The PPP DECnet Phase IV Control Protocol (DNCP)

Because Digital's serial links use DDCMP which guarantees packet delivery, the recommended default routing retransmit time is 600 seconds, compared to 10 seconds on Ethernet where delivery is not guaranteed. For PPP, the retransmit interval should be user-configurable per interface based on the underlying error rate.

The 2-octet length field is stored in VAX byte ordering (little-endian) to be consistent with DNA Phase IV Routing over Ethernet. It is required to disambiguate optional padding octets from real information.

The Hello timer and Listen timer for DNA Phase IV Routing over PPP should be set according to broadcast link recommendations (15 and 45 seconds, respectively), since the Ethernet-emulation approach is used.

Two approaches exist for running DNA Phase IV Routing over serial lines: treating the link as Ethernet (used by several router vendors) or using DDCMP with slightly different control messages (Digital's own approach). This document adopts the Ethernet-emulation approach for PPP.

DNA Phase IV Routing packets transmitted over PPP use the same format as Ethernet, but without the Ethernet header, Pad, and FCS fields. This approach treats the serial link as an Ethernet rather than using DDCMP framing.

An implementation SHOULD be prepared to wait for Authentication and Link Quality Determination to finish before timing out waiting for a Configure-Ack or other response. It is suggested that an implementation give up only after user intervention or a configurable amount of time.

Before any DNA Phase IV Routing packets may be communicated, PPP MUST reach the Network-Layer Protocol phase and the DNCP MUST reach the Opened state.

DNCP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. DNCP packets received before this phase is reached SHOULD be silently discarded.

Endnodes MUST discard all routing updates received over a PPP link.

Routers MAY omit sending routing updates if the remote node connected via the PPP link is an endnode. The node type (endnode vs. routing) is determined from received hello messages.

DNA Phase IV Routing nodes propagate topology changes immediately via Level 1 and Level 2 Routing messages with a 1-second minimum delay between updates, and periodically retransmit complete distance vectors to guard against memory corruption and packet loss.

DNCP (DNA Phase IV Routing Control Protocol) is the NCP responsible for configuring, enabling, and disabling DNA Phase IV Routing on both ends of a PPP link. It uses the same packet exchange mechanism as LCP and is identified by PPP Protocol field value hex 8027.

snmp

DNCP has no Configuration Options.

DNCP uses only Codes 1 through 7: Configure-Request, Configure-Ack, Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack, and Code-Reject. Other Codes are treated as unrecognized and result in Code-Rejects.

The maximum length of a DNA Phase IV Routing packet over PPP equals the maximum PPP Information field length minus 2 octets for the Length field.

Security issues are not discussed in this memo.

security

DNA Phase IV Routing packets are encapsulated in the PPP Information field (Protocol hex 0027) preceded by a 2-octet length field stored in VAX byte ordering (least significant byte first). The length field counts the octets in the DNA Phase IV Routing packet only.