1. CPE 401 / 601 Computer Network Systems
Lecture 9 SCTP Sockets
CPE 401 / 601 Computer Network Systems
slides are modified from Janardhan Iyengar, John Rumsey, Nimish Vartak

2. Where is SCTP in the stack?
Application
Application
kernel
user-level
SCTP
SCTP
Socket API
Socket API
Transport
UDP
TCP
UDP
TCP IP
Wifi
Eth IP
Wifi
Eth IP IP IP IP IP IP

3. SCTP – Motivation New applications
Migration from PSTN to Packet based Internet
Telephony signaling messages
Shortcomings of existing protocols
TCP
“head-of-line blocking”
Byte-oriented, not message-oriented
Multi-homing support not built in
DoS attack prone
UDP
No Reliability
Absence of congestion control
Absence of flow control
SCTP

4. SCTP – Overview
“SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. …”
RFC 2960
Has built-in support for multi-homed hosts
Is message-based – conserves the message boundaries.
Classifies messages as:
sequenced delivery of user messages within multiple streams
with an option for un-ordered delivery of individual user messages
Additional security mechanisms
SCTP

5. SCTP Feature Summary Start with TCP: reliable (retransmissions)
congestion controlled
connection oriented
Add:
4-way handshake
to reduce vulnerability to DOS attacks
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
Note: will discuss
SCTP

6. TCP Connection Setup A B SYN SYN-ACK ACK data closed t=0 listen
SYN sent
SYN-ACK
1RTT
SYN recd
(TCB created)
ACK
data
established
estab’d

7. SYN Flooding Attack attackers Flooded!! victim SYN TCB TCB SYN TCB TCB
SYN
TCB
TCB
SYN
TCB
TCB
SYN
TCB
Unavailable, reserved resources
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

8. SCTP – Features (contd …)
Connection setup
(SYN) INIT
(SYN-ACK) INIT-ACK
COOKIE-ECHO
COOKIE-ACK
End-Point A
End-Point Z
SCTP

9. What’s in a cookie? Information from original INIT
Information from current INIT-ACK
Timestamp
Life span of cookie (Time to live)
Signature for authentication (SHA-1, MD5, etc.)
SCTP

10. SCTP Association Setup
closed
V: Verification tag
I: Initiate tag A B
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

11. SCTP – Features (contd …)
Connection close
SHUTDOWN
SHUTDOWN-ACK
SHUTDOWN-CMPL
End-Point A
End-Point Z
No Half Closed State
SCTP

12. Graceful Shutdown A B App signals shutdown Shutdown pending
(pending data)
Shutdown pending
SHUTDOWN
Shutdown received
Shutdown sent
(pending data)
SHUTDOWN-ACK
Shutdown-Ack sent
SHUTDOWN-COMPLETE
Closed
Closed

13. SCTP state diagram SCTP COOKIE_WAIT CLOSED COOKIE_ECHOED ESTABLISHED
SHUTDOWN-
PENDING
SHUTDOWN-
PENDING
SHUTDOWN-
SENT
SHUTDOWN-
ACK-SENT
CLOSED
SCTP

14. SCTP Feature Summary Start with TCP: reliable (retransmissions)
congestion controlled
connection oriented
Add:
4-way handshake
to reduce vulnerability to DOS attacks
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
Note: will discuss
SCTP

15. Message Boundaries UDP honors message boundaries
Each app message becomes a datagram
TCP does not honor message boundaries
App messages become part of a byte stream
SCTP maintains message boundaries
Each app message is maintained as one or more data chunks
SCTP

16. Chunks in SCTP An SCTP packet forms the payload of an IP packet
An SCTP packet consists of a 12 byte common header and one or more “Chunks”
Control chunks bundled before
Source
Port
Destination
Verification
Tag
Checksum
Type
Length
Value
SCTP Header
Flags
Chunk 1
Chunk N N
The CHUNKS can contain control messages or user data.
SCTP

