Internet-Draft | HBH Options Processing | January 2022 |
Hinden & Fairhurst | Expires 2 August 2022 | [Page] |
This document specifies procedures for how IPv6 Hop-by-Hop options are processed. It modifies the procedures specified in the IPv6 Protocol Specification (RFC8200) to make processing of IPv6 Hop-by-Hop options practical with the goal of making IPv6 Hop-by-Hop options useful to deploy and use in the Internet. When published, this document updates RFC8200.¶
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This document specifies procedures for how IPv6 Hop-by-Hop options are processed. It modifies the procedures specified in the IPv6 Protocol Specification (RFC8200) to make processing of IPv6 Hop-by-Hop options practical with the goal of making IPv6 Hop-by-Hop options useful to deploy and use in the Internet.¶
When published this document updates [RFC8200].¶
The current list of defined Hop-by-Hop options can be found at [IANA-HBH].¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses the following loosely defined terms:¶
NOTE: This distinct separation between hardware and software processing from [RFC6398] does not apply to all router architectures. However, a router that performs all or most processing in software might still incur more processing cost when providing special processing (aka Slow Path).¶
[RFC6192] is an example of how designs can separate control plane (Slow Path) and forwarding plane (Fast Path) functions.¶
In the first version of the IPv6 specification, Hop-by-Hop options were required to be processed by all nodes: routers and hosts. This proved to not be practical in high speed routers due to several factors, including:¶
When the IPv6 Specification was updated and published in July 2017 as [RFC8200], the procedures relating to hop-by-hop options were as follows:¶
The changes meant that an implementation complied with the IPv6 specification even if it did not process hop-by-hop options, and that it was expected that routers would add configuration information to control which hop-by-hop options they would process.¶
Unfortunately, this did not improve the processing of Hop-by-Hop options and did not significantly improve deployment and use in the Internet. Essentially, it only documented how they were being used in the Internet at the time RFC8200 was published.¶
The main issues remain:¶
There has been research that discussed the general problem with dropping packets containing IPv6 extension headers, including the Hop-by-Hop Options header. For example [Hendriks] states that "dropping all packets with Extension Headers, is a bad practice", and that "The share of traffic containing more than one EH however, is very small. For the design of hardware able to handle the dynamic nature of EHs, we therefore recommend to support at least one EH".¶
This document defines a set of procedures for the hop-by-hop option header that make the processing of hop-by-hop options practical in modern transit routers.¶
This section describes several changes to [RFC8200].¶
The Hop-by-Hop Option Header as defined in Section 4.3 of [RFC8200] is identified by a Next Header value of 0 in the IPv6 header. Section 4.1 of [RFC8200] requires a Hop-by-Hop Options header to appear immediately after the IPv6 header. [RFC8200] also requires that a Hop-by-Hop Options header can only appear once in a packet.¶
The Hop-by-Hop Options Header as defined in [RFC8200] can contain one or more Hop-by-Hop options. This document updates [RFC8200] that a node MUST process the first Option in the Hop-by-Hop Header at full forwarding rate the (e.g. on the router's Fast Path) and MAY process additional Hop-by-Hop Options if configured to do so. The motivation for this change is to simplify the processing of Hop-by-Hop options as a part of normal forwarding.¶
Nodes creating packets with a Hop-by-Hop option headers SHOULD include a single Hop-by-Hop Option in the packet and MAY include more based on local configuration.¶
If there are more than one Hop-by-Hop options in the Hop-by-Hop Options header, the node MAY skip the rest of the options without having to examine these options using the "Hdr Ext Len" field in the Hop-by-Hop Options header. This field specifies the length of the Option Header in 8-octet units. The additional options do not need to be processed or verified.¶
Nodes that implement a differentiation between a Fast Path and a Slow Path MUST process all (with one exception noted below) Hop-by-Hop options in the Fast Path. The one exception to this is the Router Alert Option [RFC2711]. See Section 5.3 for discussion of the Router Alert.¶
If the node can not process an option at the full forwarding rate, it MUST behave as if it does not recognize the Option Type (as described in the next paragraph).¶
Section 4.2 of [RFC8200] defines the Option Type identifiers as internally encoded such that their highest-order 2 bits specify the action that must be taken if the processing IPv6 node does not recognize the Option Type. The text is:¶
00 - skip over this option and continue processing the header. 01 - discard the packet. 10 - discard the packet and, regardless of whether or not the packet's Destination Address was a multicast address, send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. 11 - discard the packet and, only if the packet's Destination Address was not a multicast address, send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type.¶
