Ethernet MAC Chaining
draft-fedyk-sfc-mac-chain-00

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Authors Paul Bottorff  , Don Fedyk 
Last updated 2015-07-20
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Network Working Group                                       P. Bottorff
Internet Draft                                                 D. Fedyk
Intended status: Informational                            HP Networking
Expires: January 2016                                     July 20, 2015

                           Ethernet MAC Chaining
                     draft-fedyk-sfc-mac-chain-00.txt

Abstract

   This document introduces and describes a simple and highly scalable
   service function chaining mechanism called MAC chaining which is
   built largely on existing Ethernet frame and forwarding capabilities.
   MAC chaining uses IEEE 802 Media Access Control (MAC) addresses to
   provide flexible and complete service function chains.  It is largely
   transparent to layers above Ethernet and designed to augment and
   coexist with existing virtual and physical network forwarding. MAC
   chaining is achievable in some devices and virtual switches today
   using existing protocols.

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
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   This Internet-Draft will expire on January 14, 2016.

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Copyright Notice

   Copyright (c) 2015 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. Conventions used in this document..............................4
   3. Terminology....................................................4
   4. MAC Chaining...................................................5
      4.1. MAC Chaining Packet and Address Formats...................6
      4.2. Forwarding................................................9
         4.2.1. Forwarding by Service Functions.....................11
         4.2.2. Proxy Forwarders....................................12
         4.2.3. Example MAC Chaining Walk Through...................13
         4.2.4. Destination Address MAC Chaining Operation..........14
         4.2.5. Destination and Source Address MAC Chaining.........15
         4.2.6. Forwarding by Chain Termination Functions...........15
   5. Programming a Service Chain...................................16
   6. Domain of operation...........................................16
   7. Security Considerations.......................................16
   8. IANA Considerations...........................................17
   9. References....................................................17
      9.1. Normative References.....................................17
      9.2. Informative References...................................17
   10. Acknowledgments..............................................17

1. Introduction

   Service Function Chaining (SFC) enables the creation of composite
   (network) services that consist of a directed graph of Service
   Functions (SF) which must be applied to packets selected as a result
   of classification. SFC is described in detail in the SFC architecture
   document [I-D.ietf-sfc-architecture], and is not repeated here.

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   This document describes a new highly scalable, low resource, service
   function chain (SFC) mechanism called MAC chaining that is based on
   the current IEEE 802 Ethernet header for physical and virtualized
   environments. Service function chaining is an active area in the
   standards and various proposals for how to do SFCs are being put
   forward. The basic mechanism used for MAC chaining is the use of MAC
   addresses in the Ethernet header as a mechanism both for identifying
   chains and for forwarding packets along a MAC chain. The forwarding
   mechanism used in MAC chaining is independent from virtual or overlay
   networks used to form subnets. MAC chaining addresses are terminated
   at each Service Function Forwarder (SFF) and replaced by a new set of
   MAC chaining addresses used to forward through the next Service
   Function in the chain.

             +-----------------------------------------+
            /            E2E Network                  /
           /                                         /
          +-----------------------------------------+
                   +----------------------------------+
      O     /|    /     MAC Function Chaining        /
      R    / |   /                                  /
      C   /  |  +----------------------------------+
      H  /   |     +----------------------------------+
      E |    |    /       Virtual Networks           /
      S |    |   /        Overlay/Underlay          /
      T |    |  +----------------------------------+
      R |    |     +---------------+  +---------------+
      A |    |    /   Physical    /  /    Physical   /
      T |    |   /   Networks    /  /    Networks   /
      I |    |  +---------------+  +---------------+
      O |   /
      N |  /
        | /
        |/

                     Figure 1 Service Forwarding Plane

   MAC function chaining can be viewed as a network service plane as
   shown in Figure 1. The SFC architecture document [I-D.ietf-sfc-
   architecture] describes chain forwarding in terms of 3 main
   architecture components which are the Service Classification Function
   (SCF), Service Function Forwarder (SFF) and the Service Function
   (SF). When managed with MAC chaining, Service Functions (SF) are
   simple links in the service chain and require little context of the

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   overall chain.  MAC chaining Service Function Forwarders (SFF) enable
   the chain and control the path to and from the SFs. Logically the SFF
   forms a switching layer above the existing virtual networking layers.
   For the description of MAC chaining Chain Termination Function (CTF)
   is described which is responsible for de-encapsulating the packet and
   sending it toward the final destination as a separate architectural
   component from the SFF and the SFC.

