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MPLS LDP Adjacency Graceful Shutdown Draft

Discusses network resiliency by avoiding traffic loss when disabling some parallel LDP interfaces. Includes problem definition, proposed solutions, and a new Graceful Shutdown TLV format.

nicholas-hines
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MPLS LDP Adjacency Graceful Shutdown Draft

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  1. MPLS-LDP Adjacency Graceful Shutdown Draftdraft-sboutros-mpls-ldp-gs-adj.txtSami Boutros Siva Sivabalan Syed Kamran Bob Thomas

  2. Background • This problem discussed in these slides has been identified as an area for further study in the LDP RFC (section 6 of RFC 5036 page 90): “ The current specification does not support shutting down an adjacency.  The motivation for doing it and the mechanisms for  achieving it are left for further study.”

  3. Problem Definition LDP is enabled on both interfaces LSR-1 LSR-2 • Consider the above scenario: • When enabling LDP on both interfaces, the LSRs will have: • Two Hello adjacencies; one on each interface • One TCP session Both LSRs will program the MPLS forwarding table to send MPLS traffic on both links

  4. Problem Definition Continued… When disabling LDP on one of the parallel interfaces on one LSR (e.g., LSR-1), MPLS traffic from the neighbor (e.g., LSR-2) transmitted over that interface will be black-holed until LDP Hello adjacency timer expires on the neighbor (e.g., LSR-2) Disabling LDP on this interface LSR-1 LSR-2 • The reason is that once LDP is disabled on the interface, LSR-1 will: • Stop sending Hello messages on the interface • Stop sending MPLS packets on the interface • Drop received MPLS packets on the interface • LSR-2 will stop sending MPLS packets on the interface only when the Hello timer expires

  5. Disabling LDP on this interface LSR-1 LSR-2 Proposed Solution Goal: to add network resiliency by eliminating traffic loss when disabling LDP on some of the parallel interfaces either intentionally or unintentionally • LSR-1 sends a Hello message with an optional TLV to LSR-2 to indicate that it is about to terminate adjacency on a given interface • LSR-2 re-programs the MPLS LDP forwarding entries to stop using the adjacency, and then sends an acknowledgement back to LSR-1 • Upon receiving the acknowledgement, LSR-1 can remove the corresponding forwarding entry

  6. Operation Disabling LDP on this interface LSR-1 LSR-2 On LSR-1 (originating GS) • Stop sending MPLS LDP packets on the disabled interface • Send N Hello messages to indicate LSR-2 that it is about to terminate the Hello adjacency on the interface • Stop sending any more Hello message and starts the timer • Upon receiving the Hello message with the acknowledgement TLV, remove adjacency from the forwarding table • If optional TLV is not received, wait for the timer to expire and remove adjacency from the forwarding table On LSR-2: • If optional TLV is understood, program MPLS LDP forwarding entries to stop transmitting MPLS LDP traffic over the adjacency, and send a Hello message with an acknowledgement TLV to LSR-1. • 2. Continue sending Hello messages

  7. Proposed Solution Continued... A new optional TLV called Graceful Shutdown (GS) added in the hello message. The GS TLV has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0| 0x0404 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type is to be assigned by IANA (recommended value is 0x0404) Length is set to 4 indicating that the value field is 4 Octet long R-bit: indicates whether the Hello message is for graceful shutdown request or acknowledgement. This bit is set to 1 and 0 for request and acknowledgement respectively Reserved: This field is ignored If an LSR does not support GS TLV, it should silently ignore the GS TLV and process the rest of the message. Furthermore, the LSR does not forward the GS TLV any further. Thus, the U and F bits are set to 1 and 0 respectively in accordance with RFC5036

  8. Future Enhancements • GS capability will be advertised to the neighbors to indicate that an LSR is capable of GS via the LDP capabilities. • This will help operators to identify GS-capable LSRs in maintenance scenario • This enhancement is based on the review comments from Rajiv Asati

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