Mboned                                                        J. Holland
Internet-Draft                                                   K. Rose
Intended status: Standards Track               Akamai Technologies, Inc.
Expires: September 10, 2020                               March 09, 2020

                  Asymmetric Manifest Based Integrity


   This document defines Asymmetric Manifest-Based Integrity (AMBI).
   AMBI allows each receiver or forwarder of a stream of multicast
   packets to check the integrity of the contents of each packet in the
   data stream.  AMBI operates by passing cryptographically verifiable
   hashes of the data packets inside manifest messages, and sending the
   manifests over authenticated out-of-band communication channels.

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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Comparison with TESLA . . . . . . . . . . . . . . . . . .   4
     1.2.  Threat Model  . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Buffering and Validation Windows  . . . . . . . . . . . .   6
       2.2.1.  Inter-packet Gap  . . . . . . . . . . . . . . . . . .   7
     2.3.  Packet Digests  . . . . . . . . . . . . . . . . . . . . .   8
       2.3.1.  Digest Profile  . . . . . . . . . . . . . . . . . . .   8
       2.3.2.  Pseudoheader  . . . . . . . . . . . . . . . . . . . .  11
     2.4.  Manifests . . . . . . . . . . . . . . . . . . . . . . . .  12
       2.4.1.  Manifest Layout . . . . . . . . . . . . . . . . . . .  13
     2.5.  Transitioning to Other Manifest Streams . . . . . . . . .  14
   3.  Transport Considerations  . . . . . . . . . . . . . . . . . .  15
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.2.  HTTPS . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.3.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.4.  DTLS + FECFRAME . . . . . . . . . . . . . . . . . . . . .  16
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Module  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  The YANG Module Names Registry  . . . . . . . . . . . . .  19
     6.2.  Media Type  . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Predictable Packets . . . . . . . . . . . . . . . . . . .  19
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Multicast transport poses security problems that are not easily
   addressed by the same security mechanisms used for unicast transport.

   The "Introduction" sections of the documents describing TESLA
   [RFC4082], and TESLA in SRTP [RFC4383], and TESLA with ALC and NORM
   [RFC5776] present excellent overviews of the challenges unique to
   multicast authentication, briefly summarized here:

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   o  A MAC based on a symmetric shared secret cannot be used because
      each packet has multiple receivers that do not trust each other,
      and using a symmetric shared secret exposes the same secret to
      each receiver.

   o  Asymmetric per-packet signatures can handle only very low bit-
      rates because of the computational overhead.

   o  An asymmetric signature of a larger message comprising multiple
      packets requires reliable receipt of all such packets, something
      that cannot be guaranteed in a timely manner even for protocols
      that do provide reliable delivery, and the retransmission of which
      may anyway exceed the useful lifetime for data formats that can
      otherwise tolerate some degree of loss.

   Aymmetric Manifest-Based Integrity (AMBI) defines a method for
   receivers or middle boxes to cryptographically authenticate and
   verify the integrity of a stream of packets, by communicating packet
   "manifests" (described in Section 2.4) via an out-of-band
   communication channel that provides authentication and verifiable

   Each manifest contains a message digest (described in Section 2.3)
   for each packet in a sequence of packets from the data stream,
   hereafter called a "packet digest".  The packet digest incorporates a
   cryptographic hash of the packet contents and some identifying data
   from the packet, according to a defined digest profile for the data

   Each manifest MUST be delivered in a way that provides cryptographic
   integrity guarantees of the authenticity of the manifest.  For
   example, TLS could be used to deliver a stream of manifests over a
   unicast data stream from a set of trusted senders to each receiver,
   or a protocol that asymmetrically signs each message could be used to
   transport authenticated manifests over a multicast channel.  Note
   that a UDP-based protocol might drop or reorder manifests while still
   providing authentication.

   Upon successful verification of a manifest and receipt of any subset
   of the corresponding data packets, the receiver has proof of the
   integrity of the contents of the data packets that are listed in the

   Authenticating the integrity of the data packets depends on:

   o  the authenticity of the manifests; and

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   o  the authenticity of the digest profile used for construction of
      the packet digests; and

   o  the difficulty of generating a collision for the packet digests
      contained in the manifest.