2. Conventions used in this document

   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 RFC-2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

3. Terminology

   Chain Termination Function (CTF): See [I-D.ietf-sfc-architecture].
                The chain termination function terminates a Service
                Function Path performing any de-encapsulation and
                operations required to continue forwarding to the final
                destination. The CTF may also be the final destination
                of the chain.

   CS-MAC:      A MAC address which identifies a MAC Chain Segment

   CS-MAC Authority:  CS-MAC Authority refers to the mechanism to ensure
                CS-MACs are unique but allows the optional reuse of
                MACs in different VNs. Each VN port has a single CS-MAC
                Authority. Multiple ports may share the same Authority.
                A MAC Chain may be under a single CS-MAC Authority or
                it may be split among multiple CS-MAC Authorities.

   DA:          MAC destination address

   I/G          The Individual/Group (I/G) address bit (LSB of octet
                0).

   MAC Address: IEEE 802 Media Access Control Address a 48 bit address.

   MAC Chain Segment (CS): A hop between either Service Forwarding
                Functions, a Service Classification Function and a
                Service Forwarding Function, or a Service Forwarding
                Function and a Chain Termination Function

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   VN Port:     In this document a port is the logical interface
                context for a MAC address in a virtual network (VN). A
                VN port may be implemented on any type of physical port
                or logical supporting Ethernet.

   SA:          MAC source address

   Service Function (SF):  See [I-D.ietf-sfc-architecture].

   Service Classification Function (SCF):
                See [I-D.ietf-sfc-architecture].

   Service Function Chain (SFC): See [I-D.ietf-sfc-architecture].

   Service Function Forwarder (SFF): See [I-D.ietf-sfc-architecture].

   Service Function Path (SFP): See [I-D.ietf-sfc-architecture].

   U/L:         The Universally or Locally administered (U/L) address
                bit is the bit of octet 0 adjacent to the I/G bit.

   Virtual Network (VN):    A Virtual network is used to identify a
                network segment controlled by a CS-MAC Authority.

4. MAC Chaining

   MAC chaining uses controlled assignment of Ethernet 48 bit MAC
   addresses to form the chain. Ethernet MAC addresses are selected to
   uniquely identify both the chain and the particular chain segment (or
   hop) within the identified chain. These assigned Ethernet addresses
   are called Chain Segment MAC (CS-MAC) addresses in this document.
   These CS-MACs allow MAC chaining to be implemented on existing
   Ethernet infrastructure making it broadly interoperable with the
   majority of installed base including existing Ethernet, Carrier
   Ethernet and IP equipment.

   Each MAC chain is composed of a series of Chain Segments (CS) which
   are hops between Classifiers, Service Function Forwarders and Chain
   Terminating Functions (see figure 2). Some of the chain segments
   include Service Functions while others perform forwarding between the
   SCF, SFF and CTF switches. For each chain segment, a MAC address is
   selected, from a locally administered MAC address space, to uniquely
   identify the chain segment within the SFC domain. MAC chaining uses
   these CS-MACs, in the Ethernet header, as an identifier to enable
   forwarding packets in a MAC chain.

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   +-----+  +-----+ +------+ +-----+ +-----+ +------+ +-----| +-----+
   |     |  |     |  \ SF2/  |     | |     |  \ SF4/  |     | |     |
   | SCF +--+ SFF1+---+B +---+ SFF1+-+ SFF2+---+D +---+ SFF2+-+ CTF |
   |    X| 1|A   C|2   \/   2|C    | |    E|3   \/   4|E   Y| |F    |
   +-----+  +-----+          +-----+ +-----+          +-----+ +-----+
       \...../   \............/   \.../   \............/   \..../
        CS 1          CS 2         CS 5        CS 3         CS 4
      CS-MAC=A       CS-MAC=C    CS-MAC=G    CS-MAC=E     CS-MAC=F

