What is the point of using a cryptographic sequence number if you're not using it as a nonce? OSPF features the use of Auth Crypto Sequence Number however, from reading the RFC it would appear the sequence number is not being used as a nonce.


  In addition, a non-decreasing sequence number is included in 
  each OSPF protocol packet to protect against replay attacks. 
  This provides long term protection; however, it is still 
  possible to replay an OSPF packet until the sequence number 
  changes. - OSPF MD5 Authentication Draft II


Correct me if I am wrong but isn't the whole point of a cryptographic sequence number to protect against replay attacks? If so, why would you not use a nonce? Is there any benefit to this? Or is this just a weakness which was never improved upon? 

I'm also struggling to make the distinction of "in the long term less of an issue" - how does that even make sense?
After reading the draft, I think that the purpose of using a sequence number in place of a nonce is because handling nonce in practical situations is very difficult. We need to keep a directory of the used nonces and then do a search through them. You can maintain a heap for the already seen nonces, but the problem is of storing them.

In the draft it says - "The value used in the sequence number is arbitrary, but two suggestions are the time of the message's creation or a simple message counter". This gives a very clear idea that the way they want sequence numbers to be used in practice is to use them in a strictly increasing fashion so that there is no need to store all the seen values and just the latest one can be stored. 
I also think that using increasing sequence numbers which wrap (start to repeat) around after a decent amount of time would also work because by the time the sequence numbers repeat, the validity of the message will long be expired.