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diff --git a/doc/devel/congestion-handling.dox b/doc/devel/congestion-handling.dox new file mode 100644 index 0000000..55ad17e --- /dev/null +++ b/doc/devel/congestion-handling.dox @@ -0,0 +1,452 @@ +// Copyright (C) 2018-2021 Internet Systems Consortium, Inc. ("ISC") +// +// This Source Code Form is subject to the terms of the Mozilla Public +// License, v. 2.0. If a copy of the MPL was not distributed with this +// file, You can obtain one at http://mozilla.org/MPL/2.0/. + +/** + +@page congestionHandling Congestion Handling in Kea DHCP Servers + +@section background What is Congestion? +Congestion occurs when servers are subjected to client queries +faster than they can be fulfilled. Subsequently, the servers begin +accumulating a backlog of pending queries. The longer the high rate of +traffic continues the farther behind the servers fall. Depending on the +client implementations, those that fail to get leases either give up or simply +continue to retry forever. In the former case, the server may eventually +recover. The latter case is vicious cycle from which the server is unable +to escape. + +In a well-planned deployment, the number and capacity of servers is matched +to the maximum client loads expected. As long as capacity is matched to +load, congestion does not occur. If the load is routinely too heavy, then +the deployment needs to be re-evaluated. Congestion typically occurs when +there is a network event that causes overly large numbers of clients to +simultaneously need leases such as recovery after a network outage. + +@section introduction Congestion Handling Overview + +Kea 1.5.0 introduces a new feature referred to as Congestion Handling. The +goal of Congestion Handling is to help the servers mitigate the peak +in traffic by fulfilling as many of the most relevant requests as possible +until it subsides. + +Prior to Kea 1.5.0, Kea DHCP servers read inbound packets directly +from the interface sockets in the main application thread. This meant that +packets waiting to be processed were held in socket buffers themselves. Once +these buffers fill any new packets are discarded. Under swamped conditions +the servers can end up processing client packets that may no longer be +relevant, or worse are redundant. In other words, the packets waiting in +the FIFO socket buffers become increasingly stale. + +Congestion Handling offers the ability to configure the server to use a +separate thread to read packets from the interface socket buffers. As the +thread reads packets from the buffers they are added to an internal "packet +queue". The server's main application thread processes packets from this queue +rather than the socket buffers. By structuring it this way, we've introduced +a configurable layer which can make decisions on which packets to process, +how to store them, and the order in which they are processed by the server. + +The default packet queue implementation for both Kea DHCPv4 and DHCPv6 servers +is a simple ring buffer. Once it reaches capacity, new packets get added to +the back of queue by discarding packets from the front of queue. Rather than +always discarding the newest packets, we now always discard the oldest +packets. The capacity of the buffer (i.e. the maximum number of packets the +buffer can contain) is configurable. + +@section custom-implementations Custom Packet Queues + +It is possible to replace the default packet queue implementation with a +custom implementation by registering it with your Kea server via a hook +library. The steps for doing this are listed below: + +-# Develop a derivation of the interface isc::dhcp::PacketQueue +-# Registering and un-registering your implementation via Hook library +-# Configure your Kea server to use your derivation + +(If you are not familiar with writing Kea hook libraries, you may wish to +read @ref hooksdgDevelopersGuide before continuing). + +@subsection packet-queue-derivation Developing isc::dhcp::PacketQueue Derivations + @subsection packet-queue-derivation-basics The Basics + +Your custom packet queue must derive from the class template, +isc::dhcp::PacketQueue. The class is almost