Network Working Group P. Natarajan
Internet-Draft Cisco Systems
Intended status: Standards Track P. Amer
Expires: January 10, 2010 J. Leighton
University of Delaware
F. Baker
Cisco Systems
July 9, 2009
Using SCTP as a Transport Layer Protocol for HTTP
draft-natarajan-http-over-sctp-02.txt
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Abstract
Hyper-Text Transfer Protocol (HTTP) [RFC2616] 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 innovative services
unavailable in TCP. This draft (i) specifies HTTP over SCTP's
multistreaming service, (ii) lists open issues warranting more
discussion and/or investigation, and (iii) shares some lessons
learned from implementing HTTP over SCTP. Finally, this document
highlights SCTP services that better match HTTP's needs than TCP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Designing HTTP over SCTP Streams . . . . . . . . . . . . . . . 3
3.1. Number of SCTP Streams . . . . . . . . . . . . . . . . . . 4
3.2. Mapping HTTP Transactions to SCTP Streams . . . . . . . . 5
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Definition of Pipelining . . . . . . . . . . . . . . . . . 5
4.2. How does a Web client decide between TCP vs. SCTP? . . . . 6
4.3. SCTP and Chunked Encoding . . . . . . . . . . . . . . . . 6
4.4. TCP-SCTP Gateway . . . . . . . . . . . . . . . . . . . . . 7
4.5. SCTP and Middleboxes . . . . . . . . . . . . . . . . . . . 7
5. Lessons Learned from Implementing HTTP over SCTP . . . . . . . 7
5.1. Avoiding Dependencies in Message Transmission . . . . . . 7
5.2. Order of Pipelined Requests and Responses . . . . . . . . 8
5.3. Benefits for Progressive Images . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. SCTP Services for HTTP-based Applications . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This draft specifies HTTP over SCTP. 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 new services including multistreaming, multihoming,
message-oriented data transfer etc.
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. This document presents a design for mapping HTTP
transactions over SCTP streams, and also highlights ongoing work and
open issues that require further discussion and/or investigation
within the httpbis community.
HTTP over SCTP was implemented in the Apache Server and Firefox
browser. Some of the lessons learned during this implementation
exercise are discussed. Finally, this document discusses more SCTP
services that are better suited to HTTP's needs than TCP services.
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. Designing HTTP over SCTP Streams
In this document, an HTTP request (or response) is considered
independent when its application-level processing does not depend on
the availability of other HTTP requests (or responses). The primary
objective of our design is to exploit SCTP's multistreaming service
to avoid HOL blocking between independent HTTP transactions.
An SCTP stream is a unidirectional data flow within an SCTP
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-transaction HOL blocking.
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Two guidelines govern the HTTP over SCTP streams design discussed
below: (i) to reduce deployment concerns, make no changes to the
existing HTTP specification (such as the URI syntax), 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 [N2009]. The two components of this design
are discussed next.
3.1. Number of SCTP Streams
SCTP streams are uni-directional; inbound and outbound streams carry
data to and from each end point, respectively. 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 REQUEST i (on OS a)-------->|
|<-------HTTP RESPONSE i (on OS a)--------|
| |
IS: Inbound Stream
OS: Outbound Stream
Figure 1: HTTP over SCTP Streams
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3.2. 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*.
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 either (i) increase the number of SCTP streams in the association
[I-D.stewart-tsvwg-sctpstrrst], such that, each transaction is
transmitted on a different SCTP stream, or (ii) 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 HTTP 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.
4. Open Issues
This section discusses some of the open issues that require further
discussion and/or investigation.
4.1. Definition of Pipelining
Section 8 of [RFC2616] notes that "HTTP requests and responses can be
pipelined on a connection. Pipelining allows a client to make
multiple requests without waiting for each response, allowing a
single TCP connection to be used much more efficiently, with much
lower elapsed time." Adapting this definiton of pipelining to HTTP
over SCTP implies that transmitting multiple HTTP requests over an
SCTP association (transport connection) without waiting for each
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response should be considered as pipelining, even if the requests are
transmitted on different SCTP streams. Ongoing discussion attempts
to figure out if RFC2616's definition of pipelining is generic enough
for any transport including SCTP, or if the definition is suitable
only for TCP.
4.2. How does a Web client decide between TCP vs. SCTP?
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). A web client uses one or
more of the following options to decide between TCP vs. SCTP for an
HTTP 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. This approach is explored in
[I-D.wing-http-new-tech].
