Isn't it funny that when you think of a certain country, there's always one particular definitive set that seems to be synonymous with that country? Do you have that too? So when I think of Britain, I think of Machins, just like France and La Semeuse go together, Norway and the Posthorns, Germany and Germania, etc. I suppose it's a personal thing and it just depends on your taste and you collecting interests, but still.
When thinking of Sudan, I immediately think of the Camel Postman definitives.
It's an absolute favourite of mine, even though I haven't really started collecting or studying them. I just pick up copies whenever I come across them, for future study.
The design has been around since 1898, and even though I don't think it's part of any current definitive set (I could be wrong though!), for a long time, at least one value of a definitive set used to have this design. The design has also been incorporated in various commemorative issues. And of course there's lots of varieties to collect; different sizes, design changes, watermark changes, overprints, etc.
Looks like you could be busy for a long time trying to build up a nice collection! But today I'm just showing you the stamps so you can enjoy the design!