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Versions: 00                                                            
Transport Working Group                                        R. Penno
Internet Draft                                             S. Raghunath
Intended status: Informational                         Juniper Networks
Expires: August 2010                                          R. Woundy
                                                             V. Gurbani
                                              Bell Labs, Alcatel-Lucent
                                                               J. Touch
                                                      February 26, 2010

        LEDBAT Practices and Recommendations for Managing Multiple
                        Concurrent TCP Connections

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 26, 2010.

Copyright Notice

   Copyright (c) 2010 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

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   (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
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   described in the Simplified BSD License.


   Applications routinely open multiple TCP connections.  For example,
   P2P applications maintain connections to a number of different peers
   and web browsers perform concurrent download from the same web
   server.  Application designers pursue different goals when doing so:
   P2P apps need to maintain a well-connected mesh in the swarm while
   web browsers mainly use multiple connections to parallelize requests
   that involve application latency on the web server side.  However
   this practice also has impacts to the host and the network as a
   whole. For example, an application can obtain a larger fraction of
   the bottleneck than if it had used fewer connections. Although
   capacity is the most commonly considered bottleneck resource,
   middlebox state table entries are also an important resource for an
   end system communication.

   This document clarifies the current practices of application design
   involving concurrent TCP connections and reasons behind them, and
   discusses the tradeoffs surrounding their use, whether to one
   destination or to different destinations. Other resource types may
   exist, and the guidelines are expected to comprehensively discuss

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119.

Table of Contents

   1. Introduction 3
   2. Terminology 4
   3. Multiple control versus data connections 5
   4. Multiple TCP Connections Advantages 6
      4.1. Avoiding head-of-line blocking 6
      4.2. Logical partitioning at application level 7
      4.3. Multiple streams with different properties 7

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      4.4. Signaling application layer request completion 7
      4.5. High bandwidth-delay links 7
      4.6. Error resiliency and reliability 8
      4.7. Leveraging multiple processors in a system 8
      4.8. Overcoming TCP Limitations 8
   5. Multiple TCP connections Disadvantages 8
      5.1. Additional connection setup overhead 8
      5.2. Memory Space 9
      5.3. Link Bandwidth 9
      5.4. Middleboxes 10
   6. Conclusion and Recommendations 10
      6.1. Diffserv 10
      6.2. Window scale negotiation 10
      6.3. Number of Connections 11
         6.3.1. HTTP 11
      6.4. Bi-Directional HTTP 12
   7. Security Considerations 12
   8. IANA Considerations 12
   9. Acknowledgments 12
   10. References 13
      10.1. Normative References 13
      10.2. Informative References 13
   Author's Addresses 15

1. Introduction

   The use of P2P protocols by end users is widespread. These protocols
   are meant to exchange, replicate, stream or download files with
   little human intervention, trying at the same time to minimize the
   download time of the files requested by any single peer. This is done
   by opening several connections to multiple peers and downloading one
   or more chunks of the file from each one, selecting faster peers,
   amongst others.

   If we assume that in any file transfer the bottleneck is on the
   uploading peer or server side, end users that utilize P2P clients in
   general download the file faster and consume more bandwidth within a
   specific timeframe than traditional client-server applications. P2P
   clients can overcome the server side bottleneck by opening multiple
   connections to different peers. Users of P2P applications also
   consume bandwidth throughout the whole day since even after a file is
   fully downloaded it will continue to be shared with others users
   increasing the upstream bandwidth.

   We can see then that the advantages of P2P applications come from the
   fact that they open multiple TCP connection to different peers in

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   order to download multiple pieces of a file in parallel, and that
   they always look for faster peers.

   But the use of multiple TCP connections by an application is not new.
   Web Browsers have been doing it for a decade. But these are usually
   short-lived connections as opposed to long-lived connections. A long-
   lived connection in this document should be interpreted as strictly
   defined, i.e., a TCP connection that is simply in the established
   state, but not necessarily continuously transferring data. In the
   case of P2P protocols, e.g. BitTorrent, at any point in time only a
   fraction of these connections is actually sending or receiving data,
   while the others are idle or exchange occasional control information.

