Encapsulation Methods for Transport of Ethernet over MPLS Networks
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Extracted elements (28)
Mode #4 (Tag on A, No Tag on C) is not recommended when operating with PW since it may not preserve the user's PRI bits. The recommended alternative is to use mode #2 with tag2=0 (NULL VLAN) on the PW and mode #3 at the other end, which preserves PRI bits.
The control word is needed when ECMP is active because ECMP implementations may examine the first nibble after the MPLS label stack to detect IP packets. A source MAC address beginning with 0x4 or 0x6 could be mistaken for IPv4/IPv6, causing packets on a given PW to follow different paths and increasing out-of-order delivery or causing OAM packets to diverge from traffic.
The control word is optional for Ethernet PWs because ECMP may not be present, and strict frame sequencing may not be required. Early Ethernet PW implementations were deployed without a control word, so requiring it would break backwards compatibility; hence future implementations must support operation both with and without it.
In tagged mode, if the VLAN identifier is modified by the egress PE, the Ethernet spanning tree protocol might fail to work properly. If this is significant, the VLAN identifier MUST be selected to match on the attachment circuits at both ends of the PW.
PAUSE frames received on a local Ethernet port SHOULD cause the PE to buffer or discard further Ethernet frames for that port. The implementation MUST provide a mechanism for terminating PAUSE frames locally at the PE; PAUSE frames MUST NOT be forwarded across the PW.
The Ethernet PW only supports homogeneous Ethernet frame types across the PW; both ends must be either tagged or untagged. Heterogeneous frame type support via NSP functionality is outside the scope of this document.
A PE MUST support the ability to carry the Ethernet PW as a best-effort service over the MPLS PSN. PRI bits are kept transparent between PE devices, regardless of the QoS support of the PSN.
At the ingress of the PW, the native Ethernet frame error processing mechanisms MUST be enabled. If a PE device receives an Ethernet frame containing hardware-level CRC errors, framing errors, or a runt condition, the frame MUST be discarded on input.
Future implementations MUST be able to send and receive Ethernet PW frames without the control word present, to ensure backwards compatibility with early deployments that do not include a control word.
IEEE 802.3x PAUSE frames MUST NOT be carried across the PW. PAUSE frames received on a local Ethernet port SHOULD cause the PE device to buffer or discard further Ethernet frames for that port until the PAUSE condition is cleared.
If an 802.1Q VLAN field is added at the PE, a default PRI setting of zero MUST be supported. A configured default value is recommended, or the value may be mapped from the QoS field of the PSN.
If fragmentation [FRAG] is not used and the ingress router determines that an encapsulated layer 2 PDU exceeds the MTU of the PSN tunnel, the PDU MUST be dropped.
If strict frame ordering is a requirement for a particular PW, the control word option enabling ordered delivery MUST be enabled. The PSN cannot in the general case be assumed to provide frame ordering.
If the PE detects a failure on the Ethernet physical port or the port is administratively disabled, it MUST send a PW status notification message for all PWs associated with that port.
In raw mode, service-delimiting tags are NEVER sent over the PW. If a service-delimiting tag is present when the frame is received from the attachment circuit, it MUST be stripped by the NSP before the frame is sent to the PW.
In tagged mode, every frame sent on the PW MUST have a service-delimiting VLAN tag. If the frame received from the attachment circuit does not have such a tag, the PE must prepend the frame with a dummy VLAN tag before sending it on the PW. This is the default and only REQUIRED mode.
The 802.1Q CoS to PSN CoS mapping principles adopted from 802.1Q Table 8-2 MUST be met when applying the set of PSN CoS based on user's PRI bits, ensuring proper priority ordering across available classes of service.
The egress PE MUST be aware of whether the ingress PE will send a control word over a specific PW. This awareness may be achieved by configuration of the PEs or by signaling as defined in [PWE3-CTRL].
The Ethernet PW MUST NOT be enabled unless it is known that the MTUs of the CE-PE links are the same at both ends of the PW. If an egress router receives an encapsulated PDU whose payload length exceeds the MTU of the destination layer 2 interface, the PDU MUST be dropped.
The Ethernet-specific Interface Parameter LDP Sub-TLV MUST be used to validate that the PEs and ingress/egress ports have the necessary capabilities to interoperate. If the ingress PE receives a Requested VLAN ID request it cannot fulfill and the VLAN ID does not match, the PW MUST not be enabled.
The MPLS PSN MUST be configured with an MTU large enough to transport a maximum-sized Ethernet frame encapsulated with a control word, a pseudowire demultiplexer, and a tunnel encapsulation — likely 8 or more bytes greater than the largest frame size with MPLS tunneling.
When raw mode is used, the PE that receives a frame MUST not rewrite or remove any tags that are already present in the frame; it may or may not need to add a service-delimiting tag before transmitting on the attachment circuit.
An Ethernet PW operates in one of two modes: 'raw mode' or 'tagged mode'. In tagged mode, each frame MUST contain at least one 802.1Q VLAN tag meaningful to the NSPs at both PW termination points. In raw mode, a frame MAY contain an 802.1Q VLAN tag, but it passes transparently through the NSPs.
Ethernet Tagged Mode (PW type 0x0004) uses service-delimiting tags to map input Ethernet frames to respective PWs. Ethernet Raw Mode (PW type 0x0005) transmits all Ethernet frames received on the attachment circuit of PE1 to PE2 on a single PW.
The Ethernet-specific Interface Parameter LDP Sub-TLV (0x06, Requested VLAN ID Sub-TLV) is an optional 16-bit value indicating the requested VLAN ID. It MUST be used by a PE that is incapable of rewriting the 802.1Q Ethernet VLAN tag on output, and is applicable only to PW type 0x0004.
The PWE3 Ethernet PDU consists of the Destination Address, Source Address, Length/Type, MAC Client Data, and padding extracted from a MAC frame as a concatenated octet sequence in their original order. The preamble and FCS are stripped at ingress and regenerated at egress.
The Ethernet pseudowire is subject to all general security considerations from RFC 3985 and [PWE3-CTRL]. The security of the pseudowire is only as good as the security of the underlying MPLS PSN, which can be secured by various methods described in [MPLS-ARCH]. Security via MAC address access control is out of scope.
The Ethernet PW control word is a 32-bit field: the first 4 bits MUST be set to 0 (indicating PW data), the next 12 bits are reserved (MUST be set to 0 on transmit, MUST be ignored on receipt), and the final 16 bits carry a sequence number for ordered frame delivery.