Why is my TCP Header Seen as "Data (20 Bytes)"?

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Hi Wireshark Gurus,

I am a college student working on a coding assignment. I'm writing a C program which creates a PCAP file, writes one valid network packet into the file, then exits. The point of the exercise is to learn how to format Ethernet, IP, and TCP headers in code. I pass my assignment if Wireshark can successfully open my PCAP and read the packet.

So far, my code writes a valid Ethernet and IP header (although I am skipping some IP fields, like CRC checksum and flags). My code also writes a TCP header... but Wireshark doesn't recognize it! After the IP header, it simply sees "Data, 20 bytes." I followed the RFC for TCP on the TCP header format, plus set IP_Protocol = 6, and I thought that was all that was necessary. But no.

Why might Wireshark look at my TCP header, and see raw data? I'm guessing it might be one of two reasons:

• The IP header is still malformed, leaving Wireshark to consider the IP payload as "Data"
• The TCP header format is screwed up

When Wireshark looks at where it should see a TCP header, it reports that it sees this:

Data (20 bytes)  0016 d7a6 7145 c91d 0ca8 87dd 5018 01df 0443 0000

(I added those spaces.) When I hand-check that data against what my code is writing, everything looks okay. (Field values were copied from another packet I captured in Wireshark.)

Field               octets  decimal val binary value                                hex value
=================================================================================================
src_port            2       22          0000 0000 0001 0110                         0016
dst_port            2       55206       1101 0111 1010 0110                         D7A6
tcp_seq_number      4       1900398877  0111 0001 0100 0101 1100 1001 0001 1101     7145 C91D
tcp_ack_number      4       212371421   0000 1100 1010 1000 1000 0111 1101 1101     0CA8 87DD
tcp_OffAndFlags     2       20504       0101 0000 0001 1000                         5018
tcp_window_size     2       479         0000 0001 1101 1111                         01DF
tcp_checksum        2       1091        0000 0100 0100 0011                         0443
tcp_urgent_pointer  2       0           0000 0000 0000 0000                         0000

Everything is exactly as it should be, as far as I can tell. So what is wrong?

What I see in Wireshark is included below. Many thanks in advance!

Frame 1: 54 bytes on wire (432 bits), 54 bytes captured (432 bits)
    Encapsulation type: Ethernet (1)
    Arrival Time: Dec 31, 1969 19:00:00.000000000 Eastern Standard Time
    [Time shift for this packet: 0.000000000 seconds]
    Epoch Time: 0.000000000 seconds
    [Time delta from previous captured frame: 0.000000000 seconds]
    [Time delta from previous displayed frame: 0.000000000 seconds]
    [Time since reference or first frame: 0.000000000 seconds]
    Frame Number: 1
    Frame Length: 54 bytes (432 bits)
    Capture Length: 54 bytes (432 bits)
    [Frame is marked: False]
    [Frame is ignored: False]
    [Protocols in frame: eth:ethertype:ip:data]
    [Coloring Rule Name: Broadcast]
    [Coloring Rule String: eth[0] & 1]
Ethernet II, Src: 64:96:c8:fa:fc:ff (64:96:c8:fa:fc:ff), Dst: Woonsang_04:05:06 (01:02:03:04:05:06)
    Destination: Woonsang_04:05:06 (01:02:03:04:05:06)
    Source: 64 ...