DHCP Hackaton summary

Marcin Siodelski marcin at isc.org
Wed Sep 24 11:47:29 UTC 2014

I was thinking about organizing the data in a database a bit more and I
feel like the reserved IPv6 addresses/prefixes should not be stored as a
list but they should rather be stored in a separate table and reference
the Host table (1:n) - just like we decided for options. Storing the
reservations as a list in a single field will have numerous pitfalls
listed here in no particular order:

- No easy way to search a reservation for a particular IP address or
prefix unless you use SQL functions searching by substring
- No way to use the indexes per-address because they would point to a
list of addresses. Note that indexes play important role for improving
performance of queries.
- No way to enforce uniqueness of the addresses in the database
- Number of addresses is constrained by the number of bytes allocated
for a string or a blob holding a list
- Complex notation of the prefix, as it needs to be encoded with its length.
- Can't delete a single reservation without fetching all addresses
- It is hard to count reserved resources (for monitoring/statistical
- It is hard to query for all reserved addresses
- It complicates life of implementors of future tools to manage
reservations as these tools would have to handle complex data structures
within a particular data field

So, I'd really like to see the host reservations in a separate table
as depicted here
The only reservation that may remain in the Hosts table is an IPv4
reservation (or NULL).Technically, it could also be moved to the
reservations table but since IPv4 address is only 4 bytes long it should
be rather stored in a column of a different type than the IPv6 address
or prefix. So, for performance reasons it doesn't make much sense to
store IPV4 address as a text in the same column as IPv6 address.

I also point out that the entry in the table with reservations doesn't
make sense on its own (Identifying Relationship) so there should
probably be triggers on the table with hosts, which removes the
reservations from the reservations table before the DELETE operation on
the Hosts table. This way, removal of the host entry would trigger
removal of all reservations.

In addition to the fields that Tomek listed for the options table, I
added one called "persistent" which carries a boolean value and
identifies an option as a one that should always be returned to the
client or the one that is returned only if requested (using ORO or PRL).

I am working on documenting the design for host reservation and I am
planning to use the
(or similar) database layout for this design, unless I hear objections.


