Streaming Media Supplement sa2150 and sa2250

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Chapter 2 Media-IXT Deployment Scenarios
Let’s look at how forward proxy, reverse proxy, and transparency are used:
• A proxy cache is usually either a forward proxy cache or a reverse proxy cache, but can be both
• A forward proxy cache is usually either transparent or explicit, but can be both
• A reverse proxy cache is normally neither transparent nor explicit
The HP Cache Server Appliance Administrator Guide explains the wealth of possibilities these three statements
imply. This manual supplements those explanations with specifics about proxy caching of streaming media
content.
Forward, explicit proxy caching
In explicit proxy caching, the client browser must be explicitly configured to direct its requests to the proxy
cache. Since the client knows about the proxy cache, the proxy cache is not transparent to the client; so, explicit
proxy caching is also called non-transparent proxy caching. Access providers often deploy a forward, explicit
proxy cache between upstream origin servers and downstream client players.
Forward, transparent proxy caching
When clients are not configured to explicitly direct their requests to a proxy cache, but the network is designed
so that a proxy cache server receives the client requests anyway, we say that the proxy cache is transparent to
the client.
When clients request content from a web server, a transparency device (a layer 4 switch or WCCP2-compatible
router) intercepts the request and redirects it to the proxy cache. The proxy cache checks whether the content is
in cache and fresh, and if so, serves the content to the client. Serving content from the proxy cache offloads
traffic from the original web server and provides faster delivery if the proxy cache is closer to the client than
the web server. The proxy transaction is transparent in that the client is unaware that the object is served from
a cache.
Transparency requires your network to have:
• a layer 4 switch,
- OR -
• a WCCP2-compatible router
Transparent proxy caching for HTTP and NNTP is discussed in the HP Cache Server Appliance Administrator
Guide. This manual covers forward, transparent proxy caching for RealNetworks, WMT, and QuickTime
content.
Transparency and the layer 4 switch
A layer 4 switch can examine the TCP packet to see what transport protocol is being used, and make routing
decisions based on what it finds. The switch can distinguish HTTP, RTSP, or MMS traffic from other kinds,
and redirect such traffic to Media-IXT.
The layer 4 switch provides transparency using a technique called MAC-level redirection, which works like this:
• you configure the layer 4 switch to redirect all HTTP, RTSP, and/or MMS traffic to your Media-IXT
• the layer 4 switch knows the MAC address of the next-hop device on each of its physical ports (its ethernet
ports); one of these devices is your Media-IXT
• the layer 4 switch looks in a TCP packet coming from the client; sees to what port the packet is addressed;
and therefore knows the packet’s protocol (for example, port 554 = RTSP)
• if the packet’s protocol is one that the layer 4 switch is configured to redirect, the switch replaces the
packet's next-hop MAC address with the MAC address of your Media-IXT
• the packet continues on its way, passing out of the switch to Media-IXT