70 lines
2.8 KiB
ReStructuredText
70 lines
2.8 KiB
ReStructuredText
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OSPF Neighor Retransmission List
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================================
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Overview
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--------
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OSPF neighbor link-state retransmission lists are implemented using
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both a sparse Link State Database (LSDB) and a doubly-linked list.
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Rather than previous per-neighbor periodic timer, a per-neighbor
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timer is set to the expiration time of the next scheduled LSA
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retransmission.
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Sparse Link State Database (LSDB)
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---------------------------------
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When an explicit or implied acknowledgment is recieved from a
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neighbor in 2-way state or higher, the acknowledge LSA must be
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removed from the neighbor's link state retransmission list. In order
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to do this efficiently, a sparse LSDB is utilized. LSDB entries also
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include a pointer to the corresponding list entry so that it may be
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efficiently removed from the doubly-linked list.
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The sparse LSDB is implemented using the OSPF functions is
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ospf_lsdb.[c,h]. OSPF LSDBs are implemented as an array of route
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tables (lib/table.[c,h]). What is unique of the LS Retransmission
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list LSDB is that each entry also has a pointer into the doubly-linked
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list to facilitate fast deletions.
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Doubly-Linked List
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------------------
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In addition to the sparse LSDB, LSAs on a neighbor LS retransmission
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list are also maintained in a linked-list order chronologically
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with the LSA scheduled for the next retransmission at the head of
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the list.
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The doubly-link list is implemented using the dlist macros in
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lib/typesafe.h.
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LSA LS Retransmission List Addition
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------------------------------------
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When an LSA is added to a neighbor retransmission list, it is
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added to both the sparse LSDB and the doubly-linked list with a pointer
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in the LSDB route-table node to the list entry. The LSA is added to
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the tail of the list with the expiration time set to the current time
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with the retransmission interval added. If the neighbor retransmission
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timer is not set, it is set to expire at the time of the newly added
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LSA.
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LSA LS Retransmission List Deletion
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-----------------------------------
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When an LSA is deleted from a neighbor retransmission list, it is
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deleted from eboth the sparse LSDB and the doubly-linked list with the
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pointer the LSDB route-table node used to efficiently delete the entry
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from the list. If the LSA at the head of the list was removed, then
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the neighbor retransmission timer is reset to the expiration of the
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LSA at the head of the list or canceled if the list is empty.
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Neighbor LS Retransmission List Expiration
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------------------------------------------
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When the neighbor retransmission timer expires, the LSA at the top of
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list and any in a configured window (e.g., 50 milliseconds) are
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retransmitted. The LSAs that have been retransmitted are removed from
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the list and readded to the tail of the list with a new expiration time
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which is retransmit-interval seconds in the future.
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