8. Protocol Packet Processing This section discusses the general processing of OSPF routing protocol packets. It is very important that the router topological databases remain synchronized. For this reason, routing protocol packets should get preferential treatment over ordinary data packets, both in sending and receiving. Routing protocol packets are sent along adjacencies only (with the +---+ +---+ |RT1|------------|RT2| o---------------o +---+ N1 +---+ RT1 RT2 RT7 o---------+ +---+ +---+ +---+ /|\ | |RT7| |RT3| |RT4| / | \ | +---+ +---+ +---+ / | \ | | | | / | \ | +-----------------------+ RT5o RT6o oRT4 | | | N2 * * * | +---+ +---+ * * * | |RT5| |RT6| * * * | +---+ +---+ *** | o---------+ RT3 Figure 10: The graph of adjacencies
exception of Hello packets, which are used to discover the adjacencies). This means that all routing protocol packets travel a single IP hop, except those sent over virtual links. All routing protocol packets begin with a standard header. The sections below give the details on how to fill in and verify this standard header. Then, for each packet type, the section is listed that gives more details on that particular packet type's processing. 8.1. Sending protocol packets When a router sends a routing protocol packet, it fills in the fields of the standard OSPF packet header as follows. For more details on the header format consult Section A.3.1: Version # Set to 2, the version number of the protocol as documented in this specification. Packet type The type of OSPF packet, such as Link state Update or Hello Packet. Packet length The length of the entire OSPF packet in bytes, including the standard OSPF packet header. Router ID The identity of the router itself (who is originating the packet). Area ID The OSPF area that the packet is being sent into. Checksum The standard IP 16-bit one's complement checksum of the entire OSPF packet, excluding the 64-bit authentication field. This checksum should be calculated before handing the packet to the appropriate authentication procedure. AuType and Authentication Each OSPF packet exchange is authenticated. Authentication types are assigned by the protocol and documented in Appendix D. A different authentication scheme can be used for each OSPF area. The 64-bit authentication field is set by the appropriate authentication procedure (determined by AuType). This procedure should be the last called when
forming the packet to be sent. The setting of the authentication field is determined by the packet contents and the authentication key (which is configurable on a per- interface basis). The IP destination address for the packet is selected as follows. On physical point-to-point networks, the IP destination is always set to the address AllSPFRouters. On all other network types (including virtual links), the majority of OSPF packets are sent as unicasts, i.e., sent directly to the other end of the adjacency. In this case, the IP destination is just the Neighbor IP address associated with the other end of the adjacency (see Section 10). The only packets not sent as unicasts are on broadcast networks; on these networks Hello packets are sent to the multicast destination AllSPFRouters, the Designated Router and its Backup send both Link State Update Packets and Link State Acknowledgment Packets to the multicast address AllSPFRouters, while all other routers send both their Link State Update and Link State Acknowledgment Packets to the multicast address AllDRouters. Retransmissions of Link State Update packets are ALWAYS sent as unicasts. The IP source address should be set to the IP address of the sending interface. Interfaces to unnumbered point-to-point networks have no associated IP address. On these interfaces, the IP source should be set to any of the other IP addresses belonging to the router. For this reason, there must be at least one IP address assigned to the router. Note that, for most purposes, virtual links act precisely the same as unnumbered point-to-point networks. However, each virtual link does have an IP interface address (discovered during the routing table build process) which is used as the IP source when sending packets over the virtual link. For more information on the format of specific OSPF packet types, consult the sections listed in Table 10.
