1. Stream Control Transmission Protocol (SCTP)
Acknowledgements
Prof. Paul Amer
Randall Stewart ~ Philip Conrad ~ Janardhan Iyengar
CISC 856: TCP/IP and Upper Layer Protocols
Presented By : Nikhil Shirude
November 15, 2007

2. Overview
Motivation for SCTP
SCTP PDU and Chunk Format
SCTP 4-Way Association
SCTP Association Shutdown
SCTP Multi-Homing
Summary

3. SCTP Motivation
Primary Motivation – Transportation of telephony signaling messages over IP networks
Telephony Signaling – rigid timing & reliability requirements
TCP Limitations
head-of-line blocking
does not preserve A-PDU boundaries
no support for multi-homing
vulnerable to SYN Flooding attacks
SCTP Features
4 way handshake
multihoming
multistreaming
framing
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4. SCTP Overview Services/Features SCTP TCP UDP Connection-oriented yes
Full duplex
Reliable data transfer
Partial-reliable data transfer
proposed
Flow control
TCP-friendly congestion control
ECN capable
Ordered data delivery
Unordered data delivery
Uses selective ACKs
optional
Path MTU discovery
Application PDU fragmentation
Application PDU bundling
Preserves application PDU boundaries
Multistreaming
Multihoming
Protection against SYN flooding attack
n/a
Allows half-closed connections
Reachability check
Pseudo-header for checksum
no (uses vtags)
Time wait state
for vtags
for 4-tuple

5. SCTP PDU Format Building blocks of an SCTP PDU Destination Port
Source Port
Destination Port
Verification Tag
Checksum
Chunk 1 …
Chunk N
Common Header
SCTP
PDU
Chunks
Building blocks of an SCTP PDU
Common Header which occupies the first 12 bytes
Header has a CRC-32 checksum.
Chunks are of two types: Control chunks and Data chunks

6. SCTP Chunk Format Type Flag Length Chunk Information
(Multiple of 4 bytes)
Type Data, Init, SACK, Cookie Echo, HeartBeat …
Flag Bit meanings depend on type
Length - Defines total size of the chunk including type, flags, length and data/parameters
General format for all chunks
Type determines what flag bits mean and whether they are used or not.
Length includes type, flags, length, and data. Minimum size is 4.
Data is optional.

7. Some SCTP Chunk Types Type SCTP TCP 0x00 DATA User data 0x01 INIT
~ SYN
0x02
INIT-ACK
0x03
SACK
Selective ACK
0x04
HEARTBEAT
Keep-alive message
0x05
HEARTBEAT-ACK
0x07
SHUTDOWN
~FIN
0x08
SHUTDOWN-ACK
Chunk types may have permanent parameters specific to the chunk type. Some may also have optional parameters.

8. SCTP Feature Summary What TCP and SCTP both have:
reliability (retransmissions)
congestion control
connection oriented
SCTP adds the following:
4-way handshake
to reduce vulnerability to Denial of Service attacks
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
Note: will discuss

9. First - TCP Connection Establishment
closed
t=0
SYN
SYN sent
listen
1RTT
ACK
established
SYN-ACK
SYN recd
(TCB created)
data
established

10. Security: TCP Flooding Attack
process
SYN
(victim)
TCP-based web server
(attackers)
spoofed SYN’s
Internet
SYN
TCB
SYN
TCB
SYN
SYN
TCB
SYN
SYN
flooded!!
TCB = Transport Control Block

11. The SCTP Way: 4-way handshake limits attack
process
INIT
(victim)
SCTP-based web server
(attackers)
spoofed INIT’s
Internet
INIT
INIT-ACK
INIT
INIT-ACK
INIT
No reserved resources
No flooding!!
INIT-ACK

12. SCTP: Four-way Association Setup
closed
V: Verification tag
I : Initiate tag
t=0
INIT (V=0) (I=TagA)
cookie
wait
INIT–ACK (V=TagA) (I=TagB)(StateCookie)
closed
1RTT
COOKIE–ECHO (V=TagB) (StateCookie)
cookie
echoed
COOKIE–ACK (V=TagA)
2RTT
estab’d
data (V=TagB)
established

13. What does a Cookie contain?
Information from original INIT
Information from current INIT-ACK
Timestamp
Life span of cookie (Time to Live)
Signature for authentication (MD5)

14. SCTP Association Graceful Shutdown
Upper layer
invokes SHUTDOWN
estbl’d
estbl’d
DATA
DATA
shutdown_pending
SACK
SHUTDOWN
Mandatory:
• chunk type
• chunk flags
• chunk length
• cumulative TSN
stop accepting data
shutdown_sent

15. shutdown_pending SHUTDOWN stop accepting data DATA shutdown_sent
shutdown_received
SHUTDOWN + SACK
SHUTDOWN_ACK
shutdown_ack_sent
SHUTDOWN_COMPLETE
Mandatory:
• chunk type
• chunk flags
• chunk length
closed
(delete TCB)
closed
(delete TCB)

16. SCTP Feature Summary What TCP and SCTP both have:
reliability (retransmissions)
congestion control
connection oriented
SCTP adds the following:
4-way handshake
to reduce vulnerability to Denial of Service attacks
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
Note: will discuss

17. Multi-Homing
port
Application
SCTP
132 (IANA)
...
IP addresses IP
Link
...
...
Physical
...
Multi-Homing: Technique to improve reachability of hosts
which are reachable on more than 1 destinations (interfaces)
The property of SCTP of most interest is the decoupling of the transport-layer session state from the actual IP address at each end of the connection. Lots of folk seem interested in having a transport-layer session that could have increased robustness simply by multi-homing each end-host for the session onto different networks (providing path diversity).
Maintain uninterrupted connections for FCS applications as IP addresses change/fail while nodes migrate and/or become disabled
Persistence during dynamic addition and deletion of IP addresses
Failover and changeover without degradation of performance or QoS

