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SIFTR(4)	       FreeBSD Kernel Interfaces Manual		      SIFTR(4)

NAME
     SIFTR -- Statistical Information For TCP Research

SYNOPSIS
     To	load the driver	as a module at run-time, run the following command as
     root:

	   kldload siftr

     Alternatively, to load the	driver as a module at boot time, add the fol-
     lowing line into the loader.conf(5) file:

	   siftr_load="YES"

DESCRIPTION
     The SIFTR (Statistical Information	For TCP	Research) 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.

   Compile-time	Configuration
     The default operation of SIFTR is to capture IPv4 TCP/IP packets.	SIFTR
     can be configured to support IPv4 and IPv6	by uncommenting:

	   CFLAGS+=-DSIFTR_IPV6

     in	<sys/modules/siftr/Makefile> and recompiling.

     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 for-
     mat IPv6 addresses	for logging. Note that SIFTR uses uncompressed nota-
     tion 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".

   Run-time Configuration
     SIFTR utilises the	sysctl(8) interface to export its configuration	vari-
     ables to user-space.  The following variables are available:

	   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 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 net.inet.siftr.enabled is set to
			 1.

	   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	mes-
			 sage 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.

	   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.

	   net.inet.siftr.genhashes
			 controls whether a hash is generated for each TCP
			 packet	seen by	SIFTR.	By default, the	value is set
			 to 0, which means no hashes are generated.  The
			 hashes	are useful to correlate	which TCP packet trig-
			 gered the generation of a particular log message, but
			 calculating them adds additional computational	over-
			 head into the fast path.

   Log Format
     A typical SIFTR log file will contain 3 different types of	log message.
     All messages are written in plain ASCII text.

     Note: The "\" present in the example log messages in this section indi-
     cates a line continuation and is not part of the actual log message.

     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.

	   enable_time_secs=1238556193	  enable_time_usecs=462104 \
	   siftrver=1.2.2    hz=1000	tcp_rtt_scale=32 \
	   sysname=FreeBSD    sysver=604000    ipmode=4

     Field descriptions	are as follows:

	   enable_time_secs
			 time at which the module was enabled, in seconds
			 since the UNIX	epoch.

	   enable_time_usecs
			 time at which the module was enabled, in microseconds
			 since enable_time_secs.

	   siftrver	 version of SIFTR.

	   hz		 tick rate of the kernel in ticks per second.

	   tcp_rtt_scale
			 smoothed RTT estimate scaling factor.

	   sysname	 operating system name.

	   sysver	 operating system version.

	   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 "Compile-time Configuration" subsection.

     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.

	   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

     Field descriptions	are as follows:

	   1		 Direction of packet that triggered the	log message.
			 Either	"i" for	in, or "o" for out.

	   2		 Hash of the packet that triggered the log message.

	   3		 Time at which the packet that triggered the log mes-
			 sage was processed by the pfil(9) hook	function, in
			 seconds and microseconds since	the UNIX epoch.

	   4		 The IPv4 or IPv6 address of the local host, in	dotted
			 quad (IPv4 packet) or colon-separated hex (IPv6
			 packet) notation.

	   5		 The TCP port that the local host is communicating
			 via.

	   6		 The IPv4 or IPv6 address of the foreign host, in dot-
			 ted quad (IPv4	packet)	or colon-separated hex (IPv6
			 packet) notation.

	   7		 The TCP port that the foreign host is communicating
			 via.

	   8		 The slow start	threshold for the flow,	in bytes.

	   9		 The current congestion	window for the flow, in	bytes.

	   10		 The current bandwidth-controlled window for the flow,
			 in bytes.

	   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.

	   12		 The current receive window for	the flow, in bytes.
			 The post scaled value is always reported.

	   13		 The current window scaling factor for the sending
			 window.

	   14		 The current window scaling factor for the receiving
			 window.

	   15		 The current state of the TCP finite state machine, as
			 defined in <netinet/tcp_fsm.h>.

	   16		 The maximum segment size for the flow,	in bytes.

	   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.

