.' error '. __________
<- Control Processing /_________.'or control '. A
____________________\ yes '. ? .'
'._____.'
(INT with data) no
V _V_
to "X" .' '. .
in Fig 2.2b-2 .'(estab)'. .' '.
_____.' R=S1=S2=1 '.----->.'seq.#'.--->
yes '. ? .' no '.OK .' no
'._____.' '.'
yes
_______________
Set S2=1, U=0 V
Notify user .'.
<-- with event 2 .'ACK'.
if U was 1 <-----'. OK .'--->'
_______________ yes '. .' no
'
V
to "Y"
in Fig 2.2b-2
FIGURE 2.2b-2: Input Packet Handler (continued)
"Y"
.'. _V_
.'txt'. .' '. ______________________________
.'lgth>0 '. .'within '. Use ACK to advance send window
,<----'. or DSN .'<---'. window .'--->Release ACK'ed packets from
no '. ? .' no '. ? .' yes retransmit or send queues. If
'._.' '._.' any packet had EB bit set
yes remove buffer from Packetized
________V____________________ buffer queue and inform user
Create ACK packet. Put on (success). Signal Packetizer.
<-Send packet queue. Signal OPH ______________________________
_____________________________
_____________________________________________
"X"
_V_ _V_ _____________________
.' '. .'TCB'. Put packet on
.' text '. yes .'Receive'. yes Receive packet queue
.' length>0 '.-------->.' buffer '.------>in the right order.
'. or DSN .' A '.available.' Signal Reassembler.
'. ? .' '. ? .' _____________________
'._.' '._.'
no no
_V_
________\ .' '.
/ .' seq # '. ________
.' of packet '. yes Discard
'. highest so .'---->packet ----->
'. far .' ________
'. ? .'
'._.'
no
_______V______________
Discard packet with
_____highest seq. no from
Receive packet queue.
______________________
_____________________________________________________
V
to "Begin" in Fig 2.2b-1
FIGURE 2.3-1: Reassembler
_______
/ Begin \_______/
<----------------------------------------------.
_____ yes
______V_____ .' '. __
Get ready .' Receive '. yes .'any'.
for next TCB--------->.'Packet Queue '.-------->.' more '.
____________ A '. empty ? .' A '.work?.'
'._______.' '._.'
no no
"R"------>---------' __V__ ____V____
.' is '. Go to
.' packet '. Sleep
.--<----------------------'.DSN with no.' _________
yes '. data? .'
'.___.'
no
__V__
.' '.
.' Receive '. yes
.'Buffer Queue '.--->
'. empty ? .'
________________ '._______.'
Copy from packet no <-------------"S"
to buffer until __V__
one is exhausted .'First'.
Update receive yes .' packet '. no
window. <----.'matches Recv '.--->'
________________ '.left window.'
'. edge ?.'
__V__ '.___.'
.'Send '.
.' Packet '. yes _____________________________
.' Queue empty '.---->Create ACK packet containing
'. ? .' new window. Signal OPH.
'._______.' _____________________________
no
'--------------------------->
V V
to "T" to "U"
in Fig 2.3-2 in Fig 2.3-2
FIGURE 2.3-2: Reassembler (continued)
"T" "U"
_____________
___V____ ___ __V__ Mark progress
process yes .' '. yes .'whole'. no in packet.
DSN <-----.' DSN '.<-----.' packet '.--->Return buffer--->.
________ '. set?.' '.copied?.' to user.
'._.' '.___.' _____________
no
'--------------->
__V__ __________________________
.' EOL '. yes Return buffer to user.
'. set? .'--------->Return packet to free --->
'.___.' storage. Signal Packetizer
no __________________________
A
__V__
.' full'.
'. buffer.'--------------'
'.___.' yes
no
___________________V__________________
Mark progress in buffer. Return packet
to free storage. Signal Packetizer. ,--------'
______________________________________
V V
to "R" in Fig 2.3-1 to "S" in Fig 2.3-1
FIGURE 2.4: Packetizer
_______ ________________________
/ Begin \____________\ Get ready for next TCB /___________________
\_______/ /________________________\
__V__ _____
.'Send '. .' any '.
no .' Buffer '. yes .' more '. yes
.-------------'. Queue .'---->'. work .'-----'
'.empty? .' A '. ? .'
