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Versions: 00 01

Network Working Group                                           M. Smith
Internet-Draft                                                R. Kandula
Intended status: Informational                             Cisco Systems
Expires: July 21, 2014                                  January 17, 2014


                Source-Group Tag eXchange Protocol (SXP)
                       draft-smith-kandula-sxp-00

Abstract

   This document discusses source-group tag exchange protocol (SXP), a
   control protocol to propagate IP address to Source Group Tag (SGT)
   binding information across network devices.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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 July 21, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   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.

   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  SXP Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SXP Operation . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  SXP Connection Management . . . . . . . . . . . . . . . .   4
       3.1.1.  SXP Connection  . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  SXP Message integrity/authenticity  . . . . . . . . .   5
       3.1.3.  SXP Connectivity Discovery and Connection Recovery  .   5
       3.1.4.  SXP Connection Setup Sequence . . . . . . . . . . . .   6
       3.1.5.  SXP Connection States . . . . . . . . . . . . . . . .   6
     3.2.  Binding Database  . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  SXP Learned IP-SGT Binding recovery . . . . . . . . .   8
   4.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Bit and Octet Numbering Convention  . . . . . . . . . . .   8
     4.2.  SXP Message Header  . . . . . . . . . . . . . . . . . . .   8
     4.3.  Attribute Formats . . . . . . . . . . . . . . . . . . . .   9
     4.4.  SXP OPEN and OPEN_RESP Message  . . . . . . . . . . . . .  11
       4.4.1.  Capabilities Advertisement  . . . . . . . . . . . . .  12
       4.4.2.  Keepalive and Hold Time Negotiation . . . . . . . . .  15
     4.5.  SXP UPDATE Message  . . . . . . . . . . . . . . . . . . .  20
       4.5.1.  UPDATE Attributes . . . . . . . . . . . . . . . . . .  21
       4.5.2.  UPDATE Message Samples  . . . . . . . . . . . . . . .  30
     4.6.  SXP ERROR Message . . . . . . . . . . . . . . . . . . . .  33
       4.6.1.  Error Codes . . . . . . . . . . . . . . . . . . . . .  34
     4.7.  SXP PURGE-ALL Message . . . . . . . . . . . . . . . . . .  35
     4.8.  SXP KEEPALIVE Message . . . . . . . . . . . . . . . . . .  35
   5.  Update Message Handling . . . . . . . . . . . . . . . . . . .  35
     5.1.  UPDATE Message Validation . . . . . . . . . . . . . . . .  35
     5.2.  UPDATE Message processing . . . . . . . . . . . . . . . .  37
     5.3.  Generating UPDATE Message . . . . . . . . . . . . . . . .  41
   6.  SXP Failure Scenarios . . . . . . . . . . . . . . . . . . . .  42
   7.  SXP Timers  . . . . . . . . . . . . . . . . . . . . . . . . .  42
   8.  SXP Version Negotiation . . . . . . . . . . . . . . . . . . .  45
     8.1.  SXP Versions  . . . . . . . . . . . . . . . . . . . . . .  45
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  45
   10. Implementation Note . . . . . . . . . . . . . . . . . . . . .  46
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46



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   12. IPR Disclosure  . . . . . . . . . . . . . . . . . . . . . . .  46
   13. Copyright Notice and Disclaimer . . . . . . . . . . . . . . .  46
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  47
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Introduction

   SXP stands for the Source Group Tag (SGT) eXchange Protocol.  Source
   groups are the endpoints connecting to the network that have common
   network policies.  Each source group is identified by a unique SGT
   value.  The SGT to which an endpoint belongs can be assigned
   statically or dynamically, and the SGT can be used as a classifier in
   network policies.  SXP is a control-plane mechanism used to transport
   an endpoint's SGT along with the IP address from one SGT-aware
   network device to another.  The data that SXP transports is referred
   to as the IP-SGT binding in the rest of the document.

1.1.  Terminology

   This document frequently uses the following terms:

   SGT Source Group Tag

   SXP Source-Group Tag (SGT) eXchange Protocol

   IP-SGT The IP Address to SGT binding that is exchanged over SXP
   connection

   SXP Speaker The peer that sends the IP-SGT bindings over the SXP
   connection

   SXP Listener The peer that receives the IP-SGT bindings over the SXP
   connection

   Binding Database The database of IP-SGT bindings that the SXP Speaker
   uses to export to the peer

2.  SXP Overview

   SXP uses TCP as its transport protocol to set up SXP connection
   between 2 separate network devices.  Each SXP connection has one peer
   designated as SXP speaker and the other peer as SXP listener.  The
   peers can also be configured in a bi-directional (both) mode where
   each of them act as both speaker & listener.  Connections can be
   initiated by either peers but binding information is always
   propagated from a speaker to a listener.




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   At a very high level, SXP comprises of the following -

   - SXP connection establishment

   - IP-SGT bindings propagation

   A typical deployment scenario of SXP is illustrated below -

   +---------------+                              +----------------+
   |               |                              |                |
   |               |                              |                |
   |               |                              |                |
   | SXP Speaker   |----------------------------->| SXP Listener   |
   |               |     IP-SGT Bindings          |                |
   |               |                              |                |
   |               |                              |                |
   +---------------+                              +----------------+
          |                                                |
          |                                                |
          |                                                |
          |                                                |
          +------------------------------------------------+

                           SXP Connection

3.  SXP Operation

   The SXP connections are point-to-point and uses TCP as the underlying
   transport protocol.  The summary of operations is illustrated in each
   of the sections below.

3.1.  SXP Connection Management

   An SXP connection consists of the following parts:

3.1.1.  SXP Connection

   o  The peer IP address, source IP address (the best IP address of
      local device used to reach the specified peer IP address)

      *  Used to construct TCP connection

   o  The connection mode

      *  An SXP connection can be configured for uni-directional or bi-
         directional communication.  For each direction, one peer must
         act as an sxp speaker and the other act as an sxp listener.




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      *  The SXP speaker is responsible for sending the IP-SGT bindings.

      *  The SXP listener is responsible for collecting the IP-SGT
         bindings received from the speaker peer

   o  SXP connection password

      *  If SXP data integrity and authentication are required, then
         both the peer devices must have same SXP password.

3.1.2.  SXP Message integrity/authenticity

   The connection peers on the 2 network devices supply the same SXP
   password to the TCP layer which will authenticate all further
   messages using the MD5 algorithm (RFC 2385).  The SXP payload is not
   encrypted because the main requirement is for the receiving device to
   determine that the message originated from a valid source and not to
   prevent snooping of message (discussed in security considerations
   section) payload.  SXP uses the underlying TCP MD5 option for message
   integrity/authenticity.  The TCP MD5 is exchanged (negotiated) during
   the initial TCP 3 way handshake.  Both sides (peers) are pre-
   configured with the keys (password) which will be used for forming
   the MD5 digest.  Each packet (segment) needs to be "signed" with the
   MD5 digest (16 bytes) formed using the password as the key.

   Whenever the password needs to be changed for an existing TCP
   connection, the peer resets the exising TCP connection and sets up a
   new TCP connection with the new password.  Changing the default
   password will cause all SXP connections using the default password to
   be re-established.

3.1.3.  SXP Connectivity Discovery and Connection Recovery

   SXP uses the TCP keep alive mechanism with the default behavior to
   maintain SXP connectivity.  A SXP device attempts to maintain
   connectivity with each peer.  In case of failures, the SXP device
   continues to retry connection setup with all the peers with which the
   SXP connections have not been established.  This continues until
   either a connection is established or the peer is removed from the
   peer set by a change in configuration.  This mechanism ensures
   automatic recovery of the SXP connection once the connectivity issue
   is resolved.  The SXP connection can be re-established by either
   device and hence the connection can be brought up sooner without
   relying on the retry timers of the peer devices.  Note that this is
   just an optimization and not a requirement for SXP connection
   recovery.  If both ends of an SXP connection set up the TCP
   connection at the same time, the end with source IP address higher




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   than the peer IP address wins: i.e. the TCP connection initiated from
   that end is kept and the other TCP connection is torn down.

3.1.4.  SXP Connection Setup Sequence

   When a SXP connection is configured, it initiates a TCP connection
   setup request.  SXP communication starts after TCP connection has
   been established between the two network devices.  The end that
   initiates the TCP connection starts the exchange by sending the OPEN
   message to negotiate SXP version, SXP mode, and if that end is a
   listener, SXP capabilities attribute.  Capabilities advertisement and
   Capabilities Attribute are described with additional details in
   section 5.4.1.  The receiving device responds with an OPEN_RESP
   message that includes similar attributes.  The connection can only be
   setup with one end configured as a speaker and the other end
   configured as a listener.  See the following sections regarding
   version negotiation and capability exchange process descriptions.
   Along with version negotiation, the hold time required for the
   connection is also negotiated.  The following sections explain the
   hold time negotiation in detail.

   After SXP connection is established, binding information can be sent
   from speaker to listener with UPDATE message.  Whenever the SXP
   connection has to be closed the underlying TCP connection is closed.

3.1.5.  SXP Connection States

   An SXP connection maintains following states: OFF, PENDING_ON,
   DELETE_HOLD_DOWN, ON.

   OFF: In this state, a connection initiation has not been started.
   This is the only state that an SXP connection will retry establishing
   the TCP connection.

   PENDING_ON: In this state, an SXP OPEN message has been sent to the
   peer and a response from the peer SXP is expected.

   DELETE_HOLD_DOWN: In this state, a connection that was previously in
   ON state has been terminated.  Only a listener can be in this state.

   ON: SXP is in ON state when SXP OPEN or SXP OPEN RESP message has
   been received.  SXP connection has been setup successfully.  An SXP
   connection will only propagates bindings in the ON state.








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                            +-------+
                  T7        |       |---------------+
              +------------>| OFF   |               |T1
              |             |       |<---------+    |
              |             +-------+   T2     |    |
          +--------+          |   A           +-----v-+
          |DELETE  |          |   |           |PENDING |
          |HOLDDOWN|        T5|   | T4        |  ON    |
          +---A----+          |   |           +--------+
              |             +-v-----+              |
              |      T6     |  ON   |              |T3
              +-------------|       |<-------------+
                            +-------+

   T1-T7: State Transitions

   T1: The connection sends a TCP connect request and an SXP OPEN
   message to the peer.  The connection then transits to the PENDING ON
   state.

