Internet DRAFT - draft-xu-savax-data

draft-xu-savax-data







Network Working Group                                              K. Xu
Internet-Draft                                                     J. Wu
Intended status: Standards Track                                 X. Wang
Expires: 25 May 2024                                 Tsinghua University
                                                                  Y. Guo
                                                 Zhongguancun Laboratory
                                                        22 November 2023


   Data Plane of Inter-Domain Source Address Validation Architecture
                         draft-xu-savax-data-05

Abstract

   Because the Internet forwards packets according to the IP destination
   address, packet forwarding typically takes place without inspection
   of the source address and malicious attacks have been launched using
   spoofed source addresses.  The inter-domain source address validation
   architecture is an effort to enhance the Internet by using state
   machines to generate consistent tags.  When communicating between two
   end hosts at different ADs of the IPv6 network, tags will be added to
   the packets to identify the authenticity of the IPv6 source address.

   This memo focuses on the data plane of the SAVA-X mechanism.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 25 May 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
     2.1.  Terminology and Abbreviation  . . . . . . . . . . . . . .   3
   3.  State Machine Mechanism . . . . . . . . . . . . . . . . . . .   4
   4.  Tag . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Tag Generation Algorithm  . . . . . . . . . . . . . . . .   5
       4.1.1.  Pseudo-Random Number Algorithm  . . . . . . . . . . .   5
       4.1.2.  Hash Chain Algorithm  . . . . . . . . . . . . . . . .   6
     4.2.  Tag Update  . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Packet Processing at AER  . . . . . . . . . . . . . . . . . .   8
     5.1.  Port Classification . . . . . . . . . . . . . . . . . . .   9
     5.2.  Source Address Validation . . . . . . . . . . . . . . . .   9
     5.3.  Packet Classification . . . . . . . . . . . . . . . . . .   9
     5.4.  Tag Addition  . . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Tag Verification  . . . . . . . . . . . . . . . . . . . .  10
     5.6.  Tag Replacement . . . . . . . . . . . . . . . . . . . . .  11
   6.  Packet Signature  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  MTU Consideration . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Consideration  . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The Inter-Domain Source Address Validation (SAVA-X) mechanism
   establishes a trust alliance among Address Domains (AD), maintains a
   one-to-one state machine among ADs, generates a consistent tag, and
   deploys the tag to the ADs' border router (AER).  The AER of the
   source AD adds a tag to identify the identity of the AD to the packet
   originating from one AD and sinking in another AD.  The AER of the
   destination AD verifies the source address by validating the
   correctness of the tag to determine whether it is a packet with a
   forged source address.





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   In the process of packet forwarding, if the source address and the
   destination address of this packet both belong to the trust alliance,
   but the tag is not added or incorrectly added, the AER of the
   destination AD determines that the source address is forged and
   directly discards this packet.  The destination AD forwards the
   packet directly for packets whose source address is an address
   outside the trust alliance.

   This document mainly studies the relevant specifications of the data
   plane of the inter-domain source address validation architecture
   mechanism between ADs, which will protect IPv6 networks from being
   forged source address.  See [RFC8200] for more details about IPv6.
   It stipulates the state machine, tag generation and update, tag
   processing in AER, and packet signature Its promotion and application
   can realize the standardization of the data plane in the SAVA-X to
   facilitate the related equipment developed by different manufacturers
   and organizations to cooperate to accomplish the inter-domain source
   address validation jointly.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.1.  Terminology and Abbreviation

   +==============+====================================================+
   | Abbreviation | Description                                        |
   +==============+====================================================+
   | ACS          | AD Control Server.  The server maintains           |
   |              | the state machine with other ACS and               |
   |              | distributes information to AER.                    |
   +--------------+----------------------------------------------------+
   | AD           | Address Domain.  The unit of a trust               |
   |              | alliance.  It is an address set                    |
   |              | consisting of all IPv6 addresses                   |
   |              | corresponding to an IPv6 address prefix.           |
   +--------------+----------------------------------------------------+
   | ADID         | The identity of an AD.                             |
   +--------------+----------------------------------------------------+
   | ADID_Rec     | The record of a number of an AD.                   |
   +--------------+----------------------------------------------------+
   | AER          | AD border router, which is placed at the           |
   |              | boundary of an AD of STA.                          |
   +--------------+----------------------------------------------------+



