RFC 7588

A Widely Deployed Solution to the Generic Routing Encapsulation (GRE) Fragmentation Problem

📅 published July 2015, from ietf

RFC 7588: A Widely Deployed Solution to the Generic Routing Encapsulation (GRE) Fragmentation Problem ✨

Internet Engineering Task Force (IETF) 🌈

Authors: R. Bonica (Juniper Networks), C. Pignataro (Cisco Systems), J. Touch (USC/ISI) 🫶
Category: Informational 📚
Date: July 2015 📅
ISSN: 2070-1721
Obsoletes / Updates: this title block doesn't list any RFCs that it obsoletes or updates 🙅‍♀️

Abstract 🦋

Okay so this memo is about how a bunch of vendors already solved the Generic Routing Encapsulation (GRE) fragmentation problem 💅 The fix is configurable, and it's already widely deployed on the Internet in its default configuration. Iconic, honestly 🔥

Status of This Memo 🌸

This doc is not an Internet Standards Track specification. It's published just for informational purposes 🎀

This doc is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community, and it got public review and was approved for publication by the Internet Engineering Steering Group (IESG). Not every document the IESG approves is a candidate for any level of Internet Standard. See Section 2 of RFC 5741 for the details 🧚

Info about the current status of this document, any errata, and how to give feedback on it is at http://www.rfc-editor.org/info/rfc7588 🔗

Copyright Notice 💜

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as they're in effect on the date of publication. Please read these carefully, because they describe your rights and restrictions for this document 📜 Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Simplified BSD License ⭐

Table of Contents 🍓

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
  1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
  1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
2.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .   5
  2.1.  RFC 4459 Solutions  . . . . . . . . . . . . . . . . . . .   5
  2.2.  A Widely Deployed Solution  . . . . . . . . . . . . . . .   5
3.  Implementation Details  . . . . . . . . . . . . . . . . . . .   6
  3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   6
  3.2.  GRE MTU (GMTU) Estimation and Discovery . . . . . . . . .   6
  3.3.  GRE Ingress Node Procedures . . . . . . . . . . . . . . .   7
    3.3.1.  Procedures Affecting the GRE Payload  . . . . . . . .   7
    3.3.2.  Procedures Affecting the GRE Deliver Header . . . . .   8
  3.4.  GRE Egress Node Procedures  . . . . . . . . . . . . . . .   9
4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
  5.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
  5.2.  Informative References  . . . . . . . . . . . . . . . . .  11
Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1. Introduction ✨

Generic Routing Encapsulation (GRE) [RFC2784] [RFC2890] can carry any network-layer protocol over any network-layer protocol. Main character energy 🎀 GRE has been implemented by a ton of vendors and it's widely deployed on the Internet 💖

The GRE specification doesn't describe fragmentation procedures 🙈 So, lacking guidance from the spec, vendors built their own implementation-specific fragmentation solutions. A GRE tunnel only works correctly if its ingress and egress nodes support compatible fragmentation solutions ⚠️ [RFC4459] describes several fragmentation solutions and evaluates their relative merits 📊

This memo reviews the fragmentation solutions from [RFC4459]. It also describes how many vendors have solved the GRE fragmentation problem. The solution here is configurable, and it's been widely deployed in its default configuration 🦋

This memo covers point-to-point unicast GRE tunnels that carry IPv4, IPv6, or MPLS payloads over IPv4 or IPv6. All other tunnel types are out of scope for this doc 🚫

1.1. Terminology 🧚

The following terms are specific to GRE:

GRE delivery header - an IPv4 or IPv6 header whose source address represents the GRE ingress node and whose destination address represents the GRE egress node. The GRE delivery header encapsulates a GRE header.
GRE header - the GRE protocol header. The GRE header is encapsulated in the GRE delivery header and encapsulates the GRE payload.
GRE payload - a network-layer packet that is encapsulated by the GRE header. The GRE payload can be IPv4, IPv6, or MPLS. Procedures for encapsulating IPv4 in GRE are described in [RFC2784] and [RFC2890]. Procedures for encapsulating IPv6 in GRE are described in [IPv6-GRE]. Procedures for encapsulating MPLS in GRE are described in [RFC4023]. While other protocols may be delivered over GRE, they're out of scope for this doc.
GRE delivery packet - a packet containing a GRE delivery header, a GRE header, and the GRE payload.
GRE payload header - the IPv4, IPv6, or MPLS header of the GRE payload.
GRE overhead - the combined size of the GRE delivery header and the GRE header, measured in octets.