   With the popularity of P2P applications, which maintain hundreds of
   long-lived TCP connections to multiple hosts, the issue of
   applications making use of multiple TCP connections has been gaining

   This document clarifies the current practices of application design
   and reasons behind them, and discusses the tradeoffs surrounding the
   use of many concurrent TCP connections to one destination and/or to
   different destinations. Other resource types may exist, and the
   guidelines are expected to comprehensively discuss them.

2. Terminology

   Bandwidth: A measure of the amount of data that can be transferred
   within a time period, often expressed in bits per second. Bit rate
   prefixes are expressed in decimal, so 1 kilobit per second is 1,000
   bits per second, and 1 megabit per second is 1,000,000 bits per
   second. So, if one million bits are transferred within one second,
   the average bandwidth consumption during the transfer would be 1
   megabit per second (1 Mbps). If the same amount of data were
   transferred within a day, the bandwidth would be approximately 11.574
   bits per second.

   Volume: The total number of bytes (or octets) transferred during a
   time period. Byte prefixes are expressed in binary, so 1 kilobyte is
   1,024 bytes, and 1 megabyte is 1,024 * 1,024 = 1048576 bytes. In both
   examples above the volume within a day would have been 125,000 bytes
   or about 122.07 kilobytes (122.07 KB).

   Capacity: The maximum bandwidth a link can sustain continuously.

   Long-lived connection: A TCP connection that is in the established
   state but not necessarily continuously transferring data.

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3. Multiple control versus data connections

   The traditional model of applications interacting with each other
   using TCP started off as a single socket opened between a client and
   a server for data communications.  Control signaling was usually
   passed on the same channel as well.  Telnet [RFC854] and rlogin
   protocols [RFC1282] are good examples of this approach.  File
   Transfer Protocol [RFc959] was one of the first known protocols that
   used more than one connection between a client and a server.  In FTP,
   the client in the normal client-server fashion opens the control
   connection.  This connection is used for commands from the client to
   the server and replies from the server to the client.  Distinguishing
   FTP from other protocols was its use of a second data connection.
   The client initiates this data connection passively, and the port
   number is sent to the server.  The server subsequently establishes an
   active connection to this port.  A data connection is created each
   time a file is transferred between the client and the server.
   However, unlike the control connection, it does not persist for the
   duration of the FTP session.

   These early protocols limited TCP connections between a pair of
   machines.  This changed with the advent of the Hypertext Transfer
   Protocol [RFC2616].  In HTTP, a client (browser) downloads a document
   from a server and analyses it to render the document on a display
   device of some sort.  As part of the analysis, the browser may open
   one or more connections to either the same host from which the
   original document was downloaded, or to different hosts that serve
   other content referenced in the document.  However, these connections
   were usually short lived (the current phenomenon of "long polling"
   notwithstanding).  Here, the client (browser) opens up multiple TCP
   connections to possibly multiple servers simultaneously. The Session
   Initiation Protocol [RFC3261] can use TCP connection in the same vein
   as HTTP did, namely to contact multiple servers simultaneously.
   Generally -- although there are exceptions -- in SIP just like HTTP,
   these connections are typically short-lived.

   More recent protocols like Skype (http://www.skype.com) and
   BitTorrent (http://www.bittorrent.com) have a much different view on
   the number of TCP connections they are willing to open and manage.
   While earlier protocols were parsimonious with connections, the
   modern peer-to-peer protocols do not appear to be wary of this to the
   same degree.  Part of the reason why this is the case is the
   assumption that the older protocols (Telnet, rlogin, HTTP) were
   operating under was that relatively few bytes will be transferred
   from the client to the server while many more bytes will be

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   transferred in the opposite direction.  With current peer-to-peer
   protocols, where the resource to be accessed is distributed among the
   peers, a requesting peer has to open multiple TCP connections to more
   than one peer in order to efficiently download the data represented
   by that resource.

