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Versions: (draft-natarajan-httpbis-sctp) 00 01 02

Network Working Group                                       P. Natarajan
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                                 P. Amer
Expires: September 10, 2009                                  J. Leighton
                                                  University of Delaware
                                                                F. Baker
                                                           Cisco Systems
                                                           March 9, 2009


           Using SCTP as a Transport Layer Protocol for HTTP
                 draft-natarajan-http-over-sctp-01.txt

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Abstract

   Hyper-Text Transfer Protocol (HTTP) [RFC2116] requires a reliable
   transport for end-to-end communication.  While historically TCP has
   been used for this purpose, this document proposes an alternative --
   the Stream Control Transmission Protocol (SCTP) [RFC4960].  Similar
   to TCP, SCTP offers a reliable end-to-end transport connection to
   applications.  Additionally, SCTP offers other innovative services
   unavailable in TCP.  The objectives of this draft are three-fold: (i)
   to highlight SCTP services that better match HTTP's needs than TCP,
   (ii) to propose a design for persistent and pipelined HTTP 1.1
   transactions over SCTP's multistreaming service, and (iii) to share
   some lessons learned from implementing HTTP over SCTP.  Finally, open
   issues warranting more discussion and/or investigation are listed.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  SCTP Services for HTTP-based Applications  . . . . . . . . . .  3
   4.  Designing HTTP over SCTP Streams . . . . . . . . . . . . . . .  5
     4.1.  Number of SCTP Streams . . . . . . . . . . . . . . . . . .  7
     4.2.  Mapping HTTP Transactions to SCTP Streams  . . . . . . . .  7
   5.  Lessons Learned from Implementing HTTP over SCTP . . . . . . .  8
     5.1.  Avoiding Dependencies in Message Transmission  . . . . . .  8
     5.2.  Order of Pipelined Requests and Responses  . . . . . . . .  8
     5.3.  Benefits for Progressive Images  . . . . . . . . . . . . .  9
   6.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  How does a Web client decide between TCP vs. SCTP? . . . . 10
     6.2.  TCP-SCTP Gateway . . . . . . . . . . . . . . . . . . . . . 10
     6.3.  SCTP and NATs  . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12














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1.  Introduction

   SCTP was originally developed to carry telephony signaling messages
   over IP networks.  With continued work, SCTP evolved into a general
   purpose transport protocol.  Similar to TCP, SCTP offers a reliable,
   full-duplex, congestion and flow-controlled transport connection.
   Unlike TCP, SCTP offers other innovative services including
   multistreaming, multihoming, partial-realiability, and message-
   oriented data transfer.  This document highlights some of the SCTP
   services that are better suited to HTTP's needs than TCP services.

   SCTP's multistreaming service is perhaps the most beneficial SCTP
   service for HTTP.  SCTP streams are logically separate data streams
   within an SCTP "association" (analogous to a TCP connection).
   Independent HTTP transactions, when transmitted over different SCTP
   streams, can be delivered to the application without inter-
   transaction head-of-line (HOL) blocking.  Emulation results show that
   SCTP streams eliminate HOL blocking and significantly improve web
   response times [N2008].  This document presents our design for
   persistent and pipelined HTTP 1.1 transactions over SCTP streams, and
   some of the lessons learned from implementing this design in the
   Apache server and Firefox browser.

   Finally, this document lists some of the open issues that require
   further discussion and/or investigation within the httpbis community.


2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  SCTP Services for HTTP-based Applications

   Similar to TCP, SCTP provides a reliable and in-order data transfer
   service to HTTP.  Additionally, SCTP provides other services
   unavailable in TCP.  These services are summarized below.  The HTTP
   over SCTP design proposed in Section 4 utilizes only a subset of
   these SCTP services.  The authors believe that other SCTP services
   listed below MAY help HTTP, but the details remain unclear at this
   time.


   1.  SCTP Multistreaming Avoids Head-of-Line (HOL) Blocking.

       An SCTP stream is a unidirectional data flow within an SCTP



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       association.  Each SCTP stream has its own sequencing space; data
       arriving in-order within a stream is delivered to the application
       without regard to the relative order of data arriving on other
       streams.  When independent HTTP transactions are transmitted over
       different SCTP streams, these transactions are delivered to the
       application without inter-transactoin HOL blocking.  Section 4
       discusses the benefits of HTTP over SCTP streams in more detail.