   This document defines a YANG [RFC7950] module that augments the DORMS
   [I-D.draft-jholland-mboned-dorms-01] YANG module to provide a way to
   communicate a digest profile, described in Section 2.3.1, for
   construction of the packet digests, described in Section 2.3.  When
   obtaining the digest profile by using DORMS, the authenticity of the
   data stream relies on a trust relationship with the DORMS server,
   since that anchors the authenticity of the digest profile for
   constructing packet digests.

1.1.  Comparison with TESLA

   AMBI and TESLA [RFC4082] and [RFC5776] attempt to achieve a similar
   goal of authenticating the integrity of streams of multicast packets.
   AMBI imposes a higher overhead, as measured in the amount of extra
   data required, than TESLA imposes.  In exchange, AMBI provides non-
   repudiation (which TESLA does not), and relaxes the requirement for
   establishing an upper bound on clock synchronization between sender
   and receiver.

   This tradeoff enables new capabilities for AMBI, relative to TESLA.
   In particular, when receiving multicast traffic from an untrusted
   transit network, AMBI can be used by a middle box to authenticate
   packets from a trusted source before forwarding traffic through the
   network, and the receiver also can separately authenticate the
   packets it receives.

   This use case is not possible with TESLA because the data packets
   can't be authenticated until a key is disclosed, so either the
   middlebox has to forward data packets without first authenticating
   them, so that the receiver has them prior to key disclosure, or the
   middlebox has to hold packets until the key is disclosed, at which
   point the receiver can no longer establish their authenticity.

   The other new capability is that because AMBI provides authentication
   information out of band, authentication can be retrofitted into some
   pre-existing deployments without changing the protocol of the data
   packets, under some restrictions outlined in Section 7.  By contrast,
   TESLA requires a MAC to be added to each authenticated message.

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1.2.  Threat Model

   TBD: Summarize the applicable threat model this protects against.  A
   diagram plus a cleaned-up version of the on-list explanation here is
   probably appropriate: https://mailarchive.ietf.org/arch/msg/mboned/

1.3.  Terminology

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

2.  Protocol Operation

2.1.  Overview

   In order to authenticate a data packet, AMBI receivers need to hold
   these three pieces of information at the same time:

   o  the data packet; and

   o  an authenticated manifest containing the packet digest for the
      data packet; and

   o  a digest profile defining the transformation from the data packet
      to its packet digest.

   The manifests are delivered as a stream of manifests over an
   authenticated data channel.  Manifest contents MUST be authenticated
   before they can be used to authenticate data packets.

   The manifest stream is composed of an ordered sequence of manifests
   that each contain an ordered sequence of packet digests,
   corresponding to the original packets as sent from their origin, in
   the same order.

   Note that a manifest contains potentially many packet digests, and
   its size can be tuned to fit within a convenient PDU (Protocol Data
   Unit) of the manifest transport stream, so that usually, many packet
   digests for the multicast data stream can be delivered per packet of
   the manifest transport.  The intent is that even with unicast-based
   manifest transport, multicast-style efficiencies of scale can still
   be realized, with only a relatively small unicast overhead, when
   manifests use a unicast transport.

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2.2.  Buffering and Validation Windows

   Using different communication channels for the manifest stream and
   the data stream introduces a possibility of desynchronization in the
   timing of the received data between the different channels, so
   receivers hold data packets and packet digests from the manifest
   stream in buffers for some duration while awaiting the arrival of
   their counterparts.

   While holding a data packet, if the corresponding packet digest for
   that packet arrives in the manifest stream and can be authenticated,
   the data packet is authenticated.

   While holding an authenticated packet digest, if the corresponding
   data packet arrives with a matching packet digest, the data packet is

   Once a data packet is authenticated, the corresponding packet digest
   can be discarded and the data packet can be further processed by the
   receiving application or forwarded through the receiving network.
   Authenticating a data packet consumes one packet digest and prevents
   re-learning, with a hold-down time equal to the hold time for packet
   digests.  A different manifest might provide the same packet digest
   with the same packet sequence number, but the digest remains consumed
   if it has been used to authenticate a data packet.