____________V____________ '.___.' '.___.'
Pick packet size depend- no
,-->ing on send buffer, TCB ______V______
buffer space, window, etc go to sleep
_________________________ _____________
__V__
.'Send '.
.' window '. no
'.has room ? .'--------------------->
'._______.'
yes
__V__
.' TCB '.
.' buffer '. no
.'space avail- '.---------------------'
'. able ? .' A
'._______.'
yes
_____________V____________ ________________ ____________
Copy from Send buffer to Move buffer from Set EOL bit
packet until packet full. Send queue to <--in packet
Put packet on Send packet packetized queue header
queue. Signal OPH. _________________ ____________
__________________________ A A
no
__V__ ____
.'whole'. .' EOL '.
.' Send '. yes .' set in '. yes
'. buffer .'----------->'. Send .'-----------'
'.copied?.' '.buffer?.'
'.___.' '.___.'
no
_____________V__________
Note in TCB where in
--Send buffer we stopped.
________________________
FIGURE 2.5a:
Output Packet Handler
_______
/ Begin \_______/
<--------------------------.
____________V___________
Get ready for next TCB
________________________
,------------------------------------>
__V__ _____
_____ .'Send '. .' any '.
yes .' ACK '. no .' Buffer '. yes .' more '. yes
.-----'.bit set.'<------'. Queue .'---->'. work .'-----'
'.___.' '.empty? .' A '. ? .'
no ________ '.___.' '.___.'
__________ no
____V__________________
Put latest receive left ________v______ ______V______
window edge in ACK. ->Transmit packet go to sleep
_______________________ _______________ _____________
________________ __V__
Return packet to .'pckt '. _________________
buffer pool as no .'seq # to '.
it has been <------.'rgt of Send '.
ACKed '.left window.'
________________ '. edge .'
'.___.'
yes
_______________V________________
Move packet to retransmit queue;
set new retrans. time for it.
________________________________
'---------------------->
__V__
no .'Time '. yes
-------------------------------.'to switch'.---------------------'
'.TCB's? .'
'.___.'
FIGURE 2.5b:
Retransmit Process
_______
/ Begin \_______/
<----------------------------------.
____________V___________
Get ready for next TCB
________________________
.-------------------------------->
__V__
.' Any '. _____
.'packet's '. .' any '.
.'retrans. time'. no .' more '. yes
'. has occurred .'----->'. work .'-----'
'. for this .' '. ? .'
'. TCB ? .' '.___.'
'.___.'
yes no
______V______
________V________ go to sleep
Move packet to _____________
'------------------------Send Packet
queue. Signal OPH
_________________
FIGURE 3.1:
OPEN
_______
/ Begin \_______/
__V__
.'User '. _______
.'permitted'. no
.' access to '.---->error 1------------.
'.this local .' _______
'.socket?.'
'.___.'
yes
__V__
.' fgn '.
yes .' socket '. no
.-----'. specified .'----.
'. ? .'
__V__ '.___.' __V__ _______
_______ .'conn-'. .'space'. no
yes .' ection '. '.for TCB.'---->error 4-->
,-error 6<----'. already .' '.___.' _______
_______ '.exists?.' yes
'.___.'
no ____V__________
_______ __V__ Create TCB. Set
no .'space'. S1=S2=R=F=C=1
<-error 4<-----'.for TCB.' Set U=1
_______ '.___.' _______________
yes
_________V__________
Create TCB. Set U=0
Set S1=S2=R=F=C=1
____________________
'-------------.-------------'
_____________________V__________________
Return local connection name and Success
________________________________________
-----------------------------------><--------------------------------'
____V___
/ Return \________/
FIGURE 3.2:
SEND
_______
/ Begin \_______/
__V__
.'conn-'.
.' ection '. _________
.' legal for '. no
'. this process .'----------> error 1 -----------.
