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