18. Traditional “Uni” homing
transport connection A
Internet
points of failure B
Fig – server client 2 int each. Server’s 1 interface dies. Tcp connection aborted.
client
Web server

19. Traditional “Multi” homing (TCP)
transport connection A1
Internet
points of failure B1 A2 B2
Fig – server client 2 int each. Server’s 1 interface dies. Tcp connection aborted.
client
Web server
In TCP, host choose 1 of 4 possible combinations: (A1,B1) or (A1,B2) or (A2,B1) or (A2, B2)

20. Innovative “Multi” homing in SCTP
transport “association” A1
Internet B1 A2 B2
client
Web server
SCTP Multihoming
Hosts use one association ({A1,A2}, {B1,B2})
New data sent to one primary destination
- Let B1 be the web server’s primary destination
- Let A1 be the client’s primary destination
Path status and destination reachability constantly monitored.
Fig – server client 2 int each. Server’s 1 interface dies. Tcp connection aborted.
Multiple src/dest ip addresses
Path concept
Use of different physical paths not guaranteed
Peer reachability and path status are monitored (heartbeat)
One selectable default path
Parameters per path (cwnd, RTT, RTO, PRL)

21. Multi-homing Association
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] }

22. TCP data transfer without loss
sent by application
delivered to application
data to be sent
receive buffer (6) 6 5 6 4 6 5 4 3 5 6 6 5 4 3 2 1 4 5 6 3 2 3 1 4 2 5 6 A1 B1 5 6 2 4 1 3 3 2 1 6 4 5 A2 B2 22

23. TCP data transfer with loss
sent from application
delivered to application
data to be sent
receive buffer (6) 6 6 5 5 4 4 3 3 2 1 2 6 5 4 3 1 2
retransmission
loss A1 B1 5 4 1 6 2 3 5 3 4 2 1 6 A2 B2 23

24. TCP data transfer with single path failure
sent by application
delivered to application
data to be sent
receive buffer (6) 6 6 5 5 6 4 3 4 5 6 4 5 2 3 2 4 1 3
connection fails! 2 1 3 A1 B1 5 6 6 2 3 1 5 4 4 5 4 6 1 5 4 3 6 2 A2 B2 24

25. SCTP data transfer without loss
sent by application
delivered to application
data to be sent
receive buffer (6) 6 6 5 4 6 5 4 6 5 3 4 5 2 3 6 5 6 3 4 2 1 5 6 1 3 4 2 A1 B1 5 2 6 3 1 4 6 1 4 2 3 5 A2 B2 25

26. SCTP data transfer with loss
sent from application
delivered to application
data to be sent
receive buffer (6) 6 5 6 5 4 4 3 3 2 2 1 6 5 4 3 1 2 6
loss A1 B1 3 4 1 6 2 5 6 3 1 5 4 A2 2 2 B2
retransmission 26

27. SCTP data transfer with single path failure
sent by application
delivered to application
data to be sent
receive buffer (6) 6 5 6 4 6 5 6 4 3 5 3 6 4 5 2 2 4 5 3 6 1 5 6 1 2 4 3 A1 B1
Research questions:
when to failover? sooner-faster failover detection, but greater chance of spurious failover 2 4 1 5 6 3 5 4 6 2 3 1 A2 6 5 4 6 4 5 B2
retransmission 27

28. Multihoming Example B1 A1 Internet B2 A2
Laptop connected via Ethernet and Wireless.
Both the interfaces are reachable by the peer.
Ethernet gets disconnected, transmission of data fails.
Failure detected, SCTP uses the wireless interface to transmit.
HEARTBEAT is received.
Ethernet link is restored.
Heartbeat received
Ethernet B1
Ethernet A1
Internet B2
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802.11 A2
802.11
Client Host
(SCTP)
New Transmission Path
Server Host
(SCTP)

29. SCTP Failure Detection
Host A monitors reachability of primary dest address of Host B
Host A
Host B
application
application
100
primary
200
alternates
SCTP
SCTP A1 B1 B2 B3
SACK
DATA
Host A starts the retransmission timer
If timer expires
increment error_count
If error_count > threshold
path = inactive
If Host A receives SACK before timer expires
error_count = 0 & path = active
error_count --> variable associated with each destination address of a host. (initially zero)

30. Host A monitors reachability of idle 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 idle address
When a HEARTBEAT is sent
increment error_count
If error_count > threshold
path = inactive
If Host A receives a HEARTBEAT-ACK
error_count = 0 & path = active
When primary dest. address is detected unreachable =>
SCTP sender chooses REACHABLE, alternate dest. address as primary

31. HEARTBEAT?
HEARTBEAT is a chunk that an endpoint sends to its peer endpoints to probe the reachability of a particular destination transport address.
In our case, the HEARTBEAT is sent to a destination address which has been idle for a long time to check for its reachability.
HEARTBEAT ACK is a chunk which an endpoint sends to its peer endpoints as a response to a HEARBEAT chunk.
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32. Summary of SCTP
SCTP used for applications which require data reliability and rigid timing.
SCTP provides security against DOS attacks by using cookies during association
SCTP association can bind multiple IP addresses at each endpoint
SCTP provides multi-homing for applications that require high degree of fault tolerance.

33. Reference Material Textbooks
Stream Control Transmission Protocol (SCTP)
Randall Stewart, Qiaobing Xie, Addison Wesley, 2002
TCP/IP Protocol Suite – Chapter 13
Behrouz Forouzan
RFC’s
RFC Stream Control Transmission Protocol
RFC An Introduction to SCTP
RFC SCTP Specification Errata and Issues

34. Thank You!!!!