Datagrams build upon the base I2CP to provide authenticated and repliable messages in a standard format. This lets applications reliably read the "from" address out of a datagram and know that the address really sent the message. This is necessary for some applications since the base I2P message is completely raw - it has no "from" address (unlike IP packets). In addition, the message and sender are authenticated by signing the payload.
Datagrams, like streaming library packets, are an application-level construct. These protocols are independent of the low-level transports; the protocols are converted to I2NP messages by the router, and either protocol may be carried by either transport.
Applications written in Java may use the datagram API, while applications in other languages can use SAM's datagram support. There is also limited support in i2ptunnel in the SOCKS proxy, the 'streamr' tunnel types, and udpTunnel classes.
The application designer should carefully consider the tradeoff of repliable vs. non-repliable datagrams. Also, the datagram size will affect reliability, due to tunnel fragmentation into 1KB tunnel messages. The more message fragments, the more likely that one of them will be dropped by an intermediate hop. Messages larger than a few KB are not recommended. Over about 10 KB, the delivery probablility drops dramatically. Messages over 16 KB cannot be delivered over NTCP, dropping delivery chances even more.
Also note that the various overheads added by lower layers, in particular asymmetric ElGamal/AES, place a large burden on intermittent messages such as used by a Kademlia-over-UDP application. The implementations are currently tuned for frequent traffic using the streaming library. There are a high number of session tags delivered, and a short session tag lifetime, for example. There are currently no configuration parameters available within I2CP to tune the ElGamal Session Tag parameters.
I2CP Protocol Number and Ports
The standard I2CP protocol number for datagrams is PROTO_DATAGRAM (17). Applications may or may not choose to set the protocol in the I2CP header. It is not set by default. It must be set to demultiplex datagram and streaming traffic received on the same Destination.
As datagrams are not connection-oriented, the application may require port numbers to correlate datagrams with particular peers or communications sessions, as is traditional with UDP over IP. Applications may add 'from' and 'to' ports to the I2CP (gzip) header as described in the I2CP page.
There is no method within the datagram API to specify whether it is non-repliable (raw) or repliable. The application should be designed to expect the appropriate type. The I2CP protocol number or port should be used by the application to indicate datagram type. The I2CP protocol numbers PROTO_DATAGRAM (signed) and PROTO_DATAGRAM_RAW are defined in the I2PSession API for this purpose. A common design pattern in client/server datagram applications is to use signed datagrams for a request which includes a nonce, and use a raw datagram for the reply, returning the nonce from the request.
Data integrity is assured by the gzip CRC-32 checksum implemented in the I2CP layer. There is no checksum field in the datagram protocol.
Each datagram is sent through I2P as a single message (or as an individual clove in a Garlic Message). Message encapsulation is implemented in the underlying I2CP, I2NP, and tunnel message layers. There is no packet delimiter mechanism or length field in the datagram protocol.