Chapter 23: IPv6
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
716
MDF
Multicast DSID-based Forwarding (MDF) is a key component of DOCSIS 3.0 multicast functionality. The C4/c CMTS supports
MDF-disabled mode and GMAC-Promiscuous mode, as described in the DOCSIS 3.0 MULPI specification. To change the
configuration for MDF you must first shut down the cable-mac. Execute the following command in order to enable MDF
GMAC-Promiscuous mode for the specified cable-mac:
configure interface cable-mac <mac> cable mcast-fwd-by-dsid
GMAC Explicit Mode
The C4/c CMTS supports DOCSIS 2.0+ IPv6 modems that are Multicast DSID-based Forwarding (MDF) incapable, as well as
modems that are MDF Enabled GMAC Explicit.
IPv6 Packet Structure
The structure of an IPv6 packet header is defined in RFC-2460: IPv6 Protocol Specification. The header has a fixed length of
40 octets containing the following fields:
Version (4 bits): IP protocol version. Always has the value 6.
Traffic Class (8 bits): Used to prioritize IPv6 packets. Traffic Class has the same definition as the IPv4 TOS/DSCP field.
Flow Label (20 bits): Used to identify a flow (i.e., a set of packets that require identical routing treatment).
Payload Length (2 octets): Length of the packet in octets following the header. In IPv6, each packet is divided into
exactly two portions: the header and the payload. Since the length of the Payload Length field is 2 octets, the
maximum length of an ordinary IPv6 packet is (216-1 + 40) octets (i.e., 65,575 octets). Note that RFC-2675 defines a
mechanism for sending larger IPv6 packets, called "Jumbograms". The IPv6 feature on the C4/c CMTS does not include
support for Jumbograms.
Next Header (1 octet): A protocol number or a value to indicate an extension header follows the fixed header. This
behaves somewhat similarly to the Protocol Type field in IPv4, in that it may contain a value corresponding to a layer 4
protocol such as UDP or TCP. However, it may also contain a value that tells the recipient/router that, immediately
following the fixed header, the packet contains an Extension Header. In that case the value in the Next Header field
identifies the Extension Header that follows the IPv6 header. The complete list of allowed values is obtainable at:
http://www.iana.org/assignments/protocol-numbers.