Reading some logs from my system it states

eth0: flags 4163 < up, broadcast, running, multicast> mtu

I am not sure what is the meaning of the flag here down below I see eth1,2,3 has the same flag ? I searched in google but nothing really helpful my other question is related to mtu- maximum transmitted unit- which means that the size of the largest protocol data unit that can be communicated in a single network layer transaction. however the mtu relates to but is not identical to the maximum frame size - I do not understand this one here . what is the difference between pdu and frame size I thought they are both identical


According to standard unix header file, we have up=0x1, running=0x40, broadcast=0x2, multicast=0x1000. The flag value is bitwise-OR of the values. 4163 is decimal if you convert it to hex 4163 = 0x1043 = 0x1000 + 0x40 + 0x2 + 0x1.

MTU is Maximum Transmitted Unit. It is property of specific network interface. MTU does not include header size of frames/packets. Take for example an ethernet frame, it has header size for 18 Bytes and maximum data size (payload) is 1500 Bytes. Therefore for Ethernet maximum frame size is 1518 bytes. Here 1500 Bytes is MTU. As you can see both are different.

According to wikipedia

PDU is single unit of information transmitted among peer entities

  • For Transport Layer PDU is the segment or datagram.
  • For Network Layer PDU is the packet.
  • For Data link Layer PDU is the frame.
  • For Physical Layer PDU is the bit.

In above example 1518 Bytes is PDU.

  • trying to find where did you find this standard unix header file – THEGreatGatsby Mar 30 '19 at 10:34
  • @THEGreatGatsby here you can find out about ifconfig flags. – Vineet Apr 1 '19 at 12:42

The maximum frame size or maximum L2 PDU is the largest frame a given network can transport. For standard Ethernet that's 1518 bytes. (1522 bytes with 802.1Q tag, additional Ethernet options may require larger maximum frames or eat into the payload size.)

The Ethernet L2 PDU includes the L2 overhead of 18 bytes (plus options) and can transport a maximum payload (L2 SDU) of 1500 bytes - this is a general constant for Ethernet.

Accordingly, 1500 bytes is also the maximum packet size in the network layer (L3 PDU or MTU for IP) when Ethernet is used.

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