   T2: TCP connection setup failure;

   T3: The connection received an SXP OPEN RESPONSE or SXP OPEN message.
   If the delete hold down timer is running, the timer is stopped and
   the reconciliation timer is started.

   T4: TCP connection failure; SXP message process failure; SXP
   configuration change.

   T5: The TCP connection is setup and an SXP OPEN message is received.
   If the delete hold down timer is running, the timer is stopped and
   the reconciliation timer is started.

   T6: TCP connection failure when a connection is in listener mode.
   The delete hold down timer is started with this state transition.

   T7: The delete hold down timer expired or TCP connection setup
   failure.

   In failure scenarios, the connection transits to OFF state and an
   attempt is made to re-establish the connection.

3.2.  Binding Database

   A system should have a master binding database for reconciling
   binding information from multiple SXP connections.  The database
   should consist of a single binding per IP address and there should be
   a mechanism to handle same bindings from multiple SXP connections.



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3.2.1.  SXP Learned IP-SGT Binding recovery

   When an SXP connection goes down on a network device, SXP continues
   to learn/advertise IP-SGT bindings on other SXP connections that are
   alive.  If the device has learnt any bindings from the peer where the
   SXP connection goes down, a delete hold down timer is started for
   that connection:

   o  Once the timer expires, all binding entries that were learned from
      the failed peer are deleted.  The deletion of the bindings for
      which the failed peer was the only source are reported to the
      master binding database.

   o  If the connection recovers before the delete hold down timer
      expiry, a reconcile timer is started to clean up old bindings that
      didn't get informed to be removed because of the loss of
      connectivity.

4.  Message Formats

   The SXP Messages and their formats are described in detail in this
   section.

4.1.  Bit and Octet Numbering Convention

   Throughout this section, the following numbering convention is used
   to depict the SXP message formats:

   Each diagram depicts the format and size of each field in bits.
   Implementations MUST send the bits in each diagram as they are shown,
   traversing the diagram from top to bottom and then from left to right
   within each line (which represents a 32-bit quantity).  Multi-byte
   fields representing numeric values must be sent in network (big
   endian) byte order.  Descriptions of bit field (e.g., flag) values
   are described referring to the position of the bit within the field.
   These bit positions are numbered from the most significant bit
   through the least significant bit, so a 1-octet field with only bit 0
   set has the value 0x80.

4.2.  SXP Message Header











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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    |                SXP Message Length (4 octets)                  |
    +---------------------------------------------------------------+
    |                SXP Message Type (4 octets)                    |
    +---------------------------------------------------------------+
    |                SXP payload (variable)                         |
    +---------------------------------------------------------------+

   +---------+----------+----------------------------------------------+
   | Field   | Length   | Description                                  |
   +---------+----------+----------------------------------------------+
   | SXP     | 4        | This 4-octet unsigned integer indicates the  |
   | Message |          | total length of the message, including the   |
   | Length  |          | header but excluding authentication tag. The |
   |         |          | maximum message length is 4096               |
   | SXP     | 4        | This 4-octet unsigned integer indicates the  |
   | Message |          | type code of the SXP message. The following  |
   | Type    |          | type codes are defined: 1 - OPEN 2 -         |
   |         |          | OPEN_RESP 3 - UPDATE 4 - ERROR 5 - PURGE_ALL |
   |         |          | 6 - KEEPALIVE                                |
   | SXP     | Variable | SXP message payload is an optional field.    |
   | Payload |          | Its content when present depends on the      |
   |         |          | message type                                 |
   +---------+----------+----------------------------------------------+

4.3.  Attribute Formats

   SXP messages can contain a variable number of fields.  Some fields
   have pre-defined format that every implementation shall understand.
   Other fields are called attributes and they have an extended {Type,
   Length, Value} (TLV) format in order to allows for gradual
   introduction of a new feature without requiring a protocol version
   change.  Each attribute is tagged and includes a length field.  This
   allows for newer versions of SXP to add capabilities and coexist with
   old versions of SXP in the same deployment.  Each attribute includes
   a Flags field to control processing and transitive processing of
   optional attributes.  The format of an attribute {flags, type,
   length, value} (TLV) is encoded as follows:










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   Compact TLV

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |  Type         |  TLV Length   |Reserved       |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |    Value (Variable: TLV Length octets long)                   |
      +---------------------------------------------------------------+

   Compact Extended Length TLV

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |  Type         |  TLV Extended Length          |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |    Value (Variable: TLV Extended Length octets long)          |
      +---------------------------------------------------------------+

   +----------------+------+-------------------------------------------+
   | Field          | Bits | Description                               |
   +----------------+------+-------------------------------------------+
   | Optional (O)   | 0    | 1 - Attribute is optional and could be    |
   |                |      | ignored 0 - Attribute is well-known and   |
   |                |      | must be recognized                        |
   | Non-Transitive | 1    | 0 - Attribute shall be forwarded even if  |
   | (N)            |      | it is not recognized 1 - Attribute shall  |
   |                |      | not be forwarded                          |
   | Partial (P)    | 2    | 1 - When a transitive attribute is        |
   |                |      | forwarded but not recognized 0 -          |
   |                |      | Otherwise                                 |
   | Compact (C)    | 3    | 1 - TLV is using compact Type and Length  |
   |                |      | encoding 0 - TLV is encoded using entire  |
   |                |      | 4 octets long words for Type and Length.  |
   |                |      | All other flags are 0                     |
   | Extended       | 4    | 0 - Length is a single octet long 1 -     |
   | Length (E)     |      | Length is 2 octets long                   |
   | Reserved       | 5-7  | Reserved for future use. Must be          |
   |                |      | transmitted as 0 and ignored on receive   |
   +----------------+------+-------------------------------------------+








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   +--------+----------+-----------------------------------------------+
   | Field  | Length   | Description                                   |
   +--------+----------+-----------------------------------------------+
   | Flags  | 1        | Attribute Flags                               |
   | Type   | 1 or 4   | TLV Type                                      |
   | Length | 1, 2 or  | Length (in octets) of the TLV Value field     |
   |        | 4        | (i.e. not including the 3, 4, or 8-octets     |
   |        |          | attribute header. Unsigned integer in the     |
   |        |          | following range: [0..255] - For Compact non-  |
   |        |          | Extended Length attribute. [256..4084] - For  |
   |        |          | Compact Extended Length attribute.            |
   | Value  | Variable | TLV values, which depend on the TLV type      |
   +--------+----------+-----------------------------------------------+

   Attribute Length field of Compact attributes SHOULD be only as large
   as necessary to hold an unsigned integer that specifies the number of
   octets in the attribute Value field.  This implies that Extended-
   Length field SHOULD contain a value greater than 256 (or the first
   octet of an Extended-Length field SHOULD be non-zero).  The largest
   allowed value of an Extended-Length field is 4084, considering
   maximum SXP message length of 4096.  The largest allowed value of a
   non-Compact attribute Length field is thus 4080.  SXP Listener MUST
   accept and process Compact Extended-Length attributes with Length
   field in the range [0..255] without issuing any errors.  In the rest
   of this document diagrams showing usage of Compact normal and/or
   Extended-Length attribute are only illustration of expected usage.
   They do not imply that only the illustrated class of attribute or
   attribute Length is supported.

   Attribute Value field consists of well defined sub-fields which are
   defined in this protocol spec.  Attribute Value field MAY contain
   additional, possibly optional, sub-attributes which are encoded like
   other attributes.

4.4.  SXP OPEN and OPEN_RESP Message
















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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |                Version (4 octets)                             |
      +---------------------------------------------------------------+
      |                SXP Mode (4 octets)                            |
      +---------------------------------------------------------------+
      |                SXP Node-ID Attribute (7 octets)               |
      +---------------------------------------------------------------+
      |                Capabilities Attribute (variable)              |
      +---------------------------------------------------------------+
      |                Other Optional Attributes (variable)           |
      +---------------------------------------------------------------+

   +--------------+----------+-----------------------------------------+
   | Field        | Length   | Description                             |
   +--------------+----------+-----------------------------------------+
   | Version      | 4        | Unsigned integer value. The SXP Version |
   |              |          | is set to the highest version that is   |
   |              |          | supported on the network device         |
   | SXP Mode     | 4        | Unsigned integer value used to convey   |
   |              |          | the SXP role that the device uses on    |
   |              |          | the SXP connection. The following       |
   |              |          | values are used: 1 - SXP Speaker 2 -    |
   |              |          | SXP Listener                            |
   | SXP Node-Id  | 7        | REQUIRED non-transitive attribute (in   |
   |              |          | SXP version 4 and above) an SXP         |
   |              |          | Listener SHALL include in OPEN or       |
   |              |          | OPEN_RESP to convey its unique 32 bits  |
   |              |          | Node ID. The attribute type used for    |
   |              |          | Node ID Attribute is 6                  |
   | Capabilities | Variable | REQUIRED non-transitive attribute (in   |
   |              |          | SXP version 4 and above) an SXP         |
   |              |          | Listener SHALL include in OPEN or       |
   |              |          | OPEN_RESP to convey its capabilities    |
   |              |          | for processing data on the connection   |
   +--------------+----------+-----------------------------------------+

   SXP Node-ID with value 0 is reserved for denoting bindings that are
   received from connections that use older versions of the SXP
   protocol.

4.4.1.  Capabilities Advertisement

   SXP Listeners are REQUIRED to include the Capabilities attribute when
   they send an OPEN message or when they respond to such message with
   OPEN_RESP message.  The Capabilities attribute describes the SXP



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   features supported by the Listener.  This allows the SXP Speaker to
   tailor the behavior of the connection to what the Listener could
   process.  An SXP speaker receiving Capabilities attribute from an SXP
   Listener SHALL NOT send to that listener in UPDATE messages
   information which was not indicated as supported in the Capabilities
   attribute.  If an SXP speaker receives from its listener peer a
   capability that it does not itself support or recognize, it MUST
   ignore that capability.  In particular, ERROR message MUST NOT be
   generated and the SXP session MUST NOT be terminated in response to
   reception of a capability that is not supported by the local SXP
   speaker.

