UDP Speedy Secure Shell (USSH)
draft-x1co-ussh-03

Independent Submission                                           X. x1co
Internet-Draft                                                 x1colegal
Intended status: Experimental                               17 June 2026
Expires: 19 December 2026

                     UDP Speedy Secure Shell (USSH)
                           draft-x1co-ussh-03

Abstract

   This document describes UDP Speedy Secure Shell (USSH), an
   experimental remote shell protocol built on top of USTPS.  USSH
   provides an interactive shell session over a secure UDP-based
   transport while preserving USTPS transport semantics, including
   challenge-validated session setup, per-session key establishment,
   selective retransmission, and unordered transport behavior beneath
   the shell layer.

Status of This Memo

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   This Internet-Draft will expire on 19 December 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Relationship to USTPS . . . . . . . . . . . . . . . . . . . .   2
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .   3
   5.  Message Identification  . . . . . . . . . . . . . . . . . . .   3
   6.  Protocol Model  . . . . . . . . . . . . . . . . . . . . . . .   3
   7.  Interactive Stream Handling . . . . . . . . . . . . . . . . .   4
   8.  Head-of-Line Considerations . . . . . . . . . . . . . . . . .   4
   9.  Endpoint Binding  . . . . . . . . . . . . . . . . . . . . . .   4
   10. Security Model  . . . . . . . . . . . . . . . . . . . . . . .   5
   11. Operational Status  . . . . . . . . . . . . . . . . . . . . .   5
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   13. Security Considerations . . . . . . . . . . . . . . . . . . .   5
   14. Normative References  . . . . . . . . . . . . . . . . . . . .   5
   15. Informative References  . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   USSH is an experimental secure shell protocol built directly on top
   of USTPS rather than on TCP.  Its goal is to provide interactive
   remote shell access while preserving the properties of the underlying
   USTPS transport, including packet-level authenticated protection for
   DATA packets, per-client session keys, selective retransmission, and
   lack of transport-level Head-of-Line blocking.

   USSH is not intended to be wire-compatible with SSH.  It is a
   distinct protocol with its own packetization and control semantics.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Relationship to USTPS

   USSH uses USTPS as its secure transport substrate.  The outer secure
   envelope, readable plaintext control records, and binary UPACK DATA
   framing are provided by USTPS.  USSH then defines its own application
   messages for shell authentication, PTY setup, standard input
   delivery, standard output delivery, resize events, keepalives, and
   session termination.

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   USTPS remains unordered at the transport layer.  USSH therefore
   performs application-layer reconstruction where terminal behavior
   requires ordered reconstruction of logical streams such as standard
   output.

   All datagram-size, MTU, fragmentation, retry-token, nonce, and
   transport-sequence considerations are inherited from USTPS.  USSH
   does not define a second transport layer beneath itself.

4.  Session Establishment

   A USSH client first establishes a USTPS session with the server.  In
   current public implementations, this includes a plaintext HELLO, a
   plaintext challenge containing a token and generated Base64 session
   identifier, and a validated challenge response before DATA traffic is
   accepted.

   Only after the secure USTPS session exists does the client perform
   USSH authentication and request an interactive shell or PTY-backed
   session.

5.  Message Identification

   USSH application messages are carried inside USTPS DATA packets.
   They are not identified by the outer USTPS secure-envelope magic.
   The outer encrypted USTPS envelope uses USS1, while the inner USTPS
   DATA frame uses UPAK.

   Implementers should therefore distinguish between the secure
   envelope, USTPS control, USTPS DATA framing, and the USSH application
   payload when analyzing packet captures or decrypted data.  Readable
   transport control such as ACK: ..., NACK: ..., and HELLO: ... is a
   USTPS transport feature, not a USSH application record format.

6.  Protocol Model

   Current implementations support password-based authentication after
   the secure USTPS session is established.  Once authenticated, the
   server launches a PTY-backed shell on behalf of the client.

   USSH messages currently cover at least the following categories:

   *  Authentication and session setup

   *  Standard input delivery

   *  Standard output delivery

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   *  Terminal resize events

   *  Keepalive and liveness signaling

   *  Session close and exit signaling

   USSH uses its own application-level sequence numbering where needed
   for shell-specific logical streams such as standard input ordering.
   This sequence numbering is independent from the outer USTPS transport
   seq and exists only to preserve the expected shell byte-stream
   behavior.

7.  Interactive Stream Handling

   Interactive shells are highly sensitive to stream corruption and
   terminal redraw behavior.  Although USTPS is unordered at the
   transport layer, a shell's logical input and output streams often
   require application-level ordering.  USSH therefore reconstructs
   ordered logical output where needed before presenting terminal bytes
   to the local client terminal.

   This reconstruction is an application-layer behavior of USSH.  It
   does not convert USTPS itself into an ordered transport.

8.  Head-of-Line Considerations

   USSH inherits the lack of transport-level Head-of-Line blocking from
   USTPS.  However, because terminal streams and PTY behavior often
   require coherent ordered interpretation, USSH may temporarily delay
   terminal-visible output while reconstructing the correct logical
   order of shell output fragments.

   That delay occurs at the application layer and is distinct from
   transport-level Head-of-Line blocking.

9.  Endpoint Binding

   Current public implementations bind a USSH session to the source IP
   address and UDP port that successfully completed the underlying USTPS
   retry-token challenge.  Automatic path migration has been removed
   from the current public implementation because it created practical
   reliability and security problems.

   If the client changes network and therefore changes source IP address
   or UDP port, the existing session is expected to end and the client
   is expected to reconnect cleanly.

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10.  Security Model

   USSH relies on USTPS for mandatory AEAD protection of DATA packets
   and per-client key establishment.  Current public implementations
   additionally support persistent server host keys, TOFU-based server
   identity continuity, and application-level authentication before
   granting shell access.

   Implementations SHOULD carefully protect shell spawn logic, PTY
   handling, session teardown, and terminal state restoration.  Clients
   SHOULD restore local terminal settings even when the session closes
   unexpectedly.

   Old or duplicate USTPS transport packets that arrive late are handled
   according to USTPS duplicate and stale-packet rules.  Old or
   duplicate USSH application chunks SHOULD likewise be ignored once the
   corresponding logical application sequence space has already advanced
   past them.

11.  Operational Status

   USSH has been implemented and exercised as a Beta-grade experimental
   protocol.  Public development has focused on stability over the USTPS
   substrate, interactive shell usability, session liveness, terminal
   negotiation, and making the client restore terminal state correctly
   when sessions close unexpectedly.

12.  IANA Considerations

   This document has no IANA actions.

13.  Security Considerations

   Remote shell protocols expose high-impact capabilities.  In addition
   to the security requirements inherited from USTPS, USSH
   implementations MUST authenticate clients before granting shell
   access, MUST avoid leaking shell access to unauthenticated peers, and
   SHOULD defend against terminal-state corruption and stale-session
   reuse.

   Applications and operators should understand that USSH is
   experimental and not interchangeable with SSH security review,
   ecosystem hardening, or deployment maturity.

14.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

15.  Informative References

   [USTPS-DRAFT]
              x1co, X., "UDP Speedy Transmission Protocol Secure
              (USTPS)", Work in Progress, Internet-Draft, draft-x1co-
              ustps-05, June 2026,
              <https://datatracker.ietf.org/doc/draft-x1co-ustps/>.

Author's Address

   x1co
   x1colegal
   Email: x1colegal@outlook.com.br

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