.\"
.\" Copyright (c) 2010 The FreeBSD Foundation
.\"
.\" Portions of this software were developed at the Centre for Advanced
.\" Internet Architectures, Swinburne University of Technology, Melbourne,
.\" Australia by Lawrence Stewart under sponsorship from the FreeBSD
.\" Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions, and the following disclaimer,
.\"    without modification, immediately at the beginning of the file.
.\" 2. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd October 7, 2019
.Dt SIFTR 4
.Os
.Sh NAME
.Nm SIFTR
.Nd Statistical Information For TCP Research
.Sh SYNOPSIS
To load
the driver
as a module at run-time, run the following command as root:
.Bd -literal -offset indent
kldload siftr
.Ed
.Pp
Alternatively, to load
the driver
as a module at boot time, add the following line into the
.Xr loader.conf 5
file:
.Bd -literal -offset indent
siftr_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
.Po
.Em S Ns tatistical
.Em I Ns nformation
.Em F Ns or
.Em T Ns CP
.Em R Ns esearch
.Pc
kernel module logs a range of statistics on active TCP connections to
a log file.
It provides the ability to make highly granular measurements of TCP connection
state, aimed at system administrators, developers and researchers.
.Ss Compile-time Configuration
The default operation of
.Nm
is to capture IPv4 TCP/IP packets.
.Nm
can be configured to support IPv4 and IPv6 by uncommenting:
.Bd -literal -offset indent
CFLAGS+=-DSIFTR_IPV6
.Ed
.Pp
in
.Aq sys/modules/siftr/Makefile
and recompiling.
.Pp
In the IPv4-only (default) mode, standard dotted decimal notation (e.g.
"136.186.229.95") is used to format IPv4 addresses for logging.
In IPv6 mode, standard dotted decimal notation is used to format IPv4 addresses,
and standard colon-separated hex notation (see RFC 4291) is used to format IPv6
addresses for logging.
Note that SIFTR uses uncompressed notation to format IPv6 addresses.
For example, the address "fe80::20f:feff:fea2:531b" would be logged as
"fe80:0:0:0:20f:feff:fea2:531b".
.Ss Run-time Configuration
.Nm
utilises the
.Xr sysctl 8
interface to export its configuration variables to user-space.
The following variables are available:
.Bl -tag -offset indent -width Va
.It Va net.inet.siftr.enabled
controls whether the module performs its
measurements or not.
By default, the value is set to 0, which means the module
will not be taking any measurements.
Having the module loaded with
.Va net.inet.siftr.enabled
set to 0 will have no impact on the performance of the network stack, as the
packet filtering hooks are only inserted when
.Va net.inet.siftr.enabled
is set to 1.
.El
.Bl -tag -offset indent -width Va
.It Va net.inet.siftr.ppl
controls how many inbound/outbound packets for a given TCP connection will cause
a log message to be generated for the connection.
By default, the value is set to 1, which means the module will log a message for
every packet of every TCP connection.
The value can be set to any integer in the range [1,2^32], and can be changed at
any time, even while the module is enabled.
.El
.Bl -tag -offset indent -width Va
.It Va net.inet.siftr.logfile
controls the path to the file that the module writes its log messages to.
By default, the file /var/log/siftr.log is used.
The path can be changed at any time, even while the module is enabled.
.El
.Bl -tag -offset indent -width Va
.It Va net.inet.siftr.genhashes
controls whether a hash is generated for each TCP packet seen by
.Nm .
By default, the value is set to 0, which means no hashes are generated.
The hashes are useful to correlate which TCP packet triggered the generation of
a particular log message, but calculating them adds additional computational
overhead into the fast path.
.El
.Bl -tag -offset indent -width Va
.It Va net.inet.siftr.port_filter
controls on which source or destination port siftr should capture
.Nm .
By default, the value is set to 0, which means all ports are eligible for logging.
Set to any other value, only packets where either the source or destination
port is equal to this number are logged.
.El
.Ss Log Format
A typical
.Nm
log file will contain 3 different types of log message.
All messages are written in plain ASCII text.
