The SCPS Transport Protocol
(SCPS-TP)
SCPS-FP
SCPS-TP
SCPS-SP
SCPS-NP
SCPS-FP
SCPS-TP
SCPS-SP
l Transport protocols support end-to-end
SCPS-NP
communication; only active at end points
SCPS-FP l SCPS-TP is Internet TCP and UDP plus
SCPS-TP extensions
network SCPS-SP l SCPS extensions improve operation in
SCPS-NP the space environment
l Result: end-to-end communication and
reliability to suit mission requirements
MITRE
Requirements for Reliable Data Transfer
Identified by 1993 SCPS Exploratory Study
l Support for end to end data transfer
l Transfer with full reliability (complete, correct, in sequence,
no duplication)
– Uplink for command and table loads
– Downlink for compressed data transfer
l Transfer with “best effort” reliability (correct, in sequence,
no duplication, possibly with gaps)
– For transfer of telemetry, image data, etc.
l Transfer with minimal reliability (correct, possibly
incomplete, possibly out of sequence)
– Minimal reliability must support multicast operation
l Other requirements address priority, etc.
MITRE
Evolution of SCPS-TP
ISO TP4/8602
Phase 1
Exploratory
Analysis
Internet TCP/UDP Internet TCP/UDP
XTP
NetBLT Phase 2
Standards
Development
TP4 = OSI Transport Protocol, Class 4
ISO 8602 = OSI Connectionless Transport Protocol
TCP = Transmission Control Protocol
UDP = User Datagram Protocol SCPS-TP =
XTP = Express Transfer Protocol Internet TCP/UDP +/-
NetBLT = Network Block Transfer Protocol
SCPS-TP = Space Communications Protocol Standards - Transport Protocol
MITRE
Factors affecting use of TCP in terrestrial
and spacecraft communication environments
Factor Terrestrial Communication Spacecraft Communication
Bit Error Rate < 10-9 10-5 to 10-12
Round Trip Delay Milliseconds to seconds Seconds to hours
Continuity of Continuous Intermittent: 10% (LEO) to
Connectivity 90% (TDRSS) per orbit
Forward and Reverse 1:1 (Equivalent Rates) 10:1 to 2000:1
Links Some have downlink only
CPU Capacity Unrestricted Possibly low (1750, 80186)
Memory Availability Unrestricted Possibly low
Communication Goals Fair access over time High throughput during
High aggregate throughput contact period
over time • Low horizon angles
High reliability • Data compression
Maximum link utilization
Primary Sources of Data Congestion Congestion
Loss Corruption
Link outage
Modifications to TCP to support spacecraft
communication environments
Factor Spacecraft Communication TCP Mods Required
Bit Error Rate 10-5 to 10-12 Corruption response
SNACK
End-to-end Header compression
Round Trip Delay Seconds to hours Large windows
Timer modifications
Continuity of Intermittent: 10% (LEO) to Link outage support
Connectivity 90% (TDRSS) per orbit
Forward and Reverse 10:1 to 2000:1 Rate control
Links Some have downlink only Revised Ack strategy
Header compression
CPU Capacity Possibly low (1750, 80186) Header Precomputation
Memory Availability Possibly low Increased robustness of
implementation
Communication Goals High throughput during Congestion response and
contact period slow start made optional (supported
• Low horizon angles by rate control)
• Data compression Header precomputation
Maximum link utilization Separate corruption response
SNACK
Primary Sources of Data Congestion Separate response for each type of
Loss Corruption loss and support for incoming signals
Link outage of loss type
Things to remember about SCPS-TP
l SCPS-TP is TCP and UDP with modifications to suit
the spacecraft communications environment
l Depending on configuration options selected, SCPS-
TP can be less than TCP, identical to TCP, or a
super-set of TCP
– Mission requirements dictate which options are
selected
– We anticipate that a small number of profiles (2
or 3) will emerge that cover the vast majority of
mission classes
MITRE
