Independent Submission S. Toyosawa
Request for Comments: 9401
Category: Informational 1 April 2023
The Addition of the Death (DTH) Flag to TCP
This memo specifies the incorporation of Death (DTH) flag to TCP,
including DTH's use of one bit in the TCP header. The flag is
designed to make TCP session narratives smooth and attractive.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9401
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Table of Contents 1.
Requirements Language 3.
TCP Packet Format 3.2.
When to Send 3.3.
When Not to Send 3.4.
Use with the IP Evil Bit 4.
Security Considerations 5.
IANA Considerations 6.
Normative References 6.2.
The proposed Death flag, or DTH for short, uses the fourth flag bit
in the TCP header to indicate likely termination of the TCP session.
The flag allows applications to prepare for abrupt session
terminations. Network engineers find this feature helpful in
identifying the one or more root causes of TCP RSTs. Critical end
users can use the information to better understand TCP narratives.
The flag name is adapted from the custom of anime, manga, or light
novels [NOVEL]. "Death Flags" refer to hints that a character will
die soon [CBR-FLAG].
For example, the DTH flag of an evil scientist is set when they
express too much confidence in their deadly invention. The scientist
is often killed by their own invention. This type of narrative is
also common in conventional films. A notable example is a solider in
a trench. The soldier's flag is set to 1 immediately after they
share a photograph of their fiancé and tell about the upcoming
marriage that will take place after returning from battle. Another
example is setting the flag for a couple sneaking out from an
isolated cabin for a late-night excursion. Commonly, the excursion
is violently terminated by an individual with a chainsaw.
2. Requirements Language
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
3.1. TCP Packet Format
The DTH flag uses the fourth bit in the Control bits field in TCP
header as depicted in Figure 1 [RFC9293
]. The fourth bit was
intentionally selected because "four" in Chinese is Sì; it has a
similar sound to Sǐ, which means "die".
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Source Port | Destination Port |
| Sequence Number |
| Acknowledgment Number |
| Data |D| |C|E|U|A|P|R|S|F| |
| Offset|T| Rsr |W|C|R|C|S|S|Y|I| Window |
| |H| vd |R|E|G|K|H|T|N|N| |
| Checksum | Urgent Pointer |
| [Options] |
: Data :
Note that one tick mark represents one bit position.
Figure 1: TCP Header with the DTH Flag Bit
A TCP session peer SHOULD
transmit a DTH segment when the TCP session
will likely be terminated soon. It can be sent from both the server
and client. The application or TCP stack MAY
elect not to send DTH
segments, even if it knows that the session will be terminated. This
results in a dramatic surprise for the peer; however, the end users
may perceive the end too convenient or overly simplistic. Use of the
DTH segment that is not associated with the session termination is
not encouraged but it is permitted. (This is often referred to as
"teasing" or a false-positive DTH flag.)
The DTH flag is informational. TCP software that does not implement
this feature can safely ignore this flag. However, to fully
appreciate the session, users should be aware of the subtle signs of
the session narratives.
The DTH flag itself does not change the sequence or acknowledgment
number. It does not require any acknowledgement.
The recipient of the flag is not required to act differently upon
reception; however, it is RECOMMENDED
that information be conveyed to
the application layer, so the end user can be notified of the
incident. The recipient of a DTH segment SHOULD NOT
close the socket
immediately upon reception; it SHOULD
wait for a RST or FIN segment.
This specification does not stipulate the maximum number of DTH
segments permitted in one TCP session; however, limiting them to a
few is RECOMMENDED
to maximize the dramatic effect.
3.2. When to Send
DTH can be used any time the sender considers it important to signal
its inevitable end to the TCP peer. The example scenarios below
illustrate when to send DTH segments.
