A convention is formal agreement between states and is usually an instrument negotiated under an international organisation.
For example, shipping has many conventions that were negotiated under International Maritime organisation (IMO). Some of these are
- International convention on safety of life at sea (SOLAS)
- International convention for prevention of pollution at sea (MARPOL)
- International convention on Load Lines
- International convention on Tonnage measurement of ships
- International convention on the control of harmful anti fouling system on ships
A protocol complements the convention.
In the beginning of most of the conventions that I listed above, you will find the "protocol" for that convention. This protocol are the rules of engagements.
This protocol gives the procedure for the states to follow when dealing with the convention.
For example in case of IMO conventions, protocol can give the procedure (percentage tonnage and number of states) of when the convention will be considered ratified.
Usually the amendment to an IMO convention automatically forms part of the convention after a certain fixed time from when this is adopted at the IMO.
But when some amendments requires change in the rules of engagements previously agreed as part of the convention, a new protocol can be issued.
These new protocols are called "Optional protocol".
This is because the amendments by these protocols are not binding on all the states that have ratified the original convention. The amendments by the protocols are only binding to the states that ratify the new protocol.
For example IMO introduced "harmonised system for ship certification". But this change required change in the protocol of the SOLAS 74. So IMO had to bring these changes by a new protocol to the SOLAS called 1988 protocol to the SOLAS.
Now all the states that had ratified the SOLAS 74 will not become party to this new protocol because the "rules of engagement" have changed now. So they may or may not choose to be part of this protocol.