Latest release of The Server Framework: 6.8

Version 6.8 of The Server Framework was released today.

This release includes important bug fixes, see here. It also includes lots of code change due to: the removal of support for Visual Studio 2012, adding support for Visual Studio 2017 and the results of lots of static analysis.

This release is essential for users of Release 6.7.

Bug in multi-buffer writes in 6.7


A bug has been discovered in Release 6.7 in the code that deals with TCP socket writes that involve more than a single buffer. These 'multi-buffer writes' are writes that involves either a buffer chain or a block of data passed as a pointer and a length where the length exceeds the size of the buffer allocator that the connection is using.

The bug prevents the 'multi-buffer write' from being executed as a single atomic write at the network layer and so can cause corruption of a TCP data stream if multiple sockets are writing to the same connection concurrently.

The bug is due to the removal in 6.7 of the code required to support Windows XP. In Windows XP we needed to include sequence numbers in write operations to allow for the way we always marshalled all I/O operations from the calling thread to an I/O thread to prevent I/O cancellation due to thread termination. This write sequencing code had the side effect of also protecting 'multi-buffer writes' from being interrupted by other writes.

The fix does not require the reintroduction of write sequencing but, instead, issues a single scatter/gather style write for the entire buffer chain. This is both efficient and correct.

A related bug also affects atomicity of 'multi-buffer writes' into filter layers, such as the SSL code. Similar fixes have been applied here.

The bug is fixed in Release 6.8 which will be released later today.

Supporting Visual Studio 2015 Update 3


Visual Studio Update 3 adds some new warnings for precompiled header generation. One of these, C4598, will prevent precompiled header builds of framework code due to our warnings as errors policy.

A fix for this issue is to add the new warning to the list of disabled warnings in the project file. The easiest way to do this is to do a search and replace across your source tree for *.vcxproj files and replace "4627;4654;" with "4627;4654;4598;".

I need to work out what the implications of this compiler change are on how our precompiled headers are generated.

Latest release of The Server Framework: 6.7

Version 6.7 of The Server Framework was released today.

This release is mainly a code simplification release which removes support for legacy compilers and operating systems. See here for more details. However, there are some breaking changes where smart buffers have replaced buffer references and this causes function signature changes. In addition there has been considerable churn in the Streaming Media Option Pack with knock on changes in the HTTP library code which needed to be made more complete to deal with serving HLS streams.

I hinted at the end of the last post that the 6.7 release might increase performance a little. Well, whilst the bulk of the changes in 6.7 are purely code cleaning and the removal of legacy support there is a fairly major functional change as well.

In most situations references or pointers to I/O buffers have been replaced with smart pointers. This change may cause some issues during an upgrade as you need to change some function signatures from IBuffer refs to CSmartBuffers. The advantage is that in many servers there will no longer be any need for buffer reference counting during normal I/O operations.

The Server Framework relies on reference counting to keep the objects that are used during the asynchronous operations alive until those operations complete. So we increment a counter on the socket object and also on the buffer object when we initiate an operation and then decrement the counters when the operation completes. I'm sure there are other ways to manage the object lifetime but this has worked well for us.

The problem is that these increments, although they look like cheap operations, can be quite expensive, especially on NUMA hardware.

Whilst there's not much we can do about the reference count on the socket object, the buffer doesn't really need to be reference counted most of the time. Or, more's the point. The initial reference can (and should) be passed along rather than each stage taking and releasing its own reference. With a buffer you generally only want to be accessing it from one thread at a time and so you allocate it and then issue an operation and pass the reference you have off to the operation. When the operation completes the code captures the reference and takes ownership of it and then the buffer can be processed. If you're lucky you can then use the same buffer for a response to the operation and pass it back to the framework again.

This requires a few changes to your code but it's fairly straight forward. Your OnReadCompleted() handler will give you a CSmartBuffer and if you want to retain ownership of it after the handler returns then you simply need to detach the buffer from the CSmartBuffer you were given.

This is only "potentially faster" as it really depends on the structure of your server and how you deal with our I/O buffers but the framework is no longer standing in the way of this kind of optimisation, and we've removed a couple of reference adjustments in the normal operation flow.

Another release is coming...


We've only just shipped Release 6.6.5 of The Server Framework but we already have another release that's just about to ship. This isn't because some horrible bug has slipped through our testing, it's because we've been planning to produce a 'clean up' release for some time. 6.7 is that release.

Lets be straight here, 6.7 is a release for us more than for you. The aim is to simplify our build/test and release process, remove dead code whilst introducing no new bugs and removing no functionality that you rely on.

So what does 6.7 achieve. Well, for a start we drop support for Visual Studio 2005 and 2008 and also for Windows XP. Removing support for these legacy compilers and operating systems means that we can remove all the code that was required just to support them. This massively simplifies our code base without removing anything that the code actually relies on to run on modern operating systems.

Windows Vista introduced massively important changes to asynchronous I/O and we have supported these changes for a long time (over 8 years!). The code required to jump through hoops to make code running on Windows XP behave was complex. For example, Windows XP would cancel outstanding I/O requests if the thread that issued them exited before the I/O request completed. We had a marshalling system in place to ensure that I/O operations were only ever executed on threads that we controlled so that you'd never be faced with unexpectedly cancelled operations. All of that can go now.

