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Operational Issues with Tunnel Maximum Transmission Unit (MTU)

This document addresses the operational issues related to the Maximum Transmission Unit (MTU) of tunnels in networking. The de facto Internet cell size is 1500 bytes, but tunnels often set a conservative MTU due to encapsulation overhead, making effective transmission problematic. Issues arise from Path MTU Discovery (PMTUD) failures and IP fragmentation, leading to dropped packets and communication black holes. Proposed solutions include using tunnel fragmentation prior to encapsulation and setting an indefinite MTU to handle packets flexibly. This paper emphasizes making necessary adaptations within the tunnel environment.

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Operational Issues with Tunnel Maximum Transmission Unit (MTU)

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  1. Operational Issues with Tunnel Maximum Transmission Unit (MTU) Boeing Research & Technology Fred Templin (fred.l.templin@boeing.com) 11/08/2012

  2. The Problem • De facto “Internet Cell Size” is 1500 bytes • Tunnels add encapsulation overhead that reduces the effective path MTU • Tunnels often adapt by setting a conservative and fixed MTU (e.g., 1480 bytes). However: • Path MTU Discovery messages are often filtered • IP fragmentation is problematic • Larger packets that might make it through the tunnel in one piece are discarded at the ingress • Issues apply to tunnels over both IPv4 and IPv6

  3. Path MTU Discovery (PMTUD) Issues • When a too-large packet is dropped at the tunnel ingress: • Packet Too Big (PTB) message produced by the ingress may be dropped on the path to the original source • When a too-large packet is dropped inside the tunnel: • PTB message may be dropped on the path to the tunnel ingress, or • PTB message may not contain enough information for translation into PTB to send back to the original source, or • PTB message may be fabricated by an adversarial middlebox within the tunnel

  4. IP Fragmentation Issues • Original source could use IP fragmentation *before* encapsulation • Tunnel ingress could use IP fragmentation *after* encapsulation • However: • For IPv4, IP_ID is only 16bits • For IPv6 (and probably also IPv4) middleboxes are being configured more and more to drop all IP fragments

  5. Current Mitigations • As a result, common tunnel types set a fixed and static MTU of at most 1500 minus the length of the encapsulation headers (e.g., 1480 bytes for IPv6-in-IPv4) • However: • Minimum MTU is only 1280 bytes for IPv6 and 576 (68?) bytes for IPv4 so there is no way to set a “low enough” static MTU • MTU loss within the tunnel still result in black holes • Especially problematic for tunnels-within-tunnels

  6. Alternative Approach • Tunnel ingress could use “tunnel fragmentation” *before* encapsulation • application-layer segmentation (the tunnel ingress is the “application”) • Reassembly performed by the tunnel egress • Each segment appears as an individual IP packet on the wire (i.e., and not as an IP fragment) • Extra “mid-layer” of encapsulation needed

  7. Other Considerations • The tunnel should set an indefinite MTU (i.e., admit all packets into the tunnel regardless of their size and make any necessary adaptations from within the tunnel) • “Take care of the smalls, and let the bigs take care of themselves” • Make sure packets no larger than 1500 get through • Let larger packets sink or swim on their own • Assumes that original sources that send packets larger than 1500 use RFC4821

  8. Problem Statement and Approach • Operational Issues with Tunnel Maximum Transmission Unit (MTU) • draft-generic-v6ops-tunmtu • https://datatracker.ietf.org/doc/draft-generic-v6ops-tunmtu/ • The Subnetwork Encapsulation and Adaptation Layer (SEAL) • RFC5320 (early experimental version) • draft-templin-intarea-seal (SEAL(bis)) • https://datatracker.ietf.org/doc/draft-templin-intarea-seal/

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