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   | API_Rec      | The record of the prefix of an AD or               |
   |              | STA.                                               |
   +--------------+----------------------------------------------------+
   | ARI_Rec      | The record with relevant information of            |
   |              | an AD or STA.                                      |
   +--------------+----------------------------------------------------+
   | SM           | State Machine, which is maintained by a            |
   |              | pair of ACS to generate tags.                      |
   +--------------+----------------------------------------------------+
   | TA           | Trust Alliance.  The IPv6 network that             |
   |              | uses the SAVA-X mechanism.                         |
   +--------------+----------------------------------------------------+
   | Tag          | The authentic identification of the                |
   |              | source address of a packet.                        |
   +--------------+----------------------------------------------------+

                                  Table 1

3.  State Machine Mechanism

   In SAVA-X, the state machine mechanism is used to generate, update,
   and manage the tags.

   +------+              +-------+                    +---------+
   | S_n  |     triger   | A-Box |     transition     | S_(n+1) |
   |      |------------->|       |------------------->|         |
   +------+              +-------+                    +---------+
                             | generation
                             |
                             v
                         +-------+
                         | Tag_n |
                         +-------+

                     Figure 1: State machine mechanism.

   State:  S_n and S_(n+1) represent the current state and next state of
      the SM respectively.

   Tag:  Tag_n is generated in the progress of state transiting from S_n
      to S_(n+1).

   Algorithm Box:  A-Box is Alogorithm Box. It is used to transmit the
      State and generate the tag.  It takes the current State as the
      input and the following State and current tag as the output.  The
      algorithm box consists of two parts: one is the transition
      function transit(), S_(n+1) = transit(S_n); the second is the
      function generate() to generate tags.  Tag_n = generate(S_n).  The



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      algorithm box (A-Box) is the core of the state machine.  It
      determines the data structure of state and tag, the specific mode
      of state machine implementation, as well as its security and
      complexity.

   Trigger:  It is used to trigger the transition of State.

   Transition:  It reprents the progress of state transiting from S_n to
      S_(n+1).

   Generation:  It represents the progress of calculating the current
      tag from the current State.

4.  Tag

4.1.  Tag Generation Algorithm

   There are two ways to generate tags: pseudo-random number algorithm
   and hash chain algorithm.

4.1.1.  Pseudo-Random Number Algorithm

   In the pseudo-random number generation algorithm, an initial number
   or string is usually used as the "seed", which corresponds to the
   initial state of the state machine.  Using seeds, a pseudo-random
   number sequence is generated as a tag sequence through some
   algorithm.  Next, we would take KISS (keep it simple stub), a pseudo-
   random number generation algorithm, as an example to introduce how to
   apply it to the state machine mechanism.  For the algorithm details
   of KISS, you could refer to the following reference pseudo code:

   /* Seed variables */
   uint x = 123456789,y = 362436000,z = 521288629,c = 7654321;
   uint KISS(){
      const unsigned long a = 698769069UL;
      unsigned long t;
      x = 69069*x+12345;
      y ^= (y<<13); y ^= (y>>17); y ^= (y<<5);
      t = a*z+c; c = (t>>32);
      z=cast(uint)t;
      return x+y+z;
   }

            Figure 2: KISS99: Pseudo-random number generatation







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   In this algorithm, State S can be expressed as (x, y, z, c).  The
   algorithm box is KISS().  After each calculation, the state undergoes
   a transition from S_n to S_(n+1), that is, the four variables x, y,
   z, and c are all changed.  At the same time, a pseudo-rng number (x +
   y + z) is generated.

   As the state machine shown above, the initial state is S_0 =
   (123456789, 362436000, 521288629, 7654321).  In fact, the initial
   state can be arbitrarily selected by the algorithm shown below:

   void init_KISS() {
      x = devrand();
      while (!(y = devrand())); /* y must not be zero */
      z = devrand();
      /* Don't really need to seed c as well
         but if you really want to... */
      c = devrand() % 698769069; /* Should be less than 698769069 */
   }

                 Figure 3: KISS99: Initial state selection

   The basic design goal of the pseudo-random number generation
   algorithm is mainly a long cycle and pretty distribution, however,
   without or little consideration of safety factors.  The backstepping
   security and prediction ability of the KISS algorithm have not been
   proved.

