Streaming Media Supplement sa2150 and sa2250

Chapter 8 Configuring Media-IXT for WMT

Media-IXT’s RAM cache

The variable proxy.config.cache.ram_cache_mixt_cutoff controls the size of documents stored

in Media-IXT's RAM cache. Its default value of zero (0) allows arbitrary size documents to be stored in cache,

which is appropriate for streaming media.

The only reason to change this value is a severe memory limitation that causes you to want to control the

information stored in RAM cache.

If you edit the value of proxy.config.cache.ram_cache_mixt_cutoff c, and this value is lower

than the size of an object, that object must be read off disk even if it is a cache hit.

Storing packets for retransmission

You can configure the number of packets which Media-IXT stores in case it receives a retransmission request

from the client. At one extreme, Media-IXT can store zero packets, reducing its memory footprint to a minimum

but turning off the ability to service retransmission requests. At the other extreme, Media-IXT can store the

entire window of packets (for the MMS protocol, this is 256 packets), maximizing ability to service

retransmission requests, but also maximizing memory footprint.

The configuration variable that controls storing packets for retransmission is:

proxy.config.wmt.rexmit.window

which is not in records.config by default. If you wish to tune the variable to a non-default value, you

must add the variable to records.config. When the variable is not in records.config its default value

is in effect. The default value is the maximum possible: 256.

If you wish to reduce Media-IXT's memory footprint, while not critically impairing its ability to service

retransmit requests, start by tuning the variable to 50. You may want to try a higher value after observing the

effects of 50 in your deployment.

When a client player is communicating with Media-IXT and UDP is the underlying transport protocol being

used by the data transfer protocol, the player may request retransmission if there is packet loss in the network.

The origin WMT server provides best-effort service for retransmission requests: it buffers some number of

packets after it has sent them to the player. This buffer is normally 256 or fewer packets; so the maximum

number of packets that the origin WMT server buffers is the same as the default value of

proxy.config.wmt.rexmit.window.

To sum up, if you expect Windows Media Player clients to be use UDP to communicate with Media-IXT, and

if you expect some network congestion that could cause the players to send retransmission requests for some

packets, and if you think that WMT origin servers' best-effort servicing of those requests is likely to be

inadequate, then you should leave proxy.config.wmt.rexmit.window

at or close to its default

maximum.

To tune the variable that controls storing packets for retransmission for WMT:

1. Add this line to records.config, where XXX is the number of packets you want Media-IXT to store

for service of retransmission requests:

CONFIG proxy.config.wmt.rexmit.window INT XXX

2. Restart Media-IXT.

Configuring a non-default WMT port in a forward proxy deployment

When, in a forward proxy deployment, you want Media-IXT’s WMT port to be something other than the

default, which is port 1755, you must use the proxy.config.wmt.port configuration variable.

In a forward proxy deployment:

• when Media-IXT rewrites the WMT metafile, this variable's value determines the WMT port that is in the

rewritten media content URL.