Hi Stephen,
On Thu, Oct 17, 2024 at 09:26:00PM +0100, Stephen Farrell wrote:
> On 10/17/24 20:46, Jeffrey Haas wrote:
>
> > It's just doing IP forwarding.
>
> You've said that a few times and I find it a little confusing.
>
> "Just IP forwarding" to me would not include replying to the sender
> of a BFD echo packet. ICMP yes, but I don't think BFD is a built-in
> part of an IP stack in general and "just forwarding" would in general
> be driven by rules wrt. the packet destination and not source.
That is exactly the point of "BFD Echo": there is no reply.
One and the same packet is sent out and received back again by the sender.
Destination based IP forwarding delivers this packet.
> I wonder what I'm missing?
The BFD Echo packet is sent with a destination address that belongs to
the sender. Thus the first router forwarding this packet actually returns
it to the sender. This router does not implement any BFD functionality
for this, just simple destination based IP forwarding.
That has always been the principle behind BFD Echo, as described in
RFC 5880. This principle is used by draft-ietf-bfd-unaffiliated-echo.
This principle makes it possible to have "Unaffiliated BFD Echo" in the
first place.
I'll try to illustrate this with a drawing:
network 192.0.2.0/30 network 203.0.113.0/30
| |
+[BFD endpoint]+ v +----------[IP router]---------+ v
|IP: 192.0.2.1 x----xIP: 192.0.2.1|IP: 203.0.113.1 x---...Internet...
+--------------+ +------------------------------+ ^
|
(1) BFD endpoint creates and sends BFD Echo packet |
|
BFD Echo packet network 198.51.100.0/24
Src: 198.51.100.47
Dst: 192.0.2.1
TTL: 255
-----------------> travelling from BFD endpoint to IP router
(2) FIB lookup for destination based IP forwarding at router happens
(3) IP router decrements TTL and forwards packet towards destination
BFD Echo packet
Src: 198.51.100.47
Dst: 192.0.2.1
TTL: 254
<----------------- travelling from IP router to BFD endpoint
(4) BFD endpoint receives its own BFD Echo packet
Best regards,
Erik
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