The following terms are specific to MTU discovery:

Link MTU (LMTU) - the maximum transmission unit, i.e., maximum packet size in octets, that can be conveyed over a link. LMTU is a unidirectional metric. A bidirectional link may be characterized by one LMTU in the forward direction and another LMTU in the reverse direction.
Path MTU (PMTU) - the minimum LMTU of all the links in a path between a source node and a destination node. If the source and destination nodes are connected through an Equal-Cost Multipath (ECMP), the PMTU is equal to the minimum LMTU of all links contributing to the multipath.
GRE MTU (GMTU) - the maximum transmission unit, i.e., maximum packet size in octets, that can be conveyed over a GRE tunnel without fragmentation of any kind. The GMTU is equal to the PMTU associated with the path between the GRE ingress and the GRE egress nodes minus the GRE overhead.
Path MTU Discovery (PMTUD) - a procedure for dynamically discovering the PMTU between two nodes on the Internet. PMTUD procedures for IPv4 are defined in [RFC1191]. PMTUD procedures for IPv6 are defined in [RFC1981].

The following terms are introduced by this memo:

Fragmentable Packet - a packet that can be fragmented by the GRE ingress node before being transported over a GRE tunnel. That is, an IPv4 packet with the Don't Fragment (DF) bit equal to 0 and whose payload is larger than 64 bytes. IPv6 packets are not fragmentable.
ICMP Packet Too Big (PTB) message - an ICMPv4 [RFC792] Destination Unreachable message (Type = 3) with code equal to 4 (fragmentation needed and DF set) or an ICMPv6 [RFC4443] Packet Too Big message (Type = 2).

1.2. Requirements Language 📣

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] 🫶

2. Solutions 💅

2.1. RFC 4459 Solutions 🌷

Section 3 of [RFC4459] identifies several tunnel fragmentation solutions. These solutions define procedures to be invoked when the tunnel ingress router receives a packet so large that it can't be forwarded through the tunnel without fragmentation of any kind. When applied to GRE, these procedures are:

Discard the incoming packet and send an ICMP PTB message to the incoming packet's source.
Fragment the incoming packet and encapsulate each fragment within a complete GRE header and GRE delivery header.
Encapsulate the incoming packet in a single GRE header and GRE delivery header. Perform source fragmentation on the resulting GRE delivery packet.

As per RFC 4459, Strategy 2 is applicable only when the incoming packet is fragmentable. Also as per RFC 4459, each strategy has its relative merits and costs ⚖️

2.2. A Widely Deployed Solution 🌈

Many vendors have implemented a configurable GRE fragmentation solution. In its default configuration, the solution behaves like this:

When the GRE ingress node receives a fragmentable packet with length greater than the GMTU, it fragments the incoming packet and encapsulates each fragment within a complete GRE header and GRE delivery header. Fragmentation logic is as specified by the payload protocol.
When the GRE ingress node receives a non-fragmentable packet with length greater than the GMTU, it discards the packet and sends an ICMP PTB message to the packet's source.
When the GRE egress node receives a GRE delivery packet fragment, it silently discards the fragment without attempting to reassemble the GRE delivery packet to which the fragment belongs.

In non-default configurations, the GRE ingress node can execute any of the procedures defined in RFC 4459 🎛️

The solution above is widely deployed on the Internet in its default configuration. However, the default configuration isn't always appropriate for GRE tunnels that carry IPv6 🥺

IPv6 requires that every link in the Internet have an MTU of 1280 octets or greater. On any link that can't convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6 🧱

So the default configuration is appropriate for tunnels that carry IPv6 only if the network is engineered so that the GMTU is guaranteed to be 1280 bytes or greater. In all other scenarios, a non-default configuration is required 🚨

In the non-default configuration, when the GRE ingress router receives a packet larger than the GMTU, the GRE ingress router encapsulates the entire packet in a single GRE and delivery header. It then fragments the delivery header and sends the resulting fragments to the GRE egress node, where they're reassembled 🧩

3. Implementation Details 🛠️

This section describes how many vendors have implemented the solution described in Section 2.2 💖

3.1. General 🎀

The GRE ingress nodes satisfy all of the requirements stated in [RFC2784] ✅

3.2. GRE MTU (GMTU) Estimation and Discovery ✨

okay so GRE ingress nodes can be set up with a config option that links a GMTU to a GRE tunnel 🎀. By default, the GMTU is the MTU of the next hop toward the GRE egress node, minus the GRE overhead 💅.