   In summary, trying to establish a boundary between data connections
   and control connections is something of a fool's errand.  Protocols
   evolve to match the capabilities and characteristics of the network.
   While early protocols may have opened up a pair of connections to
   communicate, more recent protocols are not inhibited in the same

   Similarly, while earlier protocols may have established different
   control channel from a data channel, this was not a design rule that
   was carried forward faithfully.  While SIP falls in the former camp
   of a control channel that is distinct from a data channel, HTTP falls
   in the latter camp (i.e., same TCP connection serves to send control
   messages and the data itself.)  BitTorrent and Skype perform control
   and data communications over the very same TCP connection as well.
   BitTorrent, in particular, attempts to open multiple connections to
   many peers, even though only a small subset of these connections are
   involved in the actual data transfer. In Skype, a peer does not open
   multiple connections to access a resource; rather multiple
   connections are opened and maintained to a discrete set of neighbors
   to help in routing of subsequent messages [Skype-analysis].

4. Multiple TCP Connections Advantages

   There are good reasons for an application to use multiple TCP
   connections. P2P apps need to maintain a well-connected mesh in the
   swarm while web browsers mainly use multiple connections to
   parallelize requests that involve application latency on the web
   server side

   But from a P2P standpoint multiple TCP connections are at the heart
   of its functionality. Multiple connections allow for multiple
   simultaneous downloads, which improve reliability and speed. Multiple
   connections also allow more effective discovery of new peers, and
   effective peer-to-peer communication, which allows exchange of
   information such as which pieces of a file a client has and is

4.1. Avoiding head-of-line blocking

   Web browsers started using multiple TCP connections partly because of
   this reason [STEVENS]. This is especially true when the multiple TCP

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   connections are between a pair of hosts. Originally, individual HTTP
   transactions each used their own TCP connection, because HTTP lacked
   a response length marker. The client sent a request to the server,
   and the server's response to the client was completed when the TCP
   connection was closed, i.e., CLOSE was interpreted as ''end of
   transaction''. This caused numerous problems, notably the buildup of
   connections in the TIME-WAIT state at the server [Fab1999]. HTTP
   added persistent connections to v1.0 (they were later included in the
   1.1 spec), and they became the default. In persistent connections,
   transactions complete but the connection remains open for subsequent
   responses. Responses are pipelined, not interleaved, however,
   resulting in Head-of-Line (HOL) blocking.

   HTTP clients currently open 4-8 connections to each endpoint. This
   partly avoids HOL blocking, but also allows increased performance.
   Separate connections open independently, increasing bandwidth, and
   also can use separate endpoint processes, increasing computational
   parallelism that maps more effectively to multiprocessors and multi-
   core systems.

4.2. Logical partitioning at application level

   Some applications such as FTP use a separate connection for control
   and data transfers. The advantage is that this allows a model where
   the data transfer is actually happening between hosts that are not
   local (see [RFC959], sections 2.3 & 5.2).

4.3. Multiple streams with different properties

   The application may need different properties on multiple streams of
   data (e.g., Nagle's algorithm, socket buffer sizes etc).

4.4. Signaling application layer request completion

   If the application assumes that connection close indicates the
   completion of a request, it becomes necessary to have new connections
   for multiple requests. This was a reason for multiple connections in
   HTTP 1.0.

4.5. High bandwidth-delay links

   In the presence of a large bandwidth-delay product, the 16-bit window
   size parameter in TCP header does not allow the application to fully
   utilize the link. In such situations, the current practice is to
   negotiate the Window Scale Option [RFC1323]. In addition multiple TCP
   connections can allow the application to achieve an effectively
   larger window size so that it can better utilize a link with high

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   bandwidth-delay product (e.g. iSCSI [SCSIREF]), although this can
   result in mutual escalation, where TCP fairness is ensured only for
   endpoints opening multiple connections.

4.6. Error resiliency and reliability

   When multiple connections are used to download a single file or
   webpage, for instance, there is lesser chance of a single failure on
   one connection having a negative impact on the whole download.
   Especially with P2P applications, this makes the network robust to
   failures and churn in participants.

