Queries Keep the Set Honest

Core Question

Core Question

Nothing in membership.py ever forgets a host on its own — so what makes a GroupTable trustworthy after a host crashes, unplugs, or roams off-link without ever calling leave()?

Outcome

Outcome

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

• explain why GroupTable alone cannot detect a host that silently disappeared
• name the two RFC 2236 timers this module models and what each one is for
• trace expire_silent()'s boundary comparison and say, at exactly the timeout, whether a host is still a member
• recognize this module as the third appearance of the expiring-dict shape, and name the other two by file

Read Order

Read Order

1. Read the module comment above QUERY_INTERVAL
2. Read QUERY_INTERVAL and MEMBERSHIP_TIMEOUT
3. Read MembershipState
4. Read on_report()
5. Read expire_silent()
6. Read report_suppression()
7. Run examples/igmp/session_02_walkthrough.py

Read It Like Code

Read It Like Code

MembershipState(
    last_report_at,
)

Fields That Matter

Fields That Matter

Decision Flow

Decision Flow

on_report(group, host, now)
  -> last_report_at[(group, host)] = now        (always overwrites; no other branch)

expire_silent(state, table, now), per (group, host) pair:
  now - last_seen >= MEMBERSHIP_TIMEOUT   -> expired: drop from last_report_at, leave(table, group, host)
  otherwise                               -> untouched, still a member

Reading Lens

Reading Lens

This is the expiring-state shape's third appearance in this course, and the module comment above QUERY_INTERVAL says so directly:

• DNS, Session 04 (src/protocol_in_code/dns/cache.py) — a CacheEntry stamped with stored_at, expiring against ttl, checked inside lookup().
• DHCP, Session 04 (src/protocol_in_code/dhcp/leases.py) — a Lease stamped with granted_at, expiring against duration, checked inside lookup_lease().
• IGMP, Session 02 (here) — a (group, host) pair stamped with last_report_at, expiring against MEMBERSHIP_TIMEOUT, checked inside expire_silent().

What varies this time is *where* the check runs. DNS and DHCP check expiry lazily, inside the read path — lookup() and lookup_lease() are also the only place staleness is ever discovered. IGMP checks it eagerly, on a schedule: expire_silent() runs once per query cycle, not once per read, because — as the docstring puts it — "nothing here is 'read' the way a cache entry is." What expires is not a cached answer or a granted address; it's interest itself. A host that stops answering queries is, for forwarding purposes, no longer there, even though nothing it did was an explicit leave().

Ask, at every line:

• what pair is this timestamp keyed by, and when was it last written?
• is this expiry check happening because someone asked (lazy), or because a cycle ran (eager)?
• did expire_silent() just call leave() on table — and would membership.py's key-deletion behavior from Session 01 now apply?

Toy Model Boundary

Toy Model Boundary

QUERY_INTERVAL = 125 and MEMBERSHIP_TIMEOUT = 260 are named constants, not derived from a robustness variable or a configurable max-response-time the way real IGMPv2 timers are (RFC 2236 computes the group membership interval from the query interval and the max response time, not as a bare literal). There is no last-member query, no leave-latency reduction, and no query retransmission — a host that leaves takes the full timeout to be noticed, with no fast path. report_suppression() always picks the first name in the tuple it's handed; there is no random suppression timer, and no reduced-timer-on-hearing-another-report simulation — just "one response is enough" modeled as a single deterministic choice.

Code Landmarks

Code Landmarks

The module comment above QUERY_INTERVAL

Names this as the expiring-state family's third member and states the theme in one sentence: "here what expires is stale INTEREST." Read it before anything else in the file — it is the thesis for the whole session.

on_report()

One line, no branches: the timestamp is always overwritten, whether this is the first report ever seen for the pair or the hundredth. There is no "already reported this cycle" special case.

expire_silent()

The landmark comparison is now - last_seen >= MEMBERSHIP_TIMEOUT. It is >=, not > — a host silent for *exactly* MEMBERSHIP_TIMEOUT seconds is expired on this call, not on the next one. The docstring calls this out explicitly as the timeout side of the same shape as ResolverCache.lookup and LeaseTable.lookup_lease.

report_suppression()

The docstring's mechanism explanation is the landmark: reports are multicast to the group address, so every other member overhears the first answer and cancels its own timer. The function models "pick the first responder" and the saving is len(group_members_who_heard) - 1 messages versus a naive every-member-answers design.

Failure Questions

Failure Questions

Use the source file to answer these:

1. At exactly now - last_seen == MEMBERSHIP_TIMEOUT, does expire_silent() treat the pair as expired or still a member? Which comparison operator in the source decides this, and what would change if it were > instead?
2. expire_silent() calls leave(table, group, host) for each expired pair. If that pair was the group's last member, what does table.groups look like immediately afterward, and which Session 01 function is responsible for that?
3. on_report() is called for a (group, host) pair that has never been seen before. Does it raise, or does it behave the same as an update to an existing pair? Where in the source is that guaranteed?
4. report_suppression() is called with an empty tuple. What does the source do, and what does that tell you about who is responsible for checking "did anyone report" before calling it?
5. The module comment names two other files that expire stale state the same way. Name both file paths and, for each, what field plays the role last_report_at plays here.

Walkthrough

Walkthrough

Run this:

PYTHONPATH=src python3 examples/igmp/session_02_walkthrough.py

The walkthrough reports a host, checks it survives one tick before MEMBERSHIP_TIMEOUT and is gone exactly at the timeout, shows a re-reported host surviving past where the first host expired, and exercises report_suppression() on both a two-member group and an empty tuple.

Done When

Done When

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

• "GroupTable never forgets a host on its own — querier.py is the only thing that tests whether a member is still really there."
• "expire_silent() expires a pair at exactly now - last_seen == MEMBERSHIP_TIMEOUT, not one tick later."
• "This is the expiring-dict shape's third appearance — DNS's cache, DHCP's leases, and now IGMP's interest — but here the check runs on a query cycle, not at read time."

References

References

• RFC 2236 Section 1 (Introduction — the querier's role in testing membership over time)
• RFC 2236 Section 3 (message formats and the timers this module names: query interval, max response time)
• RFC 2236 Section 6 (host state diagram — report suppression on hearing another member's report)

Continue

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