.Pp
Note: The
.Qq \e
present in the example log messages in this section indicates a
line continuation and is not part of the actual log message.
.Pp
The first type of log message is written to the file when the module is
enabled and starts collecting data from the running kernel.
The text below shows an example module enable log.
The fields are tab delimited key-value
pairs which describe some basic information about the system.
.Bd -literal -offset indent
enable_time_secs=1238556193    enable_time_usecs=462104 \\
siftrver=1.2.2    hz=1000    tcp_rtt_scale=32 \\
sysname=FreeBSD    sysver=604000    ipmode=4
.Ed
.Pp
Field descriptions are as follows:
.Bl -tag -offset indent -width Va
.It Va enable_time_secs
time at which the module was enabled, in seconds since the UNIX epoch.
.El
.Bl -tag -offset indent -width Va
.It Va enable_time_usecs
time at which the module was enabled, in microseconds since enable_time_secs.
.El
.Bl -tag -offset indent -width Va
.It Va siftrver
version of
.Nm .
.El
.Bl -tag -offset indent -width Va
.It Va hz
tick rate of the kernel in ticks per second.
.El
.Bl -tag -offset indent -width Va
.It Va tcp_rtt_scale
smoothed RTT estimate scaling factor.
.El
.Bl -tag -offset indent -width Va
.It Va sysname
operating system name.
.El
.Bl -tag -offset indent -width Va
.It Va sysver
operating system version.
.El
.Bl -tag -offset indent -width Va
.It Va ipmode
IP mode as defined at compile time.
An ipmode of "4" means IPv6 is not supported and IP addresses are logged in
regular dotted quad format.
An ipmode of "6" means IPv6 is supported, and IP addresses are logged in dotted
quad or hex format, as described in the
.Qq Compile-time Configuration
subsection.
.El
.Pp
The second type of log message is written to the file when a data log message
is generated.
The text below shows an example data log triggered by an IPv4
TCP/IP packet.
The data is CSV formatted.
.Bd -literal -offset indent
o,0xbec491a5,1238556193.463551,172.16.7.28,22,172.16.2.5,55931, \\
1073725440,172312,6144,66560,66608,8,1,4,1448,936,1,996,255, \\
33304,208,66608,0,208,0
.Ed
.Pp
Field descriptions are as follows:
.Bl -tag -offset indent -width Va
.It Va 1
Direction of packet that triggered the log message.
Either
.Qq i
for in, or
.Qq o
for out.
.El
.Bl -tag -offset indent -width Va
.It Va 2
Hash of the packet that triggered the log message.
.El
.Bl -tag -offset indent -width Va
.It Va 3
Time at which the packet that triggered the log message was processed by
the
.Xr pfil 9
hook function, in seconds and microseconds since the UNIX epoch.
.El
.Bl -tag -offset indent -width Va
.It Va 4
The IPv4 or IPv6 address of the local host, in dotted quad (IPv4 packet)
or colon-separated hex (IPv6 packet) notation.
.El
.Bl -tag -offset indent -width Va
.It Va 5
The TCP port that the local host is communicating via.
.El
.Bl -tag -offset indent -width Va
.It Va 6
The IPv4 or IPv6 address of the foreign host, in dotted quad (IPv4 packet)
or colon-separated hex (IPv6 packet) notation.
.El
.Bl -tag -offset indent -width Va
.It Va 7
The TCP port that the foreign host is communicating via.
.El
.Bl -tag -offset indent -width Va
.It Va 8
The slow start threshold for the flow, in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 9
The current congestion window for the flow, in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 10
The current bandwidth-controlled window for the flow, in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 11
The current sending window for the flow, in bytes.
The post scaled value is reported, except during the initial handshake (first
few packets), during which time the unscaled value is reported.
.El
.Bl -tag -offset indent -width Va
.It Va 12
The current receive window for the flow, in bytes.
The post scaled value is always reported.
.El
.Bl -tag -offset indent -width Va
.It Va 13
The current window scaling factor for the sending window.
.El
.Bl -tag -offset indent -width Va
.It Va 14
The current window scaling factor for the receiving window.
