Video Streaming with the Stream Control Transmission Protocol (SCTP) - - PowerPoint PPT Presentation

Chair for Network Architectures and Services

Department of Informatics Technische Universität München

Video Streaming with the Stream Control Transmission Protocol (SCTP)

Lothar Braun, Andreas Müller

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Internet Protocol Stack

 The Internet Protocol Stack  Why another transport layer protocol?

Network Interface (Ethernet, PPP, …) IP UDP TCP Application Physical + Data Link Layer Network Layer Transport Layer Session, Presentation, Application Layer UDP TCP SCTP

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Contents

 Recap: UDP and TCP  Problems with UDP/TCP  Stream Control Transmission Protocol (SCTP)

• Association setup / stream setup
• Message types
• Partial Reliability
• Multi-Homing support
• Congestion control

 The Lab

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Contents

Recap: UDP and TCP

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User Datagram Protocol (1)

Network Interface IP UDP Application Network Interface IP UDP Application Hello World Hello World  Message oriented

• Sending application writes a N byte message
• Receiving application reads a N byte message

 Unreliable

• Lost packets will not be retransmitted

 Unordered delivery

• Packets may be re-ordered in the network

Hello World

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User Datagram Protocol (2)

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Packet header Source Port Destination Port Length Checksum Data ...

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Application address

• n a host

Message Length: Header and Data Message

 Barely more features than IP

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Transmission Control Protocol (1)

 Connection/Stream oriented. Not message oriented  Reliable transmission

• Lost packets are retransmitted
• Retransmission will be repeated until acknowledgment is received

 In-order delivery

• Segments n + 1, n + 2, n + 3, will be delivered after segment n

 Congestion control

• TCP tries to share bandwidth equally between all end-points

Hello World Hello World

Message boundaries are not preserved

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Transmission Control Protocol (2)

 Much information encoded into a single header

• Ports  Application addressing
• Sequence Numbers  Used to identify/acknowledge segments
• Flags/Window  Signaling

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Packet header

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Source Port Destination Port Sequence Number Acknowledgement

4 bit TCP header length

6 bit unused U R G A C K P S H R S T S Y N F I N

Window Checksum Urgent Pointer Options Daten ...

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Problems

 Certain applications observe problems due to UDP and TCP

properties

 TCP: Head-of-line blocking with video streaming

• Frames 2,3,4 arrived but cannot be shown because frame 1 is missing

 Video will stop until frame 1 is delivered

 UDP:

• Unordered delivery: Second image is delivered after first image
• Packet loss: Certain frames get lost  low video/audio quality
• No congestion control

 Example: Internet-Telephony

• Two types of traffic:
• Signalling traffic: should be delivered reliable and in-order (TCP)
• Voice traffic: should not suffer from head-of-line blocking (UDP)

 Need to manage two sockets

 SCTP can deal with these problems

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Contents

SCTP Features

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 Connection and message oriented

• SCTP builds an “association” between two peers  TCP connections
• Association can contain multiple “streams”
• Messages are sent over one of the streams

 Partial reliability

• “Lifetime” defined for each message
• Retransmission of a message is performed during its lifetime
• Messages can be delivered unreliable, full reliable or partial reliable

 Multi-Homing

• SCTP can use multiple IP addresses

SCTP Features at a glance

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Contents

SCTP Message Format

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 Common header format

• 12 byte header
• In front of every SCTP message

SCTP Message Format

Ports address the application Random number which Identifies a given association: Used to distinguish new from old connections Checksum on the complete SCTP message: Common header and “chunks”

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Packet header Source Port Destination Port Verification tag

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Checksum Data („Chunks“) ...

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 Data and signaling information is transported in chunks

• One or more chunks in a SCTP message
• Each chunk type has a special meaning:
• INIT, INIT-ACK, COOKIE, COOKIE-ACK  Connection setup
• DATA  Transports user data
• SACK  Acknowledge Data
• You will learn more about the different chunk types in the pre lab

 Common chunk format  Additional formats are defined for the different chunk types

SCTP Chunk Format

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Chunk header Chunk Type Chunk Length

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Chunk Flags Chunk Data ...

