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High Performance Enabled SSH/SCP News

Chris Rapier PSC, Michael Stevens CMU
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News/Updates

Update 11/7/2012

The patches for 6.0 and 6.1 have been released. These are obviously late being that I released both of them at the same time. The lack of cycles I have to focus on HPNSSH is getting to be a serious issue that I have to come up with a solution for. Essentially, I just don't have the time or the funding to continue the work that is required. All I've been doing is forward porting the patches without really trying to optimize, fix, or add anything substatial. While what I have seem to work 'good enough' there are lot more improvements that can be made to HPN SSH (pipelining the HMAC, exploring new efficiencies, dealing with the occasionally stalling problem, etc). Someday I'd really like someone to step up but I can't expect that if I'm not willing to lead on it. So that's where we are at the moment.

UPDATE 6/17/2012

John Smith was kind enough to point out a bug in the readconf.c file dealing with the hpn_buffer_size variable. This has been fixed for the 5.9 release only. It hasn't had much impact as far as anyone can tell so we aren't going back to fix up the older patches at this point. We will if necessary. Some peopel have been asking about the much delayed 6.0 patch. There really isn't anything serious with porting the patch I've just not had the time to do it. This will be rectified in the next week.

update 1/16/2012

The new HPN and enhancement patch sets for OpenSSH 5.9 have just been released. All patches, with the exception of the the threaded AES CTR cipher work as expected. As always, the AES CTR cipher fails the forwarding test. Thanks to Netanel Shine and Timo Teras for contributing the kitchen sink and noneswitch-dynamic_window patches. Extra thanks to Netanel for encouraging me to actually update the patches!

update 8/27/2010

The new HPN and enhancement patch sets for OpenSSH 5.6 have just been released. All patches have gone through regressions tests and perfrom as expected. As always, the caveat to this is that the threaded AES CTR cipher fails the forwarding test because of problems when the application is forked into the background. As always, we'd love to get some outside help on this if anyone is interested.

Update 6/08/2010

Two new patch sets have been released today to match OpenSSH 5.4 and 5.5. Both have been tested and work as expected. The problem with the multi-threaded cipher still exists so please use caution if you are interested in that functionality. Now, these patches were significantly delayed due to some changes here. The original patch set was undertaken with the help of grants from Cisco and funding from the National Science Foundation (NSF) and the National Library of Medicine (NLM). These funding sources have long since dried up and, being that I do this as part of my day job at PSC, without that funding I could only work on that in my spare time. Unfortunately, my spare time here at work has been in short supply for the past couple of years and, as a result, I haven't been able to do anything more than occaisionally update the patches to match the current release from OpenSSH. I've been looking into getting more funding but that's a dicey thing and until that happens a lot of the work I have planned (a more functional version of the multi-threaded cipher, code optimization, file resume, and so forth) are on the far far far back burner. I've been thinking about putting all of this on Google code or SourceForge and trying to attract a team of developers interested in carrying on what I've done. However, that requires time I don't really have right now. If anyone is interested in helping maintain this patch, working on building a team, or has idea about funding sources please send some mail to This email address is being protected from spambots. You need JavaScript enabled to view it. and let me know. Thanks for your continued interest.

Update 5/14/2009

HPN13 has been upgraded to v6 for use with OpenSSH 5.2. There is one significant caveat to keep in mind if you are using the multi-threaded AES-CTR patch available as a stand alone patch or as part of the kitchen sink version. If the client is forked to the background using the -f command line option the client process will hang. Essentialyl what is happening is that -f essentially demonizes the client process after the authentication process. Since the multi-threaded AES patch starts to generate the key stream prior to authentication this move kills our threads and the main thread simply hangs waiting to hear from threads it's already killed. As such the AES-CTR multithreading patch will not pass the regression tests included with OpenSSH. This probably isn't a show stopper for *most* people but everyone should be aware of this problem. There are a couple of different ways to resolve this problem but the developer who created this the multi-threading patch doesn't have the spare cycles to devote to it right now. We do hope to have this resolved shortly but we wanted to get this out. If this is *not* acceptable to you then you can still use the a la carte patches and simply exclude the AES-CTR patch. In that case everything works without fail.

