TECHNOLOGY

Pseudowire

Encapsulation

The Pseudowire encapsulation layer provides the infrastructure necessary to adapt the specific payload that is being transported over the Pseudowire to the de-multiplexer layer, to carry the Pseudowire over the packet switched network (PSN). In the HSX 6000 platform, encapsulation of FR and Ethernet services is supported as defined by the PWE3 Martini Drafts. In all cases, the

HSX 6000 supports interpreting and then acting on the congestion indication information being carried on the Pseudowire control word when it is present.

For example, it can use the DE/CLP information for policing and marking the packet transmitted out of the attached circuit. However, generation and verification of the sequence number is not supported. In addition to the features listed below, the HSX 6000 supports customer frame checksum retention, which can be enabled or disabled per FR service port or Ethernet connection as follows:

FR one-to-one mode, as specified in: http://www.ietf.org/internet-drafts/draft-ietf-pwe3-frame-relay-05.txt.

ATM AAL5 SDU Frame format, as specified in: http://www.ietf.org/internet-drafts/draft-ietf-pwe3-atm-encap-09.txt. ATM cells of the AC are SARed at ingress to create an ATM AAL5 service data unit (SDU). The PWE3 frame encapsulates the SDU for transport.

ATM N-to-1 cell mode, as specified in: http://tools.ietf.org/wg/pwe3/draftietf-pwe3-atm-encap/, allows cells from one or more VCs to be cross connected to a single Pseudowire. Three encapsulation types are supported: atmCellNto1Vcc (default for single VPI/VCI and VCI range, atmCellNto1Vpc (default for single VPI and VPI range), and atm Transparent (default for port mode).

Ethernet (tagged mode), as specified in: http://www.ietf.org/internet-drafts/draft-ietf-pwe3-encap-10.txt. Each encapsulated Ethernet frame must contain an 802.1Q VLAN tag. An illustration of tagged mode Ethernet between a FR and Ethernet attachment circuits is provided in Figure 4-1.

Ethernet (raw mode), as specified in: http://www.ietf.org/internet-drafts/draft-ietf-pwe3-ethernet-encap-10.txt. Encapsulated Ethernet frames may contain 802.1Q VLAN tagging or be untagged.

To provide out-of-order frame detection, all control word formats contain a sequence number that can be optionally incremented for each frame of a Pseudowire circuit. Currently, the HSX 6000 does not implement order detection by using the sequence number.

The method of label distribution for the PWE3 circuits (and the supporting tunnels) may be dynamic or static. In a static configuration, the network provider configures the Pseudowire and tunnel labels to coordinated values at the PE nodes and possibly at nodes in the core of the network. In a dynamic configuration, signaling protocols distribute the labels for the Pseudowire and tunnels. Label distribution protocol (LDP) is used to signal Pseudowire labels. Each PE node is configured for LDP, including an LDP ID and one or more peer PE IP addresses with which label bindings may be exchanged. Each Pseudowire is configured with either:

32-bit PWid for PEOD-FEC based Pseudowires, or

AGI, SAII, and TAII fields for Genealized PWid FEC based Pseudowires, destination IP address, and encapsulation type. A label binding is distributed to a peer when:

An LDP session to the peer has been established.

The peer has advertised the destination IP address specified in the local Pseudowire configuration.

There are two types of label bindings, or forward equivalency classes (FEC), that are defined by the Pseudowire protocol:

The PWid FEC element, which requires provisioning at each PE node for each Pseudowire circuit, and

The generalized PWid FEC element which requires user provisioning at both nodes of the two PE nodes of the Pseudowire circuit.

However, signaling may be provisioned at a node to be “passive” and initiate signaling only when it receives a matching FEC from the PE node that is configured to be “active.”

The Pseudowire configuration for generic FEC is dual-ended. AGI, SAII, and TAII are Pseudowire identifiers for generic FEC. Ascii strings up to 80 characters may be used to configure AGI, SAII, and TAII. TAII configuration is optional, and, when not configured, the PE node initiates Pseudowire signaling only when it receives a matching FEC from a remote PE node. The PWid that is configured at each node is the same, which is how the PE nodes match label bindings from peers with local Pseudowire configuration. The HSX 6000 implements the PWid FEC element, which is a single-ended provisioning capability.

Each PWE3 is mapped to two unidirectional PWE3 labels in the data plane. A label for the PE node — to receive traffic from the MPLS core — is distributed by the PE node. So a complete Pseudowire data path requires that both Pseudowire terminating PE nodes distribute label bindings to complete a bidirectional path.