8.1.7 OSPF packet format

The protocol number for OSPF is 89, and when OSPF packet multicasting occurs, their TTL is set to 1,

1. Packet header

All OSPF packets are started by a 24-byte eight-bit group header

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Versions version

The version number of OSPF, and the version number of OSPF is 2.

Types type

Packet type following the head

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Packet Lengths Packet length

The length of the OSPF packet, including the length of the packet header, in eight-bit group bytes

Router ID Router ID

The ID of the originating router

Area ID

The region of the router where the originating packet originated

Checksum checksum

Standard IP checksum for the entire packet (including the header)

Authentication Type (Autype)

The authentication mode being used

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Certified Authentication

Autype = 0 Do not check this field, can contain any content

Autype = 1 contains a maximum of 64-bit password

Autype = 2 contains a key ID, certified data length, no reduced encryption sequence number

Key ID Key ID

Represents the authentication algorithm and the security key used to create the message digest

Encrypt serial number

Numbers that do not decrease to prevent replay attacks

2.Hello Data Packet

To form an adjacency, the parameters that the Hello packet carries must be consistent with its neighbors

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netmask netmask Network Mask

The network mask that sends the packet interface, if the mask and the network mask that receives the packet interface do not match, the packet is discarded.

Hello time interval Hello Interval

The time interval between the delivery of Hello packets on an interface, which is a periodic period of time, measured in seconds

Optional option

To ensure compatibility issues between neighbors

Router Precedence Router Priority

Used for the election of Dr and BDR, with a priority of 0, not participating in the election

Router Invalid time interval Router Dead Interval

The length of the Hello packet that the originating router will wait to emit from the neighbor router before declaring that the route is invalid

Specify the Router Dr

If there is no Dr (not selected or the network type does not require DR), this field is set to 0.0.0.0

Back up the specified router BDR

If there is no DBR, this field is set to 0.0.0.0

Neighbor Neighbor

If the originating router has received a Hello packet from some of its routers in the last router Deadinterval time from the network, then the ID of all the neighbors will be listed in this field.

3. Database Description Packet

A primary purpose of a database description packet is to describe some or all of the LSA information in the originating router database, so that the receiving router can determine if the LSA is receiving a matching LSA in its database, which can be done only by listing the LSA's head.

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Interface MTU Interface MTU

The size of the maximum IP packet that the originating router interface can send in case the packet is not fragmented, measured in eight-bit group bytes.

Optional option

Included in the database description packet, so that the router can choose not to forward some LSA to the neighbor router that does not have the necessary support capabilities

Message the first 5 bits of the next eight-bit group byte are not used and are set to 00000b

*i bit, initial bit initial bit

When sending a series of databases describing the first packet in a packet, the bit is set to 1, and subsequent database description packets will set the bit to 0,i-bit to 0

*m bit, post-succession more bit

This bit is set to 1 when the packet sent is not the last packet in a series of database description packets

*ms bit, master-slave bit master/slave bit

During database synchronization, this bit is set to 1, the router that indicates that the database describes the packet is a primary router, and the router is set to 0,ms-bit = 0

* Database Description serial numbers DD sequence number

During database synchronization, to ensure that the router receives a complete sequence of database description packets

*lsa Head LSA Header

4. Link State Request Packet

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Link status Type link state type

Link state type number, which is used to denote the type of an LSA

Link status ID link state ID

Advertise Router Advertising Router

Router ID of the router advertised by the originating LSA

5. Link-State Update packets

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OSPF packets are not allowed to leave the network from which they originated, a link-state packet can carry one or more LSA, but these LSA are only routed to the direct-attached neighbor of the router originating them, and the neighbor router receiving the LSA will be responsible for re-encapsulating the associated LSA in the new LS update packet. Thus further flooding spread to its own neighbour

LSA Quantity Number of LSA

The number of LSA contained in this packet

Link State advertisement LSA

6. Link Status Acknowledgement packet

For the LSA to reliably flood the spread

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8.1.7 OSPF packet format