Internet Draft                                                T. Herbert
<draft-herbert-gue-00.txt>                                        Google
Category: Experimental
Expires June 2014                                      December 20, 2013

                       Generic UDP Encapsulation

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on June 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.

Herbert                    Expires June 2014                    [Page 1]

Internet Draft              herbert-gue.txt            December 20, 2013


   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, can be constructed.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2. Packet formats  . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1. GUE header preamble . . . . . . . . . . . . . . . . . . . .  3
     2.2. GUE encapsulation header  . . . . . . . . . . . . . . . . .  4
   3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1. Encapsulator operation  . . . . . . . . . . . . . . . . . .  6
     3.2. Decapsulator operation  . . . . . . . . . . . . . . . . . .  7
     3.3. Router and switch operation . . . . . . . . . . . . . . . .  7
     3.4. Middlebox and NAT interactions  . . . . . . . . . . . . . .  7
     3.5. UDP checksum  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.6. MTU and fragmentation issues  . . . . . . . . . . . . . . .  8
   4. Inner flow identifier properties  . . . . . . . . . . . . . . .  8
     4.1. Flow classification . . . . . . . . . . . . . . . . . . . .  8
     4.2. Inner flow identifier properties  . . . . . . . . . . . . .  9
   5. Motivation for GUE  . . . . . . . . . . . . . . . . . . . . . . 10
   6. Security Considerations . . . . . . . . . . . . . . . . . . . . 11
   7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 11
   8. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1. Normative References  . . . . . . . . . . . . . . . . . . . 12
     8.2. Informative References  . . . . . . . . . . . . . . . . . . 12
   Appendix A: NIC processing for GUE . . . . . . . . . . . . . . . . 13
     A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . . . 13
     A.2. Checksum offload  . . . . . . . . . . . . . . . . . . . . . 14
       A.2.1. Transmit checksum offload . . . . . . . . . . . . . . . 14
       A.2.2. Receive checksum offload  . . . . . . . . . . . . . . . 14
     A.3. Transmit Segmentation Offload . . . . . . . . . . . . . . . 14
     A.4. Large Receive Offload . . . . . . . . . . . . . . . . . . . 15
   Appendix B: Privileged ports . . . . . . . . . . . . . . . . . . . 16
   Appendix C: Inner flow identifier as a route selector  . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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

Herbert                    Expires June 2014                    [Page 2]

Internet Draft              herbert-gue.txt            December 20, 2013

   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.

2. Packet formats

   The payload of a UDP packet destined to a GUE port starts with a GUE
   header. If a packet is being encapsulated it immediately follows the
   GUE header.

2.1. GUE header preamble

   The first byte of the GUE packet header contains a packet type and
   header length.
    0 1 2 3 4 5 6 7
   |  Type |  Hlen |

   Contents are:

      o Type: type of header. The rest of the fields in the header are
        defined based the type.

      o Hlen: Length in 32-bit words of the GUE header, including

Herbert                    Expires June 2014                    [Page 3]

Internet Draft              herbert-gue.txt            December 20, 2013

        optional fields but not the first four bytes of the header.
        Computed as (header_len - 4) / 4. All GUE headers are a multiple
        of four bytes in length.

2.2. GUE encapsulation header

   The GUE encapsulation header is used to encapsulate packets for
   various IP protocols. Encapsulation with a GUE header has the general

   |                               |
   |        UDP/IP header          |
   |                               |
   |                               |
   |         GUE Header            |
   |                               |
   |                               |
   |     Encapsulated packet       |
   |                               |

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

   The UDP and GUE encapsulation header format 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  |  Hlen |   Protocol    |V|R|R|R|R|R|R|R|R|R|R|R|R|R|P|P|
   |               Virtual network ID (optional)                   |
   |                                                               |
   ~                   Private fields (optional)                   ~
   |                                                               |

   The contents of the UDP header are:

Herbert                    Expires June 2014                    [Page 4]

Internet Draft              herbert-gue.txt            December 20, 2013

      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 (payload length).

      o Checksum: Either the standard UDP checksum or zero indicating no
        checksum calculated.  Zero checksum is recommended.

   The GUE header consists of:

      o Type: Set to 0x0 to indicate GUE encapsulation header.

      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. The length of the encapsulated
        packet is determined from the UDP length and the Hlen:
        encapsulated_packet_length = UDP_Length - 8 - GUE_Hlen.

      o Protocol: IP protocol number for the next header.  The next
        header begins at the offset provided by Hlen.

      o 'R' Reserved flag. Must be set to zero for sending.

      o 'V' Virtualization flag. Indicates presence of the Virtual
        Network Identifier (VNID) field. The VNID is used to tunnel
        layer 2 or layer 3 packets for network virtualization. Use and
        semantics of this field should be defined in separate documents.

      o 'P' Private flag. Indicates flags reserved for private use (as
        per private use policy specified in [RFC2434]). These flags may
        indicate the presence of private fields. These flags can only be
        used between a sender and a receiver that have agreement as to
        their meaning.

      o Virtual network ID (4 octets): Used in network virtualization to
        identify the virtual network that packet was sent on. Only
        present if virtualization bit is set.

      o Private fields: An implementation may define private fields that
        are present when a corresponding private bit 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.

