6MAN W. Kumari Internet-Draft Google Intended status: Best Current Practice J. Jaeggli Expires: January 05, 2014 Zynga R. Bonica Juniper Networks July 04, 2013 Operational Issues Associated With Long IPv6 Extension Header Chains draft-wkumari-long-headers-01 Abstract This document explains why IPv6 header chain length affects the cost of ASIC-based packet forwarding. It also explains why some network service providers discard packets with exceptionally long header chains. Finally, it identifies a reasonable header chain length. While a network service provider can enforce any filtering policy that supports its security model, a network service provider should not discard IPv6 packets based solely upon header chain length if the header chain is not longer than the value specified herein. 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]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 05, 2014. Copyright Notice Kumari, et al. Expires January 05, 2014 [Page 1] Internet-Draft long-v6-headers July 2013 Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to 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 are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Termnology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Need to see upper-layer to filter . . . . . . . . . . . . . . 5 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 8. Normative References . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction In order to make a forwarding decision, the forwarding device may require information from both the IPv6 header and an upper layer header. When a software-based forwarder encounters an IPv6 datagram that includes a long header chain, it parses the header chain, acquires the required information and makes its forwarding decision. Typically, software-based forwarders are not required to process a large number of packets per second. Therefore, they can perform the above mentioned procedure within their processing budget. By contrast, ASIC-based forwarders are engineered to forward many more packets per second. In order to fulfill this requirement, the forwarder copies a fixed number of bytes from the beginning of the packet to on-chip memory. The ASIC does this because it can access on-chip memory much more quickly than it can access off-chip memory. Once the beginning of the packet has been transferred to on-chip memory, subsequent processing and forwarding decisions can be made very quickly. Kumari, et al. Expires January 05, 2014 [Page 2] Internet-Draft long-v6-headers July 2013 The act of copying bytes from the beginning of a packet to on-chip memory consumes both wall-time and processor cycles. Therefore, the number of bytes copied to on-chip memory must be chosen wisely. If a forwarder copies more bytes than it needs, it wastes resources, significantly increasing cost and decreasing maximum performance. If it copies too few bytes, it cannot parse the header chain and make a correct forwarding decision. As currently specified in [RFC2460], the IPv6 header chain can exceed 64 kilobytes. However, packets with header chains exceeding 128 bytes are rarely observed on the Internet. Therefore, most ASIC- based forwarders copy a relatively small number of bytes from the beginning of the packet into on-chip memory. This document explains why IPv6 header chain length affects the cost of ASIC-based packet forwarding. It also explains why some network service providers discard packets with exceptionally long header chains. Finally, it identifies a reasonable header chain length. While a network service provider can enforce any filtering policy that supports its security model, a network service provider SHOULD NOT discard IPv6 packets based upon header chain length if the header chain is not longer than the value specified herein. 1.1. Termnology For the purposes of this document, the terms Extension Header, Header Chain and Upper-layer Header are used as follows: Extension Header : Extension Headers are defined in Section 4 of [RFC2460]. Currently, six extension header types are defined. [RFC2460] defines the hop-by-hop, routing, fragment and destination options extension header types. [RFC4302] defines the authentication header (AH) type and [RFC4303] defines the encapsulating security payload (ESP) header type. Header Chain : The initial portion of an IPv6 datagram containing headers. The first member of the header chain is always an IPv6 header. For a subsequent header to qualify as a member of the header chain, it must be referenced by the "Next Header" field of the previous member of the header chain. The header chain can include IPv6 headers, IPv6 extension headers and an upper-layer header. If the header chain includes two IPv6 headers, as is the case when IPv6 is tunneled over IPv6, the second IPv6 header terminates the header chain. Any headers following the second IPv6 headers are Kumari, et al. Expires January 05, 2014 [Page 3] Internet-Draft long-v6-headers July 2013 not members of the header chain. Likewise, if the header chain includes an ESP header, the ESP header terminates the header chain. Only the first 8 bytes of the ESP header contribute to the header chain length. Any headers following ESP header are not members of the header chain. Upper-layer Header : In the general case, the upper-layer header is the first member of the header chain that is neither an IPv6 header nor an IPv6 extension header. Typically, the upper-layer header represents a transport protocol (e.g., TCP, UDP, SCTP). However, it can represent a non-transport layer protocol. For example, when IPv4 is tunneled over IPv6, the upper-layer header is an IPv4 header. If the header chain includes two IPv6 headers, as is the case when IPv6 is tunneled over IPv6, the second IPv6 header is considered to be the upper-layer header and terminates the header chain. For the purposes of this document, when the upper-layer protocol is ICMPv6, the first 32 bits of the ICMPv6 message (i.e., the type, code fields and checksum fields) are considered to be the ICMPv6 header. The upper-layer payload is not part of the upper-layer header and therefore, is not part of the IPv6 header chain. For example, if the upper-layer protocol is TCP, the TCP payload is not part of the TCP header or the IPv6 header chain. 