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Academic Open Internet Journal |
Volume 14, 2005 |
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Media Streaming Over WLAN Using STREAM
CONTROL TRANSMISSION PROTOCOL |
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V.Mariappan (vbrms@yahoo.com) |
P.Narayanasamy(sam@annauniv.edu) |
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Department
of Computer Science & Engineering Anna
University, Chennai, INDIA – 600 025. |
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Abstract
Robust Media Streaming over 802.11 Wireless Local Area Network (WLAN) with Quality of Service is a very challenging task due to time-varying characteristics of wireless channel and limited resources. It has been done traditionally using Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). UDP is preferred for media applications for timely delivery but it is not reliable. TCP is not effective enough for streaming applications because its window based congestion control doesn’t provide instant rate adaptation and it has the use of byte stream and single connection by the peer ends. This paper proposes the use of Stream Control Transmission Protocol (SCTP) for video streaming over wireless LAN. The benefit of using Stream Control Transmission Protocol (SCTP) in WLAN is its potential for its increased productivity within the scope of new data services. We use SCTP for transmission of audiovisual data since it is a reliable transport protocol operating on top of a potentially unreliable connectionless packet service such as IP. Its design includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks.
Keywords: WLAN, IEEE802.11b, Media streaming, SCTP
1. Introduction
The use of IEEE 802.11 WLANs as an extension to the existing wired
infrastructure, offering the convenience of mobility and portability in the
enterprise environment, is growing at a rapid pace. The falling cost of WLAN
products has also led to their increased use in consumer homes. Although
currently WLANs are predominantly used for data transfer, the higher bandwidth
provided by new WLAN technologies such as IEEE 802.11a and IEEE 802.11g will
ultimately lead to their increasing use for multimedia transmissions. However,
to achieve a high level of acceptability and proliferation of wireless
multimedia, in particular wireless video, several key requirements need to be
satisfied in order to provide a reliable and efficient transmission
Video Transmission over
wireless LAN poses many challenges, including coping with bandwidth variations,
data losses and heterogeneity of the receivers. Currently, each network layer provides a separate solution to
these challenges by providing its own optimized adaptation and protection
mechanisms. However this layered
strategy does not always result in an optimal overall performance for the
transmission of video.
Traditional TCP does not provide the exact
functionality needed by the application or provides more functionality than is
needed. In the first case, the application needs to do extra work to be able to
use TCP, while in the latter; the extra functionality of TCP might be an
overhead. For instance, many applications need reliable message delivery, but
TCP is a byte stream oriented protocol. Message-oriented applications achieve
their required functionality by delineating the TCP stream into messages.
Additionally, TCP provides both strict ordering and reliability, but many
applications may not need both. In this case, the applications will incur an overhead
in using TCP.
It is clear that, there is a need for a protocol that
would allow both strict ordering and the partial ordering of data as required
by the application along with message oriented transmission, all of these and
some more desirable features are presented by SCTP and it serves to solve the
above-defined problem
The rest of this paper is structured as follows:
Section 2 gives brief information about SCTP, Section 3 talks about the System
design for media streaming in WLAN, Section 4 deals with Implementation of the
design given in the previous section, Section 5 is about the Conclusion and
future extension of the paper.
2. Stream Control
Transmission Protocol (SCTP)
SCTP [8,9,10,11,15,16] can
be used as the transport protocol for applications where monitoring and
detection of loss of session is required. For such applications, the SCTP
path/session failure detection mechanisms, especially the heartbeat, will
actively monitor the connectivity of the session. SCTP
distinguishes different streams of messages within one SCTP association.
This enables a delivery scheme where only the sequence of messages needs to be
maintained per stream (partial in-sequence delivery), which reduces unnecessary
head-of-line blocking between independent streams of messages.
SCTP operates on two levels:
- Within an association the reliable transfer of
datagrams is assured by using a checksum, a sequence number and a
selective retransmission mechanism. Without taking the initial sequence
into account, every correctly received data chunk is delivered to a
second, independent level.
- The second level realizes a flexible delivery
mechanism, which is based on the notion of several independent streams
of datagrams within an association. Chunks belonging to one or several streams
may be bundled and transmitted in one SCTP packet provided they are not
longer than the current path MTU.
Detection of loss and duplication of data chunks is enabled
by numbering all data chunks in the sender with the so-called Transport Sequence
Number (TSN). The acknowledgements sent from the receiver to the sender are
based on these sequence numbers. Retransmissions are timer-controlled. The
timer duration is derived from continuous measurements of the round trip delay.
Whenever such a retransmission timer expires, (and congestion control allows
transmissions) all non-acknowledged data chunks are retransmitted and the timer
is started again doubling its initial duration (like in TCP).
