|Title||IPv6 Flow Label Specification
|Author||J. Rajahalme, A. Conta, B. Carpenter,
Network Working Group J. Rajahalme
Request for Comments: 3697 Nokia
Category: Standards Track A. Conta
IPv6 Flow Label Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document specifies the IPv6 Flow Label field and the minimum
requirements for IPv6 source nodes labeling flows, IPv6 nodes
forwarding labeled packets, and flow state establishment methods.
Even when mentioned as examples of possible uses of the flow
labeling, more detailed requirements for specific use cases are out
of scope for this document.
The usage of the Flow Label field enables efficient IPv6 flow
classification based only on IPv6 main header fields in fixed
A flow is a sequence of packets sent from a particular source to a
particular unicast, anycast, or multicast destination that the source
desires to label as a flow. A flow could consist of all packets in a
specific transport connection or a media stream. However, a flow is
not necessarily 1:1 mapped to a transport connection.
Traditionally, flow classifiers have been based on the 5-tuple of the
source and destination addresses, ports, and the transport protocol
type. However, some of these fields may be unavailable due to either
fragmentation or encryption, or locating them past a chain of IPv6
option headers may be inefficient. Additionally, if classifiers
depend only on IP layer headers, later introduction of alternative
transport layer protocols will be easier.
The usage of the 3-tuple of the Flow Label and the Source and
Destination Address fields enables efficient IPv6 flow
classification, where only IPv6 main header fields in fixed positions
The minimum level of IPv6 flow support consists of labeling the
flows. IPv6 source nodes supporting the flow labeling MUST be able
to label known flows (e.g., TCP connections, application streams),
even if the node itself would not require any flow-specific
treatment. Doing this enables load spreading and receiver oriented
resource reservations, for example. Node requirements for flow
labeling are given in section 3.
Specific flow state establishment methods and the related service
models are out of scope for this specification, but the generic
requirements enabling co-existence of different methods in IPv6 nodes
are set forth in section 4. The associated scaling characteristics
(such as nodes involved in state establishment, amount of state
maintained by them, and state growth function) will be specific to
particular service models.
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 BCP 14, RFC 2119
2. IPv6 Flow Label Specification
The 20-bit Flow Label field in the IPv6 header [IPv6] is used by a
source to label packets of a flow. A Flow Label of zero is used to
indicate packets not part of any flow. Packet classifiers use the
triplet of Flow Label, Source Address, and Destination Address fields
to identify which flow a particular packet belongs to. Packets are
processed in a flow-specific manner by the nodes that have been set
up with flow-specific state. The nature of the specific treatment
and the methods for the flow state establishment are out of scope for
The Flow Label value set by the source MUST be delivered unchanged to
the destination node(s).
IPv6 nodes MUST NOT assume any mathematical or other properties of
the Flow Label values assigned by source nodes. Router performance
SHOULD NOT be dependent on the distribution of the Flow Label values.
Especially, the Flow Label bits alone make poor material for a hash
Nodes keeping dynamic flow state MUST NOT assume packets arriving 120
seconds or more after the previous packet of a flow still belong to
the same flow, unless a flow state establishment method in use
defines a longer flow state lifetime or the flow state has been
explicitly refreshed within the lifetime duration.
The use of the Flow Label field does not necessarily signal any
requirement on packet reordering. Especially, the zero label does
not imply that significant reordering is acceptable.
If an IPv6 node is not providing flow-specific treatment, it MUST
ignore the field when receiving or forwarding a packet.
3. Flow Labeling Requirements
To enable Flow Label based classification, source nodes SHOULD assign
each unrelated transport connection and application data stream to a
new flow. The source node MAY also take part in flow state
establishment methods that result in assigning certain packets to
specific flows. A source node which does not assign traffic to flows
MUST set the Flow Label to zero.
To enable applications and transport protocols to define what packets
constitute a flow, the source node MUST provide means for the
applications and transport protocols to specify the Flow Label values
to be used with their flows. The use of the means to specify Flow
Label values is subject to appropriate privileges (see section 5.1).
The source node SHOULD be able to select unused Flow Label values for
flows not requesting a specific value to be used.
