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ndd 命令中各项参数详解

[ 作者:Linux联盟收集加入时间:2006-07-19 12:49:27 来自:Linux联盟收集 ]

http://www.sean.de/Solaris/soltune.html

SolarisTM 2.x - Tuning Your TCP/IP Stack and More

Last update: 24.05.2002 (change log)

Please check your location line carefully. If you don't see http://www.sean.de/Solaris/ in your location bar, you might want to check with the original site for the most up to date information.

Important Notice!
SUN managed to publish a Solaris Tunable Parameters Reference Manual, applying to Solaris 8, HW 2/02, and Solaris Tunable Parameters Reference Manual, applying to Solaris 9, HW 9/02. You might want to check there for anything you miss here. Another good read is Solaris Operating Environment Network Settings for Security, if you are concerned about security and denial-of-service attacks.

Table of contents
Introduction
1.1 History
1.2 Quick intro into ndd
1.3 How to read this document
TCP connection initiation
Retransmission related parameters
Path MTU discovery
Further advice, hints and remarks
5.1 Common TCP timers
5.2 Erratic IPX behaviors
5.3 Common IP parameters
5.4 TCP and UDP port related parameters
Windows, buffers and watermarks
Tuning your system
7.1 Things to watch
7.2 General entries in the file /etc/system
7.3 System V IPC related entries
7.4 How to find further entries
100 Mbit ethernet and related entries

8.1 The hme interface
8.2 Other problems
Recommended patches
Literature
10.1 Books
10.2 Internet resources
10.3 RFC, mentioned and otherwise
10.4 Further material
Solaris' Future
11.1 Solaris 7
11.2 Solaris 8
11.3 Solaris 9
Uncovered material
Scripts
List of things to do
Appendices are separate documents. They are quoted from within the text, but you might be interested in them when downloading the current document. If you say "print" for this document, the appendices will not be printed. You have to download and print them separately.

Simple transactions using TCP
System V IPC parameter
Retransmission behavior
Slow start implications
The change log
Glossary (first attempt)
Index (first attempt)
1. Introduction
Use at your own risk!
If your system behaves erratically after applying some tweaks, please don't blame me. Remember to have a backup handy before starting to tune. Always make backup copies of the files you are changing. I tried carefully to assemble the information you are seeing here, aimed at improved system performance. As usual, there are no guarantees that what worked for me will work for you. Please don't take my recommendation at heart: They are starting points, not absolutes. Always read my reasoning, don't use them blindly.

Before you start, you ought to grab a copy of the TCP state transition diagram as specified in RFC 793 on page 23. The drawback is the missing error correction supplied by later RFCs. There is an easier way to obtain blowup printouts to staple to your office walls. Grab a copy of the PostScript file pocket guide, page 2 accompanying Stevens' TCP/IP Illustrated Volume 1 [4]. Or simply open the book at figure 18.12.

Please share your knowledge
I try to assemble this page and related material for everybody interested in gaining more from her or his system. If you have an item I didn't cover, but which you deem worthwhile, please write to me. A few dozen or so regular readers of this page will thank you for it. I am only human, thus if you stumble over an error, misconception, or blatant nonsense, please have me correct it. In the past, there were quite a few mistakes.

The set of documents may look a trifle colorful, or just odd, if your browser supports cascading stylesheets. Care was taken to select the formatting tags in a way that the printed output still resembles the intentions of the author, and that the set of documents is still viewable with browser like Mosaic or Lynx. Stylesheets were used as an optical enhancement. Most notable is the different color of interior and external links. Interior links are shown in greenish colors, and will be rendered within the same frame. External links on the other hand are shown in bluish colors, and all will be shown in the same new frame. If you leave it open, a new external link will be shown within the same window. Literature references within the text are often interior links, pointing to the literature section, where the external links are located.

1.1 History
This page and the related work have a long history in gathering. I started out peeking wide eyed over the shoulders of two people from a search engine provider when they were installing the German server of a customer of my former employer. My only alternative resource of tuning information was the brilliant book TCP/IP Illustrated 1 [4] by Stevens. I started gathering all information about tuning I was able to get my hands upon. The cumulation of these you are experiencing on these pages.

1.2 Quick intro into ndd
Solaris allows you to tune, tweak, set and reset various parameters related to the TCP/IP stack while the system is running. Back in the SunOS 4.x days, one had to change various C files in the kernel source tree, generate a new kernel, reboot the machine and try out the changes. The Solaris feature of changing the important parameters on the fly is very convenient.

