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Re: Path MTU Discovery
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On Sat, 8 Feb 1997, Ben Rogers wrote:
>
>Then, we would see everything perform nicely, a la:
>
>Host -> Tunnelling Router 1 -> Tunneling Router 2 --A--> ...
>
>Where each router is doing rfc1828 MD5 tunneling (20 byte IP header + 24
>bytes of AH = 44 bytes total), and path A has an MTU of 500 bytes.
This example only works for IPv4. IPv6 requires a MINIMUM MTU
of 576 Octets. Which requieres your second router to do link
specific fragmentation.
This schouldn't be a problem in your scenario since r2 has to do link
specific fragmentation anyway (MTU 500 Byte) . However if you assume a
tunneling router with a link MTU of 580 byte and you add IP in IP the
router suddenly has to do link specific fragmentation. Something he
didn't do before.
Oliver
>
>Packet 1: 1500 bytes, DF set
> Tunneled by R1 (1544 bytes) (Dropped)
>
>Packet 2: 1500 bytes, DF set
> R1 sends ICMP too big with size limit 1456 to host
> Tunneled by R1 (1544 bytes) (Dropped) (An effort to rediscover a
> "growing" MTU)
>
>Packet 3: 1456 bytes, DF set
> Tunneled by R1 (1500 bytes)
> Tunneled by R2 (1544 bytes) (Dropped)
>
>Packet 4: 1456 bytes, DF set
> Tunneled by R1 (1500 bytes)
> R2 sends ICMP too big with size limit 456 to R1
> Tunneled by R2 (1544 bytes) (Dropped)
>
>Packet 4: 1456 bytes, DF set
> R1 sends ICMP too big with size limit 412 to host
> Tunneled by R1 (1500 bytes)
> R2 sends ICMP too big with size limit 456 to R1
> Tunneled by R2 (1544 bytes) (Dropped)
>
>Packet 5: 412 bytes, DF set
> Tunneled by R1 (456 bytes)
> Tunneled by R2 (500 bytes)
> Received.
>
>
>
>ben
>
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