Internet Draft                                                T. Herbert
<draft-herbert-gue-02.txt>                                        Google
Category: Experimental                                           L. Yong
Expires April 2015                                            Huawei USA
                                                        October 24, 2014

                       Generic UDP Encapsulation

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   This specification describes Generic UDP Encapsulation (GUE), which
   is a scheme for using UDP to encapsulate packets of arbitrary IP
   protocols for transport across layer 3 networks. By encapsulating
   packets in UDP, specialized capabilities in networking hardware for
   efficient handling of UDP packets can be leveraged. GUE specifies
   basic encapsulation methods upon which higher level constructs, such
   tunnels and overlay networks for network virtualization, can be
   constructed. GUE is extensible by allowing optional meta data as part
   of the encapsulation, and is generic in that it can encapsulate
   packets of various IP protocols.

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. Packet formats . . . . . . . . . . . . . . . . . . . . . . . . . 4
      2.1. GUE header preamble . . . . . . . . . . . . . . . . . . . . 4
      2.2. GUE header  . . . . . . . . . . . . . . . . . . . . . . . . 5
      2.3. Flags and optional fields . . . . . . . . . . . . . . . . . 6
   3. Message types  . . . . . . . . . . . . . . . . . . . . . . . . . 7
      3.1. Control messages  . . . . . . . . . . . . . . . . . . . . . 7
      3.2. Data messages . . . . . . . . . . . . . . . . . . . . . . . 7
   4. Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
      4.1. Network tunnel encapsulation  . . . . . . . . . . . . . . . 8
      4.2. Transport layer encapsulation . . . . . . . . . . . . . . . 8
      4.3. Encapsulator operation  . . . . . . . . . . . . . . . . . . 8
      4.4. Decapsulator operation  . . . . . . . . . . . . . . . . . . 9
      4.5. Router and switch operation . . . . . . . . . . . . . . . . 9
      4.6. Middlebox interactions  . . . . . . . . . . . . . . . . . . 9
      4.7. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
      4.8. UDP checksum  . . . . . . . . . . . . . . . . . . . . . .  10
      4.9. MTU and fragmentation issues  . . . . . . . . . . . . . .  10
      4.10 Congestion control  . . . . . . . . . . . . . . . . . . .  11
   5. Inner flow identifier properties . . . . . . . . . . . . . . .  11
      5.1. Flow classification . . . . . . . . . . . . . . . . . . .  11
      5.2. Inner flow identifier properties  . . . . . . . . . . . .  12
   6. Motivation for GUE . . . . . . . . . . . . . . . . . . . . . .  12
   7. Security Considerations  . . . . . . . . . . . . . . . . . . .  14
      7.1. GUE security fields . . . . . . . . . . . . . . . . . . .  14
      7.2. GUE and IPsec . . . . . . . . . . . . . . . . . . . . . .  14
   8. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  15
   9. References . . . . . . . . . . . . . . . . . . . . . . . . . .  16
      9.1. Normative References  . . . . . . . . . . . . . . . . . .  16
      9.2. Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A: NIC processing for GUE  . . . . . . . . . . . . . . .  17
      A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . .  17
      A.2. Checksum offload  . . . . . . . . . . . . . . . . . . . .  18

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         A.2.1. Transmit checksum offload  . . . . . . . . . . . . .  18
         A.2.2. Receive checksum offload . . . . . . . . . . . . . .  19
      A.3. Transmit Segmentation Offload . . . . . . . . . . . . . .  19
      A.4. Large Receive Offload . . . . . . . . . . . . . . . . . .  20
   Appendix B: Privileged ports  . . . . . . . . . . . . . . . . . .  21
   Appendix C: Inner flow identifier as a route selector . . . . . .  21
   Appendix D: Hardware protocol implementation considerations . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1. Introduction

   This specification describes a general method for encapsulating
   packets of arbitrary IP protocols within User Datagram Protocol (UDP)
   [RFC0768] packets. Encapsulating packets in UDP facilitates efficient
   transport across networks. Networking devices widely provide protocol
   specific processing and optimizations for UDP (as well as TCP)
   packets. Packets for atypical IP protocols (those not usually parsed
   by networking hardware) can be encapsulated in UDP packets to
   maximize deliverability and to leverage flow specific mechanisms for
   routing and packet steering.

