serval-dna/doc/Mesh-Datagram-Protocol.md

Mesh Datagram Protocol (MDP)

Serval Project, November 2017

The Mesh Datagram Protocol is a layer 3 datagram network protocol developed for the Serval mesh network, with characteristics that make it particularly suitable for use in Ad Hoc wireless networks, which can suffer high levels of packet loss due to weak signal, interference and congestion.

MDP carries messages from sender to recipient nodes, or broadcasts to all nodes. MDP guarantees that message contents will be correct if delivered, but does not guarantee exactly one delivery (messages may be lost or delivered more than once), arrival time, or message order.

MDP can be carried over any wireless or wired data link, whether a shared medium (eg, CSMA/CA used in Wi-Fi) or a dedicated medium (eg, AX.25 packet radio, serial cable).

MDP is similar to UDP, but it uses per-link retransmission and adaptive link-state routing to boost packet delivery rates, which largely immunises it from the cumulative packet loss effect typical of multi-hop wireless networks. To carry a packet over N hops, where each hop has a probability P of dropping a packet due to interference or collision, the end-to-end loss is 1 − pow(1−P, N). For example, given a per-hop packet loss of 10%, a five hop route has a net packet loss of 41%, and a ten hop route has 75% packet loss.

The MDP retransmission scheme reduces but does not eliminate packet loss, and sometimes produces duplicate packets. However, it can squeeze useful packet delivery rates from a high-loss route that would be practically useless with Internet protocols, which rely on end-to-end retransmission.

Basic concepts

Node

A node in the Serval mesh network is a single device having one or more network interfaces, running exactly one instance of the Serval DNA daemon process that is configured to use those network interfaces.

If a single device were to run several Serval DNA daemon processes, then each daemon process would be treated as a separate node, and they should be configured to share the device's network interfaces and communicate with each other using device-local interfaces such as local sockets or pipes.

Link

A link in the Serval mesh network is a direct network connection between two nodes, making it possible for a packet sent by the Serval DNA daemon on one node to be received directly by the daemon on the other node, without traversing any other node. In other words, a link is a direct connection between nodes that supports layer 2 protocol functions.

Note that this definition does not rule out an MDP packet passing through a non-Serval multi-hop route. For example, if an overlay packet is routed through a multi-hop layer 3 or layer 4 network service, such as UDP, that is still a single link as far as MDP is concerned, because the packet does not pass through a Serval node.

MDP Address

Every node in the Serval mesh network uses a unique Serval ID (abbreviated to SID) as its MDP address.

The Serval DNA daemon can have one or many identities in its keyring, and each identity has its own unique SID. The daemon chooses the first identity that it unlocks as the principal identity, and that identity's SID becomes the node's MDP Address. If that identity is ever locked, then the daemon will choose another currently unlocked identity and change its MDP Address to be that SID.

Since each SID identifies a distinct user of the network (sometimes called a subscriber), then strictly speaking, the Serval mesh network could be said to carry messages between users not between devices. There is nothing to prevent a keyring entry from being copied from one device to another, thus it is possible for two or more devices to have the same MDP Address. At present, Serval routing does not handle this case, so it could cause unwanted effects such as route flapping or dropped messages.

In practice, the “duplicate MDP Address” problem is rare for the time being, because the Serval Mesh app for Android does not provide any way for a non-expert user to copy the keyring file from one device to another, and other Serval devices such as the Mesh Extender are not operated by end-users.

Transmitter

Whenever an MDP message is carried over a single link from one node to another, the transmitter link, identified by its MDP address, is responsible for encapsulating the message in such a way as to guarantee its content (error detection) and possibly guarantee or at least improve its probability of arrival (retransmission or error correction). The most common encapsulation is the MDP overlay packet.

Transmitter is a layer 2 concept because it concerns data transfer across a single link.

Receiver

Whenever an MDP message is carried over a single link from one node to another, the receiver link, identified by its MDP address, is responsible for decoding the encapsulated message, dealing with data corruption (error checking or correction), cooperating with the transmitter (ACK to prevent retransmission) and de-duplication.

Receiver is a layer 2 concept because it concerns data transfer across a single link.

Sender

Every MDP message originates from a single sender node, identified by its MDP address. As the message passes through many nodes to reach its recipient, it keeps the same sender address.

Sender is a layer 3 concept because it specifies an end point of a multi-link route.

Recipient

Every MDP message that is not a broadcast message is destined for a single recipient node, identified by its MDP address. The message may pass through many nodes to reach its recipient.

Recipient is a layer 3 concept because it specifies an end point of a multi-link route.

MDP message

The smallest unit of data transported by MDP is the MDP message. The MDP message is a layer 3, or end-to-end concept: the Serval mesh network carries each MDP message from its sender to its recipient, routing via as many intermediate nodes as necessary, without the messages having to specify any intermediate nodes.

An MDP message consists of a variable-length header, followed by a variable-length payload which may be encrypted and signed. Most fields in the MDP message header are optional, depending on the initial byte of bit flags:

bytes	name	present if	meaning
1	FLAGS		message flags
1..33	sender address	!SENDER_SAME	the sender's address
1..33	recipient address	!BROADCAST	the recipient's address
8	broadcast sequence	BROADCAST && !ONE_HOP	broadcast message's sequence number
1	TTL and QoS	!ONE_HOP	time-to-live counter and service type
2	payload size		number of bytes in payload
0..max	payload

MDP message flags

The single FLAGS byte at the start of the MDP message header contains the following bits (bit numbers start with 0 = LSB):

bit	symbol	meaning
0	SENDER_SAME	the transmitter is the sender
1	BROADCAST	message is broadcast; has no recipient
2	ONE_HOP	message is on last link, so the receiver is the recipient
3	(unused)	transmitter must set to zero; recipient must ignore
4	CIPHERED	payload is encrypted
5	SIGNED	payload is signed
6	ACK_SOON	transmitter will re-transmit very soon
7	(unused)	transmitter must set to zero; recipient must ignore

• The SENDER_SAME flag is set on the first outbound link of a message's trajectory if the message is encapsulated in an overlay packet; ie, the Sender SID is identical to the overlay packet's Transmitter SID. In this case the Sender SID field is omitted from the message's header, to avoid unnecessary duplication.

