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Old 10-29-2004, 02:15 AM   #1
rickthemick
Member
 
Registered: Aug 2003
Location: Sweden
Distribution: Debian
Posts: 52

Rep: Reputation: 15
"Frame check sequence"-error (802.11b Ad hoc)


I get "Frame check sequence"-error on broadcast packets between two host in a simple 2 node Ad hoc network using 802.11b and Debian/Linux. This FCS error does not appear when the packets are sent using unicast.

Here is the packet captured by Ethereal (but never delivered to my application):
(The ident is corrupted when submitting the post, tried to make it more readable)
------------------------------------------------------------
No. Time Source Destination Protocol Info
5 4.039653 10.0.4.2 255.255.255.255 UDP Source port: x11 Destination port: x11

--> Frame 5 (74 bytes on wire, 74 bytes captured)
Arrival Time: Oct 29, 2004 08:04:05.984378000
Time delta from previous packet: 1.009917000 seconds
Time since reference or first frame: 4.039653000 seconds
Frame Number: 5
Packet Length: 74 bytes
Capture Length: 74 bytes

--> Ethernet II, Src: 00:60:1d:f2:be:7c, Dst: ff:ff:ff:ff:ff:ff
Destination: ff:ff:ff:ff:ff:ff (Broadcast)
Source: 00:60:1d:f2:be:7c (10.0.3.2)
Type: IP (0x0800)
Trailer: 38FF05002100A0010030000000000000
/*** BELOW IS THE INCORRECT FCS (this line added by me) ***/
Frame check sequence: 0x00000000 (incorrect, should be 0xec31f260)

--> Internet Protocol, Src Addr: 10.0.4.2 (10.0.4.2), Dst Addr: 255.255.255.255 (255.255.255.255)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 40
Identification: 0x0000 (0)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 64
Protocol: UDP (0x11)
Header checksum: 0x2cc4 (correct)
Source: 10.0.4.2 (10.0.4.2)
Destination: 255.255.255.255 (255.255.255.255)

--> User Datagram Protocol, Src Port: x11 (6000), Dst Port: x11 (6000)
Source port: x11 (6000)
Destination port: x11 (6000)
Length: 20
Checksum: 0xf060 (correct)
Data (12 bytes)

0000 48 65 6c 6c 6f 21 20 5b 31 35 5d 00 Hello! [15].
--------------------------------------------------------------------


See that the FCS is 0x00000000? Why is this? The UDP checksum seems to be correct. This FCS eroor does not happend when using unicast! Both sockets are bound to INADDR_ANY and set to allow broadcasts.

Is it the FCS that is my problem or is it Ethereal the makes a mistake? Should ethereal be able to see packets with FCS error? Shouldnt they have been retransmitted on the hardware level without the knowledge from the software?

ADDED:
I have noticed that the FCS error does not matter, I can receive that packets even if these error are present. What matters is if there is a route back to the sending node, if it is, the packets are delivered, else not, why???


All feedback is very appreciated, I'm becoming frustrated over this problem!

Best regards
Rickard

Last edited by rickthemick; 10-29-2004 at 03:15 AM.
 
Old 10-15-2007, 03:13 AM   #2
Amandeepsinghsaini
LQ Newbie
 
Registered: Oct 2007
Posts: 1

Rep: Reputation: 0
Question

Hi rickthemick

Are you got solution of this problem.
I am also going through same kind of problem.
Could you help me.

Regards
Amandeep

Quote:
Originally Posted by rickthemick View Post
I get "Frame check sequence"-error on broadcast packets between two host in a simple 2 node Ad hoc network using 802.11b and Debian/Linux. This FCS error does not appear when the packets are sent using unicast.

Here is the packet captured by Ethereal (but never delivered to my application):
(The ident is corrupted when submitting the post, tried to make it more readable)
------------------------------------------------------------
No. Time Source Destination Protocol Info
5 4.039653 10.0.4.2 255.255.255.255 UDP Source port: x11 Destination port: x11

--> Frame 5 (74 bytes on wire, 74 bytes captured)
Arrival Time: Oct 29, 2004 08:04:05.984378000
Time delta from previous packet: 1.009917000 seconds
Time since reference or first frame: 4.039653000 seconds
Frame Number: 5
Packet Length: 74 bytes
Capture Length: 74 bytes

--> Ethernet II, Src: 00:60:1d:f2:be:7c, Dst: ff:ff:ff:ff:ff:ff
Destination: ff:ff:ff:ff:ff:ff (Broadcast)
Source: 00:60:1d:f2:be:7c (10.0.3.2)
Type: IP (0x0800)
Trailer: 38FF05002100A0010030000000000000
/*** BELOW IS THE INCORRECT FCS (this line added by me) ***/
Frame check sequence: 0x00000000 (incorrect, should be 0xec31f260)

--> Internet Protocol, Src Addr: 10.0.4.2 (10.0.4.2), Dst Addr: 255.255.255.255 (255.255.255.255)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 40
Identification: 0x0000 (0)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 64
Protocol: UDP (0x11)
Header checksum: 0x2cc4 (correct)
Source: 10.0.4.2 (10.0.4.2)
Destination: 255.255.255.255 (255.255.255.255)

--> User Datagram Protocol, Src Port: x11 (6000), Dst Port: x11 (6000)
Source port: x11 (6000)
Destination port: x11 (6000)
Length: 20
Checksum: 0xf060 (correct)
Data (12 bytes)

0000 48 65 6c 6c 6f 21 20 5b 31 35 5d 00 Hello! [15].
--------------------------------------------------------------------


See that the FCS is 0x00000000? Why is this? The UDP checksum seems to be correct. This FCS eroor does not happend when using unicast! Both sockets are bound to INADDR_ANY and set to allow broadcasts.

Is it the FCS that is my problem or is it Ethereal the makes a mistake? Should ethereal be able to see packets with FCS error? Shouldnt they have been retransmitted on the hardware level without the knowledge from the software?

ADDED:
I have noticed that the FCS error does not matter, I can receive that packets even if these error are present. What matters is if there is a route back to the sending node, if it is, the packets are delivered, else not, why???


All feedback is very appreciated, I'm becoming frustrated over this problem!

Best regards
Rickard
 
Old 08-21-2008, 11:04 AM   #3
ron11311
LQ Newbie
 
Registered: Sep 2007
Posts: 2

Rep: Reputation: 0
Sounds like a switch error

Try disabling "Spanning Tree" on the switch.
 
  


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