Reliable Entity Link (or simply 'REL'), is a system for coordinating reliable bytestreams between two Jabber entities for the purpose of keeping applications (and application specifications) simple. However, this proposal does not define any specific bytestream protocol. It is expected that there will be multiple ways to obtain a bytestream between Jabber entities (thru-server and peer-to-peer are two methods that come to mind), but applications can refer to REL instead of some particular stream transport.

A REL-compatible stream transport must have the following properties:

• Provides a reliable bytestream between two Jabber entities, which means that the bytestream transport handles all data delivery issues, such that the application need not worry about them.
• Has a the following link states:

  Code	Description
  INIT	Initiation
  GOOD	Successful initiation (connected)
  BAD	Unsuccessful initiation (stream is closed, no further state)
  CLOS	Successful closure after establishment (stream is closed, no further state)
  ERR	Link failure after establishment (stream is closed, no further state)

• Defines a stream identifier, which MUST have a unique ASCII representation. The stream protocol MUST be able to use any ASCII identifier chosen during REL negotiation, as long as the sending party doesn't use the same identifier more than once.

The following stream transports that meet these guidelines are:

Before using REL, ensure it is a supported service of the remote entity by using &xep0030;:

To use REL, the entities must obtain a REL Context ID (or cid) through some action. A cid is simply an opaque alphanumeric string. For example, perhaps the link is needed for a file transfer:

All high-level protocols that use Reliable Entity Link MUST have a way of providing such a cid. The cid must be unique among all other REL cids between the two entities.

The next step is to ask the remote entity which stream method it would like to use. We will use &xep0020; for this. The streams are listed using the short names from the table of supported streams.

The keepAlive attribute indicates that the initiator is planning on trying another method if the one selected here is to fail. An entity SHOULD use keepAlive for all attempts but the last for a given application. If keepAlive is omitted, then it is considered false.

The remote entity will then agree on a method:

Or maybe an error:

If the entity returns error, then the REL cid is invalidated and the application fails. If a stream method has been chosen successfully, then now it must be initiated using the REL cid as the stream's identifier (the stream goes into INIT state).

On GOOD: This indicates the stream is ready for use within the original context, and data exchanged over the stream is to be left up to the application.

On BAD: If the keepAlive="true" attribute was specified, then the initiator MUST repeat this section over again to attempt with a different method. If keepAlive was not specified, then the REL cid is invalidated and the application fails.

On CLOS or ERR, the REL cid is invalidated.