   Hardware devices commonly perform hash computations on packet headers
   to classify packets into flows or flow buckets. Flow classification
   is done to support load balancing (statistical multiplexing) of flows
   across a set of networking resources. Examples of such load balancing
   techniques are Equal Cost Multipath routing (ECMP), port selection in
   Link Aggregation, and NIC device Receive Side Scaling (RSS).  Hashes
   are usually either a three-tuple hash of IP protocol, source address,
   and destination address; or a five-tuple hash consisting of IP
   protocol, source address, destination address, source port, and
   destination port. Typically, networking hardware will compute five-
   tuple hashes for TCP and UDP, but only three-tuple hashes for other
   IP protocols. Since the five-tuple hash provides more granularity,
   load balancing can be finer grained with better distribution. When a
   packet is encapsulated with GUE, the source port in the outer UDP
   packet is set to reflect the flow of the inner packet. When a device
   computes a five-tuple hash on the outer UDP/IP header of a GUE
   packet, the resultant value classifies the packet per its inner flow.

   GUE provides an extensible header format for including optional meta
   data in the encapsulation header. This meta data potentially covers
   items such as virtual networking identifier, security data for
   validating or authenticating the GUE header, congestion control data,
   etc. GUE also allows private optional meta data in the encapsulation
   header. This feature can be used by a site or implementation to
   define local custom optional data. This also allows experimentation
   of options that may eventually become standard.

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2. Packet formats

   A GUE packet is comprised of a UDP packet whose payload is a GUE
   header followed by a payload which is either an encapsulated packet
   of some IP protocol or a control message (like an OAM message). A GUE
   packet has the general format:

   |                               |
   |        UDP/IP header          |
   |                               |
   |                               |
   |         GUE Header            |
   |                               |
   |                               |
   |      Encapsulated packet      |
   |      or control message       |
   |                               |

   The GUE header is variable length as determined by the presence of
   optional fields.

2.1. GUE header preamble

   The first byte of the GUE header provides the header type, indicator
   of a control or data message, and header length:

    0 1 2 3 4 5 6 7
   |Ver|C|   Hlen  |

   Contents are:

      o Ver: GUE protocol version. The rest of the fields after the
        preamble are defined based on the version. This field is three
        bits allowing four possible values.

      o Control flag: When set indicates a control message, not set
        indicates a data message.

      o Hlen: Length in 32-bit words of the GUE header, including
        optional fields but not the first four bytes of the header.
        Computed as (header_len - 4) / 4. All GUE headers are a multiple

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        of four bytes in length. Maximum header length is 132 bytes.

2.2. GUE header

   The header format for version 0x0 of GUE in UDP is:

    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 port            |      Destination port         |
   |           Length              |          Checksum             |
   |0x0|C|   Hlen  |  Proto/ctype  |            Flags            |P|
   |                                                               |
   ~                       Fields (optional)                       ~
   |                                                               |
   |                    Private flags(optional)                    |
   |                                                               |
   ~                   Private fields (optional)                   ~
   |                                                               |

   The contents of the UDP header are:

      o Source port (inner flow identifier): This should be set to a
        value that represents the encapsulated flow. The properties of
        the inner flow identifier are described below.

      o Destination port: The GUE assigned port number, XXXX.

      o Length: Canonical length of the UDP packet (length of UDP header
        and payload).

      o Checksum: Standard UDP checksum.

   The GUE header consists of:

      o Preamble byte: Version number (0x0), C bit, and header length.

      o Proto/ctype: When the C bit is set this field contains a control
        message type for the payload. When C bit is not set, the field
        holds the IP protocol number for the encapsulated packet in the
        payload. The control message or encapsulated packet begins at

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        the offset provided by Hlen.

      o Flags. Header flags that may be allocated for various purposes
        and may indicate presence of optional fields. Undefined header
        flag bits must be set to zero on transmission.

      o 'P' Private flag. Indicates presence of private flags option in
        the optional fields.

      o Fields: Optional fields whose presence is indicated by
        corresponding flags.

      o Private flags: An optional field indicated by the P bit. This
        field is set of private flags which may in turn indicate
        presence of private fields.

      o Private fields: Optional fields that are present when a
        corresponding bit in the private flags is set. A private field
        must have a length which is a multiple of four bytes, and must
        be correctly accounted for in the GUE header length.

2.3. Flags and optional fields

   Flags and associated optional fields are the primary mechanism of
   extensibility in GUE. There are sixteens flag bits in the GUE header,
   one of which is reserved to indicate the presence of a private flags
   optional field. New flags will be defined in other specifications.

   A flag may indicate presence of optional fields. The size of an
   optional field indicated by a flag must be fixed.

