rfc 2328... http://www.ietf.org/rfc/rfc2328.txt
A.3.2 The Hello packet ..................................... 193
A.3.3 The Database Description packet ...................... 195
A.3.4 The Link State Request packet ........................ 197
A.3.5 The Link State Update packet ......................... 199
A.3.6 The Link State Acknowledgment packet ................. 201
A.3.2 The Hello packet
Hello packets are OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hello
Packets are multicast on those physical networks having a multicast
or broadcast capability, enabling dynamic discovery of neighboring
routers.
All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor relationships.
A.3.3 The Database Description packet
Database Description packets are OSPF packet type 2. These packets
are exchanged when an adjacency is being initialized. They describe
the contents of the link-state database. Multiple packets may be
used to describe the database. For this purpose a poll-response
procedure is used. One of the routers is designated to be the
master, the other the slave. The master sends Database Description
packets (polls) which are acknowledged by Database Description
packets sent by the slave (responses). The responses are linked to
the polls via the packets' DD sequence numbers.
A.3.4 The Link State Request packet
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its link-state database are out-of-date. The
Link State Request packet is used to request the pieces of the
neighbor's database that are more up-to-date. Multiple Link State
Request packets may need to be used.
A router that sends a Link State Request packet has in mind the
precise instance of the database pieces it is requesting. Each
instance is defined by its LS sequence number, LS checksum, and LS
age, although these fields are not specified in the Link State
Request Packet itself. The router may receive even more recent
instances in response.
A.3.5 The Link State Update packet
Link State Update packets are OSPF packet type 4. These packets
implement the flooding of LSAs. Each Link State Update packet
carries a collection of LSAs one hop further from their origin.
Several LSAs may be included in a single packet.
Link State Update packets are multicast on those physical networks
that support multicast/broadcast. In order to make the flooding
procedure reliable, flooded LSAs are acknowledged in Link State
Acknowledgment packets. If retransmission of certain LSAs is
necessary, the retransmitted LSAs are always sent directly to the
neighbor.
A.3.6 The Link State Acknowledgment packet
Link State Acknowledgment Packets are OSPF packet type 5. To make
the flooding of LSAs reliable, flooded LSAs are explicitly
acknowledged. This acknowledgment is accomplished through the
sending and receiving of Link State Acknowledgment packets.
Multiple LSAs can be acknowledged in a single Link State
Acknowledgment packet.
Depending on the state of the sending interface and the sender of
the corresponding Link State Update packet, a Link State
Acknowledgment packet is sent either to the multicast address
AllSPFRouters, to the multicast address AllDRouters, or as a
unicast.
The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of LSA headers.
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