Network Practices - Expert Oracle RAC 12c - page 251

Fragmentation and reassembly can happen for PX messaging traffic too. As the PX execution message size
determines the size of buffer exchanged between the PX servers, the value of the PX execution message defaults to
16K in Database version 11.2 and 12c. So, RAC clusters with higher-level parallel statement execution would benefit
from Jumbo Frames configuration.
Packet Dump Analysis: MTU=1500
Packet dump analysis of cache fusion is useful to understand the intricacies of protocols, MTU, and their importance
for cache fusion traffic. The intent of this section is not to delve into network programming but to generate enough
understanding so that you can be comfortable with aspects of network engineering as it applies to cache fusion.
In this packet dump analysis, I will use UDP with a block size of 8K transferred between two nodes. I captured
these packet dumps in a two-node cluster using the Wireshark tool. The first frame is tagged as Frame 51 with a length
of 1,514 bytes. Fourteen bytes are used for the Ethernet header section.
Frame 51 (1514 bytes on wire, 1514 bytes captured)
...
Source and Destination MAC addresses follow. These MAC addresses are queried from the ARP cache. Ethernet
frame details are stored in the header section.
Ethernet II, Src: VirtualI_00:ff:04 (00:21:f6:00:ff:04),
Dst: VirtualI_00:ff:02 (00:21:f6:00:ff:02)
Destination: VirtualI_00:ff:02 (00:21:f6:00:ff:02)
Address: VirtualI_00:ff:02 (00:21:f6:00:ff:02)
IP header shows the source and target IP address. Note that these IP addresses are link local IP addresses
discussed earlier, as IP addresses are in the 169.254.x.x IP range. The length of the IP packet is 1,500 bytes, governed
by MTU of 1,500. Note that the identification is set to 21310; all fragmented packets of a segment will have same
identification number. Using this identification, kernel can reassemble all IP packets belonging to a segment.
Internet Protocol, Src: 169.254.90.255 (169.254.90.255), Dst: 169.254.87.19 (169.254.87.19)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 1500
Identification: 0x533e (21310)
Continuing the IP header section, you can see that a few flags indicate fragmentation status. In this case, more
fragments flag is set, indicating that there are more than one fragment in this stream. Fragment offset indicates the
position of this IP packet while assembling the packets.
Flags: 0x02 (More Fragments)
0... = Reserved bit: Not set
.0.. = Don't fragment: Not set
..1. = More fragments: Set
Fragment offset: 0
Time to live: 64