Many of the parameters I mention in the rest of the document you are reading are time intervals. All intervals are measured in milliseconds. Other parameters are usually bytecounts, but a few times different units of measurements are used and documented. A few items appear totally unrelated to TCP/IP, but due to the lack of a better framework, they materialized on this page.

Most tunings can be achieved using the program ndd. Any user may execute this program to read the current settings, depending on the readability of the respective device files. But only the super user is allowed to execute ndd -set to change values. This makes sense considering the sensitive parameters you are tuning. Details on the use of ndd can be obtained from the respective manual page.

ndd will become your friend, as it is the major tool to tweak most of the parameters described in this document. Therefore you better make yourself familiar with it. A quick overview will be given in this section, too. ndd is not limited to tweaking TCP/IP related parameters. Many other devices, which have a device file underneath /dev and a kernel module can be configured with the help of ndd. For instance, any networking driver which supports the Data Link Provider Interface (DLPI) can be configured.

The parameters supplied to ndd are symbolic keys indexing either a single usually numerically value, or a table. Please note that the keys usually (but not always) start out with the module or device name. For instance, changing values of the IP driver, you have to use the device file /dev/ip and all parameters start out with ip_. The question mark is the most notable exception to this rule.

1.2.1 Interactive mode
The interactive mode allows you to inspect and modify a device, driver or module interactively. In order to inspect the available keyword names associated with a parameter, just type the question mark. The next item will explain about the output format of the parameter list.

# ndd /dev/tcp
name to get/set ? tcp_slow_start_initial
value ?
length ?
2
name to get/set ? ^D
The example above queries the TCP driver for the value of the slow start feature in an interactive fashion. The typed input is shown boldface.

1.2.2 Show all available parameters
If you are interested in the parameters you can tweak for a given module, query for the question mark. This special parameter name is part of all ndd configurable material. It tells the names of all parameters available - including itself - and the access mode of the parameter.

# ndd /dev/icmp \?
?                             (read only)
icmp_wroff_extra              (read and write)
icmp_def_ttl                  (read and write)
icmp_bsd_compat               (read and write)
icmp_xmit_hiwat               (read and write)
icmp_xmit_lowat               (read and write)
icmp_recv_hiwat               (read and write)
icmp_max_buf                  (read and write)
Please mind that you have to escape the question mark with a backslash from the shell, if you are querying in the non-interactive fashion as shown above.

1.2.3 Query the value of one or more parameters (read access)
At the command line, you often need to check on settings of your TCP/IP stack or other parameters. By supplying the parameter name, you can examine the current setting. It is permissible to mention several parameters to check on at once.

# ndd /dev/udp udp_smallest_anon_port
32768
# ndd /dev/hme link_status link_speed link_mode
1

1

1
The first example checks on the smallest anonymous port UDP may use when sending a PDU. Please refer to the appropriate section later in this document on the recommended settings for this parameter.

The second example checks the three important link report values of a 100 Mbit ethernet interface. The results are separated by an empty line, because some parameters may refer to tabular values instead of a single number.

1.2.4 Modify the value of one parameter (write access)
This mode of interaction with ndd will frequently be found in scripts or when changing value at the command line in a non-interactive fashion. Please note that you may only set one value at a time. The scripts section below contains examples in how to make changes permanent using a startup script.

# ndd -set /dev/ip ip_forwarding 0
The example will stop the forwarding of IP PDUs, even if more than one non-local interface is active and up. Of course, you can only change parameters which are marked for both, reading and writing.

1.2.5 Further remarks
Andres Kroonmaa kindly supplied a nifty script to check all existing values for a network component (tcp, udp, ip, icmp, etc.). Usually I do the same thing using a small Perl script.

1.3 How to read this document
This document is separated into several chapters with little inter-relation. It is still advisable to loosely follow the order outlined in the table of contents.

The first chapter entirely focusses on the TCP connection queues. It is quite long for such small topic, but it is also meant to introduce you into my style of writing. The next chapter deals with TCP retransmission related parameters that you can adjust to your needs. The chapter is more concise. One chapter on deals with path MTU discovery, as there used to be problems with older versions of Solaris. Recent versions usually do not need any adjustments.

The fifth chapter is a kind of catch-all. Some TCP, some UDP and some IP related parameters are explained (forwarding, port ranges, timers), and a quick detour into bug 1226653 explains that some versions were capable of sending packages larger than the MTU. The following chapter in depth deals with windows, buffers and related issues.

Chapter seven detours from the ndd interface, and focusses on variables you can set in your /etc/system file, as some things can only be thus managed. Another part of that chapter deals with the hme interface and appropriate tunables. The chapter may be split in future, and parts of it are already found in the appendices.

