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tcpreq – Measuring TCP Specification Conformance

tcpreq is a modular framework for TCP Specification Conformance testing, enabling Implementors to test specific features of their individual TCP stack implementation, as well as enabling Researchers to assess the state of TCP conformance in the Internet. It was developed by Leo Blöcher at COMSYS Network Architectures Group of RWTH Aachen University. tcpreq is released according to the MIT License terms.



PAM 2020: MUST, SHOULD, DON'T CARE: TCP Conformance in the Wild, International Conference on Passive and Active Network Measurement


PAM 2020: Dataset to "MUST, SHOULD, DON’T CARE: TCP Conformance in the Wild"

Test Cases

The basic requirements of a TCP implementation are defined in RFC 793, the core TCP specification. In its over 40 years of existence, it has accumulated numerous follow-up RFCs and over 25 accepted errata culminating in RFC 793bis. All test cases are based on the requirements stated in RFC 793bis, and require active communication to the tested Host. The following test cases are available:

TCP Requirement Default [1] ALPs [2] Implementation [3]
MUST-2/3: The sender MUST generate [the checksum] and the receiver MUST check it
Reset Processing: The RST flag in a segment is processed independently from other flags
MUST-4: [End of option list, no-operation, and maximum segment size] MUST be supported OptionSupportTest
MUST-5: A TCP implementation MUST be able to receive a TCP option in any segment LateOptionTest
MUST-6: A TCP MUST ignore without error any TCP option it does not implement UnknownOptionTest
MUST-7 TCP MUST be prepared to handle an illegal option length [...] without crashing IllegalLengthOptionTest
MUST-14: TCP endpoints MUST implement both sending and receiving the MSS option MSSSupportTest
MUST-15: If an MSS option is not received at connection setup, TCP MUST assume a default send MSS of 536 [...] for IPv4 or 1220 [...] for IPv6 MissingMSSTest
MUST-16: The maximum size of a segment that TCP really sends [...] MUST be the smaller of the send MSS [...] and the largest transmission size permitted by the IP layer all tests
Rsrvd - Reserved: Must be zero in generated segments and must be ignored in received segments (optional)
MUST-30/31: TCP implementations MUST still include support for the urgent mechanism, A TCP MUST support a sequence of urgent data of any length
MUST-8: A TCP implementation MUST use a clock-driven selection of initial sequence numbers - - Meta test case: ISNs are collected and reported during handshakes

[1] Whether the test case is executed if there is no explicit selection on the command line. The most invasive test cases as well as edge cases are excluded by default.
[2] Whether the test case uses the ALP modules described in Extending tcpreq.
[3] Source code for the test case with comments detailing the steps each test case performs.


Middleboxes can alter TCP header information and thereby cause non-conformance, which should not be wrongly attributed to the probed host. tcpreq uses the tracebox approach to detect interfering middleboxes by sending and repeating its probes with increasing IP TTLs.

In every test case, the first segment is sent multiple times with increasing TTL values from 1 to 30 in parallel while capturing ICMP time exceeded messages. To distinguish the response messages and determine the hop count, the TTL is encoded in the IPv4 ID field and any combination of the TCP acknowledgment number, window size, and urgent pointer fields, as well as the TCP options using a unary coding of NOOP options.

This allows tcpreq to pinpoint and detect (non-)conformance within the end-to-end path if ICMP messages are issued by the intermediaries quoting the expired segment. Please note that alteration or removal of some of the encodings do not render the path or the specific hop non-conformant. A non-conformance is only attested, if the actual tested behavior was modified as visible through the expired segment.

Depending on the specific test case, some of the fields are not used for the TTL encoding. For example, when testing for urgent pointer adherence, the TTL is not encoded in the urgent pointer field.


  • Linux, *BSD, or comparable OS
  • Python ≥ 3.6
  • Packages in requirements.txt (pip install -r requirements.txt)
  • Either nftables or iptables
  • ethtool (for checksum tests)
  • Optional: tcpdump to record traces
  • Development: pycodestyle linter, mypy type checker


