Browser Connections Appear to Stop Responding When Accessing IIS Over SSL

Under certain conditions, a browser may appear to stop responding (hang) when you attempt to connect to an IIS Web server over an SSL connection. However, other browsers may be able to connect successfully from a different location (for example, a different physical connection than the failing browser). No error messages or events occur, and IIS works as expected.

This problem can occur if the client is on a network that does not allow TCP packets above a certain payload (for example, 1,400 bytes) and the Web server is trying to send TCP packets that are over this limit (for example, 1,460 bytes). This can be caused by a middle-tier device, such as a Load Balancer, when it is unable to forward ICMP packets that are destined for the Web server where it needs to fragment a packet (or set the packet size smaller) in order to successfully transmit it. This causes the Web server to continually send TCP Acknowledgements to the client as retransmits; however, these packets never get back to the client. This is known as a "black hole" router.

Under usual circumstances, this should be addressed at the router or middle-tier device; however, in some cases, this cannot be done and will need to be addressed at the Web server itself.

To verify that this is indeed the problem, Microsoft recommends that you issue the following ping command to the client that is failing from the Web server: It is best to test this with the largest packet size possible (for example, 1,490 bytes), and then work down to find the largest size that will pass through without fragmentation. For additional information, click the article number below to view the article in the Microsoft Knowledge Base: If you need to make a modification for TCP payload, it may be necessary to set the Max Transfer Unit (MTU) to a lower value by adding the following registry entry: In addition, it may be beneficial to implement the following registry parameters in place of the one above, so that the payload is modified if TCP Acknowledgements go unanswered: EnablePMTUBHDetectEnablePMTUDiscoveryFor additional information regarding these registry entries, click the article number below to view the article in the Microsoft Knowledge Base:

These same symptoms may occur with non-SSL connections. However, it is much less likely because SSL tends to have a larger payload. If you suspect you are having this problem with non-SSL connections, use the same procedures described in the "Resolution" section of this article, as well as the information in this section.

By obtaining a Network trace, you can determine if there is a pattern as to why this condition is occurring. To install and run Network Monitor, see:

By reviewing a Network trace between the Web server and the middleware device (Load Balancer), you may be able to determine if there is anything that could cause the condition at the client. This is an SSL enabled connection. Therefore, all data will be encrypted, but will not be a total loss. The first thing to look for is the presence of any TCP retransmits. These are identical packets that are sent repeatedly due to no response from the remote connection (client). These packets will look similar to the following example: There may be four or five (maybe more) of these exact packets that are sent to the client without any acknowledgement. This indicates that a TCP retransmit is occurring. The next thing to look for is the length of the payload denoted by len: <value> (1,460 bytes in the example above). Because all of the other data is passing through at much smaller amounts and the only thing retransmitted are these larger packets, it indicates that these packets are being dropped, most likely at a router or a middle-tier device. This can also be confirmed with a trace on the client side to determine if these packets indeed are arriving there. If not, a drop condition is occurring somewhere between the client and server. By following the steps outlined in the "Resolution" section, you may be able to determine the proper size that will pass through successfully.