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Versions: 00 01                                                         
Network Working Group                                       L. Yong Ed.
Internet Draft                                               P. L. Yang
Intended status: Standards Track                                 Huawei
Expires: Sept. 2010                                       March 5, 2010

             Enhanced ECMP and Large Flow Aware Transport

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 4, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   Section 4.e of the Trust Legal Provisions and are provided
   without warranty as described in the BSD License.


   Internet Traffic has constantly shown the pattern that a very
   small amount of the traffic flows generate a high traffic volume
   while a significant amount of small flows contribute a small
   amount of traffic volume. Differentiating such large flow and
   small flow in the packet switched network enables an enhanced
   transport method over Equal Cost Multi Paths (ECMP). This draft
   describes the enhanced ECMP transport with the large flow

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................4
      2.1. Terminology...............................................4
   3. Large Flow Recognition.........................................4
   4. Enhanced ECMP Process..........................................5
      4.1. Congestion Control........................................7
   5. Large Flow Indication..........................................8
   6. Backward Compatibility.........................................9
   7. Applicability..................................................9
      7.1. Link Aggregation Groups..................................10
      7.2. The Single Large Flow Case...............................10
      7.3. Flow Rate Difference.....................................10
      7.4. Multi-Segment Pseudowires................................11
      7.5. IP Flows.................................................11
      7.6. LSP with Entropy Label...................................11
   8. Security Considerations.......................................12
   9. IANA Considerations...........................................12
   10. References...................................................12
      10.1. Normative References....................................12
      10.2. Informative References..................................13
   11. Acknowledgments..............................................13
   Appendix A. Simulation Analysis..................................14

1. Introduction

   [FAT-PW] introduces the flow label on the label stack for some
   pseudowires (PW) to take the advantage of ECMP transport. The
   method inserts a flow label on each packet at ingress PE. The
   ECMP process in the packet switched network (PSN) hashes the
   label stack that contains the flow label. As a result, individual
   flows in a PW can be transported over different ECMP paths. Since

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   the packets that belong to the same flow have the same label
   value, the method gets ECMP transport benefit as well as
   preserves the ordering of each individual transported IP flow.

   However, the traffic over Internet today includes Web browsing
   data and audio as well as video/downloading and streaming.
   Video/downloading and streaming generates the very high rate
   flows compared to Web browsing data/audio. This causes Internet
   traffic clearly mixed with huge amount of small flows and small
   amount of very high rate flows. Internet traffic analysis [CAIDA]
   indicates that, today, ~2% of the top rate ranked flows takes
   about 30% of traffic volume while the rest of 98% flows
   contribute 70% of traffic volume. As Web HDTV and 3D TV will be
   on the Internet, the traffic volume ratio between large and small
   flows may be further higher.   Although the flow label can
   improve the load balancing per the flow basis within a
   pseudowire, under such traffic pattern, hash based distribution
   is inadequate for satisfactory load balancing.

   Hash based distribution ensures any flow to be mapped into only
   one of ECMP paths (fixed one) so the flow ordering is preserved
   in the transport. However, hash based distribution disperses all
   the possible flow identifiers over ECMP paths no matter a flow
   exists or not at the time and does not consider individual flow
   rate, i.e. it has the nature of stateless distribution. Such
   distribution method generates adequate load balancing if the
   traffic contains huge amount of flows that have similar flow
   rates. The simulation has shown that given Internet traffic
   pattern, the hash method does not evenly distribute the flows
   over ECMP paths. The load difference between two of ECMP paths
   can be significant large; the more ECMP paths exist, the more
   severe the un-balancing syndrome presents. This implies that hash
   based distribution can cause some path congested and some being
   partial filled only. This results that the packets are dropped at
   congestion point and the network reroute impacted traffic. In
   other words, this syndrome lowers the network performance and
   brings operator desires to improve load balancing over ECMP. One
   option to prevent such syndrome is to add more transport resource
   in the network. But this will lower the network utilization and
   increase the service cost.

   This draft describes an enhanced ECMP method for such traffic
   pattern and also introduces the large/small flow indication on
   the flow label to facilitate enhanced ECMP transport in PSN. The
   enhanced method uses a table for a small amount of large flow
   distribution and hashing on all other flows. The method gets
   evenly load balance by maintaining a small set of large flow

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   states. The draft states the process procedures on PE and P

   The simulation result has shown that the enhanced ECMP gets much
   better improvement on load-balancing compared to hash based ECMP
   under Internet traffic pattern. The load difference among paths
   is less than 1%.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   "OPTIONAL" in this document are to be interpreted as described in

2.1. Terminology

   Large Flow: a group of packets that contain the same "identity"
   in their header and come to the network at a high rate, i.e. the
   packet volume per time is high.