Type Packet name detailed section (transmit) _________________________________________________________ 1 Hello Section 9.5 2 Database description Section 10.8 3 Link state request Section 10.9 4 Link state update Section 13.3 5 Link state ack Section 13.5 Table 10: Sections describing OSPF protocol packet transmission. 8.2. Receiving protocol packets Whenever a protocol packet is received by the router it is marked with the interface it was received on. For routers that have virtual links configured, it may not be immediately obvious which interface to associate the packet with. For example, consider the Router RT11 depicted in Figure 6. If RT11 receives an OSPF protocol packet on its interface to Network N8, it may want to associate the packet with the interface to Area 2, or with the virtual link to Router RT10 (which is part of the backbone). In the following, we assume that the packet is initially associated with the non-virtual link. In order for the packet to be accepted at the IP level, it must pass a number of tests, even before the packet is passed to OSPF for processing: o The IP checksum must be correct. o The packet's IP destination address must be the IP address of the receiving interface, or one of the IP multicast addresses AllSPFRouters or AllDRouters. o The IP protocol specified must be OSPF (89). o Locally originated packets should not be passed on to OSPF. That is, the source IP address should be examined to make sure this is not a multicast packet that the router itself generated. Next, the OSPF packet header is verified. The fields specified in the header must match those configured for the receiving
interface. If they do not, the packet should be discarded: o The version number field must specify protocol version 2. o The 16-bit one's complement checksum of the OSPF packet's contents must be verified. Remember that the 64-bit authentication field must be excluded from the checksum calculation. o The Area ID found in the OSPF header must be verified. If both of the following cases fail, the packet should be discarded. The Area ID specified in the header must either: (1) Match the Area ID of the receiving interface. In this case, the packet has been sent over a single hop. Therefore, the packet's IP source address must be on the same network as the receiving interface. This can be determined by comparing the packet's IP source address to the interface's IP address, after masking both addresses with the interface mask. This comparison should not be performed on point-to-point networks. On point-to-point networks, the interface addresses of each end of the link are assigned independently, if they are assigned at all. (2) Indicate the backbone. In this case, the packet has been sent over a virtual link. The receiving router must be an area border router, and the Router ID specified in the packet (the source router) must be the other end of a configured virtual link. The receiving interface must also attach to the virtual link's configured Transit area. If all of these checks succeed, the packet is accepted and is from now on associated with the virtual link (and the backbone area). o Packets whose IP destination is AllDRouters should only be accepted if the state of the receiving interface is DR or Backup (see Section 9.1). o The AuType specified in the packet must match the AuType specified for the associated area. Next, the packet must be authenticated. This depends on the AuType specified (see Appendix D). The authentication procedure may use an Authentication key, which can be configured on a
per-interface basis. If the authentication fails, the packet should be discarded. If the packet type is Hello, it should then be further processed by the Hello Protocol (see Section 10.5). All other packet types are sent/received only on adjacencies. This means that the packet must have been sent by one of the router's active neighbors. If the receiving interface is a multi-access network (either broadcast or non-broadcast) the sender is identified by the IP source address found in the packet's IP header. If the receiving interface is a point-to-point link or a virtual link, the sender is identified by the Router ID (source router) found in the packet's OSPF header. The data structure associated with the receiving interface contains the list of active neighbors. Packets not matching any active neighbor are discarded. At this point all received protocol packets are associated with an active neighbor. For the further input processing of specific packet types, consult the sections listed in Table 11. Type Packet name detailed section (receive) ________________________________________________________ 1 Hello Section 10.5 2 Database description Section 10.6 3 Link state request Section 10.7 4 Link state update Section 13 5 Link state ack Section 13.7 Table 11: Sections describing OSPF protocol packet reception. 9. The Interface Data Structure An OSPF interface is the connection between a router and a network. There is a single OSPF interface structure for each attached network; each interface structure has at most one IP interface address (see below). The support for multiple addresses on a single network is a matter for future consideration. An OSPF interface can be considered to belong to the area that contains the attached network. All routing protocol packets originated by the router over this interface are labelled with the interface's Area ID. One or more router adjacencies may develop over an interface. A router's link state advertisements reflect the
