Title: Space Transport Protocols TCP, SCPSTP
1Space Transport Protocols (TCP, SCPS-TP)
- Fred Brosi
- February, 2001
2What 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
3SCPS 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
4SCPS 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
5When 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
6What 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
7The 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
8Requirements 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
9Factors affecting use of TCP in terrestrial and
spacecraft communication environments
10Modifications to TCP to support spacecraft
communication environments
11Things 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
12SCPS 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
13SCPS-FP is Internet FTP With Extensions
- SCPS-FP extensions to Internet FTP include
- Record Read
- Record Update
- Autorestart
- Manual Interrupt
- Suppression of Reply Text