4.4.1.1.  Capabilities Attribute

   This is a REQUIRED attribute that is used by an SXP Listener to
   convey to its peer SXP Speaker the list of capabilities supported by
   the listener.  The attribute contains one or more triples <Capability
   Code, Capability Length, Capability Values>:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | | Capabilities  |TLV Length (C=1|Reserved       |
      | | | | | | | | | Type 6        |    EL=0)      |               |
      |0|1|0|1| | | | |               +---------------+---------------+
      | | | | | | | | |               |TLV Extended Length (C=1 EL=1) |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |Cap Code 1     |Cap Length 1   |Reserved                       |
      |---------------+---------------+-------------------------------+
      |             Capability 1 Value (Variable)                     |
      +---------------------------------------------------------------+
      x                      *****                                    x
      x---------------------------------------------------------------x
      |Cap Code N     |Cap Length N   |Reserved                       |
      |---------------+---------------+-------------------------------+
      |             Capability N Value (Variable)                     |
      +---------------------------------------------------------------+













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   +------------+----------+-------------------------------------------+
   | Field      | Length   | Description                               |
   +------------+----------+-------------------------------------------+
   | Capability | 1        | Unsigned integer that unambiguously       |
   | Code       |          | identifies individual capabilities        |
   | Capability | 1        | Unsigned integer that contains the length |
   | Length     |          | of the Capability Value field in octets   |
   | Capability | Variable | Variable-length field that is interpreted |
   | Value      |          | according to the value of the Capability  |
   |            |          | Code field. Could have zero length        |
   +------------+----------+-------------------------------------------+

   The encoding and meaning of the capabilities defines by this version
   of the SXP protocol spec are as follows:

   +------------+------------+------------+----------------------------+
   | Capability | Capability | Capability | Description                |
   | Name       | Code       | Length     |                            |
   +------------+------------+------------+----------------------------+
   | IPv4       | 1          | 0          | Listener is capable of     |
   | Unicast    |            |            | receiving and handling     |
   |            |            |            | IPv4 unicast bindings      |
   | IPv6       | 2          | 0          | Listener is capable of     |
   | Unicast    |            |            | receiving and handling     |
   |            |            |            | IPv6 unicast bindings      |
   | Subnet     | 3          | 0          | Listener is capable of     |
   | Bindings   |            |            | receiving and handling     |
   |            |            |            | subnet bindings            |
   +------------+------------+------------+----------------------------+

4.4.1.2.  Handling of Capabilities Attribute

   Subnet Bindings - This capability indicates to the speaker that
   subnet bindings SHOULD be exported as regular bindings without any
   expansion.  A listener that fully supports bindings associated with
   IP prefixes other than host addresses (IPv4 /32 or IPv6/128) MUST
   include the Subnet Bindings capability.

   A listener that does not support subnet bindings in its forwarding
   path but has the capability to expand subnet bindings locally MUST
   send Subnet Bindings capability in order to prevent the listener from
   unnecessarily expanding subnet bindings.  A listener which does not
   support subnet bindings but might be able to support the volume of
   expanded bindings MAY include Subnet Bindings capabilities.

   A speaker receiving such capability MAY expand subnet bindings
   exhaustively or selectively and export individual bindings within a
   subnet.  The expansion of subnet bindings COULD be limited by



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   implementation to a reasonable subnet size with a limit on the total
   number of bindings expanded.  Therefore there could be subnet
   bindings which are not expanded.

   When a speaker does not expand a subnet binding it SHOULD export it
   unmodified as a subnet binding.  This will allow the listener to
   propagate such bindings even when it cannot use it locally.

   When a speaker expands a subnet binding it MUST NOT export it as both
   subnet and expanded bindings since the listener or any further SXP
   peers along the SXP propagation path cannot distinguish expanded
   bindings from other host bindings and relate them to the subnet
   bindings they are originate from.

   IPv6 Unicast - This capability indicates to the speaker that the
   listener is capable of handling and using bindings associated with
   IPv6 unicast addresses.  A listener that support IPv6 bindings MUST
   include this capability.  When this capability is not included, the
   speaker MUST NOT send IPv6 bindings within well-known attributes.  A
   speaker, however, SHOULD send IPv6 bindings within Optional
   Transitive attributes when the IPv6 Unicast capability is omitted.
   This will allow an SXP node to relay IPv6 bindings even when it could
   not process them locally.

   IPv4 Unicast - This capability should be handled in the same way as
   IPv6 Unicast capability is handled.  It is unlikely for any SXP
   listener to omit IPv4 Unicast capability at least until pure IPv6
   networks would become common.

4.4.2.  Keepalive and Hold Time Negotiation

   SXP uses TCP-based, keep-alive mechanism to determine if a connection
   is live.  This mechanism has been used in all prior versions of the
   protocol.  SXP version 4 adds an optional negotiated keep-alive
   mechanism within the protocol itself in order to provide more
   predictable and timely detection of connection loss.

   SXP connections are asymmetric with almost all of the protocol
   messages (except for OPEN/OPEN_RESP and ERROR) being sent from an SXP
   speaker to an SXP listener.

   SXP listener could also keep potentially large volume of state per
   connection which includes all the binding information learnt on a
   connection.  Therefore, it is only meaningful to have a keep-alive
   mechanism that allows a listener to detect the loss of connection
   with a speaker.

   The mechanism is based on two timers:



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   Hold Timer - Used by a listener for detection of elapsing time
   without successive KEEPALIVE and/or UPDATE messages from a speaker.

   Keep-alive Timer - Used by a speaker to trigger sending of KEEPALIVE
   messages during intervals when no other information is exported via
   UPDATE messages.

   The keep-alive timer is one-thirds of the negotiated hold-timer
   value.

   Hold Timer for the keep-alive mechanism MAY be negotiated during the
   OPEN/OPEN_RESP exchange at connection setup.

   o  A listener MAY have desirable range for Hold Time period locally
      configured or a default of [90..180] seconds.  A value of
      [0xFFFF..0xFFFF] implies that the keep-alive mechanism is not
      used.

   o  A speaker MAY have a minimum acceptable Hold Time period locally
      configured or a default of 120 seconds.  This is the shortest
      period of time a speaker is willing to send KEEPALIVE messages for
      keeping the connection alive.  Any shorter Hold Time period would
      require a faster KEEPALIVE rate from the rate the speaker is ready
      to support.  A value of 0xFFFF implies that the keep-alive
      mechanism is not used

   o  The negotiation succeeds when the speaker's minimum acceptable
      Hold Time falls below or within the desirable Hold Time range of
      the listener.  If one end turns off the keep-alive mechanism, the
      other end should also turn it off to make the negotiation success.

   o  The negotiation fails when the speaker's minimum acceptable Hold
      Time is greater than the upper bound of the listener's Hold Time
      range.

   o  The selected Hold Time period of a successful negotiation is the
      maximum of the speaker's minimum acceptable Hold Time and the
      lower bound of the listener's Hold Time range.

   o  The speaker calculates the Keep-alive Time to 1/3 of the selected
      Hold Time by default unless a different Keep-alive Time is locally
      configured.

   o  The negotiation process is designed such that the outcome is the
      same regardless of the mode of the initiator.

   The diagram below illustrates the Hold Time negotiation process.




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                     +------+                              +------+
                     | Min  |                              | Max  |
                     |      |                              |      |
                     +------+                              +------+
  Listener               |                                    |
                         |                                    |
        -----------------V<---------------------------------->V--------
  Speaker |                               |                        |
          |                               |                        |
         1|                              2|                        |3
        +-v---------+                  +--v--------+      +--------v---+
        | Selected  |                  | Selected  |      |Unacceptable|
        |  Hold     |                  |  Hold     |      | Hold       |
        |  Time     |                  |  Time     |      | Time       |
        |           |                  |           |      |            |
        +-----------+                  +-----------+      +------------+

   Connection initiated by Listener

   o  A listener MAY include a Hold-Time Attribute in the OPEN message
      with minimum and maximum values set to its configured range of
      Hold Time period.  Hold-Time Attribute with just a minimum value
      set to 0xFFFF would indicate to the speaker that the keep-alive
      mechanism is not used.

   o  When a speaker received an OPEN message it will react as follows:

      *  If the Hold-Time attribute is not present or if it contains a
         minimum value that is set to 0xFFFF, the speaker will set its
         Keepalive Time to 0xFFFF to indicate that keep-alive mechanism
         is disabled.

      *  If the received Hold-Time attribute contains a valid range, the
         speaker MUST include Hold-Time attribute in its OPEN_RESP
         message with a minimum value set as follows:

         +  0xFFFF if the speaker does not support the keep-alive
            mechanism or if the mechanism is supported but disabled due
            to local configuration which sets the Keep-alive Time to
            0xFFFF

         +  If the speaker's minimum acceptable Hold Time value is
            greater than the upper bound of the offered range, the
            speaker MUST send and Open ERROR message with sub-code set
            to Unacceptable Hold Time and terminate the connection.






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         +  Otherwise the speaker's will set the selected Hold Time to
            the maximum of its minimum acceptable Hold Time value and
            the lower bound of the offered Hold Time range.

         +  The speaker will calculate a new value for its Keep-alive
            Time as 1/3 of that selected Hold Time.

         +  The speaker will set the minimum Hold Time value of the Hold
            Time attribute to the selected Hold Time.

   o  When the listener receives the OPEN_RESP from the speaker it will
      look for Hold Time Attribute:

      *  If Hold-Time attribute is present and contains a minimum Hold
         Time value of 0xFFFF.  The speaker will set its Hold Time value
         to 0xFFFF to indicate that keep-alive mechanism is not used.

      *  If the minimum Hold Time value is within the range offered by
         the listener, the listener will set its Hold-Time period to the
         selected value it has received in the OPEN_RESP.

      *  If the minimum Hold Time value is outside the offered range,
         the listener will send Open ERROR message with sub-code set to
         Unacceptable Hold Time and terminate the connection.

   Connection initiated by Speaker

   o  A speaker MAY include a Hold-Time Attribute in the OPEN message
      with minimum value set to its minimum acceptable Hold Time period.
      Hold-Time Attribute with just a minimum value set to 0xFFFF would
      indicate to the listener that the keep-alive mechanism is not
      used.

   o  When a listener receives an OPEN message it will react as follows:

      *  If the Hold-Time attribute is not present or if it contains a
         minimum value that is set to 0xFFFF, the listener will set its
         Hold Time to 0xFFFF to indicate that keep-alive mechanism is
         disabled.