Update 7.29.2008

HPN13 has been updated to v5 for use with OpenSSH 5.1. There are two minor changes in functionality. First, the progress meter will no longer spit out an extra line with the peak throughput. Instead the peak throughput will be displayed as the last update in the meter itself. Second, I've increased the number fo outstanding requests in sftp to 256. This will give users 8MB of outstanding data. If you think you neeed more then increase the number of request of the size of the buffer (-R and -B respectively in sftp). There are some internal changes which cuts down on some of the complexity of the patch. As far as I can tell there hasn't been any change that will be noticible to the user. However, I'm often wrong so if you run into a problem please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it.

Update 4.07.2008

HPN13 has been updated to v3. This is for OpenSSH 5.0. v2 had been released for OpenSSH 4.9 but was withdrawn due to a security problem in OpenSSH 4.9 We will not be making it available and we suggest everyone move to OpenSSH 5.0.

Update 2.07.2008

We've released the HPN13 set of patches to replace the HPN12 patch set. The main addition is a multi-threaded AES-CTR mode cipher. This allows the use of multiple processing cores (using posix threads) to create a cipherstream indistinguishable from the more common single thread AES-CTR mode cipher. As such its completely compatible with existing installations. We also made a change in how we distribute the patches. We'll be providing them in a 'kitchen sink' format and as 'a la carte' so users can apply the patches that are most useful to them.

Update 11.30.2007

The previous version of the HPN12 patch (revision 19) has been removed and replaced with revision 20. In some circumstances a buffer_append_space bug resulted from using the wrong size check value. There was also a typo which may have led to inappropriately large or small HPN buffers when using the HPNBufferSize option with sshd.

Update 09.28.2007

The previous version of the HPN12 patch (revision 18) has been removed and replaced with revision 19. This revision removes some nonfunctional code that had accumulated in the patch over the past couple of years. We also change the default behaviour of the TCPRcvBufPoll option to enabled. This will end up making things a bit easier for the people with autotuning kernels - which are generally in the majority now. People without autotuning should disable this option as its just wasting cycles for them. We also made a change which was allowing peopelto specify the none cipher for authentication. We included a failsafe that will not allow the use of the none cipher unless authentication has taken place. If it is attempted the conenction will close. We've also made a change where the HPN buffer will grow somewhat more aggressively than it has in the past.

Also, we have a short paper available on the work we've done on HPN-SSH. Its should be understandable to most users of the patch. High Speed Bulk Data Transfer on the Grid Using the SSH Protocol.

Update 09.05.2007

A new version of the OpenSSH was released today and so has a new version of the HPN-SSH patch. This patch doesn't provide any new functionality *but* this should be seen as an interim patch. In the next month I expect to have a new patch available for the 4.7p1 code base which will have some improvements. Also, not to jinx anything but I think we'll have an HPN13 patch set available in a few months. This will be of *real* interest to people in multicore and SMP environments.

Update 03.27.2007

I was made aware that you could have both NoneSwitch and NoneEnabled set in the ssh_config file. While this may be convenient for those people who always use the NONE cipher switch there is no way that this is acceptable default behaviour. So I changed the configuration so that you can only use the NoneSwitch option from the command line. I also changed the maximum value of HPNBufferSize to 64MB. This is only used in HPN to Non-HPN connections. Additionally, I ran some tests recently. I wanted to make sure that all of the changes hadn't introduced any bottlenecks into the code. So I took a stock version of OpenSSH and increased the default buffer size to 64MB (which was a two line change) and compared it to HPN12v16. The end result is that they provided statistically identical throughput performance over a long fast network. I still need to rerun the tests in a local area network to see if the short RTTs have an impact. More on that when it happen.

Update 02.20.2007

Very minor change. Some of the comments were in the C++ style as opposed to plain old C. This was causing it to choke on some compilers. I've changed everything to the C style comments.