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Contents

SCTP Associations and Data Transmission

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Connection Setup

 TCP connection setup



Known Problem: TCP SYN-Flooding Client Server SYN SYN/ ACK ACK Create State for TCP connection: Store client information

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SYN Flodding

 Clients send SYN-Packets but do not respond to SYN-ACK

• Usually done by a single client that performs IP address spoofing
• Works because only a single forged packet is necessary

 Server has to store state until a TCP timeout occurs

 Leads to resource exhaustion  Server cannot accept any more connections Client 1 Client 2 Client 3 State: Client 1 Client 2 Client 3

SYN SYN SYN SYN/ACK SYN/ACK SYN/ACK

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SCTP Association Setup

 Solution to SYN-Flood problem: Cookies

Client Server INIT INIT-ACK Cookie-Echo Check if cookie is valid  Create state only on valid cookie Cookie-ACK Generate client specific cookie You will learn more about this in the Pre-Lab Send Cookie  Forget client Association is established

• No SYN-floods with spoofed

addresses possible

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Data Transmission

 Application data is transmitted in Data Chunks

• A data chunk is associated to a stream (Stream Identifier S)

 TSN (Transport Sequence Number)

• Global Sequence Number
• Similar to Sequence number in TCP  Necessary for retransmissions

 Stream sequence number

• Necessary for per-stream transmission reliability

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Transmission reliability (1)

 TCP

• Packets are transmitted fully reliable  retransmitted until received
• Packets are delivered in-order to the application
• Slow start and congestion avoidance for congestion control

 UDP

• Packets are transmitted fully unreliable  never retransmitted
• No re-ordering  packet order may be changed at the receiver
• No congestion control

 SCTP can do both and more because of its streams

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Transmission reliability (2)

 Why multiple streams?

• Solves head of line blocking
• No firewall issues (only one port for several streams)
• Partial Reliability Extension (PR-SCTP) for different reliability levels

 PR-SCTP

• Allows to set a lifetime parameter for each message
• Lifetime specifies how long the sender should try to retransmit a packet
• Allows to create reliable and unreliable streams within an association

Fully reliable streams (TCP like) Partial reliable stream unreliable stream (UDP like)

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Contents

Multihoming

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Multi-Homing: Association setup

 SCTP chooses one IP address at association setup

• IP can be specified by user

Server IP UMTS-IP DSL-IP SCTP Association DSL IP is used to setup the connection UMTS-IP is announced as backup IP at association setup

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Multi-Homing

 Heartbeat messages are periodically sent to check link

availability

Server IP UMTS-IP DSL-IP SCTP Association Heartbeat Heartbeat

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Multi-Homing

 Changes only occur when the default link is found to be broken

• Is identified because of packet loss (data or heartbeat)
• Consequence: SCTP will resume on the backup link

Server IP UMTS-IP DSL-IP SCTP Association No new association setup necessary

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Real World Deployment

 SCTP is cool -> but why don‘t we use it?  Why do we use HTTP over TCP for Video Streaming?  Why don‘t we use Multicast?  …

• Because it just works „good enough“

 Firewall and NAT issues

• Most home routers simply can‘t translate SCTP

 Implementations

• No native Windows support (only userspace lib)

 BUT: mandatory for some newly developed protocols such as IPFIX

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Contents

The Lab

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The Lab

 Pre-Lab

• Get to known SCTP in detail
• Learn how things work
• How to setup SCTP connections
• How streams are used
• What kind of SCTP chunks you have and what their purpose is

 Lab – Part 1

• Observe SCTP traffic on the wire
• Use “Wireshark” to observe the traffic from an unknown SCTP application

 Lab – Part 2

• Build a SCTP video streaming application
• Program an controller that allows to start, stop, pause, forward, rewind videos
• Stream videos over a lossy network link

– “See” and “hear” the quality differences when streaming over SCTP compared to streaming over UDP and TCP

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Contributions to the lab

 The SCTP Lab was set up in summer term 2010 (SS 2010).  Students

• Carl Denis
• Alexandra Simon

 Supervisors

• Lothar Braun
• Andreas Müller

 You are invited to contribute as well by giving us comments and

suggestions for improvement!

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Questions?