Network Working Group B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: BCP S. Amante
Expires: August 14, 2011 Level 3
February 10, 2011
Using the IPv6 flow label for equal cost multipath routing and link
aggregation in tunnels
draft-ietf-6man-flow-ecmp-01
Abstract
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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Choice of IP Header Fields for Hash Input . . . . . . . . . 3
1.2. Flow label rules . . . . . . . . . . . . . . . . . . . . . 5
2. Normative Notation . . . . . . . . . . . . . . . . . . . . . . 6
3. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. 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 routing (ECMP) and link
aggregation (LAG) [IEEE802.1AX]. There are of course numerous
possible approaches to this, but certain goals need to be met:
o 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 consistent 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 will
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 and
destination IP addresses, protocol and source and destination port
numbers {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 is, then, which header fields are used to identify a
flow and to serve as input keys to a modulo(N) hash algorithm. A
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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 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, totalling 40 bits, in the hash input makes the hash
slightly more expensive to compute but does improve the hash
distribution, due to the pseudo-random 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 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.
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 end point (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. If there is a high
proportion of tunnel traffic, traffic will not be distributed as
intended across the paths between R1 and R2.
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_____ _____ _____ _____
| TEP |_________| R1 |-------------| R2 |_________| TEP |
|__A__| |_____|-------------|_____| |__B__|
tunnel ECMP or LAG tunnel
here
As noted above, for IPv6, the 5-tuple is in any case 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."
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 today that 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
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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.
At the time of this writing, the IETF is discussing a revision of RFC
3697 [I-D.ietf-6man-flow-3697bis]. If adopted, that revision would
be fully compatible with the present document and would obviate the
concerns resulting from the above three rules. Therefore, the
present specification applies both to RFC 3697 and to its expected
successor.
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].
3. 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 end
points (TEPs).
o The flow label in the outer packet SHOULD be set by the sending
TEP to a pseudo-random 20-bit value in accordance with [RFC3697]
or its replacement. 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.
* A pseudo-random value is recommended on the basis that it will
provide uniformly distributed input values for whatever hash
function is used for load balancing.
* Note that this rule is a recommendation, to permit individual
implementers to take an alternative approach if they wish to do
so. For example, a simpler solution than a pseudo-random value
might be adopted if it was known that the load balancer would
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continue to provide uniform distribution of flows with it.
Such an alternative MUST conform to [RFC3697] or its
replacement.
o 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 ingress TEP most simply by its
{destination, source} address 2-tuple (coarse) or by its 5-tuple
{dest addr, source addr, protocol, dest port, source port} (fine).
At present, ironically, there would be little advantage for IPv6
packets in using the {dest addr, source addr, flow label} 3-tuple.
The choice of n-tuple is an implementation detail in the sending
TEP.
* It might be possible to make this classifier stateless, by
using a suitable 20 bit hash of the inner IP header's 2-tuple
or 5-tuple as the pseudo-random 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 RFC
3697 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.
o At intermediate router(s) 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}. 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. Whether this is
appropriate depends on the expected traffic mix.
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 end-points and
the ECMP or LAG paths will be part of managed infrastructure that is
well protected against on-path attacks. Off-path attackers are
unlikely to guess a valid flow label if a pseudo-random value is
used. In either case, the worst an attacker could do against ECMP or
LAG is to attempt to selectively overload a particular path. For
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further discussion, see [RFC3697] or its replacement
5. IANA Considerations
This document requests no action by IANA.
6. Acknowledgements
This document was suggested by corridor discussions at IETF76. 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 Jarno Rajahalme, Brian
Haberman, Sheng Jiang, Thomas Narten, and others.
This document was produced using the xml2rfc tool [RFC2629].
7. Change log
draft-ietf-6man-flow-ecmp-01: updated after WG Last Call, 2011-02-10
draft-ietf-6man-flow-ecmp-00: after WG adoption at IETF 79,
2010-12-02
draft-carpenter-flow-ecmp-03: clarifications after further comments,
2010-10-07
draft-carpenter-flow-ecmp-02: updated after IETF77 discussion,
especially adding LAG, changed to BCP language, added second author,
2010-04-14
draft-carpenter-flow-ecmp-01: updated after comments, 2010-02-18
draft-carpenter-flow-ecmp-00: original version, 2010-01-19
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.
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[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
8.2. Informative References
[I-D.ietf-6man-flow-3697bis]
Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification",
draft-ietf-6man-flow-3697bis-00 (work in progress),
January 2011.
[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, <http://www.cs.auckland.ac.nz/~brian/
udptcp-paper-cam-submit.pdf>.
[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.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[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.
Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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Shane Amante
Level 3 Communications, LLC
1025 Eldorado Blvd
Broomfield, CO 80021
USA
Email: shane@level3.net
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