   The private flags optional field is comprised of thirty-two flag
   bits. Private flags retain the same properties of the regular header
   flags for parsing. The semantics of the private flags are specific to
   an implementation or site that defines them.

   Flags may be paired together to allow different lengths for an
   optional field. For example, if two flag bits are paired, a field may
   possibly be three different lengths. Regardless of how flag bits may
   be paired, the lengths and offsets of optional fields corresponding
   to a set of flags must be well defined.

   Optional fields are placed in order of the flags. New flags should be
   allocated from high to low order bit contiguously without holes.
   Flags allow random access, for instance to inspect the field
   corresponding to the Nth flag bit, an implementation only considers
   the previous N-1 flags to determine the offset. Flags after the Nth
   flag are not pertinent in calculating the offset of the Nth flag.

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   Flags (or paired flags) are idempotent such that new flags cannot
   cause reinterpretation of old flags. Also, new flags can not alter
   interpretation of other elements in the GUE header nor how the
   message is parsed (for instance, in a data message the proto/ctype
   field always holds an IP protocol number as an invariant).

3. Message types

3.1. Control messages

   Control messages are indicated in the GUE header when the C bit is
   set. The payload is interpreted as a control message with type
   specified in the proto/ctype field. The format and contents of the
   control message are indicated by the type and can be variable length.

   Other than interpreting the proto/ctype field as a control message
   type, the meaning and semantics of the rest of the elements in the
   GUE header are the same as that of data messages. Forwarding and
   routing of control messages should be the same as that of a data
   message with the same outer IP and UDP header and GUE flags-- this
   ensures that a control message can be created which follows the same
   path as a data message.

   Control messages can be defined for OAM type messages. For instance,
   an echo request and corresponding echo reply message may be defined
   to test for liveness.

3.2. Data messages

   Data messages are indicated in GUE header with C bit not set. The
   payload of a data message is interpreted as an encapsulated packet of
   an IP protocol indicated in the proto/ctype field. The packet
   immediately follows the GUE header.

   Data messages are a primary means of encapsulation and can be used to
   create tunnels for overlay networks.

4. Operation

   The figure below illustrates the use of GUE encapsulation between two
   servers. Sever 1 is sending packets to server 2. An encapsulator
   performs encapsulation of packets from server 1. These encapsulated
   packets traverse the network as UDP packets. At the decapsulator,
   packets are decapsulated and sent on to server 2. Packet flow in the
   reverse direction need not be symmetric; GUE encapsulation is not
   required in the reverse path.

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   +---------------+                       +---------------+
   |               |                       |               |
   |   Server 1    |                       |    Server 2   |
   |               |                       |               |
   +---------------+                       +---------------+
          |                                        ^
          V                                        |
   +---------------+   +---------------+   +---------------+
   |               |   |               |   |               |
   | Encapsulator  |-->|    Layer 3    |-->| Decapsulator  |
   |               |   |    Network    |   |               |
   +---------------+   +---------------+   +---------------+

   The encapsulator and decapsulator may be co-resident with the
   corresponding servers, or may be on separate nodes in the network.

4.1. Network tunnel encapsulation

   Network tunneling can be achieved by encapsulating layer 2 or layer 3
   packets. In this case the encapsulator and decapsulator nodes are the
   tunnel endpoints. These could be routers that provide network tunnels
   on behalf of communicating servers.

4.2. Transport layer encapsulation

   When encapsulating layer 4 packets, the encapsulator and decapsulator
   should be co-resident with the servers. In this case, the
   encapsulation headers are inserted between the IP header and the
   transport packet. The addresses in the IP header refer to both the
   endpoints of the encapsulation and the endpoints for terminating the
   the transport protocol.

4.3. Encapsulator operation

   Encapsulators create GUE data messages, set the source port to the
   inner flow identifier, set flags and optional fields in the GUE
   header, and forward packets to a decapsulator.

   An encapsulator may be an end host originating the packets of a flow,
   or may be a network device performing encapsulation on behalf of
   servers (routers implementing tunnels for instance). In either case,
   the intended target (decapsulator) is indicated by the outer
   destination IP address.

   If an encapsulator is tunneling packets, that is encapsulating
   packets of layer 2 or layer 3 protocols (e.g. EtherIP, IPIP, ESP
   tunnel mode), it should follow standard conventions for tunneling of
   one IP protocol over another. Diffserv interaction with tunnels is

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   described in [RFC2983], ECN propagation for tunnels is described in

4.4. Decapsulator operation

   A decapsulator performs decapsulation of GUE packets. A decapsulator
   is addressed by the outer destination IP address of a GUE packet.
   The decapsulator validates packets, including fields of the GUE
   header. If a packet is acceptable, the UDP and GUE headers are
   removed and the packet is resubmitted for IP protocol processing or
   control message processing if it is a control message.