                  Figure 2 MAC Chain Segments Addressing

   In Figure 2 five chain segments are illustrated. The first chain
   segment is between the classifier and service function forwarder
   identified as SFF1. This chain segment, designated CS1, has been
   assigned CS-MAC A. (For brevity the 48 bit MAC addresses are
   identified by letters). The next chain segment is from SFF1 VN port 2
   through service function 2 and back to SFF1 VN port 2. This chain
   segment designated CS2 has been assigned CS-MAC C. SF2 on CS2 is a
   single armed SF with MAC address B attached to SFF VN port 2. (see
   section 4.2.1 for a description of the types of armed SF). Chain
   segment CS5 is between service function forwarder 1 and service
   function forwarder 2. It has an assigned CS-MAC G. Chain segment CS3
   from SFF2 VN port 3 to SFF2 VN port 4 is identified by CS-MAC E. SF4,
   lying on CS3, is a dual armed SF with MAC address D on the side
   connecting to SFF2 VN port 3. The final chain segment, CS4, of the
   path is between SFF2 and the CTF and is identified by CS-MAC F.

   MAC chaining operates in the context of Virtual Networks (VN). To
   fully describe each MAC chaining address the tuple (CS-MAC Authority,
   CS-MAC) is used which uniquely identifies each chain segment as well
   as the entire chain. Each chain segment and each VN MUST belong to a
   single CS-MAC Authority which assigns unique CS-MACs for that segment
   and VN.  If a chain segment crosses between two independent VNs, then
   both the VNs must have the same CS-MAC Authority.

4.1. MAC Chaining Packet and Address Formats

   The IEEE 802.3 frame header consists of a Destination MAC Address (6
   bytes DA) followed by a Source MAC Address (6 Bytes SA) followed by a
   number of possible fields: Ether type (2 Bytes) or other TAGs and
   Ether types. A VLAN tag (4 bytes) is a common TAG that also carries
   Priority Code Points and Discard Eligible Information for traffic
   class.   For the purpose of this document the DA and SA are the
   primary fields used for MAC chaining however the frame may optionally
   have VLAN Tags. The MAC chaining frame can also be carried inside

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   (i.e. as an NVO3 overlay) other encapsulations like VxLAN, L2VPN or
   Provider Backbone Bridging (PBB).

   Figure 3 illustrates the formats for MAC chaining used to carry the
   original IPv4 packets or L2 frames entering the classifier. Since MAC
   chaining encodes a SFC path in the MAC addresses of the Ethernet
   header it is not necessary to have the Network Service Header(NSH)
   [I-D.ietf-quinn-sfc-nsh] on every segment of the chain. The NSH is
   required on any NSH aware chain segment where meta-data is being
   carried.

   Original IPv4, MAC Chaining without NSH:
   +-------------------------------------------+--------------------+
   | Outer Ethernet, ET=0x0800                 | original IP Packet |
   +-------------------------------------------+--------------------+

   Original IPv4, MAC Chaining with NSH
   +---------------------------+---------------+--------------------+
   | Outer Ethernet, ET=0x894F | NSH, NP = 0x1 | original IP Packet |
   +---------------------------+---------------+--------------------+

   Original L2, MAC Chaining without NSH
   +-------------------------------------------+--------------------+
   | Outer Ethernet, ET=0x****                 | original L2 frame  |
   +-------------------------------------------+--------------------+

   Original L2, MAC Chaining with NSH
   +---------------------------+---------------+--------------------+
   | Outer Ethernet, ET=0x894F | NSH, NP = 0x3 | original L2 frame  |
   +---------------------------+---------------+--------------------+

       Figure 3 MAC Chaining Formats for IPv4 and L2 Service Packets

   The branch taken bit (figure 4) is a small piece of meta data that
   can be used in the MAC chaining header. For more elaborate meta data,
   the Network Service Header draft [I-D.ietf-quinn-sfc-nsh] header is
   compatible with MAC chaining. Section 11.3 [I-D.ietf-quinn-sfc-nsh]
   illustrates that the Ether type following a MAC chaining outer header
   has a registered type of 0x894F with an NSH that subsequently defines
   the payload. SFs can use the NSH within the chain. Any SCF, SF or CTF
   can remove or modify the NSH as specified in the NSH. When using the
   IETF NSH draft header each SF must either be capable of receiving an
   Ethernet frame with the NSH or must be supported by a proxy which
   removes the NSH before the SF.