3. Operation

Herbert                    Expires June 2014                    [Page 5]

Internet Draft              herbert-gue.txt            December 20, 2013

   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.

   +---------------+                       +---------------+
   |               |                       |               |
   |   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.

   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.

   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.

3.1. Encapsulator operation

   Encapsulators create encapsulation headers, 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.

Herbert                    Expires June 2014                    [Page 6]

Internet Draft              herbert-gue.txt            December 20, 2013

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

3.2. 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.

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

3.3. 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.

3.4. Middlebox and NAT 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.

   In certain instances 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.

Herbert                    Expires June 2014                    [Page 7]

Internet Draft              herbert-gue.txt            December 20, 2013

   NAT for UDP assumes bidirectional connection semantics.

   GUE primarily assumes unidirectional flow properties, there is no
   necessary correspondence between the UDP ports of GUE packet for
   encapsulated flows in different directions. GUE could be extended to
   provide bidirectional semantics, however that is outside the scope of
   this document.

3.5. UDP checksum

   GUE packets should be sent with a zero checksum if the encapsulated
   packet contains its own checksum or can be checked with some
   alternate means. Applicability Statement for the Use of IPv6 UDP
   Datagrams with Zero Checksums [RFC6936] provides analysis and
   motivation of sending zero checksums when using UDP as an
   encapsulation protocol.

   If a receiver receives a GUE packet with a non-zero checksum, it must
   perform normal UDP checksum verification.

3.6. 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 encapsulation in GUE, all the
   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. Inner flow identifier properties

4.1. 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:

Herbert                    Expires June 2014                    [Page 8]

Internet Draft              herbert-gue.txt            December 20, 2013

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

   The five-tuple hash commonly used to identify a flow in UDP will
   cover the outer source address, destination address, source port
   (inner flow identifier), and destination port. These values should be
   mostly persistent for the lifetime of an encapsulated flow, only
   changing infrequently (at most once every thirty seconds).

4.2. 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 suggest this range to be 49152 to 65535, where
        the high order two bits of the port are set to one. This
        provides fourteen bits for the inner flow identifier value.

      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

Herbert                    Expires June 2014                    [Page 9]

Internet Draft              herbert-gue.txt            December 20, 2013

        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.

5. Motivation for GUE

   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
        specific 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

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

Herbert                    Expires June 2014                   [Page 10]

Internet Draft              herbert-gue.txt            December 20, 2013

      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 can provide encapsulation for a virtual network that
        provides layer 3 connectivity. In contrast, VXLAN and NVGRE are
        defined to only provide layer 2 services (encapsulation of

      o GUE defines a 32 bit virtual networking identifier (in contrast
        to 24 bit values defined for VXLAN and NVGRE). This facilitates
        hierarchical assignment, local flag definitions in the
        identifier, and potentially obfuscation of the identifier on the

6. Security Considerations

        Encapsulation of IP protocols within GUE should not increase
        security risk, nor provide additional security in itself. As
        suggested in section 3 the source port for of UDP packets in GUE
        should be randomly seeded to mitigate some possible denial
        service attacks.

7. IANA Considerations

        A well known UDP port number assignment for GUE will be

Herbert                    Expires June 2014                   [Page 11]

Internet Draft              herbert-gue.txt            December 20, 2013

8. References

8.1. Normative References

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

   [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.

8.2. Informative References

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

   [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,
   "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.,

Herbert                    Expires June 2014                   [Page 12]

Internet Draft              herbert-gue.txt            December 20, 2013

   and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
   August 1999.

   [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

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 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
   program the queue that is used for a given flow which is identified
   either by an explicit five-tuple or by flow 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 support may be 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

Herbert                    Expires June 2014                   [Page 13]

Internet Draft              herbert-gue.txt            December 20, 2013

   may not implement such procedures to avoid fragmentation, so enabling
   UDP 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 two checksums that may be of interest, the
   payload checksum of an encapsulated packet, and the UDP checksum of
   in the outer header.

A.2.1. Transmit checksum offload

   NICs may provide a protocol agnostic method to offload transmit
   checksum 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 may seed the checksum
   with for data not covered by the NIC computation (the checksum of the
   pseudo header for instance).

   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 is desirable.

A.2.2. Receive checksum offload

   GUE is compatible with NICs that perform a protocol agnostic receive
   checksum. 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.

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 packets into separate segments and transmits each one.

Herbert                    Expires June 2014                   [Page 14]

Internet Draft              herbert-gue.txt            December 20, 2013

   This is useful to reduce CPU load on host.

   The process of TSO could be generalized as:

      1. Split the TCP payload into segments which will allow less than
         MTU size packets.

      2. For each segment, replicate the TCP header and all preceding
         headers of the original packet.

      3. For each protocol header set any payload length fields to
         reflect the length for the segment.

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

      5. 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).

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 the same five-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.

Herbert                    Expires June 2014                   [Page 15]

Internet Draft              herbert-gue.txt            December 20, 2013

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

   A 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 this decision by altering the inner flow identifier.  For
   instance, a sender 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 delivered.

Authors' Addresses

   Tom Herbert
   1600 Amphitheatre Parkway
   Mountain View, CA

Herbert                    Expires June 2014                   [Page 16]