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SFC working group                                                H. Song
Internet-Draft                                                    J. You
Intended status: Informational                                   L. Yong
Expires: March 10, 2017                                         Y. Jiang
                                                               L. Dunbar
                                                                  Huawei
                                                             N. Bouthors
                                                                  Qosmos
                                                               D. Dolson
                                                                Sandvine
                                                       September 6, 2016


                    SFC Header Mapping for Legacy SF
                  draft-song-sfc-legacy-sf-mapping-08

Abstract

   A Service Function Chain (SFC) defines a set of abstract Service
   Functions (SF) and ordering constraints that must be applied to
   packets and/or frames selected as a result of classification.  One
   assumption of this document is that legacy service functions can
   participate in service function chains without supporting the SFC
   header, or even being aware of it.  This document provides some of
   the mechanisms between an SFC proxy and an SFC-unaware service
   function (herein termed "legacy SF"), to identify the SFC header
   associated with a packet that is returned from a legacy SF, without
   an SFC header being explicitly carried in the wired protocol between
   SFC proxy and legacy SF.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 10, 2017.





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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Mechanisms  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  For Transparent Service Functions . . . . . . . . . . . .   5
       3.1.1.  VLAN  . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  VXLAN . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.3.  Ethernet MAC Address  . . . . . . . . . . . . . . . .   6
       3.1.4.  5-tuple . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  For Non-transparent Service Functions . . . . . . . . . .   7
   4.  Operation Considerations  . . . . . . . . . . . . . . . . . .   8
     4.1.  Examplar Mechanisms . . . . . . . . . . . . . . . . . . .   8
     4.2.  Challenges to Support Legacy SF . . . . . . . . . . . . .   8
     4.3.  Metadata  . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A Service Function Chain (SFC) [RFC7665] defines a set of abstract
   service functions and ordering constraints that must be applied to
   packets and/or frames selected as a result of classification.  One
   assumption of this document is that some service functions may remain
   as legacy implementations, i.e. SFC-unaware SFs.  The SFC proxy is
   proposed to act as a gateway between the SFC encapsulation and SFC-
   unaware SFs.  The SFC proxy removes the SFC header and then sends the
   packet to a legacy SF for processing, but how to associate the




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   original SFC header with the packet returned from the legacy SF needs
   to be considered.

   This document describes some of the mechanisms between an SFC proxy
   and a legacy SF, to identify the SFC header associated with a packet
   that is returned from a legacy SF.  The benefit for supporting legacy
   SF is that SFC-unaware SFs can exist in the SFC-enabled domain.  An
   SFC proxy allows a legacy SF to function in the SFC-enabled domain
   without modification of the legacy SF.

                   +----------------+
                   |SFC-unaware     |
                   |Service Function|
                   | (Legacy SF)    |
                   +----+----+------+
                        ^    |
                        |    |
                   +----+----+------+
                   |Switch(optional)|
                   +----+----+------+
                        |    |
                     (2)|    |(3)
                        |    |
                   +----+----V--------+
             (1)   |      SFC         | (4)
          -------->|      Proxy       +------->
                   +------------------+

         Figure 1: Procedure of a packet processed by a legacy SF

   Different classes of legacy SF may have variable support for
   different types of packets with respect to parsing and semantics
   (e.g., some classes of legacy SF may accept VLAN-tagged traffic;
   others may not), usually depending on device configuration.  For
   example, by creation of VLANs, traffic is steered through a firewall.

   This document focuses heavily on legacy SFs that are transparent at
   layer 2.  In particular we assume the following conditions apply in
   the class of legacy SF we are considering proxying:

      1.  Traffic is forwarded between pairs of interfaces, such that
      packets received on the "left" are forwarded on the "right" and
      vice versa.

      2.  A packet is forwarded between interfaces without modifying the
      layer 2 header; i.e., neither source MAC nor destination MAC is
      modified.