.El
.Bl -tag -offset indent -width Va
.It Va 15
The current state of the TCP finite state machine, as defined
in
.Aq Pa netinet/tcp_fsm.h .
.El
.Bl -tag -offset indent -width Va
.It Va 16
The maximum segment size for the flow, in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 17
The current smoothed RTT estimate for the flow, in units of TCP_RTT_SCALE * HZ,
where TCP_RTT_SCALE is a define found in tcp_var.h, and HZ is the kernel's tick
timer.
Divide by TCP_RTT_SCALE * HZ to get the RTT in secs.
TCP_RTT_SCALE and HZ are reported in the enable log message.
.El
.Bl -tag -offset indent -width Va
.It Va 18
SACK enabled indicator. 1 if SACK enabled, 0 otherwise.
.El
.Bl -tag -offset indent -width Va
.It Va 19
The current state of the TCP flags for the flow.
See
.Aq Pa netinet/tcp_var.h
for information about the various flags.
.El
.Bl -tag -offset indent -width Va
.It Va 20
The current retransmission timeout length for the flow, in units of HZ, where HZ
is the kernel's tick timer.
Divide by HZ to get the timeout length in seconds.
HZ is reported in the enable log message.
.El
.Bl -tag -offset indent -width Va
.It Va 21
The current size of the socket send buffer in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 22
The current number of bytes in the socket send buffer.
.El
.Bl -tag -offset indent -width Va
.It Va 23
The current size of the socket receive buffer in bytes.
.El
.Bl -tag -offset indent -width Va
.It Va 24
The current number of bytes in the socket receive buffer.
.El
.Bl -tag -offset indent -width Va
.It Va 25
The current number of unacknowledged bytes in-flight.
Bytes acknowledged via SACK are not excluded from this count.
.El
.Bl -tag -offset indent -width Va
.It Va 26
The current number of segments in the reassembly queue.
.El
.Bl -tag -offset indent -width Va
.It Va 27
Flowid for the connection.
A caveat: Zero '0' either represents a valid flowid or a default value when it's
not being set.
There is no easy way to differentiate without looking at actual
network interface card and drivers being used.
.El
.Bl -tag -offset indent -width Va
.It Va 28
Flow type for the connection.
Flowtype defines which protocol fields are hashed to produce the flowid.
A complete listing is available in
.Pa sys/mbuf.h
under
.Dv M_HASHTYPE_* .
.El
.Pp
The third type of log message is written to the file when the module is disabled
and ceases collecting data from the running kernel.
The text below shows an example module disable log.
The fields are tab delimited key-value pairs which provide statistics about
operations since the module was most recently enabled.
.Bd -literal -offset indent
disable_time_secs=1238556197    disable_time_usecs=933607 \\
num_inbound_tcp_pkts=356    num_outbound_tcp_pkts=627 \\
total_tcp_pkts=983    num_inbound_skipped_pkts_malloc=0 \\
num_outbound_skipped_pkts_malloc=0    num_inbound_skipped_pkts_mtx=0 \\
num_outbound_skipped_pkts_mtx=0    num_inbound_skipped_pkts_tcb=0 \\
num_outbound_skipped_pkts_tcb=0    num_inbound_skipped_pkts_icb=0 \\
num_outbound_skipped_pkts_icb=0    total_skipped_tcp_pkts=0 \\
flow_list=172.16.7.28;22-172.16.2.5;55931,
.Ed
.Pp
Field descriptions are as follows:
.Bl -tag -offset indent -width Va
.It Va disable_time_secs
Time at which the module was disabled, in seconds since the UNIX epoch.
.El
.Bl -tag -offset indent -width Va
.It Va disable_time_usecs
Time at which the module was disabled, in microseconds since disable_time_secs.
.El
.Bl -tag -offset indent -width Va
.It Va num_inbound_tcp_pkts
Number of TCP packets that traversed up the network stack.
This only includes inbound TCP packets during the periods when
.Nm
was enabled.
.El
.Bl -tag -offset indent -width Va
.It Va num_outbound_tcp_pkts
Number of TCP packets that traversed down the network stack.
