1 Nice: Design documentation
2 ==========================
7 For UDP candidates, one socket is created for each component and bound
8 to INADDR_ANY. The same local socket is used for the host candidate,
9 STUN candidate as well as the TURN candidate. The socket handles are
10 stored to the Component structure.
12 The library will use the source address of incoming packets in order
13 to identify from which remote candidates, if any (peer-derived
14 candidates), packets were sent.
16 XXX: Describe the subtle issues with ICMP error handling when one
17 socket is used to send to multiple destinations.
19 Real-time considerations
20 ------------------------
22 One potential use for libnice code is providing network connectivity
23 for media transport in voice and video telephony applications. This
24 means that the libnice code is potentially run in real-time context
25 (for instance under POSIX SCHED_FIFO/SHCED_RR scheduling policy) and
26 ideally has deterministic execution time.
28 To be real-time friendly, operations with non-deterministic execution
29 time (dynamic memory allocation, file and other resource access) should
30 be done at startup/initialization phase. During an active session
31 (connectivity has been established and non-STUN traffic is being sent),
32 code should be as deterministic as possible.
37 To work on platforms where available memory may be constrained, libnice
38 should gracefully handle out of memory situations. If memory allocation
39 fails, the library should return an error via the originating public
42 Use of glib creates some challenges to meet the above:
44 - A lot of glib's internal code assumes memory allocations will
45 always work. Use of these glib facilities should be limited.
46 While the glib default policy (see g_malloc() documentation) of terminating
47 the process is ok for applications, this is not acceptable for library
49 - Glib has weak support for preallocating structures needed at
50 runtime (for instance use of timers creates a lot of memory
53 To work around the above limitations, the following guidelines need
56 - Always check return values of glib functions.
57 - Use safe variants: g_malloc_try(), etc
58 - Current issues (last update 2007-05-04)
59 - g_slist_append() will crash if alloc fails
64 Management of timers is handled by the 'agent' module. Other modules
65 may use timer APIs to get timestamps, but they do not run timers.
67 Glib's timer interface has some problems that have affected the design:
69 - an expired timer will destroy the source (a potentially costly
71 - it is not possible to cancel, or adjust the timer expiration
72 timer without destroying the associated source and creating
73 a new one, which again causes malloc/frees and is potentially
75 - on Linux, glib uses gettimeofday() which is subject to clock
76 skew, and no monotonic timer API is available
78 Due to the above, 'agent' code runs fixed interval periodic timers
79 (started with g_timeout_add()) during candidate gathering, connectivity
80 check, and session keepalive phases. Timer frequency is set separately
81 for each phase of processing. A more elegant design would use dynamic
82 timeouts, but this would be too expensive with glib timer
85 Control flow for NICE agent API (NiceAgentClass)
86 ------------------------------------------------
88 The main library interface for applications using libnice is the
89 NiceAgent GObject interface defined in 'nice/agent.h'.
91 The rough order of control follow is as follows:
93 - creation of NiceAgent object instance
94 - setting agent properties such as STUN and TURN server addresses
95 - connecting the GObject signals with g_signal_connect() to application
97 - adding local interface addresses to use with
98 nice_agent_add_local_address()
100 And continues when making an initial offer:
102 - creating the streams with nice_agent_add_stream()
103 - attach the mainloop context to connect the NiceAgent sockets to
104 the application's event loop (using nice_agent_attach_recv())
105 - start candidate gathering by calling nice_agent_gather_candidates()
106 - the application should wait for the "candidate-gathering-done" signal
107 before going forward (so that ICE can gather the needed set of local
108 connectiviy candidates)
109 - get the information needed for sending offer using
110 nice_agent_get_local_candidates() and
111 nice_agent_get_local_credentials()
112 - client should now send the session offer
113 - once it receives an answer, it can pass the information to NiceAgent
114 using nice_agent_set_remote_candidates() and
115 nice_agent_set_remote_credentials()
117 Alternatively, when answering to an initial offer:
119 - the first five steps are the same as above (making initial offer)
120 - pass the remote session information to NiceAgent using
121 nice_agent_set_remote_candidates() and
122 nice_agent_set_remote_credentials()
123 - client can send the answer to session offer
125 Special considerations for a SIP client:
127 - Upon sending the initial offer/answer, client should pick one
128 local candidate as the default one, and encode it to the SDP
129 "m" and "c" lines, in addition to the ICE "a=candidate" lines.
130 - Client should connect to "new-selected-pair" signals. If this
131 signal is received, a new candidate pair has been set as
132 a selected pair (highest priority nominated pair). See
133 ICE specification for a definition of "nominated pairs".
134 - Once all components of a stream have reached the
135 "NICE_COMPONENT_STATE_READY" state (as reported by
136 "component-state-changed" signals), the client should check
137 whether its original default candidate matches the latest
138 selected pair. If not, it needs to send an updated offer
139 it is in controlling mode. Before sending the offer, client
140 should check the "controlling-mode" property to check that
141 it still is in controlling mode (might change during ICE
142 processing due to ICE role conflicts).
143 - The "remote-attributes" SDP attribute can be created from
144 the information provided by "component-state-changed" (which
145 components are ready), "new-selected-pair" (which candidates
146 are selected) and "new-remote-candidate" (peer-reflexive
147 candidates discovered during processing) signals.
148 - Supporting forked calls is not yet supported by the API (multiple
149 sets of remote candidates for one local set of candidates).
153 - ICE processing can be restarted by calling nice_agent_restart()
154 - Restart will clean the set of remote candidates, so client must
155 afterwards call nice_agent_set_remote_candidates() after receiving
156 a new offer/answer for the restarted ICE session.
157 - Restart will reinitialize the local credentials (see
158 nice_agent_get_local_credentials()).
159 - Note that to modify the set of local candidates, a new stream
160 has to be created. For the remote party, this looks like a ICE
163 Handling fallback to non-ICE operation:
165 - If we are the offering party, and the remote party indicates
166 it doesn't support ICE, we can use nice_agent_set_selected_pair()
167 to force selection of a candidate pair (for remote party,
168 the information on SDP 'm=' and 'c=' lines needs to be used
169 to generate one remote candidate for each component of the
170 streams). This function will halt all ICE processing (excluding
171 keepalives), while still allowing to send and receive media (assuming
172 NATs won't interfere).
174 Notes about sending media:
176 - Client may send media once all components of a stream have reached
177 state of NICE_COMPONENT_STATE_CONNECTED or NICE_COMPONENT_STATE_READY,
178 (as reported by "component-state-changed" signals), and a selected pair
179 is set for all components (as reported by "new-selected-pair" signals).
184 The underlying STUN library takes care of formatting and parsing STUN
185 messages (lower layer),
187 Applications should only need to use the higher layer API which then
188 uses the lower layer API.
190 The following STUN usages are currently implemented by the
192 - Binding discovery (RFC5389 with RFC3489 backward compatibility)
194 - ICE connectivity checks
196 - STUN retransmission timers
202 STUN message API provide thin wrappers to parse and format STUN
203 messages. To achieve maximum cross-architectures portability and retain
204 real-time friendliness, these functions are fully "computational" [1].
205 They also make no assumption about endianess or memory alignment
206 (reading single bytes or using memcpy()).
208 Message buffers are provided by the caller (so these can be
209 preallocated). Because STUN uses a relatively computer-friendly binary
210 format, STUN messages are stored in wire format within the buffers.
211 There is no intermediary translation, so the APIs can operate directly
212 with data received from or sent to the network.
214 [1] With one exception: The random number generated might access the
215 system entropy pool (/dev/urandom) if available.