NSURL mechanism doesn't automatically set last-modified or etag headers, so this category does if the cache contains cached responses.

NSMutableURLRequest+CacheControl.m

//
//  NSMutableURLRequest+CacheControl.m
//
//  Created by Paul Goracke on 2/26/13.
//

@implementation NSMutableURLRequest (CacheControl)

+ (instancetype) newCachePolicyRequestWithURL:(NSURL *)url timeoutInterval:(NSTimeInterval)timeoutInterval {
  // If we have cached info, tell request to ignore cache because we only want to process data when we get new from server
	// request mechanism doesn't automatically configure "If-Modified-Since" or "If-None-Match" if we have that info
	
	NSMutableURLRequest* request = [[NSMutableURLRequest alloc] initWithURL:url
					cachePolicy:NSURLRequestUseProtocolCachePolicy
					timeoutInterval:timeoutInterval];
	
	NSURLCache* cache = [NSURLCache sharedURLCache];
	NSCachedURLResponse* cachedResponse = [cache cachedResponseForRequest:request];
	NSHTTPURLResponse* urlResponse = (NSHTTPURLResponse*)[cachedResponse response];
	
	NSString* lastModified = [[urlResponse allHeaderFields] objectForKey:@"Last-Modified"];
	if ( lastModified != nil ) {
		[request setValue:lastModified forHTTPHeaderField:@"If-Modified-Since"];
		// prefer server modification handling over local caching
		request.cachePolicy = NSURLRequestReloadIgnoringCacheData;
	}
	
	NSString* etag = [[urlResponse allHeaderFields] objectForKey:@"ETag"];
	if ( etag != nil ) {
		[request setValue:etag forHTTPHeaderField:@"If-None-Match"];
		// prefer server modification handling over local caching
		request.cachePolicy = NSURLRequestReloadIgnoringCacheData;
	}
	
	return request;
}

@end