This only includes outbound TCP packets during the periods when
.Nm
was enabled.
.El
.Bl -tag -offset indent -width Va
.It Va total_tcp_pkts
The summation of num_inbound_tcp_pkts and num_outbound_tcp_pkts.
.El
.Bl -tag -offset indent -width Va
.It Va num_inbound_skipped_pkts_malloc
Number of inbound packets that were not processed because of failed
.Fn malloc
calls.
.El
.Bl -tag -offset indent -width Va
.It Va num_outbound_skipped_pkts_malloc
Number of outbound packets that were not processed because of failed
.Fn malloc
calls.
.El
.Bl -tag -offset indent -width Va
.It Va num_inbound_skipped_pkts_mtx
Number of inbound packets that were not processed because of failure to add the
packet to the packet processing queue.
.El
.Bl -tag -offset indent -width Va
.It Va num_outbound_skipped_pkts_mtx
Number of outbound packets that were not processed because of failure to add the
packet to the packet processing queue.
.El
.Bl -tag -offset indent -width Va
.It Va num_inbound_skipped_pkts_tcb
Number of inbound packets that were not processed because of failure to find the
TCP control block associated with the packet.
.El
.Bl -tag -offset indent -width Va
.It Va num_outbound_skipped_pkts_tcb
Number of outbound packets that were not processed because of failure to find
the TCP control block associated with the packet.
.El
.Bl -tag -offset indent -width Va
.It Va num_inbound_skipped_pkts_icb
Number of inbound packets that were not processed because of failure to find the
IP control block associated with the packet.
.El
.Bl -tag -offset indent -width Va
.It Va num_outbound_skipped_pkts_icb
Number of outbound packets that were not processed because of failure to find
the IP control block associated with the packet.
.El
.Bl -tag -offset indent -width Va
.It Va total_skipped_tcp_pkts
The summation of all skipped packet counters.
.El
.Bl -tag -offset indent -width Va
.It Va flow_list
A CSV list of TCP flows that triggered data log messages to be generated since
the module was loaded.
Each flow entry in the CSV list is
formatted as
.Qq local_ip;local_port-foreign_ip;foreign_port .
If there are no entries in the list (i.e., no data log messages were generated),
the value will be blank.
If there is at least one entry in the list, a trailing comma will always be
present.
.El
.Pp
The total number of data log messages found in the log file for a module
enable/disable cycle should equate to total_tcp_pkts - total_skipped_tcp_pkts.
.Sh IMPLEMENTATION NOTES
.Nm
hooks into the network stack using the
.Xr pfil 9
interface.
In its current incarnation, it hooks into the AF_INET/AF_INET6 (IPv4/IPv6)
.Xr pfil 9
filtering points, which means it sees packets at the IP layer of the network
stack.
This means that TCP packets inbound to the stack are intercepted before
they have been processed by the TCP layer.
Packets outbound from the stack are intercepted after they have been processed
by the TCP layer.
.Pp
The diagram below illustrates how
.Nm
inserts itself into the stack.
.Bd -literal -offset indent
----------------------------------
           Upper Layers
----------------------------------
    ^                       |
    |                       |
    |                       |
    |                       v
 TCP in                  TCP out
----------------------------------
    ^                      |
    |________     _________|
            |     |
            |     v
           ---------
           | SIFTR |
           ---------
            ^     |
    ________|     |__________
    |                       |
    |                       v
IPv{4/6} in            IPv{4/6} out
----------------------------------
    ^                       |
    |                       |
    |                       v
Layer 2 in             Layer 2 out
----------------------------------
          Physical Layer
----------------------------------
.Ed
.Pp
.Nm
uses the
.Xr alq 9
interface to manage writing data to disk.
.Pp
At first glance, you might mistakenly think that
.Nm
extracts information from
individual TCP packets.
This is not the case.
.Nm
uses TCP packet events (inbound and outbound) for each TCP flow originating from
the system to trigger a dump of the state of the TCP control block for that
flow.
With the PPL set to 1, we are in effect sampling each TCP flow's control block
state as frequently as flow packets enter/leave the system.
