# TRANSPORT-NG — Next-generation transport management

The current GNUnet TRANSPORT architecture is rooted in the GNUnet 0.4 design of using plugins for the actual transmission operations and the ATS subsystem to select a plugin and allocate bandwidth. The following key issues have been identified with this design:

- Bugs in one plugin can affect the TRANSPORT service and other plugins. There is at least one open bug that affects sockets, where the origin is difficult to pinpoint due to the large code base.

- Relevant operating system default configurations often impose a limit of 1024 file descriptors per process. Thus, one plugin may impact other plugin’s connectivity choices.

- Plugins are required to offer bi-directional connectivity. However, firewalls (incl. NAT boxes) and physical environments sometimes only allow uni-directional connectivity, which then currently cannot be utilized at all.

- Distance vector routing was implemented in 209 but shortly afterwards broken and due to the complexity of implementing it as a plugin and dealing with the resource allocation consequences was never useful.

- Most existing plugins communicate completely using cleartext, exposing metad data (message size) and making it easy to fingerprint and possibly block GNUnet traffic.

- Various NAT traversal methods are not supported.

- The service logic is cluttered with "manipulation" support code for TESTBED to enable faking network characteristics like lossy connections or firewewalls.

- Bandwidth allocation is done in ATS, requiring the duplication of state and resulting in much delayed allocation decisions. As a result, often available bandwidth goes unused. Users are expected to manually configure bandwidth limits, instead of TRANSPORT using congestion control to adapt automatically.

- TRANSPORT is difficult to test and has bad test coverage.

- HELLOs include an absolute expiration time. Nodes with unsynchronized clocks cannot connect.

- Displaying the contents of a HELLO requires the respective plugin as the plugin-specific data is encoded in binary. This also complicates logging.

## Design goals of TNG

In order to address the above issues, we want to:

- Move plugins into separate processes which we shall call communicators. Communicators connect as clients to the transport service.

- TRANSPORT should be able to utilize any number of communcators to the same peer at the same time.

- TRANSPORT should be responsible for fragmentation, retransmission, flow- and congestion-control. Users should no longer have to configure bandwidth limits: TRANSPORT should detect what is available and use it.

- Commnunicators should be allowed to be uni-directional and unreliable. TRANSPORT shall create bi-directional channels from this whenever possible.

- DV should no longer be a plugin, but part of TRANSPORT.

- TRANSPORT should provide communicators help communicating, for example in the case of uni-directional communicators or the need for out-of-band signalling for NAT traversal. We call this functionality backchannels.

- Transport manipulation should be signalled to CORE on a per-message basis instead of an approximate bandwidth.

- CORE should signal performance requirements (reliability, latency, etc.) on a per-message basis to TRANSPORT. If possible, TRANSPORT should consider those options when scheduling messages for transmission.

- HELLOs should be in a humman-readable format with monotonic time expirations.

The new architecture is planned as follows:

TRANSPORT’s main objective is to establish bi-directional virtual links using a variety of possibly uni-directional communicators. Links undergo the following steps:

1. Communicator informs TRANSPORT A that a queue (direct neighbour) is available, or equivalently TRANSPORT A discovers a (DV) path to a target B.

2. TRANSPORT A sends a challenge to the target peer, trying to confirm that the peer can receive. FIXME: This is not implemented properly for DV. Here we should really take a validated DVH and send a challenge exactly down that path!

3. The other TRANSPORT, TRANSPORT B, receives the challenge, and sends back a response, possibly using a dierent path. If TRANSPORT B does not yet have a virtual link to A, it must try to establish a virtual link.

4. Upon receiving the response, TRANSPORT A creates the virtual link. If the response included a challenge, TRANSPORT A must respond to this challenge as well, eectively re-creating the TCP 3-way handshake (just with longer challenge values).

## HELLO-NG

HELLOs change in three ways. First of all, communicators encode the respective addresses in a human-readable URL-like string. This way, we do no longer require the communicator to print the contents of a HELLO. Second, HELLOs no longer contain an expiration time, only a creation time. The receiver must only compare the respective absolute values. So given a HELLO from the same sender with a larger creation time, then the old one is no longer valid. This also obsoletes the need for the gnunet-hello binary to set HELLO expiration times to never. Third, a peer no longer generates one big HELLO that always contains all of the addresses. Instead, each address is signed individually and shared only over the address scopes where it makes sense to share the address. In particular, care should be taken to not share MACs across the Internet and confine their use to the LAN. As each address is signed separately, having multiple addresses valid at the same time (given the new creation time expiration logic) requires that those addresses must have exactly the same creation time. Whenever that monotonic time is increased, all addresses must be re-signed and re-distributed.

