The PPP DECnet Phase IV Control Protocol (DNCP)
obsoleted by
- rfc-1762 — The PPP DECnet Phase IV Control Protocol (DNCP)
Extracted elements (20)
Because PPP (like Ethernet) does not guarantee delivery, whereas DDCMP does, the routing update retransmit interval should be configurable per interface. Digital's default for DDCMP is 600 seconds and for Ethernet is 10 seconds; PPP interfaces should allow tuning based on actual link error rates.
The VAX byte ordering (least significant byte first) was chosen for the Length field to be consistent with DNA Phase IV Routing over Ethernet. The field itself is necessary to disambiguate optional PPP padding octets from real information.
Two approaches to running DNA Phase IV Routing over serial links exist: treating the link as Ethernet (the approach adopted here, used by several router vendors), versus Digital's own DDCMP-based approach with different control messages. This document adopts the Ethernet-emulation approach for PPP.
The format of DNA Phase IV Routing packets over PPP is the same as the format used over Ethernet, without the Ethernet header, Pad, and FCS fields. This approach treats the serial link as an Ethernet rather than using DDCMP with different control messages.
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 DNA Phase IV Routing Control Protocol must reach the Opened state.
DNCP has no Configuration Options. There are no option types defined for this NCP.
DNCP packets MUST 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 are required to discard all routing updates received over a PPP link. Routers are not required to send routing updates if the remote node connected via the PPP link is an endnode.
It is recommended that the time between routing updates be user configurable per PPP interface, because the optimal update interval depends on the error rate of the underlying data link.
Only DNCP Codes 1 through 7 (Configure-Request, Configure-Ack, Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack, and Code-Reject) are used. Other Codes should be treated as unrecognized and should result in Code-Rejects.
The maximum length of a DNA Phase IV Routing packet transmitted over a PPP link is the maximum length of the PPP Information field minus 2 octets (for the Length field).
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. They also periodically retransmit complete Level 1 and Level 2 distance vectors to guard against data corruption and packet loss.
Hello timer and Listen timer should be set to 15 and 45 seconds respectively, following the recommendations for broadcast links.
The DNA Phase IV Routing Control Protocol (DNCP) is the NCP for establishing and configuring Digital's DNA Phase IV Routing protocol over PPP. It uses the same packet exchange mechanism as the Link Control Protocol (LCP) and covers only DNA Phase IV Routing messages (data and control), not MOP, LAT, or other DNA protocols.
The type of a node (endnode versus routing node) can be determined from the hello messages received from it over the PPP link.
Security issues are not discussed in this memo. No authentication, integrity, or confidentiality mechanisms are defined for DNCP or for DNA Phase IV Routing packets carried over PPP.
DNA Phase IV Routing data packets use PPP Protocol field hex 0027. The Information field contains a 2-octet Length field (LSB first, VAX byte ordering) followed by the DNA Phase IV Routing packet. The Length field counts octets in the DNA packet only, not itself.
DNCP packets are encapsulated in the PPP Data Link Layer frame Information field with Protocol field value hex 8027 (DNA Phase IV Control Protocol). Exactly one DNCP packet appears per frame.
The octet-count (Length) field is two octets stored in VAX byte ordering (least significant byte first). It precedes the DNA Phase IV Routing payload and is needed to disambiguate optional padding octets from real information.