The chapter dealing with patches, an important topic with any OS, just points you to various sources, and only mentions some essential things for older versions of Solaris.

Literature exists in abundance. The literature sections is more a lose collection of links and some books that I consider essential when working with TCP/IP, not limited to Solaris. The RFC sections is kind of hard to keep up-to-date, but then, I reckon you know how to read the rfc-index file.

The final chapters quickly glance at new or at one time new versions of Solaris - time makes them obsolete. The chapter is there for historical reason, more or less. The scripts sections deals with the nettune script used by YaSSP. It finishes with some TODO material.

2. TCP connection initiation
This section is dedicated exclusively to the various queues and tunable variable(s) used during connection instantiation. The socket API maintains some control over the queues. But in order to tune anything, you have to understand how listen and accept interact with the queues. For details, see the various Stevens books mentioned in the literature section.

When the server calls listen, the kernel moves the socket from the TCP state CLOSED into the state LISTEN, thus doing a passive open. All TCP servers work like this. Also, the kernel creates and initializes various data structures, among them the socket buffers and two queues:

incomplete connection queue
This queue contains an entry for every SYN that has arrived. BSD sources assign so_q0len entries to this queue. The server sends off the ACK of the client's SYN and the server side SYN. The connection get queued and the kernel now awaits the completion of the TCP three way handshake to open a connection. The socket is in the SYN_RCVD state. On the reception of the client's ACK to the server's SYN, the connection stays one round trip time (RTT) in this queue before the kernel moves the entry into the

completed connection queue
This queue contains an entry for each connection for which the three way handshake is completed. The socket is in the ESTABLISHED state. Each call to accept() removes the front entry of the queue. If there are no entries in the queue, the call to accept usually blocks. BSD source assign a length of so_qlen to this queue.

Both queues are limited regarding their number of entries. By calling listen(), the server is allowed to specify the size of the second queue for completed connections. If the server is for whatever reason unable to remove entries from the completed connection queue, the kernel is not supposed to queue any more connections. A timeout is associated with each received and queued SYN segment. If the server never receives an acknowledgment for a queued SYN segment, TCP state SYN_RCVD, the time will run out and the connection thrown away. The timeout is an important resistance against SYN flood attacks.


Figure 1: Queues maintained for listening sockets.   Figure 2: TCP three way handshake, connection initiation.

Historically, the argument to the listen function specified the maximum number of entries for the sum of both queues. Many BSD derived implementations multiply the argument with a fudge factor of 3/2. Solaris <= 2.5.1 do not use the fudge factor, but adds 1, while Solaris 2.6 does use the fudge factor, though with a slightly different rounding mechanism than the one BSD uses. With a backlog argument of 14, Solaris 2.5.1 servers can queue 15 connections. Solaris 2.6 server can queue 22 connections.

Stevens shows that the incomplete connection queue does need more entries for busy servers than the completed connection queue. The only reason for specifying a large backlog value is to enable the incomplete connection queue to grow as SYN arrive from clients. Stevens shows that moderately busy webserver has an empty completed connection queue during 99 % of the time, but the incomplete connection queue needed 15 or less entries in 98 % of the time! Just try to imagine what this would mean for a really busy webcache like Squid.

Data for an established connection which arrives before the connection is accept()ed, should be stored into the socket buffer. If the queues are full when a SYN arrived, it is dropped in the hope that the client will resend it, hopefully finding room in the queues then.

According to Cockroft [2], there was only one listen queue for unpatched Solari <= 2.5.1. Solari >;= 2.6 or an applied TCP patch 103582-12 or above splits the single queue in the two shown in figure 1. The system administrator is allowed to tweak and tune the various maxima of the queue or queues with Solaris. Depending on whether there are one or two queues, there are different sets of tweakable parameters.

The old semantics contained just one tunable parameter tcp_conn_req_max which specified the maximum argument for the listen(). The patched versions and Solaris 2.6 replaced this parameter with the two new parameters tcp_conn_req_max_q0 and tcp_conn_req_max_q. A SunWorld article on 2.6 by Adrian Cockroft tells the following about the new parameters:

tcp_conn_req_max [is] replaced. This value is well-known as it normally needs to be increased for Web servers in older releases of Solaris 2. It no longer exists in Solaris 2.6, and patch 103582-12 adds this feature to Solaris 2.5.1. The change is part of a fix that prevents denial of service from SYN flood attacks. There are now two separate queues of partially complete connections instead of one.

tcp_conn_req_max_q0 is the maximum number of connections with handshake incomplete. A SYN flood attack could only affect this queue, and a special algorithm makes sure that valid connections can still get through.

tcp_conn_req_max_q is the maximum number of completed connections waiting to return from an accept call as soon as the right process gets some CPU time.

