Zequn Huang
CISC856
Sophie Che
Dept. Computer and Information Science
University of Delaware
11/20/2012
(some
slides
courtesy
of Lloyd
Wood)
Thank
Sophie
Che for
her slides
Outline
 Introduction
 DTN background
 Saratoga Protocol
 Saratoga Transactions
What’s Saratoga?
 Developed by Surrey
Satellite Technology to
transfer remote-sensing
imagery from its IPbased satellites to
ground.

Saratoga is a lightweight
transport protocol that
can also be Delay
Tolerant Network (DTN)
convergence layer.
Original Use Case
 Disaster Monitoring
Constellation (DMC)
 Used for rapid daily
large-area imaging
 Can observe effects of
natural disasters
fires in California, 28 October 2003 (UK-DMC)
DMC in use: after Hurricane Katrina, 05
 In this image,
dry land is red.
Flooded and
damaged land is
shown as brown
 DMC is working
as part of the
United Nations
International
Charter for
Space and Major
Disasters
Today’s Internet
 Successful at interconnecting communication
devices across the globe
 Based on TCP/IP protocol suite
 Connected in end-to-end, low-delay paths between
sources and destinations
 Low error rates and relatively symmetric
bidirectional data rates
Evolving Wireless Networks
Outside the Internet
 Support long and variable delays, arbitrarily long
periods of link disconnection, high error rates, and
large bidirectional data-rate asymmetries
 Examples
 Terrestrial civilian networks connecting mobile wireless
devices
 Wireless military battlefield networks connecting troops,
aircraft, satellites, and sensors
 Outer-space networks, such as the InterPlaNetary (IPN)
Internet project
Interplanetary Network
Imagine
If I say a “Hi” to you and you hear it after
9 Hours !!!!!
Interplanetary Network
Mars
Mars
Mars
Earth
Why Delay-Tolerant Network ?
 The




Internet’s underlying assumptions
Continuous, bidirectional end-to-end path
Short round-trips
Symmetric data rates
Low error rates
 The characteristics of evolving and potential
networks




Intermittent connectivity
Long or variable delay
Asymmetric data rates
High error rates
 New architectural concept is needed!
Store-And-Forward Message
Switching
 The problems of DTNs can be overcome by store-
and-forward massage switching
 DTN routers need persistent storage for their
queues because



A communication link may not be available for a long time
One node may send or receive data much faster or more
reliably than the other node
A message, once transmitted, may need to be
retransmitted for some reasons
Layers in DTN
DTN
TCP/IP Protocol Suite
Application
Application
DTN Bundle Layer
Transport Layer
Transport Layer
IP
IP
Link Layer
Link Layer
MAC/Channel
Physical Layer
Where is Saratoga in DTN
 An IP-based convergence
layer

transport protocols that it
uses to move data across
different networks
 Using UDP as a starting point
Why not using TCP?
 DTN has the features
 No congestion
 Long delay
 Asymmetric data rate
Application
DTN Bundle Layer
Convergence Layer
(Saratoga)
UDP
IP
Link Layer
MAC/Channel
Basic Saratoga Design
SNACK
Flood data
No
congestion
control
Scalable on
file size transferred
Compatible on IPv4 and
IPv6
Basic Saratoga design
 Flood data packets out as fast as possible
 ACKs are Selective Negative ACK (SNACKs)
indicating the gap to fill with resent data.
 File sender can require ACK from file reciver
TCP with SACK after timeout
Timer
TCP Sender
TCP Receiver
start
100199
stop
start
timeout
stop
s
e
n
d
e
r
loss
timer expires
r
e
c
e
i
v
e
r
100- 200199 299
100- 200199 299
100- 200199 299
300 400399 499
400- 500499 599
100- 200- 300- 400- 500199 299 399 499 599
File Offset Descriptor
 Length is fixed within a transfer, but vary
between different transfers
 How to design a scalable file transfer protocol
able to handle any size file?
 Solved this problem with 16/32/64/128-bit
pointers
Supported Size
Max File Size
16 bits
2^16 = 64 K
32 bits
2^32 = 4G
64 bits
2^64
128 bits
2^128
Saratoga Packets
BEACON
Sent periodically
Advertise nodes’ presence,
capabilities, and desires
Ask for a download, upload,
REQUEST directory listing, or deletion
operation
_get_, _put_, _getdir_, _delete_
Sent at start of transaction
METADATA Description of the file
Specify the max supported file size
Saratoga Packets (cont’d)
DATA
STATUS
Actual file data being transferred
Use chosen maximum file size
MAY request an ack
Responds to REQUEST or DATA.
Can signal list of unreceived data to
sender during a transfer
Data Packet Format
Status Packet Format
(HOLESTOFILL)
An example
Saratoga Transactions
GET without loss
REQUEST
Transfer accepted
METADATA
DATA #1
DATA #2
DATA #3
DATA #4
DATA #5
DATA #6
completed
HOLESTOFILL
Request an ack
GET with loss
REQUEST
Transfer accepted
X
Indicate a loss
METADATA
DATA #1
DATA #2
DATA #3
HOLESTOFILL for #2
DATA #2
completed
Request an ack
HOLESTOFILL
Request an ack
GET with Inactivity Timer
Start timer
REQUEST
METADATA
Reset timer
Reset timer
Reset timer
Store the file for
possible resuming later
DATA #1
HOLESTOFILL
DATA #2
No
connection
any longer
GET with Inactivity Timer (cont’d)
REQUEST
METADATA
DATA #1
HOLESTOFILL
DATA #2
DATA #3
DATA #4
Request an ack
Start timer
DATA #5
Cease to send
DATA
GET being Rejected
 Transfer rejected
 An error code is sent
 The transaction is terminated
REQUEST
METADATA
PUT
METADATA
Transfer accepted HOLESTOFILL
DATA #1
DATA #2
DATA #3
DATA #4
DATA #5
DATA #6
completed
HOLESTOFILL
Request an ack
DELETE
 A REQUEST packet including the file path to be
deleted
 Trigger a HOLESTOFILL with a status code
Reference
 Saratoga: a Delay-Tolerant Networking
convergence layer with efficient link utilization
 Delay-Tolerant Networks(DTN) A Tutorial
 TCP Selective Negative Acknowledgment over
IEEE 802.11 Wireless Networks
 Saratoga: A Scalable Data Transfer Protocol
draft-wood-tsvwg-saratoga-12
Video
 http://www.youtube.com/watch?v=XTmYm3gMYO
Q
Questions?
Ref: draft-wood-tsvwg-saratoga-02