2. Background When IPv6 was first conceived, forwarding was largely performed in software running on general-purpose processors. Due to the required performance, parsing a long header chain was not an issue. In the years between the conception of IPv6 and publication of this documentation, the Internet evolved as follows: o Throughput requirements increased dramatically, requiring the deployment of ASIC-based forwarders in both core and edge networks o New network types emerged, including very large enterprises, social networks, data centers and "clouds". Like core networks, these network require very high throughput. Like edge networks, these networks require "high-touch" edge services, which require the forwarder to access the entire header chain Kumari, et al. Expires January 05, 2014 [Page 4] Internet-Draft long-v6-headers July 2013 o Requirements for "high-touch" service, which require the forwarder to access the entire header chain, increased in networks of all kinds During those years, IPv4 [RFC0791] was the most commonly deployed protocol on the Internet. ASIC-based forwarders could meet the requirements of the evolving Internet because ASICs could predict the number of bytes that needed to be copied into on-chip memory in order to make a forwarding decision. Today, as IPv6 is being deployed, ASIC-forwarders cannot safely predict the size of the header chain. This leads to complexity and vulnerability in handling extension headers. For this reason, many network operators filter all IPv6 packets containing extension headers. 3. Need to see upper-layer to filter There is a school of thought that ISPs, content-providers and enterprises should not be performing any sort of filtering in the network and that the filtering is more appropriately performed at the end hosts. Unfortunately this solution, while clean and elegant, simply doesn't work in the real world. Filtering unknown traffic at the edge (and / or in the core) of the network is needed for a number of reasons, some of which are discussed below. o ISPs (and cloud providers) are routinely called upon to filter DoS traffic aimed at their customers (for example to block multi-Gbps DNS reflection attacks aimed at web servers, etc). At Large Edge Sites this is often a large part of the "service" provided by the network team. o IPv6 stacks are still relatively immature and operators still have concerns that stack or kernel vulnerabilities may still surface. If this turns out to be the case, a means to protect the end nodes is needed. o In many enterprises the end systems are not sufficiently hardened to be exposed on the public Internet. Even if there were no remotely exploitable operating system vulnerabilities there is significant risk that an employee may install vulnerable software, accidentally share confidential files or folders publicly or start offering (unauthorized) services. Kumari, et al. Expires January 05, 2014 [Page 5] Internet-Draft long-v6-headers July 2013 o Content providers (and similar) need to be able to filter traffic and only allow "expected" traffic into their networks. This is needed both for DoS protection, but also to ensure that development systems, databases, test systems, etc are not accidentally exposed. o While it would be nice if all employees, system and database administrators could be trusted to always ensure that only the "correct" services were listening on ports, that all software was always fully patches (and contained no vulnerabilities), that confidential data was only shared with internal addressed and that all credentials were strong and regularly changed, this simply does not reflect reality. All of these lead to the requirement that operators be able to filter at Layer 3 and Layer 4, at line rate, in the network. Obviously, to be able to filter at layers 3 and 4, the router needs to be able to see the layer 3 and 4 information. Unfortunately, if the size of extension header chain varies between 0 and 64 kilobytes, extension headers cannot be processed efficiently in ASICs. Because many implementation do not process IPv6 extension headers well, many operators filter all IPv6 packets that include them. 4. Recommendations An ISP SHOULD NOT discard IPv6 packets based solely upon header chain length if the header chain contains 128 bytes or fewer. However, it is common practice ISPs to filter IPv6 packets with long extension header chains. This document offers no recommendation regarding the maximum extension header chain length that an ISP should forward. See Section 1.1 for a formal definition of the header chain. 5. IANA Considerations This document makes no requests of the IANA 6. Security Considerations There are potential implications to the filtering or acceptances of packets which cannot be processed according to the configuration of the network operator. It is clear from the vantage point of the authors that implementors and operators should be aware of implications of relying on extension header chains or apply policies that must necessarily discard packets which contain them. Kumari, et al. Expires January 05, 2014 [Page 6] Internet-Draft long-v6-headers July 2013 7. Acknowledgements The authors wish to thank Paul Hoffman, KK and Fernando Gont. 8. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. Appendix A. Changes / Author Notes. [RFC Editor: Please remove this section before publication ] Template to -00 o Initial submission. -00 to -01 o Added maximum header chain recommendation. o Rewrite the forwarding description. Authors' Addresses Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: warren@kumari.net Kumari, et al. Expires January 05, 2014 [Page 7] Internet-Draft long-v6-headers July 2013 Joel Jaeggli Zynga 675 East Middlefield Mountain View, CA USA Email: jjaeggli@zynga.com Ronald P Bonica Juniper Networks 2251 Corporate Park Drive Herndon, VA USA Email: rbonica@juniper.net Kumari, et al. Expires January 05, 2014 [Page 8]