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Saturday, March 30, 2013

TCP Chimney Offloads and SQL Server Implementation

A lot of us often see the following type of sporadic messages although everything is running as usual. After the application or user receives the network related messages, there will be some trouble occurring with the system however it will behave normally again after each event.

Some logged messages can be found in SQL Server Error Log and Windows Event logs, some can be popped up within the application and others can be detected by querying the SQL Server Extended Event.

Some observed Error Messages:
·        ERROR [08S01] [Microsoft][SQL NativeClient]Communication link failure
·        System.Data.SqlClient.SqlException: A transport-level error has occurred when sending the request to the server. (provider: TCP Provider, error: 0 - An existing connection was forcibly closed by the remote host.)
·        Network error 0x40 occurred while sending data to the client on process ID XX batch ID 0. A common cause for this error is if the client disconnected without reading the entire response from the server. This connection will be terminated.
·        A transport-level error has occurred when receiving results from the server. (provider: TCP Provider, error: 0 - The semaphore timeout period has expired.)
·        A fatal error occurred while reading the input stream from the network. The session will be terminated (input error: XXX, output error: XXX).
·        Network error code 0x2746 occurred while establishing a connection; the connection has been closed. 

Possible mystery behind Network Latency:
There could be a number of reasons why this happens. Following are some of the potential areas where network latency and overheads can occur sporadically or continuously.

A.      Implicit Transaction: Implicit transaction mode generates a continuous chain of transactions which causes a delay to commit or rollback each transaction. This significantly degrades the performance and reduces concurrency in a high throughput OLTP system.

B.      Network bandwidth bound application: Database Mirroring, Always ON and Transitional Replication are usually Network bound application processes. Reading and writing large amounts of data or documenting continuously may increase latency and degrade network performance.

C.      TCP Chimney offloads: “TCP Chimney Offload transfers Transmission Control Protocol (TCP) traffic processing, such as packet segmentation and reassembly processing tasks, from a computer’s CPU to a network adapter that supports TCP Chimney Offload. Moving TCP/IP processing from the CPU to the network adapter can free the CPU to perform more application-level functions. TCP Chimney Offload can offload the processing for both TCP/IPv4 and TCP/IPv6 connections if supported by the network adapter”. Thus if this setting is incorrect on the both server OS and NIC level, then performance issues are guaranteed.

D.     NIC Teaming: "NIC teaming makes two or more physical NICs appear as a single virtual one to the application, which isolates application from failures of an individual NIC. There are two types of teaming:

1.    Switch independent (the switch does not know the NICs are teamed) and
2.    Switch dependent (the network switch participates in the teaming).

NIC teaming can also result in bandwidth aggregation such that, for example, four 1GB/Sec NICs can provide an aggregate 4GB/Sec throughput. In the event of problems, the teaming has to be disabled in order to verify it is not causing the problem."

E.      Jumbo Frames: Jumbo frames are only available on gigabit networks, and all devices in the network path must support them (switches, routers, NICs, and so on). If all the networking hardware doesn't support end-to-end Jumbo Frames they should not be used and should be disabled.

"Jumbo frames" extends Ethernet to 9000 bytes and is large enough to carry an 8 KB application datagram (e.g. NFS) plus packet header overhead. If Jumbo frames is supported on a Gigabit Network then “Network Packet Size” configuration in SQL Server can be increased to 8192 in a high throughput environment.

To check whether the jumbo frames is supported by the target machine, execute following command:

ping <IP or server name> -f –l 9000

Figure: Jumbo frames support check

F.       Maximum Transmission Unit (MTU): The Maximum Transmission Unit (MTU) feature is an advanced configuration that allows determining the largest data size permitted on a network connection. Generally, if the MTU is too large for the connection, computer will experience packet loss or dropping connection.

To determine the correct MTU size for a network, we have to do a specific ping test on the destination (target server).For Windows environment, use the following command for the ping test:

Syntaxping [server or IP address] –f –l xxxx
Example: ping –f –l 1472

This test can start from 1472 until we can reach the exact packet size which will not result to the prompt “Packet needs to be fragmented but DF set.” You may adjust (decrease/increase) the value by 10.

Implicit Transaction and TCP Chimney OffLoads:
The DML centric application which mostly depends on implicit transactions of SQL Server generates a lot of LOG I/O which initiates further delay on log writing process and introduces network latency.  Turning of TCP Chimney offloads is one of the “first things to do” that can be considered in order to eliminate any possibilities in this area.

“TCP Chimney Offloads” functionality does not work with virtualized Windows OS (vmware and hyper-v), therefore it can be disabled permanently.

This is one of the potential overlooked areas by many sysadmin, which needs further investigation if the received error message looks similar to the issues which have been mentioned before in the article. Although it is not the best practice, it is still recommended by many Network Experts that the “TCP Chimney Offloads” setting must be disabled in both Windows OS and NIC level to reduce latency in SQL Server implementation whether it is virtual or physical.

Windows OS and TCP Chimney offloads:
In Windows 2003 - TCP Chimney Offload is enabled by default.
In Windows 2008 - TCP Chimney Offload is disabled by default.
In Windows 2008 R2 - TCP Chimney Offload is automatic by default.
In Windows 2012 -TCP Chimney Offload is disabled by default.

