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Internet Engineering Task Force (IETF)                      B. CarpenterRequest for Comments: 6438                             Univ. of AucklandCategory: Standards Track                                      S. AmanteISSN: 2070-1721                                                  Level 3                                                           November 2011Using the IPv6 Flow Label forEqual Cost Multipath Routing and Link Aggregation in TunnelsAbstract   The IPv6 flow label has certain restrictions on its use.  This   document describes how those restrictions apply when using the flow   label for load balancing by equal cost multipath routing and for link   aggregation, particularly for IP-in-IPv6 tunneled traffic.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6438.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Carpenter & Amante           Standards Track                    [Page 1]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Choice of IP Header Fields for Hash Input . . . . . . . . .31.2.  Flow Label Rules  . . . . . . . . . . . . . . . . . . . . .42.  Normative Notation  . . . . . . . . . . . . . . . . . . . . . .53.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . . .64.  Security Considerations . . . . . . . . . . . . . . . . . . . .75.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .76.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .86.1.  Normative References  . . . . . . . . . . . . . . . . . . .86.2.  Informative References  . . . . . . . . . . . . . . . . . .81.  Introduction   When several network paths between the same two nodes are known by   the routing system to be equally good (in terms of capacity and   latency), it may be desirable to share traffic among them.  Two such   techniques are known as equal cost multipath (ECMP) routing and link   aggregation (LAG) [IEEE802.1AX].  There are, of course, numerous   possible approaches to this, but certain goals need to be met:   o  Maintain roughly equal share of traffic on each path.      (In some cases, the multiple paths might not all have the same      capacity, and the goal might be appropriately weighted traffic      shares rather than equal shares.  This would affect the load-      sharing algorithm but would not otherwise change the argument.)   o  Minimize or avoid out-of-order delivery for individual traffic      flows.   o  Minimize idle time on any path when queue is non-empty.   There is some conflict between these goals: for example, strictly   avoiding idle time could cause a small packet sent on an idle path to   overtake a bigger packet from the same flow, causing out-of-order   delivery.   One lightweight approach to ECMP or LAG is this: if there are N   equally good paths to choose from, then form a modulo(N) hash   [RFC2991] from a defined set of fields in each packet header that are   certain to have the same values throughout the duration of a flow,   and use the resulting output hash value to select a particular path.   If the hash function is chosen so that the output values have a   uniform statistical distribution, this method will share traffic   roughly equally between the N paths.  If the header fields included   in the hash input are consistent, all packets from a given flow will   generate the same hash output value, so out-of-order delivery willCarpenter & Amante           Standards Track                    [Page 2]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011   not occur.  Assuming a large number of unique flows are involved, it   is also probable that the method will avoid idle time, since the   queue for each link will remain non-empty.1.1.  Choice of IP Header Fields for Hash Input   In the remainder of this document, we will use the term "flow" to   represent a sequence of packets that may be identified by either the   source and destination IP addresses alone {2-tuple} or the source IP   address, destination IP address, protocol number, source port number,   and destination port number {5-tuple}.  It should be noted that the   latter is more specifically referred to as a "microflow" in   [RFC2474], but this term is not used in connection with the flow   label in [RFC3697].   The question, then, is which header fields are used to identify a   flow and serve as input keys to a modulo(N) hash algorithm.  A common   choice when routing general traffic is simply to use a hash of the   source and destination IP addresses, i.e., the 2-tuple.  This is   necessary and sufficient to avoid out-of-order delivery and, with a   wide variety of sources and destinations as one finds in the core of   the network, often statistically sufficient to distribute the load   evenly.  In practice, many implementations use the 5-tuple {dest   addr, source addr, protocol, dest port, source port} as input keys to   the hash function, to maximize the probability of evenly sharing   traffic over the equal cost paths.  However, including transport-   layer information as input keys to a hash may be a problem for IP   fragments [RFC2991] or for encrypted traffic.  Including the protocol   and port numbers, totaling 40 bits, in the hash input makes the hash   slightly more expensive to compute but does improve the hash   distribution, due to the variable nature of ephemeral ports.   Ephemeral port numbers are quite well distributed [Lee10] and will   typically contribute 16 variable bits.  However, in the case of IPv6,   transport-layer information is inconvenient to extract, due to the   variable placement of and variable length of next-headers; all   implementations must be capable of skipping over next-headers, even   if they are rarely present in actual traffic.  In fact, [RFC2460]   implies that next-headers, except hop-by-hop options, are not   normally inspected by intermediate nodes in the network.  This   situation may be challenging for some hardware implementations,   raising the potential that network equipment vendors might sacrifice   the length of the fields extracted from an IPv6 header.   