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Versions: 00 01 02                                                      
Network Working Group                                         J. Iyengar
Internet-Draft                             Franklin and Marshall College
Intended status: Standards Track                             S. Cheshire
Expires: January 01, 2014                                   J. Graessley
                                                                   Apple
                                                           June 30, 2013


                         Minion - Wire Protocol
                    draft-iyengar-minion-protocol-00

Abstract

   Minion uses TCP-format packets on-the-wire, for compatibility with
   existing NATs, Firewalls, and similar middleboxes, but provides a
   richer set of facilities to the application, as described in the
   Minion Service Model document.  This document specifies the details
   of the on-the-wire protocol used to provide those services.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 01, 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

1.  Conventions and Terminology Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

   This document uses terminology like "kernel" and "user-level", as
   those terms pertain to many of today's Unix-like operating systems.
   Equivalent concepts apply to software that is built using a different
   architectural model than may not include such an obvious kernel/user
   split.

2.  Introduction

   Minion uses TCP-format packets on-the-wire, to provide full
   compatibility with existing NATs, Firewalls, and similar middleboxes,
   but provides a richer set of facilities to the application, described
   in the Minion Service Model and Conceptual API document [minserv].
   This document specifies the details of the on-the-wire protocol used
   to provide those services.  Before reading this protocol
   specification document, familiarity with the Minion Service Model
   [minserv] is strongly recommended.  That information is not repeated
   here.

   Minion runs over a standard TCP connection.  Therefore, IP addresses
   and TCP ports are used just as they are with TCP [RFC0793].

   Minion is also designed to be able to use a modified TCP connection
   which supports out-of-order delivery, giving better low-latency
   performance on lossy networks, for use by the kinds of application
   that today would use UDP [RFC0768] to achieve low-latency delivery.
   The goal of providing low-latency delivery -- and consequently the
   need to be able to handle a data stream that may have gaps -- is
   reflected in various aspects of the Minion protocol design, such as
   the use of DTLS instead of TLS, and the use of Consistent Overhead
   Byte Stuffing [COBS] for reliably extracting messages from an
   incomplete data stream.  Minion is able to take advantage of out-of-
   order delivery where the network stack offers that, but Minion does
   not require it.  Minion still works correctly when the performance
   benefits of out-of-order delivery are not available.

   Minion supports messages of arbitrary size.  Large messages are
   broken into chunks a little under 16 kilobytes each (the DTLS maximum
   record size, minus a few bytes for Minion header).  At the receiving



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   end the Minion chunks are reassembled into Minion messages and
   delivered to the client application.  Small messages are sent in a
   single Minion chunk.

   Normally messages are sent by the client as a single atomic unit, and
   delivered to the receiving client as a single atomic unit.  For
   messages too large to fit conveniently in memory, the message may be
   built incrementally by the sender, and delivered to the receiving
   client incrementally, a chunk at a time.

   When a Minion message is complete, or has at least one maximum Minion
   chunk size of data accumulated, then if it is eligible to be sent
   according to the message ordering facilities offered by the Minion
   Service Model [minserv] (Sender Ordering, Receiver Ordering, and
   Chaining) a Minion chunk is generated.

   Each Minion chunk contains a Minion chunk header followed by the
   client's message data, as described in Section 3 "Minion Chunk
   Format".

   Each Minion chunk is encrypted using DTLS [RFC6347].

   Each encrypted DTLS payload is then framed using RECOBS, as described
   in Section 4 "Recursively Embeddable COBS", so that it begins with a
   00 byte and ends with an FF byte.

   The framed, encrypted chunk is then enqueued for transmission.

   If the kernel networking code supports multiple priorities, then the
   framed, encrypted chunk is placed in the transmission queue for the
   stated priority level.  Any time the TCP congestion window and/or
   receive window rules allow more data to be sent, data is drawn from
   the highest-priority non-empty transmit buffer, assigned the next
   block of unused TCP sequence numbers, formed into a TCP segment, and
   transmitted on the wire.  This just-in-time TCP sequencing mechanism
   has the effect of causing higher-priority data to be inserted right
   at the front of the conceptual combined transmit buffer, at the
   earliest possible byte boundary, unconstrained by message or chunk
   boundaries in the lower-priority messages.  This is possible because
   the RECOBS framing is robust to pre-emption at any arbitrary byte
   boundary.