   2.  Four-way Handshake during Association Establishment.

       To protect an end host from SYN-flooding DoS attacks, SCTP's
       association establishment involves a four-way handshake with a
       cookie mechanism.  Since data transfer can begin in the third
       leg, the four-way handshake does not delay data transmission any
       further than TCP's three-way handshake for connection
       establishment.

   3.  No Maximum Segment Lifetime (MSL) during Association Termination.

       SCTP's association termination does not involve a TIME_WAIT state
       [RFC0793], since the Initiation and Verification tags help to
       associate SCTP Protocol Data Units (PDUs) with the corresponding
       SCTP associations [RFC4960].  Note that TCP's TIME_WAIT state
       increases memory and processing overload at a busy web server
       [FTY1999].

   4.  SCTP Multihoming for Improved Fault Tolerance.

       Unlike TCP and UDP, an SCTP association can bind multiple IP
       addresses at each peer.  While an SCTP sender transmits data to a
       single primary destination IP address, the sender concurrently
       tracks the reachability of other destination addresses for fault-
       tolerance purposes.  If the primary address becomes unreachable,
       an SCTP sender seamlessly migrates data transmission to an
       alternate active destination address.  Multihomed clients and/or
       web servers will automatically benefit from greater fault-
       tolreance by using SCTP.

   5.  Preserving Application Message Boundaries.

       Similar to UDP, SCTP offers message-oriented data transfer.  SCTP
       preserves application message boundaries; messages are delivered
       in their entirety to the receiving application.  Applications
       using SCTP do not require explicit message delimiters, which
       simplifies message parsing.  However, the advantages of SCTP's
       message-oriented data transmission service to HTTP is unclear,
       and the proposed HTTP over SCTP design in Section 4 does not
       exploit this SCTP service.  To preserve message boundaries, SCTP



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       employs a fragmentation and reassemebly algorithm.  This
       algorithm creates dependencies in message transmission, discussed
       further in Section 5.1, [N2008].

   6.  Partial Reliability.

       Reference [RFC3758] describes PR-SCTP, an extenstion to
       [RFC4960], that enables partially reliable data transfer between
       a PR-SCTP sender and receiver.  In TCP and plain SCTP, all
       transmitted data are guaranteed to be delivered.  Alternatively,
       PR-SCTP gives an application the flexibility to notify how
       persistent the transport sender should be in trying to
       communicate a particular message to the receiver.  An application
       MAY specify a "lifetime" for each message.  A PR-SCTP sender
       tries to transmit the message during this lifetime.  Upon
       lifetime expiration, a PR-SCTP sender discards the message
       irrespective of whether or not the message was successfully
       transmitted and/or acknowledged.  This timed reliability in data
       transfer may be useful in applications that regularly generate
       new data that obsoletes earlier data, for example, online gaming
       application in which a player frequently generates new position
       coordinates or other data with ephemeral significance.  The
       proposed HTTP over SCTP design in Section 4 currently does not
       make use of this PR-SCTP service.

   7.  Unordered Data Delivery.

       Similar to UDP and unlike TCP, SCTP offers unordered data
       delivery service.  An application message, marked for unordered
       delivery, is delivered to the receiving application as soon as
       the message arrives at the SCTP receiver.  Unlike UDP, SCTP
       provides reliability for unordered data.  Note that a single SCTP
       association can transfer both ordered and unordered messages.
       The proposed HTTP over SCTP design in Section 4 does not make use
       of this SCTP service.


4.  Designing HTTP over SCTP Streams

   In this document, an HTTP GET request (or response) is considered
   independent when its application-level processing does not depend on
   the availability of other HTTP GET requests (or responses).  The
   primary objective of our design is to exploit SCTP's multistreaming
   service to avoid HOL blocking between independent HTTP transactions.

   Note that HTTP transctions do not experience HOL blocking when either
   (i) each HTTP transaction is transmitted over a different TCP
   connection (HTTP 1.0) [RFC1945], or (ii) multiple HTTP transactions



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   are transmitted in a non-pipelined fashion over a single persistent
   TCP connection [RFC2616].  Consequently, we do not expect SCTP's
   multistreaming to improve response times for an HTTP 1.0 transfer or
   a non-pipelined HTTP 1.1 transfer.  Nonetheless, these HTTP transfers
   may benefit from other SCTP features such as multihoming, four-way
   association establishment handshake etc., mentioned in Section 3.
   Note that a client or server implementing HTTP 1.0 or non-pipelined
   HTTP 1.1 over TCP can be trivially mapped to work over SCTP by
   creating an SCTP socket instead of a TCP socket
   [I-D.ietf-tsvwg-sctpsocket].