• The BROADCAST flag indicates a broadcast message that is sent to all nodes. If the BROADCAST flag is set, the Recipient SID message header field is absent, and the broadcast sequence header field is present unless the ONE_HOP flag is also set (see below).

• The ONE_HOP flag is set to indicate that the message is not to be forwarded by the receiver; this occurs in the following cases:

  • The message is on the last link of its trajectory to its recipient and is encapsulated in an overlay packet; ie, the Recipient SID is identical to the overlay packet's Receiver SID. In this case the Recipient SID field is omitted from the message's header, to avoid unnecessary duplication.

  • The message is a broadcast message that need not propagate beyond the receiver; ie, it only has one link to live (TTL = 1). In this case, the broadcast sequence and TTL and QoS fields are omitted from the message's header to save space.

• The CIPHERED flag is set if the message's payload is encrypted using the Recipient SID as public key; only the recipient possesses the private key (secret), so only the recipient can decrypt the payload. The CIPHERED and BROADCAST flags are mutually exclusive; all broadcast messages are unciphered.

• The SIGNED flag is set if the message's payload is signed by the sender; anyone can verify the signature using the Sender SID public key, but only the sender possesses the private key (secret), so only the sender can produce the signature.

• The ACK_SOON flag is set if the transmitter will re-send the message unless receiving an ACK for the message within the next few overlay packets. This flag is typically used on low-latency, high quality links to maximise throughput by avoiding redundant re-transmissions.

MDP address fields

The sender and recipient address fields in the MDP message header are encoded as a single qualifier byte Q followed by 0 ≤ N ≤ 32 bytes of data:

Q	symbol	N	resolves to...
0..31		Q	an abbreviated address in binary format
32		32	a complete SID in binary format
0xFB	SIGNKEY	32	a complete Signing ID in binary format
0xFC	P2P_ME	0	the source address of a point-to-point link
0xFD	P2P_YOU	0	the destination address of a point-to-point link
0xFE	PREVIOUS	0	the previous resolved address
0xFF	SELF	0	the transmitter of the enclosing overlay packet

SIGNKEY addresses are only used for combined IDs, ie, where the SIDs is cryptographically derived from a Signing ID. Any node running a version of the Serval DNA daemon that pre-dates the combined key upgrade will not recognise the SIGNKEY address type, and will treat it as invalid.

P2P_ME and P2P_YOU addresses are only valid in MDP messages that are being transmitted over a point-to-point link. If a P2P_ME address is received but the recipient node has not yet discovered the address of the source node at the other end of the link, then the recipient treats the address as invalid and initiates an address discovery handshake, so that subsequent P2P_ME addresses may succeed.

A PREVIOUS address resolves to the previous resolved sender or recipient address in this MDP message or in the preceding MDP message in the enclosing overlay packet. This qualifier is generally not useful and may be deprecated in future.

The SELF qualifier is only valid in MDP messages that are encapsulated within an overlay packet.

Broadcast

TODO: Describe the behaviour of broadcast messages.

Overlay Packet

MDP transmits a MDP message over a link by encapsulating it into an overlay packet (also called MDP packet or overlay frame). The MDP overlay packet is a layer 2 concept; it is only concerned with transporting MDP messages across a single link to a neighbouring peer node. Once an overlay packet arrives, the receiver unpacks all of its MDP messages, consumes those for which it (or one of its zero-hop identities) is the recipient and independently message in an overlay packet is constructed afresh when it is embedded into the packet, setting its flag bits and re-writing the address fields within the context of the overlay packet, in order to conserve link bandwidth by avoiding duplication where possible.

TODO: Describe the structure of an overlay packet in detail.

Abbreviated address

An abbreviated address is a truncated SID, ie, the initial N < 32 bytes of a whole SID.

Since SIDs are randomly allocated and only relatively few SIDs are in use within a local Serval mesh network at a given time, all SIDs in use are very likely to differ within their first few bytes. Thus, within the context of the local mesh network, there is no need to use entire SIDs to uniquely identify nodes.

SID abbreviation allows MDP messages to identify their sender and recipient using far fewer than 32 bytes, typically only 1 or 2 bytes.

TODO: Describe the the abbreviation resolution rules and the explain handshake.

MDP Client Interface

The Serval DNA daemon provides an interface that allows client applications to send and receive individual MDP packets on the Serval mesh network without having to construct and disassemble Overlay Mesh frames on their own.

MDP Client API

The MDP Client API is a C language API that an application can use to send and receive MDP packets over the Serval mesh network using the interface provided by the Serval DNA daemon.

History

MDP was designed and first prototyped in May-June 2012 as part of the first New America Foundation contract to integrate Serval security into the OpenBTS base station, and also as part of the development of release 0.90 “Shiny” of the Serval Mesh app for Android.

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Copyright 2014 Serval Project Inc.
Copyright 2016-2017 Flinders University
Available under the Creative Commons Attribution 4.0 International licence.