1. STREAM CONTROL TRANSMISSION PROTOCOL (SCTP)

2. SCTP service model connection oriented reliable data transfer
- no loss
- no duplicates
- data integrity
ordered / unordered delivery
TCP provides only ordered service.
If the application desires unordered service,
it has the option of using UDP => unreliable
SCTP separates data reliability from ordered delivery
SCTP can provide unordered service with reliability
SCTP can provide ordered service with reliability

3. SCTP preserves message boundaries
TCP is byte-oriented. Applications must add their own
record marking to delineate messages.
concept of chunks
security against SYN flooding attack
multi-homing
multi-streaming
message fragmentation and bundling
congestion control

4. Comparison between SCTP, TCP and UDP
Feature
SCTP
TCP
UDP
State required at endpoints
Yes No
Reliable Data Transfer
Multi-homed Host Support
Multi-Stream Support
Security against SYN attacks
Unordered Data Delivery
Failure detection (Heartbeat)

5. SCTP PDU format
1 common header + chunks (control or data)

6. Motivation
Many applications need reliable message delivery – they do so by delineating a TCP stream
TCP provides both strict-ordering and reliability – many applications may not need both

7. Motivation (contd) HTTP is one such application
While transferring multiple embedded files we only want
Reliable file transfer for each file
Partial ordering for the packets of each file but not total ordering amongst all the packets
TCP provides more than this (but overhead?)
SCTP may help (how? – later)

8. HTTP Server Architecture
Multiple File Transfer (Embedded files) - TCP
Client
Server
Request file 0
Fork child
Send file 0
Child
process
Request file 1..N
Send file 1,2,…N

9. HTTP Server Architecture
Multiple Files Transfer (Embedded Files) - SCTP
Client
Server
Request file 0
Fork child
Send file 0 – stream 0
Child
process
Request files 1..N
Send file 1 – stream 1
Send file N – stream N

10. Receive buffer in kernel
Reason
Server
Client 3 2 1 3 2 3 2 1 3 2 1 1
File 3
File 2
TCP
Send buffer in kernel
TCP
Receive buffer in kernel

11. Reason Server Client 3 3 2 2 3 2 1 3 2 1 1 1 File 3 File 2 SCTP
Receive buffer in kernel
SCTP
Receive buffer in kernel

12. Multi-homing

13. multi-homing single-homed SCTP endpoint multi-homed SCTP endpoint
Host B
Host A
application
application
IP1=
IP2=
IP3=
200
100
SCTP
SCTP B1 B2 B3 A1
IP=
endpoint=[ : 100]
endpoint=[ , , : 200]
SCTP association
Host A
Host B
application
application
100
IP1=
IP2=
IP3=
200
SCTP
SCTP A1 B1 B2 B3
IP=
association={ [ : 100] : [ , , : 200] }

14. X Network Host A Host B 4 possible TCP connections:
(A1,B1) or (A1,B2) or (A2,B1) or (A2,B2)
1 SCTP association:
({A1,A2},{B1,B2})
Primary destinations for A & B (e.g., A1 & B1)
Network
Host A
Host B B1 B2 A1 A2 X
What happens if a primary fails?
TCP connection is broken
whereas SCTP association can continue to transmit to an alternate destination
address

15. failure detection
Host A monitors reachability of the primary destination address of Host B
Host A
Host B
application
application
100
primary
200
alternates
SCTP
SCTP A1 B1 B2 B3
SACK
DATA
error_count is a variable associated
with each destination address of a
host. It is set to zero initially.
Host A starts the retransmission timer
If timer expires
increment error_count
If error_count > threshold
state = inactive
If Host A receives SACK before timer expires
error_count = 0 & state = active

16. Host A monitors reachability of alternate destination addresses of Host B
application
application
100
primary
200
alternates
SCTP
SCTP A1 B1 B2 B3
HEARTBEAT
HEARTBEAT-ACK
HEARTBEAT is sent periodically to each alternate address
When a HEARTBEAT is sent
increment error_count
If error_count > threshold
state = inactive
If Host A receives a HEARTBEAT-ACK
error_count = 0 & state = active
When the primary destination address is detected unreachable =>
SCTP sender chooses 1 of the REACHABLE, alternate destination addresses
as primary

17. association setup How many way handshake ? V: verification tag
I: initiation tag
Host A
Host B
closed
mandatory -
type
chunk flags
chunk length
initiation tag
a_rwnd
outbound streams
maximum inbound streams
initial TSN
optional -
addresses(IPv4,IPv6, hostname)
supported address types
ECN capable
cookie preservative
INIT (V=0) (I=TagA)
cookie
wait
closed