A source node MUST ensure that it does not unintentionally reuse Flow
Label values it is currently using or has recently used when creating
new flows. Flow Label values previously used with a specific pair of
source and destination addresses MUST NOT be assigned to new flows
with the same address pair within 120 seconds of the termination of
the previous flow. The source node SHOULD provide the means for the
applications and transport protocols to specify quarantine periods
longer than the default 120 seconds for individual flows.
To avoid accidental Flow Label value reuse, the source node SHOULD
select new Flow Label values in a well-defined sequence (e.g.,
sequential or pseudo-random) and use an initial value that avoids
reuse of recently used Flow Label values each time the system
restarts. The initial value SHOULD be derived from a previous value
stored in non-volatile memory, or in the absence of such history, a
randomly generated initial value using techniques that produce good
randomness properties [RND] SHOULD be used.
4. Flow State Establishment Requirements
To enable flow-specific treatment, flow state needs to be established
on all or a subset of the IPv6 nodes on the path from the source to
the destination(s). The methods for the state establishment, as well
as the models for flow-specific treatment will be defined in separate
To enable co-existence of different methods in IPv6 nodes, the
methods MUST meet the following basic requirements:
(1) The method MUST provide the means for flow state clean-up from
the IPv6 nodes providing the flow-specific treatment. Signaling
based methods where the source node is involved are free to
specify flow state lifetimes longer than the default 120
(2) Flow state establishment methods MUST be able to recover from
the case where the requested flow state cannot be supported.
5. Security Considerations
This section considers security issues raised by the use of the Flow
Label, primarily the potential for denial-of-service attacks, and the
related potential for theft of service by unauthorized traffic
(Section 5.1). Section 5.2 addresses the use of the Flow Label in
the presence of IPsec including its interaction with IPsec tunnel
mode and other tunneling protocols. We also note that inspection of
unencrypted Flow Labels may allow some forms of traffic analysis by
revealing some structure of the underlying communications. Even if
the flow label were encrypted, its presence as a constant value in a
fixed position might assist traffic analysis and cryptoanalysis.
5.1. Theft and Denial of Service
Since the mapping of network traffic to flow-specific treatment is
triggered by the IP addresses and Flow Label value of the IPv6
header, an adversary may be able to obtain better service by
modifying the IPv6 header or by injecting packets with false
addresses and/or labels. Taken to its limits, such theft-of-service
becomes a denial-of-service attack when the modified or injected
traffic depletes the resources available to forward it and other
traffic streams. A curiosity is that if a DoS attack were undertaken
against a given Flow Label (or set of Flow Labels), then traffic
containing an affected Flow Label might well experience worse-than-
best-effort network performance.
Note that since the treatment of IP headers by nodes is typically
unverified, there is no guarantee that flow labels sent by a node are
set according to the recommendations in this document. Therefore,
any assumptions made by the network about header fields such as flow
labels should be limited to the extent that the upstream nodes are
Since flows are identified by the 3-tuple of the Flow Label and the
Source and Destination Address, the risk of theft or denial of
service introduced by the Flow Label is closely related to the risk
of theft or denial of service by address spoofing. An adversary who
is in a position to forge an address is also likely to be able to
forge a label, and vice versa.
There are two issues with different properties: Spoofing of the Flow
Label only, and spoofing of the whole 3-tuple, including Source and
The former can be done inside a node which is using or transmitting
the correct source address. The ability to spoof a Flow Label
typically implies being in a position to also forge an address, but
in many cases, spoofing an address may not be interesting to the
spoofer, especially if the spoofer's goal is theft of service, rather
than denial of service.
The latter can be done by a host which is not subject to ingress
filtering [INGR] or by an intermediate router. Due to its
properties, such is typically useful only for denial of service. In
the absence of ingress filtering, almost any third party could
instigate such an attack.