4.1.2.  Hash Chain Algorithm

   For the design of a hash chain-based tag generating algorithm, one
   can see S/Key in [RFC1760].  In the S/Key system, there is an
   encryption end and an authentication end.  The encryption end
   generates an initial state W, and then uses some hash algorithm H()
   to iterate on W to obtain a string sequence: H_0(W), H_1(W), ...,
   H_N(W), where H_n(W) represents the iterative operation of H() on W n
   times, H_0(W) = W.  The state sequence {S} is defined as the reverse
   order of the hash chain, that is, S_n = H_(N-n)(W).  For example, the
   initial state S_0 = H_N(W) and the final state S_N = H_0(W) = W, so
   the transfer function transit() is repsented as the invere H().
   Different from the KISS pseudo-random number generation algorithm
   mentioned in the previous section, in the hash chain, the tag is the
   state itself, that is, the output and input of generate() are
   consistent, and Tag_n = S_n.  In the following discussion, S_n is
   temporarily used instead of Tag_n for the convenience of expression.

   The encryption end sends the initial state S_0 to the verification
   end, and maintains S_1 ~ S_n, which is also the tag sequence used.
   The encryption end sends S_(n+1) to the verification end every time.



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   The verification end uses the S_n maintained by itself to verify the
   tag correctness of the encryption end by calculating S_(n+1) =
   transit(S_n).  As explained above, transit() is the inversion of H().
   In practice, a secure hash algorithm is usually used as H(), such as
   SHA-256.  For these hash algorithms, it is easy to calculate H(), but
   it is difficult to calculate the inversion of H().  Therefore, the
   actual operation is as follows: after receiving S_(n+1), the
   verification end calculates whether H(S_(n+1)) is equal to S_n.  If
   it is equal, the verification is successful, otherwise, it fails.

   Hash chain algorithm has high security.  It can prevent backstepping
   and prediction well.  Not only the attacker cannot backstep or
   predict, but also the verification end cannot do that.  The
   disadvantage of the hash chain algorithm is that before using tags,
   the encryption end needs to calculate all tag sequences, and then
   send the last of the sequence to the verification end as the initial
   state.  At the same time, the encryption end needs to save a complete
   tag sequence, although it can be deleted after each tag is used up.
   The cost of storage of the hash chain algorithm can not be ignored

4.2.  Tag Update

   After the state machine is enabled, the source AD uses the initial
   state S_0 to transfer to the state S_1 through the algorithm box, and
   generates the tag Tag_1.  In the subsequent state transition
   interval, the AER of the source AD uses the same tag, Tag_1, to add
   to the message sent from this AD to the destination AD.  The source
   AD does not transfer from the state S_1 to the state S_2 until the
   transition interval passes, and starts to use tag Tag_2.  In this
   cycle, the state sequence S_1 ~ S_N and tag sequence Tag_1 ~ TAG_N
   are experienced, in which Tag_1 ~ Tag_N are used as tags in turn and
   added to the message by the source AER.  Similarly, the destination
   AER uses the same state machine to calculate the tag sequence, so as
   to verify the tag in the message.  If the source AD and the
   destination AD can ensure the synchronization of the state machine,
   it would guarantee the synchronization of the tags.  So the tags can
   be verified correctly.

   Each state machine has an activation time and an Expiration Time.
   After the expiration time comes, the current state machine is
   deactivated.  If a new state machine is available, the new state
   machine will be used and perform the same label verification process.









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5.  Packet Processing at AER

   SAVA-X does not require the intermediate router to recognize and
   process the SAVA-X option, which we will describe at Section 9, as
   long as the intermediate router correctly implements the extension
   header and option processing method described in IPv6 protocol
   [RFC8200].  The intermediate router could correctly forward the
   packet regardless of its specific content even if it does not
   recognize the SAVA-X option well.

   The border router, AER, needs to handle the tag correctly.  The AER
   of the source AD judges whether the IPv6 destination address belongs
   to the trust alliance.  If no, the packet will be forwarded directly.
   If yes, the AER continues to judge the hierarchical relationship
   between the source AD and the member ADs to which the packet's
   destination IP address belongs.  If the source AD and the destination
   AD are under the same sub-trust alliance, the AER would add the tag
   between the two ADs, otherwise add the AD_V tag.

   Note that the packet will not be processed at other AERs in the sub-
   trust alliance.

   At the AER of the boundary of the sub-trust alliance, the packet is
   classified according to the IPv6 destination address.  If the
   destination address is not within the trust alliance, it will be
   forwarded directly.  If the destination address belongs to this sub-
   trust alliance, it will be classified according to the source IP
   address.  If the source address also belongs to this sub-trust
   alliance, it will be forwarded directly.  If the source address does
   not belong to this sub-trust alliance, the AER needs to verify the
   sub-trust alliance tag and replace it with the AD_V tag in this sub-
   trust alliance for following forwarding.  If the destination IP
   address of the packet belongs to another sub-trust alliance, it SHALL
   be classified according to the source address.  If the source address
   belongs to this sub-trust alliance, verify the AD_V tag.  If
   consistent, replace with a sub-trust alliance tag.  If the source
   address is not in this sub-trust alliance, it will be forwarded
   directly.  Otherwise, the packet will be discarded.

