Feature #5138
Updated by Junxiao Shi over 3 years ago
Currently, `nfdc face create` command allows setting MTU when creating or updating a UDP face.
This allows the operator to select NDNLPv2 fragmentation instead of IP fragmentation, which is effective in working around networks that blocks/drops IPv4 fragments or IPv6 fragments.
However, MTU option is unavailable for on-demand faces: all on-demand UDP transports are created with the default MTU of `MAX_NDN_PACKET_SIZE` i.e. 8800 octets.
In global NDN testbed operations, [it has been discovered](https://named-data.slack.com/archives/C01EKSVNAB1/p1608257422016000) that some NAT middleboxes and firewalls could block egress IPv4 fragments, causing UDP connections between unrestricted end hosts and testbed routers to become unreliable or fail, when a large packet is transmitted from the testbed router to the end host.
It is necessary for the testbed router in such environments to lower the MTU of on-demand UDP faces, so that outgoing packets would use NDNLP fragmentation.
This issue is to introduce a configuration option:
```
face_system
{
udp
{
unicast_mtu 1452 mtu 1450
}
}
```
This `face_system.udp.unicast_mtu` `face_system.udp.mtu` option accepts a positive number that sets the MTU for on-demand UDP faces.
It also serves as the default MTU for UDP faces created through the `nfdc face create` command when the `mtu` option is unspecified.
It shall be an error to set a value that is smaller than `MIN_MTU` or greater than `MAX_NDN_PACKET_SIZE`.