   If a decapsulator receives a GUE packet with an unsupported version,
   unknown flag, bad header length (too small for included optional
   fields), unknown control message type, or an otherwise malformed
   header, it must drop the packet and may log the event. No error
   message is returned back to the encapsulator.

4.5. Router and switch operation

   Routers and switches should forward GUE packets as standard UDP/IP
   packets. The outer five-tuple should contain sufficient information
   to perform flow classification corresponding to the flow of the inner
   packet. A switch should not need to parse a GUE header, and none of
   the flags or optional fields in the GUE header should affect routing.

   A router should not modify a GUE header when forwarding a packet. It
   may encapsulate a GUE packet in another GUE packet, for instance to
   implement a network tunnel. In this case the router takes the role of
   an encapsulator, and the corresponding decapsulator is the logical
   endpoint of the tunnel.

4.6. Middlebox interactions

   A middle box may interpret some flags and optional fields of the GUE
   header for classification purposes, but is not required to understand
   all flags and fields in GUE packets. A middle box should not drop a
   GUE packet because there are flags unknown to it. The header length
   in the GUE header allows a middlebox to inspect the payload packet
   without needing to parse the flags or optional fields.

   A middlebox may infer bidirectional connection semantics to a UDP
   flow. For instance a stateful firewall may create a five-tuple rule
   to match flows on egress, and a corresponding five-tuple rule for
   matching ingress packets where the roles of source and destination
   are reversed for the IP addresses and UDP port numbers. To operate in
   this environment, a GUE tunnel must assume connected semantics
   defined by the UDP five tuple and the use of GUE encapsulation must

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   be symmetric between both endpoints. The source port set in the UDP
   header must be the destination port the peer would set for replies.

4.7. NAT

   IP address and port translation can be performed on the UDP/IP
   headers adhering to the requirements for NAT with UDP [RFC4787]. In
   the case of stateful NAT, connection semantics must be applied to a
   GUE tunnel as described above.

   When using transport mode encapsulation and traversing a NAT, the IP
   addresses may be changed such that the pseudo header checksum used
   for checksum calculation is modified and the checksum will be found
   invalid at the receiver. To compensate for this, A GUE option can be
   added which contains the checksum over the source and destination
   addresses when the packet is transmitted. Upon receiving this option,
   the delta of the pseudo header checksum is computed by subtracting
   the checksum over the source and and destination addresses from the
   checksum value in the option. The resultant value is then added into
   checksum calculation when validating the inner transport checksum.

4.8. UDP checksum

   When the outer IP protocol is IPv6, the UDP checksum must be set on
   transmission. A zero checksum may be used only if all the provisions
   of RFC6936 ("Applicability of Zero UDP Checksum with IPv6") are met.

   When the outer IP protocol is IPv4, the UDP checksum should be set on
   transmission. If the GUE header contains data that when corrupted can
   lead to misdirecting the packet to an incorrect receiver (for
   instance virtual network ID), then the UDP checksum must be set on
   transmission unless applicable conditions (those which would pertain
   to IPv4) of RFC6936 are met.

   A decapsulator must always validate non-zero UDP checksums for GUE
   packets following normal UDP checksum verification procedures. By
   default a decapsulator must drop IPv6 UDP packets with a zero
   checksum unless configured otherwise.

4.9. MTU and fragmentation issues

   Standard conventions for handling of MTU (Maximum Transmission Unit)
   and fragmentation in conjunction with networking tunnels
   (encapsulation of layer 2 or layer 3 packets) should be followed.
   Details are described in MTU and Fragmentation Issues with In-the-
   Network Tunneling [RFC4459]

   If a packet is fragmented before en5404]capsulation in GUE, all the

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   related fragments must be encapsulated using the same source port
   (inner flow identifier). An operator may set MTU to account for
   encapsulation overhead and reduce the likelihood of fragmentation.

4.10 Congestion control

   Per requirements of [RFC5405], if the IP traffic encapsulated with
   GUE implements proper congestion control no additional mechanisms
   should be required.

   In the case that the encapsulated traffic does not implement any or
   sufficient control, or it is not known rather a transmitter will
   consistently implement proper congestion control, then congestion
   control at the encapsulation layer must be provided. Note this case
   applies to a significant use case in network virtualization in which
   guests run third party networking stacks that cannot be implicitly
   trusted to implement conformant congestion control.

