tcp/ip thesis

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Tcp/ip thesis

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Abstract Understanding, measuring, and debugging IP networks, particularly across administrative domains, is challenging. In-path middleboxes that modify packet headers are typically transparent to a TCP, yet can impact the end-to-end performance of its connections. Of equal importance, middleboxes cause architectural ossification that hinders network protocol evolution—new options or redefined header fields are often misconstrued, modified, or disabled.

We discover over 11 thousand instances of unique non-NAT in-path packet header modifications across those flows, all with the potential to negatively affect TCP performance. Rights This publication is a work of the U. Copyright protection is not available for this work in the United States. Collections 1. Thesis and Dissertation Collection, all items. Related items Showing items related by title, author, creator and subject. Understanding, measuring, and debugging IP networks, particularly across administrative domains, is challenging.

One particularly daunting aspect of the challenge is the presence of transparent middleboxes which are now Monterey, California. Naval Postgraduate School , ;. This patch for version 3. This version of the patch was used to operate servers for the Internet Export search results.

Export citations. The list of citations that can be exported is limited to items. Export Citations. The estimate is used to set ssthresh and cwnd after a loss event and also periodically to modify the aggressiveness of the sending rate increase by increasing ssthresh.

Thereby TCP performance can be improved in the presence of transmission error induced losses that are wrongly considered congestion events. Later versions of TCP Westwood [74] also use the bandwidth estimate to find an appropriate ssthresh during the initial slow start phase. Bhandarkar et al. The results in [76] also demonstrated that TCP Westwood underutilizes the bandwidth in the presence of reordering caused by link layer retransmissions.

Our work on TCP for wireless conditions falls into the category of end-to-end solu- tions. IP makes it possible for data bundled into packets to find its way through the network. Data may be lost, reordered, or duplicated along the way. It provides the application with data in the right order, lost data is retransmitted, and duplicate data removed.

TCP also attempts to adapt to the varying network conditions by increasing its sending rate during loss free periods and decreasing it when detecting a loss. Reordering and delay spikes can trick TCP. If TCP believes that a segment was lost it decreases its sending rate, which may lead to underutilization of the available bandwidth if the segment was merely delayed or reordered.

TCP assumes some degree of stability and a low transmis- sion error rate, but a wireless channel has varying capacity. Therefore, the combination of wireless networking and TCP was problematic to begin with. WWANs have traditionally been used for voice calls, but are evolving towards a packet-switched architecture to better support data traffic and facilitate the inclusion of new services.

We have described the development and spread of existing WWAN tech- nologies and the expectations on future releases. We focus on WWAN technology in this thesis. We use an end-to-end approach, because it does not require changes to intermediate nodes making deployment easier.

Security issues with intermediate nodes inspecting packet headers can also be avoided. Chapter 8 Summary and Conclusions The Internet is continuously attracting new users and thereby businesses. To take part of the success, many network technologies that were not IP-based from the beginning have evolved to support IP-based communication. As new network types join the Internet, the heterogeneity of bandwidths, delays, error rates, and devices increases.

Therefore, the protocols should be flexible and robust against varying characteristics of heterogeneity. Resources are also more constrained. Through this separation TCP-Aix provides robustness to reordering events of one round trip time rtt and delay variations. To handle reordering events beyond one rtt, TCP-Aix uses a higher duplicate acknowledgment threshold dupthresh setting than the standard setting. We introduced the winthresh algorithm for computing dupthresh.

It minimizes the amount of spurious retransmits that a sender inserts into the network by waiting as long as possible before retransmitting a segment while at the same time avoiding window stalling. The winthresh algorithm is tuned through a parameter that relates dupthresh to the current send window swnd.

Through simulations, we found that it is important to control the delay of the conges- tion response, to enable TCP-Aix to utilize the bandwidth in dynamic scenarios where the available bandwidth varies substantially and quickly. A parameter setting correspond- ing to two swnds in the winthresh algorithm offers a good trade-off between detecting reordering and preserving the ability to rapidly adapt to such varying conditions.

With this setting, TCP-Aix can detect reordering durations of roughly three end-to-end rtts. We showed that TCP-Aix is able to maintain almost constant performance even in scenarios which frequently display long reordering durations. Performance gains are also seen in scenarios displaying only moderate reordering durations of less than one rtt. With reordering robust TCP flavors ready for deployment, the informal constraint on wireless link layers to enforce in-order delivery for TCP can be relaxed.

Thereby, the complexity of network components can be decreased. The results from our case study of a dedicated WWAN link show that a link layer that is allowed to do out-of-order delivery together with a reordering robust TCP flavor has the potential to improve smoothness considerably for short time scales compared to a standards-compliant TCP with a link layer that delivers data in-order. Smoothness plays an important role when it comes to predictability of the network traffic and the possibilities for mixing different types of traffic.