	   18		 SACK enabled indicator. 1 if SACK enabled, 0 other-
			 wise.

	   19		 The current state of the TCP flags for	the flow.  See
			 <netinet/tcp_var.h> for information about the various
			 flags.

	   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.

	   21		 The current size of the socket	send buffer in bytes.

	   22		 The current number of bytes in	the socket send
			 buffer.

	   23		 The current size of the socket	receive	buffer in
			 bytes.

	   24		 The current number of bytes in	the socket receive
			 buffer.

	   25		 The current number of unacknowledged bytes in-flight.
			 Bytes acknowledged via	SACK are not excluded from
			 this count.

	   26		 The current number of segments	in the reassembly
			 queue.

     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 mod-
     ule was most recently enabled.

	   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,

     Field descriptions	are as follows:

	   disable_time_secs
			 Time at which the module was disabled,	in seconds
			 since the UNIX	epoch.

	   disable_time_usecs
			 Time at which the module was disabled,	in microsec-
			 onds since disable_time_secs.

	   num_inbound_tcp_pkts
			 Number	of TCP packets that traversed up the network
			 stack.	 This only includes inbound TCP	packets	during
			 the periods when SIFTR	was enabled.

	   num_outbound_tcp_pkts
			 Number	of TCP packets that traversed down the network
			 stack.	 This only includes outbound TCP packets dur-
			 ing the periods when SIFTR was	enabled.

	   total_tcp_pkts
			 The summation of num_inbound_tcp_pkts and num_out-
			 bound_tcp_pkts.

	   num_inbound_skipped_pkts_malloc
			 Number	of inbound packets that	were not processed
			 because of failed malloc() calls.

	   num_outbound_skipped_pkts_malloc
			 Number	of outbound packets that were not processed
			 because of failed malloc() calls.

	   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.

	   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.

	   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.

	   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.

	   num_inbound_skipped_pkts_icb
			 Number	of inbound packets that	were not processed
			 because of failure to find the	IP control block asso-
			 ciated	with the packet.

	   num_outbound_skipped_pkts_icb
			 Number	of outbound packets that were not processed
			 because of failure to find the	IP control block asso-
			 ciated	with the packet.

	   total_skipped_tcp_pkts
			 The summation of all skipped packet counters.

	   flow_list	 A CSV list of TCP flows that triggered	data log mes-
			 sages to be generated since the module	was loaded.
			 Each flow entry in the	CSV list is formatted as
			 "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 trail-
			 ing comma will	always be present.

     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.

IMPLEMENTATION NOTES
     SIFTR hooks into the network stack	using the pfil(9) interface.  In its
     current incarnation, it hooks into	the AF_INET/AF_INET6 (IPv4/IPv6)
     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.

     The diagram below illustrates how SIFTR inserts itself into the stack.

	   ----------------------------------
		      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
	   ----------------------------------

     SIFTR uses	the alq(9) interface to	manage writing data to disk.

     At	first glance, you might	mistakenly think that SIFTR extracts informa-
     tion from individual TCP packets.	This is	not the	case.  SIFTR 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 con-
     trol block	state.

     The distinction between interrogating individual packets versus interro-
     gating the	control	block is important, because SIFTR does not remove the
     need for packet capturing tools like tcpdump(1).  SIFTR allows you	to
     correlate and observe the cause-and-affect	relationship between what you
     see on the	wire (captured using a tool like tcpdump(1)) and changes in
     the TCP control block corresponding to the	flow of	interest.  It is
     therefore useful to use SIFTR and a tool like 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.

     As	a result of needing to interrogate the TCP control block, certain
     packets during the	lifecycle of a connection are unable to	trigger	a
     SIFTR log message.	 The initial handshake takes place without the exis-
     tence of a	control	block and the final ACK	is exchanged when the connec-
     tion is in	the TIMEWAIT state.

     SIFTR 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.

     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 exclu-
     sive 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.

     In	some cases (e.g., low memory, connection termination), TCP packets
     that enter	the SIFTR pfil(9) hook function	will not trigger a log message
     to	be generated.  SIFTR refers to this outcome as a "skipped packet".
     Note that SIFTR always ensures that packets are allowed to	continue
     through the stack,	even if	they could not successfully trigger a data log
     message.  SIFTR will therefore not	introduce any packet loss for TCP/IP
     packets traversing	the network stack.