4.7. Leveraging multiple processors in a system

   With multiple processor systems, there can be higher performance with
   parallelism and multiple connections spread over different

   This presumes that the kernel is parallelized; the potential for TCP
   parallelism is limited (http://www.isi.edu/touch/pubs/pfhsn94.html)

4.8. Overcoming TCP Limitations

   The performance of a single TCP connection given a certain link is
   well understood today [PARATCPSCK], [FF99], [PFTK98] and the general
   rule of thumb is that a TCP connection can utilize 75% of its
   capacity. This means more than one connection to one or more servers
   would be needed to saturate the link.

5. Multiple TCP connections Disadvantages

   Every connected application on the Internet competes for resources.
   This is not specific to applications that open multiple TCP
   connections.  The use of multiple TCP connections just amplifies the
   issue. In the following sections we discuss these resources and how
   they are amplified by an application opening multiple connections.

5.1. Additional connection setup overhead

   The TCP's mechanisms for starting up the connection and then probing
   the available bandwidth have to be repeated for each new connection.
   So there may be lesser leverage of network information. There is also
   the overhead of additional control traffic that may have been

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5.2. Memory Space

   Each TCP connection needs a TCP control block (TCB) or equivalent to
   keep state about its connection. In operating systems where the TCP
   stack is part of the kernel, this would come from the kernel memory
   space, otherwise from userland memory.

   But irrespective of where the memory comes from a TCP control block
   requires a significant amount of memory. This is significant issue
   for devices that terminate TCP connections from multiple end hosts to
   provide functions such as Load-Balancing, Gateway and Tunneling.

   Some proposals have been put forward to reduce the amount of memory
   occupied by each TCP control block [RFC2140], but the issue remains
   significant and is amplified by applications that use multiple TCP

5.3. Link Bandwidth

   The bottlenecks for these N multiple connections could be shared or
   separate. If separate, there's no specific bottleneck where the
   connections are hogging bandwidth. But from a network resource point
   of view, the application download still gets multiple shares.

   If some/all bottlenecks are shared, then two possibilities exist for
   shared bottleneck:

   o  the bottleneck is a last-hop link (user traffic dominates link),

   o  the bottleneck is an in-network wide-area link (background traffic
      dominates link)

   If bottleneck is the last-hop, then n transport connections compete
   with each other and share link bandwidth.

   Although these connections might impact delay-sensitive traffic and
   increase delay, in the last hop they only affect the end-user, which
   is in control of which applications run on its host. In this case the
   user has the option of manually choosing when to run each
   application, configuring the end host, amongst other choices.
   Alternatively, or in conjunction with the above, the application can
   be enhanced to use Diffserv and new delay sensitive congestion

   If the shared bottleneck is in-network, then the application gets an
   unfair share of bottleneck bandwidth. This impacts flows belonging to

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   other users in general, and most importantly may impact delay-
   sensitive traffic.

5.4. Middleboxes

   Middleboxes are defined as any intermediary box performing functions
   apart from normal, standard functions of an IP router on the data
   path between a source host and destination host [RFC3234].
   Middleboxes can be stand-alone or integrated in another device such
   as a router or modem.

   The functions that are relevant to this discussion are those that
   require the middlebox to keep per session state, sometimes referred
   as transformation services. Some of these functions are, for example,
   NAT, Intrusion Detection and Load-Balancing.

   It is easy to see that the more sessions a host initiates, the more
   state the middlebox will have to keep. The relationship is at least
   1:1 but due asymmetric traffic, routing changes and other
   considerations, this can be 1:N.

   Although application traffic from most broadband subscribers today go
   through at least one middlebox (as a stand-alone device in the home
   network, or integrated into the broadband modem), it can traverse
   other middleboxes that reside within the ISP's network or close to
   the destination. These middleboxes aggregate traffic from multiple
   subscribers, and state tables within these devices can become a

6. Conclusion and Recommendations

6.1. Diffserv

   REC-1: Applications involved in bulk data transfer with low priority
   in time could mark their packets according with the guidelines of RFC
   3662 [RFC3662].

