Link-layer-assisted seamless MPEG-4 streaming over

Link-layer Assisted Seamless MPEG-4 Streaming over Mobile IPenabled Wireless LAN Chul-Ho Lee, Dongwook Lee, and JongWon Kim Networked Media Lab. Department of Information and Communications, Gwangju Institute of Science and Technology (GIST) 1 Oryong-dong, Puk-gu, Gwangju, 500-712, Republic of Korea Tel: +82-62-970-2273, Fax: +82-62-970-2204 E-mail: chlee, dulee, jongwon@gist.ac.kr ABSTRACT In the mobile IP-enabled wireless LAN (WLAN), packet flows are corrupted due to the handoff of a mobile node at the link and network layers, which results in burst packet losses. This transient behavior hurts time-critical streaming media applications so much. Many solutions have been suggested to address this handoff-related problem. However, even with recent low-latency and smooth handoff options, seamless streaming (i.e., without any playback disruption due to the handoff) is not easy to achieve in the practical situation. Since no packet will be delivered to the mobile node during the handoff process, this can cause temporary packet underflows at the streaming client. Thus, to avoid packet underflows, one has to pre-buffer the streaming client sufficiently before the handoff. Note that the required level of pre-buffering depends on the efficiency of adopted link-/IP-layer handoff options. In this paper, we are targeting seamless MPEG-4 streaming over the mobile IP-enabled WLAN. For this, we are introducing a seamless media streaming framework by estimating the accurate pre-buffering level to compensate the handoff latency and proposing a combination of schemes as a preliminary version of the proposed framework. We utilize a Layer 2 (L2) trigger (event) about a change of an associated access point (AP) with a wireless network interface to reduce the agent discovery time in total handoff latency. A packet forwarding with buffering mechanism is also realized at the foreign agent (FA). With these two options, we can nearly eliminate packet losses during the handoff period. Then, we perform pre-buffering adjustment based on handoff latency experimentally measured and analytically obtained by the handoff transient time analysis 9. The experimental results show that the proposed approach can eliminate packet losses during the handoff period and provide the feasibility of seamless MPEG-4 video streaming over the mobile IP-enabled WLAN. Keywords: Wireless LAN, Mobile IP, L2 trigger, smooth handoff, MPEG-4 video streaming, and pre-buffering.

1. INTRODUCTION Mobile devices such as PDAs and laptop computers come into wide use and the demand for IP-based multimedia services on mobile devices over wireless network increases rapidly. Mobile IP is a protocol to provide IP mobility 1. In a mobile IP-enabled wireless LAN (WLAN), mobile nodes can communicate with correspondent nodes regardless of their location and time. The mobile nodes can also move to another network while accessing the networks. To provide this IP mobility, the mobile IP defines the handoff procedure between different sub-networks. The mobile IP was originally designed without any assumption about the underlying link-layers. So, the handoff procedure of a mobile node consists of both link-layer and IP-layer handoffs. In the mobile IP-based WLAN, when the mobile node changes its associated access point (AP) to another one, it suffers some delay caused by scanning (i.e., probing), authentication, and association processes for the link-layer handoff 2. Moreover, to process the IP-layer handoff, it also spends time to discover a new foreign agent (nFA) and to perform registration process. These handoff latencies cause interruption of packet transfer. This transient behavior hurts time-sensitive streaming media applications; playback of streaming media can be stopped during the handoff period due to the lack of received data. To mitigate the impact of handoffs, several handoff ideas have been suggested to reduce the handoff latency in the mobile IP 3, 4, 5 and/or to minimize packet losses by packet buffering and forwarding (a.k.a., smooth handoff) 6, 7. However, the proposed scheme in [3] is limited since special arrangements such as a media access control (MAC) bridge,

Multimedia Systems and Applications VII, edited by Chang Wen Chen, C.-C. Jay Kuo, Anthony Vetro, Proceedings of SPIE Vol. 5600 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.571414