'. ? .' _________
'._______.'
yes
__V__
.'conn-'. _________
.' ection '. no
.' open '.-----------> error 3 ---------->
'. ? .' _________
'._______.'
yes
__V__
.' fgn '. _________
.' socket '. no
'. specified .'------------> error 5 ---------->
'.(U=0)? .' _________
'.___.'
yes
__V__
.'conn-'. _________
.' ection '. yes
'. closing ? .'------------> error 12---------->
'.(F,C=1).' _________
'.___.'
no
____________________V________________________________
Put buffer on Send Buffer queue and signal Packetizer
_____________________________________________________
<-----------------------------------------'
____V___
/ Return \________/
FIGURE 3.3:
INTERRUPT
_______
/ Begin \_______/
V
Same as SEND
____________________V_________________________
Return any pending Send buffers with code 10.
Create INT packet on outgoing packet queue.
Signal Output Packet Handler.
______________________________________________
<-----------------------------------------'
____V___
/ Return \________/
FIGURE 3.4:
RECEIVE
_______
/ Begin \_______/
__V__
.'conn-'.
.' ection '. _________
.' legal for '. no
'. this process .'----------> error 1 -----------.
'. ? .' _________
'._______.'
yes
_V_
.' '.
.' '.
.'connection '.
.' state '.
:___________________: _________
1-4 5,6 0 '--------------------> error 3 -->
'---------------------. _________
__________V__________
Put buffer on Receive _________
Buffer queue. Signal
Reassembler '-----> error 12-->
_____________________ _________
<------------------------------------------------'
____V___
/ Return \________/
FIGURE 3.5:
CLOSE
_______
/ Begin \_______/
__V__
.'conn-'.
.' ection '. _________
.' legal for '. no
'. this process .'----------> error 1 -----------.
'. ? .' _________
'._______.'
yes
_V_
.' '.
.' '.
.'connection '.
.' state '.
:___________________: _________
5 3,4 1,2,6 0
'------------------> error 3 -->
,------------' '-------------------. _________
______________V______________________
Return all buffers to user with error ___________
12; clear all packet queues, create Remove TCB
FIN packet, signal Output Packet '--->Return --->
Handler, set C=F=1 Success
_____________________________________ ___________
---------------------><----------------------------------------'
____V___
/ Return \________/
FIGURE 3.6:
STATUS
_______
/ Begin \_______/
__V__
.'conn-'.
.' ection '. _________
.' legal for '. no
'. this process .'----------> error 1 -----------.
'. ? .' _________
'._______.'
yes
__V__ __________
.'conn-'. Return
.' ection '. no state=0 or
'. open ? .'------------>error 3 --------->
'._______.' __________
yes
___________V___________
Fill in reply from TCB.
Return Success to user.
_______________________
<-----------------------------------------'
____V___
/ Return \________/
FIGURE 4.1:
SYN (no ACK)
_______
/ Begin \_______/
_V_
.' '.
.' '.
.' S1, S2, R '.
.' ? '.
:___________________: 1,1,1 _________
__________ (states 4-6)
Treat as a 1,0,1 '-------------> error 6 -->.
duplicate.<-----------' _________
Retransmit 1.0,0
SYN, ACK 0,0,0 (Syn sent) ________________
__________ (listening) '------------>Collision: Clear
S1, set timeout,
_____________________V________________ remove SYN from -->
Set R=S1=1. If U=1 set foreign socket retransmit queue
in TCB to match packet local socket. ________________
Send SYN, ACK. Signal OPH. Fill in TCB
with send window, receive sequence #.
______________________________________
'-----------------------><---------------------------------------'
___V__
/ Done \______/
FIGURE 4.2:
SYN,ACK
_______
/ Begin \_______/
__V__
.' '.
.' State 2 '. no
'.S1=1;S2=R=0.'----------------.
'. ? .'
'.___.'
yes
__V__ _______V______
.' ACK '. no
.' correct '.--------> send error 6
'. ? .' ______________
'.___.'
yes
_________V_________
Set S2=R=1. Process
ACK. Send ACK.
