9.3 IP Packets Encapsulated in non-IP Headers
9.3 IP Packets Encapsulated in non-IP Headers
This section briefly discusses the interaction of ECN with IP packets encapsulated in non-IP headers, such as MPLS (Multiprotocol Label Switching), GRE (Generic Routing Encapsulation), L2TP (Layer 2 Tunneling Protocol), and PPTP (Point-to-Point Tunneling Protocol).
The issues for ECN with these encapsulation methods are similar to those for IP-in-IP tunnels:
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Loss of congestion indication: If the encapsulating protocol does not have a mechanism to carry ECN information, congestion indications set in the inner IP header's ECN field may be lost when the outer header is removed at the tunnel exit.
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Lack of ECN support in outer header: If the encapsulating protocol does not support ECN-like functionality, routers along the tunnel path cannot signal congestion to ECN-capable flows, even if the inner IP packet is ECN-capable.
This document does not specify how ECN should be handled in these non-IP encapsulations. Such specifications must be developed separately for each encapsulation protocol, taking into account the specific characteristics and constraints of that protocol.
For example:
- MPLS: ECN support in MPLS is addressed in separate documents that define how to map ECN between IP and MPLS headers
- GRE: ECN handling in GRE tunnels would need to consider whether and how to use bits in the GRE header for ECN signaling
- L2TP and PPTP: These Layer 2 tunneling protocols present additional challenges as they may not have direct access to Layer 3 ECN information
Protocol designers working on ECN support for these encapsulations should follow principles similar to those outlined in Sections 9.1 and 9.2:
- Provide at least a limited-functionality option to avoid breaking ECN
- Consider a full-functionality option to preserve ECN benefits through the tunnel
- Address security implications of carrying congestion information across the tunnel
Until ECN support is properly specified for a particular encapsulation protocol, implementations should assume that ECN is not supported, and flows using ECN will experience packet drops rather than marks when traversing such tunnels.