   Out of band mechanisms such as rate limiting, Managed Circuit
   Breaker, traffic isolation may used to provide rudimentary congestion
   control. For finer grained congestion control that allow alternate
   congestion control algorithms, reaction time within an RTT, and
   interaction with ECN, in band mechanisms may warranted.

   DCCP may be used to provide congestion control for encapsulated
   flows. In this case, the protocol stack for an IP tunnel may be IP-
   GUE-DCCP-IP. Alternatively, GUE can be extended to include congestion
   control (related date carried in GUE optional fields). Congestion
   control mechanisms will be elaborated in other specifications.

5. Inner flow identifier properties

5.1. Flow classification

   A major objective of using GUE is that a network device can perform
   flow classification corresponding to the flow of the inner
   encapsulated packet based on the contents in the outer headers.

   To support flow classification, the source port of the UDP header in
   GUE is set to a value that maps to the inner flow. This is referred
   to as the inner flow identifier. The inner flow identifier is set by
   the encapsulator; it can be computed on the fly based on packet
   contents or retrieved from a state maintained for the inner flow.

   Examples of deriving an inner flow identifier are:

      o If the encapsulated packet is a layer 4 packet, TCP/IPv4 for
        instance, the inner flow identifier could be based on the

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        canonical five-tuple hash of the inner packet.

      o If the encapsulated packet is an AH transport mode packet with
        TCP as next header, the inner flow identifier could be a hash
        over a three-tuple: TCP protocol and TCP ports of the
        encapsulated packet.

      o If a node is encrypting a packet using ESP tunnel mode and GUE
        encapsulation, the inner flow identifier could be based on the
        contents of clear-text packet. For instance, a canonical five-
        tuple hash for a TCP/IP packet could be used.

5.2. Inner flow identifier properties

        The inner flow identifier is the value set in the UDP source
        port of a GUE packet. The inner flow identifier should adhere to
        the following properties:

      o The value set in the source port should be within the ephemeral
        port range. IANA suggests this range to be 49152 to 65535, where
        the high order two bits of the port are set to one. This
        provides fourteen bits of entropy for the inner flow identifier.

      o The inner flow identifier should have a uniform distribution
        across encapsulated flows.

      o An encapsulator may occasionally change the inner flow
        identifier used for an inner flow per its discretion (for
        security, route selection, etc). Changing the value should
        happen no more than once every thirty seconds.

      o Decapsulators, or any networking devices, should not attempt any
        interpretation of the inner flow identifier, nor should they
        attempt to reproduce any hash calculation. They may use the
        value to match further receive packets for steering decisions,
        but cannot assume that the hash uniquely or permanently
        identifies a flow.

      o Input to the inner flow identifier is not restricted to ports
        and addresses; input could include flow label from an IPv6
        packet, SPI from an ESP packet, or other flow related state in
        the encapsulator that is not necessarily conveyed in the packet.

      o The assignment function for inner flow identifiers should be
        randomly seeded to mitigate denial of service attacks. The seed
        may be changed periodically.

6. Motivation for GUE

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   This section presents the motivation for GUE with respect to other
   encapsulation methods.

   A number of different encapsulation techniques have been proposed for
   the encapsulation of one protocol over another. EtherIP [RFC3378]
   provides layer 2 tunneling of Ethernet frames over IP. GRE [RFC2784],
   MPLS [RFC4023], and L2TP [RFC2661] provide methods for tunneling
   layer 2 and layer 3 packets over IP. NVGRE [NVGRE] and VXLAN [VXLAN]
   are proposals for encapsulation of layer 2 packets for network
   virtualization. IPIP [RFC2003] and Generic packet tunneling in IPv6
   [RFC2473] provide methods for tunneling IP packets over IP.

   Several proposals exist for encapsulating packets over UDP including
   ESP over UDP [RFC3948], TCP directly over UDP [TCPUDP], VXLAN, LISP
   [RFC6830] which encapsulates layer 3 packets, and Generic UDP
   Encapsulation for IP Tunneling (GRE over UDP)[GREUDP]. Generic UDP
   tunneling [GUT] is a proposal similar to GUE in that it aims to
   tunnel packets of IP protocols over UDP.

