Wednesday, March 16, 2016

NetworkManager and multiple provisioning domains

The goal of this post is to list different options on how to introduce PvDs into the NetworkManager, i.e. what should be changed in NetworkManager and how it should handle explicit and implicit PvDs. But first we'll start with the definition of Provisioning Domain and object that could potentially be used to store/represent Provisioning Domains. The implementation this post refers to can be found on GitHub.

The term Provisioning Domain (PvD) is defined and clarified in RFC7556 as:
A consistent set of network configuration information. Classically, all of the configuration information available on a single interface is provided by a single source (such as a network administrator) and can therefore be treated as a single provisioning domain.  In modern IPv6 networks, multihoming can result in more than one provisioning domain being present on a single link.  In some scenarios, it is also possible for elements of the same PvD to be present on multiple links.
Basically it is a set of configuration information that should be treated as a single unit. Here are some examples of such units of configuration data:
  1. Static IPv4 configuration provided by a user for a server or for a network without DHCP.
  2. Data handed over to a client by DHCP server.
  3. On an IPv6 enabled local network with a single router which sends configuration data in RA to nodes attached to the network.
  4. Configuration data sent by VPN gateway upon successful connection of a client. 
In all these cases we have implicit PvDs, meaning that the sets of configuration data are implicitly bound together and there was no indication whatsoever that they should be treated as a single unit. This is in contrast to explicit PvDs which are sets of configuration data bound together by some explicit mechanism and associated with some kind of a PvD identifier sent to a client in some way. Explicit PvDs, as of time this post was written, don't exist yet, and the IETF MIF working group is trying to define necessary mechanisms to support them as well as how exactly IDs should look like.

Note that apart from explicit PvD and implicit Pvd we also differentiate between PvD and PvD instance. The difference is that PvD consists of a set of PvD instances thate are the same on some local network, while PvD instance is valid for only a single host on a given local network. In other words, PvD will include network prefix and mask, while PvD instance will include host addresses too. It is interesting to note that router advertisements communicate PvDs while DHCP communicates PvD instaces.

How to implement PvDs in the NetworkManager

As always, the same goal can be achieved in multiple ways, so here are the options on how PvDs can be implemented within NM. Basically, there are two main approaches: first, existing objects can be enhanced so that they can represent PvDs or a completely new object can be introduced.

Using NMSettingsConnection object to store PvD and PvD instance


Each network connection (which is not the same as PvD or PvD instance) is stored in NMSettingsConnection object. Those objects are generated from static files or dynamically during NetworkManager's execution. NMSettingsConnection objects are initialized from the following sources:
  1. Distribution configuration files. System dependent network configuration files (e.g. /etc/sysconfig/network-scripts for RHEL based systems) are read by NM via plugins and NMSettingsConnection objects are created as a result.
     
  2. Network manager specific configuration. NetworkManager has its own configuration files that are stored in /etc/NetworkManager/system-connections/.
  3. Dynamically created configurations. While running, NetworkManager allows new configurations to be created via D-Bus interface.
Note that NetworkManager has a concept of profiles that are used in the case of Wired networks. Basically, those are settings which are not bound to any specific network interface. Profiles can have 802.1x type of credentials assigned to them.

So, the idea of integrating PvDs into NetworkManager is for each new PvD or PvD instance to create a new NMSettingsConnection object. The modification to NMSettingsConnection should be extended with PvD ID parameter.

There are several potential problems with this approach:
  1. There is a difference between NMSettingsConnection on the one hand, and PvD and PvD instance on the other hand. For example, some NMSettingsConnection defines a network connection that should be configured using DHCP and in that case the NMSettingsConnection isn't PvD nor PvD instance. On the other hand, NMSettingsConnection can be the same as PvD instance. This is the case with static IPv4 configurations when a user specifies concrete IP addresses. Finally, NMSettingsConnection can be PvD only in the case of IPv6 when host part is generated from MAC address.
  2. When PvDs and PvD instances are received they are valid only for the interface on which they are received. But, a user can request any NMSettingsConnection object to be activated on any interface which isn't possible.
  3. Also, this can create confusion. Take for example preconfigured NMSettingsConnection which is now treated as PvD with a specific PvD ID, and it is defined to use DHCP for the configuration. Obviously, this PvD ID is expected to be valid on a certain interface on a specific attachment point. But due to the way the interface is configured (DHCP) it can actually be activated on any interface on any network that supports DHCP. Thus, it might easily happen that a user by mistake activated this particular NMSettingsConnection on a "wrong" network and so makes a user believe the network is active while in the reality it is not.

    Note that even NMSettingsConnection objects that contain credential information aren't guaranteed to retrieve the same PvD every time the connection is made. Namely, there are AAA servers and infrastructure that allow clients with a same credentials to connect to multiple networks, and thus to potentially receive multiple PvDs.
     