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      3.  When supported, VLAN-tagged or Q-in-Q packets are forwarded
      with the original VLAN tag(s) intact (S-tags and C-tags).

      4.  Traffic may be discarded by some functions (e.g., by a
      firewall).

      5.  Traffic may be injected in either direction by some functions
      (e.g., extra data coming from a cache, or simply TCP
      retransmissions).  We assume injected traffic relates to a layer 3
      or layer 4 flow, and the SF clones layer 2 headers from exemplar
      packets of the same flow.

      6.  Traffic may be modified by some functions at layer 3 (e.g.,
      DSCP marking) or higher layers (e.g., HTTP header enrichment or
      anonymization).  Note that modification can be considered a
      special case of discarding followed by injection.

      7.  Traffic may be reordered by some functions (e.g., due to
      queuing/scheduling).

   We leave the legacy SFs which modify the original layer 2 packet
   headers as an open issue for further study.

   To support this class of legacy SF, if the payload in the SFC
   encapsulation is layer 3 traffic, the SFC proxy will extract the
   layer 3 payload from SFC encapsulation and prepend a new layer 2
   header before sending the packet to the SF.  However if the payload
   in the SFC encapsulation is layer 2 traffic, the SFC proxy may
   extract the layer 2 packet from SFC encapsulation, modify the
   original source MAC address and use the new source MAC address for
   mapping to the stored SFC and layer 2 headers when the packets are
   returned to the SFC proxy.  This will not impact the SF processing.
   The SF will send the traffic back after processing.

   As shown in Figure 1, there are four steps.  The SFC proxy receives a
   packet (1) from an SFF, and removes its SFC header, which may
   optionally contain metadata, and store the SFC header locally, and
   then (2) sends the de-encapsulated packet to the SF.  After the SF
   processes the packet, the packet will be sent back (3) to the SFC
   proxy.  The SFC proxy retrieves the pre-stored SFC header
   accordingly, determines the SFC header for the next stage of the path
   and encapsulates the packet with the next SFC header, returning the
   packet to an SFF (4).








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2.  Terminology

   The terminology used in this document is defined below:

      Legacy SF: A conventional service function that does not support
      SFC header, i.e., SFC-unaware SF.

      Transparent SF: A service function that does not change any bit of
      the layer 2/3/4 packet header sent to it, but it may drop the
      packet.

      Non-transparent SF: A service function that changes some bits of
      the layer 2/3/4 packet header sent to it.

      SFC Proxy: Removes and inserts SFC encapsulation on behalf of an
      SFC-unaware service function.  SFC proxies are logical elements.

3.  Mechanisms

   The mapping mechanisms between the SFC proxy and the transparent or
   non-transparent legacy SFs are discussed in this section.  The
   mechanisms used in this document require that each forwarding entity
   (i.e., SFC proxy) and its connected service functions are in the same
   layer 2 network.  The detailed definitions of SFC proxy and SFC-
   unaware SFs is discussed in [RFC7665].

3.1.  For Transparent Service Functions

3.1.1.  VLAN

   If the service function is transparent to packet headers, for
   example, layer-2-transparent SF, then VLAN can be used for mapping
   between the SFC proxy and SF.  It is assumed that the switch between
   the SFC proxy and SF delivers traffic for all VLANs, or the SFC proxy
   and SF may be directly connected.

   The SFC proxy removes the SFC header and sends the packet to the SF,
   with encapsulating a certain VLAN ID that can represent the SFC
   header.  The legacy SF is supposed to accept VLAN-tagged packets and
   send them back on the same VLAN.  It is assumed that the SF is able
   to process Ethernet packets with VLAN tags and also accept a wide
   range of VLAN tags.  The SFC proxy locally maintains the mapping
   between VLAN ID/direction and the SFC header.