Update 11.13.2006

Yet another default buffer size bug. This time I ngelected to make sure that the hpn buffer on the server side was correctly sized. In versions prior to v14 unless the HPN buffer size was explicitly set it used a default value of 2MB. As of now it will use the current TCP buffer size. I also discovered that when the hpn disabled option is set to yes the buffer is actually 128K and not 64K. I'll be working on that this week.

Update 10.31.2006

There was a documentation bug in the usage section of scp. This has been fixed in v13 release for 4.4p1. Happy halloween kids!

Update 10.23.2006

A bug was preventing some of the options in scp from working properly. This would have affected people using the -1, -2, -4, -6, and -C options. This has been fixed in the v12 release for 4.4p1.

Update 10.12.2006

Quite a few things have happened. First we found a number of minor errors in the code and one major error. None of these errors were security related though. Minor errors were conflicting messages about the state of the NONE cipher, some documentation problems, and the like. The major error was more problematic - due to a mistake in the way I was handling reading the configuration file the HPN buffer was being set to 2MB pretty much no matter what. This has been corrected in the 4.4p1 HPNv12 patch but *not* in the 4.3p2 HPNv9 patch. This won't be an issue for most people but people on very high seepd links might have been artificially throttled because of this. While fixing this mistake I also incorporated a new method for determining the maximum size of the HPN buffer. This means that the size fo the buffer will depend on, in part, configuration options the user chooses. These are explained in the HPN12-README file. The end result of this all should be that the buffer won't be any larger than it actually has to be.

I've started collecting data on HPN12 performance in terms of throughput, CPU usage, sensitivity to packet loss and the like. I hope to have a paper on this work in the next 3 or 4 months.

Update 7.26.2006

Two minor changes. First, the version string now includes major and minor version. As of this date it should report at hpn12v8. Second, there was a bug in the documentation in that it failed to explain that the 'NONE' cipher will only work on bulk data transfers and is silently disabled for all interactive sessions. Its a good compromise betweem performance and security.

Update 7.13.2006

Turns out that there was a bug in that the NoneSwitch configuration options wasn't accepting yes/no modifiers. The NoneSwitch was just acting as a flag instead of an option. So I fixed it so that -oNoneSwitch=yes/no works correctly. Otherwise no functionality was changed.

Update 5.19.2006

New version of the patch has been released. Please see the version history and the patch information for more details. Comments or questions are appreciated.

Update 4.3.2006

We just found out that there was a typo in the patch for OpenSSHv4.3 patch with the none switch. This would have caused a segmentation fault if the -R/-r option was used. This typo has been fixed in the latest version of the patch. Sorry about that.

Update 2.1.2006

We have just released a patch for OpenSSH v4.3. This is, again, in the HPN-11 cycle so there isn't any additional functionality added. However, due to command line switch namespace collision we have have to change the -w tcp receive window switch to -r in ssh and -R in scp.

Update 9.08.2005

We've just released a patch for OpenSSH v4.2. This is still in the HPN-11 cycle so there isn't any different or new functionality at this time.

Update 6.17.2005

We've just released an entirely new set of patches and removed all of the older versions of the patches from the main site. If you'd like to see the old site please go to http://www.psc.edu/networking/projects/hpn-ssh/old-site/. This set of patches brings OpenSSH 3.9, 4.0, 4.1 up to the same level of functionality and provides some significant improvements over the old patch. A new version checking system allows us to provide almost full HPN speeds even if both ends of the connection are not using HPN clients and servers. The speed improvement will only be in the direction of whichever side is HPN. So an HPN server will get a speed boost even if nonHPN clients connect to it. Additionally, we've added the '-w' switch. This allows the user to specify a TCP receive window size for that connection up to the maximum allowed on that host. This doesn't require root privileges nor will it effect other users on the system. Additionally, we've decided to make the 'None' cipher available on all versions using the '-z' switch. We no longer consider this 'experimental' code but it shoudl still be used at the users own risk. We think we've locked it up as much as possible but we may have missed something. Please keep in mind that even with the -z switch the authentication process is still fully encrypted only the bulk data transfer will happen in the clear. Additionally, MAC will still be performed which provides greater assurance of the integrity of the data.

