Solving TCP Error Checksum Problem

If you get a TCP checksum error, this tutorial is for you.

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    This problem can occur if current network traffic can be captured and a checksum calculated. To resolve this issue, ensure that packets with incorrect TCP checksums are usually sent from the capturing virtual machine. How to disable TCP checksum verification using Wireshark: Click Settings> Change.

    Question

    If iptrace and tcpdump are collected through aix / vios host and read with Wireshark, why does it mark TCP and IP checksum fields as incorrect even though communication sometimes works fine?

    Reply

    When reading iptrace and tcpdump connected to an aix / vios host using Wireshark, this marks the TCP and IP checksum fields as incorrect, although even interacting It works fine. To understand the specific reason for this behavior, you need to understand which drives are large_send, large_receive, and which iptrace coverage is capturing the packet.

    How is TCP checksum calculated?

    The TCP checksum is calculated based on the TCP header, the TCP body concatenated with the pseudo IP header. Well, the crucial ambiguity that arises is that the checksum can be computed over the IP header, because IP comes into play directly at the lower transport layer.

    Large_send When enabled, TCP can send up to 64 KB of data bound data with a single stack call, which reduces host control and CPU utilization. The Ethernet adapter then performs TCP split offloading to segment the data based on the MTU packets, calculate the TCP and IP checksum. Large_send is a device driver attribute and is enabled by default.

    What happens if TCP checksum fails?

    If the TCP checksum is corrupted, it probably doesn’t match.Supports TCP pseudo-headers and payload. There should only be one checksum that matches our own pseudo-header and payload, but there can be multiple TCP pseudo-headers as well as payload combinations that resolve into the same checksum. This is a one-way function.

    Large_send is also known as TCP Segmentation Offload (TSO).

    What is large_recevoir?

    When big_receive is enabled, the physical layer computes the TCP IP and checksum by splitting the received packets into larger ones before passing them onto the stack. The processor should process fewer blocks than when large_receive is not working correctly, which reduces the load on the processor. The large_receive attribute for each device driver is enabled by default.

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  • LARGE_PRODUCT is also known as large reception offload (LRO).

    What is the TCP level and checksumma IP are calculated and where is iptrace logging the packet?
    error check sum tcp

    The figure below shows the layer at which TCP and IP checksums are typically computed and where iptrace / tcpdump captures all packets.

    Building:

    Does TCP have checksum?

    TCP has a checksum that covers the TCP pseudo-header as well as the payload. This is optional for UDP over IPv4, but is required for UDP over IPv6. Other transport protocols have error detection functionality while others cannot easily. If the transport protocol does not have error detection functionality, it can lead to application errors.

    Host or IP address 5.0.0.240 configured on 10GB power adapter

    Check:

    The FTP client was running 5.0.0.70 and the FTP server was 5.0.0.240.

    – The “Put” and “Get” file transfer protocols were implemented to send the received data and therefore send it.

    Large_receive = yes effect:

    Frame 453 is a Large_receive packet that is only received on 5.0.0.240.

    The effect most often associated with large_send = yes:
    error check sum tcp

    Frame 5999 is sent by Large_send packet from 5.0.0.240.

    • For the specified “large_receive” packet, dummy ffff values ​​are entered in addition to the 0’s in the IP or TCP checksum fields.
    • For a mapped large_send packet, the dummy ffff MTU and size (for example, 5a8 hex for MTU 1500) are entered in the IP Checksum and TCP Checksum fields, respectively.
    • If iptrace (or tcpdump) is read while using Wireshark, the checksum of the IP and TCP is calculated and the value in the packets is used for comparison. Since the values ​​in the packets are Stooge values, the comparison fails and the software marks the ip checksum and TCP checksum as invalid, even though the packet is often valid.
    • Large_send and large_receive are not performed on small data, and packets affect packets such as SYN, ACK, FIN, RST. Below are the ACK and SYN packet levels showing that the IP and TCP checksum is less than the invalid large_send because the combination with large_receive fails.
    • ACK:
    • Package SYN:

    Why does TCP have a checksum?

    TCP includes a checksum field in TCP headers to determine if there is a risk of problems moving through a TCP host, interconnection, or, in layman’s terms, TCP should check to see if the segment it created is corrupted (intentionally or accidentally). was en route during the segment to reach the destination.

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