What is the blocker for TLS 1.2 for Mac OS RTC Client

I think I have found the solution and there is no need to raise an enhancement request - the RTC 6.0.3 Client for Eclipse 4.2.x IDE works perfectly with TLSv1.2, and it even comes bundled with a JDK 1.7 (Oracle Java SE 1.7.0_101).

The whole thing appears to be related to the Eclipse 3.6 RCP that the older versions of RTC are based on. This version of Eclipse can only execute (Apple) Java 1.6, which does not provide TLSv1.2 support. I can confirm that it refuses to honor the "-vm" parameter in either the eclipse.ini or the Info.plist file.
https://bugs.eclipse.org/bugs/show_bug.cgi?id=339788

It is easy to see this by adding or modifying the below line in the eclipse.ini file.

-Dosgi.requiredJavaVersion=1.7

You will then get the below error message when launching RTC. Note that 1.6.0_65 is the Apple Java version, and all other installed JVMs (OpenJDK and Oracle Java) are ignored.

Version 1.6.0_65 of the JVM is not suitable for this product.

So, if you can use RTC 6.0.3 Client for Eclipse 4.2.x IDE, use it. Otherwise, use an Eclipse 4.2 base and install RTC on top of it - verify that the Eclipse base can be launched with Java 7 or 8 first.

You should raise an enhancement request to the RTC development, I believe. If I enable debugging, here is what I see in the .log file.

!ENTRY com.ibm.team.repository.transport.client.SSLUtils 0 500 2017-04-13 15:02:01.206
!MESSAGE SSL Algorithm Priority: [TLSv1.2]

!ENTRY com.ibm.team.repository.transport.client.SSLUtils 0 500 2017-04-13 15:02:01.206
!MESSAGE Creating TLSv1.2 context

!ENTRY com.ibm.team.repository.transport.client.SSLUtils 0 500 2017-04-13 15:02:01.207
!MESSAGE TLSv1.2 SSLContext not available
!STACK 0
java.security.NoSuchAlgorithmException: TLSv1.2 SSLContext not available
    at sun.security.jca.GetInstance.getInstance(GetInstance.java:142)
    at javax.net.ssl.SSLContext.getInstance(SSLContext.java:125)
    at com.ibm.team.repository.transport.client.SSLUtils.createSSLContext(SSLUtils.java:207)
    at com.ibm.team.repository.transport.client.SSLUtils.createSSLContext(SSLUtils.java:168)
    at com.ibm.team.repository.transport.client.SSLUtils.createSSLContext(SSLUtils.java:135)
    at com.ibm.team.repository.client.util.FormBasedAuth.setupSSLSupport(FormBasedAuth.java:331)
    at com.ibm.team.repository.client.util.RepositoryUtil.getRepositoryId2(RepositoryUtil.java:109)
    at com.ibm.team.repository.client.internal.TeamRepositoryService.connectToAndUpdateTeamRepository(TeamRepositoryService.java:401)
    at com.ibm.team.repository.client.internal.TeamRepositoryService.getTeamRepositoryWithOverride(TeamRepositoryService.java:180)
    at com.ibm.team.process.rcp.ui.RepositoryCreationPage.createNewRepository(RepositoryCreationPage.java:235)
    at com.ibm.team.process.rcp.ui.RepositoryCreationPage.create(RepositoryCreationPage.java:201)
    at com.ibm.team.process.rcp.ui.RepositoryCreationPage$1.run(RepositoryCreationPage.java:130)
    at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:121)

!ENTRY com.ibm.team.repository.transport.client.SSLUtils 0 500 2017-04-13 15:02:01.207
!MESSAGE Failed to create TLSv1.2 context

!ENTRY com.ibm.team.repository.transport.client.SSLUtils 0 500 2017-04-13 15:02:01.207
!MESSAGE No acceptable encryption algorithm found

!ENTRY com.ibm.team.repository.client 1 800 2017-04-13 15:02:01.208
!MESSAGE CRJAZ0053I The identifier for the repository identified by "https://clm603.example.com:9443/ccm" could not be contacted: No acceptable encryption algorithm found

(The last message is what I see on the RTC UI).

While not the exact source code, you can see the logic of the code here.
https://github.com/jenkinsci/teamconcert-plugin/blob/master/com.ibm.team.build.hjplugin/src/main/java/com/ibm/team/build/internal/hjplugin/util/SSLContextUtil.java

So the error is thrown well before an SSL handshake is made. I can confirm that there is no TCP/IP traffic happening before the error occurs.