Real-time traffic usually can not cope with too many link layer retransmissions, because the data has a limited lifetime. Out- of-order delivery could make it possible to use the same link layer configurations for both real-time and background traffic. The end-points TCP or the application would then have to deal with out-of-order delivery, but less delay variations.

We also demonstrated that out-of-order delivery at the link layer coupled with a reordering robust TCP flavor, decreases the network layer buffer requirement. As memory components become cheaper, buffer space limitations become less important.

The wireless medium is unguided and shared, which makes efficiency more important than in wired networks. We have studied how to improve TCP efficiency by reducing the acknowledgment frequency. Delayed acknowledgments were introduced to conserve network and host resources. Further reduction of the acknowledgment frequency can be motivated in the same way.

However, reducing the dependency on frequent acknowledgments in TCP is difficult be- cause acknowledgments are at the same time used for reliable delivery, loss recovery, clocking out new segments, and determining an appropriate sending rate. Our approach differs from previous work in that we study scenarios where there are no obvious advantages of reducing the TCP acknowledgment frequency.

Thereby, we investigated whether a lower acknowledgment frequency could be widely used. We proposed and evaluated an end-to-end solution, where four acknowledgments per swnd were sent and the sender compensated for the reduced acknowledgment frequency using a form of Appropriate Byte Counting. Although, we reduced the acknowledgment frequency in a symmetric wireline sce- nario, performance could be maintained.

Hence, there is a potential for reducing the acknowledgment frequency more than is done through delayed acknowledgments today. Advancements in TCP loss recovery is one of the key reasons that the dependence on frequent acknowledgments has decreased. Reducing the acknowledgment frequency increases TCP burstiness.

Unfortunately, few measurement studies of the effects of burstiness exists. We tested the effect of reducing the acknowledgment frequency and thus increasing the burstiness on network layer buffering in low multiplexing scenarios. Conclusions VoIP is an important service because it is needed for a full conversion from a circuit- switched to a packet-switched architecture in WWANs.

Shared channels, primarily de- signed for data traffic, have gained interest for VoIP. To understand which characteristics a scheduling algorithm should have for a mix of conversational traffic VoIP and interactive traffic web , we used the ns-2 simulator extended with a model of HS-DSCH to simulate a mixed traffic scenario.

We studied four scheduling algorithms: the proportional fair PF , the maximum rate MR scheduler, and two extended versions of MR for different VoIP scheduling delay budgets and varying load. Both cell throughput and user satisfaction were estimated. A more drastic increase in VoIP priority is however needed when the delay budget is short.

Furthermore, at- tempting to preserve quality for both VoIP and web traffic makes the system sensitive to overload situations. By strengthening TCP, we want to make it easier to deploy and run applications over wireless networks. We thus proposed and evaluated a number of TCP refinements. With this in mind, reordering durations of two to three times the base end-to-end rtt can be handled.

It requires sender-receiver cooperation and changes to the byte counting [30] and loss recovery [1]. For the design of reordering robust TCP flavors we found that considering a dynamic environment is important. There is always a trade-off present — when improving one characteristic of a protocol, another characteristic can be impaired.

Therefore, it is important to identify all conflicting characteristics. Relaxing the informal in-order delivery constraint can have wide reaching conse- quences. Not only the frequency with which reordering occurs is important; knowing the duration of individual reordering events is vital to estimate the effect on performance and to assess the potential of out-of-order delivery. When reducing the acknowledgment frequency, we risk reducing throughput and in- creasing burstiness. We have found that there is not enough information regarding how other applications perceive TCP burstiness to guide us in designing support for reduc- ing the acknowledgment frequency.

For systems that make an effort to provide some form of quality to the users, e. As voice traffic is also be- coming common on the traditional Internet, the interest for how various services affect each other should be growing.

With an increasing traffic load on the Internet, burstiness should be more noticeable to other applications. It is no longer sufficient to only consider co-existence and fairness towards other TCP flavors; we need to consider the effect on other applications as well. We have come across several areas where more measurements are needed to control that we are working towards a better Internet architecture.

At the moment, TCP research is focused on adapting TCP for links with high capacity, which has lead to a discussion of the entire congestion control architecture. Thus, there may be an end to the era of continuous, minor TCP modifications and instead there may be a major revision. Until then our work shows that it is possible to improve TCP robustness and reduce TCP overhead; making TCP better suited for a modern network environment, where wireless links are likely to be common.

