First of all, the pack has to be versatile, meaning that it has to able to be adapted to carry different types of equipment. For example, in a normal civilian backpack you can put different things in it, but you can't carry a couple of 20 liter Jerry cans. An external frame pack is necessary. This design is very very old as it was used in ancient Asia and America to transport goods through terrain not well suited for carts. The design consists of a frame (wood back then, now welded ultralight aluminum tubing) that is attached to straps to the wearer. Pretty much everything can be attached to this frame including a traditional bag. With a pack mounted to the frame, little or no difference can be seen from afar. This design was greatly developed for the Vietnam war, as fighting in the jungle posted a problem to conventional transport trucks. The most common to find in massive quantities in army surpluses is the ALICE (All-Purpose Lightweight Individual Carrying Equipment) pack.
The basic pack consists of the tubular frame, shoulder straps, lumbar support. From this basic design a number of official and unofficial issue add-ons are possible. The two pictures below illustrate of a basic ALICE pack with customised add-ons for various equipement.
I bought this pack from a Canadian based army surplus for 89.99$CAN but it can easily be found worldwide for the same price. Be very careful to buy an authentic frame and pack or an equal aftermarket one. I came across some very nice looking packs and close examination revealed that they were cheap Chinese copies and of very poor quality. In the case of the bag I bought, it is an aftermarket frame and bag, but of equal quality to Canadian frames. A good bag will incorporate both MOLLE and PALS attachment systems. This will enable you to add other specialty external pouches to the bag like the Smith Ballistic Goggle Pouch and the Military Issue Water Bottle. Personally I fond that the PALS ladder system is the best as it provides a secure anchorage point and a tight fit for the equipment. Keep reading for future posts!