17. SCTP Header Source Port & Destination Port Verification Tag Checksum
Uses same port concept as TCP and UDP
Verification Tag
Exchanged between endpoints at startup
To Validate the sender
Checksum
Protected by 32 bit checksum (CRC32 algorithm)
SCTP Header
Source
Port
Verification
Tag
Destination
Port
Checksum
Verification Tag
There are two Verification Tags used (one in each direction).
The initiator of the association specifies a Initiation TAG number in the INIT Chunk. The value of this TAG is used as the Verification Tag on all SCTP packets transmitted by the receiver of the INIT Chunk.
The receiver of the INIT Chunk will respond with an INIT ACL chunk which contains a Initiation TAG number in the INIT-ACK Chunk. The value of this TAG is used as the Verification Tag on all SCTP packets transmitted by the receiver of the INIT-ACK Chunk.
SCTP

18. SCTP Chunks Type Flags Length Value
Used to distinguish data chunks and different types of control chunks
Flags
Usage depends on Chunk type
Length
Required because chunks have a variable length
Value
Payload field
Chunk
Value
Flags
Length
Type N
SCTP

19. INIT Chunk Type = 1 Chunk Flags Number of Inbound Streams Chunk Length
Initiate Tag
Advertised Receiver Window Credit (a_rwnd)
Optional/Variable-Length Parameters
Initial Transmission Sequence Number (TSN)
Number of Outbound Streams
Initiate Tag: The receiver of the INIT records the value of the Initiate Tag parameter. This value MUST be placed into the Verification Tag field of every SCTP packet that the receiver of the INIT transmits within this association.
Advertised Receiver Window Credit (a_rwnd): This value represents the dedicated buffer space the sender has reserved for this association. During the life of the association this buffer space SHOULD not be lessened; however, an endpoint MAY change the value of a_rwnd it sends in SACK chunks.
Number of Outbound Streams (OS): Defines the number of outbound streams the sender of this INIT chunk wishes to create in this association. I think you can have a maximum of 65,000 streams.
Number of Inbound Streams (MIS) : Defines the maximum number of streams the sender of this INIT chunk allows the peer end to create in this association.
Note: There is no negotiation of the actual number of streams but instead the two endpoints will use the min (requested, offered).
Initial TSN (I-TSN) : Defines the initial TSN that the sender will use. The valid range is from 0 to This field MAY be set to the value of the Initiate Tag field.
Optional Parameters
IPv4 Address Parameter
IPv6 Address Parameter
Cookie Preservation Parameter
Host Name Address
SCTP

20. Data Chunk Type = 0 Reserv. Stream Sequence Number N Length
Transmission Sequence Number (TSN)
User Data (seq. n of Stream S)
Payload Protocol Identifier
Stream Identifier S U B E
U bit: The (U)nordered bit, if set to '1', indicates that this is an unordered DATA chunk, and there is no Stream Sequence Number assigned to this DATA chunk. Therefore, the receiver MUST ignore the Stream Sequence Number field. After re-assembly (if necessary), unordered DATA chunks MUST be dispatched to the upper layer by the receiver without any attempt to re-order. If an unordered user message is fragmented, each fragment of the message MUST have its U bit set to '1'.
B bit: The (B)eginning fragment bit, if set, indicates the first fragment of a user message.
E bit: The (E)nding fragment bit, if set, indicates the last fragment of a user message.
Length: This field indicates the length of the DATA chunk in bytes from the beginning of the type field to the end of the user data field excluding any padding. A DATA chunk with no user data field will have Length set to 16 (indicating 16 bytes).
TSN : This value represents the TSN for this DATA chunk. The valid range of TSN is from 0 to (2**32 - 1). TSN wraps back to 0 after reaching
Stream Identifier S: Identifies the stream to which the following user data belongs.
Stream Sequence Number n: This value represents the stream sequence number of the following user data within the stream S. Valid range is 0 to When a user message is fragmented by SCTP for transport, the same stream sequence number MUST be carried in each of the fragments of the message.
Payload Protocol Identifier: This value represents an application (or upper layer) specified protocol identifier. This value is passed to SCTP by its upper layer and sent to its peer. This identifier is not used by SCTP but can be used by certain network entities as well as the peer application to identify the type of information being carried in this DATA chunk. This field must be sent even in fragmented DATA chunks (to make sure it is available for agents in the middle of the network). The value 0 indicates no application identifier is specified by the upper layer for this payload data.
User Data: This is the payload user data. The implementation MUST pad the end of the data to a 4 byte boundary with all-zero bytes. Any padding MUST NOT be included in the length field. A sender MUST never add more than 3 bytes of padding.
SCTP