6.2. Window scale negotiation

   REC-2: Where appropriate, sender & receiver window should be scaled
   using RFC1323 based negotiation in order to make the best use of
   network resources. Recommendations to adjust window size are not new
   and have been recommended in networks where the BDP (Bandwidth Delay
   Product) is large [RFC3481].

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6.3. Number of Connections

   Multiple connections to the same or different servers provide a
   significant speedup as compared to a single connection. The
   motivation to use multiple connections is to achieve throughput
   efficiency and the cause for such deficiency could be head of line
   blocking, slow servers, server availability or simply overcoming TCP
   throughput limitations. [DYNPARACON],[PARATCPSCK].

   In the case of multiple parallel connection homogeneous connection
   sharing a link of capacity c, it was found that 6 connections are
   sufficient to reach 95% download utilization [PARATCPSCK].
   Interestingly this is comparable to the number of configured active
   transfers (5) of the most used BitTorrent client [UTORRENT]. It is
   worth noticing that during a large file transfer BitTorrent clients
   will prefer peers that provide the largest upload rate, thus
   theoretically saturating its download link. In reality, packet drops,
   upload caps and others transient effects would require clients to
   have more than 5 connections in order to saturate the link, but the
   overall effect of these issues in terms of bandwidth decrease is
   something already measured by BitTorrent clients and used in the
   (optimistic) unchoke/choke algorithm. Therefore the number of active
   connections should not be much higher than 5 since the idea is to
   saturate the link by choosing the best connections and not
   necessarily more connections.

   REC-3: Applications should only open more than 6 connections to
   download the same object if the first hop link is not saturated.

6.3.1. HTTP

   The case of web browsing (HTTP) is quite different from P2P. One
   could argue that the number of active connections used by HTTP is
   much higher than that used by BitTorrent, but the scenarios are quite
   different. In the case of dynamic pages, different objects are
   downloaded from (and exclusively available from) certain locations.
   Moreover, time is of the essence since there is an expectation that a
   page is downloaded and rendered within a few seconds.  Finally,
   objects in a webpage are quite small, with the majority (75%) below
   6KB [HTTPDATA]; therefore many connections are needed to saturate the
   link since TCP congestion avoidance never has time to ramp up to its
   maximum bandwidth.  If multiple small HTTP objects can be retrieved
   from the same server, the use of HTTP/1.1 Pipelining is recommended
   since it can dramatically reduce the number of packets and connection
   overhead between client and server [HTTPPERF].

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   REC-4: HTTP based applications should use HTTP/1.1 pipelining when
   transferring multiple small objects from the same server.

6.4. Bi-Directional HTTP

   Recent frameworks like Ajax allow application developers to write
   applications that allow a delay between when the HTTP server receives
   a request and sends the corresponding response a response.  This
   technique, called "long polling", works by having the HTTP server
   delay sending the response to a request back until it has some
   additional data to sent to the client.  HTTP streaming is a technique
   where the server keeps the connection open indefinitely by using
   chunked Transfer-Encoding mechanism to send incremental responses
   spread over time.

   Both these techniques originated as a counter mechanism to the normal
   manner of polling for events in HTTP: sending multiple requests where
   the inter-request frequency is fairly small. Such a polling mechanism
   tends to overwhelm the server if the polling frequency is set too

   Both long polling and HTTP streaming affect the number of TCP
   connections open over a period of time and the network in the
   following way:

   o  Reducing the overhead of opening/closing connections

   o  Increasing memory consumption in both clients and servers

7. Security Considerations

   None at this time

8. IANA Considerations

   None at this time

9. Acknowledgments

   J. Iyengar was one of the presenters on the first BOF and worked on
   the original version of this document.

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10. References

10.1. Normative References

   [RFC959] J. Postel and J. Reynolds, "File Transfer Protocol (FTP)",
             RFC 959 (1985).

