On 2/16/2017 1:29 PM, Stewart Bryant wrote:
On 16/02/2017 18:49, Joe Touch wrote:
On 2/16/2017 7:59 AM, Stewart Bryant wrote:
On 14/02/2017 23:00, Templin, Fred L wrote:
Unless there is operational assurance of
some size X>1280, however, tunnels have to use fragmentation to
guarantee that - at a minimum - packets up to 1280 will get through.
In that case there really needs to be a note about MPLS.
IMO, this doc shouldn't be discussing tunneling as any different from
any other link.
You can fragment into an IP tunnel, but not an MPLS tunnel, because
you cannot fragment the payload as you can in IPv4 and you cannot
fragment MPLS.
There's no such thing as an "MPLS tunnel";
In the MPLS world an "MPLS tunnel" is a common name for an MPLS LSP that
is used to carry some payload such as IP.
MPLS just adds the label tags; there's no length field. It's only a
tunnel when it goes over some other encapsulation layer, e.g., IP or
Ethernet.
at best, it's "MPLS over X",
e.g., MPLS over ethernet. MPLS doesn't indicate a message length so
while it can't support fragmentation it would never need it either. It
would be the next layer down (e.g., Ethernet, ATM, etc.) that might have
needed fragmentation. If that can't be supported, then the addition of
MPLS would just reduce the effective MTU of the MPLS-over-X link.
It was of course IPv6 over MPLS that concerned me.
IPv6 over MPLS isn't yet a tunnel. It would be IPv6 over MPLS over IPv6,
presumably (IPv6 over IPv4 isn't guaranteed to work because IPv4 EMTU_R
is only 576, and for an IPv6 tunnel it would be required to be at least
1280+encaps).
An IPv6 packet can traverse an IPv6 over MPLS over Ethernet tunnel (with
a 1500B payload) as long as the MPLS headers are smaller than 1500-1280
bytes.
An IPv6 packet can traverse an IPv6 over MPLS over IPv6 tunnel as long
as the MPLS + added IPv6 headers are smaller than 1500-1280 bytes.
However, if the arriving (transit) packet is larger than
1280-MPLS-IPv6ecaps, the MPLS tunnel ingress would need to do source
fragmentation inside the tunnel (with the egress doing reassembly).
Again, all these cases are discussed in draft-ietf-intarea-tunnels but
none have relevance IMO to this doc.
Joe