   Note that, when priorities are supported, chunks above the lowest
   priority MUST be delivered to the kernel in such a way that they are
   sent completely before the kernel resumes sending the lower-priority
   traffic.  The RECOBS framing supports interrupting a lower priority
   stream with a higher-priority chunk, but not alternating back and
   forth between two priority levels.  Once a higher-priority chunk



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   interrupts lower-priority traffic, the higher-priority chunk must be
   completed before the lower-priority traffic resumes.  Typically this
   is easily achieved by delivering the chunk to the kernel atomically
   in a single write call.

3.  Minion Chunk Format

   A Minion Chunk begins with an eight-byte header, followed by the
   client's message data:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|    Code     |Pri|     This Minion Chunk ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved      |RCP|     Referenced Minion Chunk ID            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                                                               :
   :                     Minion Chunk Data                         :
   :                                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: Minion Chunk Format

   If the Complete ('C') bit is zero, this message is incomplete; the
   receiver should expect to receive additional continuation chunks for
   this message.  If the Complete bit is one, this message is complete;
   there will be no subsequent continuation chunks for this message.

   The seven-bit chunk code identifies what type of chunk this is, as
   described below.

   The two-bit priority field indicates the priority level for this
   message, with 0 being the highest priority and 3 being the default
   (lowest-level) priority.

   Every Minion chunk has a Chunk ID.  This is a 22-bit value assigned
   from a monotonically increasing 22-bit cyclic counter.  This means
   that Chunk IDs are reused every 2^22 chunks.  At any given moment in
   time though, only a small portion of the 22-bit ID space is actively
   in use, so Chunk IDs are not ambiguous.  Each of the four priority
   levels has its own 22-bit Chunk ID space, i.e., Priority 1 Chunk 7
   and Priority 2 Chunk 7 are different chunks.  Also, the Chunk ID
   spaces in opposite directions on a connection are separate.  Each
   sender is responsible for selecting the Chunk IDs for the chunks it
   sends.





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   In some cases it is useful to refer to messages by ID, and the terms
   "Message ID" and "Chunk ID" are sometimes used interchangeably.  For
   a message that is sent using a single chunk, the Message ID is the
   same as the Chunk ID.  For a message that is sent using multiple
   chunks, the Message ID is the Chunk ID of the *final* chunk of the
   message.  One implication of this is that a message's ID is undefined
   until the message is complete.

   Because Chunk IDs are eventually reused, issues of ID lifetime must
   be carefully considered in the Minion protocol design.  For example,
   since a remote peer could, in principle, wait an arbitrary long
   length of time before replying to a message, the Message ID of a
   request that is awaiting a response MUST NOT be reused until the
   response has been received, and the client has disposed of the
   request message.  Otherwise, a reply could be ambiguous, if there
   were two outstanding request messages both using the same Message ID
   at the same time.  Likewise, the last Chunk ID of an incomplete
   message MUST NOT be reused until some subsequent chunk has been added
   to that message, referencing the previous Chunk ID.

   The Reserved field MUST be set to zero on transmission, and MUST be
   ignored on reception.

   For chunk types that need to refer to some other chunk, the
   Referenced Minion Chunk Priority (RCP) and Referenced Minion Chunk ID
   fields identify the referenced chunk.  Note that some chunk types
   refer to chunks going in the same direction (e.g. a continuation
   chunk) and some chunk types refer to chunks going in the reverse
   direction (e.g. a reply chunk).  For chunk types that do not to refer
   to any other chunk, these two fields MUST be set to zero on
   transmission, and MUST be ignored on reception.

   The Minion Chunk payload data follows the Minion Chunk Header.

   There is no explicit length field in the Minion Chunk Header, because
   the chunk length is determined implicitly in the RECOBS decoding
   step.

3.1.  Minion Chunk Codes

   The seven-bit chunk code identifies what kind of chunk this is.  128
   chunk codes are available.  The following eight chunk codes are
   currently defined:








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     00 Continuation.  This is a continuation of a previously incomplete
        message.  The Referenced Minion Chunk ID identifies what
        previous chunk this is adding to.  (If the Complete bit is one
        then this chunk is the final chunk and completes the message; no
        further chunks for this message will be arriving.)

     01 Cancellation.  This is a cancellation of a previously incomplete
        message.  The Referenced Minion Chunk ID identifies what
        previous chunk this is cancelling.  In this case Complete bit is
        unused; the Complete bit MUST be set to zero on transmission,
        and MUST be ignored on reception.