state of its interfaces and their associated adjacencies. The following data items are associated with an interface. Note that a number of these items are actually configuration for the attached network; those items must be the same for all routers connected to the network. Type The kind of network to which the interface attaches. Its value is either broadcast, non-broadcast yet still multi-access, point-to-point or virtual link. State The functional level of an interface. State determines whether or not full adjacencies are allowed to form over the interface. State is also reflected in the router's link state advertisements. IP interface address The IP address associated with the interface. This appears as the IP source address in all routing protocol packets originated over this interface. Interfaces to unnumbered point-to-point networks do not have an associated IP address. IP interface mask Also referred to as the subnet mask, this indicates the portion of the IP interface address that identifies the attached network. Masking the IP interface address with the IP interface mask yields the IP network number of the attached network. On point-to-point networks and virtual links, the IP interface mask is not defined. On these networks, the link itself is not assigned an IP network number, and so the addresses of each side of the link are assigned independently, if they are assigned at all. Area ID The Area ID of the area to which the attached network belongs. All routing protocol packets originating from the interface are labelled with this Area ID. HelloInterval The length of time, in seconds, between the Hello packets that the router sends on the interface. Advertised in Hello packets sent out this interface. RouterDeadInterval The number of seconds before the router's neighbors will declare
it down, when they stop hearing the router's Hello Packets. Advertised in Hello packets sent out this interface. InfTransDelay The estimated number of seconds it takes to transmit a Link State Update Packet over this interface. Link state advertisements contained in the Link State Update packet will have their age incremented by this amount before transmission. This value should take into account transmission and propagation delays; it must be greater than zero. Router Priority An 8-bit unsigned integer. When two routers attached to a network both attempt to become Designated Router, the one with the highest Router Priority takes precedence. A router whose Router Priority is set to 0 is ineligible to become Designated Router on the attached network. Advertised in Hello packets sent out this interface. Hello Timer An interval timer that causes the interface to send a Hello packet. This timer fires every HelloInterval seconds. Note that on non-broadcast networks a separate Hello packet is sent to each qualified neighbor. Wait Timer A single shot timer that causes the interface to exit the Waiting state, and as a consequence select a Designated Router on the network. The length of the timer is RouterDeadInterval seconds. List of neighboring routers The other routers attached to this network. On multi-access networks, this list is formed by the Hello Protocol. Adjacencies will be formed to some of these neighbors. The set of adjacent neighbors can be determined by an examination of all of the neighbors' states. Designated Router The Designated Router selected for the attached network. The Designated Router is selected on all multi-access networks by the Hello Protocol. Two pieces of identification are kept for the Designated Router: its Router ID and its IP interface address on the network. The Designated Router advertises link state for the network; this network link state advertisement is labelled with the Designated Router's IP address. The Designated Router is initialized to 0.0.0.0, which indicates the lack of a Designated Router.
Backup Designated Router The Backup Designated Router is also selected on all multi- access networks by the Hello Protocol. All routers on the attached network become adjacent to both the Designated Router and the Backup Designated Router. The Backup Designated Router becomes Designated Router when the current Designated Router fails. The Backup Designated Router is initialized to 0.0.0.0, indicating the lack of a Backup Designated Router. Interface output cost(s) The cost of sending a data packet on the interface, expressed in the link state metric. This is advertised as the link cost for this interface in the router links advertisement. There may be a separate cost for each IP Type of Service. The cost of an interface must be greater than zero. RxmtInterval The number of seconds between link state advertisement retransmissions, for adjacencies belonging to this interface. Also used when retransmitting Database Description and Link State Request Packets. Authentication key This configured data allows the authentication procedure to generate and/or verify the Authentication field in the OSPF header. The Authentication key can be configured on a per- interface basis. For example, if the AuType indicates simple password, the Authentication key would be a 64-bit password. This key would be inserted directly into the OSPF header when originating routing protocol packets, and there could be a separate password for each network. 9.1. Interface states The various states that router interfaces may attain is documented in this section. The states are listed in order of progressing functionality. For example, the inoperative state is listed first, followed by a list of intermediate states before the final, fully functional state is achieved. The specification makes use of this ordering by sometimes making references such as "those interfaces in state greater than X". Figure 11 shows the graph of interface state changes. The arcs of the graph are labelled with the event causing the state change. These events are documented in Section 9.2. The interface state machine is described in more detail in Section 9.3.