On 22/09/14 15:50, Tomek Mrugalski wrote:
> Here's the DHCP Hackaton summary. These were the discussions that I
> participated in. Couple times several things were happening in parallel,
> so I'll let others to fill in the blanks.
> We had a very interesting DHCP hackaton last week co-organized by ISC
> and a Department of Computer Communications of ETI faculty of Gdansk
> University of Technology. There were 14 participants with varied
> background - undergrad students, lecturers, professors, network
> administrators and software developers. Two participants had to drive
> from almost the other end of the country to get to Gdansk. Thanks for
> your dedication. We were even visited by a vice-dean of the faculty!
> This was a two day event where DHCP developers, users, students and
> other interested people had a chance to talk, discuss bugs, fix them,
> write new code and design new features together.
> Hackaton had started with slightly over 2 hours long participants
> introduction, DHCP introduction (about the protocol, but also its
> implementations Kea and Dibbler) and overview of ISC engineering
> processes, including trac and github repository.
> Several documentation issues were reported and fixed (thanks to Adam
> Osuchowski and Piotr Strzyzewski for reports and fixes). Couple logging
> issues were reported. Fortunately, ISC was aware of them and in two
> cases we already had branches with proposed fixes that are currently
> undergoing reviews. It's always useful to get a hands on confirmation if
> a given solution is working. Especially when it doesn't :) Fortunately,
> the code was promptly fixed.
> We started using Kea repository on github and managed to merge our first
> pull request! Yay!
> We had a discussion about implementing minimalistic server and client
> for embedded environments, based on libdhcp++ from Kea. Dominik Zeromski
> and Tomek Kleinschmidt reported that they managed to trim down libdhcp++
> to 400kb, but with additional steps it is possible to further trim it
> down to 200kb. One issue was dependency on libutil library. That
> dependency is rather minor - only OutputBuffer class is really needed.
> We discussed various possible solutions - moving OutputBuffer to
> libdhcp++, splitting OutputBuffer into its own small library etc.
> Another roadblock on making libdhcp++ is exception handling code.
> Exceptions cause compiler to generate non-trivial amount of additional
> output binary code. We had a discussion about it and the most feasible
> way forward would be to split isc_throw macro into isc_throw and
> isc_throw_fatal. The latter would be replaced by calling assert and
> terminating application. The former would be silently ignored. Those new
> macros will be used when compiling for embedded environment. There will
> be an additional target for that (libdhcp++-minimal?). Obviously, the
> regular target will remain and exceptions will be used in normal
> environments (both isc_throw and isc_throw_fatal will simply throw
> exceptions).
> Second day was focused on host reservation design. This is an upcoming
> feature for 0.9.1. After a lengthy discussion, we came up with the
> following design:
> 1. Host reservations will be defined in config file (the default mode),
> but it will possible to also store them in a database.
> 2. Proposed table layout is as follows:
> Hosts table
> - identifier (varchar) - container for client identifier. For now, it
>   will be either MAC or DUID, but soon will be adding additional
>   identifiers, like subscriber-id, remote-id etc.
> - identifier type - specifies type used in identifier (this will
>   essentially be an enum with values of TYPE_MAC, TYPE_DUID etc.)
> - ip4 - an IPv4 address (this field may be null)
> - ip6 - list of IPv6 addresses (may be null, one or several addresses)
> - ip6prefix - list of IPv6 prefixes (null, one or several prefixes)
> - hostname (optional)
> - clientclass (this field will be added at a later stage)
> - host-id (a primary key)
> We need an unified way to reserve hosts by MAC, DUID, subscriber-id,
> remote-id, docsis options etc. The design above allows that. It is also
> easily extensible. If we decide to add a support for a new identifier,
> it's only a matter of adding one more enum (database wise, it will be
> more involved DHCP engine wise, but that complexity is an independent
> problem).
> ip4 address is optional as it may be IPv6 only reservation or maybe
> mac+hostname reservation.
> ip6 and ip6prefix fields may have zero (IPv4 only reservation or maybe
> mac+hostname only), one (typical reservation) or more than one address
> or prefix. DHCPv6 spec allows for a single client to handle more than
> one address and more than one prefix (client sends several instances of
> IA_NA and/or IA_PD options)
> The hostname is obviously optional.
> Clientclass is an optional field. It allows to specify that a given host
> belongs to a client class. That is important for at least two reasons.
> First, we may get per host client classification granularity. Second, if
> there is 100 hosts that require an option, we may want to add one option
> and say that it's for class X, rather than adding 100 instances of
> exactly the same option. It seems that client class will be implemented
> at a later stage (or perhaps implemented along the rest of the fields,
> but will be ignored for now).
> host-id will be required to be able to refer for specific host instance.
> It is not clear yet if we will need it for memfile backend.
> 3. Multiple host reservations
> Since we'll be able to reserve host based on mac, DUID and others
> parameters, we can't enforce them to be unique. Therefore it may be
> possible that due to misconfiguration there will be several reservations
> that will match. It would be bad if the server randomly used one or the
> other. We need to analyze 3 cases here: 0 hosts (carry on as usual), 1