In other words, the first specifies the size of the incomplete connection queue while the second parameters assigns the maximum length of the completed connection queue. All three parameters are covered below.

You can determine if you need to tweak this set of parameters by watching the output of netstat -sP tcp. Look for the value of tcpListenDrop, if available on your version of Solaris. Older versions don't have this counter. Any value showing up might indicate something wrong with your server, but then, killing a busy server (like squid) shuts down its listening socket, and might increase this counter (and others). If you get many drops, you might need to increase the appropriate parameter. Since connections can also be dropped, because listen() specifies a too small argument, you have to be careful interpreting the counter value. On old versions, a SYN flood attack might also increase this counter.

Newer or patched versions of Solaris, with both queues available, will also have the additional counters tcpListenDropQ0 and tcpHalfOpenDrop. Now the original counter tcpListenDrop counts only connections dropped from the completed connection queue, and the counter ending in Q0 the drops from the incomplete connection queue. Killing a busy server application might increase either or both counters. If the tcpHalfOpenDrop shows up values, your server was likely to be the victim of a SYN flood. The counter is only incremented for dropping noxious connection attempts. I have no idea, if those will also show up in the Q0 counter, too.

tcp_conn_req_max
default 8 (max. 32), since 2.5 32 (max. 1024), recommended 128 <= x <= 1024
since 2.6 or 2.5.1 with patches 103630-09 and 103582-12 or above applied:
see tcp_conn_req_max_q and tcp_conn_req_max_q0

The current parameter describes the maximum number of pending connection requests queued for a listening endpoint in the completed connection queue. The queue can only save the specified finite number of requests. If a queue overflows, nothing is sent back. The client will time out and (hopefully) retransmit.

The size of the completed connection queue does not influence the maximum number of simultaneous established connections after they were accepted nor does it have any influence on the maximum number of clients a server can serve. With Solaris, the maximum number of file descriptors is the limiting factor for simultaneous connections, which just happened to coincide with the maximum backlog queue size.

From the viewpoint of TCP those connections placed in the completed connection queue are in the TCP state ESTABLISHED, even though the application has not reaped the connection with a call to accept. That is the number limited by the size of the queue, which you tune with this parameter. If the application, for some reason, does not release entries from the queue by calling accept, the queue might overflow, and the connection is dropped. The client's TCP will hopefully retransmit, and might find a place in the queue.

Solaris offers the possibility to place connections into the backlog queue as soon as the first SYN arrives, called eager listening. The three way handshake will be completed as soon as the application accept()s the connection. The use of eager listening is not recommended for production systems.

Solari < 2.5 have a maximum queue length of 32 pending connections. The length of the completed connection queue can also be used to decrease the load on an overloaded server: If the queue is completely filled, remote clients will be denied further connections. Sometimes this will lead to a connection timed out error message.

Naively, I assumed that a very huge length might lead to a long service time on a loaded server. Stevens showed that the incomplete connection queue needs much more attention than the completed connection queue. But with tcp_conn_req_max you have no option to tweak that particular length.

Earlier versions of this document suggested to tune tcp_conn_req_max with regards to the values of rlim_fd_max and rlim_fd_cur, but the interdependencies are more complex than any rule of thumb. You have to find your own ideal. When a connection is still in the queue, only the queue length limits the number of entries. Connections taken from the queue are put into a file descriptor each.

There is a trick to overcome the hardcoded limit of 1024 with a patch. SunSolve shows this trick in connection with SYN flood attacks. A greatly increased listen backlog queue may offer some small increased protection against this vulnerability. On this topic also look at the tcp_ip_abort_cinterval parameter. Better, use the mentioned TCP patches, and increase the q0 length.

echo "tcp_param_arr+14/W 0t10240" | adb -kw /dev/ksyms /dev/mem
This patch is only effective on the currently active kernel, limiting its extend to the next boot. Usually you want to append the line above on the startup script /etc/init.d/inetinit. The shown patch increases hard limit of the listen backlog queue to 10240. Only after applying this patch you may use values above 1024 for the tcp_conn_req_max parameter.

