Lecture 9

SCTP Sockets

CPE 401 / 601Computer Network Systems

slides are modified from Janardhan Iyengar, John Rumsey, Nimish Vartak

Where is SCTP in the stack?Application

IP

IP

IP

IP

IP

Application

UDP TCPUDP TCP

IP

Wifi Eth

IP

Wifi Eth

IP

Transport Transport

kernel

user-level

SCTP SCTP

Socket APISocket API

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 3

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 mechanismsSCTP 4

SCTP Feature Summary

Start with TCP:reliable (retransmissions)

congestion controlledconnection oriented

Add:4-way handshake

to reduce vulnerability to DOS attacksframing

preserve message boundaries multistreaming

instead of one ordered stream, up to 64K independent ordered streams

multihoming instead of one IP address per endpointa set of IP addresses per endpoint

SCTP 5

1RTT SYN-ACK

closed

listen

t=0

SYN

SYN sent

ACKdata established

estab’d

A B

TCP Connection Setup

SYN recd(TCB created)

SYN

victim Flooded!!

SYN Flooding Attack

TCB

TCB

TCB

TCB

TCB

• 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

attackers

128.3.4.5

192.10.2.8

221.3.5.10 SYN

SYN

190.13.4.1

228.3.14.5

130.2.4.15

Unavailable, reserved resources

SCTP – Features (contd …)Connection setup

(SYN) INIT

(SYN-ACK) INIT-ACK

COOKIE-ECHO

COOKIE-ACK

End-Point A End-Point Z

SCTP 8

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 9

V: Verification tagI: Initiate tag

1RTTINIT–ACK (V=TagA) (I=TagB) (StateCookie)

closed

closed

t=0 INIT (V=0) (I=TagA)cookiewait

COOKIE–ECHO (V=TagB) (StateCookie) cookieechoed

data (V=TagB) established

2RTTCOOKIE–ACK (V=TagA)

estab’d

A B

SCTP Association Setup

SCTP – Features (contd …)Connection close

SHUTDOWN

SHUTDOWN-ACK

End-Point A End-Point Z

SHUTDOWN-CMPL

No Half Closed State

SCTP 11

Graceful Shutdown

SHUTDOWN

SHUTDOWN-ACK

SHUTDOWN-COMPLETE

App signals shutdown

Shutdown pending

(pending data)

Shutdown sent (pending data)

Shutdown received

Shutdown-Ack sent

Closed

Closed

A B

SCTP state diagram

SHUTDOWN-SENT

SHUTDOWN-ACK-SENT

CLOSED

ESTABLISHED

SHUTDOWN-PENDING

SHUTDOWN-PENDING

CLOSED

COOKIE_WAIT

COOKIE_ECHOED

SCTP 13

SCTP Feature Summary

Start with TCP:reliable (retransmissions)

congestion controlledconnection oriented

Add:4-way handshake

to reduce vulnerability to DOS attacksframing

preserve message boundaries multistreaming

instead of one ordered stream, up to 64K independent ordered streams

multihoming instead of one IP address per endpointa set of IP addresses per endpoint

SCTP 14

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 15

Sour

cePo

rt

Des

tina

tion

Port

Ver

ific

atio

nT

ag

Che

cksu

m

Typ

e

Len

gth

Val

ue

SCTP Header

Flag

s

Chunk 1Chunk NT

ype

Len

gth

Val

ue

Flag

s

4 4 2 2 N 2 1 1 N 2 1 1

• 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

Chunks in SCTP

SCTP 16

Sour

cePo

rt

Des

tina

tion

Port

Ver

ific

atio

nT

ag

Che

cksu

m

SCTP Header

4 4 2 2

Source Port & Destination Port- 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

SCTP 17

ChunkT

ype

Len

gth

Val

ue

Flag

s

N 2 1 1

Type- 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

SCTP Chunks

SCTP 18

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

INIT Chunk

SCTP 19

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

Data Chunk

SCTP 20

DATA chunk TSN=112DATA chunk TSN=113

DATA chunk TSN=117

DATA chunk TSN=119DATA chunk TSN=120

DATA chunk TSN=116

SACK chunk Cumulative TSN=113Gap Ack Block #1 Start = +3 End = +9Gap Ack Block #2 Start = +11 End = +13

DATA chunk TSN=111DATA chunk TSN=110DATA chunk TSN=109

DATA chunk TSN=121DATA chunk TSN=122

XDATA chunk TSN=123DATA chunk TSN=124DATA chunk TSN=125DATA chunk TSN=126

DATA chunk TSN=114XDATA chunk TSN=115 X

DATA chunk TSN=118

Selective Acknowledgement

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 EndGap Ack Block #1 Start

Gap Ack Block #N EndGap Ack Block #N Start

Duplicate Transmission Sequence Number (TSN) N

SACK Chunk

SCTP 22

SCTP Feature Summary

Start with TCP:reliable (retransmissions)

congestion controlledconnection oriented

Add:4-way handshake

to reduce vulnerability to DOS attacksframing

preserve message boundaries multistreaming

instead of one ordered stream, up to 64K independent ordered streams

multihoming instead of one IP address per endpointa set of IP addresses per endpoint

SCTP 23

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 24

Head-of-Line Blocking in TCP

S RACK 2

1234

5

6

ACK 3

ACK 3

ACK 3

PDU 3 is blocking the head of the line.

1

2

R’s App

ACK 3

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 26

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

SCTP Feature Summary

Start with TCP:reliable (retransmissions)

congestion controlledconnection oriented

Add:4-way handshake

to reduce vulnerability to DOS attacksframing

preserve message boundaries multistreaming

instead of one ordered stream, up to 64K independent ordered streams

multihoming instead of one IP address per endpointa set of IP addresses per endpoint

SCTP 28

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 29

SCTP Multi-Homing

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)

IP network

IP A2

IP B2 IP B1

IP B3IP A1

SCTP 30

What is SCTP Multihoming?

Host A

A1

A2

Host B

B1

B2

InternetISP

ISP

ISP

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 31

DATA

DATA

SACK

1

2

SCTPEndpoint B

IP address B1

IP address B2

SCTPEndpoint A

IP address A1

IP address A2

Multihoming Operation

SCTP 32

SCTP – SummaryWell 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 33

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 35

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 36

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 37

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 38

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 39

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 40

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 41

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 42

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.

43

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 44

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 45

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 46