Evaluation of a Stateful Transport Protocol for Multi ...

Evaluation of a Stateful Transport Protocol for Multi-channel Wireless Mesh Networks Ashish Raniwala, Srikant Sharma, Pradipta De, Rupa Krishnan, Tzi-cker Chiueh Computer Science Department, Stony Brook University, Stony Brook, NY 11794-4400 Email: {raniwala, srikant, prade, krishnan, chiueh}@cs.sunysb.edu Abstract An effective transport protocol for a wireless mesh network (WMN) must fairly and efficiently allocate the limited network resources among multiple flows sharing the network while minimizing the performance overhead it incurs. While many transport protocols have been proposed specifically for multi-hop wireless networks, most of them refrain from keeping state in the intermediate network nodes. In this paper, we evaluate the building blocks of various stateless reliable transport protocols on a multi-channel wireless mesh network, and subsequently focus on the other extreme of the design space: Stateful Transport Protocol. We study the research question of how much performance improvement is possible if intermediate network nodes could maintain as much state as needed. We discuss the design of a stateful transport protocol, named Link-Aware Reliable Transport Protocol (LRTP), and examine how LRTP can fairly and efficiently allocate the network resources by accurately estimating the sending rate of each flow traversing the network using information about effective physical link capacity and the number of sharing flows. We investigate how LRTP can reduce the performance overhead associated with reliable packet delivery by leveraging the link-layer retransmission mechanism to eliminate per-packet end-to-end acknowledgments and unnecessary packet transmissions. Experiments conducted on an IEEE 802.11a-based multi-channel wireless mesh network testbed as well as ns-2 simulations demonstrate that LRTP can achieve significant improvements in both overall network throughput and inter-flow fairness, especially on wireless networks with channel errors, when compared with the de facto Internet transport protocol TCP, and state-of-the-art ad hoc transport protocols such as ATP. Technical Areas: Performance Evaluation, Congestion Control. Keywords: Wireless Mesh Networks, Transport Protocol, Stateful Network Core, Multi-channel Networks, Experimentation on Real Testbed.

I. I NTRODUCTION Despite advances in physical-layer transmission technologies, limited link capacity continues to be the main problem for wireless networks. Increasingly, wireless mesh networks (WMNs) use multiple channels to eliminate

adjacent links’ interference and improve end-to-end path capacity [1]. The next research question is how to enable applications to make the most of this raw network-layer capacity, a responsibility traditionally fulfilled by transport protocol. An effective transport protocol must fairly and efficiently allocate the network bandwidth among multiple competing flows, while minimizing its own overhead. Most transport protocols proposed in the literature or in use today were originally designed to work in the wired Internet. Many of them strive to scale up to gigabits/sec links and are therefore designed to be stateless in the sense that intermediate routers never need to keep transport-layer state, e.g. unacknowledged flow packets or flow’s sending rate information. While such scalability is definitely desirable for wired Internet, it is less relevant for wireless networks, because the link speed of wireless networks is much lower and the number of legacy devices are few to justify backward compatibility consideration. In this paper, we argue that it is acceptable to maintain transport-layer state in intermediate wireless routers as long as it can maximize the utilization efficiency of the limited wireless link capacity. More specifically, we explore the other extreme of the transport protocol design space: stateful transport protocol in the context of multi-channel wireless mesh networks, where individual nodes do not move and route changes occur very rarely. Although the mesh nodes do not move, end-user mobility does lead to network-layer handoffs, which is tackled through transport layer addition/deletion of flow on new/old path. Every reliable transport protocol must include the following two mechanisms: (i) a reliable end-to-end packet delivery mechanism, and (ii) a congestion control mechanism. The reliable packet delivery mechanism infers the delivery status of each data packet, and retransmits packets that are determined to be lost. The congestion control mechanism estimates the available bandwidth between the source and the destination, and allocates a fair share to each sender node. The goal of congestion control is to ensure high network utilization while avoiding congestion and maintaining inter-flow fairness. Most previous research in WMN transport protocols has mainly focused on tailoring TCP to multi-hop wireless networks while maintaining its stateless design. Through our analysis in this paper, we conclude that TCP with its stateless core design may never be able to fairly and efficiently utilize the wireless network resources. Therefore, we design a stateful wireless Link-Aware Reliable Transport Protocol (LRTP), which features a hopby-hop retransmission mechanism and an explicit rate-based congestion control. Specifically, LRTP leverages linklayer ACKs to determine each packet’s transmission status at each hop, and eliminate per-packet transport-layer ACKs. Further, packet loss at any hop immediately triggers local retransmissions, preventing unnecessary end-toend retransmissions. LRTP uses a rate-based congestion control mechanism in which each intermediate wireless router measures the effective bandwidth of each of its wireless links, fairly allocates it among those flows traversing the wireless link, and then informs the corresponding senders to adjust their sending rates.