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   The format of the MAC address used by MAC chaining is the standard
   IEEE MAC address format of 48 bits as illustrated in figure 4.

   Every MAC address is identified as either a global or a local MAC
   address. Global MAC addresses are intended to be worldwide unique
   while local address are intended for the use of local administrations
   domains and are not worldwide unique. Each global address uses a 22
   bit Organizational Unique Identifier (OUI) prefix which is assigned
   by the IEEE Registration Authority Committee to support worldwide
   uniqueness.  Recently, in response the needs of virtualization
   environments, the IEEE Registration Authority Committee has started
   assigning 22 bit Company IDs (CID) to allow independent vendors to
   share the local MAC addresses space within a domain where multiple
   un-coordinated authorities are assigning addresses. MAC chaining MAY
   make use of the new administered local MAC addresses.

       I U
       / /                                                           B
       G L                                                           T
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|n|CID1[42:47]|  CID2[32:39]  |  CID3[24:31]  |CS-ID[16:22] |X|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | CS-ID [8:15]  |  CS-ID [0:7]  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 4 MAC Chaining Ethernet MAC address Format

   Where:

   I/G      IEEE 802 Ethernet Individual/ Group address bit.

   U/L     IEEE 802 Ethernet Universal / Local Bit. Bit MAY be set
           indicating local.

   CID     Company ID - 22 Bits (Not Mandatory example only). Company
           Ids are assigned by IEEE registration to vendors who use
           Local addresses for MAC chaining or other purposes. The CIDs
           are unique and ensure that there are no collisions with
           other protocols that use local addresses. However the local
           addresses can be reused in other networks.

   BT      Branch Taken Chain indicator. Required. This bit may be set
           or reset in context of a chain.

   CS-ID    Chain Segment ID 23 bits. (Example only).

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   CS-MAC addresses, which identify chain segments, SHOULD be allocated
   by the CS-MAC Authority from the local space using a Company ID
   assigned to the CS-MAC Authority. MAC addresses also have an
   Individual (unicast) or Group (Multicast) bit I/G. MAC chaining MAY
   use individual or group addresses for the CS-MACs though restriction
   on the use of group CS-MACs may apply depending on the type of
   forwarding performed by the SFF for the particular segment.

   MAC chaining may also use global or local MAC addresses. The MAC
   address assigned to a Service Function MAY be Global or Local and can
   be assigned by any authority, not necessarily the CS-MAC Authority.

   As with other Service chaining, a packet or a frame travels through a
   network until it encounters an initial classifier. Forwarding before
   the classifier is out of the scope of this specification. The native
   packet format (L2 or L3 or tunneled, etc.) arriving at the classifier
   does not matter but the classifier (or set of classifiers ) need to
   inspect the packet and determine that the packet is part of a service
   chain.

   In all cases of MAC chaining after a frame (L2, L3, etc) has been
   classified the MAC chain begins by prepending the packet with an
   Ethernet L2 Frame header. The frame will also have a valid 4 byte CRC
   checksum.

   One advantage of MAC chaining is the MAC frame has an overhead of
   bytes that can leave the L2 MTU unaffected.  As with all Ethernet II
   frames payload must be a minimum of 64 bytes or must be padded to 64
   bytes.

4.2. Forwarding

   Forwarding of a packet is from a classifier (SCF) to the Service
   Function Forwarder (SFF) to the service function (SF) to the next SFF
   to the next SF and so on until the chain is finished. MAC chaining
   makes the distinction that the forwarding operations performed by a
   SFF and a SF are distinct and independent. However implementations
   may place SFF and SF functions as combined or separate entities. This
   makes MAC chaining particularly useful for deployment in
   virtualization environments where a virtual machine may implement one
   or more SFs and SFFs. Forwarding is a table driven operation. Note
   that all active chains are normally preprogrammed.