+----+ UnloopInd +--------+ |Down|<--------------|Loopback| +----+ +--------+ | |InterfaceUp +-------+ | +--------------+ |Waiting|<-+-------------->|Point-to-point| +-------+ +--------------+ | WaitTimer|BackupSeen | | | NeighborChange +------+ +-+<---------------- +-------+ |Backup|<----------|?|----------------->|DROther| +------+---------->+-+<-----+ +-------+ Neighbor | | Change | |Neighbor | |Change | +--+ +---->|DR| +--+ Figure 11: Interface State changes In addition to the state transitions pictured, Event InterfaceDown always forces Down State, and Event LoopInd always forces Loopback State Down This is the initial interface state. In this state, the lower-level protocols have indicated that the interface is unusable. No protocol traffic at all will be sent or received on such a interface. In this state, interface parameters should be set to their initial values. All interface timers should be disabled, and there should be no adjacencies associated with the interface. Loopback In this state, the router's interface to the network is looped back. The interface may be looped back in hardware or software. The interface will be unavailable for regular data traffic. However, it may still be desirable to gain information on the quality of this interface, either through sending ICMP pings to the interface or through something like a bit error test. For this reason, IP packets may
still be addressed to an interface in Loopback state. To facilitate this, such interfaces are advertised in router links advertisements as single host routes, whose destination is the IP interface address. Waiting In this state, the router is trying to determine the identity of the (Backup) Designated Router for the network. To do this, the router monitors the Hello Packets it receives. The router is not allowed to elect a Backup Designated Router nor a Designated Router until it transitions out of Waiting state. This prevents unnecessary changes of (Backup) Designated Router. Point-to-point In this state, the interface is operational, and connects either to a physical point-to-point network or to a virtual link. Upon entering this state, the router attempts to form an adjacency with the neighboring router. Hello Packets are sent to the neighbor every HelloInterval seconds. DR Other The interface is to a multi-access network on which another router has been selected to be the Designated Router. In this state, the router itself has not been selected Backup Designated Router either. The router forms adjacencies to both the Designated Router and the Backup Designated Router (if they exist). Backup In this state, the router itself is the Backup Designated Router on the attached network. It will be promoted to Designated Router when the present Designated Router fails. The router establishes adjacencies to all other routers attached to the network. The Backup Designated Router performs slightly different functions during the Flooding Procedure, as compared to the Designated Router (see Section 13.3). See Section 7.4 for more details on the functions performed by the Backup Designated Router. DR In this state, this router itself is the Designated Router on the attached network. Adjacencies are established to all other routers attached to the network. The router must also originate a network links advertisement for the network node. The advertisement will contain links to all routers (including the Designated Router itself) attached to the network. See Section 7.3 for more details on the functions performed by the Designated Router.
9.2. Events causing interface state changes State changes can be effected by a number of events. These events are pictured as the labelled arcs in Figure 11. The label definitions are listed below. For a detailed explanation of the effect of these events on OSPF protocol operation, consult Section 9.3. InterfaceUp Lower-level protocols have indicated that the network interface is operational. This enables the interface to transition out of Down state. On virtual links, the interface operational indication is actually a result of the shortest path calculation (see Section 16.7). WaitTimer The Wait Timer has fired, indicating the end of the waiting period that is required before electing a (Backup) Designated Router. BackupSeen The router has detected the existence or non-existence of a Backup Designated Router for the network. This is done in one of two ways. First, an Hello Packet may be received from a neighbor claiming to be itself the Backup Designated Router. Alternatively, an Hello Packet may be received from a neighbor claiming to be itself the Designated Router, and indicating that there is no Backup Designated Router. In either case there must be bidirectional communication with the neighbor, i.e., the router must also appear in the neighbor's Hello Packet. This event signals an end to the Waiting state. NeighborChange There has been a change in the set of bidirectional neighbors associated with the interface. The (Backup) Designated Router needs to be recalculated. The following neighbor changes lead to the NeighborChange event. For an explanation of neighbor states, see Section 10.1. o Bidirectional communication has been established to a neighbor. In other words, the state of the neighbor has transitioned to 2-Way or higher. o There is no longer bidirectional communication with a neighbor. In other words, the state of the neighbor has transitioned to Init or lower.
o One of the bidirectional neighbors is newly declaring itself as either Designated Router or Backup Designated Router. This is detected through examination of that neighbor's Hello Packets. o One of the bidirectional neighbors is no longer declaring itself as Designated Router, or is no longer declaring itself as Backup Designated Router. This is again detected through examination of that neighbor's Hello Packets. o The advertised Router Priority for a bidirectional neighbor has changed. This is again detected through examination of that neighbor's Hello Packets. LoopInd An indication has been received that the interface is now looped back to itself. This indication can be received either from network management or from the lower level protocols. UnloopInd An indication has been received that the interface is no longer looped back. As with the LoopInd event, this indication can be received either from network management or from the lower level protocols. InterfaceDown Lower-level protocols indicate that this interface is no longer functional. No matter what the current interface state is, the new interface state will be Down. 9.3. The Interface state machine A detailed description of the interface state changes follows. Each state change is invoked by an event (Section 9.2). This event may produce different effects, depending on the current state of the interface. For this reason, the state machine below is organized by current interface state and received event. Each entry in the state machine describes the resulting new interface state and the required set of additional actions. When an interface's state changes, it may be necessary to originate a new router links advertisement. See Section 12.4 for more details. Some of the required actions below involve generating events for