This document modifies this behaviour for the "10" and "11" values that the node MAY send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. The modified text for "10" and 11" values is:¶
10 - discard the packet and, regardless of whether or not the packet's Destination Address was a multicast address, MAY send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. 11 - discard the packet and, only if the packet's Destination Address was not a multicast address, MAY send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type.¶
The motivation for this change is to loosen the requirement to send ICMPv6 Parameter Problem messages by simplifying what the router needs to do when it performs forwarding of an Option Type it does not recognize.¶
When an ICMP Parameter Problem, Code 2, message is delivered to the source, the source can become aware that at least one node on the patch has failed to recognize the option.¶
The Router Alert option [RFC2711] purpose is to tell the node that the packet needs additional processing on the Slow Path.¶
The Router Alert option includes a two octet Value field that describes the protocol that is carried in the packet. The current values can be found in the IANA Router Alert Value registry [IANA-RA].¶
DISCUSSION¶
A Fast Path implementation SHOULD verify that a Router Alert contains a protocol, as indicated by the Value field in the Router Alert option, that is configured as a protocol of interest to that router. A verified packet SHOULD be sent on the Slow Path for processing [RFC6398]. Otherwise, the router implementation SHOULD forward within the Fast Path (subject to all normal policies and forwarding rules). As specified in [RFC2711] the top two bits of Option Type for the Router Alert option are always set to "00" indicating the node should skip over this option and continue processing the header in this case.¶
Implementations of the IP Router Alert Option SHOULD offer the configuration option to simply ignore the presence of "IP Router Alert" in IPv4 and IPv6 packets" [RFC6398].¶
A node that is configured to process a Router Alert option using the Slow Path MUST protect itself from infrastructure attack that could result from processing on the Slow Path. This might include some combination of access control list to only permit from trusted nodes, rate limiting of processing, or other methods [RFC6398].¶
Section 4 of [RFC8200] allows for a router to control it's processing of IPv6 Hop-by-Hop options by local configuration. The text is:¶
A possible approach to implementing this is to maintain a lookup table based on Option Type of the IPv6 options that are supported in the Fast Path. This would allow for a node to quickly determine if an option is supported and can be processed. If the option is not supported, then the node processes it as described in Section 5.2 of this document.¶
A node configured not to process HBH options, MUST drop the packet if the top two bits of the Option Type field of the first HBH option is non-zero.¶
The actions of the lookup table SHOULD be configurable by the operator of the router.¶
Any new IPv6 Hop-by-Hop option designed in the future should be designed to be processed at full forwarding rate (e.g., on a router's Fast Path). New options MUST NOT be defined that are not reasonably expected to be executed at full forwarding rates. New Hop-by-Hop options SHOULD have the following characteristics:¶
Any new Hop-by-Hop option that is standardized that does not meet these criteria needs to explain in detail in its specification why this can not be accomplished and that there is a reasonable expectation that it can be proceed in most Fast Path implementations.¶
There are no actions required for IANA defined in this document.¶
Security issues with IPv6 Hop-by-Hop options are well known and have been documented in several places, including [RFC6398], [RFC6192], and [RFC9098]. The main issue, as noted in Section 4, is that any mechanism that can be used to force packets into the router's Slow Path can be exploited as a denial of service attack on a transit router by saturating the resources need for router management protocols (e.g., routing protocols, network management protocols, etc.) that may cause the router to fail. Due to this it's common for transit routers to drop packets with Hop-by-Hop options headers.¶
While Hop-by-Hop options are not required to be processed in the Slow Path, the Router Alert options is designed to do just that.¶
This document changes the way Hop-by-Hop options are processed in several ways that significantly reduces the attack surface. These changes include:¶
The authors believe that these changes significantly reduces the security issues relating to IPv6 Hop-by-Hop options and will enable them to be used safely in the Internet.¶
Helpful comments were received from Brian Carpenter, Ron Bonica, Ole Troan, Mark Heard, Tom Herbert, [your name here], and other members of the 6MAN working group.¶
draft-ietf-6man-hbh-processing-00, 2022-January-29:¶
draft-hinden-6man-hbh-processing-01, 2021-June-2:¶
draft-hinden-6man-hbh-processing-00, 2020-Nov-29:¶