What does "reliable" really mean?

[stephen-derosa]

With respect to rtcReliability:

typedef struct {
	bool unordered;
	bool unreliable;
	unsigned int maxPacketLifeTime;
	unsigned int maxRetransmits;
} rtcReliability;

what does "reliable" really mean under the hood? I understand it means that messages "will get to the receiver", but whats actually happening? Are messages 100% guaranteed to be sent/received?

[paullouisageneau]

Reliable means the data channel will keep retransmitting missing parts of every message forever until it is received, so every message is guaranteed to be received at some point provided the peer connection is not lost.

[stephen-derosa]

reading more through the code i see in the reference.md:

...
    - `unreliable`: if `true`, the Data Channel will not enforce strict reliability, else it will be reliable
    - `maxPacketLifeTime`: if unreliable, time window in milliseconds during which transmissions and retransmissions may occur
    - `maxRetransmits`: if unreliable and maxPacketLifeTime is 0, maximum number of attempted retransmissions (0 means no retransmission)

And i see in reliability.hpp:

	// If both maxPacketLifeTime or maxRetransmits are unset, the channel is reliable.
	// If either maxPacketLifeTime or maxRetransmits is set, the channel is unreliable.
	// (The settings are exclusive so both maxPacketLifetime and maxRetransmits must not be set.)

given that only maxPacketLifeTime or maxRetransmits is used (not both), what does the respective other default to if the other is set? I.e. if:

• maxPacketLifeTime is set to 150 ms, what is maxRetransmits set to? is there a default re-transmit rate that will be met until maxPacketLifeTime is met?
• maxRetransmits is set to 0 retrys, what is maxPacketLifeTime set to? is there a default lifetime for a packet that will be waited for before it is determined that sending has failed?

Moreover, is it possible to have a functionality that resembles both being sent? I.e. a maxPacketLifeTime of 150 ms and 0 retrys? Or am i misunderstanding these two fields deeply?

Thanks you!

[paullouisageneau]

Both parameters are exclusive because they are implemented as different policies by SCTP. There are no default values, when you set one, the other is simply not set and therefore not a constrain (you can consider it's like the value being infinite). For instance if maxPacketLifeTime is set to 150 ms, it means the message can be sent and retransmitted as a reliable one until the 150ms duration has passed, then it is dropped.

If you would like to set both, you probably want to actually set maxPacketLifeTime because it is more restrictive. maxRetransmits only caps the number of retransmissions, so 0 ensures the packet is transmitted at least once. maxPacketLifeTime may drop the packet even if it couldn't be sent under the specified duration.

[stephen-derosa]

thank you very much for the explanation!