P2P

Index Terms:

1. Wi-Fi Direct
2. Group Formation
3. Power Saving
4. Miracast

INTRODUCTION

• Wi-Fi Direct is a new technology defined by the Wi-Fi Alliance aimed at enhancing direct device to device communications in Wi-Fi ithout requiring the presenceof an Access Point (AP).
• Wi-Fi Direct technology takes a different approach to enhance device to device connectivity. Instead of leveraging the ad-hoc mode of operation, Wi-Fi Direct builds upon the successful IEEE 802.11infrastructuremode and lets devices negotiate who will take over the AP-like functionalities

• In a typical Wi-Fi network, clients discover and associate to WLANs, which are created and announced by Access Points (APs). In this way, a device unambiguously behaves either as an AP or as a client, each of these roles involving a different set of functionality. A major novelty of Wi-Fi Direct is that these roles are specified as dynamic , and hence a Wi-Fi Direct device has to implement both the role of a client and the role of an AP

A. Architecture:

• Wi-Fi Direct devices, formally known as P2P Devices, communicate by establishing P2P Groups , which are functionally equivalent to traditional Wi-Fi infrastructure networks
• The device implementing AP-like functionality in the P2P Group is referred to as the P2P Group Owner (P2P GO), and devices acting as clients are known as P2P Clients.
• once the P2P Group is established, other P2P Clients can join the group as in a traditional Wi-Fi network

Fig. 1. Example of Wi-Fi Direct supported topologies and use c

The logical nature of the P2P roles supports different architectural deployments, two of them illustrated in Figure 1. The upper part of the figure represents a scenario with two P2P groups. The first group is created by a mobile phone sharing its 3G connection with two laptops; for this first group, the phone is acting as P2P GO while the two laptops behave as P2P Clients. In order to extend the network, one of the laptops establishes a second P2P Group with a printer; for this second group, the laptop acts as P2P GO. In order to act both as P2P Client and as P2P GO the laptop will typically alternate between the two roles by time-sharing the Wi-Fi interface; in Section II-E we will introduce the NoA protocol that can be used for this purpose. The lower part of Figure 1 illustrates the case of a laptop accessing the Internet through a legacy infrastructure AP while at the same time streaming content to a TV set by establishing a P2P Group, where the laptop acts as P2PGO.

• P2P GO announces itself through beacons, and has to support power saving services for its associated clients. The P2P GO is also required to run a Dynamic Host Configuration Protocol (DHCP) server to provide P2P Clients with IP addresses.
• Wi-Fi Direct does not allow transferring the role of P2P GO within a P2P Group.

B. Group Formation:

• Standard case, Autonomous and Persistent cases
• These three group formation cases are illustrated in Figure 2.

1). Standard:

• In this case the P2P devices have first to discover each other, and then negotiate which device will act as P2P GO.
• Wi-Fi Direct devices usually start by performing a traditional Wi-Fi scan (active or passive), by means of which they can discover existent P2P Groups and Wi-Fi networks. After this scan, a new Discovery algorithm is executed, which we describe next. First, a P2P Device selects one of the so-called Social channels, namely channels 1, 6 or 11 in the 2.4 Ghz band, as its Listen channel
• Then, it alternates between two states: a search state, in which the device performs active scanning by sending Probe Requests in each of the social channels; and a listen state, in which the device listens for Probe Requests in  its listen channel to respond with Probe Responses.
• The amount of time that a P2P Device spends on each state is randomly distributed, typically between 100 ms and 300 ms, but it is up to the each implementation to decide on the actual mechanism to e.g. trade-off discovery time with energy savings by interleaving sleeping cycles in the discovery process. An example operation of this discovery algorithm is illustrated in the first part of Figure 2
• Once the two P2P Devices have found each other, they  start the GO Negotiation phase
• using a three-way handshake, namely GO Negotiation Request/Response/Confirmation.

Fig. 2. Procedure of frame exchange for WiFi Direct.

• whereby the two devices agree on which device will act as P2P GO and on the channel where the group will operate, which can be in the 2.4 Ghz or 5 Ghz bands.
• In order to agree on the device that will act as P2P GO, P2P devices send a numerical parameter, the GO Intent value, within the three-way hand-shake, and the device declaring the highest value becomes the P2P GO.
• To prevent conflicts when two devices declare the same GO Intent, a tiebreaker bit is included in the GO Negotiation Request, which is randomly set every time a GO Negotiation Request is sent.