entirely abstract and +deliberately brief to provide developers wide latitude in the internals +of their solutions. + +The template argument, @c PacketTypePtr, is expected to be either +isc::dhcp::Pkt4Ptr or isc::dhcp::Pkt6Ptr, depending upon which +protocol the implementation will handle. Please note that the +while following text and examples largely focus on DHCPv4 out +of convenience as the concepts are identical for DHCPv6. For +completeness there are code snippets at the end of this +chapter for DHCPv6. + +The two primary functions of interest are: + +-# isc::dhcp::PacketQueue::enqueuePacket() - This function is invoked by +the receiver thread each time a packet has been read from an interface +socket buffer and should be added to the queue. It is passed a pointer to +the unpacked client packet (isc::dhcp::Pkt4Ptr or isc::dhcp::Pkt6Ptr), and +a reference to the isc::dhcp::SocketInfo describing the interface socket +from which the packet was read. Your derivation is free to use whatever +logic you deem appropriate to decide if a given packet should be added +to the queue or dropped. The socket information is passed along to be used +(or not) in your decision making. The simplest derivation would add every +packet, every time. + +-# isc::dhcp::PacketQueue::dequeuePacket() - This function is invoked by the +server's main thread whenever the receiver thread indicates that packets are +ready. Which packet is dequeued and returned is entirely up to your +derivation. + +The remaining functions that you'll need to implement are self-explanatory. + +How your actual "queue" is implemented is entirely up to you. Kea's default +implementation using a ring buffer based on Boost's boost::circular_buffer +(please refer to isc::dhcp::PacketQueueRing, isc::dhcp::PacketQueueRing4 and +isc::dhcp::PacketQueueRing6). The most critical aspects to remember when +developing your implementation are: + +-# It MUST be thread safe since queuing and dequeuing packets are done by +separate threads. (You might considering using std::mutex and std::lock_guard). + +-# Its efficiency (or lack thereof) will have a direct impact on server +performance. You will have to consider the dynamics of your deployment +to determine where the trade-off lies between the volume of packets responded +to and preferring to respond to some subset of those packets. + + @subsection packet-queue-derivation-factory Defining a Factory + +isc::dhcp::IfaceMgr using two derivations of isc::dhcp::PacketQueueMgr (one for +DHCPv4 and one for DHCPv6), to register queue implementations and instantiate +the appropriate queue type based the current configuration. In order to +register your queue implementation your hook library must provide a factory +function that will be used to create packet queues. This function will be +invoked by the server during the configuration process to instantiate the +appropriate queue type. For DHCPv4, the factory should be as follows: + +@code + PackQueue4Ptr factory(isc::data::ConstElementPtr parameters) +@endcode + +and for DHCPv6: + +@code + PackQueue6Ptr factory(isc::data::ConstElementPtr parameters) +@endcode + +The factory's only argument is an isc::data::ConstElementPtr. This is will be +an isc::data::MapElement instance containing the contents of the configuration +element "dhcp-queue-control" from the Kea server's configuration. It will +always have the following two values: + +-# "enable-queue" - used by isc::dhcp::IfaceMgr to know whether or not +congestion handling is enabled. Your implementation need not do anything +with this value. + +-# "queue-type" - name of the registered queue implementation to use. +It is used by isc::dhcp::IfaceMgr to invoke the appropriate queue factory. +Your implementation must pass this value through to the isc::dhcp::PacketQueue +constructor. + +Beyond that you may add whatever additional values you may require. In +other words, the content is arbitrary so long as it is valid JSON. It is +up to your factory implementation to examine the contents and use them +to construct a queue instance. + + @subsection packet-queue-derivation-example An Example + +Let's suppose you wish to develop a queue for DHCPv4 and