1. OSPF Header OSPF HEADER OSPF HEADER for this project Types we will use
Version (1 byte)
Pkt type (1 byte)
Length of Pkt (2 bytes)
Source Router ID (4 bytes)
Area ID (4 bytes)
Checksum (2 bytes)
Authentication type (2 bytes)
Authentication data (8 bytes) 2
Type
don’t care
IP:PORT (8 bytes!!)
Don’t care
Type = 1 -> HELLO
Type = 3 -> link state request
Type = 4 -> link state update
Type = 5 -> link state ACK
Types we will use

2. HELLO Packet standard this project
OSPF Header with
Type = 1 (HELLO)
OSPF Header with
Type = 1 (HELLO)
Network Mask (4 bytes)
Don’t care (4 bytes)
Hello Interval (2 bytes)
10 (2 bytes)
standard
Neighbors are the Ids (IP:Port) that have been send a HELLO since the in the previous 40 seconds (dead interval).
Put the senders ID last as a delimiter.
options
priority
this project
Don’t
care
Dead interval (4 bytes)
40 (4 bytes)
Designated router
(4 bytes)
Don’t care
(4 bytes)
Backup designated router
(4 bytes)
Don’t care
(4 bytes)
1st recent neighbor ID
(4 bytes)
1st Neighbor ID
(8 bytes)
last recent neighbor ID
(4 bytes)
SENDER ID
(8 bytes)

3. HELLO Protocol
Every hello interval (10 sec) you must send out a hello to ALL neighbors. The HELLOs are even sent across links that seem to be down.
During initialization, a HELLO is sent to all neighbors.
If a HELLO is received and 1. The receiving node is listed in the recent neighbors list and 2. the sending node’s BIDIRECTIONAL flag is zero, then set it to one and send immediately respond with a HELLO, with, of course, the node that just sent the HELLO on the recent neighbors list.
If a HELLO is received and the receiving node is not listed in the recent neighbors list, then immediately respond with a HELLO, with, of course, the node that just sent the HELLO on the recent neighbors list.
Whenever a HELLO is received, set the sending node’s LAST_HELLO_TIME to the current time.

4. The Link State Update packet
LSA
OSPF Header with
Type = 4 (update)
LSA Age
options
LS type
Link State ID
4 bytes
Number of LSAs
LSA1
Advertising router
4 bytes
LSA Header
LS Sequence Number
4 bytes
LSA2
checksum
length
router type
Number of Links
Link ID
4 bytes
Link Data
6 bytes
Link 1
Link type
TOS
metric

5. The Link State Update Packet for Project
LSA
LSA Age
Don’t
care
OSPF Header with
Type = 4 (update)
Link State ID
8 bytes
don’t care
4 bytes
Number of LSAs
LSA Header
LS Sequence Number
4 bytes
LSA1
don’t care
don’t care
LSA2
Don’t
care
Number of Links
Link ID
8 bytes
Link Data
6 bytes
Link 1
Link type
TOS
metric

6. Database Exchange This is not according to the OSPF standard.
It will generate more control traffic than the standard, and will not work in all situations. But, it is simpler to program and will suffice for this project.
When a link switches to bidirectional, a full database exchange should take place.
Suppose that node A has switched its bidirectional flag to node B to one,
then node A initializes a database exchange:
Node A sends an OSPF link state request packet to node B.
Node B responds by all its LSA’s in one packet.
If A does not get a LSA from B within 2 seconds, A resends an OSPF link state request packet to node B until either it gets an LSA or the link is no longer BIDIRECTIONAL.
When A receives the LSA database, it responds with a link state ACK packet.
If B fails to get a link state ACK within 2 seconds, B retransmits the LSA database. (Remember to not miss sending and receiving HELLO messages while sending a database).
B continues to retransmit the LSA database until the link is no longer BIDIRECTIONAL or the entire database has been sent.
Note, since both ends will switch the link to bidirectional, both ends will initiate a database exchange

7. Reception of link state update
A link state update is made up of one or more LSAs
When an link state update arrives, send a link state ACK to sending node (not quite the standard).
For each in the LSA in the update
if the sequence number is larger than the sequence number in the current database then
If the Link State ID of in the LSA is not the same as this node’s ID, then
Update link state database
Flood this LSA to all neighbor except the one that sent it.
else
Generate a new LSA with sequence number equal to one plus the sequence number in the arrived LSA.
Flood this LSA to ALL neighbors.
If the LSA sequence number is less than or equal to the one currently in the database, then don’t do anything.

8. Reliable Flooding
When a packet is to be flooded, put it in the ToBeFlooded list.
This list should contain the packet to be flooded, the address to where it should be sent and the time out, the next time it should be resent.
Hint: Since you should check periodically if packets need to be resent, you can initially put the packet in the list with an expired time out.
When you get an ACK, the packet should be removed from the ToBeFlooded list.

9. yes yes Initialize Send HELLOs wait for packet arrival
(but don’t wait more than 2 sec)
receive LS update
receive HELLO
Possibly clear
WaitingForUpdate
flag
Forward data packet
receive LS ACK
Clear entry on ToBeFlooded List
receive LS request
Reply to HELLO ?
does update
contains new LSAs?
Reply to HELLO
Set Bidirectional?
update LSA database
Set
compute new routing table flag
Set Bidirectional
and
Request LS update
Set waiting for LS update from XXX flag0
Add to ToBeFlooded List
Any LS update request
need to be resent?
Send HELLOs if needed
Make new routing table
If needed
send ToBeFlooded list entries
yes
Delete link
Set compute new routing
table flag
Any overdue HELLOs?
Generate new LSA
Add to be flooded list
yes
Clear LSA with MAXAGE
and add ToBeFlooded list
Age LSA
Any overdue LSA?