21. Selective Acknowledgement
DATA chunk TSN=109
DATA chunk TSN=110
DATA chunk TSN=111
DATA chunk TSN=112
DATA chunk TSN=113
DATA chunk TSN=114 X
DATA chunk TSN=115 X
DATA chunk TSN=116
DATA chunk TSN=117
DATA chunk TSN=118
DATA chunk TSN=119
DATA chunk TSN=120
DATA chunk TSN=121
DATA chunk TSN=122 X
DATA chunk TSN=123
DATA chunk TSN=124
DATA chunk TSN=125
DATA chunk TSN=126
SACK chunk Cumulative TSN=113
Gap Ack Block #1 Start = +3 End = +9
Gap Ack Block #2 Start = +11 End = +13

22. SACK Chunk Type = 3 Chunk Flags Number of Duplicate TSNs = X
Chunk Length
Cumulative Transmission Sequence Number (TSN) Ack
Advertised Receiver Window Credit (a_rwnd)
Duplicate Transmission Sequence Number (TSN) 1
Number of Gap Ack Blocks = N
Gap Ack Block #1 End
Gap Ack Block #1 Start
Gap Ack Block #N End
Gap Ack Block #N Start
Duplicate Transmission Sequence Number (TSN) N
Chunk Flags: Set to all zeros on transmit and ignored on receipt.
Cumulative TSN Ack: This parameter contains the TSN of the last DATA chunk received in sequence before a gap.
Advertised Receiver Window Credit (a_rwnd): This field indicates the updated receive buffer space in bytes of the sender of this SACK.
Number of Gap Ack Blocks: Indicates the number of Gap Ack Blocks included in this SACK.
Number of Duplicate TSNs: This field contains the number of duplicate TSNs the endpoint has received. Each duplicate TSN is listed following the Gap Ack Block list.
Gap Ack Blocks: These fields contain the Gap Ack Blocks. They are repeated for each Gap Ack Block up to the number of Gap Ack Blocks defined in the Number of Gap Ack Blocks field. All DATA chunks with TSNs greater than or equal to (Cumulative TSN Ack + Gap Ack Block Start) and less than or equal to (Cumulative TSN Ack + Gap Ack Block End) of each Gap Ack Block are assumed to have been received correctly.
Gap Ack Block Start: Indicates the Start offset TSN for this Gap Ack Block. To calculate the actual TSN number the Cumulative TSN Ack is added to this offset number. This calculated TSN identifies the first TSN in this Gap Ack Block that has been received.
Gap Ack Block End: Indicates the End offset TSN for this Gap Ack Block. To calculate the actual TSN number the Cumulative TSN Ack is added to this offset number. This calculated TSN identifies the TSN of the last DATA chunk received in this Gap Ack Block.
Duplicate TSN: Indicates the number of times a TSN was received in duplicate since the last SACK was sent. Every time a receiver gets a duplicate TSN (before sending the SACK) it adds it to the list of duplicates. The duplicate count is re-initialized to zero after sending each SACK. For example, if a receiver were to get the TSN 19 three times it would list 19 twice in the outbound SACK. After sending the SACK if it received yet one more TSN 19 it would list 19 as a duplicate once in the next outgoing SACK.
SCTP

23. SCTP Feature Summary Start with TCP: reliable (retransmissions)
congestion controlled
connection oriented
Add:
4-way handshake
to reduce vulnerability to DOS attacks
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
Note: will discuss
SCTP