   When receiving the returned packet from the SF, the SFC proxy removes
   the VLAN part from the packet and retrieves the corresponding SFC
   header according to the VLAN ID and the direction of packet travel,
   and then encapsulates SFC header into that packet before sending to



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   the next service function.  Packet direction is required because the
   SFC header for left-to-right packets is different than the SFC header
   for right-to-left packets.

3.1.2.  VXLAN

   If the SFC proxy and SF are already deployed in a nested VLAN
   network, the VLAN mapping method is not applicable.  Then VXLAN
   [RFC7348] can be used for the mapping, i.e. VNI can be used for the
   mapping between them.  VXLAN is a Layer 2 overlay scheme over a Layer
   3 network.  It uses MAC Address-in-User Datagram Protocol (MAC-in-
   UDP) encapsulation.  The drawback of this mechanism is that it
   requires both SFC proxy and SF to support VXLAN.

   This approach has similar features and drawbacks of the VLAN scheme,
   but the number of possible VNIs is larger.

3.1.3.  Ethernet MAC Address

   The MAC address also can be used to associate an SFC header between
   the SFC proxy and SF; i.e., each SFC header will be assigned a source
   MAC address on the SFC proxy.  When the SFC proxy receives the
   returned packet from the SF, it retrieves the packet's original SFC
   header by using the source MAC address as a key.  And then it
   encapsulates the packet with that SFC header and sends to the next
   hop.

   An issue with the source-MAC address approach is that there is not
   symmetry between packets going left-to-right with packets going
   right-to-left.  Such symmetry might be assumed by some legacy SFs.
   For example, if a layer-2-transparent SF responds to a TCP SYN with a
   TCP RST, it might do so by reversing the source and destination of
   the layer 2 header.  Such a packet received by the SFC proxy would
   not result in finding of the correct SFC header.  It is assumed that
   the SF passes the MAC header through without even reversal.  A
   variation that is symmetric assigns a unique source/destination pair
   for each unique SFC header.

3.1.4.  5-tuple

   The 5-tuple of a packet carried within SFC encapsulation can be used
   by the SFC proxy as a key to associate an SFC header when the 5-tuple
   is not modified by the legacy SF.  The SFC proxy maintains a mapping
   table for the 5-tuple and the SFC header.  When the packet returns
   from the SF instance, the original SFC header for this packet can be
   retrieved by inquiring the mapping table using 5-tuple as the key.
   However, this method may not work in multi-tenant scenario, as such
   uniqueness could be valid only within the scope of a single tenant.



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   So if the SFC is provided as a multi-tenant service, this method
   would fail.

3.2.  For Non-transparent Service Functions

   Non transparent service functions including NAT (Network Address
   Translation), WOC (WAN Optimization Controller) and etc, are more
   complicated, as they may change any part of the original packet sent
   to them.  It is better to analyze case by case, to utilize a specific
   field that the SF does not change for the mapping and retrieving the
   SFC header.  We would like to leave it for open discussion.

   The Figure below shows an example procedure that SFC proxy can learn
   the behavior of the SF changing the packet.  In this example, the
   following method is used for SFC header mapping.  The SF needs to
   report its mapping rules (e.g., 5-tuple mapping rules) to the control
   plane (e.g., by static configuration), and then the control plane can
   notify the SFC proxy the mapping information (step 1) via interface
   C4 [I-D.ietf-sfc-control-plane].  According to the mapping
   information, the SFC proxy can establish a mapping table for the SFC
   header, the original header, and the processed header of the packet.
   After receiving the packet from the SF (step 4), the SFC proxy
   retrieves the SFC header from the mapping table by using the
   processed header as a key.

                          +------------------------+
                          |                        |
                          |   SFC Control Plane    |
                          |                        |
                          +---^------------^-------+
                              |            |
                              |C2          |  (1)
                              |            |
                          +---V----+       |C4
                     -----+  SFF   +------ |
                          |        |       |
                          +----+---+       |
                               |       +---V----+
                               +-------+  SFC   |
                                 (2)   | Proxy  |
                                       +---^----+
                                           |
                                        (3)|(4)
                                    +------V---------+
                                    |SFC-unaware     |
                                    |Service Function|
                                    +----------------+




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4.  Operation Considerations

4.1.  Examplar Mechanisms

   The following table gives some examplar methods and the conditions to
   use.