18. SCTP INIT PDU

19. association setup V: verification tag I: initiation tag Host A Host B
closed
INIT (V=0) (I=TagA)
cookie
wait
closed
INIT-ACK
(V=TagA) (I=TagB)
mandatory –
All fields present in mandatory INIT +
state cookie
optional -
addresses(IPv4,IPv6, hostname)
ECN Capable
error reporting for unrecognized
parameters
NO TCB on server-side at this point

20. association setup V: verification tag I: initiation tag Host A Host B
closed
INIT (V=0) (I=TagA)
type
chunk flags
chunk length
state cookie
DATA chunk can be sent along with COOKIE-ECHO
cookie
wait
INIT-ACK
(V=TagA) (I=TagB)
closed
COOKIE-ECHO
(V=TagB)
cookie
echoed
Note that now we can send data to mitigate the cost of the 4-way handshake

21. TCP SYN flooding attack
why COOKIE ???
attackers
victim
Flooded!!
SYN
SYN
buffer holding half-open (pending)
connections
SYN
SYN
SYN
SYN
SYN
SYN
TCP SYN flooding attack
There is no ACK in response to the SYN-ACK, hence connection
remains half-open
Other genuine clients cannot open connections to the victim
The victim is unable to provide service

22. how does COOKIE help ? Host B receives INIT
Receiver of INIT does not make the
Transmission Control Block (TCB) [i.e no
pending connection information kept]
Remains in CLOSED state
In case of attack, COOKIE-ECHO won’t
arrive but receiver is unaffected
Send INIT_ACK
with COOKIE
Prepare
COOKIE

23. how does COOKIE help ? Host B receives INIT Send INIT_ACK Prepare
with COOKIE
Prepare
COOKIE
Host B receives
COOKIE-ECHO
invalid
Extract & Validate
COOKIE
Discard SCTP PDU
valid
yes
COOKIE expired ?
Send ERROR
chunk
Sender can request
longer cookie life
next time through the
Cookie - preservative
parameter in the
INIT chunk no
Unpack COOKIE and
build association TCB
Discard SCTP
PDU

24. association setup V: verification tag I: initiation tag Host A Host B
closed
INIT (V=0) (I=TagA)
type
chunk flags
chunk length
DATA chunk can be sent along with COOKIE-ACK
cookie
wait
INIT-ACK
(V=TagA) (I=TagB)
closed
COOKIE-ECHO
(V=TagB)
cookie
echoed
COOKIE-ACK
(V=TagA)
Note that now we can send data to mitigate the cost of the 4-way handshake
4 – way handshake !
established
established

25. association shutdown Host A Host B upper layer invokes SHUTDOWN
established
established
DATA
DATA
shutdown_pending
SACK
SHUTDOWN
shutdown_sent

26. stop accepting data
SHUTDOWN
DATA
shutdown_sent
shutdown_received
SHUTDOWN+ SACK
SHUTDOWN_ACK
shutdown_ack_sent
SHUTDOWN_COMPLETE
delete TCB
closed
delete TCB
closed

27. when should a SACK be sent ?
Host A
Host B
DATA (TSN 1)
Endpoint sends DATA to its peer, always bundle
a SACK chunk to ack any new DATA chunks
SACK 1 + DATA
DATA (TSN 2)
If no DATA to be sent to the peer, then SACK is DELAYED
Delay – 200 to 500 ms
200 ms
SACK 2
DATA (TSN 2)
Duplicate data chunks => immediately send SACK without any delay
SACK 2

28. when should a SACK be sent ?
Host A
Host B
DATA (TSN 3)
Must send a SACK for every other SCTP PDU received without any delay
DATA (TSN 4)
200 ms
SACK 4
DATA (TSN 5) X
One or more TSNs missing => immediately send SACK with Gap Ack blocks without any delay
DATA (TSN 6)
SACK 4

29. summary
SCTP PDU = 1 common header + 1 or more chunks ( control or data)
Association setup = 4 way handshake (INIT, INIT-ACK, COOKIE-ECHO,
COOKIE_ACK)
COOKIE mechanism to prevent SYN flooding attack
Graceful shutdown(SHUTDOWN, SHUTDOWN-ACK,
SHUTDOWN-COMPLETE) no half-close as in TCP
Separates reliability from ordered delivery
Preserves message boundaries
SACK chunks to ack cumulative TSN + gap ack blocks + duplicate TSNs
Achieves link / path redundancy by supporting multi-homed hosts along with
reachability check

30. References Randall R. Stewart, Qiaobing Xie.
Stream Control Transmission Protocol (SCTP) A Reference Guide
Stewart et. al. Stream Contol Transmission Protocol RFC-2960, October 2000.
URL:
SCTP for Beginners
URL: essen.de/inhalt/forschung/sctp_fb/index.html
SCTP overview
SCTP tutorial
SCTP applicability statement

31. P. Amer, P. Conrad, Sam Baskinger
References
Slides collected from various sources including Keyur Shah, Sourabh Ladha,
P. Amer, P. Conrad, Sam Baskinger