TCP is a complex protocol designed to control many mechanisms. Even small modifications may therefore have large consequences. TCP-Aix, as described in Chapter 4, is a set of modifications. We have verified that TCP-Aix works as intended and studied its impact on the network through simulations.

Thereafter, we would like to implement TCP-Aix in an operating system, estimate the complexity of the implementation, and gather more experience by observing it over the Internet and diverse networking technologies. Continuation As discussed in Chapter 5, reordering on the reverse path can be dealt with by limiting the number of segments sent in response to each acknowledgment, letting all acknowledgments clock out new segments also late acknowledgments , and using byte- counting.

Reordering on the reverse path cause the same type of problems as reducing the acknowledgment frequency. We therefore expect these changes to be helpful when only a few consecutive acknowledgments are delayed. When many acknowledgments are delayed after each other, TCP may still send large bursts. Reducing the acknowledgment frequency, as in Chapter 6, is a small change with large consequences.

The sensitivity to lost acknowledgments and delay variations on the reverse path must be studied. We also need to quantify the effects on smoothness and study burst mitigation before moving on. Another aspect is the increased acknowledg- ment frequency during times of suspected segment loss. The work in [39] provides some ideas to remedy this problem that we would like to study.

The acknowledgment strategy should be able to provide a gain in resource constrained situations. Therefore, we need to complement our evaluation with a set of scenarios exhibiting asymmetry and wireless links. At the same time, we can compare the results of our acknowledgment strategy and other acknowledgment reduction methods designed for specific environments.

It is likely that a widely deployable solution provides less improvement and we would like to quantify this cost. It is also interesting to compare congestion control of acknowledgments and a con- stantly low acknowledgment frequency. Congestion control for the acknowledgments implies that the acknowledgment overhead is only reduced if the capacity is constrained, but the error sensitivity of TCP is increased.

On the other hand, if a constantly low acknowledgment frequency is used, it is possible to perform optimizations. The dependence on the acknowledgment clock makes TCP unfair to flows with longer round trip times.

We need to investigate whether this weakness is aggravated when reducing the acknowledgment frequency. We also want to study different transfer sizes. TCP can still be refined after decades of intense research, but for shared channels in wireless cellular networks and VoIP we are at the beginning of the evolution.

For instance, it is not obvious that the wireless link layer should perform in-order delivery for TCP, and a lower acknowledgment frequency can have effects on scheduling and planning of wireless channels. It would also be interesting to study the effects of TCP smoothness on for instance VoIP and network management algorithms in this and other environments.

In general, mixing services over wireless access is an interesting area. We therefore must find out more about the application requirements, which suggests more measurement studies. We want to prevent unnecessary additions to the base protocols and leave room for important algorithms.

A related problem is to understand user behavior and expectations for different net- working environments, both to find appropriate models for evaluation and to offer suitable technology. In this thesis, we have focused on transport layer issues, except for in the HS- DPA study where we evaluated the performance from both a system and an application perspective. Looking beyond our immediate research area, efficient and robust communication is desirable from a global energy perspective.

When sending e-mails and surfing the web, we generate lots of TCP flows. This makes it interesting to also include energy efficiency not only from a battery point of view in our future work. There is work on energy consumption of different TCP flavors in multi-hop wireless networks [], which can serve as a starting point. References [1] M. Allman and V. Ludwig and R. Blanton and M. Ludwig and M. Sarolahti and M. Ludwig and A. Zhang, B. Karp, and S. Leung, V. Li, and D. Bhandarkar and A.

Springer, Apr. Bhandarkar, A. Reddy, M. Allman, and E. Bohacek, J. Hespanha, J. Lee, C. Lim, and K. Chan and R. Pelletier, K. Sandlund, L. Jonsson, and M. Balakrishnan, S. Seshan, and R. Kalampoukas, A. Varma, and K. Ming-Chit, D. Jinsong, and W. Singh and K. Magnet, and J. Lilakiatsakun and A. Altman and T. Ericson, L. Voigt, and S. Braga, E. Rodrigues, and F. Larzon, S. Ludwig, M.

Meyer, and J. Kohler, M. Handley, and S. Floyd, E. Kohler, and J. Floyd, A. Arcia, D. Ros, and J. Floyd and E. Folke, S. Bodin, and S. Schulzrinne, S. Casner, R. Frederick, and V. Saltzer, D. Reed, and D. Fall and S. Jacobson and R. Mathis, J. Mahdavi, S. Floyd, and A. Padhye and S. Blanton, M. Allman, K. Fall, and L.