                     Table 1: Mapping Examples

+-----------+--------+-------------------------------+-------------------+
|           |Methods |    Stored Key-Value           |Application        |
|           |        |                               |Scenario           |
+-----------+--------+-------------------------------+-------------------+
|           |VLAN    | (Direction, VLAN ID,          |L2 header won't    |
| For Trans-|        |  SFC header)                  |be modified by the |
| parent SF |        |e.g., assign a VLAN ID per     |SF.                |
|           |        |bidirectional path-pair        |                   |
|           +--------+-------------------------------+-------------------+
|           |VXLAN   | (Direction, VNI, SFC header)  |The SF is required |
|           |        |e.g., assign a VNI per         |to support VXLAN.  |
|           |        |bidirectional path-pair        |VNI is not modified|
|           |        |                               |by the SF.         |
|           +--------+-------------------------------+-------------------+
|           |5-tuple |(5-tuple, SFC header)          |5-tuple is not     |
|           |        |                               |modified by the    |
|           |        |The SFC proxy maintains the    |SF.                |
|           |        |mapping table for 5-tuple and  |                   |
|           |        |the SFC header.                |                   |
|           |        |Note: an SFC header for each   |                   |
|           |        |direction of a TCP flow.       |                   |
+-----------+--------+----------------- -------------+-------------------+
|           |Case-by-|Mapping rules:                 |The SFC proxy is   |
|For        |case    |e.g. 5-tuple -> 5-tuple'       |configured or is   |
|Non-trans- |        |                               |able to obtain the |
|parent SF  |        |SFC Proxy:                     |mapping rules of   |
|           |        |5-tuple -> 5-tuple'            |the SF. The SF     |
|           |        |5-tuple'-> SFC header          |modifies the       |
|           |        |                               |5-tuple based on   |
|           |        |                               |the mapping        |
|           |        |                               |rules.             |
+-----------+--------+---------------------------------------------------+

4.2.  Challenges to Support Legacy SF

   The key problem contemplated in this document is: what packet header
   should be put on the packets sent to a legacy SF such that packets
   returned from the legacy SF can be mapped to the original SFC header.
   We need to consider the relationship between an SFC path and flows



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   within the path.  Should the path act as a qualifier to the flow, or
   should a flow be allowed to change paths?  We assume flows can change
   path; this means that a given legacy SF cannot handle traffic from
   more than one routing domain.  (Private IP addresses cannot be
   qualified by the SFC header; different VPNs must use different legacy
   SFs.)

   Because we've assumed that a flow can be on multiple paths, or change
   paths, or if metadata can vary during the life of a flow, we need to
   ask to what extent packet accuracy matters.  If the SFC header used
   with a flow is changed from one path to another by the classifier,
   does it matter if packets retain exactly the original SFC header?  If
   the change is to handle routing updates or fail-over then it would be
   acceptable to put all packets returning from the legacy SF onto the
   most recently updated header.  If metadata is changed, can that
   update be applied to all packets of a flow, or does it apply to a
   specific packet?

   In the case that changes to paths and metadata are considered updates
   to the flow vs. packet properties, the SFC proxy can find the SFC
   header based on flow (e.g., the 5-tuple of the returning IP packet).
   If, in contrast, packet accuracy of SFC headers does matter, (e.g.,
   the metadata says something about the specific packet associated with
   it), then some form of per-packet bookkeeping must be done by the SFC
   proxy and the 5-tuple cannot be used for the mapping to retrieve the
   original SFC header.

