Internet DRAFT - draft-wkumari-long-headers
draft-wkumari-long-headers
6MAN W. Kumari
Internet-Draft Google
Intended status: Best Current Practice J. Jaeggli
Expires: December 17, 2015 Zynga
R. Bonica
Juniper Networks
J. Linkova
Google
June 15, 2015
Operational Issues Associated With Long IPv6 Header Chains
draft-wkumari-long-headers-03
Abstract
This memo specifies requirements for IPv6 forwarders as they process
packets with long header chains. It also provides guidance for
application developers whose applications might rely on long headers
chains.
As background, this memo explains how many ASIC-based IPv6 forwarders
process packets and why processing of packets with long header chains
might be problematic.
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 December 17, 2015.
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Copyright Notice
Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . 4
2. Forwarder Information Requirements . . . . . . . . . . . . . 4
3. Requirements For IPv6 Forwarders . . . . . . . . . . . . . . 5
4. Recommendations For Application Developers . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
IPv6 [RFC2460] forwarders can acquire information from the following
sources:
o The IPv6 header
o One or more IPv6 extension headers
o An upper-layer header
Section 2 of this document explains how IPv6 forwarders use
information from the IPv6 header and IPv6 extension headers to
provide traditional forwarding services. It also explains how IPv6
forwarders use information from the upper-layer header to provide
enhanced forwarding services.
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When a software-based forwarder processes an IPv6 datagram, it parses
the header chain, regardless of its length, acquires the required
information and makes a forwarding decision. Typically, software-
based forwarders process a relatively small number of packets per
second. Therefore, they can perform the above mentioned procedure
within the constraints of their processing budget.
By contrast, ASIC-based forwarders process many more packets per
second. In order to fulfill this requirement, ASIC-based forwarders
copy a fixed number of bytes from the beginning of the packet to on-
chip memory. Forwarders do this because they can access on-chip
memory much more quickly than they can access off-chip memory. Once
the beginning of the packet has been transferred to on-chip memory,
subsequent processing can proceed very quickly.
The act of copying bytes from the beginning of a packet to on-chip
memory consumes:
o Processor cycles
o On-chip memory
o Wall-time
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 and adversely impacts performance. If it copies too
few bytes, it may not have sufficient information to make a correct
forwarding decision.
The IPv6 header chain is a variable-length data structure, whose size
can exceed 64 kilobytes. However, packets with header chains
exceeding 256 bytes are rarely observed on the Internet. Therefore,
most ASIC-based forwarders copy a relatively small number of bytes
from the beginning of a packet into on-chip memory. While this small
number varies from platform to platform, it is generally much closer
to 256 bytes than it is to 64 kilobytes.
IPv6 forwarders MUST behave in a predictable manner when they process
a packet whose header chain length exceeds the number of bytes copied
to on-chip memory. Section 3 of this memo defines required
behaviors.
Application developers should be aware of how ASIC-based forwarders
process packets with long extension header chains. Therefore,
Section 4 of this document provides guidance to application
developers.
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1.1. Termnology
For the purposes of this document, the terms "header chain" and
"upper-layer" header are used as defined in [RFC7112].
This document also introduces the following terms:
o forwarding service - a service that accepts a packet from one
interface and forwards it through another
o traditional forwarding service - a forwarding service in which all
parameters to the forwarding algorithm are drawn from the IPv6
header, the hop-by-hop extension header, and the routing extension
header
o enhanced forwarding service - a forwarding service in which
parameters to the forwarding algorithm can be drawn from any
portion of the IPv6 header chain
2. Forwarder Information Requirements
When an IPv6 forwarder provides traditional forwarding services, it
extracts all information required by the forwarding algorithm from
the IPv6 header, the hop-by-hop extension header (if present), and
the routing extension header (if present). In the nominal case, the
IPv6 header contains all information required by the forwarding
algorithm. However, the hop-by-hop and routing extension headers can
also impact forwarding behavior.
Section 4.2 of [RFC2460] explains how the hop-by-hop extension header
impacts forwarding behavior. When the forwarder processes a hop-by-
hop extension header, it examines each option contained by the
header. If forwarder encounters an unrecognized hop-by-hop option,
and the high-order bits of the option type are "00", the forwarder
skips over the option and continues to process subsequent options.
However, if an forwarder encounters an unrecognized option, and the
high-order bits of the option type are "01", "10" or "11", the
forwarder discards the packet.
Section 4.4 of [RFC2460] explains how the routing extension header
impacts forwarding behavior. When the forwarder processes a packet
whose destination address is local to itself, it scans the header
chain, searching for a routing extension header. If the packet
contains a routing extension header and the forwarder recognizes the
routing header type, it processes the header. If the forwarder does
not recognize the routing header type, the required behavior depends
upon the Segments Left field. If the Segments Left field is equal to
zero, the forwarder ignores the routing extension header. Otherwise,
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the forwarder discards the packet. [RFC6275] and [RFC6554] describe
currently defined routing extension header types.
Some IPv6 forwarders provide enhanced forwarding services, such as
firewall filtering, rate limiting and load balancing. In order to
provide these services, the forwarder requires access to an upper
layer header. The following are examples of enhanced services that
require the forwarder to examine the upper layer header:
o Discard all packets directed to TCP port 25
o Rate limit packets destined for a particular address whose payload
is TCP and have the TCP SYN bit set
o Load balance packets across parallel links so that all packet
belonging to particular TCP session traverse the same link.