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

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
             and W. Weiss, "An Architecture for Differentiated
             Services", RFC 2475, December 1998.

10.2. Informative References

   [RFC1323] V. Jacobson, B. Braden, D. Borman, TCP Extensions for High
             Performance, RFC 1323, May 1992

   [RFC2140] J. Touch, TCP Control Block Interdependence, RFC 2140

   [RFC2616] R. Fielding et al, Hypertext Transfer Protocol HTTP/1.1,
             RFC 2616 (1999)

   [RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and F.
             Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
             Wireless Networks", BCP 71, RFC 3481, February 2003.

   [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
             Issues", RFC 3234, February 2002.

   [RFC3662]  Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
             Per-Domain Behavior (PDB) for Differentiated Services", RFC
             3662, December 2003.

   [SCSIREF] K.Z. Meth, J. Satran, Design of the iSCSI protocol, Storage
             Conference (2003)

   [STEVENS] W. Richard Stevens et al, ''Unix Network Programming, The
             Sockets Networking API'', Volume 1, Third Edition (2003),
             section 10.5, page 293.

   [HOLBLCK] Head-of-line Blocking in TCP and SCTP: Analysis and

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   [EXPPARA] S. Philopoulos and M. Maheswaran. Experimental Study of
             Parallel Downloading Schemes for Internet Mirror Sites. In
             Thirteenth IASTED International Conference on Parallel and
             Distributed Computing Systems (PDCS '01), Aug. 2001.

   [HTTPPERF] Network Performance Effects of HTTP/1.1, CSS1, and PNG.

   [DYNPARACON] P. Rodriguez and E. W. Biersack, ''Dynamic Parallel-
             Access to Replicated Content in the Internet'', IEEE/ACM
             Transactions on Networking, August 2002

   [UTORRENT] http://www.utorrent.com

   [PARATCPSCK] Altman, E., Barman, D., Tu.n, B., Vojnovic, M.Parallel
             TCP Sockets: Simple Model, Throughput and Validation. In:
             IEEE INFOCOM 2006, Barcelona, Spain (2006)

   [HTTPDATA] Y. C. Chehadeh, A. Z. Hatahet, A. E. Agamy, M. A.
             Bamakhrama, and S. A. Banawan, "Investigating distribution
             of data of http traffic: An empirical study," in
             Innovations in Information Technology, 2006.

   [PFTK98] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose. ''Modeling
             TCP Throughput: A Simple Model and its Empirical
             Validation''. SIGCOMM Symposium on Communications
             Architectures and Protocols, Aug. 1998.

   [FF99] S. Floyd and K. Fall. ''Promoting the Use of End-to-End
             Congestion Control in the Internet''. IEEE/ACM Transactions
             on Networking, Aug. 1999.

   [Fab1999] Faber, T., Touch, J. and W. Yue, "The TIME-WAIT state in
             TCP and Its Effect on Busy Servers", Proc. Infocom 1999 pp.

   [Skype-analysis] S. Baset and H. Schulzrinne, "An Analysis of the
             Skype Peer-to-Peer Internet Telephony Protocol". IEEE
             Infocom", Apr. 2006

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Author's Addresses

   Reinaldo Penno
   Juniper Networks
   1194 N Mathilda Aveue
   Sunnyvale, CA

   Email: rpenno@juniper.net

   Satish Raghunath
   Juniper Networks
   1194 N Mathilda Aveue
   Sunnyvale, CA

   Email: satishr@juniper.net

   Vijay K. Gurbani
   Bell Labs, Alcatel-Lucent
   1960 Lucent Lane
   Room 9C-533
   Naperville, IL

   Email: vkg@bell-labs.com

   Richard Woundy
   Comcast Cable Communications
   27 Industrial Avenue
   Chelmsford, MA  01824

   Email: richard_woundy@cable.comcast.com
   URI:   http://www.comcast.com

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Email: touch@isi.edu
   URL:   http://www.isi.edu/touch

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