In the presence of ingress filtering, forging a non-zero Flow Label
on packets that originated with a zero label, or modifying or
clearing a label, could only occur if an intermediate system such as
a router was compromised, or through some other form of man-in-the-
middle attack. However, the risk is limited to traffic receiving
better or worse quality of service than intended. For example, if
Flow Labels are altered or cleared at random, flow classification
will no longer happen as intended, and the altered packets will
receive default treatment. If a complete 3-tuple is forged, the
altered packets will be classified into the forged flow and will
receive the corresponding quality of service; this will create a
denial of service attack subtly different from one where only the
addresses are forged. Because it is limited to a single flow
definition, e.g., to a limited amount of bandwidth, such an attack
will be more specific and at a finer granularity than a normal
Since flows are identified by the complete 3-tuple, ingress filtering
[INGR] will, as noted above, mitigate part of the risk. If the
source address of a packet is validated by ingress filtering, there
can be a degree of trust that the packet has not transited a
compromised router, to the extent that ISP infrastructure may be
trusted. However, this gives no assurance that another form of man-
in-the-middle attack has not occurred.
Only applications with an appropriate privilege in a sending host
will be entitled to set a non-zero Flow Label. Mechanisms for this
are operating system dependent. Related policy and authorization
mechanisms may also be required; for example, in a multi-user host,
only some users may be entitled to set the Flow Label. Such
authorization issues are outside the scope of this specification.
5.2. IPsec and Tunneling Interactions
The IPsec protocol, as defined in [IPSec, AH, ESP], does not include
the IPv6 header's Flow Label in any of its cryptographic calculations
(in the case of tunnel mode, it is the outer IPv6 header's Flow Label
that is not included). Hence modification of the Flow Label by a
network node has no effect on IPsec end-to-end security, because it
cannot cause any IPsec integrity check to fail. As a consequence,
IPsec does not provide any defense against an adversary's
modification of the Flow Label (i.e., a man-in-the-middle attack).
IPsec tunnel mode provides security for the encapsulated IP header's
Flow Label. A tunnel mode IPsec packet contains two IP headers: an
outer header supplied by the tunnel ingress node and an encapsulated
inner header supplied by the original source of the packet. When an
IPsec tunnel is passing through nodes performing flow classification,
the intermediate network nodes operate on the Flow Label in the outer
header. At the tunnel egress node, IPsec processing includes
removing the outer header and forwarding the packet (if required)
using the inner header. The IPsec protocol requires that the inner
header's Flow Label not be changed by this decapsulation processing
to ensure that modifications to label cannot be used to launch theft-
or denial-of-service attacks across an IPsec tunnel endpoint. This
document makes no change to that requirement; indeed it forbids
changes to the Flow Label.
When IPsec tunnel egress decapsulation processing includes a
sufficiently strong cryptographic integrity check of the encapsulated
packet (where sufficiency is determined by local security policy),
the tunnel egress node can safely assume that the Flow Label in the
inner header has the same value as it had at the tunnel ingress node.
This analysis and its implications apply to any tunneling protocol
that performs integrity checks. Of course, any Flow Label set in an
encapsulating IPv6 header is subject to the risks described in the
5.3. Security Filtering Interactions
The Flow Label does nothing to eliminate the need for packet
filtering based on headers past the IP header, if such filtering is
deemed necessary for security reasons on nodes such as firewalls or
The discussion on the topic in the IPv6 WG mailing list has been
instrumental for the definition of this specification. The authors
want to thank Ran Atkinson, Steve Blake, Jim Bound, Francis Dupont,
Robert Elz, Tony Hain, Robert Hancock, Bob Hinden, Christian Huitema,
Frank Kastenholz, Thomas Narten, Charles Perkins, Pekka Savola,
Hesham Soliman, Michael Thomas, Margaret Wasserman, and Alex Zinin
for their contributions.
7.1. Normative References
[IPv6] Deering, S. and R. Hinden, "Internet Protocol Version 6
Specification", RFC 2460, December 1998.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to indicate
requirement levels", BCP 14, RFC 2119, March 1997.
[RND] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
7.2. Informative References
[AH] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[INGR] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP
Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[IPSec] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
Nokia Research Center
P.O. Box 407
FIN-00045 NOKIA GROUP,
3 Enterprise Drive
Shelton, CT 06484
Brian E. Carpenter
IBM Zurich Research Laboratory
Saeumerstrasse 4 / Postfach
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to
rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at email@example.com.
Funding for the RFC Editor function is currently provided by the