the neighbor state machine. For example, when an interface becomes inoperative, all neighbor connections associated with the interface must be destroyed. For more information on the neighbor state machine, see Section 10.3. State(s): Down Event: InterfaceUp New state: Depends upon action routine Action: Start the interval Hello Timer, enabling the periodic sending of Hello packets out the interface. If the attached network is a physical point-to-point network or virtual link, the interface state transitions to Point-to-Point. Else, if the router is not eligible to become Designated Router the interface state transitions to DR Other. Otherwise, the attached network is multi-access and the router is eligible to become Designated Router. In this case, in an attempt to discover the attached network's Designated Router the interface state is set to Waiting and the single shot Wait Timer is started. If in addition the attached network is non-broadcast, examine the configured list of neighbors for this interface and generate the neighbor event Start for each neighbor that is also eligible to become Designated Router. State(s): Waiting Event: BackupSeen New state: Depends upon action routine. Action: Calculate the attached network's Backup Designated Router and Designated Router, as shown in Section 9.4. As a result of this calculation, the new state of the interface will be either DR Other, Backup or DR. State(s): Waiting
Event: WaitTimer New state: Depends upon action routine. Action: Calculate the attached network's Backup Designated Router and Designated Router, as shown in Section 9.4. As a result of this calculation, the new state of the interface will be either DR Other, Backup or DR. State(s): DR Other, Backup or DR Event: NeighborChange New state: Depends upon action routine. Action: Recalculate the attached network's Backup Designated Router and Designated Router, as shown in Section 9.4. As a result of this calculation, the new state of the interface will be either DR Other, Backup or DR. State(s): Any State Event: InterfaceDown New state: Down Action: All interface variables are reset, and interface timers disabled. Also, all neighbor connections associated with the interface are destroyed. This is done by generating the event KillNbr on all associated neighbors (see Section 10.2). State(s): Any State Event: LoopInd New state: Loopback Action: Since this interface is no longer connected to the attached network the actions associated with the above InterfaceDown event are executed.
State(s): Loopback Event: UnloopInd New state: Down Action: No actions are necessary. For example, the interface variables have already been reset upon entering the Loopback state. Note that reception of an InterfaceUp event is necessary before the interface again becomes fully functional. 9.4. Electing the Designated Router This section describes the algorithm used for calculating a network's Designated Router and Backup Designated Router. This algorithm is invoked by the Interface state machine. The initial time a router runs the election algorithm for a network, the network's Designated Router and Backup Designated Router are initialized to 0.0.0.0. This indicates the lack of both a Designated Router and a Backup Designated Router. The Designated Router election algorithm proceeds as follows: Call the router doing the calculation Router X. The list of neighbors attached to the network and having established bidirectional communication with Router X is examined. This list is precisely the collection of Router X's neighbors (on this network) whose state is greater than or equal to 2-Way (see Section 10.1). Router X itself is also considered to be on the list. Discard all routers from the list that are ineligible to become Designated Router. (Routers having Router Priority of 0 are ineligible to become Designated Router.) The following steps are then executed, considering only those routers that remain on the list: (1) Note the current values for the network's Designated Router and Backup Designated Router. This is used later for comparison purposes. (2) Calculate the new Backup Designated Router for the network as follows. Only those routers on the list that have not declared themselves to be Designated Router are eligible to become Backup Designated Router. If one or more of these routers have declared themselves Backup Designated Router (i.e., they are currently listing themselves as Backup Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is declared to be Backup Designated Router. In case of a tie, the one having the highest Router ID is chosen. If no routers have declared themselves Backup Designated Router, choose the router having highest Router Priority, (again excluding those routers who have declared themselves Designated Router), and again use the Router ID to break ties. (3) Calculate the new Designated Router for the network as follows. If one or more of the routers have declared themselves Designated Router (i.e., they are currently listing themselves as Designated Router in their Hello Packets) the one having highest Router Priority is declared to be Designated Router. In case of a tie, the one having the highest Router ID is chosen. If no routers have declared themselves Designated Router, assign the Designated Router to be the same as the newly elected Backup Designated Router. (4) If Router X is now newly the Designated Router or newly the Backup Designated Router, or is now no longer the Designated Router or no longer the Backup Designated Router, repeat steps 2 and 3, and then proceed to step 5. For example, if Router X is now the Designated Router, when step 2 is repeated X will no longer be eligible for Backup Designated Router election. Among other things, this will ensure that no router will declare itself both Backup Designated Router and Designated Router. (5) As a result of these calculations, the router itself may now be Designated Router or Backup Designated Router. See Sections 7.3 and 7.4 for the additional duties this would entail. The router's interface state should be set accordingly. If the router itself is now Designated Router, the new interface state is DR. If the router itself is now Backup Designated Router, the new interface state is Backup. Otherwise, the new interface state is DR Other. (6) If the attached network is non-broadcast, and the router itself has just become either Designated Router or Backup Designated Router, it must start sending Hello Packets to those neighbors that are not eligible to become Designated Router (see Section 9.5.1). This is done by invoking the neighbor event Start for each neighbor having a Router Priority of 0.
