LTP for Reliable Telecommand-2 (NGU WG) Scott Burleigh, JPL November 1, 2011 Boulder

Licklider Transmission Protocol (LTP) supports both reliable (“red”, acknowledged) and unreliable (“green”, unacknowledged) transmission over space links. Suggestion: for Telecommand-2, utilize LTP “red” transmission in place of COP? LTP provides automatic detection and retransmission of lost content, using selective negative acknowledgment, for arbitrary data – not just for files, as CFDP does. Some additional issuance latency and transmission overhead. May be too complex for FPGA implementation. But concurrent transmission and automatic retransmission maximize effective data rate over long one-way light times. Lets uplink protocol design focus on coding and modulation. Overview CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

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. Concurrent transmission. Multiple transmissions between two peers may be in progress concurrently. How LTP Works CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

LTP Operation CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

OWLT no mutual visibility (timer suspended) timeout interval deliver block to client CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

Version Number (0x01) Segment Type Flags Session ID (Engine ID of source, session number chosen by sender) Header Extensions Segment Content Trailer Extensions Header Extension CountTrailer Extension Count Header Trailer Control-Byte Extensions CTRLEXCFlag1Flag0 LTP Segment Format CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

CTRL EXC Flag 1 Flag 0 Code Nature of segment Red data, NOT {Checkpoint, EORP or EOB} Red data, Checkpoint, NOT {EORP or EOB} Red data, Checkpoint, EORP, NOT EOB Red data, Checkpoint, EORP, EOB Green data, NOT EOB Green data, undefined Green data, undefined Green data, EOB Report segment Report-acknowledgment segment Control segment, undefined Control segment, undefined Cancel segment from block sender Cancel-acknowledgment segment to block sender Cancel segment from block receiver Cancel-acknowledgment segment to block receiver Flags CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

Some extra latency: commands would travel in LTP blocks, possibly aggregated, and the contents of an LTP block aren’t delivered until the entire block has been received. Some extra transmission overhead: – One LTP segment header plus one encapsulation packet header per command. – At least one report segment (and one report ack segment in the opposite direction) per LTP block. Complexity: difficult to put the entire protocol in FPGA, so some would remain in software – so unavailable if the processor crashes. No good for discrete hardware commands. Drawbacks CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

Automatic retransmission of corrupt data ensures command arrival. Concurrent transmission: corruption of data in one block doesn’t delay the delivery of other concurrently-issued blocks. Tolerates lengthy, irregular interruption of link without data loss. – Automatically responds to variation in round-trip time due to start and stop of contact. Tolerates arbitrarily long signal propagation times. Extension mechanism is built into the specification. – Currently defined extension segments implement security. Like TC, supports both reliable and unreliable transmission. Enables focus of uplink to be on coding and modulation. Advantages CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

Conclusions COP-1 in TC limits the effective data rate, because any data loss will suspend data delivery for at least one round trip while go-back-N procedure is executed. LTP selective NAK provides the same reliability but more efficiently: only the data that are known to have been lost are retransmitted. LTP concurrent transmission enables delivery of received data to continue during retransmission of lost data, so effective data rate is higher. LTP provides better support for high-rate uplink than COP-1. CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

Backups

Sending LTP Entity Receiving LTP Entity Sending LTP User Receiving LTP User RS CP, EORP RS CP RSReport Segment CPCheckpoint EORPEnd of Red Part Transaction.re quest SessionStart.indication GSArrival.indications RedPartReception. indication TransmissionSession Complete. indication CP, EORP RS InitialTransmission Complete.indication LTP Block Portion of block to be transmitted reliably. Portion of block to be transmitted unreliably.

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) CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

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 CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

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 CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

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. CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA

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. CCSDS Fall 2011 Meeting – NGU WG – Boulder CO, USA