============================================================================== The Collection Tree Protocol (CTP) ============================================================================== :TEP: 123 :Group: Network Working Group :Type: Documentary :Status: Draft :TinyOS-Version: 2.x :Author: Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, Sukun Kim, Philip Levis, and Alec Woo :Draft-Created: 3-Aug-2006 :Draft-Version: $Revision$ :Draft-Modified: $Date$ :Draft-Discuss: TinyOS Developer List .. Note:: This memo documents a part of TinyOS for the TinyOS Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited. This memo is in full compliance with TEP 1. Abstract ============================================================================== This memo documents the Collection Tree Protocol (CTP), which provides best-effort anycast datagram communication to one of the collection roots in a network. 1. Introduction ============================================================================== 2. Assumptions and Limitations ============================================================================== CTP is a tree-based collection protocol. Some number of nodes in a network advertise themselves as tree roots. Nodes form a set of routing trees to these roots. CTP is **address-free** in that a node does not send a packet to a particular root; instead, it implicitly chooses a root by choosing a next hop. Nodes generate routes to roots using a routing gradient. The CTP protocol assumes that the data link layer provides four things: 1) Provides an efficient local broadcast address. 2) Provides synchronous acknowledgments for unicast packets. 3) Provides a protocol dispatch field to support multiple higher-level protocols. 4) Has single-hop source and destination fields. CTP assumes that it has link quality estimates of some number of nearby neighbors. These provide an estimate of the number of transmissions it takes for the node to send a unicast packet whose acknowledgment is successfully received. CTP has several mechanisms in order to improve delivery reliability, but it does not promise 100\% reliable delivery. It is best effort, but a best effort that *tries very hard.* CTP is designed for relatively low traffic rates. Bandwidth-limited systems might benefit from a different protocol, which can, for example, pack multiple small frames into a single data-link packet. 3. Collection and CTP ============================================================================== CTP uses expected transmissions (ETX) as its routing gradient. A root has an ETX of 0. The ETX of a node is the ETX of its parent plus the ETX of its link to its parent. This additive measure assumes that nodes use link-level retransmissions. Given a choice of valid routes, CTP SHOULD choose the one with the lowest ETX value. CTP represents ETX values as 16-bit fixed-point real numbers with a precision of hundredths. An ETX value of 451, for example, represents an ETX of 4.51, while an ETX value of 109 represents an ETX of 1.09. Routing loops are a problem that can emerge in a CTP network. Routing loops generally occur when a node choose a new route that has a significantly higher ETX than its old one, perhaps in response to losing connectivity with a candidate parent. If the new route includes a node which was a descendant, then a loop occurs. CTP addresses loops through two mechanisms. First, every CTP packet contains a node's current gradient value. If CTP receives a data frame with a gradient value lower than its own, then this indicates that there is an inconsistency in the tree. CTP tries to resolve the inconsistency by broadcasting a beacon frame, with the hope that the node which sent the data frame will hear it and adjust its routes accordingly. If a collection of nodes is separated from the rest of the network, then they will form a loop whose ETX increases forever. CTP's second mechanism is to not consider routes with an ETX higher than a reasonable constant. The value of this constant is implementation dependent. Packet duplication is an additional problem that can occur in CTP. Packet duplication occurs when a node receives a data frame successfully and transmits an ACK, but the ACK is not received. The sender retransmits the packet, and the receiver receives it a second time. This can have disasterous effects over multiple hops, as the duplication is exponential. For example, if each hop on average produces one duplicate, then on the first hop there will be two packets, on the second there will be four, on the third there will be eight, etc. Routing loops complicate duplicate suppression, as a routing loop may cause a node to legitimately receive a packet more than once. Therefore, if a node suppresses duplicates based solely on originating address and sequence number, packets in routing loops may be dropped. CTP data frames therefore have an additional time has lived (THL) field, which the routing layer increments on each hop. A link-level retransmission has the same THL value, while a looped version of the packet is unlikely to do so. 4. CTP Data Frame ============================================================================== The CTP data frame format is as follows:: 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C|P| reserved | THL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ETX | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | origin | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | seqno | collect_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Field definitions are as follows: * C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next data frame it transmits. * P: Routing pull. The P bit allows nodes to request routing information from other nodes. If a node with a valid route hears a packet with the P bit set, it SHOULD transmit a routing frame in the near future. * THL: Time Has Lived. When a node generates a CTP data frame, it MUST set THL to 0. When a node receives a CTP data frame, it MUST increment the THL. If