A further warning: Changes to the value of tcp_conn_req_max parameter in a running system will not take effect until each listening application is restarted. The backlog queue length is evaluated whenever an application calls listen(3N), usually once during startup. Sending a HUP signal may or may not work; personally I prefer to TERM the application and restart them manually or, even better, use a startup script.

tcp_conn_req_max_q0
since 2.5.1 with patches 103630-09 and 103582-12 or above applied: default 1024;
since 2.6: default 1024, recommended 1024 <= x <= 10240

After installing the mentioned TCP patches, alternatively after installing Solaris 2.6, the parameter tcp_conn_req_max is no longer available. In its stead the new parameters tcp_conn_req_max_q and tcp_conn_req_max_q0 emerged. tcp_conn_req_max_q0 is the maximum number of connections with handshake incomplete, basically the length of the incomplete connection queue.

In other words, the connections in this queue are just being instantiated. A SYN was just received from the client, thus the connection is in the TCP SYN_RCVD state. The connection cannot be accept()ed until the handshake is complete, even if the eager listening is active.

To protect against SYN flooding, you can increase this parameter. Also refer to the parameter tcp_conn_req_max_q above. I believe that changes won't take effect unless the applications are restarted.

tcp_conn_req_max_q
since 2.5.1 with patches 103630-09 and 103582-12 or above applied: default 128;
since 2.6: default 128, recommended 128 <= x <= tcp_conn_req_max_q0

After installing the mentioned TCP patches, alternatively after installing Solaris 2.6, the parameter tcp_conn_req_max is no longer available. In its stead the new parameters tcp_conn_req_max_q and tcp_conn_req_max_q0 emerged. tcp_conn_req_max_q is the length of the completed connection queue.

In other words, connections in this queue of length tcp_conn_req_max_q have completed the three way handshake of a TCP open. The connection is in the state ESTABLISHED. Connections in this queue have not been accept()ed by the server process (yet).

Also refer to the parameter tcp_conn_req_max_q0. Remember that changes won't take effect unless the applications are restarted.

tcp_conn_req_min
Since 2.6: default 1, recommended: don't touch

This parameter specifies the minimum number of available connections in the completed connection queue for select() or poll() to return "readable" for a listening (server) socket descriptor.

Programmers should note that Stevens [7] describes a timing problem, if the connection is RST between the select() or poll() call and the subsequent accept() call. If the listening socket is blocking, the default for sockets, it will block in accept() until a valid connection is received. While this seems no tragedy with a webserver or cache receiving several connection requests per second, the application is not free to do other things in the meantime, which might constitute a problem.

3. Retransmission related parameters
The retransmission timeout values used by Solaris are way too aggressive for wide area networks, although they can be considered appropriate for local area networks. SUN thus did not follow the suggestions mentioned in RFC 1122. Newer releases of the Solaris kernel are correcting the values in question:

The recommended upper and lower bounds on the RTO are known to be inadequate on large internets. The lower bound SHOULD be measured in fractions of a second (to accommodate high speed LANs) and the upper bound should be 2*MSL, i.e., 240 seconds.
Besides the retransmit timeout (RTO) value two further parameters R1 and R2 may be of interest. These don't seem to be tunable via any Solaris' offered interface that I know of.

The value of R1 SHOULD correspond to at least 3 retransmissions, at the current RTO. The value of R2 SHOULD correspond to at least 100 seconds.

[...]

However, the values of R1 and R2 may be different for SYN and data segments. In particular, R2 for a SYN segment MUST be set large enough to provide retransmission of the segment for at least 3 minutes. The application can close the connection (i.e., give up on the open attempt) sooner, of course.

Great many internet servers which are running Solaris do retransmit segments unnecessarily often. The current condition of European networks indicate that a connection to the US may take up to 2 seconds. All parameters mentioned in the first part of this section relate to each other!

As a starter take this little example. Consider a picture, size 1440 byte, LZW compressed, which is to be transferred over a serial linkup with 14400 bps and using a MTU of 1500. In the ideal case only one PDU gets transmitted. The ACK segment can only be sent after the complete PDU is received. The transmission takes about 1 second. These values seem low, but they are meant as 'food for thought'. Now consider something going awry...

Solaris 2.5.1 is behaving strange, if the initial SYN segment from the host doing the active open is lost. The initial SYN gets retransmitted only after a period of 4 * tcp_rexmit_interval_initial plus a constant C. The time is 12 seconds with the default settings. More information is being prepared on the retransmission test page.

The initial lost SYN may or may not be of importance in your environment. For instance, if you are connected via ATM SVCs, the initial PDU might initiate a logical connection (ATM works point to point) in less than 0.3 seconds, but will still be lost in the process. It is rather annoying for a user of 2.5.1 to wait 12 seconds until something happens.

tcp_rexmit_interval_initial
default 500, since 2.5.1 3000, recommended >;= 2000 (500 for special purposes)

This interval is waited before the last data sent is retransmitted due to a missing acknowledgment. Mind that this interval is used only for the first retransmission. The more international your server is, the larger you should chose this interval.