When the receiver detects one or more gaps in the sequence
of data chunks, each received SCTP packet is acknowledged by sending a
Selective Acknowledgement (SACK), which reports all gaps. The SACK is contained
in a specific control chunk. Whenever the sender receives four consecutive
SACKs reporting the same data chunk missing, this data chunk is immediately
retransmitted (fast retransmit). Some of the key advantages of using SCTP are given below.
Ø
Flow Control
Ø
Multihoming
SCTP
Packets
The protocol data units
(PDU) of SCTP are called SCTP packets. SCTP runs over IP, an SCTP packet forms
the payload of an IP packet. An SCTP packet is composed of a common header and
chunks. Multiple chunks may be multiplexed into one packet up to the Path-MTU
size. A chunk may contain either control information or user data.
SCTP States
The initialization of an
association is completed on both sides after the exchange of four messages. The
passive side (let's call it server) does not allocate resources until
the third of these messages has arrived and been validated. That is to ensure
that the association setup request really does originate from the right peer
(without the possibility of blind spoofing).
SCTP streams
SCTP separates the reliable
transfer of datagrams from the delivery mechanism. This makes it possible to
adapt protocol usage to the specific needs of the applications using SCTP. Some
applications may only need partial ordering of datagrams while others might
even be satisfied with a reliable transfer that does not guarantee any
in-sequence maintenance at all.
SCTP
distinguishes different streams of messages within one SCTP association.
This enables a delivery scheme where only the sequence of messages needs to be
maintained per stream (partial in-sequence delivery), which reduces unnecessary
head-of-line blocking between independent streams of messages. Furthermore,
SCTP provides a mechanism for bypassing the sequenced delivery service, so that
messages are delivered to the user of SCTP as soon as they are completely
received (order-of-arrival delivery).
Flow control
and congestion control in SCTP have been designed in a way which assures that
SCTP traffic behaves in the network in the same way as TCP traffic does. This
enables a seamless introduction of SCTP services into existing IP networks.
3. Design
The proposed architectural design is shown in Figure 1. The transmission in SCTP was done by means
of transfer of SCTP packets. The protocol data units (PDU) of SCTP are called SCTP
packets. SCTP runs over IP and SCTP packet forms the payload of an IP packet.
An SCTP packet is composed of a common header and chunks as shown in Figure 2.
Multiple chunks may be multiplexed into one packet up to the Path-Maximum
Transmission Unit (MTU) size. A chunk may contain either control information or
user data. The structure of network packets as sent by an SCTP endpoint, as
well as the different types of data that could be sent are shown in the
diagram. The SCTP message format naturally allows support of bundling of
multiple DATA and control chunks in a single message, to improve transport
efficiency. Use of bundling is
controllable by the application, so that bundling of initial transmission can
be prohibited. Bundling will naturally
occur on retransmission of DATA chunks, to further reduce any chance of
congestion.
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Figure
1. Architectural Design
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Instead of the three-phase connection
setup for TCP, the initialization of an association is completed after the
exchange of four messages. The passive side of the as-association does not
allocate resources for the association until the third of these messages has
arrived and been validated. The Association is established between the Client and
the Server. During the course of the Association the Server and Client take
various different states. The state transition diagrams of the Server and the
Client are shown in Figure 3 and Figure 4 respectively.
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Figure 2. SCTP
Packet Format
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Figure 3 State
Transition Diagram of the Server
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4.
Implementation
The implementation was done in IEEE 802.11a WLAN
using VC++ 6.0. The media data can be captured using real time digital camera and
from the database for on demand delivery. The media data along with the audio
is mixed and is streamed in the wireless network as an SCTP packet. Since SCTP
runs over IP, an SCTP packet forms the payload of an IP Packet. An SCTP packet
is composed of a common header and chunks. Multiple chunks may be multiplexed
into one packet up to the path-MTU size. A chunk may contain either control
information or user data. Hence we multiplex the visual, audio and control
information chunks into an SCTP packet up to the said Path-
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Figure 4 State Transition Diagram of the Client
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MTU size. Hence the allowed hosts
associated with the Access Points (APs) can receive the SCTP packets. The
advantage here from the users perspective is that as soon as the SCTP packets
are being received, the media playback starts using media player, since a
packet contains all the information for the playback to start. The client host
need not wait for the completion of the reception of the entire media
information to have a playback. As the playback continues the other packets
transmitted from the server gets received, allowing a continuous playback.
Distribution of packets did not create any problem. Figure 5 shows a sample output of a video clip using SCTP
transmission.
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Figure 5 Sample Output
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5. Conclusion
This paper
presents the concept of using SCTP to develop a framework for media streaming
over wireless LAN. The issue of media streaming over WLAN has been taken care
by using SCTP and proof of concept implementation has been done. This work
overcomes delay experienced by end-hosts otherwise. Also this implementation
overcomes flow control and congestion control problems. In order to use the
system for other applications security issues must be addressed. The usefulness of SCTP can further be
exploited by using it in high loss environment and can be used in vulnerable
environment to experience the security features of the SCTP against Masquerade
Attack.
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