• next phase is the establishment of a secure communication using Wi-Fi protected Setup, which we denote as WPS Provisioning phase.
• and finally a DHCP exchange to set up the IP configuration (the Address config. phase in the figure).

2) Autonomous:

• A P2P Device may autonomously create a P2P Group, where it immediately becomes the P2P GO, by sitting on a channel and starting to beacon Other devices can discover the established group using traditional scanning mechanisms, and then directly proceed with the WPS Provisioning and Address Configuration phases.
• Compared to the previous case, then, the Discovery phase is simplified in this case as the device establishing the group does not alternate between states, and indeed no GO Negotiation phase is required. An exemplary frame exchange for this case is illustrated in the middle part of Figure 2

3) Persistent:

• During the formation process, P2P devices can declare a group as persistent, by using a flag in the P2P Capabilities attribute present in Beacon frames, Probe Responses and GO negotiation frame
• In this way, the devices forming the group store network credentials and the assigned P2P GO and Client roles for subsequent re-instantiations of the P2P group.
• after the Discovery phase, if a P2P Device recognizes to have formed a persistent group with the corresponding peer in the past, any of the two P2P devices can use the Invitation Procedure (a two-way handshake) to quickly re- instantiate the group. This is illustrated in the lower part of Figure 2, where the Standard case is assumed as baseline, and the GO Negotiation phase is replaced by the invitation exchange and the WPS Provisioning phase is significantly reduced because the stored network credentials can be reused.

C. Security:

• Security provisioning starts after discovery has taken place and, if required, the respective roles have been negotiated. Wi-Fi Direct devices are required to implement Wi-Fi Protected Setup (WPS) to support a secure connection with minimal user intervention.
• P2P GO is required to implement an internal Registrar, and the P2P Client is required to implement an Enrollee WPS is based on WPA-2 security and uses Advanced Encryption Standard (AES)-CCMP as cypher, and a randomly generated PreShared Key (PSK) for mutual authentication In the second part, depicted as “Phase 2” in Figure 2, the Enrollee (P2P Client) disassociates and reconnects using its new authentication credentials.
• In this way, if two devices already have the required network credentials (this is the case in the Persistent group formation), there is no need to trigger the first phase, and they can directly perform the authentication.

D. Power Saving:

two new power saving mechanisms: the Opportunistic Power Save protocol and the Notice of Absence (NoA) protocol.

1) Opportunistic Power Save:

• The basic idea of Opportunistic Power Save is to leverage the sleeping periods of P2P Clients

• The P2P GO advertises a time window, denoted as CTWindow, within each Beacon and Probe Response frames.

• This window specifies the minimum amount of time after the reception of a Beacon during which the P2P GO will stay awake and therefore P2P Clients in power saving can send their frames If after the CTWindow the P2P GO determines that all connected clients are in doze state, either because they announced a switch to that state by sending a frame with the Power Management (PM) bit set to 1, or because they were already in the doze state during the previous beacon interval,
• the P2P GO can enter sleep mode until the next Beacon is scheduled;  otherwise, if a P2P Client leaves the power saving mode mode (which is announced by sending a frame with the PM bit set to 0) the P2P GO is  forced to stay awake until all P2P Clients return to power saving mode. Figure 3 provides an example of the operation of the Opportunistic Power Save protocol for a scenario consisting of one P2P GO and one P2P Client.
• P2P GO does not have the final decision on whether to switch to sleep mode or not, as this depends on the activity of the associated P2P Clients. To give a P2P GO higher control on its own energy consumption Wi-Fi Direct specifies the Notice of Absence protocol

2) Notice of Absence:

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• The Notice of Absence (NoA) protocol allows a P2P GO to announce time intervals, referred to as absence periods , where P2P Clients are not allowed to access the channel, regardless of whether they are in power save or in active mod.
• In this way, a P2P GO can autonomously decide to power down its radio to save energy
• Like in the Opportunistic Power Save protocol, in the case of NoA the P2P GO defines absence periods with a signaling element included in Beacon frames and Probe Responses
• P2P GO defines a NoA schedule using four parameters:

(i) duration that specifies the length of each absence period,

(ii) interval that specifies the time between consecutive absence periods,

(iii) start time that specifies the start time of the first absence period after the current Beacon frame, and

(iv) count that specifies how many absence periods will be scheduled during the current NoA schedule

• If count is set to 255 the current NoA schedule runs permanently until it is explicitly cancelled.
• P2P Clients always adhere to the most recently received NoA schedule. Figure depicts an example operation of the NoA protocol.