It is worth noting that the possible presence of a Generic Routing   Encapsulation (GRE) header [RFC2784] and the possible presence of a   GRE key within that header creates a similar challenge to the   possible presence of IPv6 extension headers; anything that   complicates header analysis is undesirable.Carpenter & Amante           Standards Track                    [Page 3]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011   The situation is different in IP-in-IP tunneled scenarios.   Identifying a flow inside the tunnel is more complicated,   particularly because nearly all hardware can only identify flows   based on information contained in the outermost IP header.  Assume   that traffic from many sources to many destinations is aggregated in   a single IP-in-IP tunnel from tunnel endpoint (TEP) A to TEP B (see   figure).  Then all the packets forming the tunnel have outer source   address A and outer destination address B.  In all probability, they   also have the same port and protocol numbers.  If there are multiple   paths between routers R1 and R2, and ECMP or LAG is applied to choose   a particular path, the 2-tuple or 5-tuple (and its hash) will be   constant, and no load sharing will be achieved, i.e., polarization   will occur.  If there is a high proportion of traffic from one or a   small number of tunnels, traffic will not be distributed as intended   across the paths between R1 and R2, due to partial polarization.   (Related issues arise with MPLS [MPLS-LABEL].)      _____           _____               _____           _____     | TEP |_________| R1  |-------------| R2  |_________| TEP |     |__A__|         |_____|-------------|_____|         |__B__|             tunnel          ECMP or LAG         tunnel                                 here   As noted above, for IPv6, the 5-tuple is quite inconvenient to   extract due to the next-header placement.  The question therefore   arises whether the 20-bit flow label in IPv6 packets would be   suitable for use as input to an ECMP or LAG hash algorithm,   especially in the case of tunnels where the inner packet header is   inaccessible.  If the flow label could be used in place of the port   numbers and protocol number in the 5-tuple, the implementation would   be simplified.1.2.  Flow Label Rules   The flow label was left Experimental by [RFC2460] but was better   defined by [RFC3697].  We quote three rules from that RFC:   1.  "The Flow Label value set by the source MUST be delivered       unchanged to the destination node(s)."   2.  "IPv6 nodes MUST NOT assume any mathematical or other properties       of the Flow Label values assigned by source nodes."   3.  "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."Carpenter & Amante           Standards Track                    [Page 4]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011   These rules, especially the last one, have caused designers to   hesitate about using the flow label in support of ECMP or LAG.  The   fact is that today most nodes set a zero value in the flow label, and   the first rule definitely forbids the routing system from changing   the flow label once a packet has left the source node.  Considering   normal IPv6 traffic, the fact that the flow label is typically zero   means that it would add no value to an ECMP or LAG hash, but neither   would it do any harm to the distribution of the hash values.   However, in the case of an IP-in-IPv6 tunnel, the TEP is itself the   source node of the outer packets.  Therefore, a TEP may freely set a   flow label in the outer IPv6 header of the packets it sends into the   tunnel.   The second two rules quoted above need to be seen in the context of   [RFC3697], which assumes that routers using the flow label in some   way will be involved in some sort of method of establishing flow   state: "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 RFC should perhaps have made   clear that a router that has participated in flow state establishment   can rely on properties of the resulting flow label values without   further signaling.  If a router knows these properties, rule 2 is   irrelevant, and it can choose to deviate from rule 3.   In the tunneling situation sketched above, routers R1 and R2 can rely   on the flow labels set by TEP A and TEP B being assigned by a known   method.  This allows an ECMP or LAG method to be based on the flow   label consistently with [RFC3697], regardless of whether the non-   tunnel traffic carries non-zero flow label values.   The IETF has recently revisedRFC 3697 [RFC6437].  That revision is   fully compatible with the present document and obviates the concerns   resulting from the above three rules.  Therefore, the present   specification applies both toRFC 3697 and toRFC 6437.2.  Normative Notation   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 [RFC2119].Carpenter & Amante           Standards Track                    [Page 5]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 20113.  Guidelines   We assume that the routers supporting ECMP or LAG (R1 and R2 in the   above figure) are unaware that they are handling tunneled traffic.   If it is desired to include the IPv6 flow label in an ECMP or LAG   hash in the tunneled scenario shown above, the following guidelines   apply:   o  Inner packets MUST be encapsulated in an outer IPv6 packet whose      source and destination addresses are those of the tunnel endpoints      (TEPs).   o  The flow label in the outer packet SHOULD be set by the sending      TEP to a 20-bit value in accordance with [RFC6437].  The same flow      label value MUST be used for all packets in a single user flow, as      determined by the IP header fields of the inner packet.   o  To achieve this, the sending TEP MUST classify all packets into      flows once it has determined that they should enter a given tunnel      and then write the relevant flow label into the outer IPv6 header.      A user flow could be identified by the sending TEP most simply by      its {destination, source} address 2-tuple or by its 5-tuple {dest      addr, source addr, protocol, dest port, source port}.  