      *  If the received Hold-Time attribute contains a valid value, the
         speaker MUST include Hold-Time attribute in its OPEN_RESP
         message with a minimum value set as follows:

         +  0xFFFF if the listener does not support the keep-alive
            mechanism or if the mechanism is supported but disabled due
            to local configuration which sets the Keep-alive Time to
            0xFFFF



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         +  If the received Hold Time value is greater than the upper
            bound of the listener's configured Hold Time range, the
            speaker MUST send and Open ERROR message with sub-code set
            to Unacceptable Hold Time and terminate the connection.

         +  If the received Hold Time value falls within the listener's
            configured Hold Time range, the listener will make it the
            selected Hold Time.

         +  If the received Hold Time value is less than the lower bound
            of the listener's configured Hold Time range, the listener
            will set the selected Hold Time to the lower bound of its
            Hold Time range.

         +  The listener will set the minimum Hold Time value of the
            Hold Time attribute to the selected Hold Time.

   o  When the speaker receives the OPEN_RESP from the listener it will
      look for Hold Time Attribute:

      *  If Hold-Time attribute is present and contains a minimum Hold
         Time value of 0xFFFF.  The speaker will set its Hold Time value
         to 0xFFFF to indicate that keep-alive mechanism is not used.

      *  If the received Hold Time value is greater or equal to the
         speaker minimum acceptable Hold Time, the speaker will
         calculate a new value for its Keep-alive Time as 1/3 of that
         received Hold Time.

      *  If the received Hold Time value is lower than the minimum
         acceptable Hold Time, the speaker MUST send and Open ERROR
         message with sub-code set to Unacceptable Hold Time and
         terminate the connection.

4.4.2.1.  Hold-Time Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |Hold-Time (7)  | TLV Length 2/4|Reserved       |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      | Hold Time Minimum Value       |  Hold Time Maximum Value      |
      +---------------------------------------------------------------+







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   +------------------+--------+---------------------------------------+
   | Field            | Length | Description                           |
   +------------------+--------+---------------------------------------+
   | Optional(C)      | 1 Bit  | Value is 0                            |
   | Non-Transitiv(N) | 1 Bit  | Value is 0                            |
   | Partial (P)      | 1 Bit  | Value is 0                            |
   | Compact (C)      | 1 Bit  | Value is 1                            |
   | Extended Length  | 1 Bit  | Value is 0                            |
   | (E)              |        |                                       |
   | Attribute Type   | 1      | Hold-Time (7)                         |
   | Attribute Value  | 1      | 2 - when only Hold Time Minimum Value |
   | Length           |        | is present. 4 - when both Hold Time   |
   |                  |        | Minimum and Maximum values are        |
   |                  |        | present                               |
   | Hold Time        | 2      | Unsigned integer indicating the       |
   | Minimum Value    |        | number of seconds the sender proposes |
   |                  |        | as a lower bound for the Hold Timer   |
   |                  |        | period or the Hold Time period        |
   |                  |        | selected by a responder. 0 or at      |
   |                  |        | least 3 seconds                       |
   | Hold Time        | 2      | Unsigned integer indicating the       |
   | Maximum Value    |        | number of seconds the sender proposes |
   |                  |        | as a upper bound for the Hold Timer   |
   |                  |        | period. MUST be greater than Hold     |
   |                  |        | Time Minimum Value                    |
   +------------------+--------+---------------------------------------+

   Hold-Time is a well-known discretionary attribute an initiator of a
   connection MAY include in an OPEN message and a responder MAY be
   REQUIRED to include in an OPEN_RESP in order to negotiate the timer
   periods used for keep-alive mechanism.

4.5.  SXP UPDATE Message

   Prior versions of SXP protocol specify that an UPDATE message
   contains one or more SXP mapping records.  This version of SXP
   protocol generalizes the UPDATE message.  An UPDATE message MAY
   contain one or more attributes.  Some attributes are well-known and
   additional attributes COULD be added in the future without affecting
   existing implementations of SXP version 4.  This document defines the
   format of the attributes used within UPDATE message and how they are
   combined to carry the SXP binding information.









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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +---------------------------------------------------------------+
      |                  Attribute 1 (Variable)                       |
      +---------------------------------------------------------------+
      x                    *****                                      x
      +---------------------------------------------------------------+
      |                  Attribute N (Variable)                       |
      +---------------------------------------------------------------+

4.5.1.  UPDATE Attributes

4.5.1.1.  IPv4-Delete-Prefix Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |IPv4-Delete-   | TLV Length    |Reserved       |
      | | | | | | | | | Prefix        |  C = 1 EL = 0 |               |
      |0|0|0|1| | | | | Type = 13     +-------------------------------+
      | | | | | | | | |               |TLV Extended Length (C=1 EL=1) |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |                 IPv4 Prefix 1 (Variable)                      |
      +---------------------------------------------------------------+
      x                    *****                                      x
      +---------------------------------------------------------------+
      |                 IPv4 N Value (Variable)                       |
      +---------------------------------------------------------------+

   This attribute is used to convey the withdrawal of one or more IPv4
   bindings which are unambiguously identified by their IPv4 prefix.
   This attribute contains a set of IPv4 prefixes.  Each IPv4 prefix is
   encoded as a 2-tuple of the form <length, prefix>, whose fields are
   encoded as:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |  Length =     |                Reserved                       |
      |  [0..32]      |                                               |
      +---------------+-----------------------------------------------+
      |                 Prefix                                        |
      +---------------------------------------------------------------+
      | 0 < Length    | 9 <= Length   | 17 <= Length  |25 <= Length   |
      +---------------+---------------+---------------+---------------+



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   +--------+------------------------+---------------------------------+
   | Field  | Length (in Octets)     | Description                     |
   +--------+------------------------+---------------------------------+
   | Length | 1                      | Unsigned integer. The length in |
   |        |                        | bits of the IP address prefix.  |
   |        |                        | A length of zero indicates a    |
   |        |                        | prefix that matches all IP      |
   |        |                        | addresses (with Prefix field of |
   |        |                        | zero octets)                    |
   | Prefix | Variable               | An IP address prefix, followed  |
   |        | &#9121;Length/8&#9124; | by the minimum number of        |
   |        |                        | trailing bits needed to make    |
   |        |                        | the end of the field fall on an |
   |        |                        | octet boundary. The value of    |
   |        |                        | trailing bits is irrelevant     |
   +--------+------------------------+---------------------------------+

4.5.1.2.  IPv6-Delete-Prefix Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |IPv4-Delete-   | TLV Length    |Reserved       |
      | | | | | | | | | Prefix        |  C = 1 EL = 0 |               |
      |0|0|0|1| | | | | Type = 14     +-------------------------------+
      | | | | | | | | |               |TLV Extended Length (C=1 EL=1) |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |                 IPv6 Prefix 1 (Variable)                      |
      +---------------------------------------------------------------+
      x                    *****                                      x
      +---------------------------------------------------------------+
      |                 IPv6 N Value (Variable)                       |
      +---------------------------------------------------------------+

   This attribute is used to convey the withdrawal of one or more IPv6
   bindings which are unambiguously identified by their IPv6 prefix.
   This attribute contains a set of IPv6 prefixes.  Each IPv6 prefix is
   encoded as a 2-tuple of the form <length, prefix>, whose fields are
   encoded as:











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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |  Length =     |                  Reserved                     |
      |  [0..128]     |                                               |
      +---------------+-----------------------------------------------+
      |                 Prefix[1..4]                                  |
      +---------------------------------------------------------------+
      | 0 < Length    | 9 <= Length   | 17 <= Length  |25 <= Length   |
      +---------------+---------------+---------------+---------------+
      |                 Prefix[5..8]                                  |
      +---------------------------------------------------------------+
      | 33 < Length   | 41 <= Length  | 49 <= Length  |57 <= Length   |
      |               |               |               |               |
      +---------------+---------------+---------------+---------------+
      |                 Prefix[9..12]                                 |
      +---------------------------------------------------------------+
      | 65 < Length   | 73 <= Length  | 81 <= Length  |89 <= Length   |
      +---------------+---------------+---------------+---------------+
      |                 Prefix[13..16]                                |
      +---------------------------------------------------------------+
      | 97 < Length   | 105 <= Length | 113 <= Length |121 <= Length  |
      +---------------+---------------+---------------+---------------+

   +--------+------------------------+---------------------------------+
   | Field  | Length (in Octets)     | Description                     |
   +--------+------------------------+---------------------------------+
   | Length | 1                      | Unsigned integer. The length in |
   |        |                        | bits of the IP address prefix.  |
   |        |                        | A length of zero indicates a    |
   |        |                        | prefix that matches all IP      |
   |        |                        | addresses (with Prefix field of |
   |        |                        | zero octets)                    |
   | Prefix | Variable               | An IP address prefix, followed  |
   |        | &#9121;Length/8&#9124; | by the minimum number of        |
   |        |                        | trailing bits needed to make    |
   |        |                        | the end of the field fall on an |
   |        |                        | octet boundary. The value of    |
   |        |                        | trailing bits is irrelevant     |
   +--------+------------------------+---------------------------------+

4.5.1.3.  Peer-Sequence Attribute








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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |Peer-Sequence  | TLV Length    |   Reserved    |
      |0|0|0|1|0| | | | Type = 16     |    4 * N      |               |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |                         SXP ID 1                              |
      +---------------------------------------------------------------+
      |                         SXP ID 2                              |
      +---------------------------------------------------------------+
      x                         *****                                 x
      +---------------------------------------------------------------+
      |                         SXP ID N                              |
      +---------------------------------------------------------------+

   This attribute contains the SXP nodes through which exported bindings
   has traversed.  Each SXP node is identified by a 4 octet SXP ID.  SXP
   ID MUST be unique for every SXP node in an SXP deployment.  As
   exported bindings are propagated by an SXP Speaker, the speaker SHALL
   prepend its own SXP ID to the sequence of SXP IDs of the Peer-
   Sequence attribute that was received by an SXP Listener on the same
   node.  If the exported binding is originated by the local node, the
   Peer-Sequence will contain a single SXP ID identifying the local
   node.  When an UPDATE message is received by an SXP Listener, the
   first SXP ID within each Peer-Sequence attribute is the SXP node ID
   of the SXP Speaker on the remote end of the connection the message is
   received on.  The last SXP ID is the node ID of the SXP Speaker that
   originates the bindings associated with a Peer-Sequence attribute.
   UPDATE message SHALL contain one Peer-Sequence attribute and MAY
   contain multiple Peer-Sequence attributes.  Each Peer-Sequence
   attribute is associated with the bindings that follow it up to the
   next Peer-Sequence attribute if more than one Peer-Sequence attribute
   is present or up to the end of the UPDATE message.

