{"id":2219,"date":"2018-09-09T16:43:44","date_gmt":"2018-09-09T16:43:44","guid":{"rendered":"http:\/\/nenadnoveljic.com\/blog\/?p=2219"},"modified":"2018-09-14T07:45:30","modified_gmt":"2018-09-14T07:45:30","slug":"tcp-storm-in-sql-server","status":"publish","type":"post","link":"https:\/\/nenadnoveljic.com\/blog\/tcp-storm-in-sql-server\/","title":{"rendered":"TCP Storm in SQL Server"},"content":{"rendered":"

In this blog post I’ll be demonstrating how a TSQL loop can cause a massive amount of TCP packets going from SQL Server to client. I used Wireshark to record the traffic and then the Tabular Data Stream (TDS) dissector for reconstructing the TDS stream. <\/p>\n

First, let’s examine the exchange between client and server in a normal case. <\/p>\n

The following loop has a 1 second delay:<\/p>\n

WHILE 1 = 1\r\nBEGIN\r\n  IF 1 = 0 SELECT 1\r\n  WAITFOR DELAY '00:00:01'\r\nEND;<\/code><\/pre>\n
And its execution will produce the following messages:<\/p>\n
Source      Destination Protocol Length Info\r\nclient      server      TDS      112    SQL batch\r\nserver      client      TDS      91     Response\r\nclient      server      TDS      234    SQL batch\r\nserver      client      TCP      54     49501 \u2192 58205 [ACK] Seq=38 Ack=239 Win=8207 Len=0<\/code><\/pre>\n
As you can see, there were two batches sent from the client. One is the implicit “select from @@SPID” command. The other is our endless loop. After receiving the SQL batch, the server sent a zero size ACK to the client. There weren’t any other messages while the loop was running.<\/p>\n
But something strange happened after removing the delay from the loop:<\/p>\n
WHILE 1 = 1\r\nBEGIN\r\n  IF 1 = 0 SELECT 1\r\nEND;<\/code><\/pre>\n
After receiving the SQL batch from the client, the SQL Server started sending an “endless” byte stream instead of a zero size ACK.<\/p>\n
Source      Destination Protocol Length Info\r\nclient      server      TDS      112    SQL batch\r\nserver      client      TDS      107    Response\r\nclient      server      TDS      178    SQL batch\r\nserver      client      TCP      1514   49501 \u2192 63979 [ACK] Seq=54 Ack=183 Win=8211 Len=1460 [TCP segment of a reassembled PDU]\r\nserver      client      TCP      1514   49501 \u2192 63979 [ACK] Seq=1514 Ack=183 Win=8211 Len=1460 [TCP segment of a reassembled PDU]\r\nserver      client      TDS      1230   Response (Not last buffer)\r\nserver      client      TCP      1514   49501 \u2192 63979 [ACK] Seq=4150 Ack=183 Win=8211 Len=1460 [TCP segment of a reassembled PDU]\r\nserver      client      TCP      1514   49501 \u2192 63979 [ACK] Seq=5610 Ack=183 Win=8211 Len=1460 [TCP segment of a reassembled PDU]\r\nserver      client      TDS      1230   Response (Not last buffer)\r\n...<\/span><\/code><\/pre>\n
This anomaly can be reproduced on SQL Server 2014, 2016 and 2017. On SQL Server 2014, it can even cause a serious performance bottleneck when TDS encryption is turned on, because every encrypted message will result in a context switch. Fortunately, there isn’t such context switching in the later releases. <\/p>\n
By the way, I elaborated on this in my previous blog post<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
Beware of TCP storms in SQL Server, especially in SQL Server 2014 where they can cause a serious performance problem. Continue Reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[17,23],"tags":[],"class_list":["post-2219","post","type-post","status-publish","format-standard","hentry","category-sql-server","category-tcp-ip"],"yoast_head":"\nTCP Storm in SQL Server - All-round Database Topics<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/nenadnoveljic.com\/blog\/tcp-storm-in-sql-server\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"TCP Storm in SQL Server - All-round Database Topics\" \/>\n<meta property=\"og:description\" content=\"Beware of TCP storms in SQL Server, especially in SQL Server 2014 where they can cause a serious performance problem. 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