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additional resource, and corresponding signaling are needed. Also, an ad-hoc mode of operation is assumed for the WLANs 4. This makes the proposed scheme difficult to be applied to general WLANs, since current WLANs are largely utilized as an infrastructure mode for performance and management reasons 4. On the other hand, in [5], pre- and postregistration methods are proposed for low-latency handoffs in mobile IPv4, which rely on several types of Layer 2 (L2) § triggers . However, since the IEEE 802.11 link-layer handoff is hard (i.e., old AP (oAP) connection is terminated before new AP (nAP) connection is made) and forward (i.e., the handoff is not predicted and initiated through the nAP instead of the oAP), the required L2 triggers are hard to realize in the IEEE 802.11 WLANs. In addition, even with low-latency and smooth handoff options, seamless streaming (i.e., without any playback disruption due to the handoff) is not easy to achieve in a practical situation. Since no packet will be delivered to a mobile node during the handoff period, this can cause temporary packet underflows at the streaming client of the mobile node. Thus, to avoid packet underflows, one has to pre-buffer the streaming packets sufficiently before the handoff. Note that the required level of pre-buffering depends on the efficiency of adopted link-/IP-layer handoff options. So far, little studies are concerned with empirical issues regarding the practical validation of handoff options on the time-sensitive streaming media applications 3-7. In this paper, targeting seamless streaming over the mobile IP-enabled WLAN, we introduce a seamless media streaming framework by estimating the accurate pre-buffering level to compensate the handoff latency. In addition, we implement a preliminary version of the proposed framework by extending the basic mobile IPv4 system and validate the MPEG-4 streaming with the implemented mobile IP system. First of all, the L2 trigger (event) about a change of an associated AP with a wireless network interface is utilized to reduce the agent discovery time in total handoff latency. A packet buffering and forwarding mechanism (a.k.a. smooth handoff 6) is also implemented to eliminate the packet loss during the link-/IP-layer handoffs by extending the previous work in [8]. The causes of packet loss during the handoff period and solutions for the packet loss are discussed from the implementer’s perspective. Moreover, pre-buffering adjustment is performed based on the handoff latency experimentally measured and analytically obtained by the handoff transient time analysis 9, which allows that forwarded packets through packet buffering and forwarding mechanism can be utilized at appointed time for the continuous playback of streaming media. The experimental results show that the proposed approach can eliminate packet loss during the handoff period and also validate the feasibility of seamless media streaming. The rest of the paper is organized as follows. Section 2 describes the handoff-related problems and the proposed framework for seamless streaming over the mobile IP-enabled WLAN. Section 3 explains implementation as a preliminary version of the proposed framework in detail. Experimental results and discussion are presented in Section 4. Finally we conclude the paper in Section 5.

2. SEAMLESS MEDIA STREAMING FRAMEWORK To achieve seamless media streaming over the mobile IP-enabled WLAN, one should consider the following limitations. First, the network bandwidth of WLAN is scarce and limited. There exist unstable and burst packet losses due to the fading/shadowing of wireless channel and channel contention. Secondly, mobility issues including the handoff raise transient packet losses and disruption. There are other limitations such as security and power consumption of wireless terminals. In this paper, we concentrate upon solving the handoff-related problems since the transient behavior of handoff is fatal to streaming media even if the other issues are also important for seamless streaming.

2.1 Problem Statement In the mobile IP-enabled WLAN environment, when a mobile node moves to another foreign network, it experiences handoff latency due to following reasons. First, the mobile node can communicate with only one AP at each time and it thus cannot communicate with an old FA (oFA) during the link-layer handoff. This handoff latency is caused by the time taken to perform the link-layer handoff including scanning, authentication, and re-association delays. In addition, the registration process of IP-layer handoff can begin only after the link-layer handoff. Without link-layer information (i.e., L2 triggers or events), there exists delay for agent discovery because the mobile node can discover change in the point of attachment by receiving agent advertisement message from the nFA. Also, the registration process takes some time to be completed, since the registration messages should propagate to a home agent (HA). Accordingly, besides the link-layer §

L2 trigger is described as information from link-layer that informs network-layer of particular events before and after link-layer handoff 5. The description of L2 trigger is not specific to any particular link-layer.