How to check TCP Chimney Offloads:
·         In Windows 2003 use the following command:
netsh interface ip show offload
·         In Windows 2008 and above use the following command:
netsh int tcp show global

Figure: Screenshot from Windows 2008 R2

Screenshot from Windows 2012

XE Events (Extended Events) Query:
** Errors reported along with the error counts


-- Store the XML data in a temporary table
INTO    #xeTmpTbl
FROM    sys.dm_xe_session_targets xet
        JOIN sys.dm_xe_sessions xe ON xe.address = xet.event_session_address )
WHERE = 'system_health'

-- Get statistical information about all the errors reported
;WITH    myXEinfo EventXML )
          AS SELECT   C.query('.'EventXML
               FROM     #xeTmpTbl a
                        CROSS APPLY a.XMLDATA.nodes('/RingBufferTarget/event'AS T C )
        myXEErrorInfo EventTimeErrorNum )
          AS SELECT   EventXML.value('(/event/@timestamp)[1]''datetime'AS EventTime ,
                        EventXML.value('(/event/data/value)[1]''int'AS ErrorNum
               FROM     myXEinfo
               WHERE    EventXML.value('(/event/@name)[1]''varchar(255)') = 'error_reported'
    SELECT  ErrorNum ,
            MAX(EventTimeAS LastRecordedEvent ,
            MIN(EventTimeAS FirstRecordedEvent ,
            COUNT(*) AS Occurrences ,
            b.[text] AS ErrDescription
    FROM    myXEErrorInfo a
            INNER JOIN sys.messages b ON a.ErrorNum = b.message_id
    WHERE   b.language_id = SERVERPROPERTY('LCID')
    GROUP BY a.ErrorNum ,

--  Get information about each of the errors reported
WITH    myXEinfo EventXML )
          AS SELECT   C.query('.'EventXML
               FROM     #xeTmpTbl a
                        CROSS APPLY a.XMLDATA.nodes('/RingBufferTarget/event'AS T C )
               WHERE    C.query('.').value('(/event/@name)[1]''varchar(255)') = 'error_reported'
    SELECT  EventXML.value('(/event/@timestamp)[1]''datetime'AS EventTime ,
            EventXML.value('(/event/data/value)[1]''int'AS ErrNum ,
            EventXML.value('(/event/data/value)[2]''int'AS ErrSeverity ,
            EventXML.value('(/event/data/value)[3]''int'AS ErrState ,
            EventXML.value('(/event/data/value)[5]''varchar(max)'AS ErrText
            --EventXML.value('(/event/action/value)[2]', 'varchar(10)') AS Session_ID
    FROM    myXEinfo
    ORDER BY EventTime DESC
--  Drop the temporary table

** Extract Ring Buffer Information for SQL Server 2008 instances and above
SELECT  CONVERT (VARCHAR(30), GETDATE(), 121) AS run_time ,
        DATEADD(ms, a.[record_time] - sys.ms_ticks ), GETDATE()) AS [notification_time] ,
        a.* ,
        sys.ms_ticks AS [current_time]
FROM    SELECT    x.value('(//Record/Error/ErrorCode)[1]''varchar(30)'AS [error_code] ,
                    x.value('(//Record/Error/CallingAPIName)[1]''varchar(255)'AS [calling_API_name] ,
                    x.value('(//Record/Error/APIName)[1]''varchar(255)'AS [API_name] ,
                    x.value('(//Record/Error/SPID)[1]''int'AS [SPID] ,
                    x.value('(//Record/@id)[1]''bigint'AS [record_id] ,
                    x.value('(//Record/@type)[1]''varchar(30)'AS [type] ,
                    x.value('(//Record/@time)[1]''bigint'AS [record_time]
          FROM      SELECT    CAST (record AS XML)
                      FROM      sys.dm_os_ring_buffers
                      WHERE     ring_buffer_type = 'RING_BUFFER_SECURITY_ERROR'
                    ) AS R x )
        ) a
        CROSS JOIN sys.dm_os_sys_info sys
ORDER BY a.[record_time] ASC

·         Using TCP Chimney Offload
·         TCP Offloading again?!
·         Network Changes Affect Windows Server 2012
·         Information about the TCP Chimney Offload, Receive Side Scaling, and Network Direct Memory Access features in Windows Server 2008
·         Error message when an application connects to SQL Server on a server that is running Windows Server 2003: "General Network error," "Communication link failure," or "A transport-level error"
·         How to Disable TCP Chimney, TCPIP Offload Engine (TOE) or TCP Segmentation Offload (TSO).
·         TCP Chimney Offload – Possible Performance and Concurrency Impacts to SQL Server Workloads


  1. This is a terrific bit of information to know about. When sysadmins are not in the same shoes as SQL DBA's, configuration items like this can unnoticed during performance troubleshooting. Many thanks!
    Jeff Bennett
    SQL DBA, Missouri, US

  2. very good article, nice work.

  3. It is extremely interesting for me to read the article. Thank you for it. I like such topics and everything connected to them. I would like to read a bit more on that blog soon.
    Ultra-low latency MAC

  4. The two most important protocols are TCP offload (Transmission Control Protocol) and IP (Internet Protocol), which are what give the whole name.
    TCP offload.

    Thanks ...

  5. great information!
    I had a customer with a application error ID 4014
    "A fatal error occurred while reading the input stream from the network. The session will be terminated (input error: XXX, output error: XXX)"
    in ring buffer conncetivity I saw a lot of NetworkErrorFound or DisconnectDueToReadError
    after disabling NIC teaming and disabling TCP CHimney Offload in network card and operating system (server 2008R2) problem disapper
    Users are happy :]

    thanks!! great BLOG

  6. Hi,
    Is sufficient just to disable TCP chimney offload at the OS level without disabling it at the virtual network card (vmxnet3) level ?