CMTS Forwarding Rules

General CMTS Forwarding
Data forwarding through the CMTS MUST be transparent bridging, network-layer forwarding (routing, IP
switching), or a combination of the two. The CMTS MUST provide IP (v4 and v6) connectivity between hosts
attached to cable modems, and do so in a way that meets the expectations of Ethernet-attached customer equipment.
For IPv6, the CMTS is not required to deliver traffic between hosts attached to different cable modems using linklocal
scope addresses.
The CMTS SHOULD replicate broadcast packets on all primary-capable Downstream Channels of a MAC Domain.
A CMTS may provide a proxy ARP service to avoid forwarding ARP (see [DOCSIS SECv3.0]) messages. 245 A
proxy ARP service on the CMTS reduces the possibility of potential denial of service attacks because the ARP
messages are not forwarded to hosts (untrusted entities). The implementation of the proxy ARP service is vendor
dependent.
For IPv6 the CMTS SHOULD either forward Neighbor Discovery (ND) packets [RFC 2461] on primary-capable
Downstream Channels of the MAC domain or facilitate ND-based services (also known as "proxy ND service") to
avoid forwarding ND messages. A proxy ND service on the CMTS reduces the possibility of potential denial of
service attacks because the ND messages are not forwarded to hosts (untrusted entities). The implementation of the

proxy ND service is vendor dependent.

Because the CMTS is not required to track MLD messages forwarded by CMs that are not MDF-enabled, the
CMTS may have incomplete knowledge of solicited node multicast addresses in use on the CMTS RFI at any time.
For example, an initializing CM could send two MLD membership reports for Solicited Node Multicast Groups
prior to being considered MDF-enabled by the CMTS. Additionally, MDF-disabled CMs or MDF-incapable CMs
may indicate support for IPv6, and as such may operate in IPv6 provisioning mode and/or may support IPv6
eSAFEs/CPEs. When the CMTS needs to transmit a packet addressed to a solicited node multicast address, and if
the CMTS does not know which primary downstream(s) to use, the CMTS MUST transmit the packet on every
primary capable downstream that is in the link-local scope of the packet. 246
A CMTS that supports routing of IPv6 traffic is not required to support advertisement of not on-link ([RFC 4861])
prefix assignment, which eliminates the use of ND for non-link-local scope address resolution.247
If the CMTS transparently bridges data, the CMTS MUST pad out the PDU and recompute the CRC for PDUs less
than 64 bytes to be forwarded from the upstream RFI. The CMTS and CM MAY support the forwarding of other
network layer protocols other than IP. If the forwarding of other network layer protocols is supported, the ability to
restrict the network layer to IPv4 and IPv6 MUST be supported by the CMTS.
At the CMTS, if link-layer forwarding is used, then it MUST conform to the following general [IEEE 802.1D]
rules:
• The CMTS MUST NOT duplicate link-layer frames.
• The CMTS MUST deliver link-layer frames on a given Service Flow, Section 6.1.2.3, in the order they are
received subject to the skew requirements in Section 8.2.3.2.
The address-learning and -aging mechanisms used are vendor-dependent.
If network-layer forwarding is used, then the CMTS SHOULD conform to IETF Router Requirements [RFC 1812]
with respect to its CMTS-RFI and CMTS-NSI interfaces. 

A bridging CMTS applies appropriate DSID labeling and forwarding of the packets received from the NSI interface
according to the rules in Section 9.1.1.2, DSID labeling and pre-registration multicast. The NSI-side router
generates the Router Advertisement (RA) message [RFC 2461] to the RFI interface for appropriate DSID labeling
and forwarding by the bridging CMTS.
A bridging CMTS MUST forward the packets destined to the well-known IPv6 MAC addresses (see Annex A) to
the NSI-side router for processing.
A routing CMTS applies appropriate DSID labeling and forwarding of the packets received from the NSI interface
according to the rules in Section 9.1.1.2, DSID labeling and pre-registration multicast. When the routing CMTS
forwards well-known IPv6 multicast packets from the NSI to RFI, the CMTS terminates and applies appropriate
processing for these packets. The routing CMTS generates the RA message [RFC 2461] for appropriate DSID
labeling and forwarding to the RF interface.
The CMTS MUST discard IPv6 RA messages received on its RF interface.