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(a)

(b)

Figure 1. Handoff latencies: (a) handoff latency in Mobile IPv4 and (b) handoff latency in Mobile IPv4 with L2 trigger.

handoff latency, additional handoff latencies are needed for agent discovery and registration process during the IP-layer handoff. As stated above, handoff procedure and required handoff latency are presented in Fig. 1 (a). When a mobile node locates in a foreign network, its HA can intercept packets that goes toward a home address of the mobile node as a proxy. It then sends those packets through IP-in-IP tunneling to the care-of address (CoA) that indicates the termination point of a tunnel toward the mobile node side. As the mobile node performs the link-/IP-layer handoffs to move to another foreign network, it cannot receive any packets toward itself because these packets are destined to old CoA (i.e., oFA) and the mobile node cannot communicate with the oAP and the oFA during the handoff period. Therefore, there exist burst packet losses hurting streaming media during the handoff period.

2.2 Proposed Framework Fig. 2 shows the seamless media streaming framework in the mobile IP-enabled WLAN, where IEEE 802.11 devices are configured as a WLAN infrastructure. A streaming application on a mobile node receives packets from a media server, while keeping appropriate amount of packets in the client buffer to overcome resource fluctuations of network: available bandwidth, delay, jitter, and packet loss. The streaming server reacts to the feedback informed by the streaming client and performs quality adaptation and packet scheduling. The streaming client sends its status information to the server, which includes current buffer occupancy, receiving rate, error rate, and etc. Many studies on the relationship between

Figure 2. Seamless media streaming framework in Mobile IP-enabled WLAN.


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feedback and reaction have been reported in [10]. In our framework, the handoff latency is estimated before the handoff occurs. To estimate the handoff latency, the mobile client monitors network conditions such as link delay, flow rate, and queue status of the neighbor FAs. The handoff protocol and related signaling procedure are analyzed to get the handoff latency. After estimating the handoff latency, the streaming client tries to prepare enough data to compensate the estimated interruption time during the handoff period. To acquire the target buffer level, which is estimated based on the given handoff latency, the streaming client tries to collect packets, while keeping playback of the received media packets. There are two choices to boost the buffer level: increasing sending rate at the streaming server or decreasing the playback speed. It depends on the policy of pre-buffering management module. The estimated target buffer level can be varied along the time. Every FAs may have different required target buffer level to cover up the handoff latency. Thus, it is needed to maintain sufficient buffer level in order not to stop the playback during the handoff period. Here, the pre-buffering level should be estimated and adjusted periodically to match the network fluctuations. In order to validate the feasibility of seamless media streaming, we implement following features in the proposed framework.

Packet buffering and forwarding mechanism with L2 trigger in the FA.

Pre-buffering adjustment in the streaming client at the mobile node.

3. LINK-LAYER ASSISTED SEAMLESS MEDIA STREAMING 3.1 Packet Buffering and Forwarding with L2 Trigger Since the mobile IP should be independent of any link-layers, the link-layer and IP-layer are clearly separated. Thus, IPlayer and upper layers cannot catch the occurrence of the link-layer handoff without explicit link-layer notification. As mentioned above, the agent discovery time is required to sense change in the network attachment. The agent discovery time depends on the sending interval of agent advertisement message. In the mobile IP specification, the lower boundary of sending interval is recommended as 1 second. Even if the sending interval is set at 1 second, the agent discovery time is random value between 0 and 1 second and the worst case of the agent discovery time is 1 second. Thus, the delay in the agent discovery is the major component in total handoff latency. However, if the mobile node can detect the linklayer handoff through L2 trigger, after the detection of link-layer handoff, it can simply discover change in the network attachment by broadcasting an agent solicitation message without waiting for an agent advertisement message from the nFA. Therefore, agent discovery time can be reduced at about a round-trip time in the wireless link and thus total handoff latency is considerably reduced. Fig. 1 (b) shows the handoff latency in case of using L2 trigger. To avoid burst packet losses produced during the handoff period, a packet buffering and forwarding mechanism (a.k.a. smooth handoff) in the FA is proposed in [5]. Fig. 3 indicates the relevant procedures after the agent discovery process. First, the FA decapsulates and sends packets toward a mobile node and also buffers these packets in local

(a)

(b)

Figure 3. The procedure of packet buffering and forwarding mechanism in the FA.