3. Requirements For IPv6 Forwarders
The following requirements apply to all IPv6 forwarders:
o REQ-1: By default an IPv6 forwarder SHOULD NOT discard a valid
packet because of its header chain length. However, the forwarder
MAY support a configuration option that causes it to discard
packets whose header chain length exceeds a specified value.
o REQ-2: When processing packet that contains a hop-by-hop extension
header, an IPv6 forwarder MUST process the entire hop-by-hop
extension header, regardless of its length. The forwarder MUST
process each option as specified in Section 4.2 of [RFC2460]. If
an IPv6 forwarder is not able to process the entire hop-by-hop
extension header, it MUST discard the packet and SHOULD originate
an ICMPv6 Parameter Problem message to the packet's source. The
forwarder MAY have a configurable policy for sending ICMPv6
messages such as rate limiting or completely disabling them If an
IPv6 forwarder is not able to process the entire hop-by-hop
extension header, it MUST discard the packet and SHOULD originate
an ICMPv6 Parameter Problem message to the packet's source. The
forwarder MAY have a configurable policy for sending ICMPv6
messages such as rate limiting or completely disabling them.
o REQ-3: When processing a packet whose destination address is local
to itself, an IPv6 forwarder MUST scan the entire header chain,
regardless of its length, in order to determine whether the packet
contains a routing extension header. If the packet contains a
routing extension header, the forwarder MUST process routing
extension header as specified in Section 4.4 of [RFC2460]. If an
IPv6 forwarder is not able to process the entire routing extension
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header, it MUST discard the packet and SHOULD originate an ICMPv6
Parameter Problem message to the packet's source. The forwarder
MAY have a configurable policy for sending ICMPv6 messages such as
rate limiting or completely disabling them.
The length of the IPv6 header plus the length of the hop-by-hop
extension header can exceed the number of bytes that an ASIC-based
forwarder copies into on-chip memory. Therefore, in order to support
REQ-2, ASIC-based forwarders typically support a special processing
mechanism for packets containing hop-by-hop extensions.
Also, the combined length of all headers preceding the routing
extension header may exceed the number of bytes that an ASIC-based
forwarder copies into on-chip memory. Therefore, in order to support
REQ-3, ASIC-based forwarders typically support a special processing
mechanism for packets whose IPv6 destination address is local to the
forwarder. This forwarding mechanism is capable of processing the
routing extension header, even if it begins beyond of the portion of
the packet that was copied to on-chip memory.
The following requirements apply to IPv6 forwarders that provide
enhanced forwarding services:
o REQ-4: If a forwarder's ability to deliver enhanced services is
limited in any way by extension header length, that limitation
MUST be reflected in user documentation. For example, assume that
a forwarder provides a load balancing service, and that it
acquires information required by the service from the IPv6 header
and the upper-layer header. If the service behaves in one manner
when all required information is contained by the first N bytes of
the header chain and in another manner when all required
information is not contained by the first N bytes of the header
chain, user documentation MUST reflect both behaviors as well as
the value of N.
o REQ-5: If a forwarder's ability to deliver an enhanced service is
limited by extension header length, the policy specification
language used to configure the enhanced service MUST be
sufficiently robust to address the limitation. For example,
assume that the forwarder provides a firewall service. The
firewall service is capable of filtering packets directed to a
particular TCP port, but only if the TCP header is contained by
the first N bytes of the header chain. In this case, it MUST be
possible to configure one policy for packets directed to the
specified port, another policy for packet not directed to the
specified port, and a third policy for packets whose TCP
destination port is unknown.
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4. Recommendations For Application Developers
Applications developers should be aware that many ISPs and
enterprises filter or severely rate limit packets containing long
header chains. They do this because of limitations imposed by the
ASIC-based forwarders deployed at their edges. ISPs and enterprises
accept these limitations as part of an engineering trade off, in
which high-speed forwarding is achieved at the cost of limiting
enhanced services for packets with long extension headers.
For example, assume that an enterprise deploys the following firewall
filtering policy at its edge:
o Permit all packets whose destination is TCP port 80
o Discard all packets whose destination is not TCP port 80
o Discard all packets whose header chain is so long that TCP port
information is not accessible to the filtering function
In this case, the enterprise discards all packets whose destination
cannot be determined by the filtering function.
Aside from the issue of header chain length, operators may filter
packets containing extension headers that may either compromise the
network's security posture or require inordinate processing
resources.
This memo does not specify a maximum header chain length. However,
this memo does note that at the time of its publication, the number
of bytes that ASIC-based forwarders copy from the beginning of a
packet to on-chip memory varies from platform to platform. Typical
platforms copy between 128 and 384 bytes. Therefore, application
developers should avoid sending packets who header chain length is in
that range, unless they have some assurance that their packets will
not be discarded.
5. IANA Considerations
This document makes no requests of the IANA
6. Security Considerations
TBD
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7. Acknowledgements
The authors wish to thank Paul Hoffman, KK and Fernando Gont. The
authors also express their gratitude to an anonymous donor, without
whom this document would not have been written.
8. References
8.1. Normative References
[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.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", RFC 7112, January 2014.
8.2. Informative References
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, March
2012.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
Template to -00
o Initial submission.
o
-00 to -01
o Added maximum header chain recommendation.
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o Rewrite the forwarding description.
-02 to -03
o Updating REQ2 and REQ3 with sending ICMPv6 messages part.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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
J. Linkova
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: furry@google.com
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