   An HTTP transaction may be HOL blocked by another independent HTTP
   transaction only when these transactions are transmitted in a
   pipelined fashion over a single TCP connection.  Transferring these
   transactions over different streams of a single SCTP association
   elimiates the inter-transaction HOL blocking.  Emulation results show
   that persistent and pipelined HTTP 1.1 transfers over a single
   multistreamed SCTP association experience better response times when
   compared to similar transfers over a single TCP connection.  The
   difference in TCP vs. SCTP response times increases and is more
   visually perceivable in high latency and lossy browsing conditions,
   such as those found in the developing world [NAS2008].

   Apart from improving response times, SCTP streams may also reduce
   setup and memory costs at a web server/cache/proxy.  To reduce HOL
   blocking, web clients open muliple TCP connections to download
   independent HTTP transactions from the same server.  In contrast, a
   web client using SCTP eliminates HOL blocking by simply increasing
   the number of streams within a single SCTP association.  Each TCP
   connection or SCTP stream incurs additional setup and memory overhead
   at both the client and server.  However, the costs associated with a
   new SCTP stream are in general lower than those associated with a new
   TCP connection, and the cost gains from using SCTP increase as the
   number of web clients increase.  The exact difference in TCP vs. SCTP
   resource requirements depends on the respective protocol
   implementations [N2008].

   Two guidelines govern the HTTP over SCTP streams design discussed
   below: (i) make no changes to the existing HTTP specification (such
   as the URI syntax), to reduce deployment issues, and (ii) minimize
   SCTP-related state information at the server so that SCTP
   multistreaming does not further contribute to the server being a
   performance bottleneck.  Detailed discussions on various design
   decisions can be found in [N2008].  The two components of this design
   are discussed next.






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4.1.  Number of SCTP Streams

   SCTP streams are uni-directional; inbound and outbound streams carry
   data to and from each end point, respectively.  Each inbound or
   outbound stream incurs additional memory overhead in the SCTP
   Protocol Control Block, and this overhead depends on the SCTP
   implementation.  The number of inbound or outbound SCTP streams is
   negotiated during the association establishment phase (Figure 1).
   Before association establishment, the number of inbound or outbound
   streams may be modified by using appropriate SCTP socket options
   [I-D.ietf-tsvwg-sctpsocket].  The stream "reset" functionality allows
   for re-negotiating the number of streams after association
   establishment [I-D.stewart-tsvwg-sctpstrrst].  When using SCTP for
   HTTP, an SCTP association MAY employ any number of inbound or
   outbound streams (up to 65,536 [RFC4960]).  However, for every
   outbound SCTP stream with id *a* on which the client transmits
   requests, there MUST be a corresponding inbound stream with id *a*.
   Typically, this is achieved by opening an SCTP association with equal
   number of inbound and outbound streams.


                     Client                                    Server
                        |                                         |
                        |-----------INIT (IS=m,OS=m)------------->|
   #Streams = MIN (m,n) |<---------INIT-ACK (IS=n,OS=n)-----------| #Streams = MIN (m,n)
                        |                                         |
                        /                   .                     /
                        \                   .                     \
                        |                                         |
                        |----------HTTP GET i (on OS a)---------->|
                        |<---------HTTP RES i (on OS a)-----------|
                        |                                         |
   IS: Inbound Stream
   OS: Outbound Stream



                     Figure 1: HTTP over SCTP Streams

4.2.  Mapping HTTP Transactions to SCTP Streams

   To avoid incurring additional processing overhead at the web server,
   a web client determines the SCTP stream number on which each HTTP
   transaction is transmitted.  In the example shown in Figure 1, the
   web client maps HTTP transaction *i* to SCTP stream *a*.  The client
   transmits HTTP request *i* on the client's outbound (server's
   inbound) SCTP stream *a*.  The web server transmits the corresponding
   response on the server's outbound (client's inbound) SCTP stream *a*.



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   The sctp_sendmsg and sctp_recvmsg APIs, respectively, can be used to
   transmit data on a particular SCTP outbound stream, and determine the
   SCTP inbound stream number on which an application message was
   received.

