On 7/11/19 7:04 PM, Peter Schneider wrote:
Hi,
I stumbled on an incompatibility between the IP Management YANG Module
and the real world:
In the 'container ipv4', the leaf 'mtu' is declared as uint16 in the
range 68..max, which is effective the range 68..65535, as noted in rfc
7950 <https://tools.ietf.org/html/rfc7950#section-9.2> .
On the other side, the default MTU size of the loopback interface in
Linux is 65536 since several years.
I don't think there is a problem with RFC 8344 since the ipv4 protocol
MTU can not exceed 65535 bytes (RFC 791). The maximum payload supported
by an interface which is the case with the default value of the lo
interface MTU on your Linux distribution (seems this can be configured
to up to 2147483647 octets at least on mine) has to be greater then the
MTUs of all protocols (ipv4,ipv6 etc.) supported on that interface but
they do not need to be equal. It is indeed recommended that the maximum
"length of the data field of a packet" is used as defined in RFC 894
e.g. 1500 for ethernet but you can choose or be constrained (like in
this case) to not do so.
Currently there is no IETF RFC module that has data definition for
control of the interface MTU used as an upper limit for all protocol MTUs.
FYI there is a /interfaces/interface/l2-mtu leaf proposed in
draft-ietf-netmod-intf-ext-yang-07 where an uint16 type should probably
be changed to uint32.
IMO "/interfaces/interface/mtu" leaf is needed that actually
corresponds to the maximum length of the data field of a packet and
already referred to in popular software like Linux as "interface MTU"
(there is no standard this is based on just running software as far as I
am aware) should be added in addition to the l2-mtu leaf. Then in your
case this leaf would be configured to 65536 for the lo interface while
the /interfaces/interface/ipv4/mtu can be less then that.
/ Vladimir
Depending on the used netconf client, the client either reports
(correctly) an error when getting / configuring the loopback
interface, or silently reduces /raises the shown resp. configured
value for the mtu.
Are there any statements available on this issue?
Kind Regards,
Peter Schneider
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