   If the receiver's hold duration for a data packet expires without
   authenticating the packet, the packet SHOULD be dropped as
   unauthenticated.  If the hold duration of a manifest expires, packet
   digests last received in that manifest SHOULD be discarded.  (Note
   that in some cases, packet digests can be sent redundantly in more
   than one manifest.  In such cases, the latest received time for an
   authenticated packet digest should be used for the expiration time.)

   Since packet digests are usually smaller than the data packets, it's
   RECOMMENDED that senders generate and send manifests with timing such
   that the packet digests in a manifest will typically be received by
   subscribed receivers before the data packets corresponding to those
   digests are received.

   This strategy reduces the buffering requirements at receivers at, the
   cost of introducing some buffering of data packets at the sender,
   since data packets are generated before their packet digests can be
   added to manifests.

   The RECOMMENDED default hold times at receivers are:

   o  2 seconds for data packets

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   o  10 seconds for packet digests

   The sender MAY recommend different values for specific data streams,
   in order to tune different data streams for different performance
   goals.  The YANG model in Section 5 provides a mechanism for senders
   to communicate the sender's recommendation for buffering durations,
   when using DORMS.

   Receivers SHOULD follow the recommendations for hold times provided
   by the sender, subject to their capabilities and any administratively
   configured limits on buffer sizes at the receiver.

   However receivers MAY deviate from the values recommended by the
   sender for a variety of reasons.  Decreasing the buffering durations
   recommended by the server increases the risk of losing packets, but
   can be an appropriate tradeoff for specific network conditions and
   hardware constraints on some devices.

   TBD: should there be any reordering restrictions above and beyond the
   timing constraints?

2.2.1.  Inter-packet Gap

   It's RECOMMENDED that middle boxes forwarding buffered data packets
   preserve the inter-packet gap between packets, and that receiving
   libraries provide mechanisms to expose the network arrival times of
   packets to applications.

   The purpose for this recommendation is to preserve the capability of
   receivers to use techniques for available bandwidth detection or
   network congestion based on observation of packet times.  Examples of
   such techniques include paced chirping and pathrate.

   Note that this recommendation SHOULD NOT prevent the transmission of
   an authenticated packet because the prior packet is unauthenticated.
   This recommendation only asks implementations to delay the
   transmission of an authenticated packet to correspond to the
   interpacket gap if an authenticated packet was previously transmitted
   and the authentication of the subsequent packet would otherwise burst
   the packets more quickly.

   This does not prevent the transmission of packets out of order
   according to their order of authentication, only the timing of
   packets that are transmitted, after authentication, in the same order
   they were received.

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   For receiver applications, the time that the original packet was
   received from the network SHOULD be made available to the receiving

2.3.  Packet Digests

2.3.1.  Digest Profile

   A packet digest is a message digest for a data packet, built
   according to a digest profile defined by the sender.

   The digest profile is defined by the sender, and specifies:

   1.  A cryptographically secure hash algorithm (REQUIRED)

   2.  A manifest stream identifier

   3.  Whether to hash the IP payload or the UDP payload. (see

   The hash algorithm is applied to a pseudoheader followed by the
   packet payload, as determined by the digest profile.  The computed
   hash value is the packet digest.

   TBD: there should also be a way to specify that only packets to a
   specific UDP port are applicable.  I think this is not quite right
   today and probably should be done with a grouping in the yang model,
   so that the profile appears either inside a "protocol" container
   inside the (S,G) or inside the udp-stream inside the "protocol", but
   am not sure.  Follow-up on this after the first reference
   implementation...  Payload Type  UDP vs. IP payload validation

   When the digest profile indicates that UDP payloads are validated,
   the IP protocol for the packets MUST be UDP (0x11) and the payload
   used for calculating the packet digest includes only the UDP payload,
   with length as the number of UDP payload octets, as calculated by
   subtracting the size of the UDP header from the UDP payload length.