   GUE has the following discriminating features:

      o UDP encapsulation leverages specialized network device
        processing for efficient transport. The semantics for using the
        UDP source port as an identifier for an inner flow are defined.

      o GUE permits encapsulation of arbitrary IP protocols, which
        includes layer 2 3, and 4 protocols. This potentially allows
        nearly all traffic within a data center to be normalized to be
        either TCP or UDP on the wire.

      o Multiple protocols can be multiplexed over a single UDP port
        number. This is in contrast to techniques to encapsulate
        protocols over UDP using a protocol specific port number (such
        as ESP/UDP, GRE/UDP, SCTP/UDP). GUE provides a uniform and
        extensible mechanism for encapsulating all IP protocols in UDP
        with minimal overhead (four bytes of additional header).

      o GUE is extensible. New flags and optional fields can be defined.

      o The GUE header includes a header length field. This allows a
        network node to inspect an encapsulated packet without needing
        to parse the full encapsulation header.

      o Private flags and fields allow local customization and
        experimentation while being compatible with processing in
        network nodes (routers and middleboxes).

      o GUE includes both data messages (encapsulation of packets) and

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        control messages (such as OAM).

7. Security Considerations

        Encapsulation of IP protocols within GUE should not increase
        security risk, nor provide additional security in itself. As
        suggested in section 5 the source port for of UDP packets in GUE
        should be randomly seeded to mitigate some possible denial
        service attacks.

        GUE is most useful when it is in the outermost header of a
        packet which allows for flow hash calculation as well as making
        GUE header data (such as virtual network identifier) visible to
        switches and middleboxes. GUE must be amenable to encapsulating
        (and being encapsulated within) IPsec. Also, we allow provisions
        to secure the GUE header itself without external protocol.

7.1. GUE security fields

        Security fields should be used to provide integrity and
        authentication of the GUE header. Security negotiation
        (interpretation of security field, key management, etc.) is
        expected to be negotiated out of band between two communicating
        hosts. Security fields will be specified in future documents.

7.2. GUE and IPsec

        GUE may be used to encapsulate IPsec packets. This allows the
        benefits of deriving a flow hash for the inner, potentially
        encrypted, packet. In this case the protocol stack may be:

        |                               |
        |        UDP/IP header          |
        |                               |
        |                               |
        |         GUE Header            |
        |                               |
        |                               |
        |     ESP/AH/private security   |
        |                               |
        |                               |
        |       Encapsulated packet     |
        |                               |

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        Note that the security does not cover the GUE header (does not
        authenticate it for instance). GUE security optional fields may
        be used to provide authentication or integrity of the GUE

8. IANA Considerations

        A well known UDP port number assignment for GUE will be

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9. References

9.1. Normative References

   [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
   August 1980.

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
   IANA Considerations Section in RFCs", RFC 2434, October 1998.

   [RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983,
   October 2000.

   [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
   Notification", RFC 6040, November 2010.

   [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
   for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936,
   April 2013.

   [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
   Network Tunneling", RFC 4459, April 2006.

   [RFC4787] Audet, F., Ed., and C. Jennings, "Network Address
   Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
   RFC 4787, January 2007.

   [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
   for Application Designers", BCP 145, RFC 5405, November 2008.

9.2. Informative References

   [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
   October 1996.

   [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
   Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC 3948, January

   [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
   Locator/ID Separation Protocol (LISP)", RFC 6830, January 2013.

   [RFC3378] Housley, R. and S. Hollenbeck, "EtherIP: Tunneling Ethernet
   Frames in IP Datagrams", RFC 3378, September 2002.

   [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,

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   Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

   [RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed., "Encapsulating
   MPLS in IP or Generic Routing Encapsulation (GRE)", RFC 4023, March

   [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
   and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
   August 1999.

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

   [NVGRE] NVGRE: Network Virtualization using Generic Routing
   Encapsulation draft-sridharan-virtualization-nvgre-03

   [VXLAN] VXLAN: A Framework for Overlaying Virtualized Layer 2
   Networks over Layer 3 Networks draft-mahalingam-dutt-dcops-vxlan-06

   [TCPUDP] Encapsulation of TCP and other Transport Protocols over UDP

   [GREUDP] Generic UDP Encapsulation for IP Tunneling draft-yong-tsvwg-

   [GUT] Generic UDP Tunnelling (GUT) draft-manner-tsvwg-gut-02.txt

   [MNGCB] Network Transport Circuit Breakers draft-ietf-tsvwg-circuit-

   [REMCSUM] Remote Checksum Offload draft-herbert-remotecsumoffload-00

Appendix A: NIC processing for GUE

   This appendix provides some guidelines for Network Interface Cards
   (NICs) to implement common offloads and accelerations to support GUE.
   Note that most of this discussion is generally applicable to other
   methods of UDP based encapsulation.

