Group PJMEDIA_TRANSPORT

group PJMEDIA_TRANSPORT

Transports.

The media transport (pjmedia_transport) is the object to send and receive media packets over the network. The media transport interface allows the library to be extended to support different types of transports to send and receive packets.

The media transport is declared as pjmedia_transport “class”, which declares “interfaces” to use the class in pjmedia_transport_op structure. For the user of the media transport (normally the user of media transport is media stream, see Streams), these transport “methods” are wrapped with API such as pjmedia_transport_attach(), so it should not need to call the function pointer inside pjmedia_transport_op directly.

The connection between Streams and media transport is shown in the diagram below:

pjproject/pjmedia/docs/xml/media-transport.PNG

Basic Media Transport Usage

The media transport’s life-cycle normally follows the following stages.

Creating the Media Transport

Application creates the media transport when it needs to establish media session to remote peer. The media transport is created using specific function to create that particular transport; for example, for UDP media transport, it is created with pjmedia_transport_udp_create() or pjmedia_transport_udp_create2() functions. Different media transports will provide different API to create those transports.

Alternatively, application may create pool of media transports when it is first started up. Using this approach probably is better, since application has to specify the RTP port when sending the initial session establishment request (e.g. SIP INVITE request), thus if application only creates the media transport later when media is to be established (normally when 200/OK is received, or when 18x is received for early media), there is a possibility that the particular RTP port might have been occupied by other programs. Also it is more efficient since sockets don’t need to be closed and re-opened between calls.

Attaching and Using the Media Transport.

Application specifies the media transport instance when creating the media session (pjmedia_session_create()). Alternatively, it may create the media stream directly with pjmedia_stream_create() and specify the transport instance in the argument. (Note: media session is a high-level abstraction for media communications between two endpoints, and it may contain more than one media streams, for example, an audio stream and a video stream).

When stream is created, it will “attach” itself to the media transport by calling pjmedia_transport_attach(), which is a thin wrapper which calls “attach()” method of the media transport’s “virtual function pointer” (pjmedia_transport_op). Among other things, the stream specifies two callback functions to the transport: one callback function will be called by transport when it receives RTP packet, and another callback for incoming RTCP packet. The pjmedia_transport_attach() function also establish the destination of the outgoing RTP and RTCP packets.

When the stream needs to send outgoing RTP/RTCP packets, it will call pjmedia_transport_send_rtp() and pjmedia_transport_send_rtcp()

of the media transport API, which is a thin wrapper to call send_rtp() and send_rtcp() methods in the media transport’s “virtual function

pointer” (

pjmedia_transport_op).

When the stream is destroyed, it will “detach” itself from the media transport by calling pjmedia_transport_detach(), which is a thin wrapper which calls “detach()” method of the media transport’s “virtual function pointer” (pjmedia_transport_op). After the transport is detached from its user (the stream), it will no longer report incoming RTP/RTCP packets to the stream, and it will refuse to send outgoing packets since the destination has been cleared.

Reusing the Media Transport.

After transport has been detached, application may re-attach the transport to another stream if it wants to. Detaching and re-attaching media transport may be preferable than closing and re-opening the transport, since it is more efficient (sockets don’t need to be closed and re-opened). However it is up to the application to choose which method is most suitable for its uses.

Destroying the Media Transport.

Finally if application no longer needs the media transport, it will call pjmedia_transport_close()

function, which is thin wrapper which calls “destroy()” method of the media transport’s “virtual function

pointer” (

pjmedia_transport_op). This function releases all resources used by the transport, such as sockets and memory.

Interaction with SDP Offer/Answer

For basic UDP transport, the Basic Media Transport Usage above is sufficient to use the media transport. However, more complex media transports such as Secure RTP (SRTP) Media Transport and ICE Media Transport requires closer interactions with SDP offer and answer negotiation.

The media transports can interact with the SDP offer/answer via these APIs:

The usage of these API in the context of SDP offer answer will be described below.

Initializing Transport for New Session

Application must call pjmedia_transport_media_create() before using the transport for a new session.

Creating SDP Offer and Answer

The pjmedia_transport_encode_sdp() is used to put additional information from the transport to the local SDP, before the SDP is sent and negotiated with remote SDP.

When creating an offer, call pjmedia_transport_encode_sdp() with local SDP (and NULL as rem_sdp). The media transport will add the relevant attributes in the local SDP. Application then gives the local SDP to the invite session to be sent to remote agent.

When creating an answer, also call pjmedia_transport_encode_sdp(), but this time specify both local and remote SDP to the function. The media transport will once again modify the local SDP and add relevant attributes to the local SDP, if the appropriate attributes related to the transport functionality are present in remote offer. The remote SDP does not contain the relevant attributes, then the specific transport functionality will not be activated for the session.

The pjmedia_transport_encode_sdp() should also be called when application sends subsequent SDP offer or answer. The media transport will encode the appropriate attributes based on the state of the session.

