Dynamic Placement of Multi-Segment Pseudowires
updates
- rfc-6073 — Segmented Pseudowire
Extracted elements (29)
AII Type 2 addressing uses a Global ID, Prefix, and AC ID with a length-mask aggregation scheme analogous to IPv4 CIDR (mask 0–96 bits), explicitly to reduce the scaling burden on PW routing while allowing varying levels of summarization.
BGP advertises only the T-PE Global ID, Prefix, and prefix length — not a fully-qualified per-AC 96-bit address — because the ST-PE already knows the AC ID on the terminating T-PE by provisioning, eliminating the need for per-PW auto-discovery advertisements and reducing BGP state.
Interactions with IP PSNs using L2TPv3 (as described in RFC 6073 Section 8) are explicitly left for further study and are outside the scope of this document, which covers only MPLS PSNs using FEC 129.
This document updates RFC 6073 by correcting the Length field of PW Switching Point PE Sub-TLV Type 0x06 from 12 to 14. Implementations following the original RFC 6073 value will interoperate incorrectly with implementations following RFC 7267.
A PSN tunnel MUST be selected that can support at least the required class of service and has sufficient bandwidth. When an S/T-PE receives a PW Bandwidth TLV, once the PW next hop is selected, it MUST request the appropriate resources from the PSN.
BGP advertisements received with invalid prefix lengths (128–159 bits) MUST be rejected as having a bad packet format. The minimum valid Length is 96 bits except for the default route (0-length prefix).
Each S-PE MUST be assigned an address (at least a Global ID + Prefix in AII Type 2 format) that uniquely identifies it for the PW Switching Point PE TLV. An S-PE without such an address receiving a dynamic MS-PW Label Mapping message MUST return a Label Release with status code 'Resources Unavailable' (0x38).
If an S-PE cannot obtain sufficient PSN resources to set up a segment, a Label Release MUST be returned to the previous hop with status 'Bandwidth resources unavailable'. The T-PE receiving this status MUST also withdraw the Label Mapping for the opposite direction if it was already set up.
If the PW AII routing table lookup does not yield a next-hop IP address, the destination AII is unreachable, PW setup MUST fail, and a Label Release MUST be returned with status 'AII Unreachable'. All PEs capable of dynamic MS-PW path selection MUST build a PW AII routing table.
Implementations MUST interpret the AII type to determine the meaning of the address format irrespective of the number of segments. All segments of the PW MUST be signaled with the same AII type.
In the PW Switching Point PE TLV for dynamically placed MS-PWs using FEC 129, sub-TLV type 0x06 MUST be used, and its Length field MUST be set to 14 (correcting RFC 6073 which incorrectly stated 12).
Individual globally unique addresses MUST be allocated to all attachment circuits (ACs) and S-PEs of an MS-PW. The AII MUST be globally unique across all PSNs spanned by the MS-PW.
On receiving a Label Mapping that loops (originated from a different peer than the one the forward Label Mapping was sent to, or where the received SAII is local to the S-PE), the S-PE MUST send a Label Release with status code 'PW Loop Detected'.
One T-PE MUST initiate PW signaling (active/ST-PE role) while the other waits (passive/TT-PE role). Both T-PEs MUST be identified before signaling begins, and MUST use the same method for this determination.
PSN tunnel failures MUST be handled by PSN mechanisms such as MPLS fast reroute. A T-PE receiving an unsolicited Label Release with a failure code MUST throttle PW setup retry attempts using exponential backoff.
PW segments of the same MS-PW MUST be of the same PW type. PSN tunnels carrying different segments may be of the same or different technology.
When tearing down an MS-PW due to AII reachability change, the unsolicited Label Release messages accompanying the Label Withdraw MUST carry status code 'AII Unreachable'. The teardown procedure at an S-PE triggered by reachability change is OPTIONAL.
PW AII Routing Table: each S-PE and T-PE capable of dynamic MS-PW maintains a table keyed on (Global ID, Prefix, AC ID) with a length mask (0–96 bits) analogous to IPv4 CIDR. Longest-match lookup determines the next signaling hop. Entries may be static or BGP-learned.
PW Switching Point PE Sub-TLV Type 0x06 ('L2 PW address of PW Switching Point') with corrected Length=14, used in the PW Switching Point PE TLV to record each switching point traversed by a dynamically placed MS-PW identified via FEC 129.
New BGP SAFI value 6 registered: 'Network Layer Reachability Information used for Dynamic Placement of Multi-Segment Pseudowires'. Added to IANA 'Subsequent Address Family Identifiers (SAFI) Parameters' registry.
New LDP TLV type registered: value 0x096E, description 'Bandwidth TLV', defined in RFC 7267. Added to the IANA 'TLV Type Name Space' registry under 'Label Distribution Protocol (LDP) Parameters'.
Three new LDP Status Codes registered: 0x00000037 (E=0) 'Bandwidth resources unavailable', 0x00000038 (E=0) 'Resources Unavailable', 0x00000039 (E=0) 'AII Unreachable'. Added to IANA 'Status Code Name Space' registry.
Dynamic path selection automatically selects S-PEs without operator intervention. Operators SHOULD use VCCV trace mechanisms (RFC 6073) to verify the path taken by an MS-PW and confirm it does not introduce additional security risks.
The PW control protocol may be used to establish MS-PWs across administrative boundaries. RFC 6073 Section 13 security considerations — including LDP message integrity and authentication — apply to all protocol extensions in this document.
Active/Passive T-PE election: both T-PEs compare SAII and TAII as unsigned integers (Global ID first, then Prefix, then AC ID). If SAII > TAII the T-PE is active (ST-PE, initiates signaling); if SAII < TAII the T-PE is passive (TT-PE, waits). If all fields are equal, there is a configuration error.
S-PE Label Mapping processing: on receipt, the S-PE determines forward vs reverse direction by checking whether the FEC is already installed and whether the message came from the same peer as the forward Label Mapping. Forward direction triggers next-hop lookup, PSN tunnel check, label allocation, and forwarding; reverse direction installs the FEC and relays a new Label Mapping toward the ST-PE.
AII Type 2 TLV (from RFC 5003): 1-octet Type=0x02, 1-octet Length, 4-octet Global ID, 4-octet Prefix, 4-octet AC ID. Used to globally identify attachment circuits and S-PEs for MS-PW routing and signaling.
BGP MS-PW NLRI (AFI=25, SAFI=6): 1-octet Length (prefix length in bits), 8-octet Route Distinguisher, 4-octet Global ID, 4-octet Prefix, 4-octet AC ID. Lengths 128–159 bits are invalid; minimum is 96 bits (except the 0-length default route); maximum is 160 bits.
PW Bandwidth TLV (type 0x096E): 2-octet type with F=1/B=0 bits, 2-octet Length (value=64), 32-octet Forward SENDER_TSPEC (ST-PE to TT-PE direction), 32-octet Reverse SENDER_TSPEC (TT-PE to ST-PE direction) as defined in RFC 2210 Section 3.1.