Tail Call Execution Semantics

The five-minute read

One action, walked through

1. Open BIP 117 and read the status line before the examples.
2. Write down the rule in one sentence. A fair version of that sentence is: BIP16 (Pay to Script Hash)[1] and BIP141 (Segregated Witness)[2] provide mechanisms by which script policy can be revealed at spend time as part of the execution witness.
3. Name the object that changes: BIP16, BIP141, BIP116.
4. Ask what an old client, an old contract, or an offline counterparty does. If the document is silent, the silence is part of the design.

The argument, unpacked

What has to be true

• You are implementing BIP 117 at the status the text itself states: Draft.
• The object that has to change is BIP16, BIP141, BIP116. A neighbouring document with a similar name is not this one.
• Activation is a separate mechanism from the opcode or the sighash. This page does not pick a height.

What happened after the paper

The source marks this draft. It is not a live consensus rule just because it has a number. Later documents can narrow, replace, or ignore this one. Cite the number you mean.

What to check before you use the idea

• Which bytes become invalid under Tail Call Execution Semantics, and which old transactions stay valid?
• Is enforcement in consensus, or only in the mempool policy of one client?
• Which of these objects does the text actually define: BIP16, BIP141, BIP116?

Terms

BIP 117

The public text titled Tail Call Execution Semantics.

Draft

The document's own label for how finished the text is. It is not a market fact.

The problem the paper names

Bitcoin script and block validity only change when a soft fork says an old pattern is now invalid. Without that rule, every node is free to accept what this document wants to reject.

What the design proposes

• BIP16 (Pay to Script Hash)[1] and BIP141 (Segregated Witness)[2] provide mechanisms by which script policy can be revealed at spend time as part of the execution witness.
• In both cases only a single script can be committed to by the construct.
• While useful for achieving the goals of these proposals, they still require that all policies be specified within the confine of a single script, regardless of whether the policies are needed at the time of spend.

How the mechanism is specified

• Taken from the specification, the next constraint is: In both cases only a single script can be committed to by the construct.
• Locate BIP16, BIP141, BIP116 in BIP 117 and apply it to one transaction or call.
• Then check the failure the class of rule always has: a node that did not upgrade, a reverted call, a replayed signature, or a peer that does not speak the message.

What this page does not treat as proven

• A soft fork does not bind a node that never upgrades. It binds the nodes that enforce the new rejection.
• Activation is a separate mechanism from the opcode or the sighash. This page does not pick a height.
• The source marks this draft. It is not a live consensus rule just because it has a number.

Why a venture studio still reads it

Use BIP 117 when a pitch says 'Tail Call Execution Semantics' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.