National Aeronautics and Space Administration 1 Licklider Transmission Protocol (LTP): An Overview Scott Burleigh Jet Propulsion Laboratory California Institute of Technology 23 April 2009

National Aeronautics and Space Administration 2 What it is A retransmission protocol for delay-tolerant reliable communication between two adjacent points in a network. Descended from the acknowledged transmission procedures of CFDP, the CCSDS File Delivery Protocol. Originally designed to serve as a “convergence-layer” protocol for the interplanetary leg(s) of an end-to-end path in a delay- tolerant network (DTN). –Runs just above link layer, e.g., CCSDS Telemetry/Telecommand. May also be useful for some kinds of terrestrial applications, running above UDP/IP.

National Aeronautics and Space Administration Bundle Protocol: routing, custody transfer 3 Where it fits in TCP CCSDS TM/TC file transfer, messaging, etc. PPP, Ethernet, , SONET… CCSDS Prox-1 LTP encap IP UDP

National Aeronautics and Space Administration 4 Chronology Late 2000 – initial LTP design drafts circulated within DTN team. February 2002 – CFDP approved by CCSDS. December 2003 – Mani Ramadas (Ohio University) volunteers to work on an Internet Draft for LTP. March 2004 – Stephen Farrell (Trinity College, Dublin) joins up. 12 May 2004 – first LTP Internet Draft posted. Summer of 2004 – Stephen writes first implementation in C August 2005 – Mani releases reference implementation in Java. 1 December 2006 – Chris Krupiarz (Johns Hopkins University Applied Physics Lab) reports on MESSENGER flight software testbed exercise of an LTP implementation in C. October 2008 – Deep Impact Network Experiment demonstrates the use of LTP on-board a spacecraft in interplanetary space

National Aeronautics and Space Administration 5 Features Can handle very large bandwidth-delay product. Tolerates lengthy, irregular interruption of link without data loss. –Handle variation in round-trip time due to start and stop of contact. Minimizes overhead on low-capacity and/or asymmetric links. –Selective NAKs. Aggregation of small client service data units into larger blocks, acknowledged at block granularity. Optional accelerated retransmission: multiple checkpoints per transmitted block, or interim reports prior to reception of checkpoint. Partial reliability: checkpointing and retransmission for only the first N bytes of a block, and N can be zero. Extension mechanism is built into the specification. –Currently defined extension segments implement security.

National Aeronautics and Space Administration 6 How it works A block of client service data to be transmitted is divided into segments. When the segments are transmitted, one or more are flagged as checkpoints. When a checkpoint is received, the receiver returns a report of cumulative reception for that block. –Reports acknowledge checkpoints and either signal successful reception or else trigger retransmission. –Reports are explicitly acknowledged. Reports and checkpoints are on timers, are retransmitted if not acknowledged. Known changes in remote peer’s transmission state may dynamically revise timers. Deferred transmission. Multiple transmissions between two peers may be in progress concurrently.

National Aeronautics and Space Administration 7 OWLT no mutual visibility (timer suspended) timeout interval deliver block to client

National Aeronautics and Space Administration 8 OWLT Original countdown timer (A) transmit original segment (B) receive original segment, queue ACK (C) transmit ACK (D) receive ACK (S) remote engine suspends transmission (R) remote engine resumes transmission (sender) (receiver) queuing (etc.) margin time signal propagation time delay for suspended transmission Timer revision (1 of 3)

National Aeronautics and Space Administration 9 OWLT Original countdown timer (C) transmit ACK (S) remote engine suspends transmission (R) remote engine resumes transmission (sender) (receiver) (B) receive original segment, queue ACK queuing (etc.) margin time signal propagation time delay for suspended transmission Timer revision (2 of 3) (A) transmit original segment (D) receive ACK

National Aeronautics and Space Administration 10 OWLT Original countdown timer (S) remote engine suspends transmission (R) remote engine resumes transmission OWLT adjusted countdown timer queuing (etc.) margin time signal propagation time delay for suspended transmission (C’) transmit ACK (D’) receive ACK (sender) (receiver) (C) transmit ACK (B) receive original segment, queue ACK Timer revision (3 of 3) (A) transmit original segment (D) receive ACK

National Aeronautics and Space Administration 11 LTP vs TCP (1 of 2) TCPLTP architectural elements One durable, unbounded connection per pair of ports. “Window” is buffer of bytes in transit on connection. One temporary, bounded session per transmission unit. “Block” is buffer of bytes in transit within session. acknowledgmentsACKs on ranges of bytes in window; SACK optional. Selective NAKs on ranges of bytes in block. configuration of communication Connections are dynamically opened, parameters negotiated. No connection protocol. Parameters are managed and asserted. demuxPort number. Different port number at receiver for each connection. Session number. Any number of sessions may be delivering to the same client. concurrencyNumber of concurrent open connections is typically limited by number of FDs. Number of concurrently open sessions is limited by available space, possibly management.

National Aeronautics and Space Administration 12 LTP vs TCP (2 of 2) TCPLTP sites of retransmission End-to-end. Retransmission sites are co-located with applications. Point-to-point. Retransmission sites are co-located with routers. delivery orderBytes delivered in-order within connection. Bytes delivered in-order within session, but sessions may complete out of order. timersTimeout interval computed from RTT history. Timeout interval computed from known OWLT and link state schedule. flow controlNumber of unacknowledged bytes in buffer is limited by each connection’s window size. Number of unacknowledged bytes in all blocks is limited by max block size and max number of sessions. congestion controlControl window size for each connection; slow start, AIMD. No congestion control; bundle protocol may do rate control.

National Aeronautics and Space Administration 13 Status of specifications Three LTP Internet Drafts began IRSG review in April of 2007, were published as Internet RFCs in September of 2008: –"Licklider Transmission Protocol - Motivation", RFC 5325 –"Licklider Transmission Protocol - Specification", RFC 5326 –"Licklider Transmission Protocol - Extensions", RFC 5327

National Aeronautics and Space Administration 14 Implementations Stephen Farrell’s implementation in C++ Mani Ramadas’s implementation in Java APL implementation in C JPL implementation in C, used for DINET Successful interoperation tests: –Between C++ and Java implementations at November 2006 IETF meeting in San Diego –Between C++ and JPL implementations at March 2008 IETF meeting in Philadelphia

National Aeronautics and Space Administration 15 Tracking the work Home page – Mailing list –

National Aeronautics and Space Administration 16 Acknowledgment Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.