   Figure 5 illustrates the table driven forwarding operation of a MAC
   chaining SFF. Every frame arriving on the ingress VN port is matched
   to the MAC chaining filtering database. On arrival at the SFF the DA
   always contains a CS-MAC for the chain segment just crossed. The DA

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   is looked up in the context of Port 1 (a VN port) and the subsequent
   DA prime (DA' a CS-MAC used as the new frame DA if this segment is DA
   forwarding) and SA' (a CS-MAC used as the new frame SA if this
   segment is DA/SA forwarding) and egress Port 1 prime (1') are
   determined by the lookup.

   Ingress Port1                                    Egress Port
   +------+------+-----------+     +------+------+-----------+
   |CS_DA | SA   |           | ->  | DA'  | SA'  |           |
   +--+---+------+-----------+     +------+------+-----------+
      |                            \............/
      |                                   ^
      |                                   |
      |                                +--+
      |                         ....../........
      |                        /               \
      |        +-------+-------+-----+---------+-------+
       `------>|Port1  |CS_DA1 |DA1' | SA1'    |Port1' |
               |Port2  |CS_DA2 |DA2' | SA2'    |Port2' |
               |Port3  |CS_DA3 |DA3' | SA3'    |Port3' |
               |       |       |     |         |       |
               |       |       |     |         |       |
               +-------+-------+-----+---------+-------+
                  MAC Chaining Filtering Database

                   Figure 5 MAC Chaining Table Operation

   Any frame which doesn't exactly match an entry in the MAC chaining
   filtering database MUST be discarded. The filtering database itself
   is configured under the control of a network controller (see section
   6) which is responsible for creating the chain by programming the MAC
   chaining filtering database. The MAC chaining filtering database is
   an exact match database which may use existing Bridge match logic.
   The exact match filtering with hash implementation allows the
   filtering database to easily scale to a large number of chains.

   The Branch Taken (BT) Operation bit (figure 4) allows an SF to branch
   the chain by changing the BT bit. Not all service chains are branch
   capable. If a simple branch chain is desired it must be programmed in
   the SFF filtering database. A branch operation is indicated by
   setting a reserved bit in the CS-MAC address. This bit is not read as
   a bit but as a paired address to the forward direction. The context

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   of the BT bit may be maintained in both the SA and the DA once the
   bit has been set. An SFF receiving a frame with the BT bit set will
   look up the exact match address and forward the frame in the context
   of the received VN.

   More complicated branching requires SF chain awareness. The next hop
   addresses may be overridden by chain aware SFs to perform more
   advance branching. A SF must be provided with the allocated addresses
   for larger branches.

   Any frame that arrives at an SFF and is not found in the forwarding
   table is dropped.  Devices in the path that are not MAC service
   chaining aware are free to bridge the frame normally or to route
   using any underlay Layer 3 or 2.5 VN encapsulation.

4.2.1. Forwarding by Service Functions

   MAC chaining Service Functions (SFs) must be able to pass Ethernet
   DA/SA addresses through the SF unless the SF is supported by a Proxy
   Forwarder (see Proxy Forwarders section below). SFs are not required
   to pass VLAN Tags. Service Functions supported by MAC chaining can be
   classified by how they attach to the network as single armed, dual
   armed or multi-armed. Single arm SFs receive and send all packets on
   the same VN port. Single armed SFs are typically used when the
   direction of travel is unimportant to the SF. Dual arm SFs have two
   VN ports and pass packets between the two VN ports. Each VN port of a
   dual arm SF must attach to a different virtual or physical network.
   Dual arm SFs are typically used when the SF needs to know the
   direction of travel. Multi-arm SFs have more than two VN ports. In a
   multi-arm (two or more) the SF selects the egress VN port based on
   its' re-classification of the packet. Each VN port of a multi-arm SF
   must attach to a different VN or the SF must be MAC Chaining aware.
   These SFs allow the SFs to branch the chain based on re-
   classification or to replicate in the chain.