     02 Unordered Message.  This chunk begins a new unordered message.
        The Referenced Minion Chunk ID is unused, and MUST be set to
        zero on transmission, and MUST be ignored on reception.

     03 Ordered Message.  This chunk begins a new ordered message.  This
        message is subject to Receiver Ordering; it MUST NOT be
        delivered to the receiving client until the message indicated by
        the Referenced Minion Chunk ID field has been delivered.

     04 Chained Message.  This chunk begins a new message that chains on
        after a preceding message.  The Referenced Minion Chunk ID
        identifies the preceding message.  This message MUST NOT be
        delivered to the receiving client until the previous message of
        the chain as been delivered to the receiving client, and this
        message MUST be delivered to the receiving client in a manner
        that indicates to the client that it is related to the previous
        message.

     05 Reply/Acknowledge.  This chunk begins a new message which is an
        explicit reply to a previously received message.  The Referenced
        Minion Chunk ID identifies the received message to which this is
        a reply.  A reply may be empty, in which case it serves as a
        simple acknowledgement that the request was received and
        accepted, or it may contain data.  It is anticipated that future
        Minion protocol development will create additional Minion chunk
        codes to negotiate future protocol features.  For these
        capability negotiation messages, an empty reply referencing the
        request serves as an acknowledgement that the requested protocol
        feature is supported.

     06 Reject.  A Minion Reject code indicates that the referenced
        received message had an error or was not accepted for some other
        reason.  A Reject Message may be empty, or may contain data
        giving information concerning the reason for the rejection.  It
        is possible to reject an incomplete message that is still
        arriving, by sending a Reject referencing the most recent Chunk



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        ID for that partial message.  The sender will respond by sending
        a Cancellation for that message, confirming that no further
        chunks will be sent.  When used for Minion protocol capability
        negotiation, a Reject message referencing the request indicates
        that the requested protocol feature is not supported.

     07 End Minion.  It is anticipated that there will be existing
        application protocols that initially add Minion as an optional
        feature, which they use only when the remote peer indicates it
        also has Minion support, and otherwise they will communicate
        using the existing protocol without the Minion features.  Such
        application protocols typically will first connect using their
        existing protocol, and then negotiate an "upgrade" to Minion
        framing.  For symmetry, it would be good if such an "upgrade"
        were not an irreversible one-way path.  We would like to offer
        the ability for applications to connect over raw TCP, switch to
        Minion for some message exchanges, and then drop back to raw TCP
        for some subsequent communication.  For this reason the Minion
        chunk code 8 exists, which signals, "This is the final Minion-
        format message you will receive in this particular Minion
        session; after this you're on your own."

4.  Recursively Embeddable COBS

   Consistent Overhead Byte Stuffing [COBS] allows complete messages to
   be reliably located within an incomplete data stream that may contain
   gaps.

   COBS works by transforming the payload data to eliminate all
   occurrences of zero bytes.  This is like PPP byte stuffing, but more
   efficient; COBS has a worst-case data size overhead below 0.5%.
   Having created a zero-free payload, the payloads can then be
   concatenated into a single byte stream, separated by single zero
   bytes, and the zero bytes unambiguously mark the boundaries between
   payloads, because we know the payloads themselves no longer contain
   any zero bytes.  At the receiving end the transformation is reversed
   to recreate the original payload data.

   The transformation process [COBS] is, in effect, a simple run length
   encoding.  An extremely simplified summary of the original 1997 COBS
   encoding is as follows:

   o  If the payload begins with three nonzero bytes followed by a zero,
      then the output is the byte value 4 (the run length) followed by
      the three nonzero bytes, and the subsequent zero is skipped.






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   o  If that is followed by fifty nonzero bytes followed by a zero,
      then the output is the byte value 51 (the run length) followed by
      the fifty nonzero bytes, and the subsequent zero is skipped.

   o  This process is repeated until the entire payload has been
      replaced by its zero-free equivalent.

   Recursively Embeddable COBS (RECOBS) is a derivative of the original
   1997 COBS encoding.  RECOBS code bytes have the following meanings:

     00 New payload begins
     01 Represents a single zero byte
     02 Two bytes: a single nonzero byte, followed by a single zero byte
     03 Three bytes: two nonzero bytes, followed by a single zero byte
      n Represents n bytes: n-1 nonzero bytes, followed by a zero byte
     FD 253 bytes: 252 nonzero bytes, followed by a single zero byte
     FE 253 bytes: 253 nonzero bytes, with *no* following zero byte
     FF Payload ends

   This has the effect that, after encoding, every payload has
   unambiguous bookends; every payload begins with a single 00, and ends
   with a single FF.  Using this encoding, recursive embedding becomes
   possible.  At *any* point in the encoded byte stream it is now
   possible to interrupt the byte stream, insert a new RECOBS-encoded
   payload, and then resume the previous byte stream.

