Table of content
ODR protocol
ODR (On Demand Routing) - the protocol for a routing information propagation used in a star network topology. The point-to-multipoint topology is a star, so ODR is widely used in wireless networks. The ODR algorithm unlike the other dynamic routing protocols, is very simple, so its operation is not resource-intensive.
Let's look at the ODR protocol operation on the example (Figure 1). The network scheme consists of a base station BS1 and three subscriber devices CPE2, CPE3 and CPE4, connected to BS1. There is a local network segment with its own addressing after each wireless device. An IP subnet is also assigned to the MINT area.
- Step 1: pre-configuration. It is necessary to establish a radio link between wireless devices, assign IP addresses to network interfaces, and globally disable switching. The devices routing table after preconfiguration is shown in Table 1.
- Step 2: roles distribution. The ODR protocol operates with devices of two roles types: hub - central device, spoke - end devices. Usually, the base station sector, which has a connection to the backhaul network, acts as the central device, and subscriber stations with the user devices connected, act as end devices.
- Step 3: default route sending. Devices with the hub role configured send messages to inform all devices about their role. To send such messages, the service fields of the MINT protocol are used (see InfiLINK 2x2 and InfiMAN 2x2: Switching), so all message recipients are situated in the MINT area. Receiving such a message, devices with the spoke role form a hub list. Since in a star topology a device with a hub role has a connection to the backhaul, a device with a spoke role can add a default route to the routing table by specifying the hub address as the gateway (Table 2). One MINT area can have several devices with the hub role, therefore each spoke can have several default routes in the RIB.
- Step 4: default route selection. The MINT protocol is used to distribute information about devices with the hub role, which has an internal metric that indicates the radio parameters of links and their current load, so the criteria for choosing a default route and adding it to FIB is the smallest metric value. The metrics analysis is performed permanently for each device with the hub role, therefore, if the metric for the used route grows, it can be replaced by another. Thus, the fault tolerance and balancing functions are implemented.
- Step 5: routing information distribution. Each device with the spoke role generates service messages for each device with the hub role. These messages contain information about directly connected and static routes in the device FIB. Note that information about networks common with hub is not included in such messages. For example, CPE3 will report network 192.168.3.0/24 to BS1, but will not report 172.16.0.0/29, because the BS1 radio interface is associated with this network and the BS1 FIB already has information about it (Table 1).
- Step 6: connectivity between all LAN segments is established. The necessary routes have been added to the routing tables of wireless devices (Table 2).
Figure 1 - The ODR protocol operation scheme in a star topology network
Table 1 - Wireless devices routing tables after pre-configuration
Table 2 - Wireless devices routing tables after ODR configuration
Чаще всего протокол ODR используется в схеме PTMP (рис. 1), однако для лучшего понимания усложним схему, реализовав функцию балансировки и отказоустойчивости (рис.2):
- два сектора БС1 и БС2, коммутаторы InfiMUX1 и InfiMUX2 подключены в Коммутатор 1;
- к каждому из секторов подключены по два абонентских устройства: АС11 и АС12 к БС1, АС21 и АС22 к БС2;
- все устройства объединены в единую область MINT (см. Коммутация в устройствах семейств InfiLINK 2x2 и InfiMAN 2x2.);
- устройствам InfiMUX1 и InfiMUX2 назначены роли hub, т.к. они имеют подключение к внешней сети WAN, беспроводным устройствам - роль spoke.
Следует пояснить, что роль - характеристика не устройства, а интерфейса, поддерживающего работу протокола MINT. Например, беспроводное устройство может играть роль hub в области MINT, к которой оно подключено интерфейсом rf5.0, и роль spoke в области MINT, подключенной через интерфейс prf0. В рассматриваемом примере используется одна общая область MINT, поэтому на устройствах БС1 и БС2 достаточно активировать поддержку протокола ODR на одном из интерфейсов, поддерживающих работу MINT
После предварительной настройки и распределения ролей, устройства АС11, АС12, АС21 и АС22 добавят в таблицу маршрутизации маршрут по умолчанию, указав в качестве шлюза InfiMUX1 или InfiMUX2. Выбор шлюза будет продиктован наименьшим значением метрики до этого устройства. Каждое из устройств с ролью hub получит маршрутную информацию о пользовательских сетях LAN-11, LAN-12, LAN-21 и LAN-22 от устройств с ролями spoke. Устройства БС1 и БС2 являются в данной схеме промежуточными и не сообщают маршрутную информацию устройствам с ролью hub.
Отказоустойчивость: допустим, что весь трафик, проходящий через БС1, также проходит через InfiMUX1, а трафик БС2 - через InfiMUX2. В случае выхода из строя InfiMUX1, устройства БС1, АС11 и АС12 удалят из таблицы маршрутизации запись со шлюзом InfiMUX1, а вместо него добавят другой маршрут по умолчанию через InfiMUX2. Аналогичная ситуация произойдёт при выходе из строя InfiMUX 2.
Балансировка: метрика маршрутов по умолчанию в ODR рассчитывается исходя из радиопараметров и уровня загрузки каналов. Это позволяет выполнять балансировку трафика на стороне устройств с ролью spoke и равномерно утилизировать устройства и каналы связи в области MINT.
Figure 2 - Redundancy scheme using ODR protocol
ODR protocol features
Unlike the static routing, ODR protocol has following features:
- easy configuration: basic ODR configuration consists of roles distribution;
- scalability: an increase in the devices number on the network does not require a larger amount of work, it is enough to perform preliminary configuration on devices and define a role;
- rapidity: the state of devices and communication channels is constantly monitored by the ODR protocol, so changes in the network topology will instantly be reflected in the routing table;
- fault tolerance and traffic balancing: schemes implementation for increasing reliability and optimal links utilization does not require additional manipulations and is performed automatically;
- topology limitation: ODR cannot be used in arbitrary topology networks and is intended for star type networks;
- proprietary implementation: ODR protocol is supported by other hardware manufacturers, however these implementations will not be compatible with Infinet devices. This because the transport technology for ODR is not standardized and in Infinet devices the proprietary MINT protocol is used.
ODR practice
The examples of ODR configuration are on the document child page: engНастройка протокола ODR.