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+# Cache Server
+
+The cache server is an HTTP server that is to sit on your local network.
+
+It responds to the Apple Cache API, which is actually fairly simple.
+
+When testing a cache server, the client seems to fetch / with a few specific headers.
+As you will see later, Apple makes a large use of the headers. Almost too much, but I can see why they would, they want to make sure they don't alter the response content.
+
+```http
+GET / HTTP/1.1
+Host: 10.0.0.60:54459
+Max-Forwards: 0
+X-Apple-Locator-Tag: #773c067b
+Accept: */*
+User-Agent: AssetCacheLocatorService/106.1 CFNetwork/1111 Darwin/19.0.0 (x86_64)
+Accept-Language: en-us
+Accept-Encoding: gzip, deflate
+Connection: keep-alive
+
+HTTP/1.1 200 OK
+Date: Sat, 12 Oct 2019 07:53:44 GMT
+Accept-Ranges: bytes
+Content-Length: 0
+```
+
+A sample download flow is (taken from an iPhone downloading Skate City on Apple Arcade):
+
+```http
+GET /itunes-assets/Purple113/v4/8e/ce/1d/8ece1d03-212c-0cd1-c1ec-00e5d2f38152/pre-thinned16169977081049726707.thinned.signed.dpkg.ipa?accessKey=LONG_ACCESS_TOKEN_HERE&source=iosapps.itunes.apple.com HTTP/1.1
+Host: 10.0.0.60:54459
+Apple-Download-Type: redownload
+Accept: */*
+User-Agent: appstored/1 CFNetwork/1115 Darwin/19.0.0
+Accept-Language: en-us
+Accept-Encoding: gzip, deflate
+Connection: keep-alive
+
+HTTP/1.1 200 OK
+Age: 618151
+X-Apple-MS-Content-Length: 285978974
+Access-control-allow-methods: HEAD, GET, PUT
+Server: ATS/8.0.5
+Cache-Control: private
+Strict-Transport-Security: max-age=31536000; includeSubDomains;
+X-Apple-Cached-Ranges: D885102C-110C-49BD-8672-7EF84A49C9AA bytes=0-154236588
+X-iCloud-Content-Length: 285978974
+Accept-Ranges: bytes
+Access-Control-Allow-Origin: *
+X-Responding-Server: massilia_protocol_035:735003803:ms11p01if-qufw20163201.ms.if.apple.com:8082:19T4:nocommit
+Access-control-allow-headers: range
+Content-Length: 285978974
+Via: http/1.1 ussjc2-vp-vst-008.ts.apple.com (ApacheTrafficServer/8.0.5), https/1.1 ussjc2-vp-vfe-008.ts.apple.com (ApacheTrafficServer/8.0.5), http/1.1 usscz2-edge-lx-002.ts.apple.com (ApacheTrafficServer/8.0.5), http/1.1 usscz2-edge-bx-042.ts.apple.com (ApacheTrafficServer/8.0.5)
+Date: Sat, 05 Oct 2019 04:17:18 GMT
+X-DLB-Upstream: 10.117.108.197:8082
+x-icloud-versionid: 7cfb0240-e4b1-11e9-a9f3-d8c497a3592d
+Connection: close
+Access-Control-Expose-Headers: *
+X-Apple-Cache-Session: SZOOPklKmDIW
+X-Apple-Request-UUID: cc5994da-d626-4a31-be5f-bc3c8ac905e4, cc5994da-d626-4a31-be5f-bc3c8ac905e4
+Access-control-max-age: 3000
+CDNUUID: 18fac378-7a8d-496d-9c22-1d09eeb6b56b-990994976
+Content-Type: application/octet-stream
+X-Cache: miss, hit-fresh, miss, hit-fresh
+Access-control-allow-credentials: false
+Last-Modified: Wed, 02 Oct 2019 01:10:56 GMT
+Etag: "04E6B949F0F06043F454792436ACF09D-3"
+```
+
+It seems that the hostname is in the `source` query parameter, and the access key is somehow passed along (I have yet to figure out the end result URL that the cache server primes it's cache with).
+
+A few things to note, the path of the request seems to be the path of the asset on the source, I haven't quite figured out how that works yet, as it seems like an odd choice.
+
+The actual download protocol is still in the works, but it seems as though the client can download while the server is also caching something. Also, note the availability of range requests, which means the client can request ranges of bytes from the request.
+
+All in all, it seems like a really interesting caching API. More to come soon.