   Small Flow: a group of packets that contain the same "identity"
   in their header and come to the network at a low rate, i.e. the
   packet volume per time is very low. Single packet can be
   considered as a special small flow in the context of this

3. Large Flow Recognition

   The high technology now enables router devices to inspect the
   received the packets and identifies the large flows from huge
   amount of packets that belong to many flows (both large and
   small). Large flow recognition process may use protocol
   inspection, flow volume measurement, or other methods to detect
   the large flows. If a router can differentiate packets that
   belong to the high rate flows from all the received packets, it
   can perform differentiated transports for the large flows and
   small flows in PSN as described in section 4.

   It is possible for hosts to insert a large flow indication on the
   packet header. However, there is a huge security concern for a
   network to perform on the customer inserted indication.

   Typically, a large flow has the context for an entire packet
   switched network. It has obvious benefit that if ingress PE
   performs the large flow recognition and inserts a large flow
   indication on the packets, then all the P nodes within PSN can

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   distinguish the large flow packets by checking this indication.
   This can largely reduce the implementation cost and the impact on
   the performance.

   The native service processing function (NSP) [RFC3985] in the
   ingress PE can identify the flow or groups of flows in the
   service, and insert the flow (group) identity of each packet
   before it is passed to the pseudowire forwarder. When ingress PE
   performs large flow recognition, the pseudowire forwarder
   [RFC3985] can perform packet inspection and detect the large flow
   packets.  The design method for the large flow recognition is
   outside the scope of this document. The pseudowire forwarder can
   insert a large flow indication on all the packets that belong to
   the large flow once it is recognized as a large flow. The large
   flow indication encoding schema is described in section 5. Since
   a large flow comes and disappears when it is transported
   completely, the list of large flows could dynamically change.

   Large Flow Recognition has the assumption that a large flow
   sustains for certain time on the network. This assumption applies
   video, streaming, and file download applications. Although
   application rate may vary over the time, its lowest rate value is
   still much high compared to the small flows. Operator can set the
   large flow criteria.

   It is worth to mention that a large flow recognition process may
   or may not need a time to recognize a large flow. If it needs and
   even the time is very short, during this period, some packets
   belonging to a large flow may be treated as small flow packets,
   which may cause the packets for a large flow traversing different
   paths during the transition. Thus this may cause a bit packet
   disordering at a destination. To prevent the packet disordering,
   Large Flow Recognition Process can use some temporary caching
   technology to hold the large flow packets for short time at the
   time the flow is recognized as a new large flow. Another factor
   to consider is that today packet based applications at the end
   points normally have a buffer to deal with packet delay variance
   and loss/disordering, thus the seldom disordering during
   transport is no longer a BIG issue for Internet traffic. Some
   large flow recognition may not need time to detect the large
   flow; it does not generate the ordering issue.

4. Enhanced ECMP Process

   Label switched routers can implement the enhanced ECMP for
   distributing flows over ECMP paths. The enhanced ECMP process
   separates the packets that belong to a large flow from the

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   packets that belong to a small flow and applies different
   treatments on these two types of packets. The process uses
   hashing to select the path from equal cost multi paths for all
   the small flow packets and uses a large flow table to select the
   path for all the large flow packets. Figure 1 illustrates the
   enhanced ECMP processing diagram.

                                    +-------------+    | 4 ECMP Paths
                                    |  Small-Flow |    |
                               +--->|  Forwarding |--->|=========
              +------------+   |    |  Process    |    |
       Packets| Packet     |   |    +-------------+    |=========
       ------>| Separation |---+                       |
              | Process    |---+                       |=========
              +------------+   |    +-------------+    |
                               |    | Large-Flow  |    |=========
                               +--->| Forwarding  |--->|
                                    | Process     |    |
                                    +-------------+    |

                Figure 1 Enhanced ECMP Process Diagram

   Figure 1 depicts three function elements. There are four equal
   cost paths shown as an example. Small-Flow Forwarding Process is
   used for forwarding all the small flow packets, which can be the
   same as existing ECMP process. Packet Separation Process and
   Larger-Flow Forwarding Process are the new elements in the
   enhanced ECMP proposed in this document. The Packet Separation
   Process receives all the transported packets and evaluates all
   the income packets; it uses the first nibble to distinguish
   labeled packets or IP packets. If a labeled packet is marked as a
   large flow, it will be sent to Large-Flow Forwarding Process; if
   not, it will be sent to Small-Flow Forwarding Process. As a
   result, the small flow transport path will be determined by
   hashing method; the large flow transport path will be determined
   by Large-Flow Forwarding Process. Since this draft focuses on the
   labeled packets, IP packet process is described in section 7.5.

