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Requirements for IP-in-IP Tunnel MTU Assurance

Requirements for IP-in-IP Tunnel MTU Assurance. V6OPS Working Group - IETF 64 Fred L. Templin fred.l.templin@boeing.com. Problem Statement. IP-in-IP tunnels span multiple L2 segments but are seen by L3 as ordinary links that must present an assured MTU

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Requirements for IP-in-IP Tunnel MTU Assurance

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  1. Requirements for IP-in-IP Tunnel MTU Assurance V6OPS Working Group - IETF 64 Fred L. Templin fred.l.templin@boeing.com

  2. Problem Statement • IP-in-IP tunnels span multiple L2 segments but are seen by L3 as ordinary links that must present an assured MTU • Common tunneling mechanisms set fixed MTU (e.g.,1280 bytes or larger for IPv6), but cannot assure delivery for packets of that size. Current approaches: • don’t set the DF bit and allow IPv4 fragmentation • set the DF bit and watch for ICMPv4 fragmentation needed msgs, i.e., use IPv4 Path MTU Discovery

  3. Problems with IPv4 Fragmentation • No mechanism for determining decapsulator’s MRU • Network-based IPv4 fragmentation has negative impact on performance • IPv4 fragmentation can result in black holes when firewalls/NATs in the path

  4. Problems with IPv4 PMTUD • ICMPv4 fragmentation needed messages can be spoofed by on/off-path adversaries; dropped or altered by on-path adversaries • ICMPv4 fragmentation needed messages can’t always be translated into ICMPv6 packet too big messages

  5. Requirements for New Mechanism • tunnel endpoint negotiation (means for encapsulator to determine whether decapsulator implements scheme) • Backwards compatibility with IPv4 fragmentation; IPv4 PMTUD • “Above-IPv4” host-based segmentation at the encapsulator • “Above-IPv4” reassembly at the decapsulator

  6. Requirements for New Mechanism • Packet splicing error detection • Accommodate out-of-order delivery • Means for encapsulator to probe PMTU • Means for decapsulator to send authenticated probe response • Proactive path probing to determine best MTU; detect MTU-related black holes • Means to discover decapsulator’s MRU

  7. Summary • Existing tunnel mechanisms have no means of assuring tunnel MTU • Most problematic for tunnels that traverse NATs; Firewalls • Tunnel MTU assurance needed for tunnels that span NATs; Firewalls

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