# Micropython - TCP Header - Maximum Segment Size - MSS

**URL:** <https://forums.digi.com/t/micropython-tcp-header-maximum-segment-size-mss/21411>\
**Category:** XBee Micropython & IDE plugin\
**Tags:** tcp-sockets, micropython, tcp, payload, xbee\
**Created:** [May 28, 2020, 9:39am UTC](https://forums.digi.com/t/micropython-tcp-header-maximum-segment-size-mss/21411 "2020-05-28T09:39:40Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![gopinaath](https://avatars.discourse-cdn.com/v4/letter/g/59ef9b/32.png) [@gopinaath](https://forums.digi.com/u/gopinaath)\
**Post date:** [May 28, 2020, 9:39am UTC](https://forums.digi.com/t/micropython-tcp-header-maximum-segment-size-mss/21411/1 "2020-05-28T09:39:40Z")

</div>

Trying to communicate using MQTT using Micropython on Xbee3 Cellular Modem.

It appears that Micropython’s usocket library sends with TCP Header Option- Maximum Segment Size(MSS) that is very low (less than 50). Such low values are considered a security vulnerability and discarded by Layer 4 appliances and firewalls causing failures and problems.

Can someone please let me know how I can update the TCP Options Header of MSS in Micropython, / usocket library or even outside?

Linking below information for TCP MSS pointers and the IETF standards for additional pointers on TCP MSS:

[https://blog.apnic.net/2019/07/31/tcp-mss-values-whats-changed/](https://blog.apnic.net/2019/07/31/tcp-mss-values-whats-changed/)

[https://tools.ietf.org/html/rfc879](https://tools.ietf.org/html/rfc879)