Hi,
i am doing a modification to the TCP protocol and want to use an
additional bit of the TCP header's Reserved field. I chose to use the
bit next to the Bits used by the ECN mechanism. Therefore i changed the
TCP header definition in include/linux/tcp.h of a 2.6.16.29 Kernel to
struct tcphdr {
__u16 source;
__u16 dest;
__u32 seq;
__u32 ack_seq;
#if defined(__LITTLE_ENDIAN_BITFIELD)
__u16 res1:3,
doff:4,
fin:1,
syn:1,
rst:1,
psh:1,
ack:1,
urg:1,
ece:1,
cwr:1,
elcn:1;
#elif defined(__BIG_ENDIAN_BITFIELD)
__u16 doff:4,
res1:3,
elcn:1,
cwr:1,
ece:1,
urg:1,
ack:1,
psh:1,
rst:1,
syn:1,
fin:1;
#else
#error "Adjust your <asm/byteorder.h> defines"
#endif
__u16 window;
__u16 check;
__u16 urg_ptr;
};
I added the ecln bit instead of one bit of the res1 field.
I used tcpdump to check the TCP traffic when using the modified version
of TCP and i ran into the problem that on connection setup the SYN
packet is dropped by the receiver because of a bad checksum.
(I tried the unmodified kernel first, and it worked ;) )
An example output of such a bad packet from tcpdump is as follows:
IP (tos 0x0, ttl 64, id 15794, offset 0, flags [DF], proto: TCP (6),
length: 60) sender > receiver: S, cksum 0xaa78 (incorrect (-> 0x0272),
2824515850:2824515850(0) win 5840 <mss 1460,sackOK,timestamp 294938686
0,nop,wscale 1>
0x0000: 4500 003c 3db2 4000 4006 c448 89e2 12f9 E..<=.@.@..H....
0x0010: 89e2 1204 0bf0 006f a85a b10a 0000 0000 .......o.Z......
0x0020: a002 16d0 aa78 0000 0204 05b4 0402 080a .....x..........
0x0030: ffff 903e 0000 0000 0103 0301 ...>........
I cut the problem down to the tcp_v4_send_check and the tcp_v4_check
method respectively as these are responsible for calculating the TCP
checksum. However i am not able to follow the csum_partial method which
apparently calculates the checksum for the TCP packet without the pseudo
header.
My questions are
Why does the checksum calculation fail in case of the modified header
structure ?
Does the calculation method assume the Reserved field to be of size 4
and equal 0 ?
If yes, how can you fix the checksum calculation to consider the new
bit and the new size of the reserved field ?
Thx in advance for your help
greets
Michael
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