We can observe the performance on TCP as mentioned above. Now we change the service to UDP, the result is presented as follows. We set the UDP related traffic value in the following:
$cbr set type_ CBR
$cbr set packet_size_ 1000
$cbr set rate_ 1mb
$cbr set random_ false
We will discuss and compare the effect of various data rates on UDP services. UDP service is different from TCP service because it is a connectionless service, so the lost UDP packets will not be retransmitted again. If the packet loss rate is high, there will be a noticeable gap, as shown in the following figures.
Fig. 49: Original & with & without Transient Auth. under Fast Handover.
Figure 49 shows the difference between three cases under UDP services. We could observe that the transient authentication curve is similar to the original fast handover. So, the packet loss rate is almost equal, while the packet loss rate of fast handover without transient authentication is high.
no-transient timestamp no-transient udp seqno 40.429763 442
41.069763 450
Table 4: No-transient with UDP 100Kbps during handoff.
400
Fig. 50: UDP with 100Kbps Data Rate.
In order to analyze the UDP packet loss rate, we add a sequence number to each packet to observe the growth of packet loss. First, we set the UDP data rate to 100kbps as shown in Fig. 50, and there are 8 lost packets during re-authentication without transient authentication in 640ms. This elapsed time is too large to offer a good Multimedia Streaming or VOIP service. Actually, when users of a multimedia streaming application move from the coverage area of an AP (access point) to the other, the connection must be handed off in approximately 150 milliseconds, otherwise the user will feel the jitter affect. So, we use the transient authentication during handoff to reduce the authentication time in 150ms, as shown in Fig. 50. It can improve the QoS for multimedia streaming application.
Fig. 51: UDP Data Rate 1Mbps.
no-transient timestamp no-transient udp seqno
40.725603 4428
41.33824 4532
Table 5: No-transient with UDP 1Mbps during handoff.
We increase the data rate to 1Mbps to observe the difference. First, we address the fast handover without transient authentication case. At 40.73sec, the mobile node receives UDP sequence number 4428 and continues to receive UDP packets at 41.34sec with UDP sequence number 4532. So, the total number of lost packets is 104 in 0.6sec, we can see that there is a very large gap shown in Fig. 51.
Compared with TCP experiment, the packet loss rate of UDP experiment is higher because TCP will adjust its window to slow down its sending rate. UDP keeps its constant sending rate at 1Mb, so the packet loss rate is higher if the mobile node doesn’t perform transient authentication mechanism to get a temporary certificate to pass the authentication in the new domain. We try to use the fast handover protocol to reduce the handoff time, however the authentication process still causes a significant delay which is approximately 330ms or even higher. We can realize that the authentication process does affect the handoff performance drastically if we use more complex authentication mechanism. Then, we calculate the packet loss rate in 2 seconds during handoff period as follows.
Sending rate Packet drops Packet loss rate
100kbps 8 2 32%(no-transient) 8%(transient)
1Mbps 104 34 41.6%(no-transient) 13.6%(transient) Table 6: UDP Packet Loss Rate (no-transient & transient).
8% (transient) 32% (no-transient)
14% (transient) 42% (no-transient)
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
no-transient transient
authentication scheme
UDP Packet loss rate(%)
100Kbps 1Mbps
Fig. 52: Packet Loss Rate with Different UDP Data Rate.
Figure 52 show that the UDP data rate and transient authentication indeed affect the packet loss rate. Next, we show the final figure regarding increasing the authentication time in 300ms under UDP service in data rate 1Mbps.
4300
Figure 53 shows that the authentication processing time also increase the packet loss rate.
Auth. processing time Packet drops Packet loss rate
100ms (no-transient) 104 41.6%
100ms (transient auth) 34 13.6%
300ms (no-transient) 258 100%
300ms (transient auth) 86 34.4%
Table 7: Packet drops with authentication processing time 100ms & 300ms, sending rate 1Mbps.
3.9 Section Conclusion
In this project, we investigated a two-stage authentication scheme which includes transient authentication and re-authentication mechanism. The user authentication is called re-authentication.
The re-authentication signaling consists of 4-way handshaking and we use it to simulate the authentication time during handoff period. In the original structure of Mobile IP, it needs approximately 3-4 sec to complete the handoff process excluding authentication. If we add authentication process on it, the handoff time will increase drastically due to the complex authentication mechanism. Since a more complex authentication mechanism needs longer time to process, the transient authentication becomes important if we try to reduce the authentication time during handoff period. In our experiment as discussed in section 3.8, we demonstrate that the packet loss rate increases when UDP sending rate increases. Packet loss rate is reduced to 8% with transient authentication when UDP sending rate is 100Kbps. Packet loss rate is reduced to 13.6% when UDP sending rate is 1Mbps. If we combine transient authentication with fast handover protocol, it is able to reduce the packet loss rate and perform well as original fast handover protocol without authentication mechanism. It has no protocol overhead and is feasible to implement. The re-authentication signaling is mainly to simulate an authentication process, and we can understand how the packet loss rate will change if we add the authentication mechanism in this project. Finally, the proposed transient authentication method piggybacks on authentication information through fast handover protocol without additional signaling overhead. We know that IEEE 802.1x is a MAC layer authentication mechanism. It takes more than 1200ms to
complete the authentication process. If we count the total disconnection time such as scan, authentication and association, the time value is too large to be acceptable for certain applications.
Nowadays, many proposed methods are trying to improve the performance in different phases. Even if we use the best available method in each phase, the total disconnection time is still too large to offer a high Quality of Service for VOIP or Multimedia streaming. So, the tradeoff between authentication and fast handoff is difficult. In this project, we perform the user authentication to open the filtering table for MN on new access router, which will drop or forward the packets toward MN according to the authentication table. We use transient authentication concept early to get the access right in the new domain when MN roams to it based on fast handover protocol.
