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Michael Uray
Asked: 2019-01-26 18:15:12 +0800 CST

Is there a TCP Maximum Segment Size (MSS) Minimum?

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I have there a OpenVPN VPN network connection, where the MSS size 60 gets send out for some reason (don't know why yet). EDIT: Reason: OpenVPN mssfix was mistakenly set to 1

36  92.064383   10.102.0.43 10.1.151.50 TCP 52  49991 → 63760 [SYN] Seq=0 Win=65535 Len=0 MSS=60 WS=128 SACK_PERM=1
37  92.064763   10.1.151.50 10.102.0.43 TCP 52  63760 → 49991 [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=1460 WS=256 SACK_PERM=1

There are two devices on this network, which behave different.

  • Win 10 PC with Filezilla FTP Server
  • Embedded device (PLC) based on VxWorks with internal FTP server

The PC sets the MSS size to 536, but the PLC sets it down to 60 like requested.

41  92.171676   10.1.151.50 10.102.0.43 FTP-DATA    576 FTP Data: 536 bytes (PASV) (RETR TCData.br)
67  17.385576   10.1.151.1  10.102.0.43 FTP-DATA    100 FTP Data: 60 bytes (PASV) (RETR TCData.br)

Is that implemented on the right way on both sides?

I know that the default TCP MSS is 536, but is there a required minimum size?

ip

1 Answers

  1. The minimum MSS is controlled by the minimum IP datagram MTU of 576 bytes. Since TCP uses 40 bytes of overhead, then the minimum MSS is 536 bytes. If more bytes are used, say for timestamps, then the MSS would be smaller

    This can be found in the IP RFC 791 Section 3.1. Internet Header Format - > Total Length.

    Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to be up to 65,535 octets. Such long datagrams are impractical for most hosts and networks. All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole or in fragments). It is recommended that hosts only send datagrams larger than 576 octets if they have assurance that the destination is prepared to accept the larger datagrams.

    The number 576 is selected to allow a reasonable sized data block to
be transmitted in addition to the required header information.  For
example, this size allows a data block of 512 octets plus 64 header
octets to fit in a datagram.  The maximal internet header is 60
octets, and a typical internet header is 20 octets, allowing a
margin for headers of higher level protocols.
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