Lecture 9 SCTP Sockets

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Lecture 9SCTP Sockets

• CPE 401 / 601Computer Network Systems

slides are modified from Janardhan Iyengar, John
Rumsey, Nimish Vartak
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Where is SCTP in the stack?
Application
Application
Socket API
Socket API
UDP
TCP
UDP
TCP
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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

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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

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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

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TCP Connection Setup
A
B
closed
t0
listen
SYN
SYN sent
SYN-ACK
1RTT
SYN recd (TCB created)
ACK
data
established
estabd
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SYN Flooding Attack
attackers
Flooded!!
victim
130.2.4.15
128.3.4.5
TCB
SYN
TCB
228.3.14.5
192.10.2.8
SYN
TCB
TCB
190.13.4.1
SYN
TCB
221.3.5.10
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

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SCTP Features (contd )

• Connection setup

(SYN) INIT
(SYN-ACK) INIT-ACK
COOKIE-ECHO
COOKIE-ACK
End-Point A
End-Point Z
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Whats 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.)

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SCTP Association Setup
V Verification tag I Initiate tag
closed
A
B
t0
INIT (V0) (ITagA)
cookie wait
INITACK (VTagA) (ITagB) (StateCookie)
closed
1RTT
COOKIEECHO (VTagB) (StateCookie)
cookie echoed
COOKIEACK (VTagA)
2RTT
estabd
data (VTagB)
established
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SCTP Features (contd )

• Connection close

SHUTDOWN
SHUTDOWN-ACK
SHUTDOWN-CMPL
End-Point A
End-Point Z
No Half Closed State
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Graceful Shutdown
A
B
App signals shutdown
(pending data)
Shutdown pending
SHUTDOWN
Shutdown received
Shutdown sent
(pending data)
SHUTDOWN-ACK
Shutdown-Ack sent
SHUTDOWN-COMPLETE
Closed
Closed
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SCTP state diagram
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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

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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

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Chunks in SCTP
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SCTP Header

• 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
Source Port
Destination Port
Verification Tag
Checksum
4 4 2 2
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SCTP Chunks

• 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

Chunk
Flags
Type
Value
Length
N 2 1 1
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INIT Chunk
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Data Chunk
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Selective Acknowledgement
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SACK Chunk
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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

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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

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Head-of-Line Blocking in TCP
S
Rs App
R
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.
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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

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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

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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

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Multi-homing
Internet
End-Point A
End-Point Z

• In TCP, connections made between
  and
• 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.

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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)

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What is SCTP Multihoming?

• 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

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Multihoming Operation
SACK
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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

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(No Transcript)
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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?

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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).

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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)

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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.

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One-to-many Example Server

• int sd, newfd, sosz, msg_flags
• struct sockaddr_in6 sin6
• struct sndrcvinfo snd_rcv
• char buf8000
• 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)

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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.

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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
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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
• ...

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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.

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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.

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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).

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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