   Important Behaviours
     The behaviour of a	log file path change whilst the	module is enabled is
     as	follows:

     1.	  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.

     2.	  Assuming the new path	is valid and opened successfully:

	  -   Flush all	pending	log messages to	the old	file path.

	  -   Close the	old file path.

	  -   Switch the active	log file pointer to point at the new file
	      path.

	  -   Commence logging to the new file.

     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	writ-
     ing, the accumulated log messages will be written out to the file.

EXAMPLES
     To	enable the module's operations,	run the	following command as root:
     sysctl net.inet.siftr.enabled=1

     To	change the granularity of log messages such that 1 log message is gen-
     erated for	every 10 TCP packets per connection, run the following command
     as	root: sysctl net.inet.siftr.ppl=10

     To	change the log file location to	/tmp/siftr.log,	run the	following com-
     mand as root: sysctl net.inet.siftr.logfile=/tmp/siftr.log

SEE ALSO
     tcpdump(1), tcp(4), sysctl(8), alq(9), pfil(9)

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.

HISTORY
     SIFTR first appeared in FreeBSD 7.4 and FreeBSD 8.2.

     SIFTR 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, Aus-
     tralia, which was made possible in	part by	a grant	from the Cisco Univer-
     sity Research Program Fund	at Community Foundation	Silicon	Valley.	 More
     details are available at:

     http://caia.swin.edu.au/urp/newtcp/

     Work on SIFTR v1.2.x was sponsored	by the FreeBSD Foundation as part of
     the "Enhancing the	FreeBSD	TCP Implementation" project 2008-2009.	More
     details are available at:

     http://www.freebsdfoundation.org/

     http://caia.swin.edu.au/freebsd/etcp09/

AUTHORS
     SIFTR was written by Lawrence Stewart <lstewart@FreeBSD.org> and James
     Healy <jimmy@deefa.com>.

     This manual page was written by Lawrence Stewart <lstewart@FreeBSD.org>.

BUGS
     Current known limitations and any relevant	workarounds are	outlined
     below:

     -	 The internal queue used to pass information between the threads of
	 operation is currently	unbounded.  This allows	SIFTR 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.

     -	 If using SIFTR	on a machine that is also running other	modules	util-
	 ising the pfil(9) framework e.g.  dummynet(4),	ipfw(8), pf(4),	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	SIFTR "sees" and processes them.

     -	 There is a known, harmless lock order reversal	warning	between	the
	 pfil(9) mutex and tcbinfo TCP lock reported by	witness(4) when	SIFTR
	 is enabled in a kernel	compiled with witness(4) support.

     -	 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.

     -	 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 SIFTR is
	 deleted whilst	the module is set to use the file.  Switching to a new
	 log file using	the 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.

     -	 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 colli-
	 sions 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.

     -	 There is no garbage collection	performed on the flow hash table.  The
	 only way currently to flush it	is to disable SIFTR.

     -	 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.

     -	 At the	time of	writing, there was no simple way to hook into the TCP
	 layer to intercept packets.  SIFTR's use of IP	layer hook points
	 means all IP traffic will be processed	by the SIFTR 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, SIFTR should
	 intercept packets after they have been	processed by the TCP layer
	 i.e.  intercept packets coming	up the stack after they	have been pro-
	 cessed	by tcp_input(),	and intercept packets coming down the stack
	 after they have been processed	by 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.

     -	 The "inflight bytes" value logged by SIFTR does not take into account
	 bytes that have been SACK'ed by the receiving host.

     -	 Packet	hash generation	does not currently work	for IPv6 based TCP
	 packets.

     -	 Compressed notation is	not used for IPv6 address representation.
	 This consumes more bytes than is necessary in log output.

FreeBSD	9.2		       November	12, 2010		   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | IMPLEMENTATION NOTES | EXAMPLES | SEE ALSO | ACKNOWLEDGEMENTS | HISTORY | AUTHORS | BUGS

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