## Priorities and preferences

In the new design, TRANSPORT adopts a feature (which was previously already available in CORE) of the MQ API to allow applications to specify priorities and preferences per message (or rather, per MQ envelope). The (updated) MQ API allows applications to specify one of four priority levels as well as desired preferences for transmission by setting options on an envelope. These preferences currently are:

- GNUNET_MQ_PREF_UNRELIABLE: Disables TRANSPORT waiting for ACKS on unreliable channels like UDP. Now it is fire and forget. These messages then cannot be used for RTT estimates either.

- GNUNET_MQ_PREF_LOW_LATENCY: Directs TRANSPORT to select the lowest-latency transmission choices possible.

- GNUNET_MQ_PREF_CORK_ALLOWED: Allows TRANSPORT to delay transmission to group the message with other messages into a larger batch to reduce the number of packets sent.

- GNUNET_MQ_PREF_GOODPUT: Directs TRANSPORT to select the highest goodput channel available.

- GNUNET_MQ_PREF_OUT_OF_ORDER: Allows TRANSPORT to reorder the messages as it sees fit, otherwise TRANSPORT should attempt to preserve transmission order.

Each MQ envelope is always able to store those options (and the priority), and in the future this uniform API will be used by TRANSPORT, CORE, CADET and possibly other subsystems that send messages (like LAKE). When CORE sets preferences and priorities, it is supposed to respect the preferences and priorities it is given from higher layers. Similarly, CADET also simply passes on the preferences and priorities of the layer above CADET. When a layer combines multiple smaller messages into one larger transmission, the `GNUNET_MQ_env_combine_options()` should be used to calculate options for the combined message. We note that the exact semantics of the options may differ by layer. For example, CADET will always strictly implement reliable and in-order delivery of messages, while the same options are only advisory for TRANSPORT and CORE: they should try (using ACKs on unreliable communicators, not changing the message order themselves), but if messages are lost anyway (e.g. because a TCP is dropped in the middle), or if messages are reordered (e.g. because they took different paths over the network and arrived in a different order) TRANSPORT and CORE do not have to correct this. Whether a preference is strict or loose is thus dened by the respective layer.

## Communicators

The API for communicators is defined in `gnunet_transport_communication_service.h`. Each communicator must specify its (global) communication characteristics, which for now only say whether the communication is reliable (e.g. TCP, HTTPS) or unreliable (e.g. UDP, WLAN). Each communicator must specify a unique address prex, or NULL if the communicator cannot establish outgoing connections (for example because it is only acting as a TCP server). A communicator must tell TRANSPORT which addresses it is reachable under. Addresses may be added or removed at any time. A communicator may have zero addresses (transmission only). Addresses do not have to match the address prefix.

TRANSPORT may ask a communicator to try to connect to another address. TRANSPORT will only ask for connections where the address matches the communicator’s address prefix that was provided when the connection was established. Communicators should then attempt to establish a connection. No response is provided to TRANSPORT service on failure. The TRANSPORT service has to ask the communicator explicitly to retry.

If a communicator succeeds in establishing an outgoing connection for transmission, or if a communicator receives an incoming bi-directional connection, the communicator must inform the TRANSPORT service that a message queue (MQ) for transmission is now available. For that MQ, the communicator must provide the peer identity claimed by the other end, a human-readable address (for debugging) and a maximum transfer unit (MTU). A MTU of zero means sending is not supported, SIZE_MAX should be used for no MTU. The communicator should also tell TRANSPORT what network type is used for the queue. The communicator may tell TRANSPORT anytime that the queue was deleted and is no longer available.

The communicator API also provides for flow control. First, communicators exhibit back-pressure on TRANSPORT: the number of messages TRANSPORT may add to a queue for transmission will be limited. So by not draining the transmission queue, back-pressure is provided to TRANSPORT. In the other direction, communicators may allow TRANSPORT to give back-pressure towards the communicator by providing a non-NULL `GNUNET_TRANSPORT_MessageCompletedCallback` argument to the `GNUNET_TRANSPORT_communicator_receive` function. In this case, TRANSPORT will only invoke this function once it has processed the message and is ready to receive more. Communicators should then limit how much traffic they receive based on this backpressure. Note that communicators do not have to provide a `GNUNET_TRANSPORT_MessageCompletedCallback`; for example, UDP cannot support back-pressure due to the nature of the UDP protocol. In this case, TRANSPORT will implement its own TRANSPORT-to-TRANSPORT flow control to reduce the sender’s data rate to acceptable levels.

TRANSPORT may notify a communicator about backchannel messages TRANSPORT received from other peers for this communicator. Similarly, communicators can ask TRANSPORT to try to send a backchannel message to other communicators of other peers. The semantics of the backchannel message are up to the communicators which use them. TRANSPORT may fail transmitting backchannel messages, and TRANSPORT will not attempt to retransmit them.