A.1. Receive multi-queue

   Contemporary NICs support multiple receive descriptor queues (multi-
   queue). Multi-queue enables load balancing of network processing for
   a NIC across multiple CPUs. On packet reception, a NIC must select
   the appropriate queue for host processing. Receive Side Scaling is a
   common method which uses the flow hash for a packet to index an
   indirection table where each entry stores a queue number. Flow
   Director and Accelerated Receive Flow Steering (aRFS) allow a host to

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   program the queue that is used for a given flow which is identified
   either by an explicit five-tuple or by the flow's hash.

   GUE encapsulation should be compatible with multi-queue NICs that
   support five-tuple hash calculation for UDP/IP packets as input to
   RSS. The inner flow identifier (source port) ensures classification
   of the encapsulated flow even in the case that the outer source and
   destination addresses are the same for all flows (e.g. all flows are
   going over a single tunnel).

   By default, UDP RSS support is often disabled in NICs to avoid out of
   order reception that can occur when UDP packets are fragmented. As
   discussed above, fragmentation of GUE packets should be mitigated by
   fragmenting packets before entering a tunnel, path MTU discovery in
   higher layer protocols, or operator adjusting MTUs. Other UDP traffic
   may not implement such procedures to avoid fragmentation, so enabling
   UDP RSS support in the NIC should be a considered tradeoff during

A.2. Checksum offload

   Many NICs provide capabilities to calculate standard ones complement
   payload checksum for packets in transmit or receive. When using GUE
   encapsulation there are at least two checksums that may be of
   interest: the encapsulated packet's transport checksum, and the UDP
   checksum in the outer header.

A.2.1. Transmit checksum offload

   NICs may provide a protocol agnostic method to offload transmit
   checksum (NETIF_F_HW_CSUM in Linux parlance) that can be used with
   GUE. In this method the host provides checksum related parameters in
   a transmit descriptor for a packet. These parameters include the
   starting offset of data to checksum, the length of data to checksum,
   and the offset in the packet where the computed checksum is to be
   written. The host initializes the checksum field to pseudo header

   In the case of GUE, the checksum for an encapsulated transport layer
   packet, a TCP packet for instance, can be offloaded by setting the
   appropriate checksum parameters.

   NICs typically can offload only one transmit checksum per packet, so
   simultaneously offloading both an inner transport packet's checksum
   and the outer UDP checksum is likely not possible. In this case
   setting UDP checksum to zero (per above discussion) and offloading
   the inner transport packet checksum might be acceptable.

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   If an encapsulator is co-resident with a host, then checksum offload
   may be performed using remote checksum offload [REMCSUM]. Remote
   checksum offload relies on NIC offload of the simple UDP/IP checksum
   which is commonly supported even in legacy devices. In remote
   checksum offload the outer UDP checksum is set and the GUE header
   includes an option indicating the start and offset of the inner
   "offloaded" checksum. The inner checksum is initialized to the pseudo
   header checksum. When a decapsulator receives a GUE packet with the
   remote checksum offload option, it completes the offload operation by
   determining the packet checksum from the indicated start point to the
   end of the packet, and then adds this into the checksum field at the
   offset given in the option. Computing the checksum from the start to
   end of packet is efficient if checksum-complete is provided on the

A.2.2. Receive checksum offload

   GUE is compatible with NICs that perform a protocol agnostic receive
   checksum (CHECKSUM_COMPLETE in Linux parlance). In this technique, a
   NIC computes a ones complement checksum over all (or some predefined
   portion) of a packet. The computed value is provided to the host
   stack in the packet's receive descriptor. The host driver can use
   this checksum to "patch up" and validate any inner packet transport
   checksum, as well as the outer UDP checksum if it is non-zero.

   Many legacy NICs don't provide checksum-complete but instead provide
   an indication that a checksum has been verified (CHECKSUM_UNNECESSARY
   in Linux). Usually, such validation is only done for simple TCP/IP or
   UDP/IP packets. If a NIC indicates that a UDP checksum is valid, the
   checksum-complete value for the UDP packet is the "not" of the pseudo
   header checksum. In this way, checksum-unnecessary can be converted
   to checksum-complete. So if the NIC provides checksum-unnecessary for
   the outer UDP header in an encapsulation, checksum conversion can be
   done so that the checksum-complete value is derived and can be used
   by the stack to validate an checksums in the encapsulated packet.

A.3. Transmit Segmentation Offload

   Transmit Segmentation Offload (TSO) is a NIC feature where a host
   provides a large (>MTU size) TCP packet to the NIC, which in turn
   splits the packet into separate segments and transmits each one. This
   is useful to reduce CPU load on the host.