   The AER of the destination AD classifies the packet according to the
   source address of the packet to be forwarded to determine whether it
   originates from a member AD.  If yes, enter the label check.
   Otherwise, it will be forwarded directly.  Tag verification process:
   if the tag carried by the packet is consistent with the tag used by
   the source AD, remove the tag and forward the packet.  Otherwise, the
   packet will be discarded.





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5.1.  Port Classification

   In order to classify packets correctly to complete tag addition,
   inspection, and packet forwarding, it is necessary to classify the
   ports (interfaces) of AER.  Any connected port of AER must belong to
   and only belong to the following types of ports:

   *  Ingress Port: Connect to the port of the non-SAVA-X router in this
      AD.  Generally connected to IGP router in the domain.

   *  Egress Port: Connect to other AD ports.

   *  Trust Port: Connect to the port of the SAVA-X router in this AD.

5.2.  Source Address Validation

   In SAVA-X, AER must check the source address of the packet.  Only the
   packet passing the check will be subject to the Section 5.3 step, and
   the packet using the fake source IP address will be discarded.  The
   source address is checked using the ingress filtering method.  AER
   only checks the source address according to the following three
   rules:

   *  The packet entering an AER from the Ingress Port SHALL only carry
      the source address prefix belonging to this AD.

   *  The packet entering an AER from the Egress Port SHALL NOT carry
      the source address prefix belonging to this AD.

   *  Packets entering an AER from Trust Port are not checked.

   The prefix of IP address owned by one AD SHALL be configured by the
   administrator or obtained from the control plane and deployed to AER
   by ACS of this AD.

5.3.  Packet Classification

   It SHALL be classified after the packet entering an AER passes the
   source address validation.  There are three types of packets: packets
   that SHOULD be tagged, packets that SHOULD check tags and other
   messages.  The judgment rules of the three packets are as follows:










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   *  Packets entering AER from Ingress Port.  If the source address
      belongs to this AD and the IPv6 destination address belongs to
      another AD in the same sub-trust alliance, the tag must be added.
      If the source IP address belongs to another AD in the same sub-
      trust alliance and the IPv6 destination address belongs to another
      sub-trust alliance, the tag must be verified and replaced with the
      sub-trust alliance tag.  Other packets are forwarded directly.

   *  Packets entering AER from the Egress Port.  The tag must be
      checked if the source address belongs to another AD in the same
      sub-trust alliance and the IPv6 destination address belongs to
      this AD.  If the source address belongs to another sub-trust
      alliance and the IPv6 destination address belongs to another AD in
      the same sub-trust alliance, the tag must be checked and replaced.
      And other packets can be forwarded directly.

   *  Packets entering AER from Trust Port.  These packets SHOULD be
      forwarded directly.

   The relationship between IP address and ADs SHALL be obtained from
   the control plane and deployed to the AER by the ACS of the AD.  When
   the SAVA-X option of the packet received from the progress port
   carries the active AD number, you can skip the "mapping from address
   to AD number" process and directly use the AD number carried in the
   message.

5.4.  Tag Addition

   AER SHOULD add a destination option header and add the SAVA-X option
   into the packet according to the requirements of IETF [RFC8200].

   According to [RFC8200], the destination option header SHOULD be
   filled so that its length is an integer multiple of 8 bytes,
   including the Next Hader and Hdr Ext Len fields of the destination
   option header, the Next Header and Payload Length fields of the IPv6
   packet header, and the upper protocol header (such as TCP, UDP,
   etc.).  If it is necessary, AER SHOULD recalculate the Checksum
   field.

5.5.  Tag Verification

   AER will process the first option with Option Type equal to the
   binary code of 00111011 in the destination header.  It is detailed
   described at Section 9.

   1.  If the packet does not contain a destination option header or
       SAVA-X option. the packet SHOULD be discarded.




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   2.  If the packet contains the SAVA-X option but the parameters or
       tag are incorrect, the packet SHOULD be discarded.

   3.  If the packet contains the SAVA-X option, and the parameters and
       tag are correct, AER must replace the tag or remove the tag when
       needed before forwarding the message.

