Databases Reference
In-Depth Information
LAN versus WAN
Because communication across a WAN typically requires more network hops
than communication across a LAN, your application is more likely to encounter
varying MTU sizes, resulting in packet fragmentation. In addition, if data has to
travel over VPN within a WAN, packet fragmentation further reduces the MTU
size. If you are unable to avoid packet fragmentation by setting the client and the
database server to the path MTU (see “Understanding Maximum Transmission
Unit (MTU),” page 99), it becomes even more important to reduce the number
of network round trips between the client and server to preserve performance.
Detecting and Resolving Network Packet Fragmentation
If you don't have privy knowledge of the MTUs of every network node along the
network path, how can you tell if packet fragmentation occurs? Operating system
commands, such as the
ping
command (Windows) and the
traceroute
com-
mand (UNIX/Linux), can help you determine if packets are being fragmented
along a particular network path. In addition, with a little persistence and detec-
tive work, you can determine the optimal packet size for the network path, a size
that doesn't require packet fragmentation.
For example, suppose your client is a Windows XP machine, and data
requests are made from this machine to a UNIX database server located in
London. You know from the following trace report that three network hops are
involved to reach the server:
Tracing route to UK-server-03 [10.131.15.289]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 10.30.4.241
2 <1 ms <1 ms <1 ms 10.30.4.245
3 112 ms 111 ms 111 ms 10.168.73.37
4 113 ms 112 ms 116 ms 10.131.15.289
Therefore, the network path looks similar to the configuration shown in
Figure 4-19. If the MTU of the client is set to a value of 1500, the client sends
1500-byte packets across the network. The MTU of the other network nodes is
unknown.
