tp.4

早期freebsd实现

implementation, despite the fact that
the frozen reference function is required only for
class 4.
.It Ar p_inact_ticks
nonzero short integer [various]
.br
Number of clock ticks without an incoming packet from the peer after which 
.Tn TP
close the connection.
This parameter applies only to class 4.
.It Ar p_keepalive_ticks
nonzero short integer [various]
.br
Number of clock ticks between acknowledgments that are sent
to keep an inactive connection open (to prevent the peer's
inactivity control function from closing the connection).
This parameter applies only to class 4.
.It Ar p_winsize
short integer between 128 and 16384. [4096 bytes]
.br
The buffer space limits in bytes for incoming and outgoing data.
There is no way to specify different limits for incoming and outgoing
paths.
The actual window size at any time
during the lifetime of a connection
is a function of the buffer size limit, the negotiated
maximum TPDU
size, and the 
rate at which the user program receives data.
This parameter applies only to class 4.
.It Ar p_tpdusize
unsigned char between 0x7 and 0xd. 
[0xc for class 4] [0xb for class 0]
.br
Log 2 of the maximum TPDU size to be negotiated.
The
.Tn TP
standard
.Pf ( Tn ISO
8473) gives an upper bound of 
0xd for class 4 and 0xb for class 0.
The
.Tn ARGO
implementation places upper bounds of
0xc on class 4 and 0xb on class 0.
.It Ar p_ack_strat
.Dv TPACK_EACH
or
.Dv TPACK_WINDOW.
.Bq Dv TPACK_WINDOW
.br
This parameter applies only to class 4.
Two acknowledgment strategies are supported:
.Pp
.Dv TPACK_EACH means that each data TPDU
is acknowledged
with an AK TPDU.
.Pp
.Dv TPACK_WINDOW
means that upon receipt of the packet that represents
the high edge of the last window advertised, an AK TPDU is generated.
.It Ar p_rx_strat
4 bit mask
.Bq Dv TPRX_USE_CW No \&|\  Dv TPRX_FASTSTART
over
connectionless network protocols]
.Pf [ Dv TPRX_USE_CW
over
connection-oriented network protocols]
.br
This parameter applies only to class 4.
The bit mask may include the following values:
.Pp
.Dv TPRX_EACH :
When a retransmission timer expires, retransmit
each packet in the send window rather than
just the first unacknowledged packet.
.Pp
.Dv TPRX_USE_CW :
Use a "congestion window" strategy borrowed
from Van Jacobson's congestion window strategy for TCP.
The congestion window size is set to one whenever
a retransmission occurs.
.Pp
.Dv TPRX_FASTSTART :
Begin sending the maximum amount of data permitted
by the peer (subject to availability).
The alternative is to start sending slowly by 
pretending the peer's window is smaller than it is, and letting
it slowly grow up to the peer window's real size.
This is to smooth the effect of new connections on a congested network
by preventing a transport connection from suddenly 
overloading the network with a burst of packets.
This strategy is also due to Van Jacobson.
.It Ar p_class
5 bit mask
.Bq Dv TP_CLASS_4 No \&|\  Dv TP_CLASS_0
.br
Bit mask including one or both of the values
.Dv TP_CLASS_4
and
.Dv TP_CLASS_0 .
The higher class indicated is the preferred class.
If only one class is indicated, negotiation will not occur
during connection establishment.
.It Ar p_xtd_format
Boolean.
[false]
.br
Boolean indicating that extended format is negotiated.
This parameter applies only to class 4.
.It Ar p_xpd_service
Boolean.
[true]
.br
Boolean indicating that 
the expedited data transport service will be negotiated.
This parameter applies only to class 4.
.It Ar p_use_checksum
Boolean.
[true]
.br
Boolean indicating the the use of checksums will be negotiated.
This parameter applies only to class 4.
.It Ar p_use_nxpd
Reserved for future use.
.It Ar p_use_rcc
Reserved for future use.
.It Ar p_use_efc
Reserved for future use.
.It Ar p_no_disc_indications
Boolean.
[false]
.Pp
Boolean indicating that the local
.Tn TP
entity will not issue
indications (signals) when a
.Tn TP
connection is disconnected.
.It Ar p_dont_change_params
Boolean.  [false]
.br
If
.Em true
the
.Tn TP
entity will not override
any of the other values given in this structure.
If the values cannot be used, the
.Tn TP
entity will drop, disconnect,
or refuse to establish the connection to which this structure pertains.
.It Ar p_netservice
One of {
.Dv ISO_CLNS ,
.Dv ISO_CONS ,
.Dv ISO_COSNS ,
.Dv IN_CLNS } .
.Pf [ Dv ISO_CLNS ]
.br
Indicates which network service is to be used.
.Pp
.Dv ISO_CLNS
indicates the connectionless network service provided
by CLNP 
.Pf ( Tn ISO
8473).