   When the number of available SCTP streams is greater than or equal to
   the number of HTTP transactions, a web client SHOULD NOT pipeline
   transactions intra-stream, i.e., each HTTP transaction SHOULD be
   mapped to a different SCTP stream.  When the number of available SCTP
   streams is less than the number of HTTP transactions, the web client
   MAY employ a scheduling policy to pipeline transactions intra-stream.
   Our implementation employs a round-robin scheduling policy, where
   HTTP transactions are mapped to available SCTP streams in a round-
   robin fashion.  Other scheduling policies MAY be considered.  For
   example, in a lossy network environment, such as wide area wireless
   connectivity through GPRS, a better scheduling policy might be
   'smallest pending object first' where the next GET request goes on
   the SCTP stream that has the smallest sum of object sizes pending
   transfer.  Such a policy reduces the probability of intra-stream HOL
   blocking, i.e., HOL blocking between responses downloaded on the same
   SCTP stream.


5.  Lessons Learned from Implementing HTTP over SCTP

   HTTP over SCTP was implemented in the Apache server and Firefox
   browser at the University of Delaware's Protocol Engineering Lab.
   Some lessons learned during this experience are discussed below.
   More details can be found in [N2008].

5.1.  Avoiding Dependencies in Message Transmission

   SCTP's fragmentation and reassembly algorithm creates dependencies in
   message transmission, i.e., a fragment of message i+1 cannot be
   transmitted until all fragments of message i have been transmitted.
   If messages i and i+1 are of sizes 100KB and 1KB respectively, the
   100KB message transmission can unnecessarily block transmission of
   the 1KB message.  The client or server application can overcome this
   by splitting each HTTP request or response into multiple messages,
   such that, each message at the SCTP layer results in a PMTU-sized
   SCTP PDU, and is not fragmented further by SCTP.  An application can
   use either the SCTP_PEER_ADDR or the SCTP_STATUS socket options to
   obtain an SCTP association's PMTU [I-D.ietf-tsvwg-sctpsocket].

5.2.  Order of Pipelined Requests and Responses

   Section 8 of [RFC2616] mandates that in HTTP 1.1 with pipelining, "a
   server MUST send its responses to those requests in the same order



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   that the requests were received."  Since TCP always delivers data in-
   order, the order of HTTP requests received by the server, and
   therefore, the order of HTTP responses generated by the server match
   the order of transmitted HTTP requests from the client.
   Consequently, a web client can assume that, within a TCP connection,
   the order of HTTP responses from the server always matches the order
   of transmitted HTTP requests.  Unlike TCP, SCTP's multistreaming
   feature delivers out-of-order data at both the server and client.
   When HTTP requests from client to server are lost, requests
   transmitted over different SCTP streams will be delivered out-of-
   order at the server, and therefore, the order of generated HTTP
   responses will be different from the order of transmitted HTTP
   requests.  Also, the loss of an HTTP response will affect the order
   of HTTP responses from the server.  Our experience with the FreeBSD
   SCTP implementation revealed that HTTP requests and responses can be
   received out-of-order even under no loss conditions [N2008].
   Therefore, web client implementations MUST be aware that within an
   SCTP assocation, the order of pipelined responses from the server may
   not match the order of transmitted HTTP reqeusts.  However, in case
   of intra-stream pipelining, the order of HTTP responses within an
   inbound SCTP stream *a* MUST match the order of transmitted HTTP
   requests within the corresponding outbound SCTP stream *a*.
   Consequently, within each SCTP stream, a web server MUST send its
   responses to those reqeusts in the same order that the requests were
   received.

5.3.  Benefits for Progressive Images

   Progressive images (e.g., JPEG, PNG) are coded such that the initial
   bytes approximate the entire image, and successive bytes gradually
   improve the image's quality/resolution.  Simple experiments have
   shown that user-perceived response time improvements for HTTP 1.1
   (persistent and pipelined) transfers consisting of progressive images
   are more significant than for similar transfers consisting of non-
   progressive images.  When each progressive image is downloaded on a
   different SCTP stream, the Firefox implementation over FreeBSD SCTP
   renders a good quality version of each progressive image
   significantly earlier than the page download time [NAS2008].  These
   page downloads were captured as movies and can be viewed at [Movies].


6.  Open Issues

   This section discusses some of the open issues that require further
   discussion and/or investigation.






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6.1.  How does a Web client decide between TCP vs. SCTP?