Update 5.11.2005

I've just put out the patch set for OpenSSH 4.0p1. This is *experimental* and should be used with caution. We have found that under some circumstances one of the buffers used by scp can grow quite large rather quickly. This seems to be because the scp process run asymetrically from the ssh transport. However, there does seem to be a method to tell ssh to slow down if scp can't write the data to disk fast enough. We've also added a new feature we're callinh user adjustable windows. The -w [size of receive window in bytes] will set the local receive buffer of that specific connection to a user defined size. The size of this window cannot exceed the system defined maximum buffer size though. No special permissions are necessary to set the buffer size. It is very important to note that this will *only* set the the local receive buffer size. Its unlikley that users will ever be able to set the remote receive buffer size for obvious reasons.

Update 3.25.2005

In all likelihood this is the last OpenSSH 3.9 patch we'll be releasing. in this patch we remove all references to the 'unlimited' element we inserted in the buffer struct found in buffer.c. It turns out that this was basically unnecessary once we resolved some issues in channels.c. We've also updated channels.c to handle the largewindow bug we discovered in a more practical manner. Thanks to Darren Tucker for his help on that. Lastly, we made a few minor modifications to scp.c to bring the size of its read/write pipe buffers into closer alignment with the ssh read/write pipe/buffers. The end result of this is a 60% reduction in read/write syscalls when scp is the data source and a 20% reductions when it is the data sink.

Update 1.15.2005

We discovered a minor problem in that the experimental none cipher switching patch did not perform as expected. A failsafe which I thought was in place to prevent switching to the none cipher during interactive sessions wasn't performing as expected. The result being that it was possible to send data in the clear. However to do this the user would have to explicitly pass the -z switch to ssh. So if you didn't do that you did not send data in plaintext. However, I urge all users of the experimental patch to upgrade to the latest version as soon as possible. In the new patch I explicitly test for the tty_flag before the none cipher switch takes place. If the tty_flag is true then the cipher switch fails silently and the session continues with the original encryption cipher (this silent failure mode may change in future releases of this patch). Additionally, we now also check to make sure the -T (no_tty_flag) switch is not set before enabling the none cipher.

Update 1.12.2005

We've started putting hpn-ssh on more production systems so we've be able to get a better idea of how it might perform in the wild. Initial results look good but it did illustrate the need to have your network buffers tuned properly. hpn-ssh will not make ssh run faster if your system is mistuned. Essentially it can only work with what it has. Also, never forget that disk I/O operations will be a likely bottleneck in some systems. An interesting test is to transfer a large file to disk and then transfer the same file to /dev/null. If there is a notable difference in throughput you are disk bound. Also, we're about to start up more active development again. We hope to tighten up some code and rethink some assumptions. This will likely just be incremental improvements. However, we also have some other ssh projects we'll be persuing. More on that later in a couple of months.

Update 11.23.2004

Quick note here. The previously discovered problem with corrupted MACs on input actually stems from a hardware and/or driver bug with the intel e1000 cards. We think this fully justifies our continued use of HMAC even without data encryption - we'd not have found this bug without it. We have seen some problem with the syskonnect interface we replaced it with though. It turns out that under high loads the vm.min_free_kbytes is too low and crashes the interface. We set it to 12MB and it seems to work fine now.

Update 11.19.2004

Last night we conducted an edurance test using the cipher switching version of hpn-ssh. Using pipes we pushed data from /dev/zero on a host at NCSA to /dev/null on a host at PSC. We were able to move 4.343 terabytes of data in 17 hours 38 minutes at an average rate of 71MBps. Instantaneous tranfer rates over 100MBps were seen with some regularity. Please note that this test did not make use of the disk subsystem and therefore the results should be viewed as having occured under laboratory conditions. In more practical situations users will see their throughput being limited by the speed of their disk IO.