At present,      there would be little point in using the {dest addr, source addr,      flow label} 3-tuple of the inner packet, but doing so would be a      future-proof option.  The choice of n-tuple is an implementation      choice in the sending TEP.      *  As specified in [RFC6437], the flow label values should be         chosen from a uniform distribution.  Such values will be         suitable as input to a load-balancing hash function and will be         hard for a malicious third party to predict.      *  The sending TEP MAY perform stateless flow label assignment by         using a suitable 20-bit hash of the inner IP header's 2-tuple         or 5-tuple as the flow label value.      *  If the inner packet is an IPv6 packet, its flow label value         could also be included in this hash.      *  This stateless method creates a small probability of two         different user flows hashing to the same flow label.  Since         [RFC6437] allows a source (the TEP in this case) to define any         set of packets that it wishes as a single flow, occasionally         labeling two user flows as a single flow through the tunnel is         acceptable.Carpenter & Amante           Standards Track                    [Page 6]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011   o  At intermediate routers that perform load distribution, the hash      algorithm used to determine the outgoing component-link in an ECMP      and/or LAG toward the next hop MUST minimally include the 3-tuple      {dest addr, source addr, flow label} and MAY also include the      remaining components of the 5-tuple.  This applies whether the      traffic is tunneled traffic only or a mixture of normal traffic      and tunneled traffic.      *  Intermediate IPv6 router(s) will presumably encounter a mixture         of tunneled traffic and normal IPv6 traffic.  Because of this,         the design should also include {protocol, dest port, source         port} as input keys to the ECMP and/or LAG hash algorithms, to         provide additional entropy for flows whose flow label is set to         zero, including non-tunneled traffic flows.   o  Individual nodes in a network are free to implement different      algorithms that conform to this specification without impacting      the interoperability or function of the network.   o  Operations, Administration, and Maintenance (OAM) techniques will      need to be adapted to manage ECMP and LAG based on the flow label.      The issues will be similar to those that arise for MPLS [RFC4379]      and pseudowires [RFC6391].4.  Security Considerations   The flow label is not protected in any way and can be forged by an   on-path attacker.  However, it is expected that tunnel endpoints and   the ECMP or LAG paths will be part of a managed infrastructure that   is well protected against on-path attacks (e.g., by using IPsec   between the two tunnel endpoints).  Off-path attackers are unlikely   to guess a valid flow label if an apparently pseudo-random and   unpredictable value is used.  In either case, the worst an attacker   could do against ECMP or LAG is attempt to selectively overload a   particular path.  For further discussion, see [RFC6437].5.  Acknowledgements   This document was suggested by corridor discussions at IETF 76.  Joel   Halpern made crucial comments on an early version.  We are grateful   to Qinwen Hu for general discussion about the flow label.  Valuable   comments and contributions were made by Miguel Garcia, Brian   Haberman, Sheng Jiang, Thomas Narten, Jarno Rajahalme, Brian Weis,   and others.Carpenter & Amante           Standards Track                    [Page 7]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 20116.  References6.1.  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.   [RFC3697]      Rajahalme, J., Conta, A., Carpenter, B., and S.                  Deering, "IPv6 Flow Label Specification",RFC 3697,                  March 2004.   [RFC6437]      Amante, S., Carpenter, B., Jiang, S., and J.                  Rajahalme, "IPv6 Flow Label Specification",RFC 6437,                  November 2011.6.2.  Informative References   [IEEE802.1AX]  Institute of Electrical and Electronics Engineers,                  "Link Aggregation", IEEE Standard 802.1AX-2008, 2008.   [Lee10]        Lee, D., Carpenter, B., and N. Brownlee, "Observations                  of UDP to TCP Ratio and Port Numbers", Fifth                  International Conference on Internet Monitoring and                  Protection ICIMP 2010, May 2010.   [MPLS-LABEL]   Kompella, K., Drake, J., Amante, S., Henderickx, W.,                  and L. Yong, "The Use of Entropy Labels in MPLS                  Forwarding", Work in Progress, May 2011.   [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.   [RFC2784]      Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.                  Traina, "Generic Routing Encapsulation (GRE)",RFC 2784, March 2000.   [RFC2991]      Thaler, D. and C. Hopps, "Multipath Issues in Unicast                  and Multicast Next-Hop Selection",RFC 2991,                  November 2000.   [RFC4379]      Kompella, K. and G. Swallow, "Detecting Multi-Protocol                  Label Switched (MPLS) Data Plane Failures",RFC 4379,                  February 2006.Carpenter & Amante           Standards Track                    [Page 8]

RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011   [RFC6391]      Bryant, S., Filsfils, C., Drafz, U., Kompella, V.,                  Regan, J., and S. Amante, "Flow-Aware Transport of                  Pseudowires over an MPLS Packet Switched Network",RFC 6391, November 2011.Authors' Addresses   Brian Carpenter   Department of Computer Science   University of Auckland   PB 92019   Auckland  1142   New Zealand   EMail: brian.e.carpenter@gmail.com   Shane Amante   Level 3 Communications, LLC   1025 Eldorado Blvd   Broomfield, CO  80021   USA   EMail: shane@level3.netCarpenter & Amante           Standards Track                    [Page 9]