   When packet accuracy does matter, packets injected by the legacy SF
   pose a fundamental problem.  Is there any correct SFC header that can
   be added?  Observation: the same problem exists for a normal (not
   legacy) SF that wishes to modify or inject a packet.

   Because the SFC proxy needs to keep dynamic state by storing packet
   headers, an expiration time should be used for each mapping entry in
   the SFC proxy.  If the SFC header in that entry has not been
   witnessed or retrieved after the expiration time, the entry will be
   deleted from the entry table.

   Observation: if metadata is not used, the number distinct SFC headers
   is known at configuration time, equivalent to the number of paths
   configured to pass through the SF.  The mappings between SFC headers
   and layer 2 encodings could be configured at this time vs. at run
   time.  However, if metadata is used, a combinatorial explosion of
   distinct SFC headers may result, which is a problem for any device
   attempting to store them for later retrieval.






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4.3.  Metadata

   Some classes of SF may need to inject new packets, for example a
   transparent cache sending content from its disk.  The legacy SF
   usually encapsulates the new packets with the same encapsulation with
   the related received packets, e.g. with the same 5-tuple, or V-LAN
   ID.  The SFC proxy would associate the new packet with the
   corresponding SFC header based on the mechanisms discussed in
   Section 3.  However, per-packet metadata should be prohibited for
   this case.

   Some classes of SF may need to inject a packet in the opposite
   direction of a received packet, for example a firewall responding to
   a TCP SYN with a RST.  If the RST generator is VLAN-type legacy, it
   may know what VLAN to use; then the SFC proxy would translate VLAN
   into a reverse SFP and attach a corresponding SFC header insetad of
   the original SFC header.  In this case, the SFC proxy should be
   configured with the bidirectional SFP, i.e. SFC proxy needs to be
   designed according to the properties of the SF.  Similarly, packet-
   specific metadata is not recommended to be used.

   We leave the metadata model as an open issue that will be documented
   in other documents.  In some cases this information will also assist
   normal (non-legacy) SFs that wish to modify or inject packets.

5.  Security Considerations

   When the layer 2 header of the original packet is modified and sent
   to the SF, if the SF needs to make use of the layer 2 header, it may
   cause security threats.  There may be security issues with state
   exhaustion on the SFC proxy, e.g., exhausting VLAN IDs, or exhausting
   5-tuple state memory.

6.  Acknowledgement

   The authors would like to thank Ron Parker and Joel Halpern for their
   valuable comments.

7.  References

7.1.  Normative References

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <http://www.rfc-editor.org/info/rfc7348>.



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   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <http://www.rfc-editor.org/info/rfc7665>.

7.2.  Informative References

   [I-D.ietf-sfc-control-plane]
              Boucadair, M., "Service Function Chaining (SFC) Control
              Plane Components & Requirements", draft-ietf-sfc-control-
              plane-07 (work in progress), August 2016.

Authors' Addresses

   Haibin Song
   Huawei
   101 Software Avenue, Yuhuatai District
   Nanjing, Jiangsu  210012
   China

   Email: haibin.song@huawei.com


   Jianjie You
   Huawei
   101 Software Avenue, Yuhuatai District
   Nanjing,  210012
   China

   Email: youjianjie@huawei.com


   Lucy Yong
   Huawei
   5340 Legacy Drive
   Plano, TX  75025
   U.S.A.

   Email: lucy.yong@huawei.com


   Yuanlong Jiang
   Huawei
   Bantian, Longgang district
   Shenzhen  518129
   China

   Email: jiangyuanlong@huawei.com



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   Linda Dunbar
   Huawei
   1700 Alma Drive, Suite 500
   Plano, TX  75075
   U.S.A.

   Email: ldunbar@huawei.com


   Nicolas Bouthors
   Qosmos

   Email: nicolas.bouthors@qosmos.com


   David Dolson
   Sandvine
   408 Albert Street
   Waterloo, ON N2L 3V3
   Canada

   Email: ddolson@sandvine.com





























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