The Symbian OS (779886), страница 52
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It supports the Core ETelAPI, Multimode ETel API and ETel Packet API requests.Telephony Reference Platform CollectionThese components support a standard reference platform telephonyimplementation. See Figure 9.20.230THE COMMS SERVICES BLOCKTelephony ReferencePlatformTRPTSYTRPCSYFigure 9.20 Telephony Reference Platform componentsTable 9.11 Telephony Reference Platform ComponentsComponent NameDevelopment NameTRP TSYTRPTRP CSYTRPBaseband Channel Adaptor for C32C32BCA• The TRP TSY is a reference TSY designed to run on developmentboard hardware, as part of a wider effort to make easier licenseedevelopment on phones using Symbian OS.• The TRP CSY is used to manage the internal channel between thetelephony hardware (a dedicated phone-side core running the TIphone stack) and the application hardware (an ARM core runningSymbian OS).
The logical driver on the TI H2 board presents theinternal serial bus as a standard serial port.• The Baseband Channel Adaptor for C32 is a reference plug-in providing a serial communications implementation of the BasebandChannel Adapter interface (see Section 9.6). For example, the Telephony Reference Platform provides a serial communications BCAplug-in implementing the BCA interface.9.8 Networking ServicesWeb browsing and email were the functions that motivated the inclusionof networking services in the first releases of Symbian OS, although thepotential for more exotic applications such as network news readers andmultiplayer games was.It is worth remembering in this context that the devices at whichthe first ER5 release was targeted were not phones, although they wereconnected and they were telephony-enabled in the sense that they weredesigned to interoperate with phones.
That interoperation, however, wasunderstood in terms of phone-as-modem, dialing up an ISP to accessan email account, a corporate intranet or the Internet, or even the web.Neither email nor the Internet were ubiquitous in the way that they bothnow are and the web was still very much a novelty.NETWORKING SERVICESTCP/IP SecurityESock APIExtensionsTCP/IPUtilities231WAP StackSubconnection InterfaceNetworkingServicesNetwork Protocol PluginsNetworking PluginsLink Layer ControlFigure 9.21 Networking Services componentsThe core of the networking implementation remains the TCP/IP v4/v6networking stack, implemented as a PRT Socket Server plug-in moduleand the network interface plug-ins that support it and which are, in turn,supported by link-layer plug-ins.While the ESock Socket Server and Network Interface Manager havemigrated out into the Comms Framework to provide generic socketsupport for all communications services (and not just for networking),networking services have expanded to encompass TCP/IP enhancementssuch as IPSec, telephony-driven networking enhancements includingpacket-data services (for GPRS and UMTS) and Quality of Service (QoS,required for 3G services), as well as completely new technologies suchas WAP and most recently (in Symbian OS v9) Wi-Fi.
See Figure 9.21.Networking StackSymbian OS Networking Services are based on a TCP/IP protocol implementation, TCP/IPv4/v6 PRT (in effect the transport and network layers ofthe OSI seven-layer model), together with IP extensions that implementvarious packet-level services including QoS and IPSec. See Figure 9.22.However, TCP/IP packets and the stack itself are not directly available.TCP/IP packets are encapsulated within the stack and there are no visibleTCP/IP packet classes, for example. The stack is implemented as a Socket232THE COMMS SERVICES BLOCK7Application6Presentation5Session4Transport3NetworkIPv4 / IPv62Datalink1PhysicalDevice driverandHardwareprotocols7 layer OSI modelStevens' 4 layer modelApplicationprotocolsTCP'High level' protocolsUDP'Low level' protocolsFigure 9.22The OSI Seven-Layer model and simplified layer modelServer PRT protocol plug-in, and network services are made availablethrough the sockets interface, by requesting a TCP/IP socket.The stack does however support a hook mechanism provided by IPHook to enable packets to be accessed within the stack on the inboundand outbound paths, for example to allow pre- and post-processing andother packet transformations; that, for example, is the mechanism used toimplement IPSec packet-level encryption and decryption.A socket is a session-based abstraction that sits logically above thenetworking protocol implementation, which provides the transport andnetwork layers implementation.The bottom interface of the stack relies on the Network InterfaceManager to select a suitable outgoing interface, which in turn relies onthe Network Controller to find a network agent to negotiate the chosenconnection (for Network Interface Manager and the Network Controllersee Section 9.6).A number of network agents (AGT files) are available: CSD.AGT,to establish circuit-switched data connections; PSD.AGT, to establishpacket-switched data connections; and NULL.AGT, which implements aminimal agent that is used with Ethernet.A number of network interface implementations (NIF files) are available, including for PPP and Ethernet, as well as a QoS Test NIF that isused in conjunction with the QoS Framework.At these levels, the architecture has evolved quite significantly sinceSymbian OS v7, which implemented a rather simplistic view of networksand connections.