Special laboratory environments working in LAN-only environments might be better off with 500 ms or even less. If you are doing measurements involving TCP (which is almost always a bad idea), you should consider lowering this parameter.

Why do I consider TCP measurements a bad idea? If ad-hoc approaches are used, or there is no deeper knowledge of the mechanics of TCP, you are bound to arrive at wrong conclusions. Unless there are TCP dumps to document that indeed what you expect is actually happening, results may lead to wrong conclusions. If done properly, there is nothing wrong with TCP measurements. The same rules apply, if you are measuring protocols on top of TCP.

There are lots of knobs and dials to be fiddled with - all of which need to be documented along with the results. Scientific experiments need to be repeatable by others in order to verify your findings.

tcp_rexmit_interval_min
default 200, recommended >;= 1000 (200 for special purposes)
Since 8: default 400

After the initial retransmission further retransmissions will start after the tcp_rexmit_interval_min interval. BSD usually specifies 1500 milliseconds. This interval should be tuned to the value of tcp_rexmit_interval_initial, e.g. some value between 50 % up to 200 %. The parameter has no effect on retransmissions during an active open, see my accompanying document on retransmissions.

The tcp_rexmit_interval_min doesn't display any influence on connection establishment with Solaris 2.5.1. It does with 2.6, though. The influence on regular data retransmissions, or FIN retransmissions I have yet to research.

tcp_ip_abort_interval
default 120000, since 2.5 480000, recommended 600000
This interval specifies how long retransmissions for a connection in the ESTABLISHED state should be tried before a RESET segment is sent. BSD systems default to 9 minutes.

You don't want your connections to fail too quickly once they are in the ESTABLISHED state. A reader reported that Veritas backup clients might fail with "socket write failed". Veritas recommends not to set above parameter below 8 minutes.

tcp_ip_abort_linterval
default ?, recommended ?

According to an unconfirmed user report, the parameter is the abort interval for passive connections, i.e. those received on ports in the LISTEN state. Refer to the tcp_ip_abort_cinterval for details, as there is some confusion between what SunSolve says and what can be read in Stevens.

tcp_ip_abort_cinterval
default 240000, since 2.5 180000, recommended ?

This interval specifies how long retransmissions for a remote host are repeated until the RESET segment is sent. The difference to the tcp_ip_abort_interval parameter is that this connection is about to be established - it has not yet reached the state ESTABLISHED. This value is interesting considering SYN flood attacks on your server. Proxy server are doubly handicapped because of their Janus behavior (like a server towards the downstream cache, like a client towards the upstream server).

According to Stevens this interval is connected to the active open, e.g. the connect(3N) call. But according to SunSolve the interval has an impetus on both directions. A remote client can refuse to acknowledge an opening connection up to this interval. After the interval a RESET is sent. The other way around works out, too. If the three-way handshake to open a connection is not finished within this interval, the RESET Segment will be sent. This can only happen, if the final ACK went astray, which is a difficult test case to simulate.

To improve your SYN flood resistance, SUN suggests to use an interval as small as 10000 milliseconds. This value has only been tested for the "fast" networks of SUN. The more international your connection is, the slower it will be, and the more time you should grant in this interval. Proxy server should never lower this value (and should let Squid terminate the connection). Webservers are usually not affected, as they seldom actively open connections beyond the LAN.

tcp_rexmit_interval_max
default 60000, RFC 1122 recommends 240000 (2MSL), recommended 1...2 * tcp_close_wait_interval or tcp_time_wait_interval
Since 2.6: default 240000
Since 8: default 60000

All previously mentioned retransmissions related interval use an exponential backoff algorithm. The wait interval between two consecutive retransmissions for the same PDU is doubled starting with the minimum.

The tcp_rexmit_interval_max interval specifies the maximum wait interval between two retransmissions. If changing this value, you should also give the abort interval an inspection. The maximum wait interval should only be reached shortly before the abort interval timer expires. Additionally, you should coordinate your interval with the value of tcp_close_wait_interval or tcp_time_wait_interval.

tcp_deferred_ack_interval
default 50, BSD 200, recommended 200 (regular), 50 (benchmarking), or 500 (WAN server)
Since 8: default 100

This parameter specifies the timeout before sending a delayed ACK. The value should not be increased above 500, as required by RFC 1122. This value is of great interest for interactive services. A small number will increase the "responsiveness" of a remote service (telnet, X11), while a larger value can decrease the number of segments exchanged.