(7) If the above calculations have caused the identity of either the Designated Router or Backup Designated Router to change, the set of adjacencies associated with this interface will need to be modified. Some adjacencies may need to be formed, and others may need to be broken. To accomplish this, invoke the event AdjOK? on all neighbors whose state is at least 2-Way. This will cause their eligibility for adjacency to be reexamined (see Sections 10.3 and 10.4). The reason behind the election algorithm's complexity is the desire for an orderly transition from Backup Designated Router to Designated Router, when the current Designated Router fails. This orderly transition is ensured through the introduction of hysteresis: no new Backup Designated Router can be chosen until the old Backup accepts its new Designated Router responsibilities. The above procedure may elect the same router to be both Designated Router and Backup Designated Router, although that router will never be the calculating router (Router X) itself. The elected Designated Router may not be the router having the highest Router Priority, nor will the Backup Designated Router necessarily have the second highest Router Priority. If Router X is not itself eligible to become Designated Router, it is possible that neither a Backup Designated Router nor a Designated Router will be selected in the above procedure. Note also that if Router X is the only attached router that is eligible to become Designated Router, it will select itself as Designated Router and there will be no Backup Designated Router for the network. 9.5. Sending Hello packets Hello packets are sent out each functioning router interface. They are used to discover and maintain neighbor relationships. On multi-access networks, Hello Packets are also used to elect the Designated Router and Backup Designated Router, and in that way determine what adjacencies should be formed. The format of an Hello packet is detailed in Section A.3.2. The Hello Packet contains the router's Router Priority (used in choosing the Designated Router), and the interval between Hello Packets sent out the interface (HelloInterval). The Hello Packet also indicates how often a neighbor must be heard from to remain active (RouterDeadInterval). Both HelloInterval and
RouterDeadInterval must be the same for all routers attached to a common network. The Hello packet also contains the IP address mask of the attached network (Network Mask). On unnumbered point-to-point networks and on virtual links this field should be set to 0.0.0.0. The Hello packet's Options field describes the router's optional OSPF capabilities. There are currently two optional capabilities defined (see Sections 4.5 and A.2). The T-bit of the Options field should be set if the router is capable of calculating separate routes for each IP TOS. The E-bit should be set if and only if the attached area is capable of processing AS external advertisements (i.e., it is not a stub area). If the E-bit is set incorrectly the neighboring routers will refuse to accept the Hello Packet (see Section 10.5). The rest of the Hello Packet's Options field should be set to zero. In order to ensure two-way communication between adjacent routers, the Hello packet contains the list of all routers from which Hello Packets have been seen recently. The Hello packet also contains the router's current choice for Designated Router and Backup Designated Router. A value of 0.0.0.0 in these fields means that one has not yet been selected. On broadcast networks and physical point-to-point networks, Hello packets are sent every HelloInterval seconds to the IP multicast address AllSPFRouters. On virtual links, Hello packets are sent as unicasts (addressed directly to the other end of the virtual link) every HelloInterval seconds. On non- broadcast networks, the sending of Hello packets is more complicated. This will be covered in the next section. 9.5.1. Sending Hello packets on non-broadcast networks Static configuration information is necessary in order for the Hello Protocol to function on non-broadcast networks (see Section C.5). Every attached router which is eligible to become Designated Router has a configured list of all of its neighbors on the network. Each listed neighbor is labelled with its Designated Router eligibility. The interface state must be at least Waiting for any Hello Packets to be sent. Hello Packets are then sent directly (as unicasts) to some subset of a router's neighbors. Sometimes an Hello Packet is sent periodically on a timer; at other times it is sent as a response to a received Hello Packet. A router's hello-sending behavior varies depending
on whether the router itself is eligible to become Designated Router. If the router is eligible to become Designated Router, it must periodically send Hello Packets to all neighbors that are also eligible. In addition, if the router is itself the Designated Router or Backup Designated Router, it must also send periodic Hello Packets to all other neighbors. This means that any two eligible routers are always exchanging Hello Packets, which is necessary for the correct operation of the Designated Router election algorithm. To minimize the number of Hello Packets sent, the number of eligible routers on a non-broadcast network should be kept small. If the router is not eligible to become Designated Router, it must periodically send Hello Packets to both the Designated Router and the Backup Designated Router (if they exist). It must also send an Hello Packet in reply to an Hello Packet received from any eligible neighbor (other than the current Designated Router and Backup Designated Router). This is needed to establish an initial bidirectional relationship with any potential Designated Router. When sending Hello packets periodically to any neighbor, the interval between Hello Packets is determined by the neighbor's state. If the neighbor is in state Down, Hello Packets are sent every PollInterval seconds. Otherwise, Hello Packets are sent every HelloInterval seconds. 10. The Neighbor Data Structure An OSPF router converses with its neighboring routers. Each separate conversation is described by a "neighbor data structure". Each conversation is bound to a particular OSPF router interface, and is identified either by the neighboring router's OSPF Router ID or by its Neighbor IP address (see below). Thus if the OSPF router and another router have multiple attached networks in common, multiple conversations ensue, each described by a unique neighbor data structure. Each separate conversation is loosely referred to in the text as being a separate "neighbor". The neighbor data structure contains all information pertinent to the forming or formed adjacency between the two neighbors. (However, remember that not all neighbors become adjacent.) An adjacency can be viewed as a highly developed conversation between two routers.
