nginx错误日志分析，求大神帮助！！

Question

如下的日志中，client、server和host字段都是什么含义？

2017/06/21 13:00:10 [error] 2229#2229: *3708369 upstream prematurely closed connection while reading response header from upstream, client: 172.17.81.1, server: i.snssdk.com, request: "POST /video/app/stream/v42/?category=video_new&iid=11445996226&device_id=36158742811&ac=wifi&channel=toutiaodsp29&aid=32&app_name=video_article&version_code=600&version_name=6.0.0&device_platform=android&user_version=2.0.0&ab_version=132561%2C135627%2C140259%2C131047%2C135779%2C138001%2C139491&ab_feature=z1&ssmix=a&device_type=H15&device_brand=alps&language=zh&os_api=19&os_version=4.4.2&uuid=862401024512458&openudid=6dd743d94992848&aliyun_uuid=8D62E0C3EA3ADB9C743D94F079A39941&manifest_version_code=200&resolution=480*854&dpi=240&update_version_code=6003&_rticket=1498021208099 HTTP/1.1", upstream: "http://127.0.0.1:8425/video/app/stream/v42/?category=video_new&iid=11445996226&device_id=36158742811&ac=wifi&channel=toutiaodsp29&aid=32&app_name=video_article&version_code=600&version_name=6.0.0&device_platform=android&user_version=2.0.0&ab_version=132561%2C135627%2C140259%2C131047%2C135779%2C138001%2C139491&ab_feature=z1&ssmix=a&device_type=H15&device_brand=alps&language=zh&os_api=19&os_version=4.4.2&uuid=862401024512458&openudid=6dd743d94992848&aliyun_uuid=8D62E0C3EA3ADB9C743D94F079A39941&manifest_version_code=200&resolution=480*854&dpi=240&update_version_code=6003&_rticket=1498021208099", host: "ic.snssdk.com"