   Since the bottom label at the label stack (S bit = 1) can be PW
   label, LSP label, or Flow Label, it is necessary for Packet
   Separation Process to differ Flow Label from PW Label and LSP
   Label. Since Flow Label is never on the top of the label stack
   and is not processed by the forwarder, it has its unique
   position. The draft suggests setting FL TTL to 0. Thus, when S

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   bit =1 and TTL = 0, the label is Flow Label. Otherwise, it is
   either PW Label or LSP label.

   Large-Flow Forwarding Process uses a flow table for packet
   forwarding. The flow table has an entry for each "live" flow.
   When the process receives a packet, it retrieves the flow ID from
   the packet and performs the table lookup by using flow ID. It
   forwards the packets to the path indicated in the table. If the
   process does not find an entry that matches the flow ID on a
   packet, it calls the path selection algorithm. The algorithm can
   select a path for the flow, say A, based on current path load,
   i.e. select the path that has least load at the time. Then the
   process forwards the packet to the selected path and inserts a
   new entry for the flow A in the table. The following packets of
   flow A will be forwarded to the path indicated in the table. When
   a flow is transported completely, the process no longer receives
   the packets that belong to the flow; the age function in the
   process can delete the flow entry from the table, which prevents
   the table size from the unnecessary growth. The age process
   frequency is configurable based on operation needs. If one of
   ECMP paths is down the algorithm will map impacted large flows to
   other ECMP paths. If a new ECMP path is added, the new flows can
   be assigned to the new path; it is optional for the process to
   perform the "live" large flow reassignment since the "live" flows
   may disappear itself anyway. The design method of Large-Flow
   Forwarding Process is outside the scope of this document.

   Note: Large-Flow Forwarding Process can work with any hash-key
   generation scheme. Large-Flow distribution method using few large
   flows effectively compensates the uneven distribution caused by
   hashing and traffic pattern. It is worth to mention the
   differences between DiffDerv and large and small flow
   differentiation. Although both relate to traffic classification,
   they aim the different purposes. DiffDerv is for the network to
   perform differentiated service treatments. Large and small flow
   differentiation is to improve the network utilization in ECMP.

4.1. Congestion Control

   The enhanced ECMP also brings an advance in congestion control.
   The congestion happens when the traffic volume exceeds the path
   capacity. Since the large flows take much more bandwidth,
   dropping few large flows can efficiently rescue the congestion
   condition and keep the rest of services running normally. As a
   result, the congestion control only impacts few services. Large-
   Flow Process can easily select the large flows and block them
   during the congestion. Whether it is worth to cache these blocked

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   flows or not is for further study and outside the scope of this

5. Large Flow Indication

   This draft specifies the protocol to encode a large flow
   indication on the flow label specified in [FAT-PW]. Figure 2
   illustrates current flow label format [RFC3031] with the
   amendment given in [RFC5462]. Label field is filled with the flow
   identity. Since the flow label is never on the top of label
   stack, TTL field is not used. However, to prevent any
   provisioning error, TTL filed is recommended to set as 1. S bit
   is used to indicate the bottom of stack and set to 1 for the flow
   label. 3 Traffic Class bits are not used in current ECMP
   processing now. The document suggests using the first bit in the
   Traffic Class bits to indicate the large flow or small flow, and
   suggests value 1 for the large flow and value 0 for the small
   flow. The two other bits reserve for the future. Figure 3 shows
   proposed format.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                Label                  | TC  |S|       TTL     |

                    Label:  Label Value, 20 bits
                    TC:     Traffic Class, 3 bits;
                    S:      Bottom of Stack, 1 bit
                    TTL:    Time to Live, 8 bits

                  Figure 2 Current Flow Label Formant

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                Label                  |F| RV|S|       TTL     |

                    Label:  Label Value, 20 bits
                    F:      Flow Characteristics Indication, 1 bit;

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                    RV:     Reserved Bit, set to 0
                    S:      Bottom of Stack, 1 bit
                    TTL:    Time to Live, 8 bits

         Figure 3 Flow Label Format with Large-Flow Indication

   When Flow Label is used on a PW, ingress PE can insert the flow
   label and a large flow indication on each packet; egress PE will
   trim off the flow label before sending the packets to the right
   AC. The procedure for informing flow label presence and label
   insertion procedure remains the same as [FAT-PW].