   When the digest profile indicates that IP payloads are validated, the
   IP payload of the packet is used, using the outermost IP layer that
   contains the (S,G) corresponding to the (S,G) protected by the
   manifest.  There is no restriction on the IP protocols that can be
   authenticated.  The length field in the pseudoheader is calculated by

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   subtracting the IP Header Length from the IP length, and is equal to
   the number of octets in the payload for the digest calculation.  Motivation

   Full IP payloads often aren't available to receivers without extra
   privileges on end user operating systems, so it's useful to provide a
   way to authenticate only the UDP payload, which is often the only
   portion of the packet available to many receiving applications.

   However, for some use cases a full IP payload is appropriate.  For
   example, when retrofitting some existing protocols, some packets may
   be predictable or frequently repeated.  Use of an IPSec
   Authentication Header [RFC4302] is one way to disambiguate such
   packets.  Even though the shared secret means the Authentication
   Header can't itself be used to authenticate the packet contents, the
   sequence number in the Authentication Header can ensure that specific
   packets are not repeated at the IP layer, and so it's useful for AMBI
   to have the capability to authenticate such packets.

   Another example: some services might need to authenticate the UDP
   options [I-D.ietf-tsvwg-udp-options].  When using the UDP payload,
   the UDP options would not be part of the authenticated payload, but
   would be included when using the IP payload type.

   Lastly, since (S,G) subscription operates at the IP layer, it's
   possible that some non-UDP protocols will need to be authenticated.  TBD: Packet contents?

   TBD: Determine whether we need to support packet contents in the
   packet digest.  If so, add to above list in Section 2.3.1:

   o  A set of bits from the packet contents (potentially empty)

   The packet contents are a sequence of bits composed from a sequence
   of fixed bit (offset, length) pairs, as specified in xxxxxx.  A
   useful choice for packet contents is to use sequence numbers in the
   application level protocol, such as with RTP [RFC3550], but any
   contents from the packet with a fixed bit offset and length can be

   Providing variable packet contents in the packet digest increases the
   difficulty of attacking the hash by limiting the scope of legitimate
   data packets that can be matched when attempting to generate a hash

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   The basic idea is to put an encoding here so that for example the RTP
   sequence number or the sequence number in an Authentication Header
   can be provided here in bulk (you give "value starts at bit 80 and is
   16 bits long unsigned and increases by 1 per packet for the packets
   in the manifest with starting value 10", indicating that the 100
   packets in the manifest have values 10-110 in their contents at the
   given location.  Now those contents are prepended to the packet
   digest, and can be verified against the packets, as well as the hash
   of the contents).

   For packet streams without a sequence number, we can instead
   incorporate a few high-entropy bits from the packet contents and NOT
   provide the value as a sequence number, but rather incorporate it in
   the digest values themselves.  (Is this useful?)

   Before defining this, I want to calculate how much overhead it buys
   us- how much can we truncate a good hash algorithm if we use this to
   add collision resistance?  Might not be worthwhile, it's a
   significant increase in complexity.  -jake 2019-08-31

   If we need it, tentative addition to yang for the data profile looks

       list packet-contents {
         key offset;
         description "contents from the packet for the packet
         leaf offset {
           type uint16;
           mandatory true;
           description "offset of the contents, in number of bits";
         leaf length {
           type uint16;
           mandatory true;
           description "length of the contents, in number of bits";
         leaf manifest-delivery {
           type enumeration {
             enum sequence;
             enum digest;
           mandatory true;
           description "the way these content bits are delivered in
               the manifest";

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   The manifest-delivery would indicate whether the bits are a sequence
   number (in which case a section for a manifest with a start+step
   would be added ahead of the digests), or digest (indicating the bits
   appear inside each digest, ahead of the hash), and they would prepend
   in order to the packet digest, with sequence number bits inserted at
   the right bit location for the digest, based on earlier-appearing
   values, if any.

2.3.2.  Pseudoheader

   When calculating the hash for the packet digest, the hash algorithm
   is applied to a pseudoheader followed by the payload from the packet.
   The complete sequence of octets used to calculate the hash is
   structured as follows:

    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
   |         Source Address (32 bits IPv4/128 bits IPv6)           |
   |                             ...                               |
   |       Destination Address (32 bits IPv4/128 bits IPv6)        |
   |                             ...                               |
   |     Zeroes    |   Protocol    |            Length             |
   |          Source Port          |        Destination Port       |
   |                     Manifest Identifier                       |
   |                        Payload Data                           |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  Source Address

   The IPv4 or IPv6 source address of the packet.  Destination Address

   The IPv4 or IPv6 destination address of the packet.  Zeroes

   All bits set to 0.

