Network Working Group P. Natarajan
Internet-Draft P. Amer
Intended status: Standards Track J. Leighton
Expires: May 3, 2009 University of Delaware
F. Baker
Cisco Systems
October 30, 2008
Using SCTP as a Transport Layer Protocol for HTTP
draft-natarajan-http-over-sctp-00.txt
Status of this Memo
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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
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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
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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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 two options for how the web client can decide between using
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: 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,
"httpsctp://" or "httpssctp://". At this point, the authors believe
that option 1 is more desireable than option 2, and will have less
repercussions than option 2.
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.stewart-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-17 (work in progress),
July 2008.
[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.stewart-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-stewart-behave-sctpnat-04 (work in progress),
July 2008.
[I-D.stewart-tsvwg-sctpstrrst]
Stewart, R., Lei, P., and M. Tuexen, "Stream Control
Transmission Protocol (SCTP) Stream Reset",
draft-stewart-tsvwg-sctpstrrst-00 (work in progress),
June 2008.
[Movies] "Movies Comparing HTTP over TCP vs. HTTP over SCTP
Streams", 2008, <http://www.cis.udel.edu/~amer/PEL/
leighton.movies/index.html>.
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[N2008] Natarajan, P., "Leveraging Innovative Transport Layer
Services for Improved Application Performance", PhD
Dissertation, in progress, Computer & Information Sciences
Department, University of Delaware, USA , 2008.
[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.
[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
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone:
Email: nataraja@cis.udel.edu
Paul D. Amer
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone: 302-831-1944
Email: amer@cis.udel.edu
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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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