Floyd and T. Floyd, T. Henderson, and A. Kurose and K. Pearson Education, Rappaport, Wireless Communications - Principles and Practice, 2nd ed. Pren- tice Hall, Hui and K. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Farrell, V. Cahill, D. Geraghty, I. Humphreys, and P. Tsaoussidis and I.

Bakre and B. Kent and K. Wei, C. Zhang, H. Zang, D. Towsley, and J. Casetti, M. Gerla, S. Lee, S. Mascolo, and M. Wang, K. Yamada, M. Sanadid, and M. Bhandarkar, N. Sadry, A. Reddy, and N. Leung and C. Floyd and V. Allman and A. Jacobson, R. Braden, and D. Floyd, M. Mahdavi, M. Mathis, and M. Paxson and M. Allman, H. Balakrishnan, and S. Ramakrishnan, S. Floyd, and D. Allman, S. Floyd, and C. Handley, J. Padhye, and S.

Allman, A. Jain, and P. Balakrishnan, V. Padmanabhan, S. Seshan, M. Stemm, and R. Ladha, P. Amer, A. Medina, M. Allman, and S. Savage, N. Cardwell, D. Wetherall, and T. Semke, J. Mahdavi, and M. Available: citeseer. Padmanabhan, G. Fairhurst, and M. Sarolahti, M. Kojo, and K. C, Feb. Gurtov and S. Li, D. Leith, and R. Wei, P. Cao, and S. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering, D. Estrin, S.

Floyd, V. Jacobson, G. Minshall, C. Partridge, L. Peterson, K. Shenker, J. Wroclawski, and L. Shenker, L. Zhang, and D. Zhang, S. Shenker, and D. Grieco and S. Chen, P. Huang, C. Huang, and C. Bansal, H. Floyd, and S. Padhye, and J. Floyd and M. Wang, Y. Xia, and D. Andrew, C. Marcondes, S. Floyd, L. Dunn, R. Guillier, W. Gang, L. Eggert, S.

Ha, and I. Bennett, C. Partridge, and N. Loguinov and H. Jaiswal, G. Iannaccone, C. Diot, J. Kurose, and D. Gharai, C. Perkins, and T. Wang, G. Lu, and X. Zhou and P. Pearlman, Z. Haas, P. Sholander, and S. Kandula, D. Katabi, S. Sinha, and A. Chen and R.

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You do not have access to any existing collections. You may create a new collection. Citation Request Accessible Version. MLA Sanders, Sean. APA Sanders, S. Chicago Sanders, Sean. Share on Facebook. Share on Twitter. For traffic traces to remain useful for analysis, techniques must be developed to glean insight using this limited header information. The context in which web page classification is defined and evaluated in this dissertation is different from prior traffic classification methods in three ways.

First, the impact of diversity in client platforms browsers, operating systems, device type, and vantage point on network traffic is explicitly considered. Second, the challenge of overlapping traffic from multiple web pages is explicitly considered and demultiplexing approaches are evaluated web page segmentation.

And lastly, unlike prior work on traffic classification, four orthogonal labeling schemes are considered genre-based, device-based, navigation-based, and video streaming-based — these are of value in several web-related applications, including privacy analysis, user behavior modeling, traffic forecasting, and potentially behavioral ad-targeting. We conduct evaluations using large collections of both synthetically generated data, as well as browsing data from real users.

Our analysis shows that the client platform choice has a statistically significant impact on web traffic. It also shows that change point detection methods, a new class of segmentation approach, outperform existing idle time-based methods. Overall, this work establishes that web page classification performance can be improved by: i incorporating client platform differences in the feature selection and training methodology, and ii utilizing better performing web page segmentation approaches.

This research increases the overall awareness on the challenges associated with the analysis of modern web traffic. There are also stochastic effects like slow shadow fading and fast multipath fading. Large objects, such as buildings, can cause shadow fading effects such that two users at the same distance from a base station may experience different channel conditions. Reflecting objects and scatters in the channel give rise to multiple signal waves and so do the movements of the users and the surrounding objects.

These signal waves are replicas of the original signal, which arrive from many different angles, at slightly different times with varying amplitudes and phases. The result is rapidly varying signal strength. The ability to accurately decode a radio signal also depends on the presence and strength of other, interfering signals.

Self-interference is caused by the desired signal itself through multipath propagation or from replicas of the signal arriving from for instance different base stations. These codes enable the receivers to separate signals and thereby facilitates decoding of the desired signal. But, multipath propagation can destroy the orthogonality of the channelization codes that enables the receivers to separate signals. Thus, when there is multipath propagation there may also be intra cell interference when data is sent in parallel to more than one user in the same cell using different channelization codes.