   We see three options for how the web client can decide between TCP
   vs. SCTP for an HTTP (1.0 or 1.1) transfer.  Option 1: The web client
   tries in tandem to establish both a TCP connection and an SCTP
   association to the server.  The web client chooses TCP vs. SCTP
   depending on which transport connection gets established first.
   Option 2: [RFC3263] describes how SIP proxies and clients can select
   the transport protocol using SRV records [RFC2782].  A similar
   solution can be conceived for HTTP.  Option 3: The web client selects
   TCP vs. SCTP based on the URI.  URIs starting with "http://" or
   "https://" imply TCP and a new URI scheme could be established for
   similar services over SCTP, such as, "http-sctp://" or
   "https-sctp://" [I-D.wood-tae-specifying-uri-transports].  While
   option 1 is simplistic, the other options require support from new
   mechanisms and/or protocols.

   Web client implementations MUST be aware that an end user or the
   other end-point (server/proxy) MAY choose to override the client's
   default choice of transport (TCP vs. SCTP).  Also, web clients SHOULD
   cache information on which servers support SCTP, for later re-use.

6.2.  TCP-SCTP Gateway

   Research has shown that SCTP streams enable perceivable improvements
   to web response times, especially in high latency and/or lossy last
   hops such as VSAT links [N2008].  A TCP-SCTP gateway allows web
   clients in such last hops to experiance the benefits of SCTP streams
   even if the web server runs over TCP.  Additionally, the gateway also
   ensures that, web clients connecting to the Internet via the gateway
   MAY always assume SCTP as the default transport instead of trying to
   choose between TCP vs. SCTP as discussed in Section 6.1.

6.3.  SCTP and NATs

   The end-to-end path between a client and server MAY consist of one or
   more Network Address Translators (NATs) that manipulate address and
   port information in IP and SCTP headers.  SCTP's association
   establishment and multihoming mechanisms pose unique challenges in
   the context of NATs.  These issues are discussed in
   [I-D.ietf-behave-sctpnat].


7.  Acknowledgments


8.  References




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8.1.  Normative References

   [I-D.ietf-tsvwg-sctpsocket]
              Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P.
              Lei, "Sockets API Extensions for Stream Control
              Transmission Protocol (SCTP)",
              draft-ietf-tsvwg-sctpsocket-19 (work in progress),
              February 2009.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC2116]  Apple, C. and K. Rossen, "X.500 Implementations
              Catalog-96", RFC 2116, April 1997.

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

8.2.  Informative References

   [FTY1999]  Faber, T., Touch, J., and W. Yue, "The TIME_WAIT state in
              TCP and its effect on busy servers", INFOCOM '99:
              Proceedings of the IEEE INFOCOM Conference, pp. 1573-
              1583 , 1999.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-01 (work in progress),
              February 2009.

   [I-D.stewart-tsvwg-sctpstrrst]
              Stewart, R., Lei, P., and M. Tuexen, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              draft-stewart-tsvwg-sctpstrrst-01 (work in progress),
              February 2009.

   [I-D.wood-tae-specifying-uri-transports]
              Wood, L., "Specifying transport mechanisms for retrieval
              or delivery of URIs",
              draft-wood-tae-specifying-uri-transports-04 (work in



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              progress), February 2009.

   [Movies]   "Movies Comparing HTTP over TCP vs. HTTP over SCTP
              Streams", 2008, <http://www.cis.udel.edu/~amer/PEL/
              leighton.movies/index.html>.

   [N2008]    Natarajan, P., "Leveraging Innovative Transport Layer
              Services for Improved Application Performance", PhD
              Dissertation, Computer & Information Sciences Department,
              University of Delaware, USA , 2009.

   [NAS2008]  Natarajan, P., Amer, P., and R. Stewart, "Multistreamed
              Web Transport for Developing Regions", NSDR '08:
              Proceedings of the second ACM SIGCOMM workshop on
              Networked systems for developing regions, Seattle, WA,
              USA , 2008.

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, May 2004.


Authors' Addresses

   Preethi Natarajan
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone:
   Email: prenatar@cisco.com








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Internet-Draft                SCTP for HTTP                   March 2009


   Paul D. Amer
   University of Delaware
   Computer and Information Sciences Department
   Newark, DE  19716
   USA

   Phone: 302-831-1944
   Email: amer@cis.udel.edu


   Jonathan Leighton
   University of Delaware
   Computer and Information Sciences Department
   Newark, DE  19716
   USA

   Phone:
   Email: leighton@cis.udel.edu


   Fred Baker
   Cisco Systems
   1121 Via Del Rey
   Santa Barbara, CA  93117
   USA

   Phone:
   Email: fred@cisco.com























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