From owner-freebsd-questions Wed Feb 19 6:14:28 2003 Delivered-To: freebsd-questions@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 13A3A37B401 for ; Wed, 19 Feb 2003 06:14:26 -0800 (PST) Received: from angelo.kcl.ac.uk (angelo.kcl.ac.uk [137.73.66.5]) by mx1.FreeBSD.org (Postfix) with ESMTP id D726643F3F for ; Wed, 19 Feb 2003 06:14:24 -0800 (PST) (envelope-from dev.dhas@kcl.ac.uk) Received: from ctr-Dev.kcl.ac.uk (EE077.eee.kcl.ac.uk [137.73.10.124]) by angelo.kcl.ac.uk with ESMTP id h1JDXse2000036 for ; Wed, 19 Feb 2003 13:33:57 GMT Message-Id: <5.2.0.9.0.20030219134120.00b0caa8@pop2.kcl.ac.uk> X-Sender: kkqd2740@pop2.kcl.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.2.0.9 Date: Wed, 19 Feb 2003 13:41:57 +0000 To: freebsd-questions@FreeBSD.ORG From: Audsin Subject: packet length 1284 instead of 1280 Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed X-MailScanner: Found to be clean Sender: owner-freebsd-questions@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG Sir / Madam I am doing my research on fragmentation avoidance technique for mip6. I am using FreeBSD4.4 with kame snap and ethereal to capture packets I have a query regarding the packet length If i am correct , packet length = IPHdr +extHdr+TCPHdr+TCPOpt+Data =IPHdr+Routing Header + Frag Header + TCP Header+ Tcpopt+ Data 40+24+8+20+12+1176=1280 (Which is the MTU of the gif0 interface) But my ethereal capture says the packet length to be 1284 btyes. Can anyone please let me know what that 4 bytes is accounted for? With this mail, i am pasting a capture packet of my experiment Frame 973 (1284 on wire 1284 captured) Arrival Time: Dec 6 2002 39:54.8 Time delta from previous packet: 0.000052 seconds Time relative to first packet: 97.69262 seconds Frame Number: 973 Packet Length: 1284 bytes Capture Length: 1284 bytes Null/Loopback Family: IPv6 (0x0000001c) Internet Protocol Version 6 Version: 6 Traffic class: 0x00 Flowlabel: 0x6017f Payload length: 1240 Next header: IPv6 routing (0x2b) Hop limit: 64 Source address: 2001:618:400::8949:b94 (2001:618:400::8949:b94) Destination address: 3ffe:400b:6004:3:200:39ff:fe11:f32 (3ffe:400b:6004:3:200:39ff:fe11:f32) Routing Header Type 0 Next header: IPv6 fragment (0x2c) Length: 2 (24 bytes) Type: 0 Segments left: 1 address 0:00 3ffe:327e:2:1:200:39ff:fe11:f32 (3ffe:327e:2:1:200:39ff:fe11:f32) Fragmention Header Next header: TCP (0x06) Offset: 0 More fragments: Yes Identification: 0xef9a0000 Transmission Control Protocol Src Port: 49165 -49165 Dst Port: iad2 -1031 Seq: 2.68E+09 Ack: 1.12E+09 Source port: 49165 -49165 Destination port: iad2 -1031 Sequence number: 2.68E+09 Next sequence number: 2.68E+09 Acknowledgement number: 1.12E+09 Header length: 32 bytes Flags: 0x0010 (ACK) 0... .... = Congestion Window Reduced (CWR): Not set .0.. .... = ECN-Echo: Not set ..0. .... = Urgent: Not set ...1 .... = Acknowledgment: Set .... 0... = Push: Not set .... .0.. = Reset: Not set .... ..0. = Syn: Not set .... ...0 = Fin: Not set Window size: 16912 Checksum: 0x7783 Options: (12 bytes) NOP NOP Time stamp: tsval 1931258 tsecr 159233 Data (1176 bytes) Here the packet length is 1284 instead of 1280. Can anyone please explain me the situation . Is it because of any header or something else. Best Regards Dev To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message