The parameter might also interest to HTTP servers which transmit small amounts of data after a very short retrieval time. With a heavy-duty servers or in laboratory banging environment, you might encounter service times answering a request which are well above 50 ms. An increase to 500 might lead to less PDUs transferred over the network, because TCP is able to merge the ACK with data. Increases beyond 500 should not be even considered.

SUN claims that Solaris recognizes the initial data phase of a connection. An initial ACK (not SYN) is not delayed. As opposed to the simplistic approach mentioned in the SUN paper, a request for a webservice (both, server or proxy) which does not fit into a single PDU can be transmitted faster. Also check the tcp_slow_start_initial Parameter.

The tcp_deferred_ack_interval also seems to be used to distinguish full-sized segments between interactive traffic and bulk data transfer. If a sender uses MSS sized segments, but sends each segment further apart than approximately 0.9 times the interval, the traffic will be rated interactive, and thus every segment seems to get ACKed.

tcp_deferred_acks_max
Since 2.6: default 8, recommended ?, maximum 16

This parameter features the maximum number of segments received after which an ACK just has to be sent. Previously I thought this parameter solely related to interactive data transfer, but I was mistaken. This parameter specifies the number of outstanding ACKs. You can give it a look when tuning for high speed traffic and bulk transfer, but the parameter is controversial. For instance, unless you employ selective acknowledgments (SACK) like Solaris 7, you can only ACK the number of segments correctly received. With the parameter at a larger value, statistically the amount of data to retransmit is larger.

Good values for retransmission tuning don't beam into existence from a white source. Rather you should carefully plan an experiment to get decent values. Intervals from another site can not be carried over to another Solaris system without change. But they might give you an idea where to start when choosing your own values.

The next part looks at a few parameters having to do with retransmissions, as well.

tcp_slow_start_initial
Since 2.5.1 with patch 103582-15 applied: default 1
Since 2.6: default 1, recommended 2 or 4 for servers
Since 8: default 4, no recommendations

This parameter provides the slow-start bug discovered in BSD and Windows TCP/IP implementations for Solaris. More information on the topic can be found on the servers of SUN and in Stevens [6]. To summarize the effect, a server starts sending two PDUs at once without waiting for an ACK due to wrong ACK counts. The ACK from connection initiation being counted as data ACK - compare with figure 2. Network congestion avoidance algorithms are being undermined. The slow start algorithm does not allow the buggy behavior, compare with RFC 2001.

Setting the parameter to 2 allows a Solaris machine to behave like it has the slow start bug, too. Well, IETF is said to make amends to the slow start algorithm, and the bug is now actively turned into a feature. SUN also warns:

It's still conceivable, although rare, that on a configuration that supports many clients on very slow-links, the change might induce more network congestions. Therefore the change of tcp_slow_start_initial should be made with caution.

[...]

Future Solaris releases are likely to default to 2.

You can also gain performance, if many of your clients are running old BSD or derived TCP/IP stacks (like MS). I expect new BSD OS releases not to figure this bug, but then I am not familiar with the BSD OS family. A reader of this page told me about cutting the latency of his server in half, just by using the value of 2.

If you want to know more about this feature and its behavior, you can have a look at some experiments I have conducted concerning that particular feature. The summary is that I agree with the reader: A BSDish client like Windows definitely profits from using a value of 2.

tcp_slow_start_after_idle
Since 2.6: default 2, no recommendations
Since 8: default 4, no recommendations

I reckon that this parameter deals with the slow start for an already established connection which was idle for some time (however the term idle is defined here).

tcp_dupack_fast_retransmit
default 3, no recommendations

Something to do with the number of duplicates ACKs. If we do fast retransmit and fast recovery algorithms, this many ACKs must be retransmitted until we assume that a segment has really been lost. A simple reordering of segments usually causes no more than two duplicate ACKs.

There are a couple of parameters which require some elementary familiarity with RFC 2001, which covers TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms, as well as ssthresh and cwnd.

tcp_rtt_updates
default 0, BSD 16, recommended: (see text)
Since 8: 20, no recommendations

This parameter controls when things like rtt_sa (the smoothed RTT), rtt_sd (the smoothed mean deviation), and ssthresh (the slow start threshold) are cached in the routing table. By default, Solaris does not cache any of the parameters. It is claimed that you can set it to a value you like, but to be the same as BSD, use 16.