TCP Conn

End−user Mobile

LRTP Conn

Ingress Gateway

Mesh Router

Mesh Router

TCP Conn

Wired Egress Gateway

Internet Host

Multi−channel Wireless Mesh Network

Fig. 1.

Transparent splitting of end-user TCP connections into three sub-connections. Note that TCP ACKs never traverse the WMN.

While the WMN nodes in the proposed architecture run the LRTP protocol, the end-user mobile nodes as well as the nodes on the wired network still run the original TCP. To achieve this, each ingress/egress WMN node employs a TCP-LRTP proxy that transparently converts an end-to-end TCP connection into three sub-connections: a TCP sub-connection running from end-user mobile to the ingress WMN node, an LRTP sub-connection from ingress WMN node to egress WMN node, and another TCP sub-connection from egress WMN node up to the final end-point of the original connection. This architecture is shown in Figure 1. We make the following contributions in this paper – 1) We study the performance and fairness characteristics of the de facto Internet transport protocol, TCP, and a state-of-the-art MANET transport protocol, ATP, in the context of multi-channel multi-hop wireless mesh networks, and show that they are deficient in one aspect or another. 2) We devise and evaluate an explicit rate-based congestion control mechanism that measures the effective wireless link bandwidth and allocates bandwidth shares among flows sharing a wireless link by taking into account inherent bandwidth demands of end nodes. The evaluations are done on a 9-node multi-channel multi-hop wireless mesh testbed. 3) We implement and evaluate a hop-by-hop reliable packet delivery mechanism that utilizes link-layer ACK information for packet loss detection and local retransmission, and show how it can largely eliminate transportprotocol ACKs that currently consume substantial bandwidth of a multi-hop wireless path. The rest of the paper is organized as follows. Section II studies congestion control and reliability mechanisms used in the existing transport protocols. Section III details the stateful congestion control of LRTP. Section IV focuses on LRTP’s hop-by-hop reliable packet delivery mechanism and the corresponding implementation details. Section V presents results from a performance study conducted on a 9-node IEEE 802.11a-based multi-channel WMN testbed as well as ns-2 simulations of larger-topology multi-channel WMNs. Section VI concludes the paper.

II. E XISTING T RANSPORT L AYER M ECHANISMS In this section, we evaluate various transport protocols on an IEEE 802.11a-based multi-channel WMN testbed. Use of multiple channels eliminates the degradation due to self-interference effects and also alleviates the hidden terminal problem [26]. We first measure the transport layer acknowledgment overhead introduced by TCP and evaluate techniques proposed to reduce this overhead. We then discuss the techniques used to reduce the end-toend retransmission overhead associated with TCP. The subsequent subsection studies some of the key bandwidth estimation techniques in context of multi-channel WMNs. The final subsection evaluates the fairness aspect of TCP and ATP. A. Reducing Acknowledgment Overhead One of the characteristics of IEEE 802.11 wireless networks is high per-packet overhead. Each packet incurs large MAC contention, PLCP header, and link-layer ACK overhead. Any transport protocol that uses per-packet end-to-end acknowledgment therefore incurs substantial control overhead. Original TCP design incurred per-packet end-to-end acknowledgment; the delayed-ACK scheme reduces this to one ACK for every two segments. To maintain TCP ACK clocking, delayed-ACK cannot indefinitely wait for a second segment to arrive. Delayed-ACK, therefore, helps at higher transmission rates, as segments arrive with time gap less than delayed-ACK timeout. Further reduction in ACK traffic is achieved using ACK-filtering technique [19]. Here intermediate nodes suppress ACKs for previous packet in a connection, if the ACK for a later packet is pending. Even when delayed-ACK is enabled, we found that TCP’s ACK traffic imposes substantial overhead on connection throughput. Our experiments on the 802.11a mesh network substantiate this fact. Table I shows how ACK overhead increases for each hop as higher transmission rates are used. The optimal throughput was measured by sending a UDP packet stream at different rates. Beyond 18 Mbps, the Delayed-ACK helps in limiting the ACK overhead. In the Appendix, we derive the following relation for the percentage ACK overhead in an 802.11-a mesh network.

1/2 ∗ (162 + 720/r) ∗ 100/(162 + 12400/r)