Update 11.12.2004

We're back from the Supercomputing Conference and I want to thank everyone who saw the poster and talked to me during the course of the past week. There has been a lot of interest and, hopefully, a lot of support will be forthcoming. In the meantime I've decided to release the combined hpn and none cipher patch. This is the first iteration of the patch and the none cipher support might be changing the near future. So long term compatablity is not assured. However, what this does is allow for midstream cipher switching. In particular the undocumented '-z' switch to scp will provide for encrypted authentication and unencrypted data transfer. We've seen speeds of over 692Mbps using this patch. This will *not* work with interactive (TTY) sessions - which you don't really want anyway. If the server doesn't support the none switch then the connection will fail silently. Regardless, use this patch at your own risk and view this as an experimental patch. Lastly, the use of the none cipher is *within* spec for the SSH v2.0 protocol. OpenSSH simply decided to not implement it for obvious reasons.

Update 10.28.2004

We'll be presenting this work at the SuperComputing 2004 Conference in Pittsburgh, Pennsylvania November 9-12. The poster presentation is officialy from 5-7pm on the 9th so if you want to stop by and say hello we'll be there. Also, you can see some of the throughput rates we've been getting by going to the Internet2 Weekly Top Flow Reports. We start showing up during the week of September 20, 2004. Look in the Non-Measurement flow section for flows between PSC-NCNE and NCSA using port 52222. We've recently hit 50MB/s and we were strictly processor limited.

Update 9.27.2004

We have uncovered some problems with using 9000 byte packets on the linux system. When we pull data from the linux system we consistently get MAC errors (ssh checksum indicating data corruption). We were able to recreate with an unpatched SSH server so it doesn't seem to be a result of our patch. Also, the previously mentioned user report of asymmetrical transfer rates was determined to be an issue with the system buffers and not the SSH code. If anyone else is seeing this behavior please let us know.

Update 9.24.2004

We resolved the problem on the linux 2.4 autotuning kernel by upgrading to 2.6. There also seems to be a minor problem with the way linux does memory accounting for the windows which might cause a problem with the rcv_ssthresh. However, users with non-autotuning kernels should not see a problem. As of this evening we were able to sustain 35MBps (280Mbps) in both directions. The OS X issue is still being explored.

Update 09/24/2004

We have recently heard of and experienced some problems with asymmetrical performance. The first is on a linux 2.4 autotuning kernel. In this case it seems that a tcp bug *might* be preventing the window from updating properly. On Mac OS X (10.3.4 1.33Ghz CPU) we were able to get through asymmetry down to a 2:1 ratio (8.0 MBps sending v. 3.5 MBps receiving)but there is indication of some CPU bounding issues. We have another report of a user also seeing highly asymmetrical throughput but we've yet to be able to determine the cause. Reports from other users would be *very* useful at this stage.