The value to this parameter is the number of RTT samples that had to be sampled, so that an accurate enough value can be stored in the routing table. If you chose to use this feature, use a value of 16 or above. Using 16 allows the smoothed RTT filter to converge within 5 % of the correct value, compare Stevens [4], chapter 21.9.

ip_ire_cleanup_interval
default 30000, no recommendatations
Since 8: the parameter has a new name: ip_ire_arp_interval

The parameters may do more than described here. If a routing table entry is not directly connected and not being used, the cache for things like rtt_sa, rtt_sd and ssthresh associated with the entry will be flushed after 30 seconds. The parameter tcp_rtt_updates must be greater than zero to enable the cache.

I could imagine that external helper programs invoked by MRTG on a regular basis connecting to a far-away host might benefit from increasing this value slightly above the invocation interval.

4. path MTU discovery
Whenever a connection is about to be established, the three-way handshake open negotiation, the segment size used will be set to the minimum of (a) the smallest MTU of an outgoing interface, and (b) from MSS announced by the peer. If the remote peer does not announce a MSS, usually the value 536 will be assumed. If path MTU discovery is active, all outgoing PDUs have the IP option DF (don't fragment) set.

If the ICMP error message fragmentation needed is received, a router on the way to the destination needed to fragment the PDU, but was not allowed to do so. Therefore the router discarded the PDU and did send back the ICMP error. Newer router implementations enclose the needed MSS in the error message. If the needed MSS is not included, the correct MSS must be determined by trial and error algorithm.

Due to the internet being a packet switching network, the route a PDU travels along a TCP virtual circuit may change with time. For this reason RFC 1191 recommends to rediscover the path MTU of an active connection after 10 minutes. Improvements of the route can only be noticed by repeated rediscoveries. Unfortunately, Solaris aggressively tries to rediscover the path MTU every 30 seconds. While this is o.k. for LAN environments, it is a grossly impolite behavior in WANs. Since routes may not change that often, aggressive repetitions of path MTU discoveries leads to unnecessary consumption of channel capacity and elongated service times.

Path MTU discovery is a far reaching and controversial topic when discussing it with local ISPs. Still, pMTU discovery is at the foundation of IPv6. The PSC tuning page argues pro path MTU discovery, especially if you maintain a high-speed or long-delay (e.g. satellite) link.

The recommendation I can give you is not to use the defaults of Solaris < 2.5. Please use path MTU discovery, but tune your system RFC conformant. You may alternatively want to switch off the path MTU discovery all together, though there are few situations where this is necessary.

I was made aware of the fact that in certain circumstances bridges connecting data link layers of differing MTU sizes defeat pMTU discovery. I have to put some more investigation into this matter. If a frame with maximum MTU size is to be transported into the network with the smaller MTU size, it is truncated silently. A bridge does not know anything about the upper protocol levels: A bridge neither fragments IP nor sends an ICMP error.

There may be work-arounds, and the tcp_mss_def is one of them. Setting all interfaces to the minimum shared MTU might help, at the cost of losing performance on the larger MTU network. Using what RFC 1122 calls an IP gateway is a possible, yet expensive solution.

ip_ire_pathmtu_interval
default 30000, recommended 600000
Since 2.5 600000, no recommendations

This timer determines the interval Solaris rediscovers the path MTU. An extremely large value will only evaluate the path MTU once at connection establishment.

ip_path_mtu_discovery
default 1, recommended 1

This parameter switches path MTU discovery on or off. If you enter a 0 here, Solaris will never try to set the DF bit in the IP option - unless your application explicitly requests it.

tcp_ignore_path_mtu
default 0, recommended 0

This is a debug switch! When activated, this switch will have the IP or TCP layer ignore all ICMP error messages fragmentation needed. By this, you will achieve the opposite of what you intended.

tcp_mss_def
default 536, recommended >;= 536
Since 8: split into tcp_mss_def_ipv4 and tcp_mss_def_ipv6

This parameter determines the default MSS (maximum segment size) for non-local destination. For path MTU discovery to work effectively, this value can be set to the MTU of the most-used outgoing interface descreased by 20 byte IP header and 20 byte TCP header - if and only if the value is bigger than 536.

tcp_mss_def_ipv4
Since 8: default 536
tcp_mss_def_ipv6
Since 8: default 1460

Solaris 8 supports IPv6. Since IPv6 uses different defaults for the maximum segment size, one has to distinguish between IPv4 and IPv6. The default for IPv6 is close to what is said for tcp_mss_def.

tcp_mss_min
default 1, see text, (88 in Linux 2.4 and Win2k)
Since 8 with patch 108528-14 or above: 108

This parameter defines the minimum for the maximum segment size. While I still ponder the impl