A malicious actor can send the flag when it suddenly repents; for
example, when a sender suddenly regrets their part in a DDoS attack
and unexpectedly ceases the attack. The archvillain generally
terminates the sender cruelly and mercilessly soon after the change
in behavior (or they are killed for protecting the hero). The timing
of DTH transmission is implementation dependent. It can be sent
anytime from the early signs of betrayal to just prior to the
The flag can be sent when the sender stops using cryptographic
protections and reveals its plain-text content, for example, a
mysterious character with a mask that often dies after they expose
their face. In this example, the DTH segment would be sent just
before sending the redirect (30x) from HTTPS to HTTP [RFC9110
Similarly, the flag can be set when the forged User-Agent or Server
HTTP header field is changed to the actual value, when their true
identity would be revealed (for example, "I am your long-lost twin",
"I am a spy", etc.). This occasionally leads to the death of the
The TCP peer is RECOMMENDED
to send the flag when it notices resource
issues, e.g., diminishing memory space or bandwidth. An AI bot,
cyborg, sorcerer application with forbidden protocols, etc., SHOULD
consider sending the flag when it starts to heavily cough error
An application less capable of performing its task MAY
send the flag
from time to time. It will be killed by the OS (the archvillain) or
CTRL-C (the end user) sooner or later due to its inefficiency. The
same is likely to occur with a memory-hogging application, for
example, an unscrupulous character that attempts to take all the
treasure often dies accidentally (e.g., falls from a cliff).
An application SHOULD
really think twice before accessing a
"honeypot" or haunted server. If your choices are limited (e.g.,
your favorite server breaks down in the middle of nowhere and the
dark server that is not on the DNS is the only place you can
shelter), sending the flag periodically is a good idea. The session
is most likely cursed.
3.3. When Not to Send
The DTH flag SHOULD NOT
be piggybacked on the FIN flag. If present,
the recipient SHOULD
silently ignore DTH flag. The only exception is
when the recipient is an expert at Hokuto-Shinken ("Big Dipper Divine
Fist") [WIKI-FNS]. In that circumstance, the sender is already dead
but remains active for a few seconds (which is unofficially called
the "half-zombie open" state).
The DTH flag SHOULD NOT
be sent with the URG flag [RFC6093
]. The use
of the URG flag is not recommended in new implementations [RFC9293
Use of the flag in the early state of a TCP session is NOT
. Characters that die in the early stage are considered
nonessential, hence their death does not contribute to the quality of
the session. (Obviously, there are exceptions.)
3.4. Use with the IP Evil Bit
Some experimental implementations use the Evil bit [RFC3514
] of the
IP header to indicate if the session portrays an evil character. The
DTH flag is not designed to characterize a TCP session. It is
intended to show the fate of the session irrespective of the nature
of the session. When both Evil bit and DTH flag are present, they MUST
be interpreted independently.
4. Security Considerations
Precursors to the inevitable death (often violent) of a TCP session
are useful for upper-layer applications and end users; however, the
security vs. usability balance should also be considered. Since DTH
flags may expose the internal state of the TCP session, they can be
exploited by attackers (e.g., naming the murderer before the
detective points out the suspect). Spoilers are an act of evil.
Those who wish to keep the story secret should use the flag mildly.
5. IANA Considerations
This document defines the behavior of the one of the currently
reserved (Rsrvd) control bits in the TCP header. It is used as an
informative indicator of the fate of a TCP session. The fourth bit
(counting from the beginning of the thirteenth octet in a TCP header)
is intentionally selected to signify its meaning; however, a change
in the bit position does not cause any functional deterioration.
This feature may already be implemented in different manners in
Hollywood and/or Japanese animation studio networks; however, to the
author's knowledge, the technology is not yet patented.
6.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Bellovin, S., "The Security Flag in the IPv4 Header", RFC 3514
, DOI 10.17487/RFC3514
, April 2003,
] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093
, DOI 10.17487/RFC6093
January 2011, <https://www.rfc-editor.org/info/rfc6093
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293
, DOI 10.17487/RFC9293
, August 2022,
6.2. Informative References
[CBR-FLAG] Stalberg, A., "10 Death Flags That Mean An Anime Character
is Probably Going To Die", 2023,
[NOVEL] Wikipedia, "Light novel", February 2023,
] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110
, June 2022,
[WIKI-FNS] Wikipedia, "List of Fist of the North Star characters",
March 2023, <https://en.wikipedia.org/w/index.php?title=Li