I. Introduction
The two parties in the WebRTC audio and video call need to exchange candidate information and then perform connectivity detection. Therefore, each party in the communication needs to collect its own host address, mapping address, etc. By collecting network card information, we can know the address information of the network card configuration.
2. Collect network card information
1. When to collect network card information
The time for WebRTC to collect network card information is to call PeerConnection::SetLocalDescription, which will call JsepTransportController::MaybeStartGathering to collect candidates.
JsepTransportController::MaybeStartGathering will call P2PTransportChannel::MaybeStartGathering, which then calls PortAllocator::CreateSession to create BasicPortAllocatorSession.
BasicNetworkManager::StartUpdating is called in the BasicPortAllocatorSession constructor, which sends KUpdateNetworksMessage message to the network thread.
After receiving KUpdateNetworksMessage, call BasicNetworkManager::UpdateNetworksContinually, then call BasicNetworkManager::UpdateNetworksOnce, and finally call CreateNetworks to start collecting network card information.
2. CreateNetworks
Windows uses GetAdaptersAddresses to obtain the address information associated with the network card. For a detailed introduction to this API, please see here . The following only introduces the key parameter information used in WebRTC.
IPHLPAPI_DLL_LINKAGE ULONG GetAdaptersAddresses(
[in] ULONG Family,
[in] ULONG Flags,
[in] PVOID Reserved,
[in, out] PIP_ADAPTER_ADDRESSES AdapterAddresses,
[in, out] PULONG SizePointer
);The values and meanings of Family are as follows.
| value | meaning |
| AF_UNSPEC | Return IPv4 and IPv6 type address information |
| OF_INET | Return only IPv4 address information |
| AF_INET6 | Return only IPv6 address information |
Some values and corresponding meanings of Flags are as follows (only the Flags used in WebRTC are introduced).
| value | meaning |
| GAA_FLAG_SKIP_DNS_SERVER |
Do not return DNS address |
| GAA_FLAG_SKIP_ANYCAST |
Do not return IPv6 arbitrary addresses |
| GAA_FLAG_SKIP_MULTICAST |
does not return a multicast address |
| GAA_FLAG_INCLUDE_PREFIX |
Returns a list of IP address prefixes on the adapter |
Reserved : Reserved parameters, just set to 0.
AdapterAddress : address output parameter, it is the pointer of IP_ADAPTER_ADDRESSES data, all IP_ADAPTER_ADDRESSES information can be traversed through the Next pointer in this data structure, for the system whose Windows version is greater than or equal to VISTA, IP_ADAPTER_ADDRESSES is IP_ADAPTER_ADDRESSES_LH, otherwise IP_ADAPTER_ADDRESSES is IP_ADAPTER_ADD RESSES_XP, below IP_ADAPTER_ADDRESSES_LH will be used as an example to illustrate the meaning of important information in the data structure.
SizePointer : Output parameter, indicating the size of the buffer pointed to by AdapterAddress.
The code for WebRTC to call GetAdaptersAddresses to obtain network card information is as follows.
Each address information is represented by a Network structure, and all Networks are finally stored in NetworkMap, and the logic of traversing to obtain IP is as follows.
3. Obtain the default address information
After obtaining the NetworkList information, it will obtain the local default IPv4 and IPv6 addresses. The acquisition method is to create a local socket, and then connect to the Google DNS service. After the connection between the socket and the remote address is completed, the local socket can be obtained through getsockname. the address of.
