Databases Reference
In-Depth Information
In most modern systems, packet fragmentation is not automatic but occurs
as a result of a process known as
path MTU discovery
, a technique for determin-
ing the
path MTU
, which is the lowest MTU of any network node along a partic-
ular network route. Because packet fragmentation requires additional
communication between network nodes to negotiate the correct packet size and
significant CPU processing to divide communication into smaller packets and
reassemble them, it degrades performance. The following sections explain why
packet fragmentation has a negative impact on performance and provide guide-
lines for detecting and resolving packet fragmentation.
Understanding Maximum Transmission Unit (MTU)
MTU is the maximum packet size that can be sent across a particular network
link as determined by the network type. See Table 4-2 for the MTU values of
some common network types.
Table 4-2
MTU Values of Common Network Types
Network
MTU
16 MB/second Token Ring
17914
4 MB/second Token Ring
4464
FDDI
4352
Ethernet
1500
IEEE 802.3/802.2
1492
PPPoE (WAN miniport)
1480
X.25
576
Each network node has one or multiple network adapters installed, one for
each network it connects to. The operating system on each node provides an
MTU setting for each network adapter. The MTU setting determines the size of
network packets sent from that node. By default, this MTU setting is set to the
MTU of the network type and can be set to another value, but that value cannot
exceed the MTU of the network type. For example, if a network node is con-
nected to an Ethernet network, the MTU setting for that machine's network
adapter must be set to a value of 1500 (MTU for Ethernet networks) or less.
How does MTU affect network packets? Let's consider a simple example
where only two network nodes, a client and database server, send and receive net-
work packets as shown Figure 4-16. In this case, Node A has an MTU setting of
1500, meaning that it sends 1500-byte packets across the network to Node B.
