[Differential] D19422: if_vxlan(4) Allow set MTU more than 1500 bytes.

[aleksandr.fedorov_itglobal.com (Aleksandr Fedorov)]

aleksandr.fedorov_itglobal.com updated this revision to Diff 59753.
aleksandr.fedorov_itglobal.com added a comment.


  VXLAN encapsulate ethernet frames within UDP/IP packets. So, we can calculate maximum overhead for IPv4:
  
  - IP_MAXPACKET = 65K - constant from netinet/ip.h.
  - Maximum IP header length IP_MAX_HDR_LEN (There is no suitable constant for it.) = 60 as the Internet Header Length field is the unsigned 4-bit number of 32-bit words - 15 * 32 = 480 bit = 60 bytes.
  - sizeof(struct udphdr) = 8 bytes.
  - sizeof(struct vxlan_header) = 8 bytes.
  - Inner frame ETHER_HDR_LEN = 14 bytes.
  - Inner frame ETHER_CRC_LEN = 4 bytes.
  - Inner frame ETHER_VLAN_ENCAP_LEN = 4 bytes.
  
  The result VXLAN_MAX_MTU = IP_MAXPACKET - IP_MAX_HDR_LEN - sizeof(struct udphdr) - sizeof(struct vxlan_header) - ETHER_HDR_LEN - ETHER_CRC_LEN - ETHER_VLAN_ENCAP_LEN = 65535 - 60 - 8 - 8 - 14 - 4 - 4 = 65437
  So overhead is 98 bytes.
  
  Unfortunately for IPv6, the maximum header length is not fixed and can be extended in the future. It is usually 40 bytes.
  
  Revision changes:
  
  - Calculate VXLAN_MAX_MTU using existing constants.

CHANGES SINCE LAST UPDATE
  https://reviews.freebsd.org/D19422?vs=59684&id=59753

CHANGES SINCE LAST ACTION
  https://reviews.freebsd.org/D19422/new/

REVISION DETAIL
  https://reviews.freebsd.org/D19422

AFFECTED FILES
  sys/net/if_vxlan.c

EMAIL PREFERENCES
  https://reviews.freebsd.org/settings/panel/emailpreferences/

To: aleksandr.fedorov_itglobal.com, bryanv, hrs, #network, rgrimes, krion, jhb
Cc: evgueni.gavrilov_itglobal.com, olevole_olevole.ru, ae, freebsd-net-list, krzysztof.galazka_intel.com
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