> host (use that reservation), more than one (log error).
> The easiest way would be to do SELECT with LIMIT 2. We're not really
> interested in how many are there, just if there is anything more than
> one. We will then log warning and probably use whatever MySQL or
> Postgres returned as first host.
> 4. Options in DB
> We will also need a table for storing options in database. The immediate
> need is to allow storing options for a specific host, but soon after
> that we'll need options associated with a given class or with a given
> subnet (once we get the capability to store subnets in a DB).
> The proposed table looks as follows:
> Options table
> - option code (uint16, option code in DHCPv4 or DHCPv6 sense)
> - value string (options have diverse formats. using string will allow
>   values to be stored in an uniform way, e.g. for format uint32, bool,
>   string we'll store something like 1234,true,"hello world".
> - option space (see explanation below)
> - host-id - foreign key (a reference to hosts table)
> - client-class string (explanation below)
> - subnet-id uint32_t
> option space will be the same as we have in Kea now. There are 2 defined
> out of box: "dhcp4" and "dhcp6", but we can easily define vendor option
> namespaces. Also, for custom options it is now possible to define that
> option A belongs to "dhcp4" and contains option space "foo" and options
> B and C belong to option space "foo".
> It is understood and agreed that for any custom or vendor options, there
> will be definitions needed. Those will be kept in the config file.
> Client-class will be an optional field that will specify that a given
> option is for hosts that belong to that class.
> Subnet-id performs similar selective role for subnets.
> Host-id, client-cass and subnet-id can be considered selectors, i.e.
> properties that will be used to find appropriate options. There was a
> discussion on how the query should look like:
> host-id=X AND client-class=Y AND subnet-id=X
> host-id=X OR client-class=Y OR subnet-id=X
> There was also a brief discussion how MySQL treats NULLs, as not all
> values will be specified at all times. This particular topic will need
> further thoughts.
> 5. Hosts cached or not
> In theory, hosts can be kept either in memory or in database only. In
> memory approach, hosts information is read from the DB and kept in
> memory. This is good from performance perspective, but requires
> administrative action (admin needs to tell the server that hosts
> reservations changed) every time hosts information has changed in the
> DB. On the other hand, it is possible to not keep anything in memory and
> check with the DB every time. This will be less performant (extra query
> for each packet processing), but will have the benefit of being always
> up to date (any changes to the DB will be immediately picked up).
> Adam proposed a mixed mode, where the server has a cache that is
> gradually filled with data that is read from the DB. This is an
> interesting proposal as it gives fast startup and ultimately high
> performance. We will likely implement it one day, but it is probably not
> feasible in 0.9.1 timeframe. Nevertheless, we need to make the design
> extensible to allow adding it at some later time.
> 6. Required and proposed commands
> If we implement hosts kept in the memory strategy, then we'll need at
> least the following commands:
> - delete-host - deletes one host. It would be useful to use
> identifier/identifier type as parameters for consistency
> - flush-case - drops existing list of hosts from the in-memory cache
> - reload-hosts - drop all hosts from memory and reload from DB
> We also talked about reloading hosts from DB. In typical case, there
> will be thousands of mostly static host reservations, with a handful new
> ones being added or deleted. Naive approach of dropping all from memory
> and reloading will cause the server to reload mostly the same with
> minimal delta, which will be a performance problem. It may be useful to
> has one extra command: add-host. Full implications are not yet fully
> understood, though. The behavior must be well defined: is add-host only
> adds a host to in-memory cache and will be lost after restart? Or will
> it also insert the host into DB?
> 7. Feature request
> One missing feature is config file inclusion. Adam requested this
> capability as it is very commonly used. Kea doesn't have such capability
> yet. It is supported by ISC DHCP, though.
> 8. DHCPv6 Bulk leasequery in Dibbler
> Damian Manelski is working on bulk leasequery support in Dibbler.
> Dibbler server was extended with TCP sockets and bulk leasequery
> processing. There are bits and pieces still missing, though. Several
> bulk leasequery scenarios rely on options inserted by relay agants. In
> particular, on remote-id and relay-id options. ISC DHCP does not insert
> those options. Dibbler inserts only remote-id, but not relay-id. We
> managed to implement support for relay-id. Even though this is not
> directly related to Kea, it may still be useful as a testing tool, even
> before we start implementing bulk leasequery in Kea. Even as of today,
> we do have code for extracting options inserted by relays. Using
> remote-id or relay-id for testing its correctness may be useful.
> 9. MAC addresses support in DHCPv6
> Adam Kalmus started work on extracting MAC addresses from DUID. This is
> one of the tickets we planned for 0.9.1. This work is in its early
> stages, but the environment was set up and first lines of code were
> written. That's a good start. We're keeping our fingers crossed.
> No slides were shown during the event.
> Thanks to doc. Krzysztof Nowicki and Department of Computer
> Communications from Gdansk University of Technology for hosting us!
> Tomek Mrugalski
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