4.5.1.4.  Source-Group-Tag Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      |O|N|P|C|E| | | |SGT Type = 17  | TLV Length 2  |    Reserved   |
      |0|0|0|1|0| | | |  Group Tag    |     2         |               |
      +-+-+-+-+-+-+-+-+---------------+---------------+---------------+
      |       SGT Value               |            Reserved           |
      +-------------------------------+-------------------------------+





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   +--------------------+-----------------+----------------------------+
   | Field              | Length (in      | Description                |
   |                    | Octets)         |                            |
   +--------------------+-----------------+----------------------------+
   | Attribute Type     | 1               | Source-Group-Tag (17)      |
   | Attribute Value    | 1               |                            |
   | Length             |                 |                            |
   | SGT Value          | 2               | Unsigned integer value of  |
   |                    |                 | an SGT                     |
   +--------------------+-----------------+----------------------------+

4.5.1.5.  IPv4-Add-Prefix Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | |IPv6-Add-Prefix|TLV Length (C=1|     Reserved  |
      |O|N|P|C|E| | | |  Type = 11    |    EL=0)      |               |
      | | | | | | | | |               +---------------+---------------+
      |0|0|0|1| | | | |               |TLV Extended Length (C=1 EL=1) |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |             IPv4 Prefix 1 (Variable)                          |
      +---------------------------------------------------------------+
      x                      *****                                    x
      |---------------------------------------------------------------+
      |             IPv4 N Value (Variable)                           |
      +---------------------------------------------------------------+

   This attribute is used to convey the binding of one or more IPv4
   prefixes with an SGT value or other information.  The encoding of
   each IPv4 prefix is described above in IPv4-Delete-Prefix Attribute
   section.

   The unbundling of IP prefixes from the information associated with
   them achieves two major objectives:

   o  Allows for efficient sharing of common attributes such as Peer-
      Sequence, and Source-Group-Tag with multiple IP prefixes.

   o  Allows for future introduction of new information for association
      with IP prefixes via optional attributes.

   The associated SGT is the value specified by the latest preceding
   occurrence of Source-Group-Tag attribute within the same UPDATE
   message.  An IPv4-Add-Prefix attribute SHALL be preceded with an
   occurrence of Peer-Sequence attribute.  The latest preceding




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   occurrence provides the path along which the bindings of all the
   prefixes contained in the IPv4-Add-Prefix have been traversed.

4.5.1.6.  IPv6-Add-Prefix Attribute

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | |IPv6-Add-Prefix|TLV Length (C=1|     Reserved  |
      |O|N|P|C|E| | | |  Type = 12    |    EL=0)      |               |
      | | | | | | | | |               +---------------+---------------+
      |0|0|0|1| | | | |               |TLV Extended Length (C=1 EL=1) |
      +-+-+-+-+-+-+-+-+---------------+-------------------------------+
      |             IPv6 Prefix 1 (Variable)                          |
      +---------------------------------------------------------------+
      x                      *****                                    x
      |---------------------------------------------------------------+
      |             IPv6 N Value (Variable)                           |
      +---------------------------------------------------------------+

   This attribute is used to convey the binding of one or more IPv6
   prefixes with an SGT value or other information.  The encoding of
   each IPv6 prefix is described above in IPv6-Delete-Prefix Attribute
   section.  The handling of this attribute and its relationship with
   preceding attributes such as Peer-Sequence or Source-Group-Tag are
   the same as what is described above in IPv4-Add-Prefix section except
   for a different attribute type (12) and possibly longer IPv6
   prefixes.

4.5.1.7.  IPv4-Add-Table Attribute

   This attribute provides a flexible tabular representation of bindings
   information.  It is provided for efficient aggregation of multiple
   bindings information when an implementation cannot aggregate multiple
   IP prefixes that are associated with the same SGT or other common
   attributes.














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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
     | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
     |O|N|P|C|E| | | | IPv4-Add-Table| TLV Length    |  Reserved     |
     |0|0|0|1| | | | |  Type = 21    | (C = 1 EL = 0)|               |
     | | | | | | | | |               |-------------------------------+
     | | | | | | | | |               | TLV Extended Length (C=1 EL=1)|
     +-+-+-+-+-+-+-+-+---------------+---------------+---------------+
     | Number of     |                 Reserved                      |
     | Columns C     |                                               |
     +---------------+---------------+---------------+---------------+
     |'1'Column Type | '1'Column Type| '1'Column     |'2'Column Type |
     |   Flags       |               |   Width       |   Flags       |
     +---------------+---------------+---------------+---------------+
     |'2'Column Type | '2'Column     |                               |
     |               |    Width      |           ***                 |
     +---------------+---------------+---------------+---------------+
     x                           ***                                 x
     +---------------+---------------+---------------+---------------+
     |'C'Column Type | 'C'Column Type| 'C'Column     |   Reserved    |
     |   Flags       |               |   Width       |               |
     +---------------+---------------+------------------------------+--+
     | '1''1' Value  |     ***       |'1''C'Value    |'1'IPv4 Prefix   |
     |               |     ***       |               |    (Variable)   |
     +---------------+---------------+---------------+-----------------+
     x         ***   x     ***       x    ***        x         ***     x
     x         ***   x     ***       x    ***        x         ***     x
     x---------------+---------------+---------------+-----------------x
     | 'R''1' Value  |     ***       |'R''C'Value    |'R'IPv4 Prefix   |
     |               |     ***       |               | (Variable)      |
     +---------------+---------------+---------------+-----------------+



















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   +---------+----------+----------------------------------------------+
   | Field   | Bits     | Description                                  |
   +---------+----------+----------------------------------------------+
   | Number  | 1        | 1 - Unsigned integer which specifies the     |
   | of      |          | number of columns of information (not        |
   | Columns |          | including the trailing IP prefix) associated |
   |         |          | with each prefix                             |
   | [i]     | 1        | 0 - An Attribute Type Flags value defining   |
   | Column  |          | the flag of the attribute in the I'th column |
   | Type    |          | on each row                                  |
   | Flags   |          |                                              |
   | [i]     | 1        | 1 - An Attribute Type value defining what    |
   | Column  |          | each row have in the i'th column position    |
   | Type    |          |                                              |
   | [i]     | 1        | 1 - The length in octets of the attribute    |
   | Column  |          | value in the i'th column on each row         |
   | Width   |          |                                              |
   | [j]     | Variable | 0 - An IP prefix which is associated with    |
   | IPv4    |          | the C-tuple of attribute values on the j'th  |
   | Prefix  |          | row of the table. The format of is the same  |
   |         |          | as the IP prefix field in the IPv4-Add-      |
   |         |          | Prefix attribute                             |
   +---------+----------+----------------------------------------------+

   Below is an example of an IPv4-Add-Table attribute which contain a
   single SGT attribute value on each row.

























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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    |O|N|P|C|E| | | | IPv4-Add-     | TLV Length    |   Reserved    |
    | | | | | | | | |  Table        | (C = 1 EL = 0)|               |
    |0|0|0|1| | | | |  Type = 21    |-------------------------------+
    | | | | | | | | |               | TLV Extended Length (C=1 EL=1)|
    +-+-+-+-+-+-+-+-+---------------+---------------+---------------+
    | Number of     |                Reserved                       |
    | Columns 1     |                                               |
    +-------------------------------+-------------------------------+
    | Source-Group  | Column        |           Reserved            |
    | Tag Type = 17 | Width = 2     |                               |
    +-------------------------------+-------------------------------+--+
    |     '1'SGT Value              |'1'Prefix      |'1'IPv4 Prefix    |
    |                               | Length        |                  |
    +-------------------------------+---------------+------------------+
    x         ***                   x    ***        x         ***      x
    x-------------------------------+---------------+------------------x
    |     'R'SGT Value              |'R'Prefix      |'R'IPv4 Prefix    |
    |                               | Length        |                  |
    +-------------------------------+---------------+------------------+


4.5.1.8.  IPv6-Add-Table Attribute

   This attribute provides a flexible tabular representation of IPv6
   bindings information.






















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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
    |O|N|P|C|E| | | | IPv4-Add-     | TLV Length    |    Reserved   |
    | | | | | | | | |  Table        | (C = 1 EL = 0)|               |
    |0|0|0|1| | | | |  Type = 22    |-------------------------------+
    | | | | | | | | |               | TLV Extended Length (C=1 EL=1)|
    +-+-+-+-+-+-+-+-+---------------+-------------------------------+
    | Number of     |                Reserved                       |
    | Columns C     |                                               |
    +---------------+---------------+---------------+---------------+
    |'1'Column Type | '1'Column Type| '1'Column     |'2'Column Type |
    |   Flags       |               |   Width       |   Flags       |
    +---------------+---------------+---------------+---------------+
    |'2'Column Type | '2'Column     |                               |
    |               |    Width      |           ***                 |
    +---------------+---------------+---------------+---------------+
    x                           ***                                 x
    +---------------+---------------+---------------+---------------+
    |'C'Column Type | 'C'Column Type| 'C'Column     |    Reserved   |
    |   Flags       |               |   Width       |               |
    +---------------+---------------+-------------------------------+--+
    | '1''1' Value  |     ***       |'1''C'Value    |'1'IPv6 Prefix    |
    |               |     ***       |               |  (Variable)      |
    +---------------+---------------+---------------+------------------+
    x         ***   x     ***       x    ***        x         ***      x
    x---------------+---------------+---------------+------------------x
    | 'R''1' Value  |     ***       |'R''C'Value    |'R'IPv6 Prefix    |
    |               |     ***       |               |  (Variable)      |
    +---------------+---------------+---------------+------------------+


   The format and handling of IPv6-Add-Table attribute are the same as
   what is described above in IPv4-Add-Table section except that the
   attribute type (22) is used and the trailing IPv6 Prefix field in
   each row could be potentially longer than the corresponding trailing
   IPv4 Prefix fields.