your implementation +requires two configurable parameters: capacity and threshold. Your class +declaration might look something like this: + +@code +class YourPacketQueue4 : public isc::dhcp::PacketQueue<isc::dhcp::Pkt4Ptr> { +public: + + // Logical name you will register your factory under. + static const std::string QUEUE_TYPE; + + // Factory for instantiating queue instances. + static isc::dhcp::PacketQueue4Ptr factory(isc::data::ConstElementPtr params); + + // Constructor + YourPacketQueue4(const std::string& queue_type, size_t capacity, size_t threshold) + : isc::dhcp::PacketQueue<isc::dhcp::Pkt4Ptr>(queue_type) { + + // your constructor steps here + } + + // Adds a packet to your queue using your secret formula based on threshold. + virtual void enqueuePacket(isc::dhcp::Pkt4Ptr packet, const dhcp::SocketInfo& source); + + // Fetches the next packet to process from your queue using your other secret formula. + virtual isc::dhcp::Pkt4Ptr dequeuePacket(); + + : // Imagine you prototyped the rest of the functions + +}; +@endcode + +Your factory implementation would then look something like this: + +@code + +const std::string QUEUE_TYPE = "Your-Q4"; + +isc::dhcp::PacketQueue4Ptr +YourPacketQueue4::factory(isc::data::ConstElementPtr parameters) { + + // You need queue-type to pass into the base class. + // It's guaranteed to be here. + std::string queue_type = isc::data::SimpleParser::getString(parameters, "queue-type"); + + // Now you need to fetch your required parameters. + size_t capacity; + try { + capacity = isc::data::SimpleParser::getInteger(parameters, "capacity"); + } catch (const std::exception& ex) { + isc_throw(isc::dhcp::InvalidQueueParameter, "YourPacketQueue4:factory:" + " 'capacity' parameter is missing/invalid: " << ex.what()); + } + + size_t threshold; + try { + threshold = isc::data::SimpleParser::getInteger(parameters, "threshold"); + } catch (const std::exception& ex) { + isc_throw(isc::dhcp::InvalidQueueParameter, "YourPacketQueue4:factory:" + " 'threshold' parameter is missing/invalid: " << ex.what()); + } + + // You should be all set to create your queue instance! + isc::dhcp::PacketQueue4Ptr queue(new YourPacketQueue4(queue_type, capacity, threshold)); + return (queue); +} + +@endcode + +Kea's configuration parser cannot know your parameter requirements and thus +can only flag JSON syntax errors. Thus it is important for your factory to +validate your parameters according to your requirements and throw meaningful +exceptions when they are not met. This allows users to know what to correct. + +@subsection packet-queue-registration Registering Your Implementation + +All hook libraries must provide a load() and unload() function. Your hook +library should register you queue factory during load() and un-register it +during unload(). Picking up with the our example, those functions might +look something like this: + +@code +// This function is called when the library is loaded. +// +// param - handle library handle (we aren't using it) +// return - 0 when initialization is successful, 1 otherwise +int load(LibraryHandle& /* handle */) { + try { + // Here you register your DHCPv4 queue factory + isc::dhcp::IfaceMgr::instance().getPacketQueueMgr4()-> + registerPacketQueueFactory(YourPacketQueue4::QUEUE_TYPE, + YourPacketQueue::factory); + } catch (const std::exception& ex) { + LOG_ERROR(your_logger, YOUR_LOAD_FAILED) + .arg(ex.what()); + return (1); + } + + LOG_INFO(your_logger, YOUR_LOAD_OK); + return (0); +} + +// This function is called when the library is unloaded. +// +// return - 0 if deregistration was successful, 1 otherwise +int unload() { + + // You need to remove your queue factory. This must be done to make sure + // your queue instance is destroyed before your library is unloaded. + isc::dhcp::IfaceMgr::instance().getPacketQueueMgr4()-> + unregisterPacketQueueFactory(YourPacketQueue4::QUEUE_TYPE); + + LOG_INFO(your_logger, YOUR_UNLOAD_OK); + return (0); +} +@endcode + +@subsection packet-queue-factory Configuring Kea to use YourPacketQueue4 + +You're almost there. You developed your implementation, you've unit tested it +(You did unit test it right?). Now you just have to tell Kea to load it