State The functional level of the neighbor conversation. This is described in more detail in Section 10.1. Inactivity Timer A single shot timer whose firing indicates that no Hello Packet has been seen from this neighbor recently. The length of the timer is RouterDeadInterval seconds. Master/Slave When the two neighbors are exchanging databases, they form a master/slave relationship. The master sends the first Database Description Packet, and is the only part that is allowed to retransmit. The slave can only respond to the master's Database Description Packets. The master/slave relationship is negotiated in state ExStart. DD Sequence Number A 32-bit number identifying individual Database Description packets. When the neighbor state ExStart is entered, the DD sequence number should be set to a value not previously seen by the neighboring router. One possible scheme is to use the machine's time of day counter. The DD sequence number is then incremented by the master with each new Database Description packet sent. The slave's DD sequence number indicates the last packet received from the master. Only one packet is allowed outstanding at a time. Neighbor ID The OSPF Router ID of the neighboring router. The Neighbor ID is learned when Hello packets are received from the neighbor, or is configured if this is a virtual adjacency (see Section C.4). Neighbor Priority The Router Priority of the neighboring router. Contained in the neighbor's Hello packets, this item is used when selecting the Designated Router for the attached network. Neighbor IP address The IP address of the neighboring router's interface to the attached network. Used as the Destination IP address when protocol packets are sent as unicasts along this adjacency. Also used in router links advertisements as the Link ID for the attached network if the neighboring router is selected to be Designated Router (see Section 12.4.1). The Neighbor IP address is learned when Hello packets are received from the neighbor. For virtual links, the Neighbor IP address is learned during the routing table build process (see Section 15).
Neighbor Options The optional OSPF capabilities supported by the neighbor. Learned during the Database Exchange process (see Section 10.6). The neighbor's optional OSPF capabilities are also listed in its Hello packets. This enables received Hello Packets to be rejected (i.e., neighbor relationships will not even start to form) if there is a mismatch in certain crucial OSPF capabilities (see Section 10.5). The optional OSPF capabilities are documented in Section 4.5. Neighbor's Designated Router The neighbor's idea of the Designated Router. If this is the neighbor itself, this is important in the local calculation of the Designated Router. Defined only on multi-access networks. Neighbor's Backup Designated Router The neighbor's idea of the Backup Designated Router. If this is the neighbor itself, this is important in the local calculation of the Backup Designated Router. Defined only on multi-access networks. The next set of variables are lists of link state advertisements. These lists describe subsets of the area topological database. There can be five distinct types of link state advertisements in an area topological database: router links, network links, and Type 3 and 4 summary links (all stored in the area data structure), and AS external links (stored in the global data structure). Link state retransmission list The list of link state advertisements that have been flooded but not acknowledged on this adjacency. These will be retransmitted at intervals until they are acknowledged, or until the adjacency is destroyed. Database summary list The complete list of link state advertisements that make up the area topological database, at the moment the neighbor goes into Database Exchange state. This list is sent to the neighbor in Database Description packets. Link state request list The list of link state advertisements that need to be received from this neighbor in order to synchronize the two neighbors' topological databases. This list is created as Database Description packets are received, and is then sent to the neighbor in Link State Request packets. The list is depleted as