.Pp
.Dv ISO_CONS
indicates the connection-oriented network service provided
by X.25
.Pf ( Tn ISO
8208) and
.Tn ISO
8878.
.Pp
.Dv ISO_COSNS
indicates the 
connectionless network service running over a
connection-oriented subnetwork service: CLNP 
.Pf ( Tn ISO
8473) over X.25
.Pf ( Tn ISO
8208).
.Pp
.Dv IN_CLNS
indicates the 
DARPA Internet connectionless network service provided by IP (RFC 791).
.It Ar p_dummy
Reserved for future use.
.El
.Pp
The
.Dv TPOPT_FLAGS
option is used for obtaining
various boolean-valued options.
Its meaning is as follows.
The bit numbering used is that of the RT PC, which means that bit
0 is the most significant bit, while bit 8 is the least significant bit.
.sp 1
.Em Values for TPOPT_FLAGS:
.Bl -tag -width Bitsx
.It Sy Bits
.Sy Description [Default]
.It \&0
.Dv TPFLAG_NLQOS_PDN :
set when the quality of the 
network service is
similar to that of a public data network.
.It \&1
.Dv TPFLAG_PEER_ON_SAMENET :
set when the peer
.Tn TP
entity
is considered to be on the same network as the local
.Tn TP
entity.
.It \&2
Not used.
.It \&3
.Dv TPFLAG_XPD_PRES :
set when expedited data are present
[0]
.It 4\&..7
Reserved.
.El
.Sh ERROR VALUES
.Pp
The
.Tn TP
entity returns
.Va errno
error values as defined in
.Aq Pa sys/errno.h
and
.Aq Pa netiso/iso_errno.h .
User programs may print messages associated with these value by
using an expanded version of
.Xr perror
found in the
.Tn ISO
library,
.Pa libisodir.a .
.Pp
If the
.Tn TP
entity encounters asynchronous events
that will cause a transport connection to be closed,
such as
timing out while retransmitting a connect request TPDU,
or receiving a DR TPDU,
the
.Tn TP
entity issues a
.Dv SIGURG
signal, indicating that
disconnection has occurred.
If the signal is issued during a 
a system call, the system call may be interrupted,
in which case the
.Va errno
value upon return from the system call is
.Er EINTR.
If the signal
.Dv SIGURG
is being handled by reading
from the socket, and it was an
.Xr accept 2
that
timed out, the read may result in
.Er ENOTSOCK ,
because the
.Xr accept
call had not yet returned a
legitimate socket descriptor when the signal was handled.
.Dv ETIMEDOUT
(or a some other errno value appropriate to the
type of error) is returned if
.Dv SIGURG
is blocked
for the duration of the system call.
A user program should take one of the following approaches:
.Bl -tag -width Ds
.It Block Dv SIGURG
If the program is servicing
only one connection, it can block or ignore
.Dv SIGURG
during connection 
establishment.
The advantage of this is that the
.Va errno
value
returned is somewhat meaningful.
The disadvantage of this is that
if ignored, disconnection and expedited data indications could be
missed.
For some programs this is not a problem.
.It Handle Dv SIGURG
If the program is servicing more than one connection at a time
or expedited data may arrive or both, the program may elect to
service
.Dv SIGURG .
It can use the
.Fn getsockopt ...TPOPT_FLAGS...
system 
call to see if the signal
was due to the arrival of expedited data or due to a disconnection.
In the latter case, 
.Xr getsockopt
will return
.Er ENOTCONN .
.El
.Sh SEE ALSO
.Xr tcp 4 ,
.Xr netstat 1 ,
.Xr iso 4 ,
.Xr clnp 4 ,
.Xr cltp 4 ,
.Xr ifconfig 8 .
.Sh BUGS
The protocol definition of expedited data is slightly problematic,
in a way that renders expedited data almost useless,
if two or more packets of expedited data are send within
time \(*e, where \(*e
depends on the application.
The problem is not of major significance since most applications
do not use transport expedited data.
The problem is this:
the expedited data acknowledgment TPDU
has no field for conveying
credit, thus it is not possible for a
.Tn TP
entity to inform its peer
that "I received your expedited data but have no room to receive more."
The
.Tn TP
entity has the choice of acknowledging receipt of the
XPD TPDU:
.Bl -tag -width Ds
.It "when the user receives the" XPD TSDU
which may be a fairly long time,
which may cause the sending
.Tn TP
entity to retransmit the packet,
and possibly to close the connection after retransmission, or
.It "when the" Tn TP No "entity receives it"
so the sending entity does not retransmit or close the connection.
If the sending user then tries to send more expedited data
.Dq soon ,
the expedited data will not be acknowledged (until the
receiving user receives the first XPD TSDU).
.El
.Pp
The
.Tn ARGO
implementation acknowledges XPD TPDUs
immediately,
in the hope that most users will not use expedited data frequently
enough for this to be a problem.