4.5.2.  UPDATE Message Samples

   Single IPv4 host binding as exported by the first Speaker away from
   the originating node:








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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
 | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+------
 |              SXP Message Length = 32                          |SXP
 +---------------------------------------------------------------+Msg
 |              SXP Message Type = UPDATE (3)                    |Header
 +---------------------------------------------------------------+------
 |O|N|P|C|E| | | |Peer-Sequence  | TLV Length    |   Reserved    |
 |0|0|0|1|0| | | | Type = 16     |    8          |               |
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+Peer-
 |                  Local SXP ID                                 |Seq-
 |                                                               |Attr
 +---------------------------------------------------------------+
 |SXP ID of Speaker end of the connection on which it was        |
 |             originally received                               |
 +---------------------------------------------------------------+-----
 |O|N|P|C|E| | | |Source-Group-| TLV Length    |    Reserved   |
 | | | | | | | | | Tag           |    2          |               |Src-
 |0|0|0|1|0| | | | Type = 17     |               |               |Grp-
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+Tag
 |    SGT Value = <sgt#>         |             Reserved          |Attr
 |                               |                               |
 +---------------------------------------------------------------+-----
 |O|N|P|C|E| | | |IPv4-Add-Prefix| TLV Length    |Prefix Length  |IPv4-
 | | | | | | | | | Type = 11     |    5          |     32        |Add-
 |0|0|0|1|0| | | |               |               |               |Pre-
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+fix
 |                  IPv4 Host Address                            |Attr
 +---------------------------------------------------------------+-----

   Multiple IPv4 host and subnet bindings sharing the same Peer-Sequence
   as exported by the first Speaker away from the originating node. 11
   subnets bindings of length between 17 and 24 and as many host
   bindings as SXP total message size allowed are packed into this
   message:

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
 | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+------
 |              SXP Message Length = 4096                        |SXP
 +---------------------------------------------------------------+Msg
 |              SXP Message Type = UPDATE (3)                    |Header
 +---------------------------------------------------------------+------
 |O|N|P|C|E| | | |Peer-Sequence  | TLV Length    |   Reserved    |
 |0|0|0|1|0| | | | Type = 16     |    8          |               |
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+Peer-



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 |                  Local SXP ID                                 |Seq-
 |                                                               |Attr
 +---------------------------------------------------------------+
 |SXP ID of Speaker end of the connection on which it was        |
 |             originally received                               |
 +---------------------------------------------------------------+-----
 |O|N|P|C|E| | | |IPv4-Add-Table | TLV Length    |    Reserved   |IPv4-
 | | | | | | | | |               |3 + 11x6 +572x7|               |Add-
 |0|0|0|1|0| | | | Type = 21     | = 4073        |               |Table
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+Attri-
 |Number of      |               Reserved                        |-bute
 |Columns 1      |                                               |
 +---------------------------------------------------------------+------
 |Source         | Column        |         Reserved              |Col
 |Group Tag      |  Width = 2    |                               |Head
 |Type = 17      |               |                               |ers
 +-+-+-+-+-+-+-+-+---------------+-------------------------------+------
 | '1' SGT Value                 |'1'Prefix      |   Reserved    |
 |                               |Length  =      |               |
 |                               | [17..24]      |               |
 |-------------------------------+---------------+---------------+
 |        '1' IPv4 Subnet Prefix                 |   Reserved    |Subnet
 +-----------------------------------------------+---------------+Bind-
 *                                               *               |ings
 *                                               *   Reserved    |Rows
 * '2..10' <SGT Value, IPv4 Subnet Prefix> rows  *               |
 +-----------------------------------------------+---------------+
 | '11' SGT Value                |'11'Prefix     |    Reserved   |
 |                               |Length  =      |               |
 |                               | '17..24'      |               |
 |-------------------------------+---------------+---------------+
 |        '11' IPv4 Subnet Prefix                |   Reserved    |
 +-----------------------------------------------+---------------+----
 | '1' SGT Value                 |'1'Prefix      |  Reserved     |
 |                               |Length  = 32   |               |
 |-------------------------------+---------------+---------------+
 |                  '1'IPv4 Host Address                         |
 +-----------------------------------------------+---------------+
 x                                               |   Reserved    |Host
 x '2..571' <SGT Value, 32, IPv4 Host Address>   |               |Bind-
 x                                               +---------------+ing
 x                                                               |Rows
 x---------------------------------------------------------------+
 | '572' SGT Value               |'572'Prefix    |   Reserved    |
 |                               |Length  = 32   |               |
 +-------------------------------+---------------+---------------+
 |                  '572'IPv4 Host Address                       |
 +---------------------------------------------------------------+-----



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4.6.  SXP ERROR Message

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | | | | | | | | | | |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-++-+-+-+-+-+-+-+-+-+
      | |   0 (E=0)   |  0 (E=0)      |  non-Extended Error Code (E=0)|
      |E+-------------+---------------+-------------------------------|
      | |Error Code   |Error Sub-code |                               |
      | |  (E=1)      |  (E=1)        |                               |
      +-+-----------------------------+                               |
      |             Data (Variable)                                   |
      +---------------------------------------------------------------+

   +--------------+--------+----------+--------------------------------+
   | Field        | Bits   | Length   | Description                    |
   |              |        | (in      |                                |
   |              |        | Octets)  |                                |
   +--------------+--------+----------+--------------------------------+
   | Extended     | 0      |          | 1 - Extended Error format,     |
   |              |        |          | which includes code, sub-code, |
   |              |        |          | and variable data 0 - Legacy   |
   |              |        |          | non-Extended Error             |
   | Error Code   | [1..7] | 0        | Unsigned integer in the range  |
   |              |        |          | [0..127] - indicates the type  |
   |              |        |          | of ERROR                       |
   | Error Sub-   |        | 1        | Unsigned integer in the range  |
   | Code         |        |          | [0..255] - Provides extended   |
   |              |        |          | information about the nature   |
   |              |        |          | of the reported error. Each    |
   |              |        |          | Error code may have one or     |
   |              |        |          | more Error Sub-codes           |
   |              |        |          | associated with it. If no      |
   |              |        |          | appropriate Error Sub-code is  |
   |              |        |          | defined, then zero             |
   |              |        |          | (Unspecified) value is used    |
   | non-Extended |        | 2        | Error code values: 0 - No      |
   | Error Code   |        |          | error 1 - Version Mismatch 2 - |
   |              |        |          | Message Parse Error            |
   | Data         |        | Variable | Additional data for diagnosing |
   |              |        | Message  | the reason for the ERROR       |
   |              |        | Length - | message.  The content depends  |
   |              |        | 10       | on the values of Error Code    |
   |              |        |          | and Error Sub-code             |
   +--------------+--------+----------+--------------------------------+






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4.6.1.  Error Codes

       +-----------+-----------+-------------------------------------+
       | Error     | Error     |          Symbolic Name              |
       |  Code     | Sub-code  |                                     |
       +-----------+-----------+-------------------------------------+
       |    1      |    -      |     Message Header Error            |
       |           |           |                                     |
       +-----------+-----------+-------------------------------------+
       |    2      |    -      |    OPEN Message Error               |
       |           |           |                                     |
       +-----------+-----------+-------------------------------------+
       |    3      |    -      |     UPDATE Message Error            |
       |           |           |                                     |
       +-----------+-----------+-------------------------------------+
       |           |    1      |  Malformed Attribute List           |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    2      |  Unrecognized Well-known Attribute  |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    3      |  Missing Well-known Attribute       |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    4      |  Attribute Flags Error              |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    5      |  Attribute Length Error             |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    6      |  Malformed Attribute                |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    7      |  Optional Attribute Error           |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    8      |  Unsupported Version Number         |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    9      |  Unsupported Optional Attribute     |
       |           |           |                                     |
       +           +-----------+-------------------------------------+
       |           |    10     |  Unacceptable Hold Time             |
       |           |           |                                     |
       +-----------+-----------+-------------------------------------+






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4.7.  SXP PURGE-ALL Message

   There is no payload corresponding to this message type.

   When an SXP connection on which the local SXP node is a speaker is
   administratively deleted, SXP MUST send a PURGE-ALL message to the
   SXP listener at the remote end.  This will provide the speaker timely
   knowledge that the connection is about to be torn down and that it is
   not an intermittent loss of communication.  The listener will
   immediately delete all bindings that were received on that connection
   and will not go through Delete Hold-Down timer before removal of
   bindings.

   When SXP feature gets administratively disabled, SXP MUST send PURGE-
   ALL message on all the connections on which the local end is a
   speaker.

   Upon receiving a PURGE-ALL message, an SXP speaker MUST immediately
   delete all the bindings that were received on that connection.
   PURGE-ALL message should have the same effect as an UPDATE message
   with IPv4-Delete-Prefix or IPv6-Delete-Prefix attributes that contain
   all bindings which has been exported on the connection.  A PURGE-ALL
   is simply an optimization of such delete at-once case.

4.8.  SXP KEEPALIVE Message

   There is no payload corresponding to this message type.  This message
   is send by the speaker to the listener side when KEEPALIVE timer
   expires.

5.  Update Message Handling

   An SXP UPDATE message may be received only in the ON state.
   Receiving an UPDATE message in any other state is an error.  When an
   UPDATE message is received, each field is checked for validity.

5.1.  UPDATE Message Validation

   UPDATE message SHALL be organized as illustrated below:












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      +-------------------------------------------------------------+
      |Zero or more global optional attributes which are unrelated  |
      |to any of the binding delete or add attributes or groups     |
      |             below them                                      |
      +-------------------------------------------------------------+
      |At most one each IPv4-Delete-Prefix or IPv6-Delete-Prefix    |
      +-------------------------------------------------------------+
      |Zero or more Del-IPv4 or Del-IPv6 (non-zero only when neither|
      |  IPv4-Delete-Prefix nor IPv6-Delete-Prefix are present)     |
      +-------------------------------------------------------------+

   All errors detected while processing the UPDATE message MUST be
   indicated by sending the ERROR message with the Error Code UPDATE
   Message Error to the SXP speaker from which the UPDATE message was
   received.  The Error Sub-code provides additional information about
   the error.  UPDATE message is a sequence of attributes.  Attributes
   could be further classified as compact, compact with extended length,
   or non-compact attributes.  Each attribute consists of a fixed-size
   header, 3, 4, or 8 octets long respectively, which contains TLV
   Length field.  The total length of each attribute is thus: Attribute
   Length = 3/4/8 + TLV Length If the length of any attribute is larger
   than the SXP Message length or the sum of Attribute Length of all
   attributes is larger than the SXP Message length, the Error Sub-code
   MUST be set to Malformed Attribute List.  If any recognized attribute
   has Attribute Flags that conflicts with its Attribute Type, then the
   Error Sub-code MUST be set to Attribute Flags Error.  The Data field
   MUST contain the erroneous attribute (flags, type, length, and
   value).  If any recognized attribute has Attribute Length that
   conflicts with the expected length (based on its Attribute Type),
   then the Error Sub-code MUST be set to Attribute Length Error.  The
   Data field MUST contain the erroneous attribute (flags, type, length,
   and value).

