From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hagen Paul Pfeifer Subject: Re: Miscalculated TCP ACK ? Date: Mon, 22 Aug 2011 19:58:43 +0200 Message-ID: <20110822175843.GC3294@nuttenaction> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org To: Gabriel Beddingfield Return-path: Received: from alternativer.internetendpunkt.de ([88.198.24.89]:40585 "EHLO geheimer.internetendpunkt.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751174Ab1HVR6q (ORCPT ); Mon, 22 Aug 2011 13:58:46 -0400 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: * Gabriel Beddingfield | 2011-08-22 10:34:14 [-0500]: >I'm having trouble with a particular server via HTTPS. It appears >that my local linux machines are sending incorrect ACK. However, I >don't have enough expertise to know for sure. > >Using wireshark, the server sends: > > Transmission Control Protocol, Src Port: https (443), Dst Port: >36015 (36015), Seq: 27658, Ack: 827, Len: 18 > >Local machine replies: > > Transmission Control Protocol, Src Port: 36015 (36015), Dst Port: >https (443), Seq: 827, Ack: 27677, Len: 0 > >It appears to me that the ACK is off-by-one (should have been 27676). No, it is absolutely correct: ACK -> last seen Seq plus one (27658 + 18 + 1): RFC 793: If the ACK control bit is set this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always sent. Cheers, Hagen