   In the following scenarios, the tag needs to be removed.  If there
   are only the SAVA-X option, Pad1, and PadN options in the destination
   option header of the message, AER SHOULD remove the whole destination
   option header.  If there are other options besides the SAVA-X option,
   Pad1 and PadN option in the destination option header, AER SHOULD
   remove the SAVA-X option and adjust the alignment of other options
   according to the relevant protocols of IPv6.  In order to remove the
   SAVA-X option, the destination option header may also be filled, or
   some Pad1 and PadN may be removed, to make its length multiple of 8
   bytes.  At the same time, the Next Header field and Payload Length
   field deployed in the IPv6 message header, and the Checksum field of
   the upper protocol header (such as TCP, UDP, etc.)  SHALL be
   rewritten as necessary.

5.6.  Tag Replacement

   The tag needs to be replaced when the packet passes through different
   sub-trust alliances.  Tag replacement needs to be done on the AER of
   the boundary address domain of the sub-trust alliance.  This feature
   is not necessary to realize the AER of each non-boundary address
   domain in the sub-trust alliance.

   When the packet arrives at the AER of the sub-trust alliance
   boundary, it is classified according to the destination address.

   1.  If the destination address does not belong to the trust alliance,
       it will be forwarded directly.

   2.  If the destination address belongs to this sub-trust alliance, it
       will be classified according to the source address of the packet.

       *  If the source address also belongs to this sub-trust alliance,
          the packet will be forwarded directly.

       *  If the source address does not belong to this sub-trust
          alliance, AER should verify the sub-trust alliance tag and
          replace it with the AD_V tag in this sub-trust alliance for
          forwarding.






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   3.  If the destination address of the packet belongs to another sub-
       trust alliance, it shall be classified according to the source
       address.

       *  If the source address belongs to this sub-trust alliance, AER
          should verify the AD_V tag and replace the tag with the sub-
          trust alliance tag when it is consistent.

       *  If the source address is not in this sub-trust alliance, it
          will be forwarded directly.

   4.  Otherwise, the packet will be discarded.

   Alliance tag will be used when the packet crosses the upper AD which
   is at the higher level of source AD and destination AD.  Alliance tag
   is the tag maintained between the source AD corresponding to the AD
   in the parent AD and the destination AD corresponding to the address
   domain in the parent AD.

6.  Packet Signature

   It is difficult to accurately synchronize time among the trust
   alliance members.  So we propose a shared time slice, which means
   that there are two tags affecting at the same time in a period of
   time.  But it may suffer from a replay attack.  Therefore, a packet
   signature mechanism is proposed to prevent replay attacks and conceal
   the original tag.

   The tag is time-dependent.  The state machine triggers state
   transition by time and generates a new tag.  In a short period of
   time, all data packets are labeled with the same tag.  Moreover, due
   to the subtle differences in time synchronization, both old and new
   tags can be used for this short period of time, so attackers can
   reuse tags for replay attacks by simply copying tags.

   The packet signature mechanism joins the 8-bit part of the payload in
   the packet and the tags generated by the state machine.  Then it
   calculates the hash value with the parameters above to achieve the
   effect of packet-by-packet signature and resist the attacker's reuse
   of tags.  Its processing flow is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet by Packet Signature                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Level       |    Length     |           Reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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             Figure 4: Format of the packet by packet signature

   Packet by Packet Signature:  The hash value of the original tag,
      source address, destination address, first 8-bit of payload,
      credible level, and credible prefix length.

   Level:  8-bit of credible level.

   Length:  8-bit of credible prefix length.

   Reserved:  16-bit of reserved field. 0 will be padded.

   Firstly, it takes the source address, destination address and the
   first 8-bit of the data part of the data packet from the data packet,
   joins them in the way of (src-ip, dst-ip, first 8-bit of payload),
   and then joins the tag generated by the state machine at this time,
   the credible level of the SAVA architecture adopted by this AD and
   the length of the credible prefix to hash the concatenated string
   with the hash algorithm to get a new message digest.  Then it is
   reduced to a 32-bit packet signature by clipping and folding
   algorithm.  The AER adds the 32-bit packet signature together with
   the 2-bit credible level and the 7-bit credible prefix length to the
   SAVA-X option, fills the option into 64-bit, and forwards it.  At the
   AER of the destination AD, the same splicing and the same hash
   operation are performed to verify whether the generated string is
   consistent with the signature of the data packet.  If they are
   consistent, they are forwarded.  Otherwise, it is considered that the
   source address is forged and the data packet is discarded.