   At the receiving end, the decoder is part-way through decoding a
   payload when the interruption occurs.  The decoder sees a 00, which
   is not legal in RECOBS-encoded data, so the decoder knows a new
   payload is beginning.  Because the decoder has not yet seen the FF
   end-marker for the previous payload, it knows that payload is
   incomplete, so it saves its decoding state for later resumption.  The
   decoder then proceeds to decode the embedded payload.  When the
   decoder sees the FF end-marker for the embedded payload, it delivers
   that fully decoded payload to the waiting client, and then resumes
   its decoding of the previously interrupted payload.

   In principle this recursive embedding could be nested arbitrarily
   deeply, limited only by the amount of storage the decoder has
   available for partially-received payloads and their associated
   decoding state.

   In practice, Minion limits RECOBS embedding to four levels (the base
   level plus three levels of nested interruption) to establish a
   defined upper bound on the amount of storage required by a decoder.

5.  Flow Control




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   TCP [RFC0793] implements flow control in the form of the advertised
   receive window.  This is to prevent a faster sender from overwhelming
   a slower receiver.  Minion requires similar protection to prevent a
   slower receiver running out of memory trying to buffer messages
   arriving faster than it can handle them.

   For a pure user-level library implementation of Minion, this is
   achieved by having the library set an upper bound on the amount of
   memory it will use for storing received messages that have not yet
   been handled by the client.  Once this limit is met, the library
   ceases reading TCP data from the kernel, which causes the TCP receive
   window to fill up, which causes the sender to stop sending.  Once the
   client consumes some messages, the library then reads more data from
   the kernel, the TCP receive window opens up, and the sender is
   permitted to send more data.

   However, this means that there is some duplication of buffering --
   the TCP receive window in the kernel and additional buffering in the
   user-level library.  For this reason a kernel extension is proposed
   where a client (the Minion library in this case) can read data from
   the connection *without* raising the TCP receive window.  In a sense
   it is reading the data "secretly", without admitting to the sender at
   the other end that it has been read.  Those bytes, even though read
   into user space, are still counted against the TCP receive window.
   Later, after the client application has actually consumed the
   message, another kernel call is made to acknowledge consumption of
   those bytes, and the TCP receive window is raised.

   This mechanism integrates message-level flow control with TCP's byte-
   level flow control, rather than having two independent flow control
   mechanisms happening concurrently at different levels, in ways that
   might interact badly with each other.

   Note that the Minion protocol design will have to consider possible
   deadlock situations.  For example, suppose one Minion host is
   refusing to consume any more Minion Chunks because it wishes to send
   a Reject message for them, but it cannot, because the peer's receive
   window is closed.  Suppose also that the reason the peer's receive
   window is closed is because the peer also is sitting on a pile of
   unwanted Minion Chunks that it refuses to consume until it can send a
   Reject message for them.  Possible deadlocks such as these need to be
   considered, and mechanisms to avoid them created.









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6.  Retransmission Policy

   One of the main arguments that is often presented to justify why a
   particular application protocol is built on UDP instead of TCP is
   that, "UDP is better for 'real time' applications."  The supporting
   reasoning for this is often that, "TCP insists on continuing to
   retransmit data long after the client doesn't need any more."  In
   truth the real problem is not retransmission; it is that the
   conventional TCP APIs don't allow received data to be delivered out
   of order.  Suppose a TCP sender has 50 packets in flight at any given
   time (e.g. the bandwidth x delay product is 75 kB) then the loss of a
   single packet causes all 49 following packets to stall at the
   receiver because the API doesn't allow for them to be delivered to
   the client until the missing packet has been received.

   Minion solves this problem by allowing data to be delivered as it
   arrives, even if there are gaps.  But the argument still remains that
   even after removing the ordering requirement at the receiver, it may
   still be a waste of bandwidth to retransmit data that will arrive too
   late to be useful.  And indeed, it is possible with TCP to
   fraudulently acknowledge segments that were in fact not received, and
   this will cause the sender to not retransmit those segments.

