Space Transport Protocols TCP, SCPSTP

1
Space Transport Protocols (TCP, SCPS-TP)

• Fred Brosi
• February, 2001

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What is SCPS?

• The Space Communications Protocol Standards
  (SCPS) project was developed to improve
  communications performance in a satellite
  environment
• Intended environments include GEO, HEO, and LEO
  (including constellations such as SDI and
  Teledesic)
• SCPS extended the internet protocol suite

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SCPS extends the internet protocol suite
Internet Stack
OSI Stack
Sample Protocols
7
Application
FTP, SCPS-FP
Application
Presentation
6
Session
5
TCP, SCPS-TP
End-to-End
Transport
4
IPSEC, SCPS-SP
3.5
IP, SCPS-NP
Internetwork
Network
3
Network Access
Various protocols (e.g., CCSDS commercial,
military)
Data Link
2
1
Physical
4
SCPS Capabilities
Record read record update
Core
File record Integrity
Automatic restart
Interoperable
/-
User suspend/resume
Internet FTP
Suppress ASCII reply codes.
SCPS FILE TRANSFER PROTOCOL
7
Congestion control appropriate formixed-loss
environments (congestion, corruption,
outage) Selective negative acknowledgment Robust
header compression Window scaling (long/fat
pipes) Partial Reliability service
(BETS) Delimitation of record boundaries Timesta
mping for high ratesequencing, delay measurement.
(SCPS-FP)
(Optional)
Core
Interoperable
SCPS TRANSPORT PROTOCOL
/-
Internet
4
(SCPS-TP)
TCP/UDP
SCPS SECURITY PROTOCOL
3.5
(SCPS-SP)
(Optional)
Authentication guarantee of the identity of a
source Access Control prevention of
unauthorized access Integrity protection
against modification Confidentiality protection
from disclosure.
SCPS NETWORK PROTOCOL
3
(SCPS-NP)
(Optional)
Provide both connectionless and managed-
connection routing Support precedence
(priority) based handling Offer multiple routing
options Signal errors to the layer
above Support packet lifetime control Scaleable
- tailor capability to need, e.g.,
highcommunications efficiency in
constrainedbandwidth conditions.
2
Underlying Link Protocol
SCPS sits on top of existing link capabilities,
and augments them
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When does the Internet stack have problems?

• When there is loss due to sources other than
  congestion
• When there are shorter round trip times in the
  network than yours (other users recover fast and
  get first in line)
• When sessions span spacecraft contacts or require
  handovers between ground stations (intentionally
  or unintentionally)
• When systems support varying bandwidthdelay
  products
• When there is significant bandwidth asymmetry
• When there is limited bandwidth, CPU, or memory

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What basic extensions to TCP does SCPS make?

• Selective NACK for faster, bit-efficient loss
  recovery
• Loss-tolerant congestion control
• Rate control (with or in lieu of congestion
  control)
• Loss-tolerant header compression
• Partial reliability service
• Ability to use SCPS-SP and/or SCPS-NP and/or IP
• Retains interoperability with standard TCP
  its extensions

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The SCPS Transport Protocol (SCPS-TP)

• Transport protocols support end-to-end
  communication only active at end points
• SCPS-TP is Internet TCP and UDP, plus extensions
• SCPS extensions improve operation in the space
  environment
• Result end-to-end communication and reliability
  to suit mission requirements

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Requirements for Reliable Data Transfer

• Support for end to end data transfer
• Transfer with full reliability (complete,
  correct, in sequence, no duplication)
• Uplink for command and table loads
• Downlink for compressed data transfer
• Transfer with best effort reliability (correct,
  in sequence, no duplication, possibly with gaps)
• For transfer of telemetry, image data, etc.
• Transfer with minimal reliability (correct,
  possibly incomplete, possibly out of sequence)
• Minimal reliability must support multicast
  operation
• Other requirements address priority, etc.

1993 SCPS Exploratory Study
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Factors affecting use of TCP in terrestrial and
spacecraft communication environments
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Modifications to TCP to support spacecraft
communication environments
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Things to remember about SCPS-TP

• SCPS-TP is TCP and UDP with modifications to suit
  the spacecraft communications environment
• 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
• A few profiles (2 or 3) will cover the majority
  of mission classes

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SCPS File Transfer Protocol (SCPS-FP)

• Files are named collections of data - could be
  in memory, mass storage, etc.
• File transfer protocols coordinate movement of
  files (or parts of files) between systems
• SCPS-FP is derived from FTP
• Extensions support movement of file records,
  addition of integrity checking, and
  interrupt/restart support

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SCPS-FP is Internet FTP With Extensions

• SCPS-FP extensions to Internet FTP include
• Record Read
• Record Update
• Autorestart
• Manual Interrupt
• Suppression of Reply Text