   MAC chaining SFs can perform one of two types of frame forwarding
   which are called DA/SA forwarding and DA forwarding. A SF that does
   DA/SA forwarding forwards received frames to the entity identified by
   the received SA. A SF that does DA forwarding passes the frame from
   ingress VN port to the egress VN port without modifying the DA. The

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   choice of DA or DA/SA forwarding may be different for each chain
   segment within a chain.
   Service Functions performing DA/SA MAC chaining use explicit
   receivers (standard Ethernet station interfaces) since the frames
   directly address the SF. Service Functions using DA/SA MAC chaining
   require only a single MAC address regardless of the number of chains
   passing through them. Service Functions performing DA MAC chaining
   must use a promiscuous receiver since the frames passing through them
   will not be explicitly addressed to the SF.

   Existing MAC chaining un-aware SFs that support transparent or Bridge
   modes can support MAC chaining using DA forwarding. A MAC chaining
   un-aware SF forwards a received frame using the same DA and SA as
   received. MAC chaining Service Functions may also be MAC chaining
   aware. A MAC chaining aware SF forwards received frames using the
   received SA as the transmit DA, exchanging DA for SA in the frame.
   For virtual SFs (i.e. VNFs) a MAC chaining unaware SF which copies
   Ethernet SA and DA from ingress to egress can be converted into a MAC
   chaining aware SF by running the SF in a guest OS equipped with MAC
   chaining aware forwarding library.

   To allow SF with more elaborate branching operations a SF may be
   given a set of MAC chaining addresses corresponding to a set of
   branches (enumerated). Details of the branch semantics such as load
   balancing or branch to chain identifiers will be added in a future
   revision of this document.

4.2.2. Proxy Forwarders

   Proxy forwarding is typically for legacy devices or other devices
   that do not have an ability to support MAC chaining by passing
   through L2 headers.

   Some service functions may reside on devices that do not understand
   MAC chaining. Legacy functions on middle boxes are one example. In
   this case a proxy forwarding function is used. Proxies may be
   integrated with the SFF or located in the switches attaching to the
   FS. The proxy will removing the MAC chaining header and forwarding
   the packet in an appropriate format to the SF. The SF then returns
   the packet to the proxy upon completion of its operation. The
   Specific formats of frames between the proxy and the SF, when using a
   proxy is out of the scope of this document.

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   The most basic proxy is a transparent proxy, which must be located
   between the SF and any underlay entity. A transparent proxy provides
   a provisioned Ethernet header which is used for forwarding all frames
   egressed by a SF at a specific VN port. The use of a transparent
   proxy limits the utility of the service chain in which it is inserted
   since no chain state is passed through the SF by the proxy.

4.2.3. Example MAC Chaining Walk Through

   Figure 6 outlines the general path and operations of a MAC chaining.

   The Service Classification Function (SCF) determines if a packet
   matches a predetermined policy for the chain by inspecting the packet
   then selecting the chain by encoding the frame with next destination
   equal to the chain segment 1 MAC Address A and itself as the Source
   Address (SA) designated as H in figure 6.

   SFF1 receives a frame from the SCF with Destination Address (DA)
   equal to A and finds the next chain segment by looking up A to find
   the next DA equal to SF2 MAC Address B and sets the SA equal to chain
   segment 2 MAC Address C.

   SF2 is a single armed Service Function which receives and sends all
   data on a single network interface. The single SF2 network interface
   normally connects to a single virtual or physical network. SF2
   receives a frame from SFF1 performs its function and then returns the
   frame to C. This process requires the SF to forward back to the
   frame's SA, by swapping DA and SA, on the same VN port.

                        One Arm            Two Arm
     +-----+   +-----+  +------+  +-----+  +------+  +-----| +-----+
     |     |   |     |   \ SF2/   |     |   \ SF4/   |     | |     |
     | SCF +---+ SFF1+----+B +----+ SFF1+----+D +----+ SFF2+-+ CTF |
     |    H|   |A   C|     \/     |C    |     \/x    |E   T| |F    |
     +-----+   +-----+            +-----+            +-----+ +-----+
          \...../   \............../   \............../   \..../
           CS 1         CS2                 CS3            CS4

          +----+    +----+   +----+     +----+   +----+   +----+
          |A,H |    |B,C |   |C,B |     |D,E |   |E,x |   |F,T |
          +----+    +----+   +----+     +----+   +----+   +----+
          Frames showing MAC DA,SA

                    Figure 6 MAC Service Chain Example

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   SCF:     Service Classification Function
   CTF:     Chain Termination Function
   SF:      Service Function
   SFF:     Service Function Forwarder
   CSx:     Chain Segment x.