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   The IP Protocol field from IPv4, or the Next Header field for IPv6.
   When UDP payload is indicated, this value MUST be UDP (0x11).  Length

   The length in octets of the Payload Data field, expressed as an
   unsigned 16-bit integer.  Source Port

   The source port of the packet.  Zeroes if using a protocol that does
   not use source ports.  Destination Port

   The destination port of the packet.  Zeroes if using a protocol that
   does not use destination ports.

   TBD: there's something I hate about the source and destination ports.
   Maybe it should only be active in UDP-payload mode, instead of zeroes
   when not UDP?  But I suspect there's a better approach than UDP-or-
   not, so it's this way for now, with hopes of finding something better
   in the next version.  Manifest Identifier

   The 32-bit identifier for the manifest stream.  Payload Data

   The payload data includes either the IP payload or the UDP payload,
   as indicated by the digest profile.

   The payload type is configurable because when sending UDP, some
   legacy networks may strip the UDP option space, and it's necessary to
   provide a manifest stream capable of authentication that can
   interoperate with these networks.  However, for non-UDP traffic or in
   order to authenticate the UDP options, some use cases may require
   support for authenticating the full IP payload.

2.4.  Manifests

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2.4.1.  Manifest Layout

    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
   |                  Manifest Stream Identifier                   |
   |                   Manifest sequence number                    |
   |                 First packet sequence number                  |
   |       Refresh Deadline        |      Packet Digest Count      |
   |                 ... Packet Content Expansions ...             |
   |                      ... Packet Digests ...                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  Manifest Stream Identifier

   A 32-bit unsigned integer chosen by the sender.  Manifest Sequence Number

   A monotonically increasing 32-bit unsigned integer.  Each manifest
   sent by the sender increases this value by 1.  On overflow it wraps
   to 0.

   It's RECOMMENDED to expire the manifest stream and start a new stream
   for the data packets before a sequence number wrap is necessary.  First Packet Sequence Number

   A monotonically increasing 32-bit unsigned integer.  Each packet in
   the data stream increases this value by 1.

   It's RECOMMENDED to expire the manifest stream and start a new stream
   for the data packets before a sequence number wrap is necessary.

   Note: for redundancy, especially if using a manifest stream with
   unreliable transport, successive manifests MAY provide duplicates of
   the same packet digest with the same packet sequence number, using
   overapping sets of packet sequence numbers.  When received, these
   reset the hold timer for the listed packet digests.

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   A 16-bit unsigned integer number of seconds.

   A zero value means the current digest profile for the current
   manifest stream is stable.

   A nonzero value means that the authentication is transitioning to a
   new manifest stream, and the set of digest profiles SHOULD be
   refreshed by receivers that might stay joined longer than this
   duration, and a different manifest stream SHOULD be selected, before
   this many seconds have elapsed, in order to avoid a disruption.  See
   Section 2.5.  Packet Digest Count

   The count of packet digests in the manifest.  Packet Digests

   Packet digests appended one after the other, aligned to 8-bit
   boundaries with zero padding (if the bit length of the digests are
   not multiples of 8 bits).

2.5.  Transitioning to Other Manifest Streams

   It's possible for multiple manifest streams authenticating the same
   data stream to be active at the same time.  The different manifest
   streams can have different hash algorithms, manifest ids, and current
   packet sequence numbers for the same data stream.  These result in
   different sets of packet digests for the same data packets, one
   digest per packet per digest profile.

   It's necessary sometimes to transition gracefully from one manifest
   stream to another.  The Refresh Deadline field from the manifest is
   used to signal to receivers the need to transition.

   When a receiver gets a nonzero refresh deadline in a manifest the
   sender SHOULD have an alternate manifest stream ready and available,
   and the receiver SHOULD learn the alternate manifest stream, join the
   new one, and leave the old one before the number of seconds given in
   the refresh deadline.  After the refresh deadline has expired, a
   manifest stream MAY end.

