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Church Of The
Swimming Elephant

What is... Packets/MTU?
Packet/MTU Theory.

Page 2

By John Holstein, Cotse Help Desk Coordinator

Packet/MTU Size

Packet size, also referred to as MTU (Maximum Transmission Unit) is the largest amount of data that can be transferred in one physical frame on the network.

Standard MTU Sizes:
Ethernet MTU = 1500 bytes
PPPoE MTU = 1492 bytes
Dial-Up MTU = 576 bytes

A packet or "MTU" consists of a header and the actual data. The actual data is referred to as the Maximum Segment Size or "MSS". The MSS is the largest segment of TCP data that can be transmitted. In other words: MTU = MSS + TCPIP headers.

  • MSS = MTU - (TCP20 + IP20) or MSS = MTU - 40
  • no packet fragmentation
  • zero packet loss
  • zero router congestion

    Packet Size vs. Latency

    Let's examine a transfer of 1,500,000 bytes of data using different packet size over a T1 line (T1=1,544,000 bits/sec) using the following formula:

    ( MSS + header ) * 8 bits/byte
    ------------------------------ = latency (per hop)
    1,544,000 bits/sec.

    Then, using different MTU values, we can calculate the relevance of packet size to latency.

    If MTU = 1500, then: (1460+40) * 8 / 1,544,000 = 7.772 ms delay per hop
    If MTU = 576, then: (536+40) * 8 / 1,544,000 = 2.924 ms delay

    Assuming a transfer over 10 hops, the 1500 MTU would wield 77.72 ms delay, while a 576 MTU would take 29.24 ms to transfer over a T1 line.

    So what are we getting at? Well, it takes more time to send/receive larger packets. DUH!? How much mathematics do we need to understand this?

    Of course we also need to look at it this way:

    Even though we are receiving the smaller packets and they are transmitting faster, the overall speed in which we receive the TOTAL file will vary. That is the point in changing your MTU.

    In some instances, the Total File will be received faster by smaller packet sizes and vice versa. Larger packets will present the Total File to you in certain circumstances.

    Best piece of advise:

    Play with your MTU / MRU size to get the best overall results.

    Continue to Page 3

    Comments? Questions? Bugs? John Holstein

    Return to the Help Desk

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