   SFF1 receives a frame from SF2 with DA equal to chain segment 2 MAC
   Address C, finds the next chain segment by looking up C to find the
   next destination equal to SF4 MAC Address D and the SA equals chain
   segment 3 MAC Address E.

   SF4 is a dual armed Service Function which receives and sends data on
   two network interfaces. SF4 always forwards frames between its two
   interfaces. The two interfaces of SF4 are normally connected to
   separate virtual or physical networks. SF4 receives the frame from
   SFF1 with DA equals D and SA equals E performs its function then
   forwards to E by swapping DA with SA and sends out the packet to the
   other VN port (A VN port that supports address E as a destination).

   SFF2 receives a frame from SF4 with DA equals chain segment 3 MAC
   Address E, finds the next chain segment by looking up E to find the
   next destination equals chain segment 4 MAC Address F and SA equals
   SFF2 MAC Address T.

   The CTF receives a frame from SFF2 with destination equals F. The CTF
   must perform any required packet header adjustment and egress VN port
   determination based on the destination equals F and the frame payload
   (i.e. uses the IP address to route the packet).

4.2.4. Destination Address MAC Chaining Operation

   Destination MAC address chaining uses only the Destination MAC
   address to key on and implement a chain. Destination Address MAC
   chaining is used to operate with a MAC chaining un-aware SF. A
   Classifier/Service Function Forwarder (SFF), composing a DA MAC
   chaining hop, encodes the Chain Segment MAC in the frame DA and an
   address for the SFF in the frame SA. This encoding will address the
   next SFF or CTF in the chain. DA MAC chaining may only be used with
   dual-arm or multi-arm SFs since an unmodified frame can't be returned
   to the same network where it was received. Any SF along a DA MAC
   chaining segment must be operating in Transparent or in Bridging mode
   so it behaves as a "bump in the wire".

   For a Service Function to participates in a DA MAC chaining it must
   operate in promiscuous receiver, like an Ethernet Bridge, rather than

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   explicit receiver used by Ethernet stations. A MAC chaining Service
   Function Forwarder uses promiscuous receiver on its' VN ports just
   like every Ethernet Bridge and most Routers. In promiscuous receiver
   the switch receives and inspects every frame presented to it
   independent of the addressing on the frame.

   DA MAC chaining is determined by the configuration of the forwarding
   table in the SFF. After the initial classification the packet is
   passed to the first SFF (this may be a virtual operation completely
   within a single chaining switch). The SFF formats the Ethernet frame
   with a DA of the next hop in the Chain. At each hop the MAC address
   is looked up in a table similar to figure 2.

4.2.5. Destination and Source Address MAC Chaining

   DA and SA MAC chaining is a variation of MAC chaining that allows MAC
   chaining aware SFs to use explicit receiver and to support single
   armed as well as dual and multi-armed SFs. When using DA/SA MAC
   chaining the SF is individually addressed by the DA and therefore
   does not need to operate using a promiscuous learning receiver. Such
   a SF does not need a MAC lookup table and may be provisioned with a
   single global or local address under any administration authority
   (not necessarily the MAC chaining address authority). Service
   Functions using DA/SA MAC chaining require only a single MAC address
   regardless of the number of chains passing through them. DA/SA MAC
   chaining is particularly advantageous for virtual service functions
   (VNFs) since it reduces the need to flood frames into the virtual NIC
   supporting the SFs virtual machines and server I/O accelerators.

   DA/SA MAC chaining uses both addresses in the Ethernet L2 Header. The
   DA is use for the next hop device and the SA is used for the
   subsequent next hop device of the chain. A SF receives a frame;
   processes the frame; replaces the DA with the received SA and uses
   resulting DA (received SA) to forward the frame. By specifying 2 hops
   in a chain the SF can be a very generic operation. The original SA of
   the received frame does not have to be the address at the SFF that
   created the header, allowing forwarding flexibility.

