A Segment Carries the Conversation State

Core Question

Core Question

What does a TCP segment actually carry, and what makes a combination of flags and payload nonsensical before anyone even looks at what state the connection is in?

Outcome

Outcome

By the end of this session, the learner should be able to:

• name the five fields that make up a Segment
• explain why flags is a frozenset[str] instead of five booleans or a bitmask
• list every way validate_segment() can reject a segment, and in what order the checks run
• explain why "RST carrying a payload" is treated as invalid rather than merely unusual

Read Order

Read Order

1. Read Segment
2. Read is_syn / is_ack / is_fin / is_rst
3. Read flags_label()
4. Read SegmentValidity
5. Read validate_segment()
6. Run examples/tcp/session_01_walkthrough.py

Read It Like Code

Read It Like Code

Segment(
    seq,
    ack,
    flags,
    payload_len,
    window,
)

Fields That Matter

Fields That Matter

Decision Flow

Decision Flow

flags not subset of FLAG_NAMES        -> BadFlagName
SYN and FIN both present              -> SynFinTogether
payload_len < 0                       -> NegativeLength
payload_len > 0 and RST present       -> PayloadWithoutSeqMeaning
otherwise                             -> Valid

Reading Lens

Reading Lens

The important move in this session is to stop thinking of a segment as "a packet with some flags on it" and start asking:

• which flags are present, and does that combination mean anything?
• does the payload agree with what the flags say this segment is for?
• is this check about the segment alone, or does it need the connection's state too?

Notice that validate_segment() never looks at a ConnectionState. Everything it checks is a fact about the segment by itself — state-dependent legality is Session 02's problem, not this one's.

Toy Model Boundary

Toy Model Boundary

Real TCP segments carry a checksum, a data offset, options (MSS, window scaling, SACK, timestamps), and urgent-pointer plumbing tied to the URG flag. This toy keeps only the five fields that the rest of the course actually reasons about: seq, ack, flags, payload_len, window. FLAG_NAMES is deliberately just {SYN, ACK, FIN, RST} — no URG, no PSH, no ECE/CWR — because those four are what drive every state transition in Session 02.

payload_len is an integer count, not real bytes. The toy never constructs an actual payload buffer, because the reading target here is what the length means, not how to move bytes.

Code Landmarks

Code Landmarks

Segment

A frozen dataclass. Once built, a segment cannot be mutated — which matters because Session 02 passes segments around and builds replies from their fields without ever needing to worry that a segment changed underneath it.

is_syn / is_ack / is_fin / is_rst

Four one-line predicates, each just a membership test on flags. Small enough to not need comments, and every other module in this track calls them instead of touching segment.flags directly.

flags_label()

Renders flags in "the conventional wire order" (SYN, ACK, FIN, RST), not set-iteration order. A frozenset has no order of its own — this function is where the human-readable rendering imposes one.

validate_segment()

The main reading target. Four early-return checks, evaluated in a fixed sequence. Notice the check order: an unknown flag is rejected before the SYN+FIN check even runs, and negative length is rejected before the RST-payload check.

Failure Questions

Failure Questions

Use the source file to answer these:

1. A segment has flags=frozenset({"SYN", "FIN", "URG"}). Which SegmentValidity comes back, and why does that check win over the SYN+FIN check?
2. Why is payload_len < 0 treated as a distinct validity outcome from payload_len > 0 on an RST, instead of being folded into the same check?
3. Is a segment with flags=frozenset() and payload_len=0 valid? Trace validate_segment() to confirm.
4. Does validate_segment() reject a segment with ack set but no ACK flag present? Read the function body to be sure, not the docstring.
5. Why does an RST with payload_len=0 pass validation, but the same RST with payload_len=1 fail?

Walkthrough

Walkthrough

Run this:

PYTHONPATH=src python3 examples/tcp/session_01_walkthrough.py

The walkthrough builds one valid control segment, one valid data segment, and one segment for each of the four ways validate_segment() can reject input, then checks flags_label() renders in wire order.

Done When

Done When

The learner can say all of the following without looking at notes:

• "A segment is five fields — flags is a set of names, not a bitmask I have to decode."
• "Validity is checked before state; SYN+FIN together is always wrong, no matter what state the connection is in."
• "An RST carrying a payload is rejected because a reset has no sequence meaning for data it claims to carry."

References

References

• RFC 9293 Section 3.1 (Header Format)
• RFC 9293 Section 3.4 (Sequence Numbers — payload/control-flag sequence semantics)
• RFC 9293 Section 3.8.6 (Managing the Window — the window field this segment carries)

Continue

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