In order to provide the functions of authentication and security, definitely, the original handoff time will be increased, this in turn increases packet loss rate and degrades the quality of service. So, how to maintain good service under the framework of providing authentication and security will be an essential issue in the future.
The proposed method in this project is designed under Mobile IP architecture. It is a user authentication mechanism in which we send the identity of the mobile node and user authentication information before handoff really starts, this could reduce the total authentication time during the handoff process. Also, we may modify the layer 2 authentication of IEEE 802.1x to support the pre re-authentication function or enhance its security and key distribution, this will be studied in the future research.
五、結論與討論
The speedy handover mechanism is proposed to meet one of the objectives of this subproject. It can enhance the performance of wireless handover. Different from the famous fast handover scheme that a mobile node must discover the movement by itself, an access point take charge of detecting the movements of mobile nodes in speedy handover mechanism. As a result, the proposed scheme can work no matter the radio covering area of the two neighbored FAs are overlapped or not. We are planning to refine the speedy handover further and submit it to an international conference.
In the second year of this subproject, we will develop a scheme for location management of mobile hosts. Load balance and QoS provision in wireless access networks will be the targets of our new scheme.
In this project, the concept of “discrete scan” is newly proposed. Based on the discrete scan scheme, the next AP in a handoff can be discovered prior to the occurrence of handoffs. With the survey of fast handoff schemes based on the prediction of next APs, a series of handoff schemes, including context caching, proactive key distribution, buffering and forwarding, inter layers handoff design, are suitable to cooperate with discrete scan scheme to fasten layer-2 and layer-3 handoff to achieve the goal of seamless handoffs.
The principle of discrete scan lies on the decomposition of the passive scan procedure in a traditional layer-2 handoff into discrete pieces of sniffing periods prior to the occurrence of handoff. A mobile node utilizes the possible idle time of its NIC as the sniffing periods so that the disruption caused by a sniffing activity is controlled less than 50 ms to fulfill the minimal requirement of seamless handoff features.
With the application of discrete scan scheme, a mobile node can further select an appropriate next AP among the next-AP candidates with help of information extracted from MAC headers in those frames collected in sniffing periods. Several mechanisms to select the next AP with desired features are proposed in this project, such as the ratio of stations locating within coverage of the mobile node to that in the BSS and the station count discovered in the last sniffing period before a handoff are initiated to select the nearest AP as the next AP. Besides, a scheme to identify an approached AP and an indicator to predict the available bandwidth of concerned BSS are briefly discussed in this project.
With the benefits of simplicity and nearly real-time characteristic, the station count mechanism is elaborated and evaluated. The setting of a sniffing period for discrete scan scheme, performance for the mechanism to hit the nearest AP as well as the impact on service QoS caused by the absence from working channel in sniffing periods for discrete scan scheme in a DCF wireless environment are
demonstrated with both analytical model and numerical results. For further verification, those for EDCA wireless environment are analyzed with results generated in ns-2 simulations.
The numerical as well as the simulation results show that discrete scan with station count mechanism provides considerable high hit ratio on the selection of the nearest AP among the next-AP candidates discovered by a mobile node. Although the imbalance of number of stations in the next-AP candidates is still an inevitable factor to affect the selection accuracy for the nearest AP, its effects can be significantly reduced by setting a proper length of sniffing periods.
The nature that an AP, responsible for all downlink traffic, has the same transmission opportunity as an arbitrary station, taking care only its own uplink traffic, in a BSS may causes insufficient downlink bandwidth for a symmetric bidirectional connection such as a VoIP application. The downlink problem for VoIP traffic is supposed to resolve with the piggyback scheme reported in 802.11e standard, because it is an inherent QoS problem instead of being caused by discrete scan scheme. However, the impact on the service QoS caused by discrete scan scheme is discussed in view of uplink flow for VoIP traffic.
Simulation results show the induced disruptions being maintained within a tolerable level in the EDCA environment for all cases of reasonable number of stations in a BSS; However, numerical analysis shows a mobile node with discrete scan scheme in DCF wireless LAN can sustain acceptable QoS when less than six stations are in a BSS.
In the future, we will further improve the scheme by investigating a scenario that a mobile VoIP node with discrete scan scheme works in EDCA environment. The observation will focus on the QoS degradation induced by discrete scan scheme. Besides, an integrated selection mechanism with sniffing function and an algorithm to identify a station via reception of its transmitting frame will be developed.
Finally, we will plan to implement a complete set of discrete scan as well as selection mechanism into a real WiFi phone. The WiFi phone shall be proved with experiments the capability of fast layer-2 handoff because latency of probe phase has been eliminated. The performance to select a desired next AP shall be verified with experiments. Eventually, the strategies addressed for fast handoff in the related works, such as context caching, buffering-and-forwarding and inter layer handoff design will be implemented into the APs and FAs to cooperate with a mobile node with discrete scan scheme, so that a complete wireless environment can be configured to support seamless inter-domain handoff for a next generation WiFi services.
In order to provide the functions of authentication and security, definitely, the original handoff time will be increased, this in turn increases packet loss rate and degrades the quality of service. So, how to maintain good service under the framework of providing authentication and security will be an essential issue in the future.
The proposed method in this project is designed under Mobile IP architecture. It is a user authentication mechanism in which we send the identity of the mobile node and user authentication information before handoff really starts, this could reduce the total authentication time during the handoff process. Also, we may modify the layer 2 authentication of IEEE 802.1x to support the pre re-authentication function or enhance its security and key distribution, this will be studied in the future research.
六、參考文獻
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draft-ietf-aaa-diameter-mobileip-20.txt