Removing XP also means we no longer need to maintain an XP machine in our build farm. It's one less configuration that needs to be built and tested before a release.

Dropping support for VS2005 and 2008 removes 4 complete sets of builds (x86 and x64 for each compiler) plus all of the conditional code that was required to support the older compilers. At last we can start moving towards a slightly more modern C++, perhaps.

Some old code has been removed; there's no need, on modern operating systems, to share locks. This worked really well back in the day, but, well, we were running on Windows NT at the time and resources were much more limited than they are now. All of the "Shared Critical Section" code is now gone. This has knock on effects into the Socket Tools library where all of the shared lock socket code has been removed. Nobody should be using that in 2016 anyway! You can no longer set a critical section's spin count in the socket allocator, it never really worked anyway as the lock was used for too many different things.

Some experimental code has also been removed; The TLS and Low Contention buffer allocators are gone. The horrible "dispatch to all threads" cludge has been removed from the I/O pools (it was only there to support pre-Vista CancelIO() calls which are no longer needed now that we have CancelIOEx()).

The original callback timer queue that was based on GetTickCount() and which spawned Len's "Practical testing" series of blog posts (back in 2004!) has gone. There's no need for the complexity when all supported operating systems have GetTickCount64().

Finally we've slimmed down our set of example servers. Removing servers which didn't add much value or which duplicated other examples. Again, this speeds our release process by speeding up the build and test stage as there are fewer servers to build and fewer tests to run.

So, what's in it for you? Well, a faster build/test/release cycle so new functionality and bug fixes can be released quicker and potentially faster code in some circumstances. There's no great rush to upgrade if you don't want to, but we'll be focusing on the 6.7 code base going forwards.

Version 6.6.5 of The Server Framework was released today.

This release is mainly a feature release with a few bug fixes.

A while back a client of mine had an issue with a TLS 1.2 connection failing during the TLS handshake. We couldn't see any issues with the code and if we only enabled TLS 1.1 on the server then the connection handshake worked just fine.

Eventually we tracked the issue down to the fact that the certificate in use had been signed with MD5 and that MD5 isn't a valid hash algorithm for TLS 1.2 and so the handshake failed when SChannel attempted to validate the certificate and found that it was unacceptable. There's a Microsoft blog posting about this problem here.

Annoyingly the error message for this is: "The client and server cannot communicate, because they do not possess a common algorithm." which is bang on the money, but doesn't help much until after you've worked out what the problem is. We were already dumping out the enabled algorithms and protocols and couldn't understand why the connection either wasn't working or wasn't downgrading to TLS 1.1.

I've now added some certificate investigation code to the point where my example SChannel servers set up their contexts. This at least allows me to warn and degrade to TLS 1.1 if the certificate is not going to work with TLS 1.2. In production systems I expect we'd just fail to start up.

   CCredentials::Data data;

   data.cCreds = 1;
   data.paCred = &pCertContext;

   // Note that for TLS 1.2 you need a certificate that is signed with SHA1 and NOT MD5.
   // If you have an MD5 certificate then TLS 1.2 will not connect and will NOT downgrade
   // to TLS 1.1 or something else...

   DWORD dataSize = 0;

   if (::CertGetCertificateContextProperty(
      ByteBuffer buffer(dataSize);

      BYTE *pData = buffer.GetBuffer();

      if (::CertGetCertificateContextProperty(
         const _tstring signHashDetails(reinterpret_cast<tchar *>(pData), dataSize);

         if (signHashDetails.find(_T("MD5")) != _tstring::npos)
            if (enabledProtocols == 0 || enabledProtocols & SP_PROT_TLS1_2)
               OutputEx(_T("Certificate is signed with: ") + signHashDetails);
               OutputEx(_T("MD5 hashes do not work with TLS 1.2 and connection downgrade does NOT occur"));
               OutputEx(_T("This results in failure to connect. Disabling TLS 1.2"));

               if (enabledProtocols)
                  enabledProtocols &= ~SP_PROT_TLS1_2;

               if (!enabledProtocols)
                  // if enabledProtocols is zero then all protocols are
                  // enabled, so we need to explicitly enable everything
                  // except TLS1.2

                  enabledProtocols = SP_PROT_SSL3TLS1;

   data.grbitEnabledProtocols = enabledProtocols;

Latest release of The Server Framework: 6.6.4

Version 6.6.4 of The Server Framework was released today.

This release is mainly a bug fix release for clients using WebSockets over SSL.

Latest release of The Server Framework: 6.6.3

Version 6.6.3 of The Server Framework was released today.

This release is mainly a bug fix release but it also adds support for Visual Studio 2015 and Windows 10 (though we don't explicitly use any Windows 10 APIs). There are quite a lot of small changes due to us running Gimpel Lint over the code. Most of the changes will have fixed potential issues rather than issues that have actually been reported.

However, there are two serious fixes for the SChannel Option Pack for an issue that was introduced in Release 6.6.2 and which could cause corruption of the data flow in some situations and stalling and CPU pegging in another. There is also a fix which may prevent UDP datagram corruption, See here for more details.

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About this Blog

I usually write about the development of The Server Framework, a super scalable, high performance, C++, I/O Completion Port based framework for writing servers and clients on Windows platforms.

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