Network Working Group                                       J. RajahalmeRequest for Comments: 3697                                         NokiaCategory: Standards Track                                       A. Conta                                                              Transwitch                                                            B. Carpenter                                                                     IBM                                                              S. Deering                                                                   Cisco                                                              March 2004                     IPv6 Flow Label SpecificationStatus 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 Notice   Copyright (C) The Internet Society (2004).  All Rights Reserved.Abstract   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   positions.1.  Introduction   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.Rajahalme, et al.           Standards Track                     [Page 1]RFC 3697             IPv6 Flow Label Specification            March 2004   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   are used.   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   [KEYWORDS].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   this specification.   The Flow Label value set by the source MUST be delivered unchanged to   the destination node(s).Rajahalme, et al.           Standards Track                     [Page 2]RFC 3697             IPv6 Flow Label Specification            March 2004   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   key.   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 avoidsRajahalme, et al.           Standards Track                     [Page 3]RFC 3697             IPv6 Flow Label Specification            March 2004   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   specifications.   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        seconds.   (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 otherRajahalme, et al.           Standards Track                     [Page 4]RFC 3697             IPv6 Flow Label Specification            March 2004   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   explicitly trusted.   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   Destination Address.   The former can be done inside a node which is using or transmitting