Particularly with UMTS packet-switched 3G networks,the networking world becomes more complex. For example, multihoming means that devices can have multiple IP addresses (multipleNETWORKING SERVICES233network interfaces may be active, each with its own IP address andpotentially each on a different network) and packet-switched phone dataservices mean that multiple interfaces and networks may provide accessto a single network destination. However, from an application perspectivethese changes are mostly invisible and impact only systems developers.Note that Bluetooth and wireless LAN are not supported by default butrequire comparable drivers to abstract the hardware for the (overlying)Ethernet NIF implementation.
The NIF can support one lower-layer packetdriver, loaded during initialization.PPP NIF has been part of the networking delivery since the firstreleases of Symbian OS but was significantly enhanced in Symbian OS v8to improve interoperability with MS Windows, for example by supportingMicrosoft extensions to CHAPS dialup authentication.The Tunneling NIF was introduced with VLAN support and IPSecreimplementation in Symbian OS v8.Network SecurityNetwork Security protocols operate at different levels in the overall networking stack. TLS and SSL (the two can be considered synonyms) operateat the transport level, providing per-packet encryption and decryption.IPSec on the other hand operates at the network level and is principally designed to support secure networks, for example Virtual PrivateNetworks (VPN) based on policy.The TLS component implements TLS v1.0 (Transport Level Security)and SSL v3.0 (Secure Sockets Layer), providing more or less transparent,per-packet encryption-based security to client applications, for exampleHTTPS or SyncML.
TLS is implemented as a number of separate DLLsexposing client APIs to applications, which enable sockets to be securedand internal APIs used by networking and security components. TLS wasfirst introduced in Symbian OS v7 and was redesigned and enhanced inSymbian OS v8. A typical use of SSL is to enable secure browser-basedtransactions.The IPSec implementation provides security policy management,including support for multi-homed clients (so that different security policies can be associated with the different IP addresses in use by the device)and multiple active policies. IPSec is implemented as a policy serverand supporting libraries, as well as a protocol-level PRT Core IPSec PRTplug-in.
In effect, it sits between the Socket Server and clients requestingsecure sockets. The IPSec PRT does not implement the full interfacerequired by the Symbian OS v9 Socket Server architecture based on theComms Framework and therefore is considered to be a ‘pseudo-PRT’.IPSec uses the networking stack Hook interface to inspect all incomingand outgoing packets and apply the required cryptographic transformations. The actual security algorithms and libraries are implemented bycryptography and security services components.234THE COMMS SERVICES BLOCKThe VPN component uses IPSec to manage VPN policies and connections, including VPN password management and is implemented as aVPN manager server and supporting libraries.IPSec was first introduced into Symbian OS in v7 and was redesignedand enhanced in Symbian OS v8.Quality of ServiceQuality of Service enables performance characteristics to be specified fora communications channel in a packet-based network, to ensure that therequired data rates for a given application are met.The QoS Framework PRT implements QoS policy setting for opensockets (Socket Server sub-sessions) that are treated as QoS channels,providing generic and UMTS-specific APIs for use by applications.
WhileGPRS supports general QoS principles, UMTS defines four traffic classes(Conversational, Streaming, Interactive and Background). Like IPSec, QoSis considered to be a ‘pseudo-PRT’.Networking DaemonsA number of standard networking daemons are implemented as part ofnetworking support.• DND (i.e. DNS) and DHCP are implementations of the Internetstandard protocols for domain-name resolution and dynamic hostaddress assignment.• DND makes DNS queries to the (remote) network and listens for andresponds to local queries. Like its Unix counterpart, it is implementedas a server ‘daemon’. It is accessed through the RHostResolverclass by Socket Server clients (i.e. as a Socket Server sub-session) andsupports GetByName and GetByAddress queries.• DHCP enables a device to obtain an IP address and network parameters dynamically from the network, so that having a fixed IP addressbecomes unnecessary.
The DHCP implementation consists of a serverdaemon and a client-side interface and provides a limited API sufficient for the Network Interface Manager to configure an appropriatenetwork interface. (It is not intended for other users.)WAP SupportWireless Application Protocol (WAP) evolved out of work started by theUnwired Planet consortium, which evolved into the WAP Forum in 1998at around the time that Symbian joined it and into the Open MobileAlliance (OMA) in 2002. WAP was a deliberate attempt to create a WebNETWORKING SERVICES235standard targeted at mobile devices in general and phones in particular,to make web content browsable on those devices.WAP defines a protocol stack, much like TCP/IP, with transport anddatagram layers defined over a variety of possible mobile phone networkbearers.
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