   However, we chose not to use fraudulent acknowledgements to suppress
   retransmissions, because certain NATs, Firewalls and other
   middleboxes may block traffic if they observe implausible protocol
   actions which they find suspicious.  One of the important goals of
   Minion is 100% compatibility with today's existing Internet devices,
   not 99% compatibility.

   We expect packet loss to be about 1% (at most a few percent) in a
   functioning network, and the cost of retransmitting those lost
   packets, even in the extreme case where *all* the retransmissions
   turn out to be unnecessary, is an overhead of about 1%. We argue that
   an overhead of about 1% is an acceptable price to pay in exchange for
   100% compatibility with existing NATs, Firewalls and other
   middleboxes.

7.  Optional Kernel Extensions

   While Minion can be implemented entirely as a user-level library
   built on top of existing standard networking APIs like BSD sockets,
   it can also benefit from some optional kernel extensions:

   Send Priorities
      Normal TCP APIs transmit data strictly in the order is is given to
      the kernel.  The addition of priority support allows a sendmsg()
      call to be used in conjunction with cmsg ancillary data to



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      indicate the priority level of the data.  For normal applications
      this capability would be of little use because it would most
      likely result in corruption of the data stream, but it is useful
      with Minion because the RECOBS encoding is robust against message
      insertion at arbitrary byte boundaries.  An alternative way to
      achieve a similar effect is, instead of buffering data in the
      kernel, to keep the data in the user-space library for as long as
      possible.  When the TCP congestion window and/or receive window
      rules allow more data to be sent, the kernel generates some kind
      of upcall (e.g., a kevent notification) to the user-space library
      informing it of the ability to transmit, and the user-space
      library responds by selecting which particular block of data to
      hand to the kernel next.

   Immediate Receive
      Normal TCP APIs deliver data only in TCP sequence number order.
      The addition of support for new cmsg ancillary data in the
      recvmsg() call allows the user-space library to request *any*
      available data, not only in-order data.  The cmsg ancillary data
      returned from the recvmsg() call indicates to the user-space
      library where in the TCP sequence space this particular block of
      data lies.

   Integrated Receive Window
      Normal TCP APIs raise the receive window any time data is read out
      of the kernel into user space.  The addition of new cmsg ancillary
      data in the recvmsg() call allows the user-space library to
      request that the kernel return received data *without* reflecting
      this in its receive window calculation.  After the client
      application has consumed the message data from the user-space
      Minion library, the Minion library makes a subsequent recvmsg()
      call with appropriate cmsg ancillary data to inform the kernel how
      many bytes to add back into its receive window.  In essence, the
      receive window boundary is stretched outside the kernel to account
      for data held by *both* the kernel *and* the user-space Minion
      library.

   These optional kernel extensions are a key part of what makes Minion
   compelling.  Minion can be adopted today by any application, using
   Minion as a purely user-space library.  Such an application performs
   as well as any application can when it is built on top of standard
   TCP.  However, unlike an application built on top of standard TCP,
   Minion offers the promise of future kernel support for even better
   performance.  Any given application with its own application-specific
   protocol is unlikely to receive special kernel support to make just
   that one application work better.  But when many applications all use
   the Minion protocol, it then becomes reasonable to add kernel support
   to improve all of those applications.



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8.  IANA Considerations

   No IANA actions are required by this document.

9.  Security Considerations

   No new security risks occur as a result of using this protocol.

10.  Acknowledgements

   Many thanks to Bryan Ford, Padma Bhooma and Anumita Biswas for their
   contributions to the development of Minion.

11.  References

11.1.  Normative References

   [COBS]     Cheshire, S. and M. Baker, "Consistent Overhead Byte
              Stuffing", September 1997,
              <http://stuartcheshire.org/papers/COBSforToN.pdf>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

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

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [minserv]  Iyengar, J., "Minion - Service Model and Conceptual API",
              draft-iyengar-minion-concept-00 (work in progress), June
              2013.

11.2.  Informative References

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

Authors' Addresses











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   Janardhan Iyengar
   Franklin and Marshall College
   Mathematics and Computer Science
   PO Box 3003
   Lancaster, Pennsylvania  17604-3003
   USA

   Phone: +1 717 358 4774
   Email: janardhan.iyengar@fandm.edu


   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com


   Josh Graessley
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 408 974 5710
   Email: jgraessley@apple.com






















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