Transmissions in other cells, where channelization codes are reused, cause inter cell interference. The further apart the senders using the same channelization codes are, the weaker the interfering signals are. When there is no central management, as in many IEEE Collisions occur when there are several signals interfering at a receiver such that at least one of the signals can not be perceived. To detect collisions, a node must be able to send and receive at the same time.

The incoming signal usually has lower signal strength than the outgoing signal, making it costly to build hardware for detecting collisions in IEEE Channels can also be unidirectional, if the environment is such that the signal can propagate further in one direction than the other. Therefore, acknowledgments are important to ensure that data have been correctly received. Furthermore, in multihop wireless networks, collisions can occur at node A if node B and C, which are outside each others communication range, transmits to node A simultaneously.

This is called the hidden node problem. The requirement for mobility naturally led to wireless technology. WWANs are predomi- nantly used for mobile telephony. Their distinguishing feature is that a user can move at a relatively high speed over a wide area while being engaged in a real-time session.

Pure voice calls are relatively predictable and have low bitrate requirements. Compared to 1G, 2G technologies offers at least a three-times increase in spectrum efficiency [63]. The enormous interest in data-based services resulted in an effort to provide data ser- vices over 2G technologies.

However, the low bitrate channels 9. The de- mand for data transmission has therefore driven the evolution of WWAN systems towards higher bitrates, lower latencies, and an IP-based infrastructure. It builds on existing 2G technology, like GSM, but is more adapted for packet-based data transmission.

In Table 2. It is the global standard for mobile telecommuni- cation standardized by 3GPP [62]. There are also requirements regarding latency, cost of deployment, spectrum efficiency, and capacity. The improved systems allow new services to be provided. PoC calls are half duplex communication that allow a single person to reach an active talk group by pushing a button, instead of making several calls to coordinate with a group.

MBMS makes it possible to broadcast content over a cellular network to small terminals, e. In March , they reported nearly 2. In the design of release 6 and also more recent releases, efficient solutions for the co-existence of voice and data traffic are offered. Voice services remain important in the foreseeable future, which means that the manufacturers are constantly striving to increase the capacity for voice.

At the same time, Voice over IP VoIP is replacing earlier technologies for supporting voice services and other IP-based applications continue to grow, which affects latencies and bitrate requirements. It should allow users to access any system at any time anywhere. From a user perspective, multimedia services at a low transmission cost, integrated services, and cus- tomized services are expected [65].

The underlying technologies for wireless and wireline systems must therefore function together to achieve seamless transitions for the user when it changes environments. This is a challenge because there are many players that want to protect their revenues and enter new markets. Most of our wireless scenarios in this thesis are derived from WWANs, but there are also other complementary and competing wireless technologies.

Compared to WWANs, they provide higher data rates at the expense of mobility and service quality. Many people have a wireless router at home, which operates using a standard from the IEEE The first widely accepted standard was IEEE The brand name for some of the IEEE The IEEE No permit to set up a network is therefore required. Thus, the cost of setting up the network is access to the Internet, any usual wireline subscription will do, and a wireless router.

The drawback is that other people may also set up wireless networks and thereby cause interference prob- lems when sending on the same frequency. Other devices, such as microwave ovens and cordless telephones, are allowed to use the same frequency band too causing interference.

Two user devices can also communicate directly with each other if they are within communication reach. MANETs allow communication within an area without an access point and can be quickly deployed. The network can also be connected to the Internet. For quick deployment and good performance, self-configuration is essential as users move around. The data can also take different routes through a MANET as the topology changes, which may lead to both reordering and delay variations.

Thus, the research on TCP-Aix and a reduction of the acknowledgment frequency as presented in this thesis are relevant for these network technologies. The network consists of spatially distributed devices that collect information from sen- sors. There are strict requirements regarding the size and cost of the sensors, because a large number is often needed. Furthermore; energy consumption, memory capability, computational speed, and network bandwidth are important issues in the design of a sensor network.

The limited memory makes it difficult to maintain state as required by TCP [66]. From an energy perspective, handshakes at the beginning and end of a transfer, acknowledgments, and the large headers of TCP are a lot of overhead for trans- mitting only small amounts of data. TCP is also designed for global addressing, whereas attribute-based naming is more suitable for the specifics of a sensor network.

Thus, even though accessing a sensor network through the Internet is often desirable, TCP is not suitable for sensor networking. A split connection approach is more likely. Instead the situation where an end-to-end path through the network rarely, or never, exists between two entities is considered. The packets consequently have to take one hop at a time towards the destination, residing for longer time periods at each node along the path. In this thesis, we do not consider DTNs and sensor networks.