  1. Choose an interface to run tcpreq on. Example: eth1.
  2. Set up the firewall rules preventing kernel interference.
    1. You will likely have to customize the user ID/name in the templates:
      • nftables: Replace skuid tcpreq with skuid <username> in tcpreq-nft.conf.
      • iptables: Replace --uid-owner 1001 with --uid-owner <user ID> in tcpreq-ipt.rules.
    2. Optional: If you want to limit the firewall rules to eth1, insert:
      • nftables: meta oif != eth1 accept on a new line before meta skuid ... in tcpreq-nft.conf.
      • iptables: -o eth1 before -j tcpreq in tcpreq-ipt.rules.
    3. Load the rules:
      • nftables: nft -f tcpreq-nft.conf
      • iptables: iptables-apply tcpreq-ipt.rules (IPv4) or ip6tables-apply tcpreq-ipt.rules (IPv6)
  3. Disable NIC offloads: ethtool -K eth1 tx off rx off tso off gso off gro off lro off
    • tx off rx off: Disables checksum offloads (necessary for checksum tests)
    • tso off gso off gro off lro off: Disables segmentation offloads in tcpdump traces (optional)
  4. Run python -m tcpreq and wait for it to finish. See python -m tcpreq -h for details on its CLI.
    • -B <eth1's IP address> to set the IP address
    • -r 10k to limit tcpreq to 10 000 packets per second
    • -b <path/to/blacklist> to enforce a blacklist (in CIDR notation, like ZMap)
    • -T * to add all test cases, -T ZeroChecksumTest to add just the ZeroChecksumTest, -T !ZeroChecksumTest to remove the ZeroChecksumTest to/from the selection of test cases
    • -o <path/to/results.json> to specify the output file name
    • --json/--nmap/--zmap <path/to/input> to specify the input file name(s)
  5. Re-enable the NIC offloads: ethtool -K eth1 tx on rx on tso on gso on gro on lro on
  6. Remove the firewall rules added previously: see the comments in the respective rules file

tcpreq can run as an unprivileged user, in which case the Python executable requires the CAP_NET_RAW and CAP_NET_ADMIN capabilities. These can be granted with setcap cap_net_admin,cap_net_raw+ep <path/to/python>, though this should only be done within a tcpreq-specific virtualenv. Otherwise, other Python scripts may also open raw IP sockets and intercept traffic. Without these capabilities in place, tcpreq must be run as root. In this case, the user ID above is 0 and the username is root.

The JSON input format expects one object per line with keys for ip, port, and optionally host (used in some ALP modules). Other keys are ignored by tcpreq and passed through to the output module. The default (JSON Lines) output module dumps all the information available to tcpreq in one JSON object per line. A sample of JSON input and of the default output format can be found below:

Input: {"ip":"2001:db8::248:1893:25c8:1946","port":80,"host":"","custom_rank":9107}

Output (comments and multi-line for readability):

  "ip": "2001:db8::248:1893:25c8:1946",
  "port": 80,
  "host": "",
  "custom_rank": 9107,
  "results": [
      "test": "OptionSupportTest",
      "timestamp": "2020-04-03T09:12:40Z",  # UTC
      "src": {"ip":"2001:db8::1", "port": 49506, "host": null},
      "path": [
        # Hop count, IP
        [1, "2001:db8::7925:ca2c:bb71:0c22"],
        [3, "2001:db8::248:1893:0:1"]
      "isns": [
        # Monotonic timestamp, ISN
        [8602715.399565088, 3785391440],
        [8602929.02997876,  1514644230]
      "status": "PASS",
      "stage": null,
      "reason": null,
      "custom": null
status Semantics
PASS Test case succeeded
UNK Conformance couldn't be determined. More information in reason.
FAIL Test case failed due to non-conformant behavior. More information in reason.
ERR Python exception during test case execution. More information in reason.
FLTR Target IP is blacklisted or target IP and port are duplicates
DISC Target IP address type is not supported (e.g., IPv6 on an IPv4-only host)

Extending tcpreq

Test Cases

In addition to the included test cases, custom test cases can be created in the tcpreq.tests package. Every test case must derive from BaseTest, which provides the infrastructure to send and receive segments among other things. This base class includes methods for sending and receiving segments, for the three-way handshake (3WH), and for detecting middleboxes according to the pattern described above.

A new test case only needs to implement BaseTest's abstract asynchronous run method. The RSTACKTest in tcpreq.tests.rst_ack is a simple example of such a test and takes advantage of the convenience methods described above. More complex examples including middlebox detection and the use of ALPs can be found in, e.g., tcpreq.tests.options and tcpreq.tests.mss. The finished test case must then be imported in the module's so that it can be selected on the command line. It may also be added to the selection of default test cases in that file.

Application-layer Protocols (ALPs)

Some of tcpreqs test cases require payload data, which is provided by pluggable ALP instances. Two popular ALPs come included in tcpreq: HTTP (from scratch) and TLS/SSL (via Python's ssl module/OpenSSL). Additional ALPs can be added by creating classes derived from BaseProtocol in tcpreq.alp and importing them in the module's The documentation in the base class explains the interface currently used in the included test cases. Both tcpreq.alp.http and tcpreq.alp.tls serve as examples.

Input/Output Formats

By default, JSON (lines) and console output are supported. To produce output in your preferred format, a custom output module can be added in tcpreq.output. Create a class derived from _StreamOutput and register it with the file extensions it is supposed to handle in _OUTPUT_TBL. Unknown/missing file extensions default to JSON Lines. As with the ALP modules, the base class documents the interface and the included _JSONLinesOutput class serves as an example. Console output is only used in absence of an output file.

There is currently no similar mechanism to extend the selection of input formats because of a lack of naming conventions. Instead, tcpreq.opts needs to be modified to include a new CLI parameter with an associated parsing function. The Nmap and ZMap formats may serve as examples.


Modular framework for TCP specification conformance testing








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