   Due to the problem of time synchronization, when both old and new
   tags are valid, both old and new tags need to be verified.  As long
   as one of them passes the verification, the packet should be
   forwarded.  The original tag generated by the state machine will not
   appear in the packet.  The attackers do not know the tag generated by
   the state machine at this time, so they can not forge the packet
   signature in the same way, which ensures the security of the data
   communication plane.

7.  MTU Consideration

   As the AER adds an option to the packet, the length of this packet is
   increasing, causing the MTU problem.  This problem could taken place
   in source AER or the link between source AER and destination AER.  If
   it occurs on the source AER, the source AER returns the ICMP message
   of "packet too big" to the source host and informs the host to reduce
   the packet size.  Otherwise, if it occurs on other links from the
   source AER to the destination AER, which means the packet size
   exceeds the MTU of other links from the source AER to the destination



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   AER after adding the tag, the corresponding router will return the
   ICMP message of "packet too big" to the source host.  However, after
   the source host adjusts its own MTU, the problem MAY still exist
   because the root cause is AER causing packet size to exceed MTU, and
   the host does not know it.  This problem can be solved by the
   following two methods.  First, the MTU of the external link is set to
   1280 at the source AER as this is the minimum value of MTU under
   IPv6.  Then the MTU of the source host end will be set to the minimum
   value of MTU subtracting the maximum value of the SAVA-X option.
   This method can solve the problem, but it greatly limits the packet
   size and wastes the available MTU.  The second is to monitor the ICMP
   message of "packet too big" sent to the host in the domain at the
   source AER.  If such a message is monitored, the expected MTU value
   in the message minus the maximum value of the SAVA-X option will be
   forwarded.  This method makes good use of MTU value to a certain
   extent, but it causes a large monitoring cost.

8.  Security Consideration

   TBD.

9.  IANA Considerations

   SAVA-X is designed for IPv6-enabled networks.  It takes a destination
   option, SAVA-X option, and header to carry the Tag. We recommend
   using 00111011, i.e. 59, for the SAVA-X option.  Here we give our
   SAVA-X option format in use.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  | Opt Data Len  |Tag Len|AI Type|   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              TAG                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                     Additional Information                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5: Format of SAVA-X option.

   Option Type:  8-bit field.  The destination option type of SAVA-X =
      59.

   Opt Data Len:  8-bit field.  The bytes length of the SAVA-X option.
      Its value is 2 + LenOfAI + (TagLen + 1), where LenOfAI is 2 when
      AI Type is 1, or 4 when AI Type is 2, or 0 default.

   Tag Len:  4-bit field.  The bytes length of TAG equals to (Tag Len +



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      1) * 8, e.g. if Tag Len = 7, it means SAVA-X uses 64 bits long
      TAG.  It guarantees the length of TAG would be an integral
      multiple of 8 bits.  The maximum length of TAG is 128 bits and the
      minimum length of TAG is 32 bits.

   AI Type:  4-bit field.  The type of Additional Information. 0 for no
      Additional Information, 1 for 16-bit long Additional Information,
      and 2 for 32-bit long Additional Information.  Others are not
      assigned.

   Reserved:  These bits are not used now and must be zero.

   TAG:  Variable-length field The actual tag, and its length is
      determined by the Tag Len field.

   Additional Information:  As defined in the AI Type field.

10.  Normative References

   [RFC1760]  Haller, N., "The S/KEY One-Time Password System",
              RFC 1760, DOI 10.17487/RFC1760, February 1995,
              <https://www.rfc-editor.org/rfc/rfc1760>.

   [RFC5210]  Wu, J., Bi, J., Li, X., Ren, G., Xu, K., and M. Williams,
              "A Source Address Validation Architecture (SAVA) Testbed
              and Deployment Experience", RFC 5210,
              DOI 10.17487/RFC5210, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5210>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/rfc/rfc8200>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

Acknowledgments

   TODO acknowledge.





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Authors' Addresses

   Ke Xu
   Tsinghua University
   China
   Email: xuke@tsinghua.edu.cn


   Jianping Wu
   Tsinghua University
   China
   Email: jianping@cernet.edu.cn


   Xiaoliang Wang
   Tsinghua University
   China
   Email: wangxiaoliang0623@foxmail.com


   Yangfei Guo
   Zhongguancun Laboratory
   China
   Email: guoyangfei@zgclab.edu.cn



























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