Renewal Happens Before the End

Core Question

Core Question

If a lease is just a dict entry that expires, why does RFC 2131 make a client start asking for a new one long before the old one runs out — and who is it even allowed to ask, at each point in that countdown?

Outcome

Outcome

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

• compute T1 and T2 for a given lease by hand, including the integer-floor rounding
• name all four RenewalState values and the exact boundary that separates each pair
• explain why RENEWING asks one specific server while REBINDING asks anyone
• explain why T1 and T2 both round down, and why that costs the client time rather than gaining it

Read Order

Read Order

1. Read the module comment above RenewalState (RFC 2131 §4.4.5 defaults)
2. Read RenewalState
3. Read t1()
4. Read t2()
5. Read renewal_state()
6. Read renewal_target()
7. Run examples/dhcp/session_05_walkthrough.py

Read It Like Code

Read It Like Code

renewal_state(lease, now) -> RenewalState
renewal_target(state)     -> "granting-server" | "any-server" | "none"

Fields That Matter

Fields That Matter

Decision Flow

Decision Flow

now >= granted_at + duration   -> Expired    (renewal_target: none)
now >= t2(lease)               -> Rebinding  (renewal_target: any-server)
now >= t1(lease)                -> Renewing   (renewal_target: granting-server)
otherwise                      -> Fresh      (renewal_target: none)

Reading Lens

Reading Lens

The move this session asks for is to stop treating "lease expiry" as one deadline and start reading it as three, checked in a fixed order from latest to earliest — renewal_state() checks expiry first, then T2, then T1, falling through to FRESH only if none of those >= comparisons fire. Read the >= in each branch specifically: at exactly T1, the state is already RENEWING, not FRESH; at exactly T2, the state is already REBINDING, not RENEWING. Nothing in this module ever uses >.

Ask, at every state:

• who does renewal_target() say to ask, and does that match what the docstring on renewal_state() claims — "quietly, unicast" versus "louder now, broadcast"?
• is now past T1 but not yet T2, or past T2 but not yet expiry — which comparison in renewal_state() is the one that actually fires first for this now?
• what does the client lose, in seconds, to the // floor at T1 and T2, compared to if the division were exact?

The who-do-you-ask table is the thesis of this module, and it is worth writing out explicitly because renewal_target()'s three-line body is easy to skim past:

Toy Model Boundary

Toy Model Boundary

Real DHCP clients can request a specific lease time via option 51, and T1/T2 can be set explicitly by the server via options 58 and 59 rather than always falling back to the RFC 2131 defaults of 50% and 87.5%. This module hard-codes those two fractions inside t1() and t2() — there is no server override path, so every lease in this course renews on the same schedule regardless of what a real server might have configured.

There is also no actual renewal *request* modeled here — Session 06's server loop only implements DORA (DISCOVER/OFFER/REQUEST/ACK), not the unicast REQUEST a real client sends at T1 to renew in place. renewal_state() and renewal_target() tell you what a client *should* do and *who* to ask; nothing in this course's source sends that message.

Code Landmarks

Code Landmarks

The module comment above RenewalState

States the two fractions from RFC 2131 §4.4.5 directly — 50% for T1, 87.5% for T2 — and flags up front that both round down, so a lease can end up "very slightly short-changed rather than ever granted extra time." That asymmetry is worth carrying into t1() and t2() before reading their bodies.

t1() and t2()

Two one-line functions, both granted_at + (some fraction of duration), both using //. t2()'s lease.duration * 7 // 8 multiplies before dividing — read it as "seven eighths, floored," not as two separate steps that could reorder.

renewal_state()'s docstring

Spells out the whole state machine in prose before the code does: FRESH does nothing, RENEWING asks quietly (unicast, the granting server), REBINDING asks loudly (broadcast, any server), EXPIRED is just over. The four-line if chain below it is that paragraph compiled.

renewal_target()

Three lines, and the entire "who do you ask" answer for the whole module. Notice it takes a RenewalState, not a Lease or a now — by the time you call this, renewal_state() has already done all the time arithmetic.

Failure Questions

Failure Questions

Use the source file to answer these:

1. For a lease with duration=100, what integer does t1() return, and what integer does t2() return? Which operator — // or / — makes the difference between "half the lease" and the number t1() actually computes?
2. At now exactly equal to t1(lease), does renewal_state() return FRESH or RENEWING? Which specific comparison in the function decides this, and is it > or >=?
3. At now exactly equal to t2(lease), does renewal_state() return RENEWING or REBINDING? Why does the function check the T2 condition before the T1 condition rather than after?
4. What does renewal_target() return for RenewalState.EXPIRED, and what does it return for RenewalState.FRESH — are they the same string, and does the function distinguish those two states at all?
5. For a lease with duration=101 (an odd number), does t1() round down evenly, or does the client lose a fraction of a second of "fresh" time to the floor? What is t1()'s return value for granted_at=0, duration=101?

Walkthrough

Walkthrough

Run this:

PYTHONPATH=src python3 examples/dhcp/session_05_walkthrough.py

The walkthrough builds one lease, checks RenewalState at a point before T1, at exactly T1, at exactly T2, and at expiry, and prints renewal_target() for all four states as the who-do-you-ask table.

Done When

Done When

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

• "T1 is half the lease, T2 is seven-eighths, both computed with integer floor division from granted_at."
• "At exactly T1 the state is already RENEWING, and at exactly T2 it's already REBINDING — every boundary in this module is >=, never >."
• "RENEWING asks the one server that granted the lease; REBINDING gives up on that server and asks anyone; EXPIRED asks nobody."

References

References

• RFC 2131 Section 4.4.5 (T1 and T2: default renewal at 50% of lease time, rebinding at 87.5%)

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