4.2.6. Forwarding by Chain Termination Functions

   The forwarding to the final destination by the CTF typically does not
   use MAC chaining. The CTF is responsible for receiving frames
   addresses to the termination CS-MAC for each chain, de-encapsulating
   the packets, and forwarding the packets toward their final
   destination. One common method which may be used by the CTF for
   forwarding to the final destination is to route the packets using the
   IP address of the service packet.

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   If the service packet (data payload) is an L2 packet then the CTF may
   use either the IP network addresses or the L2 addresses to forward
   the packet. The choice between these two CTF forwarding models will
   depend on the application. Other CTF forwarding models are possible
   using by using the CS-MAC or meta-data for forwarding.

5. Programming a Service Chain

   The capability exists today with open flow enabled switches to
   specify MAC match criteria and actions that match MAC forwarding all
   operations. However not all switches are Openflow enabled.

   A Yang model could be specified to enable the MAC Chaining operations
   using an I2RS agent.

   Chains must be preprogrammed. Care must be taken to ensure that
   service chain loops are not created. The policy is to drop a frame
   that is not an exact match on any MAC chaining aware SFF.

   MAC chaining may be programmed be allowed to pass through bridges
   that are not MAC chaining aware. It is recommended that this
   operation be explicitly controlled by setting up port based VLANs
   designed for this purpose. Ports can add a VLAN tag as part of their
   forwarding operation. This can be usually be achieved with existing
   Ethernet controls that allow ports to have service tags added.  The
   VLAN tagging is independent of MAC chaining in this regard.

6. Domain of operation

   MAC chaining requires connectivity of L2 virtual networks over the
   service chain path.  (This may include multiple VNs that are
   interconnected.) In many networks this is readily available. Data
   centers for example can use MAC chain within a physical site that has
   L2 connectivity.

   If Virtualization of the L2 domain is enabled MAC chaining could
   operate over L2 networks such as NVO3 or Ethernet EVPN and an
   existing L2 Overlay.

7. Security Considerations

   MAC chaining is an Ethernet based forwarding operation that follows
   standard Ethernet rules.  VN ports should be qualified with VLANs
   that limit the scope of MAC chaining frames.  This prevents MAC
   chaining messages from being flooded to external parts of the network
   or injected into a network from external sources.  Programming the

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   VLAN that support MAC chaining is controlled and access to those
   VLANs is allowed only by trusted devices.

   MAC chaining is IP agnostic but like any tunneling protocol it will
   deliver IP frames to other parts of a network.

8. IANA Considerations

   There are no IANA considerations for this document.

9. References

9.1. Normative References.

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

   [802-2001] "Standard for Local and Metropolitan Area Networks:
             Overview and Architecture", IEEE 802, Standard 2014.

9.2. Informative References

   [I-D.ietf-sfc-architecture]
             Halpern, J., Pignataro, C. Editors, "Service Function
             Chaining (SFC) Architecture", draft-ietf-sfc-architecture-
             09(work in progress), June 2015.

   [I-D.ietf-quinn-sfc-nsh]
             P.Quinn et al., "Network Service Header",  draft-quinn-
             sfc-nsh-07 work in progress), February 2015.

10. Acknowledgments

   This document was prepared using 2-Word-v2.0.template.dot.

   Copyright (c) 2015 IETF Trust and the persons identified as authors
   of the code. All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:

   o  Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

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   o  Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in
      the documentation and/or other materials provided with the
      distribution.

   o  Neither the name of Internet Society, IETF or IETF Trust, nor the
      names of specific contributors, may be used to endorse or promote
      products derived from this software without specific prior written
      permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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Authors' Addresses

   Don Fedyk
   Hewlett Packard Networking
   153 Taylor Street
   Littleton, MA
   Email: don.fedyk@hp.com

   Paul Bottorff
   Hewlett Packard Networking
   8000 Foothills Blvd.
   Roseville, CA
   Email: paul.bottorff@hp.com

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