   If any recognized attribute does not conform to the attribute
   specification in UPDATE Attributes section, then Error Sub-code MUST
   be set to Malformed Attribute.  The Data field MUST contain the
   erroneous attribute (flags, type, length, and value).  Cases of
   malformed attributes:

   o IP Prefix length field is outside of the permitted range [0..32]/
   [0..128] for IPv4/IPv6 respectively.

   o Implied length of IP Prefix field which extends the prefix beyond
   the extent of the containing attribute.

   If any attribute appears more than once when at most one occurrence
   is permitted then the Error Sub-code MUST be set to Malformed
   Attribute List.



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   If any attribute appears in a location where it is not expected (such
   as IPv4-Add-Prefix/IPv6-Add-Prefix without preceding Source-Group-
   Tag) then the Error Sub-code MUST be set to Malformed Attribute List.

   If an optional non-transitive attribute is unrecognized, it is
   quietly ignored.

   If an optional transitive attribute is unrecognized, the Partial bit
   (the third high-order bit) in the attribute flags octet is set to 1,
   and the attribute is retained for export according to the scope in
   which the attribute appears.  A global attribute is exported along
   every binding specified in this UPDATE message on all connections for
   which the local peer is a speaker.  A path attribute is exported
   along the bindings from a single path.  A per <path, source-group>
   attribute is exported along bindings received from a single path
   which share a single Source-Group-Tag attribute.  If an optional
   attribute is recognized and has a valid value, then, depending on the
   type of the optional attribute, it is processed locally, retained,
   and updated, if necessary, for possible export to listener peers.  A
   Peer-Sequence attributes is checked for syntactic correction.  If the
   path is syntactically incorrect (e.g. length is not a multiple of 4),
   then an ERROR message MUST be sent to the speaker with Error Sub-code
   set to Malformed Attribute.  SXP listener learns the SXP Node-ID from
   the SXP Node-ID in the OPEN or OPEN_RESP it receives at connection
   establishment.  This Node-ID is used to validate subsequent Peer-
   Sequence attributes from the same peer.  SXP MUST check whether the
   leftmost (with respect to the position of octets in the protocol
   message) SXP Node-ID in the Peer-Sequence is equal to the SXP Node-ID
   of the peer that sent the UPDATE message.  If the checks determines
   that this is not the case, then an ERROR message MUST be sent to the
   speaker with Error Sub-code set to Malformed Attribute.

5.2.  UPDATE Message processing

   Processing of an UPDATE message by an SXP listener follows the
   organization of the message according to the following high level
   steps:

   1.  UPDATE Message validation as described in section 6.1.

   2.  Processing of global optional attributes.

   3.  Processing of binding delete attributes.  Binding delete
       attributes include any of IPv4-Del-Prefix, IPv6-Del-Prefix, Del-
       IPv4, or Del-IPv6 attributes.

   4.  Processing path-groups.




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       A.  Process per-path common optional attributes

       B.  Add-Prefix groups.  Each Add-Prefix group starts with a
           Source-Group-Tag attribute.

              i. Process per <path, SGT> optional attributed.

              ii.  Process IPv4-Add-Prefix attribute

              iii.  Process IPv6-Add-Prefix attribute

       C.  Processing IPv4-Add-Table attribute

       D.  Processing IPv6-Add-Table attribute

       E.  Processing Add-IPv4 attributes

       F.  Processing Add-IPv6 attributes

   5.  Processing trailing optional non-transitive attributes

   Each path-group starts with an instance of Peer-Sequence attribute.

   Processing Binding Delete Attributes (step 3)

   If the UPDATE message contains a non-empty IPv4-Del-Prefix/IPv6-Del-
   Prefix or one or more Del-IPv4/Del-IPv6 attributes, the previously
   received bindings from the remote speaker peer, whose IP/IPv6
   addresses are contained in any of those attributes, SHALL be removed
   from the SXP input bindings database.  Additional processing of a
   deleted binding depends on whether it was the SXP contributed binding
   for its IPv4/IPv6 address and whether it was the selected and
   exported binding by the master binding data-base:

   o  Non-contributed binding - No further processing is needed.

   o  Contributed but not exported - The binding was the preferred
      binding by SXP and was reported to the master binding database.
      However, the binding was not exported by SXP due to higher
      priority contributors in the master binding database.  SXP SHALL
      select a new contribution for the IPv4/IPv6 address of the deleted
      binding from the input bindings database, and, upon change of SGT
      or other associated data that are reported to the master bindings
      database, report the newly selected binding to the master binding
      database.  The newly selected contribution will not affect the
      exported binding since an existing higher priority contributor is
      unaffected.




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   o  Contributed and Exported - The binding was the preferred binding
      by SXP and was the selected binding for the IPv4/IPv6 address by
      the master binding database.  SXP contribution was either the only
      binding contributor or the highest priority contributor.

   SXP SHALL select a new contribution for the IPv4/IPv6 address of the
   deleted binding among the bindings for that address from other peers.

   SXP SHALL apply the following rules for selecting a binding for
   reporting to the master binding database:

   1.  Shortest Path rule

   Choose a binding which has the shortest Peer-Sequence among all the
   bindings for the same IP address.  A binding received without Peer-
   Sequence attribute (i.e. from an earlier version speaker) is
   considered as being received with a Peer-Sequence that contains a
   single NULL SXP Node-ID (value 0).

   1.  Most Recently Received rule

   If there are more than one bindings with the shortest Peer-Sequence,
   the binding which has been most recently received is selected.

   If the deleted binding was the only binding for its IP address in the
   SXP input binding database, SXP SHALL report it to the master binding
   database.  The external outcome when no other contributors to the
   same IP address is present, is that a deletion of the binding using
   IPv4-Del-Prefix/IPv6-Del-Prefix or Del-IPv4/Del-IPv6 attribute MUST
   be exported on all connections on which the local end is a speaker.

   If there is one or more additional contributors in the master binding
   database, a new binding is selected and it MUST be exported using
   IPv4-Add-Prefix/IPv6-Add-Prefix, IPv4-Add-Table/IPv6-Add-Table, or
   Add-IPv4/Add-IPv6 and associated with a Peer-Sequence that includes
   only the SXP Node ID of the local instance.

   If the binding selection rule has yielded a newly selected binding
   from SXP input binding database it SHALL be compared to the binding
   it is replacing, and

   o  upon change of SGT, or, other associated data that are reported to
      the master binding database, report the newly selected binding to
      the master binding database.  The newly selected contribution will
      become the selected contribution and will trigger export.

   o  If the SGT of the newly selected binding, and all other associated
      data that are reported to the master binding database, remain the



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      same as those of the deleted binding, SXP SHOULD NOT report a new
      contribution to the master binding database.  However, the newly
      selected binding has a different Peer-Sequence attribute because
      binding from a different peer is selected.  There could also be
      changes in optional transitive attributes.  SXP SHALL export the
      newly selected binding directly and include all transitive
      attributes, thus bypassing the usual notification path from the
      master bindings database.

   Processing Binding Add Attributes (step 4)

      I. Implicit delete of current bindings

      If the UPDATE message contains a non-empty IPv4-Add-Prefix/IPv6
      -Add-Prefix/IPv4-Add-Table/IPv6-Add-Table or one or more Add-IPv4/
      Add-IPv6 attributes, the previously received bindings from the
      remote speaker peer (if any), whose IP/IPv6 addresses are
      contained in any of those attributes, SHALL be removed from the
      SXP input bindings database and replaced with the new bindings for
      those IPv4/IPv6 addresses.  An added binding is an implicit delete
      of an existing binding for the same IP address from the same peer.
      The external behavior of the implicit deletion is as described for
      the processing of IPv4-Del-Prefix/IPv6-Del-Prefix or Del-IPv4/Del-
      IPv6 attributes where the deleted binding is the last binding for
      its IP address.

      II.  Loop Detection

      SXP MUST check the Peer-Sequence associated with each added
      binding for the presence of loops before accepting the binding as
      a new binding or as a replacement for an implicitly deleted
      binding.  SXP loop detection is done by scanning the entire Peer-
      Sequence attribute, and checking that the SXP Node ID of the local
      system does not appear in the Peer-Sequence.

      III.  New or Replaced Bindings

      SXP SHALL keep the most recently received binding for each IP
      address in the input binding database.  SXP SHALL keep all the
      mandatory attributes and any optional transitive attributes and
      MAY keep other non-transitive attributes.  A subset of the
      attributes, such as Source-Group-Tag, are used locally on the
      system and have to be reported to the master binding database.
      The master binding database arbitrates among multiple binding
      contributors for the same IP address.  SXP SHALL report new
      bindings to the master binding database along with the attributes
      the master database is aware of.  The arbitration process in the
      master binding database yields a selected binding contributor for



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      each IP address.  The binding contributor could be a binding
      source other than SXP.  SXP MUST NOT export any binding it has
      received in UPDATE message unless it has survived the arbitration
      process in the master binding database to become the selected
      binding.

      SXP SHALL become aware of the outcome of the arbitration process
      within the master binding database.  The arbitration process could
      be triggered by binding add or delete reported by SXP or binding
      changes from other binding contributors.  SXP NEED NOT be aware of
      the presence of other binding contributors, their nature, or how
      many such contributors exist.  SXP is only aware of the possible
      existence of such binding contributors and only need to know
      whether a selected binding was contributed by SXP itself or by
      some other contributor.  The mechanism by which SXP becomes aware
      of changes of selected bindings in the master binding database is
      an implementation detail.

