structure: nginx<->backend server

the nginx and backendserver enable gzip.

nginx.conf:

worker_processes 1;

error_log /var/log/nginx/error.log warn; pid /var/run/nginx.pid;

worker_rlimit_nofile 1024;
events {
worker_connections 1024;
}
http {
include /etc/nginx/mime.types; default_type application/octet-stream;

log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';

access_log /var/log/nginx/access.log main;
sendfile on;
tcp_nopush on;
keepalive_timeout 120;
gzip on;
gzip_types application/json text/plain application/x-javascript application/javascript text/javascript text/css application/xml text/xml;
gzip_min_length 1k;
server_tokens off;
upstream backend{
server xxx.xxx.xxx.xxx;
keepalive 120;
}
proxy_temp_path /tmp/cache_tmp;
proxy_cache_path /tmp/cache levels=1:2 keys_zone=cache1:100m inactive=7d max_size=10g;
server{
listen 80;
location / {
proxy_pass http://backend;
proxy_http_version 1.1;
proxy_set_header Range $http_range;
proxy_set_header If-Range $http_if_range;
proxy_set_header Connection "";
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header Accept-Encoding "gzip";
proxy_cache cache1;
proxy_cache_key $uri$is_args$args;
proxy_cache_revalidate on;
}
}
}
Now, I use iftop watch, RX>TX, why ? When i disable proxy_cache , the RX=TX