Authors: Garrett D’Amore
This document defines a scalability protocol used for bidirectional stateless communications between a pair of nodes.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
This document describes a PAIR protocol, which is to be used between a pair of nodes for message oriented but otherwise free form bidirectional communication between two parties.
The use case for this protocol is one where a pair of potential peers wishes to communicate bidirectionally with little or no constraint on message contents, or replies. As such this protocol itself is unacknowledged. These are typical use cases:
One client sends and receives messages with one server. This is the historical model.
Many clients send and receive messages with one server. The server sends and receives messages with clients, but must identify which client to which it desires to send a message.
One client sends and receives messages with many servers. The client must determine to which server it desires to send any particular message.
Combining the above, many clients and many servers communicate, forming a kind of mesh, but every sender identifies the intended recipient explicitly.
Head of Line Blocking
Note that in the usage scenarios involving polyamrous relationships, since a single socket is being used to communicate to potentially many peers, we cannot use a blocking paradigm; that is we must not allow a slow peer to block communications with other peers. To prevent that, when in polyamorous mode, the socket must act as a "best-effort" delivery, dropping messages (or possibly returning them to the user with an error) when the remote peer is unable to handle them. (A per-peer outgoing configurable buffer MAY be used to allow some queuing to occur before messages are dropped.)
Best Effort Delivery
Note that even in monogamous partnerships, message delivery cannot be guaranteed as there is no acknowledgement. This is true even when the underlying transport(s) are themselves reliable, as intermediate routing steps may drop messages; an implementation may also fail to deliver messages to an application after the transport message has already been received.
Applications which need reliable delivery may build acknowledgements on top of this protocol.
At the transport layer, even in polyamorous mode, all of the relationships are peer wise and unicast; therefore this protocol can be run over any supported unicast transport.
This protocol is assigned the protocol number
17, which represents
major protocol number
1, subprotocol number
1. (The historical version
of the protocol used number
This protocol is assigned the name
Overview of the Algorithm
A PAIR endpoint can be connected only to another PAIR endpoint. If some other peer attempts to connect to a PAIR endpoint, then the PAIR endpoint MUST close the underlying transport channel.
When creating more complex topologies, PAIR endpoints are paired in the intermediate nodes to form a forwarding component; we call this component a device. Multiple devices can be chained together.
In this way, complex topologies can be built, and forwarding chains can be created to allow messages to pass through complex underlying transport boundaries in a manner that is transparent to the application.
Each peer need only know it’s own nearest partner(s).
It is also possible to create device implementations that support polyamorous relationships. In this case, the device must decide to which of several partners a message should be forwarded.
Given the above, it is apparent that in the general case a polyamorous device must retain enough information to ensure that once two remote peers are connected, the same peers are used for further communication. This is necessary as there may be implied state in the messages sent and received, such that a given peer may not expect or tolerate a change in with which remote peer it is communication.
It is possible for implementations to have more detailed knowledge of the application protocol(s) in use above this PAIR protocol. In such a case, a device implementation MAY forward a message however it chooses.
Monogamous mode peers only communicate with at most one other peer. Furthermore, they are strictly monogamous, in that they will reject attempts to form additional relationships.
Accordingly, when another peer attempts to establish communication with a monogamous peer, that peer MUST drop the connection or perform other transport-specific appropriate actions to reject the connection.
Because there is exactly one other party with him the peer is communicating, there is no need for applications to specify the destination of the message.
Implementations MUST support monogamous mode.
Polyamorous mode peers are willing to establish multiple relationships with peers, and communicate amongst them. However applications must be aware of this mode of operation. Implementation of polyamorous mode is OPTIONAL.
Polyamorous implementations MUST provide a means for polyamorous applications to choose to which particular remote peer a message shall be sent.
In either case, if a remote peer is specified, the message MUST be delivered to that peer if possible; it MUST NOT be delivered to any other peer. In such a case implementations SHOULD provide an indication of a failure to send to the application.
Polyamorous implementations MUST offer the ability for applications to determine from which remote peer a message was received, in the same format that would be used for sending a message to the same peer.
If no peer is specified when sending a message, then the implementation MAY choose any peer at it’s discretion. Implementations SHOULD default to sending to the same peer when none is specified. (As an exception, when an implementation knows that the upper protocol and applications are stateless, then if no peer is specified they MAY choose a remote peer at their own discretion.)
In order to prevent the creation of forwarding loops, a hop-count is added to the message header, allowing implementations to determine through how many intermediate device nodes a message has passed.
Implementations MUST have a configurable maximum hop-count, and MUST discard any message which has exceeded it’s hop count. Implementations SHOULD NOT disconnect channels underneath though, since a given device peer may be polyamorous and disconnecting would potentially impact peers that would otherwise be unaffected.
The default limit for hops SHALL be 8.
Per Message Header
Each message is prefixed with a 32-bit header. The header consists of the following:
This is a 32-bit word (big-endian). The upper 24 bits MUST be zero. The lower 8 bits contain SHALL current hop count.
Each time the message is sent, the current hop count (which starts at 0) SHALL be incremented.
Therefore every message transmitted shall have a hop count of at least 1. When this value exceeds the implementation defined hop limit (see Loop Avoidance), it is discarded.
Implementations MUST discard any message with a hop count of zero, since that may represent a wrap from 255, as well as any message where the reserved bits are not zero.