Usually ingress nodes level up their GMTU guess by running PMTUD procedures 🦋. But if an implementation supports PMTUD for GRE tunnels, it also gets a config option that disables PMTUD 🙈. This option is required to mitigate certain denial-of-service attacks (see Section 4), so it's not optional drama 💀.

The GRE ingress node's GMTU estimate won't always be accurate 😭. It's only an estimate, bestie. When the GMTU changes, the ingress node won't find out right away. And if the ingress node runs PMTUD and interior nodes can't deliver ICMP feedback to it, the GMTU estimates may be inaccurate 🫠.

3.3. GRE Ingress Node Procedures 🌸

This section defines the procedures GRE ingress nodes run when they get a packet whose size is bigger than the relevant GMTU 🧚.

3.3.1. Procedures Affecting the GRE Payload 🍓

3.3.1.1. IPv4 Payloads 🌈

By default, if the payload is fragmentable, the GRE ingress node fragments the incoming packet and wraps each fragment in a complete GRE header and GRE delivery header 🎁. So the GRE egress node gets a bunch of complete, non-fragmented delivery packets, and each one carries a fragment of the GRE payload. The egress node forwards those payload fragments to their ultimate destination, where they get reassembled ✨.

Also by default, if the payload is not fragmentable, the GRE ingress node discards the packet and sends an ICMPv4 Destination Unreachable message to the packet's source 🙅‍♀️. The code in that message is 4 (fragmentation needed and DF set). The ICMPv4 Destination Unreachable message also has a next-hop MTU (as specified by [RFC1191]), and that next-hop MTU equals the GMTU associated with the tunnel 💜.

The GRE ingress node supports a non-default configuration option that invokes an alternative behavior. If that option is configured, the GRE ingress node fragments the delivery packet. See Section 3.3.2 for the details 🔥.

3.3.1.2. IPv6 Payloads 🦋

By default, the GRE ingress node discards the packet and sends an ICMPv6 [RFC4443] Packet Too Big message to the payload source 🫶. The MTU in that Packet Too Big message equals the GMTU associated with the tunnel 👑.

3.3.1.3. MPLS Payloads 🧚

By default, the GRE ingress node discards the packet. It's literally impossible to reliably identify the payload source, so the GRE ingress node does not attempt to send an ICMP PTB message to the payload source 🙈.

3.3.2. Procedures Affecting the GRE Deliver Header 🎀

3.3.2.1. Tunneling GRE over IPv4 🌸

By default, the GRE ingress node does not fragment delivery packets 🙅‍♀️. BUT the GRE ingress node includes a config option that allows delivery packet fragmentation, so it's not a total lockdown 🔓.

By default, the GRE ingress node sets the DF bit in the delivery header to 1 (Don't Fragment) 💜. The GRE ingress node also supports a config option that invokes the following behavior:

When the GRE payload is IPv6, the DF bit on the delivery header is set to 0 (Fragments Allowed).
When the GRE payload is IPv4, the DF bit is copied from the payload header to the delivery header.

When the DF bit on an IPv4 delivery header is set to 0, any router between the GRE ingress and egress nodes can fragment the GRE delivery packet 🦋.

If the GRE egress node is configured to support reassembly, it will reassemble fragmented delivery packets 🧩. Otherwise, the GRE egress node will discard delivery packet fragments 🥲.

3.3.2.2. Tunneling GRE over IPv6 🌈

By default, the GRE ingress node does not fragment delivery packets. However, the GRE ingress node includes a config option that allows this 🎀.

If the GRE egress node is configured to support reassembly, it will reassemble fragmented delivery packets ✨. Otherwise, the GRE egress node will discard delivery packet fragments 💀.

3.4. GRE Egress Node Procedures 👑

By default, the GRE egress node silently discards GRE delivery packet fragments 🤫. It does this without trying to reassemble the GRE delivery packets those fragments belong to 🙈.

BUT the GRE egress node supports a config option that lets it reassemble GRE delivery packets 🧩.

4. Security Considerations 🔐

Okay, this section is serious, so buckle up 😭. In the GRE fragmentation solution described above, either the GRE payload or the GRE delivery packet can be fragmented. If the GRE payload gets fragmented, it's typically reassembled at its ultimate destination. If the GRE delivery packet gets fragmented, it's typically reassembled at the GRE egress node 🫶.