24. Multi-streaming
A.k.a. partial ordering. Eliminates Head of Line (HOL) blocking
In TCP, all data must be sent in order; loss at head of line delays delivery of subsequent data
In SCTP, you can send over up to 64K independent streams, each ordered independently
A loss on one stream does not delay the delivery on other streams i.e. multi-streaming eliminates HOL blocking
SCTP

25. Head-of-Line Blocking in TCPS R
R’s App 1 2
ACK 2 3 1 4
ACK 3 2 5
ACK 3 6
ACK 3
ACK 3
PDU 3 is blocking the head of the line.

26. Head-of-line Blocking
TCP provides a single data stream
When a segment is lost, subsequent segments must wait to be processed.
Problem for some applications (telephony)
SCTP provides multiple independent streams per association
SCTP

27. SCTP Multistreaming Logical separation of data within an assoc
Designed to prevent head-of-line blocking
Can be used to deliver multiple objects belonging to the same assoc
Eg: objects on a webpage, multimedia streams (audio/video/text), files in an FTP mget

28. SCTP Feature Summary Start with TCP: reliable (retransmissions)
congestion controlled
connection oriented
Add:
4-way handshake
to reduce vulnerability to DOS attacks
framing
preserve message boundaries
multistreaming
instead of one ordered stream, up to 64K independent ordered streams
multihoming
instead of one IP address per endpoint a set of IP addresses per endpoint
Note: will discuss
SCTP

29. Multi-homing Internet End-Point A End-Point Z
In TCP, connections made between <IP addr,port> and <IP addr, port>
If a host is multi-homed, you have to choose ONE IP Addr only, at each end
If that interface goes down, so does the connection
With SCTP, you can list as many IP addresses per endpoint as you like
If host is still reachable through ANY of those addresses, connection stays up.
SCTP

30. SCTP Multi-Homing Multiple src/dest ip addresses
IP network
IP A2
IP B2
IP B1
IP B3
IP A1
Multiple src/dest ip addresses
Use of different physical paths not guaranteed
Peer reachability and path status are monitored (heartbeat)
One selectable default destination
Parameters per path (cwnd, ssthresh, RTT)
SCTP

31. What is SCTP Multihoming?
Host A A1 A2
Host B B1 B2
Internet
ISP
Hosts pick 1 of 4 possible TCP connections:
{(A1, B1), (A1, B2), (A2, B1), (A2, B2)}
Hosts use 1 SCTP association:
({A1,A2}, {B1,B2})
Selectable “primary” dest: Host A → B1 ; Host B → A1
New data sent only to primary destination
SCTP

32. Multihoming Operation
SCTP
Endpoint A
IP address A1
IP address A2
SCTP
Endpoint B
IP address B1
IP address B2
DATA 1 2
SACK
SCTP

33. SCTP – Summary Well suited for Multimedia Like TCP
Provides connection establishment
Ensures Reliability
Provisions for ordered and un-ordered data
Provides Congestion Control
In addition to TCP features
Provides multi-homing
Provides multi-streaming
Has security features
SCTP

35. SCTP Socket Types
SCTP socket API comes in two forms: one-to-one and one-to-many.
The one-to-many at one time was known by the “UDP style” socket. The one-to-one used to be called the a “TCP style” socket.
So what is the purpose of each socket style and how can it be used?
SCTP

36. One-to-One style
The purpose of the one-to-one style socket is to provide a smooth transition mechanism for those applications running on TCP and wishing to move to SCTP.
The same semantics used in TCP are used with this style.
A server will typically open the socket, make a call to listen (to accept associations), and call accept, blocking upon the arrival of a new association.
The only notable difference between a TCP socket and a SCTP socket is the socket call uses IPPROTO_SCTP instead of IPPROTO_TCP (or 0).
SCTP