   The receivers SHOULD use a random value between now and one half the
   number of seconds in the deadline field, to spread the spike of load
   on the DORMS server during a large multicast event.

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3.  Transport Considerations

3.1.  Overview

   AMBI manifests MUST be authenticated, but any transport protocol
   providing authentication can be used.  This section discusses several
   viable options for the use of an authenticating transport, and some
   associated design considerations.

   TBD: extend the 'manifest-transport' in the YANG model to make an
   extensible mechanism to advertise different transport options for
   receiving manifest streams.

   TBD: add ALTA to the list when and if it gets further along
   [I-D.draft-krose-mboned-alta-01].  Sending an authenticatable
   multicast stream (instead of the below unicast-based proposals) is a
   worthwhile goal, else a 1% unicast authentication overhead becomes a
   new unicast limit to the scalability.

3.2.  HTTPS

   This document defines a new media type 'application/ambi' for use
   with HTTPS.

   An HTTPS stream carrying the 'application/ambi' media type is
   composed of a sequence of binary AMBI manifests.  It is RECOMMENDED
   to use Chunked encoding.

   Complete packet Digests from partially received manifests MAY be used
   by the receiver for authentication, even if the full manifest is not
   yet delivered.

3.3.  DTLS

   TBD: DTLS [RFC6347] can provide authentication for datagrams, so if
   manifests can be constructed to fit within datagrams, it is an
   appropriate choice.  (IPSec is similar-worth adding as an option?).

   This option provides no native redundancy or retransmission, but
   packet digests can be repeated in different manifests to provide some
   resilience to loss.  Lost manifests that result in the loss of blocks
   of packet digests can be expensive, since they would make received
   data packets unauthenticatable.  TBD: should we therefore not support
   this case?

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   DTLS for manifests that do not fit into single-packet payloads can
   still be delivered by using FECFRAME [RFC6363], particularly Reed-
   Solomon [RFC6865] or possibly Raptor [RFC6681].  This has some
   advantages compared to HTTPS because of the absence of HOL-blocking,
   while providing for tunable redundancy.  This has some advantages
   relative to DTLS because of overhead reduction and non-integer
   redundancy tunability (e.g. 1.5 becomes a viable redundancy factor).

   TBD: define this method, possibly in another RFC.

4.  Examples

   TBD: walk through some examples as soon as we have a build running.
   Likely to need some touching up.

5.  YANG Module

5.1.  Tree Diagram

   module: ietf-ambi
     augment /dorms:metadata/dorms:sender/dorms:group/dorms:udp-stream:
       +--rw manifest-stream* [id]
          +--rw id                     uint32
          +--rw manifest-transport*    inet:uri
          +--rw hash-algorithm         ct:asymmetric-key-algorithm-t
          +--rw payload-type           enumeration
          +--rw data-hold-time-ms?     uint32
          +--rw digest-hold-time-ms?   uint32

5.2.  Module

<CODE BEGINS> file ietf-ambi@2020-03-09.yang
module ietf-ambi {
    yang-version 1.1;

    namespace "urn:ietf:params:xml:ns:yang:ietf-ambi";
    prefix "ambi";

    import ietf-dorms {
        prefix "dorms";
        reference "I-D.jholland-mboned-dorms";

    import ietf-inet-types {
        prefix "inet";

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        reference "RFC6991 Section 4";

    import ietf-crypto-types {
        prefix "ct";
        reference "draft-ietf-netconf-crypto-types";

    organization "IETF";

        "Author:   Jake Holland

    "Copyright (c) 2019 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents

     This version of this YANG module is part of RFC XXXX
     (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
     for full legal notices.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
     'MAY', and 'OPTIONAL' in this document are to be interpreted as
     described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
     they appear in all capitals, as shown here.