For example, setting PPL to 2 halves the sampling rate i.e., every second flow
packet (inbound OR outbound) causes a dump of the control block state.
.Pp
The distinction between interrogating individual packets versus interrogating the
control block is important, because
.Nm
does not remove the need for packet capturing tools like
.Xr tcpdump 1 .
.Nm
allows you to correlate and observe the cause-and-affect relationship between
what you see on the wire (captured using a tool like
.Xr tcpdump 1 Ns )
and changes in the TCP control block corresponding to the flow of interest.
It is therefore useful to use
.Nm
and a tool like
.Xr tcpdump 1
to gather the necessary data to piece together the complete picture.
Use of either tool on its own will not be able to provide all of the necessary
data.
.Pp
As a result of needing to interrogate the TCP control block, certain packets
during the lifecycle of a connection are unable to trigger a
.Nm
log message.
The initial handshake takes place without the existence of a control block and
the final ACK is exchanged when the connection is in the TIMEWAIT state.
.Pp
.Nm
was designed to minimise the delay introduced to packets traversing the network
stack.
This design called for a highly optimised and minimal hook function that
extracted the minimal details necessary whilst holding the packet up, and
passing these details to another thread for actual processing and logging.
.Pp
This multithreaded design does introduce some contention issues when accessing
the data structure shared between the threads of operation.
When the hook function tries to place details in the structure, it must first
acquire an exclusive lock.
Likewise, when the processing thread tries to read details from the structure,
it must also acquire an exclusive lock to do so.
If one thread holds the lock, the other must wait before it can obtain it.
This does introduce some additional bounded delay into the kernel's packet
processing code path.
.Pp
In some cases (e.g., low memory, connection termination), TCP packets that enter
the
.Nm
.Xr pfil 9
hook function will not trigger a log message to be generated.
.Nm
refers to this outcome as a
.Qq skipped packet .
Note that
.Nm
always ensures that packets are allowed to continue through the stack, even if
they could not successfully trigger a data log message.
.Nm
will therefore not introduce any packet loss for TCP/IP packets traversing the
network stack.
.Ss Important Behaviours
The behaviour of a log file path change whilst the module is enabled is as
follows:
.Bl -enum
.It
Attempt to open the new file path for writing.
If this fails, the path change will fail and the existing path will continue to
be used.
.It
Assuming the new path is valid and opened successfully:
.Bl -dash
.It
Flush all pending log messages to the old file path.
.It
Close the old file path.
.It
Switch the active log file pointer to point at the new file path.
.It
Commence logging to the new file.
.El
.El
.Pp
During the time between the flush of pending log messages to the old file and
commencing logging to the new file, new log messages will still be generated and
buffered.
As soon as the new file path is ready for writing, the accumulated log messages
will be written out to the file.
.Sh EXAMPLES
To enable the module's operations, run the following command as root:
sysctl net.inet.siftr.enabled=1
.Pp
To change the granularity of log messages such that 1 log message is
generated for every 10 TCP packets per connection, run the following
command as root:
sysctl net.inet.siftr.ppl=10
.Pp
To change the log file location to /tmp/siftr.log, run the following
command as root:
sysctl net.inet.siftr.logfile=/tmp/siftr.log
.Sh SEE ALSO
.Xr tcpdump 1 ,
.Xr tcp 4 ,
.Xr sysctl 8 ,
.Xr alq 9 ,
.Xr pfil 9
.Sh ACKNOWLEDGEMENTS
Development of this software was made possible in part by grants from the
Cisco University Research Program Fund at Community Foundation Silicon Valley,
and the FreeBSD Foundation.
.Sh HISTORY
.Nm
first appeared in
.Fx 7.4
and
.Fx 8.2 .
.Pp
.Nm
was first released in 2007 by Lawrence Stewart and James Healy whilst working on
the NewTCP research project at Swinburne University of Technology's Centre for
Advanced Internet Architectures, Melbourne, Australia, which was made possible
in part by a grant from the Cisco University Research Program Fund at Community
Foundation Silicon Valley.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Pp
Work on
.Nm
v1.2.x was sponsored by the FreeBSD Foundation as part of
the
.Qq Enhancing the FreeBSD TCP Implementation
project 2008-2009.
