Which attribute must exist in the BGP update packet?

Assalam-o-Alaikum 

Its AS-Path a.k.a Mandatory well known Attribute, and it also includes Origin and Next HOP (these attritubes should exist in a BGP packet). 

And all the other options are Optional Attributes and they will only exist if you have made some tuning in your BGP network.     

also you can check the below :

Explanation:Origin, AS_Path, Next_Hop are well-known mandatory BGP attributes that all BGP Updatesmust include.Note:There are4 BGP attribute types:+ Well-known Mandatory: recognized by all implementations of BGP and mustappear in aBGP update message. If missing, a notification error will be generated.+ Well-Known Discretionary: recognized by all implementations of BGP but may not besent in the BGP update message (include LOCAL_PREF,ATOMIC_AGGREGATOR).+ Optional Transitive: may or may not be recognized by all BGP implementations.Because the attribute is transitive, BGP accepts and advertises the attribute even if it is notrecognized (include Community attribte).+ Optional Nontransitive: may or may not be recognized by all BGP implementations.Whether or not the receiving BGP router recognizes the attribute, it is nontransitive and isnot passed along to other BGP peers (include MED).

AS path, Origin and Next hop is mandatory attributes which must exists on every BGP updates.

C. AS_Path