Agent Tcl: A Flexible and Secure Mobile-Agent System
Reference: Robert S. Gray (1996). Proceedings of the 1996 USENIX Tcl/Tk Workshop, pp. 9-23. Source file: gray-security.pdf. URL
Summary
Gray introduces Agent Tcl, a mobile-agent system built on the Tcl scripting language plus Safe Tcl that supports migration with a single agent_jump instruction, transparent inter-agent communication, and pluggable transport mechanisms. The paper argues that Tcl’s simplicity and ease of embedding make it more accessible than Telescript’s object-oriented language while the Safe Tcl extension provides the sandboxing needed for secure execution — a better balance than the ARA, Tacoma, or SodaBot alternatives.
The architecture has four layers (transport, server, interpreter, agents) and supports PGP-signed agents, resource-manager-based authorization, direct-connection and message-passing communication, and a flat namespace of named agents. Gray details how the Tcl core was modified to capture complete interpreter state (stack, procedure frames, variables) for transparent migration at arbitrary points, and outlines a plan to add Java and Scheme as additional agent languages.
Key Ideas
- Single-instruction migration (
agent_jump) that transparently captures/restores state.
- Safe Tcl for sandboxed execution with policy decoupled from mechanism.
- Layered architecture supporting multiple languages and transports.
- Security approach: PGP-signed code + resource managers + Safe Tcl.
- Explicit interpreter stack rewrite to enable mid-execution migration.
Connections
Conceptual Contribution
- Claim: A mobile-agent system can be both simple and secure by building on an interpreted scripting language (Tcl) augmented with Safe Tcl sandboxing, exposing migration as a single instruction that transparently captures/restores complete interpreter state.
- Mechanism: Modifies the Tcl core to use an explicit command stack (replacing recursive Tcl_Eval) so full execution state is capturable; primitives agent_begin/submit/jump/send/receive/meet/accept provide uniform mobility and messaging; security via PGP-signed agents + per-identity resource managers + Safe Tcl trusted/untrusted dual interpreters with link substitution.
- Concepts introduced/used: Mobile Agent, agent_jump, State Capture, Safe Tcl, Sandboxing, Explicit Command Stack, Flat Namespace, Resource Managers
- Stance: engineering
- Relates to: Precursor to DAgents Security Book Chapter which formalises the security model; contemporary alternative to Telescript/ARA/Tacoma (discussed in the D’Agents chapter); shares interpreter-level security philosophy with Agents Secure Interaction in Data Driven Languages.
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