GRE Generic Associated
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Internet Area Working GroupINTAREARFC 5586 defines a Generic Associated Channel (G-ACh) mechanism for
MPLS paths that enables multiplexing of auxiliary traffic over such
paths along with the user traffic they carry. Such auxiliary traffic is
commonly used for Operations, Administration, and Maintenance protocols
that enable monitoring and management of the path. This document
describes the applicability of the G-ACh mechanism to Generic Routing
Encapsulation (GRE) tunnels.Generic Routing Encapsulation (GRE) is a
means of encapsulating one network layer protocol over another. This
practice is commonly referred to as "tunneling". GRE deployments
typically take the form of one or more logical point-to-point or
point-to-multipoint tunnels.As with other kinds of network links, a problem with logical tunnels
is how they are monitored and managed. In some cases no special
functionality is needed for this purpose beyond that provided by the
underlying network layer. In other cases, however, more robust
Operations, Administration, and Maintenance (OAM) functionality may be
required. For example, a tunnel may be carrying critical traffic that
is subject to a strict service level agreement, one that requires the
service provider to monitor the tunnel continuously for connectivity
faults and performance degradations.A mechanism to facilitate such OAM functionality, the Generic
Associated Channel (G-ACh), has been defined for tunnels based on
Multiprotocol Label Switching (MPLS) in . The
G-ACh provides an auxiliary "side-channel" associated with each tunnel
that can be used to carry a variety of OAM protocols over the tunnel so
that it can be monitored and managed. Examples of OAM protocols defined
for use over the G-ACh include Bidirectional Forwarding Detection (BFD)
and protocols for precision measurement of
packet loss, delay, and throughput .This document describes how the existing G-ACh mechanism can be used
for GRE tunnels. The scope of this document is limited to description
of the mechanism itself, and does not include discussions on
applicability of specific G-ACh protocols to GRE tunnels.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .As shown in , a GRE packet has the
following form, with the outermost encapsulation layer shown at the
top:The Delivery Header is the header for the underlying network layer
over which the GRE packet is transported. It may be an IPv4 header or
an IPv6 header. The GRE Header follows the
Delivery Header and identifies the format of the payload packet.When the payload is a G-ACh message, the GRE packet has the
following form:As specified in , the Associated Channel
Header identifies the format of the G-ACh payload that follows.The presence of the Associated Channel Header (ACH) is indicated by
the Protocol Type field in the GRE header (
and ). The value of the Protocol Type field
is an EtherType as used for next-layer protocol type identification in
Ethernet frames. The EtherType registry is maintained by the
Institute of Electrical and Electronics Engineers (IEEE) and also
documented in the IANA "Ethernet Numbers" registry. The IEEE has
allocated an EtherType for G-ACh packets as follows:EtherTypeMeaning(TBD)Generic Associated Channel (G-ACh) packet The format of the GRE header as documented in , when the payload is a G-ACh packet beginning with
an Associated Channel Header, is:IANA is requested to verify that the IANA "Ethernet Numbers" registry
reflects the IEEE allocation for the G-ACh EtherType.This document indicates how a Generic Associated Channel protocol
packet can be carried inside a GRE packet. This encapsulation itself
poses no security risks beyond those already documented for GRE and the
G-ACh. When a G-ACh protocol is used for Operations, Administration,
and Maintenance of GRE tunnels, the security considerations of that
protocol also apply.