Appendix A. Topology Discovery Use Case
This section preserves the RFC text for DHCPv4 over DHCPv6 with Relay Agent support, including relay behavior, topology-discovery diagrams, deployment notes, security considerations, and IANA status.
Appendix A. Example Use Case: Topology Discovery for IPv4-Only Radio
Unit in 3GPP RAN with Switched Fronthaul
In 3GPP mobile network architecture, the User Equipment (UE) is
connected via a Radio Access Network (RAN). RAN is built up with
Baseband Units (BBUs) and Radio Units (RUs). A radio Fronthaul (FH)
network connects RUs and BBUs. Each RU and BBU is an IP host, and
they may support IPv4 only, IPv6 only, or both, depending on the
vendor and the model. Each RU is unique as it is tied to a set of
antennas, and each antenna is serving a specific Cell and Sector.
Each RU is configured by the BBU depending on the Cell and Sectors it
serves. However, that dependency is only specified by the cabling
between RUs and antennas. BBUs can be cabled to RUs directly or via
a Layer 2 switched network.
+--------+
| RU2 +-----+
| | |
+--------+ |
|
+--------+ |
| RU3 | |
| +--+ | +-----------+
+--------+ | +--+ |
+-----+ Baseband |
| |
+--------+ +-----+ Unit |
| RU4 +--+ +--+ |
| | | +-----------+
+--------+ |
|
+--------+ |
| RU2 +-----+
| |
+--------+
Figure 5: 3GPP RAN Where RUs Are Cabled Directly to BBUs
In Figure 5, the BBU is directly cabled to a set of RUs, and the BBU
can recognize the relationship between RUs and Cell/Sectors based on
the cabling between the RUs and antennas.
When BBUs and RUs are connected via a Layer 2 switched network, the
added level of complexity requires the BBUs to have a deeper
knowledge of the topology in order to properly configure the RUs,
involving knowledge of all the cabling in the switched network.
Examples for switched networks are shown in Section 3 of [RFC7969]
and demonstrate the different levels of complexity. An example of a
FH is depicted in Figure 6.
+--------+
| RU1 | P1 +-+------+ | |
| +--------+ | L2RA | | +----+------+ |
+--------+ | +------+ | | | L3RA | |
| L2 | +--+ +------+ |
+--------+ P2 | Switch | | | | |
| RU2 +--------+ #1 +-----+ | Router +----+
| | +--------+ | +-----------+ | +---------+
+--------+ | | | |
| +--+ DHCP |
+--------+ | | | Server |
| RU3 | P1 +-+------+ | | | #1 |
| +--------+ | L2RA | | +-----------+ | +---------+
+--------+ | +------+ | | | |
| L2 | +--+ Baseband | |
+--------+ P2 | Switch | | | Unit | |
| RU4 +--------+ #2 +-----+ | +----+
| | +--------+ | +-----------+ |
+--------+ | |
Figure 6: 3GPP RAN with Layer 2 Switched Fronthaul Example
If IPv6 is used and all RUs are capable of DHCPv6 in Figure 6, DHCP
topology knowledge can be used for solving the RU configuration
problem. Such solution would use the topology discovery mechanisms
described in Section 3.2 of [RFC7969].
If RUs are capable of IPv4 only but implement a DHCP 4o6 client
according to [RFC7341], the same topology discovery mechanisms are
applicable.
If RUs are capable of IPv4 only and cannot implement a DHCP 4o6
client according to [RFC7341], the topology discovery mechanisms
described in Section 3.2 of [RFC7969] can be used by introducing
4o6RA in the switches as described in this document.