Offer/Answer Completion

Once both local and remote SDP have been negotiated by the SDP Negotiation State Machine (Offer/Answer Model, RFC 3264) (normally this is part of PJSIP invite session), application should give both local and remote SDP to pjmedia_transport_media_start() so that the settings are activated for the session. This function should be called for both initial and subsequent SDP negotiation.

Stopping Transport

Once session is stop application must call pjmedia_transport_media_stop() to deactivate the transport feature. Application may reuse the transport for subsequent media session by repeating the pjmedia_transport_media_create(), pjmedia_transport_encode_sdp(), pjmedia_transport_media_start(), and pjmedia_transport_media_stop() above.

Implementing Media Transport

To implement a new type of media transport, one needs to “subclass” the media transport “class” (pjmedia_transport) by providing the “methods” in the media transport “interface” (pjmedia_transport_op), and provides a function to create this new type of transport (similar to pjmedia_transport_udp_create() function).

The media transport is expected to run indepently, that is there should be no polling like function to poll the transport for incoming RTP/RTCP packets. This normally can be done by registering the media sockets to the media endpoint’s IOQueue, which allows the transport to be notified when incoming packet has arrived.

Alternatively, media transport may utilize thread(s) internally to wait for incoming packets. The thread then will call the appropriate RTP or RTCP callback provided by its user (stream) whenever packet is received. If the transport’s user is a stream, then the callbacks provided by the stream will be thread-safe, so the transport may call these callbacks without having to serialize the access with some mutex protection. But the media transport may still have to protect its internal data with mutex protection, since it may be called by application’s thread (for example, to send RTP/RTCP packets).

Typedefs

typedefPJ_BEGIN_DECL struct pjmedia_transport pjmedia_transport

Forward declaration for media transport.

Enums

enum pjmedia_tranport_media_option

This enumeration specifies the general behaviour of media processing

Values:

enumerator PJMEDIA_TPMED_NO_TRANSPORT_CHECKING

When this flag is specified, the transport will not perform media transport validation, this is useful when transport is stacked with other transport, for example when transport UDP is stacked under transport SRTP, media transport validation only need to be done by transport SRTP.

enum pjmedia_transport_type

Media transport type.

Values:

enumerator PJMEDIA_TRANSPORT_TYPE_UDP

Media transport using standard UDP

enumerator PJMEDIA_TRANSPORT_TYPE_ICE

Media transport using ICE

enumerator PJMEDIA_TRANSPORT_TYPE_SRTP

Media transport SRTP, this transport is actually security adapter to be stacked with other transport to enable encryption on the underlying transport.

enumerator PJMEDIA_TRANSPORT_TYPE_USER

Start of user defined transport.

Functions

void pjmedia_transport_info_init(pjmedia_transport_info *info)

Initialize transport info.

Parameters

info – Transport info to be initialized.

pj_status_t pjmedia_transport_get_info(pjmedia_transport *tp, pjmedia_transport_info *info)

Get media transport info from the specified transport and all underlying transports if any. The transport also contains information about socket info which describes the local address of the transport, and would be needed for example to fill in the “c=” and “m=” line of local SDP.

Parameters

  • tp – The transport.

  • info – Media transport info to be initialized.

Returns

PJ_SUCCESS on success.

void *pjmedia_transport_info_get_spc_info(pjmedia_transport_info *info, pjmedia_transport_type type)

Utility API to get transport type specific info from the specified media transport info.

Parameters

  • info – Media transport info.

  • type – Media transport type.

Returns

Pointer to media transport specific info, or NULL if specific info for the transport type is not found.

pj_status_t pjmedia_transport_attach(pjmedia_transport *tp, void *user_data, const pj_sockaddr_t *rem_addr, const pj_sockaddr_t *rem_rtcp, unsigned addr_len, void (*rtp_cb)(void *user_data, void *pkt, pj_ssize_t), void (*rtcp_cb)(void *usr_data, void *pkt, pj_ssize_t))

Attach callbacks to be called on receipt of incoming RTP/RTCP packets. This is just a simple wrapper which calls attach() member of the transport.

Parameters

  • tp – The media transport.

  • user_data – Arbitrary user data to be set when the callbacks are called.

  • rem_addr – Remote RTP address to send RTP packet to.

  • rem_rtcp – Optional remote RTCP address. If the argument is NULL or if the address is zero, the RTCP address will be calculated from the RTP address (which is RTP port plus one).

  • addr_len – Length of the remote address.

  • rtp_cb – Callback to be called when RTP packet is received on the transport.

  • rtcp_cb – Callback to be called when RTCP packet is received on the transport.

Returns

PJ_SUCCESS on success, or the appropriate error code.

void pjmedia_transport_detach(pjmedia_transport *tp, void *user_data)

Detach callbacks from the transport. This is just a simple wrapper which calls detach() member of the transport. After the transport is detached, it will ignore incoming RTP/RTCP packets, and will refuse to send outgoing RTP/RTCP packets. Application may re-attach the media transport to another transport user (e.g. stream) after the transport has been detached.

Parameters

  • tp – The media transport.