   The process of TSO can be generalized as:

      - Split the TCP payload into segments which allow packets with
        size less than or equal to MTU.

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      - For each created segment:

        1. Replicate the TCP header and all preceding headers of the
           original packet.

        2. Set payload length fields in any headers to reflect the
           length of the segment.

        3. Set TCP sequence number to correctly reflect the offset of
           the TCP data in the stream.

        4. Recompute and set any checksums that either cover the payload
           of the packet or cover header which was changed by setting a
           payload length.

   Following this general process, TSO can be extended to support TCP
   encapsulation in GUE.  For each segment the Ethernet, outer IP, UDP
   header, GUE header, inner IP header if tunneling, and TCP headers are
   replicated. Any packet length header fields need to be set properly
   (including the length in the outer UDP header), and checksums need to
   be set correctly (including the outer UDP checksum if being used).

   To facilitate TSO with GUE it is recommended that optional fields
   should not contain values that must be updated on a per segment
   basis-- for example the GUE fields should not include checksums,
   lengths, or sequence numbers that refer to the payload. If the GUE
   header does not contain such fields then the TSO engine only needs to
   copy the bits in the GUE header when creating each segment and does
   not need to parse the GUE header.

A.4. Large Receive Offload

   Large Receive Offload (LRO) is a NIC feature where packets of a TCP
   connection are reassembled, or coalesced, in the NIC and delivered to
   the host as one large packet. This feature can reduce CPU utilization
   in the host.

   LRO requires significant protocol awareness to be implemented
   correctly and is difficult to generalize. Packets in the same flow
   need to be unambiguously identified. In the presence of tunnels or
   network virtualization, this may require more than a five-tuple match
   (for instance packets for flows in two different virtual networks may
   have identical five-tuples). Additionally, a NIC needs to perform
   validation over packets that are being coalesced, and needs to
   fabricate a single meaningful header from all the coalesced packets.

   The conservative approach to supporting LRO for GUE would be to
   assign packets to the same flow only if they have identical five-

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   tuple and were encapsulated the same way. That is the outer IP
   addresses, the outer UDP ports, GUE protocol, GUE flags and fields,
   and inner five tuple are all identical.

Appendix B: Privileged ports

   Using the source port to contain an inner flow identifier value
   disallows the security method of a receiver enforcing that the source
   port be a privileged port. Privileged ports are defined by some
   operating systems to restrict source port binding. Unix, for
   instance, considered port number less than 1024 to be privileged.

   Enforcing that packets are sent from a privileged port is widely
   considered an inadequate security mechanism and has been mostly
   deprecated. To approximate this behavior, an implementation could
   restrict a user from sending a packet destined to the GUE port
   without proper credentials.

Appendix C: Inner flow identifier as a route selector

   An encapsulator generating an inner flow identifier may modulate the
   value to perform a type of multipath source routing. Assuming that
   networking switches perform ECMP based on the flow hash, a sender can
   affect the path by altering the inner flow identifier.  For instance,
   a host may store a flow hash in its PCB for an inner flow, and may
   alter the value upon detecting that packets are traversing a lossy
   path. Changing the inner flow identifier for a flow should be subject
   to hysteresis (at most once every thirty seconds) to limit the number
   of out of order packets.

Appendix D: Hardware protocol implementation considerations

   A low level protocol, such is GUE, is likely interesting to being
   supported by high speed network devices. Variable length header (VLH)
   protocols like GUE are often considered difficult to efficiently
   implement in hardware. In order to retain the important
   characteristics of an extensible and robust protocol, hardware
   vendors may practice "constrained flexibility". In this model, only
   certain combinations or protocol header parameterizations are
   implemented in hardware fast path. Each such parameterization is
   fixed length so that the particular instance can be optimized as a
   fixed length protocol. In the case of GUE this constitutes specific
   combinations of GUE flags, fields, and next protocol. The selected
   combinations would naturally be the most common cases which form the
   "fast path", and other combinations are assumed to take the "slow

   In time, needs and requirements of the protocol may change which may

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   manifest themselves as new parameterizations to be supported in the
   fast path. To allow allow this extensibility, a device practicing
   constrained flexibility should allow the fast path parameterizations
   to be programmable.

Authors' Addresses

   Tom Herbert
   1600 Amphitheatre Parkway
   Mountain View, CA


   Lucy Yong
   Huawei USA
   5340 Legacy Dr.
   Plano, TX  75024


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