     This module contains the definition for the AMBI data types.
     It provides metadata for authenticating SSM channels as an
     augmentation to DORMS.";

    revision 2019-08-25 {
        description "Initial revision as an extension.";

      "/dorms:metadata/dorms:sender/dorms:group/dorms:udp-stream" {

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        description "Definition of the manifest stream providing
            integrity info for the data stream";

        list manifest-stream {
            key id;
            description "Definition of a manifest stream.";
            leaf id {
                type uint32;
                mandatory true;
                description "The Manifest ID referenced in a manifest.";
            leaf-list manifest-transport {
                type inet:uri;
                description "A URI that provides a location for the
                    manifest stream";
            leaf hash-algorithm {
                type ct:asymmetric-key-algorithm-t;
                mandatory true;
                    "The hash algorithm for the packet hashes within
                     manifests in this stream.";
            leaf payload-type {
                type enumeration {
                    enum udp {
                      description "The hash includes only the UDP
                    enum ip {
                      description "The hash includes the full IP
                mandatory true;
                description "The contents of the payload for the
                    digest profile";
            leaf data-hold-time-ms {
                type uint32;
                default 2000;
                description "The number of milliseconds to hold data
                    packets waiting for a corresponding digest before
            leaf digest-hold-time-ms {
                type uint32;
                default 10000;

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                description "The number of milliseconds to hold packet
                    digests waiting for a corresponding data packet
                    before discarding";


6.  IANA Considerations

6.1.  The YANG Module Names Registry

   This document adds one YANG module to the "YANG Module Names"
   registry maintained at <https://www.iana.org/assignments/yang-
   parameters>.  The following registrations are made, per the format in
   Section 14 of [RFC6020]:

         name:      ietf-ambi
         namespace: urn:ietf:params:xml:ns:yang:ietf-ambi
         prefix:    ambi
         reference: I-D.draft-jholland-mboned-ambi

6.2.  Media Type

   TBD: Register 'application/ambi' according to advice from:

   TBD: check guidelines in https://tools.ietf.org/html/rfc5226

7.  Security Considerations

7.1.  Predictable Packets

   Protocols that have predictable packets run the risk of offline
   attacks for hash collisions against those packets.  When
   authenticating a protocol that might have predictable packets, it's
   RECOMMENDED to use a hash function secure against such attacks or to
   add content to the packets to make them unpredictable, such as an
   Authentication Header ([RFC4302]), or the addition of an ignored
   field with random content to the packet payload.

   TBD: explain attack from generating malicious packets and then
   looking for collisions, as opposed to having to generate a collision
   on packet contents that include a sequence number and then hitting a
   match (especially expand on this if we do add Section

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   TBD: follow the rest of the guidelines: https://tools.ietf.org/html/

8.  Acknowledgements

   Many thanks to Daniel Franke, Eric Rescorla, Christian Worm
   Mortensen, Max Franke, and Albert Manfredi for their very helpful
   comments and suggestions.

9.  References

9.1.  Normative References

              Holland, J., "Discovery Of Restconf Metadata for Source-
              specific multicast", draft-jholland-mboned-dorms-01 (work
              in progress), September 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", RFC 6363,
              DOI 10.17487/RFC6363, October 2011,

   [RFC6681]  Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
              Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
              DOI 10.17487/RFC6681, August 2012,

   [RFC6865]  Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
              Matsuzono, "Simple Reed-Solomon Forward Error Correction
              (FEC) Scheme for FECFRAME", RFC 6865,
              DOI 10.17487/RFC6865, February 2013,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

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   [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/info/rfc8174>.

9.2.  Informative References

              Rose, K. and J. Holland, "Asymmetric Loss-Tolerant
              Authentication", draft-krose-mboned-alta-01 (work in
              progress), July 2019.

              Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
              udp-options-08 (work in progress), September 2019.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
              June 2005, <https://www.rfc-editor.org/info/rfc4082>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              DOI 10.17487/RFC4383, February 2006,

   [RFC5776]  Roca, V., Francillon, A., and S. Faurite, "Use of Timed
              Efficient Stream Loss-Tolerant Authentication (TESLA) in
              the Asynchronous Layered Coding (ALC) and NACK-Oriented
              Reliable Multicast (NORM) Protocols", RFC 5776,
              DOI 10.17487/RFC5776, April 2010,

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

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Authors' Addresses

   Jake Holland
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144
   United States of America

   Email: jakeholland.net@gmail.com

   Kyle Rose
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144
   United States of America

   Email: krose@krose.org

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