Jumbo frame with MTU

　　Recently there are tests asked me jumbo and the MTU limit is what I asked to live, on-line check some information, found that most of the copy down on the encyclopedia, did not speak of the point, the key moment or colleagues, and now according to their understanding to tidy up.

First look at the basic concepts of both.

• Mtu

The Maximum Transmission unit (Maximum transmission Unit,MTU) is the maximum datagram size (in bytes) that a communication protocol can pass above a layer, which is usually closely related to the link layer protocol. The ethernetii frame structure is as follows:

DMAC (6)

SMAC (6)

Type (2)

Data (46-1500)

CRC (4)

Due to the electrical limitations of Ethernet transmission, each Ethernet frame has a minimum size of 64bytes, the maximum cannot exceed 1518bytes, and for Ethernet frames less than or greater than this limit, we can view the data as the wrong frame. These data frames are discarded by the generic Ethernet forwarding device. (Note: Data frames less than 64Bytes are generally caused by Ethernet collisions, or line interference or bad Ethernet interface, for data frames greater than 1518Bytes we generally call it giant frame, which is generally due to line interference or bad Ethernet port generation).

　　Since the maximum data frame for Ethernet ethernetii is 1518Bytes, remove the frame header of the Ethernet frame (Dmac destination MAC address 48bit=6bytes+smac source MAC address 48bit=6bytes+ Type domain 2bytes) 14Bytes and frame tail CRC checksum part 4Bytes (this part is sometimes called FCS), then the rest of the upper layer protocol is the data domain is the largest can only have 1500Bytes, this value we call it the MTU.

This MTU is a very important part of the network layer protocol, because the network layer protocol, such as the IP protocol, determines whether or not to fragment the data passed down from the top. Just like a box can't fit a chunk of bread, we need to cut the bread into pieces and put it in multiple boxes. When two remote PC interconnection, their data need to pass through a lot of routers and a variety of network media to reach the end of the network, the different media MTU are not the same, like a long period of water pipe, by different thickness of the water pipe composition (MTU different) through the water pipe the maximum amount of water is determined by the middle of the most fine water.

• Jumbo Frame

Let's take a look at the encyclopedia's conceptual explanation:

　　Jumbo frames are Ethernet frames with a frame length greater than 1522 bytes. This is a manufacturer-standard, ultra-long frame format designed specifically for Gigabit Ethernet. The length of jumbo frames varies by vendor, ranging from 9000 bytes ~64000 bytes. The use of jumbo frames can make Gigabit Ethernet performance fully, so that the efficiency of data transmission increased 50%~100%. In the application environment of networked storage, jumbo frames have an extraordinary significance.

(Note: The 1522 here should be added on the basis of the 1518 4bytes Vlan-tag)

Let's talk about the pros:

reduce the overhead of frames, increase system throughput, and reduce CPU utilization. (others to be added)

Again, the downside:

(1) because the transmission of a jumbo frame takes a long time, taking up the length of the line, resulting in network delay.

(2) Once a jumbo frame transfer fails, it is a more serious network event, and retransmission takes more time.

(3) All devices in the network must support jumbo frame for normal use.

(4) Internet links do not support jumbo frame.

• The relationship between the two　

In the actual network equipment, the MTU limit is the software level limit, but the jumbo frame is the limit hardware level, generally applies in the gigabit port or above. In principle, the minimum value of the jumbo frame is greater than the maximum frame length of the Ethernet.

Raise chestnuts: (here does not count the frame head, also does not consider the situation which does not have the Shard, knows that meaning to be OK)

(1) Jumbo frame is less than MTU: three layer physical port MTU is set to 900,jumbo frame set to 1700.

Send 500 message: Sent directly.

Send 1600 message: First Shard, then 900+700 normal issue.

(2) Jumbo frame greater than MTU: three layer physical port MTU set to 3000,jumbo frame set to 1700.

Send 500 message: Sent directly.

Send 1750 message: The Jumbo frame is limited to do packet loss processing.

Send 4000 message: Shard 3000+1000,3000 part lost, 1000 parts issued, peer check failed, discard (doubtful).

　　　

Jumbo frame with MTU