37. One-to-One Example Server
int sd, newfd, sosz;
struct sockaddr_in6 sin6;
sosz = sizeof(sin6);
sd = socket(AF_INET6, SOCK_STREAM, IPPROTO_SCTP);
listen(sd, 1);
while (1) {
newfd = accept(sd, (struct sockaddr *)&sin6, &sosz)
do_child_stuff(newfd, &sin6, sosz); }
SCTP

38. One-to-Many style
A typical server using a one-to-many style socket will do a socket() call, followed by a listen() and recvfrom().
A typical client will just sendto() the server of his choice.
Note that the connect() and accept() call are not needed.
The connect() call can be done by either side (server or client) but it is not needed.
Note that this style is more like what a UDP client/server would look like thus the previous name.
SCTP

39. One-to-many Example Server
int sd, newfd, sosz, msg_flags;
struct sockaddr_in6 sin6;
struct sndrcvinfo snd_rcv;
char buf[8000];
sosz = sizeof(sin6);
sd = socket(AF_INET6, SOCK_SEQPKT, IPPROTO_SCTP);
listen(sd, 1);
while (1) {
len = sctp_recvmsg(sd, buf, sizeof(buf), (sockaddr *)&sin6, &sosz, &snd_rcv, &msg_flags);
do_child_stuff(newfd, buf, len, &sin6, &snd_rcv, msg_flags); }
SCTP

40. SCTP Notifications
The SCTP stack, at times, has information it may wish to share with its application (or Upper Layer Protocol ... ULP).
The ULP can turn off and on specific notifications via a socket options call.
By default ALL notifications are off.
We can get a notification
By reading data and looking at the msg_flags, if the message read is a notification, then "MSG_NOTIFICATION” is contained within the msg_flags argument upon return.
SCTP

41. Deciphering Notifications
Every Notification uses a TLV format as illustrated below:
Type of notifications
SCTP_ASSOC_CHANGE
SCTP_PEER_ADDR_CHANGE
SCTP_REMOTE_ERROR
SCTP_SEND_FAILED
SCTP_SHUTDOWN_EVENT
....
struct sctp_tlv {
u_int16_t sn_type;
u_int16_t sn_flags;
u_int32_t sn_length; };
SCTP

42. Socket Options
SCTP provides a host of socket options to perform a mirad of operations.
Some have unique structures others just turn things on and off with boolean's or integers.
SCTP_NODELAY
SCTP_MAXSEG
SCTP_ASSOCINFO
SCTP_AUTOCLOSE
SCTP_ADAPTION_LAYER
SCTP_DEFAULT_SEND_PARAM
SCTP_DISABLE_FRAGMENTS
...
SCTP

43. Extended “system calls”.
sctp_connectx
Allows a user to specify multiple address to attempt to connect too.
sctp_bindx
Allows an application to bind a set of addresses instead of one or all addresses.
sctp_opt_info
Some implementations do not support a getsockopt() call that allows data to be passed both ways. This call is compatible with all implementations.
sctp_peeloff
this call is used to convert a single association that is part of a one-to-many socket into an individual new socket descriptor that is a one-to-one socket.

44. Extended “system calls”
sctp_getpaddrs
This call will return a block of memory holding the peers addresses currently part of the association.
sctp_freepaddrs
This call is used to release the memory back that the sctp_getpaddrs call allocated.
sctp_getladdrs
This call will return a block of memory holding the local addresses bound to an association.
sctp_freeladdrs
This call should be used to release the memory allocated by sctp-getladdrs back to the system.
SCTP

45. Extended “system calls”
sctp_sendmsg
This call will allow the caller to specify on the command line things like the stream number and other SCTPish information to be sent with a message.
sctp_send
This call has a similar purpose to sctp_sendmsg but instead of a large number of command line options, a sctp_sendrcvinfo structure is used to pass the relevant information.
sctp_recvmsg
This call (as we saw previously) is used to receive a message but also a sctp_sendrcvinfo structure with details on the message (e.g. The stream number and stream sequence number).
SCTP

46. Summary SCTP is a new transport protocol
available now in bleeding edge Linux and BSD kernels, and will make its way into the mainstream
It has some cool new features
SCTP