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All about RAC and MTU with a video

Let’s first discuss how RAC traffic works before continuing. Environment for the discussion is: 2 node cluster with 8K database block size, UDP protocol is used for cache fusion. (BTW, UDP and RDS protocols are supported in UNIX platform; whereas Windows uses TCP protocol).

UDP protocol, fragmentation, and assembly

UDP Protocol is an higher level protocol stack, and it is implemented over IP Protocol ( UDP/IP). Cache Fusion uses UDP protocol to send packets over the wire (Exadata uses RDS protocol though).

MTU defines the Maximum Transfer Unit of an IP packet. Let us consider an example of MTU set to 1500 in a network interface. One 8K block transfer can not be performed with just one IP packet  as the IP packet size (1500 bytes) is less than 8K. So, one transfer of UDP packet of 8K size is fragmented to 6 IP packets and sent over the wire. In the receiving side, those 6 packets are reassembled to create one UDP buffer of size 8K. After the assembly, that UDP buffer is delivered to an UDP port of a UNIX process. Usually, a foreground process will listen on that port to receive the UDP buffer.

Consider what happens If MTU is set to 9000 in the network interface:  Then 8K buffer can be transmitted over the wire with just one IP packet. There is no need for fragmentation or reassembly with MTU=9000 as long as the block size is less than 8K. MTU=9000 is also known as jumbo frame configuration.  ( But, if the database block size is greater than jumbo frame then fragmentation and reassembly is still required. For example, for 32KB size, with MTU=9000,  there will three 9K IP packets  and one 5K IP packet to be transmitted).

Fragmentation and reassembly is performed at OS Kernel layer level and hence it is the responsibility of Kernel and the stack below to complete the fragmentation and assembly. Oracle code simply calls the send and receive system calls, passes the buffers to populate.

Few LMS system calls in Solaris platform:

0.6178  0.0001 sendmsg(30, 0xFFFFFFFF7FFF7060, 32768)          = 8328
0.6183  0.0004 sendmsg(30, 0xFFFFFFFF7FFFABE0, 32768)          = 8328
0.6187  0.0001 sendmsg(36, 0xFFFFFFFF7FFFBA10, 32768)          = 144
0.7241  0.0001 recvmsg(27, 0xFFFFFFFF7FFF9A10, 32768)          = 192
0.7243  0.0001 recvmsg(27, 0xFFFFFFFF7FFF9A10, 32768)          = 192


If you talk to a network admin about use of UDP for cache fusion, usually, there will be few eyebrows raised about the use of UDP. From RAC point of view, UDP is the right choice over TCP for cache fusion traffic. With TCP/IP, for every packet transfer has overhead, connection need to be setup, packet sent, and the process must wait for TCP Acknowledgement before considering the packet send as complete. In a busy RAC systems, we are talking about 2-3 milli-seconds for packet transfer and with TCP/IP, we probably may not be able to achieve that level of performance. With UDP, packet transfer is considered complete, as soon as packet is sent and error handling is done by Oracle code itself. As you know, reliable network is a key to RAC stability, if much of packets (closer to 100%) are sent without any packet drops, UDP is a good choice over TCP/IP for performance reasons.

If there are reassembly failures, then it is a function of unreliable network or kernel or something else, but nothing to do with the choice of UDP protocol itself. Of course, RDS is better than UDP as the error handling is offloaded to the fabric, but usually require, infiniband fabric for a proper RDS setup. For that matter, VPN connections use UDP protocol too.

IP identification?

In a busy system, there will be thousands of IP packets traveling in the interface, in a given second. So, obviously, there will be many IP packets from different UDP buffers received by the interface. Also, because these ethernet frames can be delivered in any order, how does Kernel know how to assemble them properly? More critically, how does the kernel know that 6 IP packets from one UDP buffer belongs together and the order of those IP packets?

Each of these IP packet has an IP identification and fragment offset. Review the wireshark files uploaded in this blog entry, you will see that all 6 IP packets will have the same IP identification. That ID and the fragment offset is used by the kernel to assemble the IP packets together to create UDP buffer.

Identification: 0x533e (21310)
Fragment offset: 0

Reassembly failures

What happens if an IP packet is lost, assuming MTU=1500 bytes?

From the wireshark files with mtu1500, you will see that each of the packet have a Fragment offset. That fragment offset and IP identification is used to reassemble the IP packets to create 8K UDP buffer. Consider that there are 6 puzzle pieces, each puzzle piece with markings, and Kernel uses those markings( offset and IP ID) to reassemble the packets. Let’s consider the case, one of 6 packet never arrived, then the kernel threads will keep those 5 IP packets in memory for 64 seconds( Linux kernel parameter ipfrag_time controls that time) before declaring reassembly failure. Without receiving the missing IP packet, kernel can not reassemble the UDP buffer, and so, reassembly failure is declared.

Oracle foreground process will wait for 30 seconds (it used to be 300 seconds or so in older version of RAC) and if the packet is not arrived within that timeout period, FG process will declare a ‘GC lost packet’ and re-request the block. Of course, kernel memory allocated for IP fragmentation and assembly is constrained by Kernel parameter ipfrag_high_thres and ipfrag_low_thres and lower values for these kernel parameters can lead to reassembly failures too (and that’s why it is important to follow all best practices from RAC installation guides).

BTW, there are few other reasons for ‘gc lost packets’ too. High CPU usage also can lead to ‘gc lost packets’ failures too, as the process may not have enough cpu time to drain the buffers, network buffers allocated for that process becomes full, and so, kernel will drop incoming packets.

It is probably better to explain these concepts visually. So, I created a video. When you watch this video, notice that there is HD button on the top of the video. Play this in HD mode so that you will have better learning experience.

You can get the presentation file from the video here: MTU

Wireshark files explained in the video can be reviewed here:

BTW, when you review the video, you will see that I had little bit trouble identifying the packet in the wireshark output initially. I understood the reason for not seeing the packets filled with DEADBEEF characters. Why do you think I didn’t see the packets initially?

Also, looks like, video quality is not that great when embedded. If you want actual mp4 files, let me know, may be I can upload to a drop box and let you download, email me.

MTU Vidoe