2. Related Works
2.3. P2P-MANET application
For the orientation of service, the P2P-MANET application is similar to wired P2P application.
The application can be divided into three kinds: file sharing, voice communication, and video streaming.
2.3.1. File sharing
The kind of file sharing is the first application of wired P2P service. The file searching is completed quickly via the P2P overlay, which is constructed by finger table. The mobile P2P application inherits the advantage to develop the file sharing function.
M-CAN (Mobile-CAN) [14] is a modification based on CAN, which provides the P2P functions of registering, grouping, file transmission, caching, as well as the DHT functions.
Every node is registered on one or several nearby super nodes, which are organized by CAN.
The goal of M-CAN is to provide a fast lookup for file sharing on ad hoc network. The super node must index all neighborhood available files and handle all lookups, so the overhead of super node is obviously high.
MADPastry (Mobile AD hoc Pastry) [15] combines the DHT overlay, Pastry, and the ad hoc routing protocol, AODV, to reduce the overhead of peer lookup and routing maintenance.
MADPastry uses Pastry proximity awareness to reduce the overhead without flooding.
MADPastry is implemented at the network layer. MADPastry maintains three routing tables:
Pastry routing table, Pastry leaf set, and AODV routing table; those tables complete the indirect routing. Pastry routing table indexes and hashes the mobile nodes; Pastry leaf set knows the neighbors on overlay; AODV routing table is extracted from standard AODV.
These routing tables can be updated via overhearing data packets, and the routing path without data flow is not monitored. The extension of MADPastry integrates three routing tables to provide interfaces to P2P application layer and MAC layer as Figure 2.5 illustrated.
Its motivation is to avoid AODV route discovery, minimize overlay maintenance, and maximize available packet information. Because Pastry is suitable for the wired network, MADPastry performs poorly at high speed.
M-Chord (Mobile-Chord) [16] is a modification based on Chord. M-Chord is very similar to M-CAN and is inspired from the hierarchical architecture of M-CAN. M-Chord also uses the super node to associate with the ordinary nodes and manage the file indexes.
Every super node needs to maintain the routing finger table and sharing files directory to achieve Chord's lookup and file sharing. The joining/leaving/routing/updating process of peer follows Chord and is similar with M-CAN. M-Chord combines the real and virtual split strategies to achieve a load balance and low overlay overhead.
2.3.2. Voice communication
The application connection of voice communication or group conference is established via Session Initiation Protocol (SIP) based on flooding-based multicast. A peer always floods the received data to other peers immediately. The hierarchy-based or cluster-based architecture is used to improve the efficiency of flooding delivery.
Pastry
Pastry leaf set Pastry routing table
AODV
AODV routing table
Network layer
Application layer
MAC layer interface
interface
Figure 2.5: The integrated network stack of MADPastry.
Skype [17] can hold the voice communication or group conference through Internet.11 It classifies peers into two types: super peer12 and ordinary peer. The super peer is responsible for building P2P overlay and optimizing the routing path. Hence, a super peer is like a leader, which synchronizes voice data then sends it to ordinary peers through multicast.
Audio Conferencing Testbed (ACT) [18] is based on OLSR to set up a one-to-one or many-to-many communication in WiFi MANET. ACT uses the minimum spanning tree13 to minimize the latency of audio dissemination to the whole network. Every peer must maintain its minimum spanning tree to deliver data by itself. ACT predicts the disconnection and mobility to shorten the service interruption time.
CLAPS (Cross-Layer And P2P based Solution) [19] inherits the tree-based overlay and OLSR extension from MOST14 for real-time video streaming. CLAPS assumes that the physical routing topology can be provided by OLSR, which sends cross-layer message to optimize the overlay. The source peer maintains a minimum spanning tree as its overlay. The minimum cost is computed via link distance packaged in cross-layer message, and the spanning tree is recomputed periodically to keep the overlay proximity. To optimize the overlay proximity and avoid the overlap path in ad hoc routing, the candidate peers are selected as relay nodes on multicast paths according to the equal link distance. The cross-layer architecture of CLAPS is shown in Figure 2.6. In the simulation, CLAPS adopts UDP as transport layer protocol and WiFi as MAC layer protocol.
11 Skype cannot work if MANET cannot access Internet.
12 Any peer with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a super peer.
13 Broadcast tree is the buzzword in ACT.
14 MOST (Multicast Overlay Spanning Tree Protocol) extends the OLSR unicast to support multicast routing. It is a multicast routing protocol, not an overlay protocol. Although MOST uses the minimum spanning tree as its overlay, it does not consider P2P issues. However, the overlay of CLAPS is inspired from the spanning tree of MOST due to the multicast purpose.
2.3.3. Video streaming
Both live streaming and video on demand (VoD) are the applications of video distribution.
However, supporting live streaming is more difficult than supporting VoD due to the instant factor.15 The audiences cannot tolerate any sensible lags in such live programs. Therefore, how to continue the stream smoothly and deliver it efficiently among the peers is important to use P2P solution.
Smart Gnutella [20] enhances the original Gnutella for MANET and the real-time application.16 Smart Gnutella inherits the ultrapeer from the wired Gnutella. The ultrapeer must manage the neighborhood leaf peers and support the multicast communication in Smart Gnutella. In order to meet the wireless and mobile environment, Smart Gnutella defines three message types (ping/pong, welcome, and broadcast) and four node states (full, stable, connecting, and idle). The application-layer QoS can be monitored via ping/pong; the peer discovery and connection maintenance can be established via welcome; the node states can be updated via broadcast.
As Figure 2.7 illustrated, MP2PS (Mesh-based P2P Streaming) [21] provides real-time
15 The live streaming service usually provides the live sports games, the first-hand stock information, or the latest news.
16Gnutella is designed for file sharing in the wired network. Smart Gnutella is designed for QoS-sensitive collaborative virtual environment on MANET.
MOST UDP Live video application
OLSR WiFi
Link distance
Figure 2.6: The cross-layer architecture of CLAPS.
streaming with scalability and availability over MANET.17 MP2PS adopts mesh-based live streaming application, Joost,18 and no retransmission on wireless network, UDP, and on-demand ad hoc routing protocol, AODV.