appropriate Link State Update packets are received. 10.1. Neighbor states The state of a neighbor (really, the state of a conversation being held with a neighboring router) is documented in the following sections. The states are listed in order of progressing functionality. For example, the inoperative state is listed first, followed by a list of intermediate states before the final, fully functional state is achieved. The specification makes use of this ordering by sometimes making references such as "those neighbors/adjacencies in state greater than X". Figures 12 and 13 show the graph of neighbor state changes. The arcs of the graphs are labelled with the event causing the state change. The neighbor events are documented in Section 10.2. The graph in Figure 12 shows the state changes effected by the Hello Protocol. The Hello Protocol is responsible for neighbor +----+ |Down| +----+ | | Start | +-------+ Hello | +---->|Attempt| Received | +-------+ | | +----+<-+ |HelloReceived |Init|<---------------+ +----+<--------+ | | |2-Way |1-Way |Received |Received | | +-------+ | +-----+ |ExStart|<--------+------->|2-Way| +-------+ +-----+ Figure 12: Neighbor state changes (Hello Protocol) In addition to the state transitions pictured, Event KillNbr always forces Down State, Event InactivityTimer always forces Down State, Event LLDown always forces Down State
acquisition and maintenance, and for ensuring two way communication between neighbors. The graph in Figure 13 shows the forming of an adjacency. Not every two neighboring routers become adjacent (see Section 10.4). The adjacency starts to form when the neighbor is in state ExStart. After the two routers discover their master/slave status, the state transitions to Exchange. At this point the neighbor starts to be used in the flooding procedure, and the two neighboring routers begin synchronizing their databases. When this synchronization is finished, the neighbor is in state Full and we say that the two routers are fully adjacent. At this point the adjacency is listed in link state advertisements. For a more detailed description of neighbor state changes, together with the additional actions involved in each change, see Section 10.3. +-------+ |ExStart| +-------+ | NegotiationDone| +->+--------+ |Exchange| +--+--------+ | Exchange| Done | +----+ | +-------+ |Full|<---------+----->|Loading| +----+<-+ +-------+ | LoadingDone | +------------------+ Figure 13: Neighbor state changes (Database Exchange) In addition to the state transitions pictured, Event SeqNumberMismatch forces ExStart state, Event BadLSReq forces ExStart state, Event 1-Way forces Init state, Event KillNbr always forces Down State, Event InactivityTimer always forces Down State, Event LLDown always forces Down State, Event AdjOK? leads to adjacency forming/breaking
Down This is the initial state of a neighbor conversation. It indicates that there has been no recent information received from the neighbor. On non-broadcast networks, Hello packets may still be sent to "Down" neighbors, although at a reduced frequency (see Section 9.5.1). Attempt This state is only valid for neighbors attached to non- broadcast networks. It indicates that no recent information has been received from the neighbor, but that a more concerted effort should be made to contact the neighbor. This is done by sending the neighbor Hello packets at intervals of HelloInterval (see Section 9.5.1). Init In this state, an Hello packet has recently been seen from the neighbor. However, bidirectional communication has not yet been established with the neighbor (i.e., the router itself did not appear in the neighbor's Hello packet). All neighbors in this state (or higher) are listed in the Hello packets sent from the associated interface. 2-Way In this state, communication between the two routers is bidirectional. This has been assured by the operation of the Hello Protocol. This is the most advanced state short of beginning adjacency establishment. The (Backup) Designated Router is selected from the set of neighbors in state 2-Way or greater. ExStart This is the first step in creating an adjacency between the two neighboring routers. The goal of this step is to decide which router is the master, and to decide upon the initial DD sequence number. Neighbor conversations in this state or greater are called adjacencies. Exchange In this state the router is describing its entire link state database by sending Database Description packets to the neighbor. Each Database Description Packet has a DD sequence number, and is explicitly acknowledged. Only one Database Description Packet is allowed outstanding at any one time. In this state, Link State Request Packets may also be sent asking for the neighbor's more recent advertisements. All adjacencies in Exchange state or greater are used by the flooding procedure. In fact, these
adjacencies are fully capable of transmitting and receiving all types of OSPF routing protocol packets. Loading In this state, Link State Request packets are sent to the neighbor asking for the more recent advertisements that have been discovered (but not yet received) in the Exchange state. Full In this state, the neighboring routers are fully adjacent. These adjacencies will now appear in router links and network links advertisements.