  4. Finally, the problem is that on a single network interface only one NMSettingsObject might be activated and so this prevents having multiple PvDs on a single interface.
Those problems are not unsolvable, i.e. they could be solved by modifying certain aspects of the NetworkManager in general, and NMSettingsObject in particular.

Treating NMActiveConnection object as PvD instance and PvD


Whenever a connection is made in NetworkManager an object is created. Basically, there are two classes for the object, both of which inherit from NMActiveConnection base class. Which class is used depends on the type of the connection. Basically,  the only distinction is made between VPN connections that are represented by NMVPNConnection objects and other connections that are represented by NMActRequest objects. The main task of NMActiveConnection is to bind NMSettingsConnection with NMDevice objects.

The idea in this case is to treat NMActiveConnection as a PvD or a PvD instance, i.e. on each new PvD or PvD instance received new NMActiveConnection is created.

But, there are still some problems:
  1. Since NMActiveConnection objects are transient that means that there would be no history of PvDs used. This might, or might not be a problem, depending on whether we need this history or not.

    The cases when the history would be necessary is if we cache some information for the next time we connect to the given PvD. The second case is if there are processes still using PvD through API and thus the information about PvD must live until the process dies. Note that this letter problem could be solved with delayed removal of NMActiveConnections or by some asynchronous mechanism informing applications that specific NMActiveConnection isn't available any more.
     
  2. The second problem is the question if there could exist two ActiveConnection objects that were created from the same NMSettingsConnection object, i.e. can NMSettingsConnections be shared.
     
  3. The third problem is that it will happen from within a single NMActiveConnection that two or more PvDs are received and this requires that NMActiveConnection is a factory for itself.

Using NMIP4Config and NMIP6Config objects for PvDs and PvD instances


NetworkManager has object/classes for storing IPv4 (libnm-core/nm-setting-ip4-config.c) and IPv6 (libnm-core/nm-setting-ip6-config.c) settings. More precisely, those objects are used to expose network settings of devices to the rest of the NetworkManager. So, in some way they are PvDs in a sense that each of them contains enough information to allow connection to the network.

The problem is that internally NetworkManager keeps a single IPv4/IPv6 configuration object per device and in addition it merges all received configuration data on a single interface.

Specifically, in case of configuration data received in RAs everything is kept in the object NMRdisc defined in src/rdisc/nm-rdisc.h. There you'll find arrays of received configuration data. NetworkManager assumes that a single router sends all the configuration data. This assumption is not valid on a multihomed network, or a network that can send multiple provisioning domains within each RA. What would be necessary is to change this structure so that configuration data is kept separate for each router and provisioning domain.

The problems in this case are:
  1. NMIPxConfig objects were not intended to keep information about available IPv4 and IPv6 addresses but to make available addresses configured on device. So, it reverses the purpose of those objects which isn't accepted so well.
  2. Again, those are transient objects and thus there is no history. It is possible to keep every object alive, but NM isn't designed to behave in such way.
  3. It seems that in libnm there is no way to obtain a list of IPv4 and IPv6 objects.

Having separate PvD structures


This is the final alternative and the most intrusive one. The idea is that settings, active connections and IPv6 and IPv6 objects/classes stay as is, but instead, when each new connection is established a new PvD data structure is created. PvD is inferred from configuration settings or the NetworkManager received explicit PvD.

This would solve the problem that some settings might be used to obtain different PvDs which isn't known until connection is established. For example, if we are using DHCP to configure the interface, then, PvD received depends on the PoA.

It would also solve the problem that the user might try to instantiate one PvD, while some other is actually in use. This way, after the connection is established, appropriate PvD is searched for, or new one is created.

This is most intrusive change that would require change in APIs and thus break compatibility with the existing applications (or require a completely new API).

Current PvD Support Implementation


The first implementation of PvDs was done using NMIP6Config as a PvD container. Before describing the implementation we have to state that the only mechanism currently able to carry PvDs is RA messages. NMIP6Config objects are extended with PvD ID field. At first, there was support for different types of PvD IDs and the first implemented type was UUID stored in ASCII format. Later in the development process PvD ID types were removed and the only possible type is UUID. It seems that this doesn't make implementations less flexible and in the same time substantially reduces complexity.

When RA is received, and after it is processed as usual, a new implicit PvD is created from data in RA. If there are two or more routers on the local network, each sending its own configuration data, then a separate PvD is created for each RA. Also, in case there are PVD container option in RA it is parsed and additional PvD is created from that data.

This information is then handled to NMDevice object which merges data from implicit PvDs (as it does in the unmodified version) but now there is also a hash table with set of PvDs received on the given interface. This information is then exposed through NMActiveConnection object.



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