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TCP
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1996-07-27
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tcp <subcommand>
These commands are used for the Transmission Control Protocol
service. All TCP parameters are configurable per interface. Commands
of the form 'tcp <command>' set the default or global values. Use
the 'ifconfig <iface> tcp <command>' form to set or show the interface-
specific values.
To set the system default TCP parameters, you must do so BEFORE
attaching interfaces. After attachment, you must use the 'ifconfig
<iface> tcp' command form to show or change values for that interface.
THIS IS A CHANGE FROM THE BEHAVIOUR EXHIBITED PRIOR TO JNOS VERSION
1.10X16, RELEASED 08FEB94.
Notes:
Attempting outgoing connections to addresses without an
existing route results in Error number 219.
tcp access <permit|deny|delete> <ipaddr[/bits]|all> [loport [hiport]]
Display or set tcp access controls, which determine which TCP services
(ports) are accessible to which IP addresses. Default is to permit
all hosts to access all ports, until the first TCP access command is
entered, at which point all other ports and addresses are denied
unless specifically permitted by subsequent tcp access commands.
This subcommand adds or deletes an access control entry maintained
in an internal table. Entries are scanned in the order that they
were added, to determine if access will be granted. Access is granted
only if an entry with matching ipaddr or range, and ports, is found
with "permit" set before a match with "deny" or no match is found.
The optional /bits suffix to the ipaddr specifies how many leading
bits in the ipaddr are to be considered significant in the address
comparisons. If not specified, 32 bits (i.e., full significance)
is assumed. All addresses can be specified by "all". Loport and
hiport specify the port or range of TCP ports for which the access
control command applies. If "all" is given as the loport, or if no
port range is specified, all ports are assumed, ie, 1 to 65534.
"tcp access" will display the table of current access control entrys.
Access commands should be entered from the most specific ipaddr to the
least specific, since the first match (permit or deny) encountered
in the internal table is returned.
#Example:
#Allow a specific AMPRnet host SMTP access
tcp access permit 44.76.1.199 25
#but deny all other services to him
tcp access deny 44.76.1.199
#Allow all other AMPRnet hosts full access to TCP services
tcp access permit 44.76.1/24 all
#Allow a specific subnet access to ports 1 through 25,
#which includes echo, discard, ftp, telnet, and smtp.
tcp access permit 23.1.46/24 1 25
#Note that all other hosts not matched above, are denied access
tcp blimit [<value>] Default: 31
Display or set the default tcp retransmission backoff limit.
Normally each successive tcp retransmission is delayed a time
value that increases exponentially or linearly. The backoff
limit <value> serves to set the maximum backoff delay allowed.
See also tcp timertype and tcp maxwait.
tcp clean
Reset all tcp connections that are in a "FIN wait 2" state. This
is useful to release memory resources held by Jnos for connections
that were not properly closed.
tcp irtt [<milliseconds>]
(B) Display or set the initial round trip time estimate, in
milliseconds, to be used for new TCP connections until they can
measure and adapt to the actual value. The default is 5000
milliseconds (5 seconds). Increasing irtt when operating over
slow channels will avoid the flurry of re-transmissions that
would otherwise occur as the smoothed estimate settles down at
the correct value. Note that this command should be given before
servers are started in order for it to have effect on incoming
connections.
TCP also keeps a cache of measured round trip times and mean
deviations (MDEV) for current and recent destinations. Whenever
a new TCP connection is opened, the system first looks in this
cache. If the destination is found, the cached IRTT and MDEV
values are used. If not, the default IRTT value mentioned above
is used, along with a MDEV of 0. This feature is fully
automatic, and it can improve performance greatly when a series
of connections are opened and closed to a given destination (e.g.
a series of FTP file transfers or directory listings).
tcp kick <tcb_addr>
If there is unacknowledged data on the send queue of the
specified TCB, this command forces an immediate retransmission.
<tcb addr> can be found with the 'tcp status' command.
tcp maxwait [<msec>]
Set or show the maximum time for retry timeout in milliseconds.
Default = 0, no maximum.
tcp mss [<size>]
(B) Display or set the TCP Maximum Segment Size in bytes that
will be sent on all outgoing TCP connect request (SYN segments).
This tells the remote end the size of the largest segment
(packet) it may send. Changing MSS affects only future
connections; existing connections are unaffected. See also the
section 'Of PACLEN, MTU, MSS, and More' in the JNOS40
Configuration Manual.
tcp reset <tcb_addr>
Deletes the TCP control block at the specified address.
tcp retries [<num>]
(B) Display or set the number of retries before a tcp connection
will be reset. Default is 10. This is useful to eliminate idle
connections that have not been properly shut down. If set to zero,
there is no maximum, i.e. a connection will never retry out.
tcp rtt <tcb_addr> <milliseconds>
Replaces the automatically computed round trip time in the
specified TCB with the rtt in milliseconds. This command is
useful to speed up recovery from a series of lost packets since
it provides a manual bypass around the normal backoff
retransmission timing mechanisms.
tcp status [<tcb_addr> | all]
Without arguments, displays several TCP-level statistics, plus a
summary of all existing TCP connections, including TCB
address, send and receive queue sizes, local and remote sockets,
and connection state. If <tcb addr> is specified, a more
detailed dump of the specified TCB is generated, including send
and receive sequence numbers and timer information. If "all" is
given, the summary will also include TCBs in a listening state
(awaiting a connection). In this case, a (S) will indicate that
a server process is to be spawned when a connection occurs.
tcp syndata [yes | NO]
(B) Display or set the tcp syn + data piggybacking flag. Some
tcp systems cannot handle syn + data together.
tcp timertype [linear | exponential]
(B) Display the current setting or set the timer type backoff
algorithm. Default is linear.
tcp trace [yes | NO]
(B) Display or set the tcp trace flag on or off.
tcp window [<size>]
(B) Displays or sets the default receive window size in bytes to
be used by TCP when creating new connections. Existing
connections are unaffected.