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circular buffer. When the mobile node moves to another foreign network, it appends a Previous Foreign Agent Notification extension to the registration request message as an extension of the registration process and sends them to an nFA. The nFA then sends a Binding Update message to an oFA as well as the registration request message to the HA. When the oFA receives the Binding Update message, it validates the message; if validated, the oFA updates its binding cache and sends a Binding ACK message to the mobile node through the nFA. Then, the oFA re-tunnels buffered packets, along with any future packets tunneled to the oFA, to the mobile node’s new CoA (i.e., nFA). Thus, burst packet losses generated during the handoff period can be nearly eliminated by the packet buffering and forwarding mechanism in the FA. The mobile node cannot anticipate and notify the movement to another foreign network before it initiates the handoff since the link-layer handoff is hard and forward handoff in the mobile IP-enabled WLAN as mentioned above. Therefore, the old FA cannot know the exact moment when the mobile node disconnects and thus should continuously buffer packets for the MN in a circular buffer. At this point, how many packets should be buffered at the oFA is an important issue. It is related to the handoff latency. To avoid some packet losses, the circular buffer size should be set to the worst case of handoff latency. Thus, some packet duplications are possible. The estimation of the circular buffer size is similar to pre-buffering adjustment in the streaming client, which is handled in following subsection.

3.2 Pre-buffering Adjustment In general, pre-buffering in the streaming client of a mobile node can compensate network jitter and delay. In addition, the forwarded packets, which are buffered in the oFA during the handoff period, can be utilized at a proper time for the continuous playback of streaming media through the per-buffering. However, if the amount of pre-buffering is not sufficient to overcome the discontinuance of data transmission generated during the handoff period, although buffered packets are forwarded to the mobile node, the playback disruption may happen. Also, over-sized pre-buffering causes both memory waste and undesired latency overhead. Therefore, to find the effective pre-buffering time is very important. The streaming client should be able to play streaming media without playback disruption while it consumes time for link-layer handoff, agent discovery, and the end-to-end messaging for binding update among oFA/nFA/mobile node (i.e., the mobile node cannot receive any packets during the handoff period). Therefore, based on the handoff transient time analysis, the pre-buffering time can be calculated as a function of link-layer handoff latency, the propagation delay of wireless link, the link delay between the oFA and the nFA, and the processing and queueing delay in the mobile agents and APs 9. Fig 4. indicates the total delay (ttotal), which should be considered to estimate pre-buffering time, between the last received packet in the old foreign network and the first forwarded packet via the nFA from the oFA in the new foreign network. We denote link-layer handoff latency, agent discovery time, binding update delay, and additional delay as tL2, td, tBU, and tadd, respectively. In addition, agent discovery time (td) and binding update delay (tBU) can be denoted by td = 2tw, tBU = 2tf + 2tw,

Figure 4. The transmitted packets of old stream before/after the handoff period.


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Figure 5. Mobile IP-enabled WLAN environment.

where tw is the propagation delay of wireless link and tf is the forward link (between the oFA and the nFA) delay. The additional delay (tadd) is the consumed time taken to receive duplicated packets transmitted via the nFA from the oFA. To simply estimate the pre-buffering time, we don’t consider processing delay and queueing delay in mobile agents and APs. In addition, since the packets of new stream will be decoded after forwarded packets via the nFA from the oFA are decoded, the packets of new stream via the nFA aren’t considered to estimate pre-buffering time and thus aren’t presented in the Fig. 3. Finally, to estimate pre-buffering time, we consider the total delay and the margin (tmargin) to smooth playback in the streaming client. Therefore, the total delay and estimated pre-buffering time (tpre-buffering) can be denoted by ttotal = tL2 + td + tBU + tadd = tL2 + 2tf + 4tw + tadd,

(1)

tpre-buffering = ttotal + tmargin= tL2 + 2tf + 4tw + tadd + tmargin.