  • user_data – User data which must match the previously set value on attachment.

pj_status_t pjmedia_transport_send_rtp(pjmedia_transport *tp, const void *pkt, pj_size_t size)

Send RTP packet with the specified media transport. This is just a simple wrapper which calls send_rtp() member of the transport. The RTP packet will be delivered to the destination address specified in pjmedia_transport_attach() function.

Parameters

  • tp – The media transport.

  • pkt – The packet to send.

  • size – Size of the packet.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_send_rtcp(pjmedia_transport *tp, const void *pkt, pj_size_t size)

Send RTCP packet with the specified media transport. This is just a simple wrapper which calls send_rtcp() member of the transport. The RTCP packet will be delivered to the destination address specified in pjmedia_transport_attach() function.

Parameters

  • tp – The media transport.

  • pkt – The packet to send.

  • size – Size of the packet.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_send_rtcp2(pjmedia_transport *tp, const pj_sockaddr_t *addr, unsigned addr_len, const void *pkt, pj_size_t size)

Send RTCP packet with the specified media transport. This is just a simple wrapper which calls send_rtcp2() member of the transport. The RTCP packet will be delivered to the destination address specified in param addr, if addr is NULL, RTCP packet will be delivered to destination address specified in pjmedia_transport_attach() function.

Parameters

  • tp – The media transport.

  • addr – The destination address.

  • addr_len – Length of destination address.

  • pkt – The packet to send.

  • size – Size of the packet.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_media_create(pjmedia_transport *tp, pj_pool_t *sdp_pool, unsigned options, const pjmedia_sdp_session *rem_sdp, unsigned media_index)

Prepare the media transport for a new media session, Application must call this function before starting a new media session using this transport.

This is just a simple wrapper which calls media_create() member of the transport.

Parameters

  • tp – The media transport.

  • sdp_pool – Pool object to allocate memory related to SDP messaging components.

  • options – Option flags, from pjmedia_tranport_media_option

  • rem_sdp – Remote SDP if local SDP is an answer, otherwise specify NULL if SDP is an offer.

  • media_index – Media index in SDP.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_encode_sdp(pjmedia_transport *tp, pj_pool_t *sdp_pool, pjmedia_sdp_session *sdp, const pjmedia_sdp_session *rem_sdp, unsigned media_index)

Put transport specific information into the SDP. This function can be called to put transport specific information in the initial or subsequent SDP offer or answer.

This is just a simple wrapper which calls encode_sdp() member of the transport.

Parameters

  • tp – The media transport.

  • sdp_pool – Pool object to allocate memory related to SDP messaging components.

  • sdp – The local SDP to be filled in information from the media transport.

  • rem_sdp – Remote SDP if local SDP is an answer, otherwise specify NULL if SDP is an offer.

  • media_index – Media index in SDP.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_media_start(pjmedia_transport *tp, pj_pool_t *tmp_pool, const pjmedia_sdp_session *sdp_local, const pjmedia_sdp_session *sdp_remote, unsigned media_index)

Start the transport session with the settings in both local and remote SDP. The actual work that is done by this function depends on the underlying transport type. For SRTP, this will activate the encryption and decryption based on the keys found the SDPs. For ICE, this will start ICE negotiation according to the information found in the SDPs.

This is just a simple wrapper which calls media_start() member of the transport.

Parameters

  • tp – The media transport.

  • tmp_pool – The memory pool for allocating temporary objects.

  • sdp_local – Local SDP.

  • sdp_remote – Remote SDP.

  • media_index – Media index in the SDP.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_media_stop(pjmedia_transport *tp)

This API should be called when the session is stopped, to allow the media transport to release its resources used for the session.

This is just a simple wrapper which calls media_stop() member of the transport.

Parameters

tp – The media transport.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_close(pjmedia_transport *tp)

Close media transport. This is just a simple wrapper which calls destroy() member of the transport. This function will free all resources created by this transport (such as sockets, memory, etc.).

Parameters

tp – The media transport.

Returns

PJ_SUCCESS on success, or the appropriate error code.

pj_status_t pjmedia_transport_simulate_lost(pjmedia_transport *tp, pjmedia_dir dir, unsigned pct_lost)

Simulate packet lost in the specified direction (for testing purposes). When enabled, the transport will randomly drop packets to the specified direction.

Parameters

  • tp – The media transport.

  • dir – Media direction to which packets will be randomly dropped.

  • pct_lost – Percent lost (0-100). Set to zero to disable packet lost simulation.

Returns

PJ_SUCCESS on success.

struct pjmedia_transport_op
#include <transport.h>

This structure describes the operations for the stream transport.

See

pjmedia_transport_op.

struct pjmedia_transport
#include <transport.h>

This structure declares media transport. A media transport is called by the stream to transmit a packet, and will notify stream when incoming packet is arrived.

struct pjmedia_transport_specific_info
#include <transport.h>

This structure describes storage buffer of transport specific info. The actual transport specific info contents will be defined by transport implementation. Note that some transport implementations do not need to provide specific info, since the general socket info is enough.

struct pjmedia_transport_info
#include <transport.h>

This structure describes transport informations, including general socket information and specific information of single transport or stacked transports (e.g: SRTP stacked on top of UDP)

Forward declaration for media transport info.