Database Reference
In-Depth Information
This section shows that this segment uses UDP and specifies source and target ports. In this example, an LMS
process uses source port and a foreground process uses destination port. In Linux, the lsof command can be used to
identify the port used by a process.
User Datagram Protocol, Src Port: 20631 (20631), Dst Port: 62913 (62913)
..
Data (8328 bytes)
0000 04 03 02 01 22 7b 14 00 00 00 00 00 4d 52 4f 4e ...."{......MRON
After receiving all required IP packets (six in this example), kernel thread reassembles the IP packets to a UDP
segment and schedules a foreground process listening on the destination port 62913. User process will copy the
buffers to application buffers (either PGA or buffer cache).
Packet Dump Analysis: MTU=9000
In this section, we will review the packet dump with a path MTU of 9,000 bytes. As an 8K segment can be transmitted
using just one IP packet, there is no need for fragmentation or reassembly.
Frame 483 (8370 bytes on wire, 8370 bytes captured)
...
Frame Number: 483
Frame Length: 8370 bytes
Capture Length: 8370 bytes
The IP header section shows the source and destination IP addresses of the IP packet. The size of one IP packet
is 8,356 bytes. Also, note that
Don't Fragment
flag is set, indicating that this buffer should not be fragmented as path
MTU exceeds the IP packet size.
Internet Protocol, Src: 169.254.90.255 (169.254.90.255), Dst: 169.254.87.19 (169.254.87.19)
Version: 4
...
Total Length: 8356
Identification: 0x0000 (0)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
The UDP section identifies the source and destination ports. In this example, the length of the buffer is 8,336
bytes, with a data payload of 8,328 bytes.
User Datagram Protocol, Src Port: 20631 (20631), Dst Port: 62913 (62913)
Source port: 20631 (20631)
Destination port: 62913 (62913)
Length: 8336
Essentially, one IP packet of 8,356 bytes is used to transport a payload of 8,328 bytes, avoiding fragmentation
and reassembly.