___________________
<----------------------'
___V__
/ Done \______/
FIGURE 4.3:
INT (from net)
_______ ____________
/ Begin \____\Process ACK
\_______/ /(may set S2)------.
____________
__V__
____________ .' in '.
Discard no .' state 4 '.
.<------- (or queue) <-------'. S1=S2=R=1 .'
____________ '. F=0 ? .'
'.___.'
yes
__V__
____________ .' '.
ACK and no .' within '.
<------- discard <-------'. window .'
____________ '. ? .'
'.___.'
yes
____________________________V_______________
Move Receive Left window edge to sequence
number of INT. Return event 10 with any
pending Receive buffers. Ruturn event 11 to
user. Send ACK for INT.
____________________________________________
__V__
see yes .'data '.
Figure<----------.' in this '.
2.2 '.packet?.'
'.___.'
no
'------------------------------------>
___V__
/ Done \______/
FIGURE 4.4:
FIN
_______ ____________
/ Begin \____\Process ACK
\_______/ /(may set S2)------.
____________
__V__
.' '.
no .'S1=S2=R=1'.
.--------------'. (estab- .'
'.lished).'
'.___.'
yes
__V__
______V_____ .' '.
no .' within '.
.----------------- discard <-------'. window .'
____________ '. ? .'
'.___.'
yes
__V__
(state 4) 0 .'F bit'. 1 (state 5)
.------------'. value .'------------.
'.___.'
_________________________V________
Return all user buffers (event 12) _____________________V__
Clear all packet queues. Send FIN Return success to User's
packet. Set F=1. Inform user CLOSE. Remove TCB.
"connection closing" (event 12) ________________________
__________________________________
'-----------------><-----------------------------------'
___V__
/ Done \______/
FIGURE 4.5:
Error 6 (bad SYN)
_______
/ Begin \_______/
__V__
.' '.
.'refers to'.
.'current pckt?'. _________
.'(ACK matches seq '. no
'. # of packet on .'-----------------> discard -----------.
'.retrans or send.' _________
'. queues?) .'
'._______.'
yes
_V_
.' '. 1 (state 3)
.' value '.--------------------------------.
'. of R.' bad SYN,ACK
'._.'
0 (state 2)
bad SYN
__________________V__________________ _______V______
Other side is established. Send RESET Clear S1, R
(put error packet's seq. # in ACK Remove SYN,ACK
field. Return all user buffers with from retrans
code 14. Inform user with event 14 queue.
_____________________________________ ______________
V
<--------------------------------------------------'
___V__
/ Done \______/
FIGURE 4.6:
Error 7,8
_______
/ Begin \_______/
__V__
.' '.
.'refers to'. _________
.' current '. no
'. packet (check .'----------------> discard -----------.
'. ACK)? .' A _________
'._______.'
yes
_V_
.' '.
.' '.
.'connection '.
.' state '.
:___________________:
4 5 3 2 6
.-------' '------'
'-----------------------------.
'-------------.
___V___ ____V_______ ______V_______ ________V_________
Pass to Remove TCB. Clear S1, R. Discard. SYN will
user Return Remove SYN,ACK be retrans to
_______ success to from transmit avoid receiver
user's CLOSE queue (go to having to queue it
____________ state 1). __________________
______________
V V
'------------------------------><---------------------------------'
___V__
/ Done \______/
FIGURE 4.7:
RESET
_______
/ Begin \_______/
__V__
no .'Reset'. yes
.------------'. All ? .'------------------.
'.___.'
_________V_________
Clear all TCB's for
foreign TCP. Inform
users with event 14
___________________
__V__
.' Is '. _________
.' RESET '. no
.'believable ? '.-------> discard ------------->
'.(check ACK .' _________
'.field) .'
'.___.'
yes
________________V________________
Clear all queues for this TCB.
Return event 14 for user buffers.
Inform User with event 14.
_________________________________
<----------------------------------------'
___V__
/ Done \______/
[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Alex McKenzie with ]
[ support from GTE, formerly BBN Corp. 2/2000 ]