a node receives a THL of 255, it increments it to 0. * ETX: The ETX routing metric of the single-hop sender. When a node transmits a CTP data frame, it MUST put the ETX value of its route through the single-hop destination in the ETX field. If a node receives a packet with a lower gradient than its own, then it MUST schedule a routing frame in the near future. * origin: The originating address of the packet. A node forwarding a data frame MUST NOT modify the origin field. * seqno: Origin sequence number. The originating node sets this field, and a node forwarding a data frame MUST NOT modify it. * collect_id: Higher-level protocol identifier. The origin sets this field, and a node forwarding a data frame MUST NOT modify it. * data: the data payload, of zero or more bytes. A node forwarding a data frame MUST NOT modify the data payload. Together, the origin, seqno and collect_id fields denote a unique ***origin packet.*** Together, the origin, seqno, collect_id, and THL denote a unique ***packet instance*** within the network. The distinction is important for duplicate suppression in the presence of routing loops. If a node suppresses origin packets, then if asked to forward the same packet twice due to a routing loop, it will drop the packet. However, if it suppresses packet instances, then it will route succesfully in the presence of transient loops unless the THL happens to wrap around to a forwarded packet instance. A node MUST send CTP data frames as unicast messages with link-layer acknowledgments enabled. 5. CTP Routing Frame ============================================================================== The CTP routing frame format is as follows:: 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C|P| reserved | parent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | parent | ETX | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ETX | +-+-+-+-+-+-+-+-+ The fields are as follows: * C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next routing frame it transmits. * P: Same as data frame. * parent: The node's current parent. * metric: The node's current routing metric value. When a node hears a routing frame, it MUST update its routing table to reflect the address' new metric. If a node's ETX value changes significantly, then CTP SHOULD transmit a broadcast frame soon thereafter to notify other nodes, which might change their routes. The parent field acts as a surrogate for the single-hop destination field of a data packet: a parent can detect when a child's ETX is significantly below its own. When a parent hears a child advertise an ETX below its own, it MUST schedule a routing frame for transmission in the near future. 6. Implementation ============================================================================== An implementation of CTP can be found in the tos/lib/net/ctp directory of TinyOS 2.0. This section describes the structure of that implementation and is not in any way part of the specification of CTP. This implementation has three major subcomponents: 1) A **link estimator**, which is responsible for estimating the single-hop ETX of communication with single-hop neighbors. 2) A **routing engine**, which uses link estimates as well as network-level information to decide which neighbor is the next routing hop. 3) A **forwarding engine**, which maintains a queue of packets to send. It decides when and if to send them. The name is a little misleading: the forwarding engine is responsible for forwarded traffic as well as traffic generated on the node. 6.1 Link Estimation ------------------------------------------------------------------------------ The link estimator estimates the ETX to single-hop neighbors. The implementation uses two mechanisms to estimate the quality of a link: periodic broadcast packets and data packets. The estimator itself only generates broadcast packets. For data traffic, it depends on other components telling it about acknowledged and unacknowledged transmissions. The periodic broadcast packets have sequence numbers, which the estimator uses to estimate the sender-to-receiver packet reception rate (PRR). The data payload of periodic broadcast packets contain these estimates. Therefore, when node A receives a link estimation broadcast message from node B, it can use the packet header to estimate the B-to-A PRR and the packet payload to update B's estimate of the A-to-B PRR. Multiplying these two values gives a *bidirectional* PRR, or an estimate of the probability that if A transmits a packet to B, B will successfully hear and acknowledge the packet and A will hear the acknowledgment. The inverse of the bidirecitonal PRR is the ETX. CTP link estimation adapts its beaconing rate to be slow when its routing table is stable and faster when changes occur. It adjusts the beaconing interval using an algorithm similar to the trickle dissemination protocol[2_]. CTP sends beacons more often when one of three conditions occurs: 1) The routing table is empty (this also sets the P bit) 2) The node's routing ETX increases by >= 1 trasmission 3) The node hears a packet with the P bit set CTP also estimates link quality using data transmissions. This is a direct measure of ETX. Whenever the data path transmits a packet, it tells the link estimator the destimation and whether it was successfully acknowledged. The estimator produces an ETX estimate every 5 such transmissions, where 0 successes has an ETX of 6. The estimator combines the beacon and data estimates by incorporating them into an exponentially weighted moving average. Beacon-based estimates seed the neighbor table. The expectation is that the low beacon rate in a stable network means that for a selected route, data estimates will outweigh beacon estimates. Additionally, as the rate at which CTP collects data estimates is proportional to the transmission rate, then it can quickly detect a broken link and switch to another candidate neighbor. 6.2 Routing Engine ------------------------------------------------------------------------------ The