An In-Band Data Communication Network For the MPLS Transport Profile
Extracted elements (18)
The G-ACh-based DCN is intended for use only where the MPLS-TP network is not capable of encapsulating or delivering native DCN messages, accommodating nodes without direct IP encapsulation support on line-side interfaces or out-of-fiber data communication network access.
The G-ACh header uses PPP protocol identifiers (RFC 1661, RFC 3818) for the PID field to indicate which layer 3 PDU (IPv4, IPv6, or OSI) is contained in the payload, enabling delivery to the appropriate layer 3 process.
Two separate channels (MCC and SCC) are provided rather than a single channel to facilitate easy demultiplexing of control and management traffic, enable separate rate limiters and traffic-shaping functions, and allow separate or overlapping address spaces in management and control planes.
The G-ACh used as described creates a 'data channel associated CCh' (type 6) when associated with an MPLS-TP section, and an 'in-band CCh' (type 7) when associated with an MPLS-TP LSP or PW spanning one or more hops, as classified in the NM-REQ appendix.
There is no requirement for MPLS-TP devices to support IP or OSI forwarding in the fast path; a node MAY apply layer 3 forwarding in the slow or fast path and MAY discard messages it cannot forward. Protocols using the DCN should make no assumptions about forwarding capabilities unless determined a priori or via routing protocol.
According to RFC 5586, a receiver MUST NOT forward a GAL packet based on the GAL label as is normally the case for MPLS packets; if the GAL appears at the bottom of the label stack, it MUST be processed locally.
Protocols that operate over the MCN or SCN are REQUIRED to include adequate security mechanisms, and implementations MUST allow operators to configure the use of those mechanisms.
Provider Edge nodes wishing to set up PWs using a signaling protocol with remote adjacencies (such as LDP) in the absence of an IP-based control plane MUST first set up an LSP tunnel across the MPLS-TP network.
The ACH MUST NOT include the ACH TLV Header, meaning that no ACH TLVs can be included in MCC/SCC messages sent on the G-ACh.
The DCN channel MUST NOT be used to transport user traffic and SHALL only be used to carry management or control plane messages.
When a receiver receives a G-ACh packet with Channel Type set to MCC or SCC and the PID value is unknown, the receiver SHALL silently discard the received packet, MAY increment a discard counter, and MAY log an event.
When the G-ACh is associated with an MPLS section or LSP, the TTL field of the GAL MUST be set to 1, and the S-bit of the GAL MUST be set to 1.
The Channel Type field in the ACH header determines whether the message is an MCC or SCC message. The PID field identifies the payload layer 3 protocol (IPv4, IPv6, OSI) using PPP protocol identifiers from RFC 1661 and RFC 3818.
The Generic Associated Channel Label (GAL) is added to MCC/SCC messages when the G-ACh is associated with an MPLS section (GAL only) or an LSP (LSP label pushed on top of GAL), as defined in RFC 5586.
The Management Communication Channel (MCC) and Signaling Communication Channel (SCC) are two independent channels provided by the G-ACh to build the MCN and SCN respectively, enabling separate address spaces and duplicate protocol instances in management and control planes.
IANA allocated two Channel Types in the PW Associated Channel Type registry (originally defined in RFC 4446, updated by RFC 5586): 0x0001 for Management Communication Channel (MCC) and 0x0002 for Signaling Communication Channel (SCC).
The G-ACh provides a virtual link between MPLS-TP nodes and might be used to induce many forms of security attack. The MPLS data plane does not include any security mechanisms of its own; protocols using the DCN must apply their own security mechanisms.
The G-ACh MCC/SCC packet consists of a 4-byte ACH header (0001|Version|Reserved|Channel Type), followed by a 2-byte Protocol Identifier (PID) field indicating the payload protocol type using PPP protocol identifiers, followed by the MCC/SCC message payload.