Thus, the characteristics of the network are no longer the same as they were when the notion of congestion control and avoidance was born. At that time, transmission errors were infrequent, since wired links were mostly used. Most nodes were stationary resulting in relatively stable rtt conditions and capacity. The bandwidth was much lower than it is today and power and interference concerns were limited. The research area has been popular for almost twenty years. Most of the solutions proposed for TCP over wireless networks fall into one of three broad categories: split connection approaches, link layer schemes, or end-to-end solutions.

In a split connection approach, the original end-to-end connection is split into two separate connections. An intermediate node prior to the wireless link act as the receiver for the connection over the wireline part of the path and as the sender for a second connection covering the wireless link.

In cellular networks, the intermediate node is often the base station or another node belonging to the wireless core network. As topologies become more complex, it becomes more difficult to find suitable points at which to split connections. The motivation behind split connections, given in early proposals like I-TCP [70], was to separate congestion control and flow control functionality over the wireless link from that across the wireline network.

A split also allows a protocol specifically developed for wireless links to be used instead of TCP over the wireless section of the network. This protocol could be designed to have lower overhead and handle mobility better. In addition, the intermediate node can compress or adapt the content, which sometimes requires the first connection to finish before the second part of the transfer is initiated.

A drawback is that the end-to-end semantics of TCP are broken when a connection is split. To begin with, the packets must be examined to determine whether it is a TCP flow and which type of application that generated it. This prevents the use of IP security IPsec [71] end-to-end. Other drawbacks are the need for keeping state in the network and taking on transport layer responsibilities at an internal node. Still, split connections seem to be relatively common in practice.

In [72], the use of split connections was inferred through measurements. Link layer solutions can be motivated by for instance high transmission error rate being a local problem and should therefore be solved locally. Many wireless link layers perform retransmissions to reduce the observable transmission error rate. These retrans- missions cause segments to arrive at the receiver out-of-order, thus generating dupacks that may falsely trigger congestion control.

A common solution is to implement in-order delivery at the link layer. Both link layer retransmissions and in-order delivery add to the delay variations over the link. Summary and Discussion 21 link layers require access to the TCP headers which can cause security issues.

When only the end devices have to be modified it is called an end-to-end solution. This approach allows full use of IPsec and the transfers do not have to pass through any particular intermediate node. But, end-to-end solutions may also have to have competi- tive performance in scenarios where the problem that they are targeting is not present. Because they work without network support, their usage is often not restricted to specific situations.

An example of an end-to-end solution to the problem with transmission errors is TCP Westwood [73]. By measuring the time between acknowledgments, Westwood derives an estimate of the eligible bandwidth. The estimate is used to set ssthresh and cwnd after a loss event and also periodically to modify the aggressiveness of the sending rate increase by increasing ssthresh. Thereby TCP performance can be improved in the presence of transmission error induced losses that are wrongly considered congestion events.

Later versions of TCP Westwood [74] also use the bandwidth estimate to find an appropriate ssthresh during the initial slow start phase. Bhandarkar et al. The results in [76] also demonstrated that TCP Westwood underutilizes the bandwidth in the presence of reordering caused by link layer retransmissions. Our work on TCP for wireless conditions falls into the category of end-to-end solu- tions.

IP makes it possible for data bundled into packets to find its way through the network. Data may be lost, reordered, or duplicated along the way. It provides the application with data in the right order, lost data is retransmitted, and duplicate data removed. TCP also attempts to adapt to the varying network conditions by increasing its sending rate during loss free periods and decreasing it when detecting a loss. Reordering and delay spikes can trick TCP. If TCP believes that a segment was lost it decreases its sending rate, which may lead to underutilization of the available bandwidth if the segment was merely delayed or reordered.

TCP assumes some degree of stability and a low transmis- sion error rate, but a wireless channel has varying capacity. Therefore, the combination of wireless networking and TCP was problematic to begin with. WWANs have traditionally been used for voice calls, but are evolving towards a packet-switched architecture to better support data traffic and facilitate the inclusion of new services.

We have described the development and spread of existing WWAN tech- nologies and the expectations on future releases. We focus on WWAN technology in this thesis. We use an end-to-end approach, because it does not require changes to intermediate nodes making deployment easier. Security issues with intermediate nodes inspecting packet headers can also be avoided.

Chapter 8 Summary and Conclusions The Internet is continuously attracting new users and thereby businesses. To take part of the success, many network technologies that were not IP-based from the beginning have evolved to support IP-based communication. As new network types join the Internet, the heterogeneity of bandwidths, delays, error rates, and devices increases. Therefore, the protocols should be flexible and robust against varying characteristics of heterogeneity. Resources are also more constrained.