More details are available at:
.Pp
http://www.freebsdfoundation.org/
.Pp
http://caia.swin.edu.au/freebsd/etcp09/
.Sh AUTHORS
.An -nosplit
.Nm
was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org
and
.An James Healy Aq Mt jimmy@deefa.com .
.Pp
This manual page was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
.Sh BUGS
Current known limitations and any relevant workarounds are outlined below:
.Bl -dash
.It
The internal queue used to pass information between the threads of operation is
currently unbounded.
This allows
.Nm
to cope with bursty network traffic, but sustained high packet-per-second
traffic can cause exhaustion of kernel memory if the processing thread cannot
keep up with the packet rate.
.It
If using
.Nm
on a machine that is also running other modules utilising the
.Xr pfil 9
framework e.g.
.Xr dummynet 4 ,
.Xr ipfw 8 ,
.Xr pf 4 Ns ,
the order in which you load the modules is important.
You should kldload the other modules first, as this will ensure TCP packets
undergo any necessary manipulations before
.Nm
.Qq sees
and processes them.
.It
There is a known, harmless lock order reversal warning between the
.Xr pfil 9
mutex and tcbinfo TCP lock reported by
.Xr witness 4
when
.Nm
is enabled in a kernel compiled with
.Xr witness 4
support.
.It
There is no way to filter which TCP flows you wish to capture data for.
Post processing is required to separate out data belonging to particular flows
of interest.
.It
The module does not detect deletion of the log file path.
New log messages will simply be lost if the log file being used by
.Nm
is deleted whilst the module is set to use the file.
Switching to a new log file using the
.Em net.inet.siftr.logfile
variable will create the new file and allow log messages to begin being written
to disk again.
The new log file path must differ from the path to the deleted file.
.It
The hash table used within the code is sized to hold 65536 flows.
This is not a
hard limit, because chaining is used to handle collisions within the hash table
structure.
However, we suspect (based on analogies with other hash table performance data)
that the hash table look up performance (and therefore the module's packet
processing performance) will degrade in an exponential manner as the number of
unique flows handled in a module enable/disable cycle approaches and surpasses
65536.
.It
There is no garbage collection performed on the flow hash table.
The only way currently to flush it is to disable
.Nm .
.It
The PPL variable applies to packets that make it into the processing thread,
not total packets received in the hook function.
Packets are skipped before the PPL variable is applied, which means there may be
a slight discrepancy in the triggering of log messages.
For example, if PPL was set to 10, and the 8th packet since the last log message
is skipped, the 11th packet will actually trigger the log message to be
generated.
This is discussed in greater depth in CAIA technical report 070824A.
.It
At the time of writing, there was no simple way to hook into the TCP layer
to intercept packets.
.Nm Ap s
use of IP layer hook points means all IP
traffic will be processed by the
.Nm
.Xr pfil 9
hook function, which introduces minor, but nonetheless unnecessary packet delay
and processing overhead on the system for non-TCP packets as well.
Hooking in at the IP layer is also not ideal from the data gathering point of
view.
Packets traversing up the stack will be intercepted and cause a log message
generation BEFORE they have been processed by the TCP layer, which means we
cannot observe the cause-and-affect relationship between inbound events and the
corresponding TCP control block as precisely as could be.
Ideally,
.Nm
should intercept packets after they have been processed by the TCP layer i.e.
intercept packets coming up the stack after they have been processed by
.Fn tcp_input ,
and intercept packets coming down the stack after they have been
processed by
.Fn tcp_output .
The current code still gives satisfactory granularity though, as inbound events
tend to trigger outbound events, allowing the cause-and-effect to be observed
indirectly by capturing the state on outbound events as well.
.It
The
.Qq inflight bytes
value logged by
.Nm
does not take into account bytes that have been
.No SACK Ap ed
by the receiving host.
.It
Packet hash generation does not currently work for IPv6 based TCP packets.
.It
Compressed notation is not used for IPv6 address representation.
This consumes more bytes than is necessary in log output.
.El