5.3.  Generating UPDATE Message

   Binding Removal Event

   If SXP becomes aware of a deletion of a binding for an IPv4/IPv6
   prefix it then processes the event as follows:

   o  The IPv4/IPv6 prefix SHALL be included in either an IPv4-Del-
      Prefix/IPv6-Del-Prefix or a single Del-IPv4/Del-IPv6 attribute of
      an outgoing UPDATE message.  SXP MAY delay exporting of such event
      for a small and bound duration in order to allow for aggregation
      as long as such aggregation will not exhibit different external
      behavior from individually exported events.

   Binding Change Event

   If SXP becomes aware of a new or changed binding for an IPv4/IPv6
   prefix it then processes the event as follows:

   o  If the selected contributor is not SXP, SXP SHALL associate the
      binding with a Peer-Sequence that contains the local SXP Node ID
      as the only peer and export the binding in one of IPv4-Add-Prefix/
      IPv6-Add-Prefix, IPv4-Add-Table/IPv6-Add-Table, or Add-IPv4/Add-
      IPv6.  SXP SHALL include in the outgoing UPDATE message the
      attributes that are available from the master binding database.

   o  If the selected contributor is SXP, SXP MUST locate the
      contributed binding in its input binding database and extract the
      Peer-Sequence that was associated with the binding when it was
      originally received.  SXP SHALL prepend the local SXP Node ID to



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      the Peer-Sequence when it is added to an outgoing UPDATE message.
      SXP SHALL include in the UPDATE message the mandatory attributes
      reported by the master binding database and any additional well-
      known or optional transitive attributes that were associated with
      the binding upon its arrival.  SXP MAY add additional optional
      transitive or non-transitive attributes.

   SXP SHALL flag an SXP originated binding in its input bindings
   database in order to support UPDATE message processing as specified
   in this section.

6.  SXP Failure Scenarios

   The following SXP failure scenarios are analyzed in this section:

   o  SXP Connection Failure

   o  Operational (Hardware) Failures

   SXP connection can fail for various reasons:

   o  Peer device offline

   o  Loss of network connectivity

   o  Loss message authentication key synchronization with the peer.  In
      this case the device re-attempts message exchange for a fixed
      number of times and if the problem persists then the connection is
      torn down.

   o  Software processing failures, e.g. SXP message decode failure.
      SXP version mismatch and SXP message parse error will result in
      the device sending an Error TLV.  When sending an error message,
      the error code is passed as part of the TLV and the action
      decision is left up to the peer device.  This error code COULD be
      simply logged for debugging purposes.  The SXP connection is
      closed and retries to setup after sending the SXP ERROR message.

7.  SXP Timers

   There are 5 SXP timers defined in SXP protocol:

   o  Retry open timer

      *  Retry open timer is triggered as long as there is one SXP
         connection on the device that is not up.





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      *  The default timer value is 120 seconds.  Value 0 means retry
         timer will not be started.

      *  The retry continues until the SXP connection is setup or the
         retry timer is configured to be 0.

   o  Delete Hold Down Timer

      *  Delete hold down timer is trigger when a connection is on
         listener side is torn down.  The bindings learnt are not
         deleted immediately but being held off for the delete hold down
         timer period.

      *  The bindings are deleted upon the expiry of this timer.

      *  The timer value is set to 120 seconds and it is not
         configurable.

   o  Reconciliation Timer

      *  If a SXP connection is brought up within the delete hold down
         timer period, bindings are re-populated from the speaker side.
         At the same time, the old bindings learnt before the connection
         goes down still holds.

      *  Reconciliation timer starts right after the connection is
         brought up to wait for the new bindings to be forwarded from
         the peer.

      *  Upon the timer expiry, SXP checks the bindings in its input
         binding database and delete any stale bindings.  Those are
         bindings that could have been deleted on the remote side while
         the connection was down.

      *  The default timer value is 120 seconds.  Value 0 means
         reconciliation timer will not be started.

   o  Hold Timer

      *  Hold Timer MAY be used by an SXP Listener for detect when a
         connection is no longer live.

      *  The usage of Hold Timer and KEEPALIVE is negotiated during OPEN
         /OPEN_RESP exchange.

      *  The Hold Timer is started when the connection reached ON state.
         The interval is set to the negotiated Hold Time.




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      *  The Hold Timer is restarted whenever a listener receives a
         KEEPALIVE or an UPDATE message.

      *  If a listener does not receive successive KEEPALIVE, and/or
         UPDATE messages within the period negotiated for the Hold Time
         of a connection, the Hold Timer expires.

      *  Upon the timer expiry, SXP MUST send an ERROR message with Hold
         Timer Expired code and tear down the connection.

      *  The rest of the behavior is the same as when TCP indicates to
         SXP the loss of a connection.  The Delete Hold Down timer is
         restarted for delayed deletion of bindings as described above.

      *  The suggested period of the Hold Timer is 90 seconds.  SXP
         implementation MAY allow for local configuration of Hold Time
         for a listener.  The actual value of the Hold Time used for a
         connection is negotiated.

   o  Keepalive Timer

      *  Keepalive Timer MAY be used by an SXP Listener for triggering
         sending KEEPALIVE messages to the listener peer which monitors
         a connection using Hold Timer.

      *  The usage of Keepalive Timer and KEEPALIVE messages is
         negotiated during OPEN/OPEN_RESP exchange.

      *  The Keepalive Timer is restarted for the first time when the
         connection reaches its ON state.

      *  The Keepalive Timer is restarted thereafter, when a speaker
         sends an UPDATE or KEEPALIVE message.

      *  Upon timer expiry, SXP speaker MUST send KEEPALIVE message in
         order to indicate to the listener that the connection remains
         live.

      *  The suggested period of the Keepalive Timer is 30 seconds.  SXP
         implementation MAY allow for local configuration of Keepalive
         Time for a speaker.  The actual value of the Keepalive Time
         used for a connection is set to 1/3 of the negotiated Hold
         Time.

      *  The Keepalive Timer is restarted with a random jitter.  The
         actual period of the Keepalive Timer is set to a random value
         between 0.75 to 1.0 of the negotiated Keepalive Time which is
         re-calculated every time the timer is restarted.



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8.  SXP Version Negotiation

   In a software release, SXP always supports the highest version in
   that release and any version lower than that.  SXP version
   negotiation is per connection base.  On the same device, the
   connections established between this device and the peer devices may
   have difference versions, depending on the SXP version running on the
   peer devices.

   If the two ends of a SXP connection run the same SXP version, the
   running version is used.  If the two ends are running different
   versions, the version of the connection is decided with following
   process:

   When the initiator of a connection sends a SXP OPEN message, the
   highest supported version is coded in the OPEN message.

   On the receiver side, if the version in the OPEN message is higher
   than the version it can support, it sends back OPEN RESPONSE message
   with the highest version it supports (which is lower than the version
   in OPEN message).  When the OPEN RESPONSE message is received by the
   initiator, it accepts the version coded in the OPEN RESPONSE.  The
   receiver side's version is used as the connection's version.

   In summary, SXP connection picks the highest version that is
   supported on both sides.

8.1.  SXP Versions

   SXP supports three versions starting from version 2 to 4.  The
   details of what is supported in each of the version follows -

   +-----+----------+----------+------------+-----------+--------------+
   | Ver | IPv4     | IPv6     | Subnet     | Loop      | SXP          |
   |     | Bindings | Bindings | Binding    | Detection | Capability   |
   |     |          |          | Expansion  |           | Exchange     |
   +-----+----------+----------+------------+-----------+--------------+
   | 2   | Yes      | Yes      | No         | No        | No           |
   | 3   | Yes      | Yes      | Yes        | No        | No           |
   | 4   | Yes      | Yes      | Yes        | Yes       | Yes          |
   +-----+----------+----------+------------+-----------+--------------+

9.  Security Considerations

   SXP carries mappings between IP addresses and source groups.  An
   adversary that can modify the data can potentially gain an advantage
   over the system.  This specification defines the use of TCP MD5
   Signature Option [RFC5925] to provide integrity protection for the



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   information.  The key used with TCP MD5[RFC1321] should be chosen to
   be long and have high entropy to provide as much protection from
   dictionary attacks as possible.

   MD5 has been shown to contain weaknesses so this protection may not
   be sufficient in many environments.  In addition, an adversary who
   can observe the data may be able to use the information learned to
   identify weak or highly valued targets so it is desirable that
   confidentiality also be provided.  A future revision of this
   specification will provide more robust security mechanisms such as
   ones based on IPSEC or TLS.

10.  Implementation Note

   Current implementations of the protocol support source and peer IP
   addresses to be IPv4 only.  However, the implentation should take
   care of being able to support IPv6 addresses as well in the future.
   As such, there are not changes expected to be done in the protocol
   itself to support IPv6 address based connections.

   The SXP password can be upto 80 ASCII characters.

11.  IANA Considerations

   TBD

12.  IPR Disclosure

   "By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79."

13.  Copyright Notice and Disclaimer

   "Copyright (C) The IETF Trust (year).  This document is subject to
   the rights, licenses and restrictions contained in BCP 78, and except
   as set forth therein, the authors retain all their rights."
   Additional copyright notices are not permitted in IETF Documents
   except in the case where such document is the product of a joint
   development effort between the IETF and another standards development
   organization or the document is a republication of the work of
   another standards organization.  Such exceptions must be approved on
   an individual basis by the IAB.

   "This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY , THE



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   IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
   WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
   WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY
   RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
   PARTICULAR PURPOSE."

14.  Acknowledgements

   Special acknowledgements are due to the following contributors.  The
   protocol was originally developed by Michael Smith, Michael Fine and
   Awais Nemat.  Enhancements have been made over the years by Ronen
   Arad, Rajesh Bhandari, Lei Fu, and Paddy Nallur.  Darrin Miller
   provided significant input into requirements for later versions of
   the protocol.

15.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

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

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

Authors' Addresses

   Michael Smith
   Cisco Systems
   210 West Tasman Drive
   San Jose, California  95134
   United States

   Email: michsmit@cisco.com


   Rakesh Reddy Kandula
   Cisco Systems
   Cessna Business Park, Kadubeesanahalli Village
   Sarjapur/Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: krreddy@cisco.com






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