Through this separation TCP-Aix provides robustness to reordering events of one round trip time rtt and delay variations. To handle reordering events beyond one rtt, TCP-Aix uses a higher duplicate acknowledgment threshold dupthresh setting than the standard setting. We introduced the winthresh algorithm for computing dupthresh. It minimizes the amount of spurious retransmits that a sender inserts into the network by waiting as long as possible before retransmitting a segment while at the same time avoiding window stalling.

The winthresh algorithm is tuned through a parameter that relates dupthresh to the current send window swnd. Through simulations, we found that it is important to control the delay of the conges- tion response, to enable TCP-Aix to utilize the bandwidth in dynamic scenarios where the available bandwidth varies substantially and quickly.

A parameter setting correspond- ing to two swnds in the winthresh algorithm offers a good trade-off between detecting reordering and preserving the ability to rapidly adapt to such varying conditions. With this setting, TCP-Aix can detect reordering durations of roughly three end-to-end rtts. We showed that TCP-Aix is able to maintain almost constant performance even in scenarios which frequently display long reordering durations.

Performance gains are also seen in scenarios displaying only moderate reordering durations of less than one rtt. With reordering robust TCP flavors ready for deployment, the informal constraint on wireless link layers to enforce in-order delivery for TCP can be relaxed.

Thereby, the complexity of network components can be decreased. The results from our case study of a dedicated WWAN link show that a link layer that is allowed to do out-of-order delivery together with a reordering robust TCP flavor has the potential to improve smoothness considerably for short time scales compared to a standards-compliant TCP with a link layer that delivers data in-order.

Smoothness plays an important role when it comes to predictability of the network traffic and the possibilities for mixing different types of traffic. Real-time traffic usually can not cope with too many link layer retransmissions, because the data has a limited lifetime.

Out- of-order delivery could make it possible to use the same link layer configurations for both real-time and background traffic. The end-points TCP or the application would then have to deal with out-of-order delivery, but less delay variations. We also demonstrated that out-of-order delivery at the link layer coupled with a reordering robust TCP flavor, decreases the network layer buffer requirement.

As memory components become cheaper, buffer space limitations become less important. The wireless medium is unguided and shared, which makes efficiency more important than in wired networks. We have studied how to improve TCP efficiency by reducing the acknowledgment frequency. Delayed acknowledgments were introduced to conserve network and host resources.

Further reduction of the acknowledgment frequency can be motivated in the same way. However, reducing the dependency on frequent acknowledgments in TCP is difficult be- cause acknowledgments are at the same time used for reliable delivery, loss recovery, clocking out new segments, and determining an appropriate sending rate. Our approach differs from previous work in that we study scenarios where there are no obvious advantages of reducing the TCP acknowledgment frequency.

Thereby, we investigated whether a lower acknowledgment frequency could be widely used. We proposed and evaluated an end-to-end solution, where four acknowledgments per swnd were sent and the sender compensated for the reduced acknowledgment frequency using a form of Appropriate Byte Counting. Although, we reduced the acknowledgment frequency in a symmetric wireline sce- nario, performance could be maintained.

Hence, there is a potential for reducing the acknowledgment frequency more than is done through delayed acknowledgments today. Advancements in TCP loss recovery is one of the key reasons that the dependence on frequent acknowledgments has decreased. Reducing the acknowledgment frequency increases TCP burstiness.

Unfortunately, few measurement studies of the effects of burstiness exists. We tested the effect of reducing the acknowledgment frequency and thus increasing the burstiness on network layer buffering in low multiplexing scenarios. Conclusions VoIP is an important service because it is needed for a full conversion from a circuit- switched to a packet-switched architecture in WWANs.

Shared channels, primarily de- signed for data traffic, have gained interest for VoIP. To understand which characteristics a scheduling algorithm should have for a mix of conversational traffic VoIP and interactive traffic web , we used the ns-2 simulator extended with a model of HS-DSCH to simulate a mixed traffic scenario. We studied four scheduling algorithms: the proportional fair PF , the maximum rate MR scheduler, and two extended versions of MR for different VoIP scheduling delay budgets and varying load.

Both cell throughput and user satisfaction were estimated. A more drastic increase in VoIP priority is however needed when the delay budget is short. Furthermore, at- tempting to preserve quality for both VoIP and web traffic makes the system sensitive to overload situations. By strengthening TCP, we want to make it easier to deploy and run applications over wireless networks.

We thus proposed and evaluated a number of TCP refinements. With this in mind, reordering durations of two to three times the base end-to-end rtt can be handled. It requires sender-receiver cooperation and changes to the byte counting [30] and loss recovery [1]. For the design of reordering robust TCP flavors we found that considering a dynamic environment is important. There is always a trade-off present — when improving one characteristic of a protocol, another characteristic can be impaired.