and +use it. Continuing with the example, your kea-dhcp4 configuration would need +to look something like this: + +@code +{ +"Dhcp4": +{ + ... + + "hooks-libraries": [ + { + # Loading your hook library! + "library": "/somepath/lib/libyour_packet_queue.so" + } + + # any other hook libs + ], + + ... + + "dhcp-queue-control": { + "enable-queue": true, + "queue-type": "Your-Q4", + "capacity" : 100, + "threshold" : 75 + }, + + ... +} +@endcode + +@subsection packet-queue-example-dhcpv6 DHCPv6 Example Snippets + +For completeness, this section includes the example from above +implemented for DHCPv6. + +DHCPv6 Class declaration: + +@code +class YourPacketQueue6 : public isc::dhcp::PacketQueue<isc::dhcp::Pkt6Ptr> { +public: + + // Logical name you will register your factory under. + static const std::string QUEUE_TYPE; + + // Factory for instantiating queue instances. + static isc::dhcp::PacketQueue6Ptr factory(isc::data::ConstElementPtr params); + + // Constructor + YourPacketQueue6(const std::string& queue_type, size_t capacity, size_t threshold) + : isc::dhcp::PacketQueue<isc::dhcp::Pkt6Ptr>(queue_type) { + + // your constructor steps here + } + + // Adds a packet to your queue using your secret formula based on threshold. + virtual void enqueuePacket(isc::dhcp::Pkt6Ptr packet, const dhcp::SocketInfo& source); + + // Fetches the next packet to process from your queue using your other secret formula. + virtual isc::dhcp::Pkt6Ptr dequeuePacket(); + + : // Imagine you prototyped the rest of the functions + +}; +@endcode + +DHCPv6 Factory implementation: + +@code +const std::string QUEUE_TYPE = "Your-Q6"; + +isc::dhcp::PacketQueue6Ptr +YourPacketQueue6::factory(isc::data::ConstElementPtr parameters) { + + // You need queue-type to pass into the base class. + // It's guaranteed to be here. + std::string queue_type = isc::data::SimpleParser::getString(parameters, "queue-type"); + + // Now you need to fetch your required parameters. + size_t capacity; + try { + capacity = isc::data::SimpleParser::getInteger(parameters, "capacity"); + } catch (const std::exception& ex) { + isc_throw(isc::dhcp::InvalidQueueParameter, "YourPacketQueue6:factory:" + " 'capacity' parameter is missing/invalid: " << ex.what()); + } + + size_t threshold; + try { + threshold = isc::data::SimpleParser::getInteger(parameters, "threshold"); + } catch (const std::exception& ex) { + isc_throw(isc::dhcp::InvalidQueueParameter, "YourPacketQueue6:factory:" + " 'threshold' parameter is missing/invalid: " << ex.what()); + } + + // You should be all set to create your queue instance! + isc::dhcp::PacketQueue6Ptr queue(new YourPacketQueue6(queue_type, capacity, threshold)); + return (queue); +} +@endcode + +DHCPv6 Hook load/unload functions + +@code +// This function is called when the library is loaded. +// +// param - handle library handle (we aren't using it) +// return - 0 when initialization is successful, 1 otherwise +int load(LibraryHandle& /* handle */) { + try { + // Here you register your DHCPv6 queue factory + isc::dhcp::IfaceMgr::instance().getPacketQueueMgr6()-> + registerPacketQueueFactory(YourPacketQueue6::QUEUE_TYPE, + YourPacketQueue::factory); + } catch (const std::exception& ex) { + LOG_ERROR(your_logger, YOUR_LOAD_FAILED) + .arg(ex.what()); + return (1); + } + + LOG_INFO(your_logger, YOUR_LOAD_OK); + return (0); +} + +// This function is called when the library is unloaded. +// +// return - 0 if deregistration was successful, 1 otherwise +int unload() { + + // You need to remove your queue factory. This must be done to make sure + // your queue instance is destroyed before your library is unloaded. + isc::dhcp::IfaceMgr::instance().getPacketQueueMgr6()-> + unregisterPacketQueueFactory(YourPacketQueue6::QUEUE_TYPE); + + LOG_INFO(your_logger, YOUR_UNLOAD_OK); + return (0); +} +@endcode + +Server configuration for kea-dhcp6: + +@code +{ +"Dhcp6": +{ + ... + + "hooks-libraries": [ + { + # Loading your hook library! + "library": "/somepath/lib/libyour_packet_queue.so" + } + + # any other hook libs + ], + + ... + + "dhcp-queue-control": { + "enable-queue": true, + "queue-type": "Your-Q6", + "capacity" : 100, + "threshold" : 75 + }, + + ... +} +@endcode + +*/ |