Version History

April 7, 2008: Released OpenSSH HPN-13 Patch version 3
Spooning up to OpenSSH5.0.
March 28, 2008: Released OpenSSH HPN-13 Patch version 2
Spooning up to OpenSSH4.9.
January 19, 2008: Released OpenSSH HPN-13 Patch version 1
Release of the MT-AES-CTR mode cipher.
Sept 28, 2007: Released OpenSSH HPN-12 Patch version 19
Addresses problems with none cipher in preauthentication contexts, removed some nonfunctional code, change defalt behaviour of TCPRcvBufPoll to enabled. Please see the above update for this date fr some more details. We'll also be trying to maintain the Version History a little more rigourously.
July 13, 2006: Released OpenSSH HPN-12 Patch version 7
Addresses minor problem with parsing command line. Please see the above update for this date.
May 19, 2006: Released OpenSSH HPN-12 Patch
This patch only covers version 4.3p2 at this time. Other versions will be available shortly. This patch was motivated by the desire to bring the code more in line with the OpenSSH style and nomenclature. Unfortunately these changes completely changed the command line structure but that was for the best as it helps avoid namespace collisions. Additional functionality was added by giving the user more control over buffer size, hpn functionality and hpn to non-hpn behaviour. These changes helped address a performance problem seen in some local area networks. Lastly, we no longer have a seperate NONE patch as we've integrated this into the main patch tree. NONE can be disabled on the server with a runtime anf configuration option so there was no need to have seperate patches. The HPN12-README file included in the patch explains the use and function in more detail.
June 17, 2005: Released OpenSSH HPN-11 Patch
This patch set covers OpenSSH 3.9p1, 4.0p1, and 4.1p1. Each patch provides the same functionality and full interoperability with other patches and standard SSH servers (OpenSSH and others). Changes have been made to buffer.c in order to increase the maximum allowable buffer size. We also rewrote a lot of the version checking system version.c in order to be smarter about how to handle interactions between HPN and STD SSH clients and servers. Its likely that no more major functionality changes will be incorporated in future patch versions.
May 11, 2005: Released OpenSSH 4.0 HPN Patch
This patch incorporates the previous improvements, works against the 4.0 code base, and introduces the -w option to set the local receive buffer size. No special permissions are needed to do this. Usage is -w [size of receive buffer in bytes]. This works for ssh and scp but not yet sftp. Please see the news section for an important message regarding a possible bug.
March 28, 2005: Fixed Typo in HPN Patch for 3.9
I had a comma in the wrong place. Sorry if you had problems with the original version. I decided not to update the version number for this very minor fix.
March 25, 2005: Updated HPN Patch for OpenSSH version 3.9p1
This patch removes some unnecessary changes in the buffer.c code specifically the buffer->unlimited member of the buffer struct. It also makes some minor changes to the buffer sizes used on the scp ssh pipe. The net result being a 20 to 60% decrease in the number of read/write syscalls in a typical transfer. This version of the 3.9p1 patch is also aware of OpenSSH version 4.0(p1).
January 15, 2005: Security Fix for HPN/None Cipher Experimental Patch
This is a security fix to the combined hpn/none cipher patch referenced in the experimental patch release of November 12, 2004. This patch explicitly checks to see if the tty_flag is set prior to switching to the none cipher. It also *disables* the none cipher switch if the -T (no_tty_flag) switch is used. Lastly, it also sends a warning to stderr when the switch to the none cipher takes place.
If you are not using the combined hpn/none patch then you do not need to apply this patch.
November 12, 2004: Combined HPN and None Cipher Experimental Patch
This is a combined patch for high performance networking through the use of dynamically resized buffers and the none cipher. This will only work with non-interactive sessions created by scp. With this patch the authentication IS encrypted and only after the auhentication key exchange is complete will the cipher switch to none for the bulk data transfer.
USAGE: scp -z file user@destination:path
September 15, 2004: Patch for OpenSSH 3.9p1
This patch will work cleanly with the OpenSSH 3.9p1 source code. Otherwise no changes to functionality were made.
July 20, 2004: Compatability Testing fix
This patch includes a test to check the version of the server. If the server does not incorporate the enhanced windowing routine it will be disabled. This will prevent the sshd buffer bug from being triggered in older versions of sshd.
July 13, 2004: sshd input buffer fix
Our code uncovered a bug in the manner in which the input buffer in sshd grew. It was possible to make the input buffer grow larger than a set maximum bound leading to a fatal exception for that sshd process. We’ve addressed this by explicitly checking the size of the buffer before allowing it to grow.
July 12, 2004: Window Size fix
  1. Based on coversations on the openssh developers mailing list we’ve imposed a maximum size on the network buffer of 2^30-1 bytes. Because of how the ssh code uses buffers this is an effective limit of 2^29-1 bytes or 512 Megabytes. This should be sufficient for all but the longest and fattest network paths. It might be possible to increase this by 1 bit in the future but this should be sufficient for now.
  2. The CVS and Portable patches have been rolled into one patch. It might throw a warning against the Portable version but it shouldn’t cause any problems.
July 7, 2004: Initial release
First release of openssh-3.8.1p1-dynwindow patch v. 0.1

This work made possible in part by grants from The National Science Foundation and Cisco Systems, Inc.