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 [H2008].
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 simple and end-to-end, the other options require
support from new protocols and/or infrastructure. Also, using
options 2 and 3, a web client can identify whether the web server
supports SCTP but cannot determine if the middleboxes en-route
support SCTP (discussed below).
4.3. SCTP and Chunked Encoding
SCTP's message-based transmission could be leveraged to avoid HTTP's
chunked encoding feature. Chunked encoding [RFC2616] allows
dynamically generated HTTP messages to be transferred as a series of
chunks, each with its own size indicator. The current proposal is to
use SCTP's Payload Protocol ID (PPID) -- an optional value set by the
sender and read by the receiver, to avoid chunked encoding in HTTP
over SCTP. The approach would be to define two new SCTP PPID values
(allocated by IANA) -- HTTP_MESSAGE_PIECE and HTTP_MESSAGE_END. The
sender sets PPID to HTTP_MESSAGE_PIECE for all but the last chunk of
an HTTP object, and sets the last chunk's PPID to HTTP_MESSAGE_END.
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This proposal is under investigation.
4.4. TCP-SCTP Gateway
Research has shown that SCTP streams enable perceivable improvements
to throughput and web response times, especially in high latency
and/or lossy last hops such as satellite links [N2009]. A TCP-SCTP
gateway allows web clients in such last hops to experience 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 or SCTP as
discussed in Section 4.2. Note that web proxies can also function as
TCP-SCTP gateways.
4.5. SCTP and Middleboxes
SCTP's association establishment and multihoming mechanisms pose
unique challenges in the context of NATs. These issues are addressed
in [I-D.ietf-behave-sctpnat]. The end-to-end path between a client
and server MAY consist of one or more NATs and/or firewalls that do
not support SCTP. Until middleboxes support SCTP, UDP encapsulation
is a possible solution [I-D.tuexen-sctp-udp-encaps].
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 [N2009].
5.1. Avoiding Dependencies in Message Transmission
Similar to UDP, SCTP preserves message boundaries and employs a
fragmentation and reassembly algorithm to accomplish this. 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 delay transmission of
the 1KB message. The client or server application can avoid this
delay 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, thus requiring no further fragmentation 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].
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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
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 [N2009].
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
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. Acknowledgments
Thanks to Henrik Nordstorm, Dan Wing, and Andrew Yourtchenko for
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helping with the Open Issues Section.
7. References
7.1. 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.
7.2. 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.
[H2008] Hardie, T., "Email Post to the APPS-DISCUSS Mailing List",
(work in progress) , 2008.
[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
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Transmission Protocol (SCTP) Stream Reconfiguration",
draft-stewart-tsvwg-sctpstrrst-01 (work in progress),
February 2009.
[I-D.tuexen-sctp-udp-encaps]
Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
Packets", draft-tuexen-sctp-udp-encaps-02 (work in
progress), November 2007.
[I-D.wing-http-new-tech]
Wing, D., Yourtchenko, A., and P. Natarajan, "Happy
Eyeballs: Successful Introduction of New Technology to
HTTP", (work in progress) , 2009.
[I-D.wood-tae-specifying-uri-transports]
Wood, L., "Specifying transport mechanisms in Uniform
Resource Identifiers",
draft-wood-tae-specifying-uri-transports-06 (work in
progress), May 2009.
[Movies] "Movies Comparing HTTP over TCP vs. HTTP over SCTP
Streams", 2008, <http://www.cis.udel.edu/~amer/PEL/
leighton.movies/index.html>.
[N2009] 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)
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Partial Reliability Extension", RFC 3758, May 2004.
Appendix A. SCTP Services for HTTP-based Applications
This draft discussed how SCTP's multistreaming and message-based
transmission could be adapted for HTTP. This section lists other
SCTP services. The authors believe that the SCTP services listed
below MAY help HTTP, but the details remain unclear at this time.
1. 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.
2. 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].
3. 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.
4. Partial Reliability.
Reference [RFC3758] describes PR-SCTP, an extenstion to
[RFC4960], that enables partially reliable data transfer between
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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 3 currently does not
make use of this PR-SCTP service.
5. 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 3 does not make use
of this SCTP service.
Authors' Addresses
Preethi Natarajan
Cisco Systems
425 East Tasman Drive
San Jose, CA 95134
USA
Phone:
Email: prenatar@cisco.com
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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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