(2)

In addition, the circular buffer size in the oFA should be long enough to store the packets, which will be forwarded to the nFA, during the total delay (ttotal) except additional delay (i.e., pure handoff latency) because the mobile node only cannot receive any packet during the pure handoff period.

3.3 Implementation Details The experimental environment to provide the seamless MPEG-4 streaming over the mobile IP-enabled WLAN is depicted in Fig. 5. Cisco Aironet 350 series APs and client adapters supporting IEEE 802.11b are used and operated in an infrastructure mode. The HUT mobile IP and the MPEG4IP implementations for Linux are adopted for the mobile IP system and MPEG-4 clients, respectively 11,12. In the considered environment, the FA and AP entities are distinctively separated. We also adopt Apple Darwin Quicktime streaming server as a streaming server 12. On top of these, we utilize the L2 trigger about a change of an associated AP with a wireless network interface to reduce the agent discovery time in total handoff latency. In addition, we implement the packet buffering and forwarding mechanism and extend the related messages (such as Previous Foreign Agent Notification extension, Binding Update, and Binding ACK 7). The used device driver of wireless network interface card (NIC) supports the Linux Wireless Extension 13, a generic API allowing a driver to expose to the user space configuration and statistics specific common wireless LANs, which make it possible to signal the mobile IP software for L2 triggers (events), in particular at the event about a change of an associated AP with a wireless network interface. The event can be received through the rtnetlink socket when the a wireless network interface has joined an nAP, which trigger broadcasting of an agent solicitation message to know whether associated network is changed or not. If the mobile node can know the change in the network attachment through the agent advertisement message as the response of the agent solicitation, it initiates the registration process. The packet buffering and forwarding mechanism is realized based on the FA architecture depicted in Fig. 6. First of all, after decapsulating tunneled packets, the FA forwards them to the mobile node and save them in local circular buffer

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Figure 6. FA architecture for packet buffering and forwarding mechanism.

simultaneously. In order to transparently capture packets passing through the FA, we use Linux Divert socket 14. The Linux Divert socket is also used to forward the buffered packets to the nFA, since it can enable IP packet interception and injection on end nodes as well as on routers. Through the Linux Divert socket, packets can be intercepted in IP layer and available for user processes outside the kernel via a modified version of raw sockets. Divert sockets rely on the IP firewall mechanism for packet filtering. At this time, the circular buffer size and related buffering parameters are based on the worst-case time of link-/IP-layer handoffs. In addition, in order to make the tunnel between the oFA and the nFA for packet forwarding, we utilize the routing and IP-in-IP tunneling capabilities of the Linux operating system. In the oFA, the tunnel toward the nFA is established as an outgoing interface. A routing entry is also added for the tunnel. On the other hand, in the nFA, the tunnel between the oFA and the nFA is established as an incoming interface. Routing rules and tables are also added to the nFA for a connection between the incoming tunnel interface and the outgoing Ethernet interface. Using this tunneling connection, buffered packets locating in the oFA can be re-tunneled to the nFA and all packets arriving at the oFA, destined to the mobile node, are immediately re-tunneled to the nFA. These re-tunneled packets are forwarded to the mobile node via the nFA.

4. EXPERIMENTAL RESULTS AND DISCUSSION We test the performance of MPEG-4 video streaming with the modified mobile IP system. The experimental testbed is depicted in Fig. 5. For the experiment, we set the period of agent advertisement broadcasting to 1 second, which is the

Forwarded packets

Around 1370ms

(a)

Around 680ms

(b)

Figure 7. Received RTP sequence: (a) Original handoff and (b) Packet buffering and forwarding with L2 trigger.


