Nine DHCP options that are particularly useful for VoIP and UC

Network engineer defining DHCP options in a server room - TeleDynamics blog

DHCP is best known as the protocol that automatically assigns IP addresses to hosts that connect to a network. It is one of those protocols that we don’t often think about because it functions in the background — users are completely oblivious to its operation, just enjoying its convenience and flexibility.

However, DHCP is about more than automatic address allocation, especially for VoIP and UC devices such as IP phones and video terminals. In this article, we explore the operation of DHCP and the specialized options it provides for VoIP and UC applications, some of which you might not know about!

Brief review of DHCP

The Dynamic Host Configuration Protocol (DHCP) automatically assigns network parameters to a host connecting to an IP-based network. Network parameters can include an IP address, subnet mask, default gateway, and DNS server. If you’ve ever set up a PC to automatically assign these elements, you know what I’m talking about (though most PCs today are designed to use DHCP “out of the box”).

DHCP operates based on the client-server model. Within the network, a DHCP server is set up and deployed. Any network device that has been configured to acquire its network settings from a DHCP server will seek out such a server on the network.

Obtaining network parameters from a DHCP server involves exchanging DHCP messages between client and server. The following diagram shows these messages and describes this process.

DHCP server to client 4-way exchange diagram - TeleDynamics blog

Step 1: When a DHCP client is activated, it sends out a DHCP Discover message. As its name suggests, the Discover message is used to find a DHCP server on the local network segment. It also informs any DHCP server that sees it of the specific parameters the client is requesting. Since the client doesn’t know the addresses of any DHCP servers that may be listening, this message is broadcast to all devices on the local network segment.
Step 2: A DHCP server that receives the Discover message responds with a DHCP Offer message. This message contains the parameters the DHCP server offers based on what the client asked for in the Discover message.
Step 3: The client examines the offered parameters and responds with a DHCP Request message, essentially approving and requesting that those parameters be assigned to it.
Step 4: The DHCP server receives the Request and responds with a DHCP Acknowledgement (Ack). This is simply an acknowledgment that informs the DHCP client that the parameters have been attributed to the client.

Once this four-step process is complete, the DHCP client automatically configures itself with the parameters sent by the DHCP server and then establishes network connectivity.

DHCP options

Each parameter that you can configure using DHCP is called an option. The IP address, subnet mask, DNS server, and default gateway are all DHCP options the DHCP provides. However, these fundamental parameters needed to establish connectivity are only the most basic ones the protocol can set up — DHCP was designed to configure hundreds of options for network hosts automatically.

Each option is assigned an 8-bit number, so you can have up to 256 different options. The Internet Assigned Numbers Authority (IANA) has standardized and published these options and their numerical values.

DHCP option use cases

For basic workstation and PC connectivity, you only need a little from DHCP that goes beyond the fundamental parameters required to establish network connectivity.

However, VoIP and UC systems may use additional DHCP options to better coordinate and automate their operation over the network. Here are some options that are particularly useful for VoIP:

TFTP Server Name (Option 66): This option allows VoIP phones to obtain the address of a TFTP server, which you can use to download configuration files and firmware updates.
TFTP Server Address (Option 150): Like Option 66, Option 150 can provide one or more IP addresses of TFTP servers. Unlike Option 66, Option 150 is specifically designed to support a list of servers, offering redundancy and load balancing.
URL for HTTPS Server (Option 160): This is similar to options 66 and 150 but for HTTPS servers. It can also be used for secure downloading of configuration files.
Network Time Protocol Servers (Option 42): VoIP phones require accurate time for call logging, billing, and other functions. This option requests the IP address of an NTP server for time synchronization.
Vendor-Specific Information (Option 43): This is a flexible option for providing vendor-specific configuration parameters to VoIP phones. It can include information such as the voice VLAN ID, call server addresses, and other critical settings specific to a VoIP service provider or equipment vendor.
SIP Servers (Option 120): This option specifies the address of one or more SIP servers, which is particularly useful for VoIP systems based on the Session Initiation Protocol (SIP).

Many options above are also helpful for UC systems, which integrate voice, video, messaging, and other real-time communication services across multiple devices and applications.

There are additional DHCP options that can be particularly useful for UC beyond the standard set used for VoIP telephones. These options facilitate the seamless operation and management of UC systems by providing critical configuration information to client devices, such as these:

Vendor-Identifying Vendor-Specific Information (Option 125): This option allows clients to obtain more detailed vendor-specific information. For UC systems that often involve complex configurations and many services, this option can provide device-specific settings, policies, and features pertinent to the UC system’s vendor.
VLAN ID Configuration (Option 133): These options are used to assign VLAN IDs and VLAN priorities. Separating voice and video traffic onto dedicated VLANs can enhance UC system quality of service (QoS) and security.
Certificate Provisioning (Option 157): UC systems that use certificates for device authentication and secure communication can employ this option to specify the URL from which a client device can obtain its certificates.

These are just some of the available DHCP options instrumental in automating device configuration in a VoIP and UC environment. These capabilities enhance the user experience by providing seamless connectivity, optimal performance, and security across various communications channels.

The magic of DHCP is that much of its operation takes place without any intervention from the user—or even from the network admin. These options are typically preconfigured on many devices and software applications, so little tweaking is necessary.

That said, if a particular enterprise network uses its own DHCP server to serve VoIP phones and UC clients, then it may be necessary to configure those servers to deliver the correct option codes and parameters to the end devices.

A word about DHCPv6

IPv6 is gaining in adoption, so it would be amiss not to briefly discuss DHCPv6, the counterpart of DHCP that serves the IPv6 address space.

Although similar in functionality and philosophy to its IPv4 counterpart, DHCPv6 utilizes a different messaging and operational framework. While both protocols serve the primary purpose of assigning addresses and network configuration parameters to clients, the design and implementation of DHCPv6 reflect the architectural differences between IPv4 and IPv6 and lessons learned from years of deploying DHCPv4.

In the same way, DHCPv6’s options framework and its option codes are different. For one thing, the option code is a 16-bit number, allowing for over 65,000 code values. So far, the IANA has defined fewer than 150 codes, so there’s a lot of room for future codes, both standardized and proprietary.

Conclusion

DHCP is an indispensable feature used by networks to ensure the quick, easy, and seamless deployment and maintenance of network parameters on network devices such as VoIP telephones and UC clients.

By leveraging the benefits that these options provide, both VoIP and UC systems can dynamically provide client devices with all the settings and parameters they need, reducing manual configuration efforts and the potential for errors.