Re: rfc5837 and rfc8335

[David Ahern <dsahern@gmail.com>]

On 3/23/21 10:39 AM, Ron Bonica wrote:
> Hi Folks,
> 
>  
> 
> The rationale for RFC 8335 can be found in Section 5.0 of that document.
> Currently, ICMP ECHO and ECHO RESPONSE messages can be used to determine
> the liveness of some interfaces. However, they cannot determine the
> liveness of:
> 
>  
> 
>   * An unnumbered IPv4 interface
>   * An IPv6 interface that has only a link-local address
> 
>  
> 
> A router can have hundreds, or even thousands of interfaces that fall
> into these categories.
> 
>  
> 
> The rational for RFC 5837 can be found in the Introduction to that
> document. When a node sends an ICMP TTL Expired message, the node
> reports that a packet has expired on it. However, the source address of
> the ICMP TTL Expired message doesn’t necessarily identify the interface
> upon which the packet arrived. So, TRACEROUTE can be relied upon to
> identify the nodes that a packet traverses along its delivery path. But
> it cannot be relied upon to identify the interfaces that a packet
> traversed along its deliver path.
> 
>  

It's not a question of the rationale; the question is why add this
support to Linux now? RFC 5837 is 11 years old. Why has no one cared to
add support before now? What tooling supports it? What other NOS'es
support it to really make the feature meaningful? e.g., Do you know what
Juniper products support RFC 5837 today?

More than likely Linux is the end node of the traceroute chain, not the
transit or path nodes. With Linux, the ingress interface can lost in the
layers (NIC port, vlan, bond, bridge, vrf, macvlan), and to properly
support either you need to return information about the right one.
Unnumbered interfaces can make that more of a challenge.