   TC bits in MPLS Label are typically used for DiffServ purpose.
   [RFC3270] [RFC5129] Since Flow Label is never used in the top of
   Label Stack, DiffServ function does not apply to the Flow Label.
   Hence the proposed Flow Label TC bit usage in this document does
   not impact DiffServ function.

6. Backward Compatibility

   The enhanced ECMP fully support backward compatibility in PSN. If
   ingress PE does not support Large Flow Recognition, it SHALL set
   flow label F bit to 0. Then all the flows are treated as small
   flows in PSN. P routers with existing ECMP or enhanced ECMP
   capability use hashing to discriminate the flows and distribute
   those flows over ECMP paths. If ingress PE supports Large Flow
   Recognition, it will insert the indication on the flow label. The
   P routers with existing ECMP capability will ignore the
   indication and just perform hashing on all the flows. The P
   routers with enhanced ECMP capability will separate the large and
   small flows and perform different treatments as proposed in this
   document. Although P router with existing ECMP capability gets
   uneven load balancing over its ECMP paths, it maintains the same
   performance as today's network. If ingress PE does not support
   the flow label on PW, when enhanced ECMP applies, it will treat
   PW packets or LSP packets as small flow packets.

7. Applicability

   Carriers have desires to improve transport network capability via
   certain service awareness in packet transport and not be
   constrained in just "pipe" transport service.[FAT-PW]brings such
   potential by introducing the flow label in the label stack, which
   enables ECMP transport discriminates traffic at flow granularity.
   The large flow aware transport further enables ECMP transport to
   distinguish the large and small flows and perform different

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   treatments on two types of flows, which can improve the load
   balancing when traffic pattern contains very small percentage of
   large flows.

   The method described in this document requires the new capability
   from the PSN and applies to packet switched routers. It requires
   ingress PE to perform the large flow recognition and inserts a
   large flow indication on the flow label; and P or PE routers
   perform the enhanced ECMP function. Since each router node
   performs ECMP function independently, a packet switched network
   can work well even when some nodes support the enhanced ECMP
   capability and some do not. This allows operator to gradually
   upgrade the network.

7.1. Link Aggregation Groups

   A Link Aggregation Group (LAG) is used to bond together several
   physical circuits between two adjacent nodes so they appear to
   higher-layer protocols as a single, higher bandwidth "virtual"
   pipe. These may co-exist in various parts of a given network. The
   enhanced ECMP proposed in this document can assist in producing a
   more uniform flow distribution and controlling the congestion in

7.2. The Single Large Flow Case

   [FAT-PW] has suggested several options for the single large flow
   in a PW. With the enhanced ECMP capability, it has beneficial to
   insert a flow label even for a single large flow. Then ingress PE
   can insert a large flow indication. P routers in PSN can treat it
   as a large flow.

7.3. Flow Rate Difference

   The enhanced ECMP method uses the different treatments between
   large flows and small flows. Neither of treatments considers the
   flow rate in the distribution process. This is because that even
   load balance is achieved by hashing on the small flows and
   selecting the least used the path for a new "live" flow. The
   latter distribution using few large flows effectively compensates
   the uneven balance caused by the former and is unnecessary to
   consider individual flow rate. This is nice that enhanced ECMP
   keeps the nature of statistical balancing. Therefore, the
   enhanced ECMP method works well even flow rates are broad.

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7.4. Multi-Segment Pseudowires

   The flow label mechanism described in this document works on
   multi-segment PWs [MS-PW] without requiring modification to the
   Switched PEs (S-PEs). This is because the flow label is
   transparent to the label swap operation. There is no need to
   perform Large Flow Recognition at Switched PEs.

7.5. IP Flows

   Today's ECMP method applies to both IP flows and MPLS labeled
   flows in PSN. Typically, Hash method uses IP source and
   destination address pair plus other elements to discriminate IP
   flows and distribute them over ECMP paths. If PE can insert a
   large flow indication in the packets of IP flows, the proposed
   method can apply to IP flows as well. IPv6 protocol [RFC2460]
   already has the flow label field. 3-tuple: source, destination,
   and flow label is used in ECMP.[RFC3697] The method proposed here
   meets the restriction on the flow label.[FLOW-ECMP] IETF just
   needs to specify one bit in TC field (8 bits) to indicate small
   and large flow. By default, all TC bits are set as 0 [RFC2460],
   which is compatible to this solution. Although IPv4 protocol does
   not have such flow label, IETF can decide if it is necessary to
   improve IPv4 protocol to have the large flow indication or just
   wait the time for IPv6 to take over. The IP large flow
   recognition and indication is outside the scope of this document.

