3.1.Multiple Highest PC Values

Instead of maintaining a single highest PC value for each neighbour, animplementation of the procedure described in this section uses two values:the highest multicast value PCm and the highest non-multicast (unicast)value PCu. More precisely, the (Index, PC) pair contained in theneighbour table (Section 3.2 of [RFC8967]) is replacedby a triple (Index, PCm, PCu), where:¶

• Index is an arbitrary string of 0 to 32 octets, and¶
• PCm and PCu are 32-bit (4-octet) integers.¶

When a Challenge Reply is successful, both highest PC values are updatedto the value contained in the PC TLV from the packet containing thesuccessful challenge. More precisely, the last sentence of the fourthbullet point ofSection 4.3 of [RFC8967] is replacedas follows:¶

OLD:¶

If the packet contains a successful Challenge Reply, then the PCand Index contained in the PC TLVMUST be stored in theneighbour table entry corresponding to the sender (which already exists inthis case), and the packet is accepted.¶

NEW:¶

If the packet contains a successful Challenge Reply, then the Indexcontained in the PC TLVMUST be stored in the Index field of the neighbourtable entry corresponding to the sender (which already exists in thiscase), the PC contained in the TLVMUST be stored in both the PCm andPCu fields of the neighbour table entry, and the packet is accepted.¶

When a packet that does not contain a successful Challenge Reply isreceived, the PC value that it contains is compared to either the PCu orthe PCm field of the corresponding neighbour entry, depending on whetheror not the packet was sent to a multicast address. If the comparison issuccessful, then the same value (PCm or PCu) is updated. More precisely,the last bullet point ofSection 4.3 of [RFC8967] isreplaced as follows:¶

OLD:¶

At this stage, the packet contains no successful Challenge Reply, and the Index contained in the PC TLV is equal to the Index in the neighbour table entry corresponding to the sender. The receiver compares the received PC with the PC contained in the neighbour table; if the received PC is smaller or equal than the PC contained in the neighbour table, the packetMUST be dropped and processing stops (no challenge is sent in this case, since the mismatch might be caused by harmless packet reordering on the link). Otherwise, the PC contained in the neighbour table entry is set to the received PC, and the packet is accepted.¶

NEW:¶

At this stage, the packet contains no successful Challenge Reply andthe Index contained in the PC TLV is equal to the Index in the neighbourtable entry corresponding to the sender. The receiver compares thereceived PC with either the PCm field (if the packet was sent to a multicastIP address) or the PCu field (otherwise) in the neighbour table. If thereceived PC is smaller than or equal to the value contained in the neighbourtable, the packetMUST be dropped and processing stops. Note that no challenge issent in this case, since the mismatch might be caused by harmless packetreordering on the link. Otherwise, the PCm (if the packet was sent toa multicast address) or the PCu (otherwise) field contained in theneighbour table entry is set to the received PC, and the packet isaccepted.¶

3.2.Window-Based Verification

Window-based verification is similar to what is described inSection 3.4.3 of [RFC4303]. When using window-based verification,in addition to retaining within its neighbour table the highest PC valuePCh seen from every neighbour, an implementation maintains a fixed-sizewindow of booleans corresponding to PC values directly below PCh. Moreprecisely, the (Index, PC) pair contained in the neighbour table (Section 3.2 of [RFC8967]) is replaced by:¶

• a triple (Index, PCh, Window), where Index is an arbitrary string of0 to 32 octets, PCh is a 32-bit (4-octet) integer, and Window is a vectorof booleans of size S (the default value S=128 isRECOMMENDED).¶

The window is a vector of S boolean values numbered from 0 (the "leftedge" of the window) up to S-1 (the "right edge"); the boolean associatedwith the index i indicates whether a packet with a PC value of (PCh - (S-1) + i)has been seen before. Shifting the window to the left by an integeramount k is defined as moving all values so that the value previously atindex n is now at index (n - k); k values are discarded at the left edge,and k new unset values are inserted at the right edge.¶

Whenever a packet is received, the receiver computes its indexi = (PC - PCh + S - 1). It then proceeds as follows:¶

1. If the index i is negative, the packet is considered too old, and itMUST be discarded.¶
2. If the index i is non-negative and strictly less than the window size S, the window value at the index is checked. If this value is already set, the received PC has been seen before and the packetMUST be discarded. Otherwise, the corresponding window value is marked as set, and the packet is accepted.¶
3. If the index i is larger or equal to the window size (i.e., PC is strictly larger than PCh), the windowMUST be shifted to the left by (i - S + 1) values (or, equivalently, by the difference PC - PCh), and the highest PC value PChMUST be set to the received PC. The value at the right of the window (the value with index S - 1)MUST be set, and the packet is accepted.¶

When receiving a successful Challenge Reply, the remembered highest PCvalue PChMUST be set to the value received in the Challenge Reply, andall of the values in the windowMUST be reset except the value at indexS - 1, whichMUST be set.¶

3.3.Combining the Two Techniques

The two techniques described above serve complementary purposes:¶

• splitting the state allows multicast packets to be reordered with respect to unicast ones by an arbitrary number of PC values, while¶
• the window-based technique allows arbitrary packets to be reordered but onlyby a bounded number of PC values.¶

Thus, they can profitably be combined.¶

An implementation that uses both techniquesMUST maintain, for everyentry of the neighbour table, two distinct windows, one for multicast andone for unicast packets. When a successful Challenge Reply is received,both windowsMUST be reset. When a packet that does not containa Challenge Reply is received, if the packet's destination address isa multicast address, the multicast windowMUST be consulted and possiblyupdated, as described inSection 3.2. Otherwise, the unicastwindowMUST be consulted and possibly updated.¶