The packet reassembly process is resource intensive and vulnerable to several denial-of-service attacks 💀. In the simplest attack, the attacker sends fragmented packets faster than the victim can reassemble them. In a variation on that attack, the first fragment of each packet is missing, so no packet can ever be reassembled 🙅‍♀️.

Given that the packet reassembly process is resource intensive and vulnerable to denial-of-service attacks, operators should decide where reassembly is best performed. Once they've made that decision, they should decide whether to fragment the GRE payload or the GRE delivery packet accordingly 🎯.

Some IP implementations are vulnerable to the Overlapping Fragment Attack [RFC1858]. This vulnerability is not specific to GRE, and it needs to be considered in all environments where IP fragmentation is present 🚨. [RFC3128] describes a procedure by which IPv4 implementations can partially mitigate the vulnerability. [RFC5722] mandates a procedure by which IPv6-compliant implementations are required to mitigate the vulnerability. The procedure described in RFC 5722 completely mitigates the vulnerability 🙌. Operators SHOULD ensure that the vulnerability is mitigated to their satisfaction on equipment that they deploy 🫡.

PMTUD is vulnerable to two denial-of-service attacks (see Section 8 of [RFC1191] for details). Both attacks are based upon a malicious party sending forged ICMPv4 Destination Unreachable or ICMPv6 Packet Too Big messages to a host 😱. In the first attack, the forged message indicates an inordinately small PMTU. In the second attack, the forged message indicates an inordinately large MTU. In both cases, throughput is adversely affected 📉. To mitigate such attacks, GRE implementations include a config option to disable PMTUD on GRE tunnels. They can also include a config option that conditions the behavior of PMTUD to establish a minimum PMTU 🛡️.

5. References 📚

5.1. Normative References 💖

[RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
           RFC 792, DOI 10.17487/RFC0792, September 1981,
           <http://www.rfc-editor.org/info/rfc792>.

[RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
           DOI 10.17487/RFC1191, November 1990,
           <http://www.rfc-editor.org/info/rfc1191>.

[RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
           Considerations for IP Fragment Filtering", RFC 1858,
           DOI 10.17487/RFC1858, October 1995,
           <http://www.rfc-editor.org/info/rfc1858>.

[RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
           for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
           1996, <http://www.rfc-editor.org/info/rfc1981>.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119,
           DOI 10.17487/RFC2119, March 1997,
           <http://www.rfc-editor.org/info/rfc2119>.

[RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
           Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
           DOI 10.17487/RFC2784, March 2000,
           <http://www.rfc-editor.org/info/rfc2784>.

[RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
           RFC 2890, DOI 10.17487/RFC2890, September 2000,
           <http://www.rfc-editor.org/info/rfc2890>.

[RFC3128]  Miller, I., "Protection Against a Variant of the Tiny
           Fragment Attack (RFC 1858)", RFC 3128,
           DOI 10.17487/RFC3128, June 2001,
           <http://www.rfc-editor.org/info/rfc3128>.

[RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
           "Encapsulating MPLS in IP or Generic Routing Encapsulation
           (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
           <http://www.rfc-editor.org/info/rfc4023>.

[RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
           Control Message Protocol (ICMPv6) for the Internet
           Protocol Version 6 (IPv6) Specification", RFC 4443,
           DOI 10.17487/RFC4443, March 2006,
           <http://www.rfc-editor.org/info/rfc4443>.

[RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
           RFC 5722, DOI 10.17487/RFC5722, December 2009,
           <http://www.rfc-editor.org/info/rfc5722>.

5.2. Informative References 🦋

[IPv6-GRE] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
           for Generic Routing Encapsulation (GRE)", Work in
           Progress, draft-ietf-intarea-gre-ipv6-10, June 2015.

[RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
           Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
           2006, <http://www.rfc-editor.org/info/rfc4459>.

Acknowledgements 🫶

The authors would like to thank Fred Baker, Fred Detienne, Jagadish Grandhi, Jeff Haas, Brian Haberman, Vanitha Neelamegam, Masataka Ohta, John Scudder, Mike Sullenberger, Tom Taylor, and Wen Zhang for their constructive comments 💖. The authors also express their gratitude to Vanessa Ameen, without whom this memo could not have been written 👑.

Authors' Addresses 💌

Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia  20170
United States

Email: rbonica@juniper.net


Carlos Pignataro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, North Carolina  27709
United States

Email: cpignata@cisco.com


Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, California  90292-6695
United States

Phone: +1 (310) 448-9151
Email: touch@isi.edu
URI:   http://www.isi.edu/touch

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