   The Packet Separation Process in the enhanced ECMP uses the first
   nibble to differentiate IP flows and non IP flows before
   evaluating the large flow indication. When PSN does not support
   large and small IP flow distinction, the enhanced ECMP treats all
   IP flows as small flows.

7.6. LSP with Entropy Label

   Entropy Label [Entropy] is inserted in LSP traffic at ingress LSR
   to gain better ECMP load balancing at transit LSRs. Entropy label
   is very similar as PW flow label and is used to differentiate
   "microflow" within a LSP so ECMP process can get better
   dispersion granularity. Enhanced ECMP and Large Flow Aware
   Transport can apply to LSP with entropy label. Traffic class
   field in the Entropy can use the same encoding scheme described
   in this document. If ingress LSR does not support large flow
   recognition, then it SHOULD set Large Flow indication bit to 0.

   The same approach applies to Application Label [RFC4928] as well.

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8. Security Considerations

   Since the number of large flows is very small compared to the
   number of small flows; packet switched routers only need to
   maintain a small size of table or flow states. Notes operator can
   use the large flow criteria to control the large flow volume. The
   method won't create the scalability and performance issue.

9. IANA Considerations

   IANA is for the further study.

10. References

10.1. Normative References

   [RFC2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1995.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3031] Rosen, E., Viswanathan, A., and Callon, R., "Multiple
             protocol Label Switching Architecture", RFC3031, January
   [RFC3270] Faucheur F. Le, etc, "Multi-Protocol Label Switching
             (MPLS) Support of Differentiated Services", RFC3270, May

   [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
             "IPv6 Flow Label Specification", RFC 3697, March 2004.

   [RFC3985] Bryant, S., Pate P., "Multiprotocol Label Switching
             (MPLS) Label Stack Entry: "EXP" Field Renamed to
             "Traffic Class" Field", RFC3985, March 2005

   [RFC4928] Swallow, G., Bryant, S., and Andersson, L., "Avoiding
             Equal Cost Multipath Treatment in MPLS Network",
             RFC4928, June 2007.
   [RFC5129] Davie, B., Briscoe, B., and Jay, J., "Explicit
             Congestion Marking in MPLS", RFC5129, January 2008

   [RFC5462] Andersson, L. etc, "Multiprotocol Label Switching
             (MPLS) Label Stack Entry: EXP Field Renamed to Traffic
             Class Field", October 2009

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10.2. Informative References

   [FAT-PW] Bryant, S., Drafz, U Kompella, V., etc, "Flow Aware
            Transport of Pseudowires over an MPLS PSN", draft-ietf-
            pwe3-fat-pw-03, (work in progress), Jan. 2010

   [Entropy] Kompella K, Amante S., "The use Entropy Labels in MPLS
            Forwarding", draft-kompella-mpls-entropy-label-01,
            January 2009

   [MS-PW] Bocci, M. Bryant, S., "An Architecture fro Multi-Segment
             Pseudowire Emulation Edge-to-Edge", RFC5659 October
   [FLOW-ECMP] Carpenter B., "Using the IPv6 flow label for equal
             cost multipath routing in tunnels", draft-carpenter-
             flow-ecmp-01, February 18, 2010

   [CAIDA]  Caida Internet Traffic Analysis,

11. Acknowledgments

   Authors like to thank Stewart Bryan, Frederic Jounay, Simon
   Delord, Raymond Key for the review and suggestions.

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Appendix A.                 Simulation Analysis

   We create Internet Traffic Generator based on observed Internet
   Traffic pattern. The generator randomly generates 98% of small
   traffic flows and 2% of large traffic flows up to 10G traffic.
   The traffic volume for the large flows and small flows are 30%
   and 70%.  Simulator uses hash based distribution to disperse the
   traffic over 4 paths and 10 paths, respectively; and also uses
   enhanced ECMP method to disperse the traffic over 4 paths and 10
   paths. The results show the performance between ECMP and enhanced
   ECMP from 6 simulations. Enhanced ECMP gets <1% load differences
   among paths while ECMP have up to 15% load differences. It shows
   how the simple distribution on few large flows can effectively
   compensate the uneven load balance caused by hashing and the
   traffic pattern.

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Authors' Addresses

   Lucy Yong
   Huawei Technologies Co., Ltd.
   1700 Alma Dr.
   Plano, TX 75075

   Phone: +14692295387
   Email: lucyyong@huawei.com

   Peilin Yang
   Huawei Technologies Co., Ltd.
   No.91, Baixia Road, Nanjing 210001
   P. R. China

   Phone: +86-25-84565881
   EMail: yangpeilin@huawei.com

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