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(a)

(b)

Figure 8. Decoded frame sequence: (a) Original handoff and (b) Packet buffering and forwarding with L2 trigger.

lower boundary of the sending interval of agent advertisement messages in mobile IP specification. We also configure that the radio coverage of APs between the adjacent different sub-networks can be overlapped. Handoff latency is computed as a time interval between the last packet transferred from the oFA and the first packet transferred from the nFA (except for the mobile IP signaling). To accurately measure the handoff latency, additional monitoring computers are used to capture L2 management frames from the used wireless channels. In addition, to measure required link delays of both the propagation delay of wireless link (tw) and the link delay between the oFA and the nFA (tf), we add the probing packet and measurement functions in the HUT mobile node and agent. The pre-buffering time can be estimated and applied to streaming client before playback of streaming media. In our testbed, we measure each delay several times and take the maximum value among them: tL2 = 600ms, tf = 2.21ms, tw = 4.86ms, and tadd = 100ms. According to Eq. (2), the pre-buffering time is estimated at 1023.86ms, where we set tmargin = 300ms. Also, in our experiment, we simply use a fixed buffering time of 700ms to the circular buffer in the oFA, which is set by considering the measured maximum linklayer handoff and safe margin. To evaluate the quality improvement for streaming media, we experiment MPEG-4 video streaming with the sending rate of about 1 Mbps and the frame rate is 25 frames/sec. The pre-buffering time is fixed to 1 second in the original mobile IP system case. To show seamless playback of streaming, we measure the sequence number of RTP packet and the decoded frame sequence. The trace of sequence number of received RTP packets is depicted in Fig. 7. From the comparison between Fig. 7(a) and Fig. 7(b), by using L2 trigger, we can know that the handoff latency is reduced, mostly thanks to the delay reduction for agent discovery. Also, burst packet losses during the handoff period can be canceled out by the packet buffering and forwarding mechanism. In the Fig. 7(a), the handoff latency is around 1370ms including link-layer handoff latency and the latency for the agent discovery. Since the mobile node can discover change in the network attachment by receiving agent advertisement message from the nFA, the latency for agent discovery cause increment of the total handoff latency. Thus, there exist burst packet losses (i.e., around 100 RTP packets) during the handoff period. However, in the Fig. 7(b), the handoff latency can be reduced at around 680ms since the latency for the agent discovery become a round-trip time in wireless link by using L2 trigger. In addition, burst packet losses generated during the handoff period are eliminated by the packet buffering and forwarding mechanism. The trace of decoded frame sequence is depicted in Fig. 8. From the Fig. 8(a), we can know that the media playback is blocked by the burst packet losses generated during around 1370ms of the handoff latency because the any packets cannot be received during the handoff period. In addition, since the pre-buffering time is smaller than the handoff latency, there exists buffer underflow at the streaming client. Thus, additional quality degradation is generated after the handoff and even the streaming client doesn’t recover the smooth playback of the streaming media and doesn’t synchronize

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between audio and video of the streaming media for a long time. However, in the Fig. 8(b), there does not exist playback disruption during the handoff period because the forwarded packets can be applied to playback by the sufficient prebuffering in the MPEG-4 client before the buffer of streaming client becomes underflow. So, we can know that the applied our measurement result to the pre-buffering time is reasonable.

5. CONCLUSION In this paper, we presented the causes of burst packet losses during the handoff period and the proposed approach to reduce the handoff latency and mitigate the playback disruption due to the packet losses during the handoff period from the implementation perspective. First, we introduced the seamless streaming framework by estimating the accurate buffer level for pre-buffering to compensate the handoff latency. We calculated the handoff latency in the application point of view for the mobile IP-enabled WLAN environment. Also, we implemented a preliminary version of the proposed framework by extending the basic mobile IPv4 system and validated the feasibility of seamless media streaming. The experiment results show that the handoff-aware streaming has no playback discontinuity while keeps a minimal prebuffer size. For the future work, we will implement and experiment the whole proposed framework for seamless media streaming in the mobile IP-enabled WLAN.

ACKNOWLEDGEMENTS This research was supported in part by University IT Research Center Project and in part by grant R05-2004-000-109870 from the Basic Research Program of the Korea Science and Engineering Foundation.

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