Therefore, it is important to identify all conflicting characteristics. Relaxing the informal in-order delivery constraint can have wide reaching conse- quences. Not only the frequency with which reordering occurs is important; knowing the duration of individual reordering events is vital to estimate the effect on performance and to assess the potential of out-of-order delivery.

When reducing the acknowledgment frequency, we risk reducing throughput and in- creasing burstiness. We have found that there is not enough information regarding how other applications perceive TCP burstiness to guide us in designing support for reduc- ing the acknowledgment frequency. For systems that make an effort to provide some form of quality to the users, e. As voice traffic is also be- coming common on the traditional Internet, the interest for how various services affect each other should be growing.

With an increasing traffic load on the Internet, burstiness should be more noticeable to other applications. It is no longer sufficient to only consider co-existence and fairness towards other TCP flavors; we need to consider the effect on other applications as well. We have come across several areas where more measurements are needed to control that we are working towards a better Internet architecture.

At the moment, TCP research is focused on adapting TCP for links with high capacity, which has lead to a discussion of the entire congestion control architecture. Thus, there may be an end to the era of continuous, minor TCP modifications and instead there may be a major revision. Until then our work shows that it is possible to improve TCP robustness and reduce TCP overhead; making TCP better suited for a modern network environment, where wireless links are likely to be common.

TCP is a complex protocol designed to control many mechanisms. Even small modifications may therefore have large consequences. TCP-Aix, as described in Chapter 4, is a set of modifications. We have verified that TCP-Aix works as intended and studied its impact on the network through simulations. Thereafter, we would like to implement TCP-Aix in an operating system, estimate the complexity of the implementation, and gather more experience by observing it over the Internet and diverse networking technologies.

Continuation As discussed in Chapter 5, reordering on the reverse path can be dealt with by limiting the number of segments sent in response to each acknowledgment, letting all acknowledgments clock out new segments also late acknowledgments , and using byte- counting. Reordering on the reverse path cause the same type of problems as reducing the acknowledgment frequency.

We therefore expect these changes to be helpful when only a few consecutive acknowledgments are delayed. When many acknowledgments are delayed after each other, TCP may still send large bursts. Reducing the acknowledgment frequency, as in Chapter 6, is a small change with large consequences. The sensitivity to lost acknowledgments and delay variations on the reverse path must be studied.

We also need to quantify the effects on smoothness and study burst mitigation before moving on. Another aspect is the increased acknowledg- ment frequency during times of suspected segment loss. The work in [39] provides some ideas to remedy this problem that we would like to study. The acknowledgment strategy should be able to provide a gain in resource constrained situations.

Therefore, we need to complement our evaluation with a set of scenarios exhibiting asymmetry and wireless links. At the same time, we can compare the results of our acknowledgment strategy and other acknowledgment reduction methods designed for specific environments. It is likely that a widely deployable solution provides less improvement and we would like to quantify this cost.

It is also interesting to compare congestion control of acknowledgments and a con- stantly low acknowledgment frequency. Congestion control for the acknowledgments implies that the acknowledgment overhead is only reduced if the capacity is constrained, but the error sensitivity of TCP is increased.

On the other hand, if a constantly low acknowledgment frequency is used, it is possible to perform optimizations. The dependence on the acknowledgment clock makes TCP unfair to flows with longer round trip times. We need to investigate whether this weakness is aggravated when reducing the acknowledgment frequency.

We also want to study different transfer sizes. TCP can still be refined after decades of intense research, but for shared channels in wireless cellular networks and VoIP we are at the beginning of the evolution. For instance, it is not obvious that the wireless link layer should perform in-order delivery for TCP, and a lower acknowledgment frequency can have effects on scheduling and planning of wireless channels. It would also be interesting to study the effects of TCP smoothness on for instance VoIP and network management algorithms in this and other environments.

In general, mixing services over wireless access is an interesting area. We therefore must find out more about the application requirements, which suggests more measurement studies. We want to prevent unnecessary additions to the base protocols and leave room for important algorithms.

A related problem is to understand user behavior and expectations for different net- working environments, both to find appropriate models for evaluation and to offer suitable technology. In this thesis, we have focused on transport layer issues, except for in the HS- DPA study where we evaluated the performance from both a system and an application perspective.

Looking beyond our immediate research area, efficient and robust communication is desirable from a global energy perspective. When sending e-mails and surfing the web, we generate lots of TCP flows. This makes it interesting to also include energy efficiency not only from a battery point of view in our future work. There is work on energy consumption of different TCP flavors in multi-hop wireless networks [], which can serve as a starting point.

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