5. PEP Environment Examples The following sections describe examples of environments where PEP is currently used to improve performance. The examples are provided to illustrate the use of the various PEP types and PEP mechanisms described earlier in the document and to help illustrate the motivation for their development and use. 5.1 VSAT Environments Today, VSAT networks are implemented with geosynchronous satellites. VSAT data networks are typically implemented using a star topology. A large hub earth station is located at the center of the star with VSATs used at the remote sites of the network. Data is sent from the hub to the remote sites via an outroute. Data is sent from the remote sites to the hub via one or more inroutes. VSATs represent an environment with highly asymmetric links, with an outroute typically
much larger than an inroute. (Multiple inroutes can be used with each outroute but any particular VSAT only has access to a single inroute at a time, making the link asymmetric.) VSAT networks are generally used to implement private networks (i.e., intranets) for enterprises (e.g., corporations) with geographically dispersed sites. VSAT networks are rarely, if ever, used to implement Internet connectivity except at the edge of the Internet (i.e., as the last hop). Connection to the Internet for the VSAT network is usually implemented at the VSAT network hub site using appropriate firewall and (when necessary) NAT [RFC2663] devices. 5.1.1 VSAT Network Characteristics With respect to TCP performance, VSAT networks exhibit the following subset of the satellite characteristics documented in [RFC2488]: Long feedback loops Propagation delay from a sender to a receiver in a geosynchronous satellite network can range from 240 to 280 milliseconds, depending on where the sending and receiving sites are in the satellite footprint. This makes the round trip time just due to propagation delay at least 480 milliseconds. Queueing delay and delay due to shared channel access methods can sometimes increase the total delay up to on the order of a few seconds. Large bandwidth*delay products VSAT networks can support capacity ranging from a few kilobits per second up to multiple megabits per second. When combined with the relatively long round trip time, TCP needs to keep a large number of packets "in flight" in order to fully utilize the satellite link. Asymmetric capacity As indicated above, the outroute of a VSAT network is usually significantly larger than an inroute. Even though multiple inroutes can be used within a network, a given VSAT can only access one inroute at a time. Therefore, the incoming (outroute) and outgoing (inroute) capacity for a VSAT is often very asymmetric. As outroute capacity has increased in recent years, ratios of 400 to 1 or greater are becoming more and more common. With a TCP maximum segment size of 1460 bytes and delayed acknowledgments [RFC1122] in use, the ratio of IP packet bytes for data to IP packet bytes for ACKs is only (3000 to 40) 75 to 1.
Thus, inroute capacity for carrying ACKs can have a significant impact on TCP performance. (The issue of asymmetric link impact on TCP performance is described in more detail in [BPK97].) With respect to the other satellite characteristics listed in [RFC2488], VSAT networks typically do not suffer from intermittent connectivity or variable round trip times. Also, VSAT networks generally include a significant amount of error correction coding. This makes the bit error rate very low during clear sky conditions, approaching the bit error rate of a typical terrestrial network. In severe weather, the bit error rate may increase significantly but such conditions are rare (when looked at from an overall network availability point of view) and VSAT networks are generally engineered to work during these conditions but not to optimize performance during these conditions. 5.1.2 VSAT Network PEP Implementations Performance Enhancing Proxies implemented for VSAT networks generally focus on improving throughput (for applications such as FTP and HTTP web page retrievals). To a lesser degree, PEP implementations also work to improve interactive response time for small transactions. There is not a dominant PEP implementation used with VSAT networks. Each VSAT network vendor tends to implement their own version of PEP functionality, integrated with the other features of their VSAT product. [HNS] and [SPACENET] describe VSAT products with integrated PEP capabilities. There are also third party PEP implementations designed to be used with VSAT networks. These products run on nodes external to the VSAT network at the hub and remote sites. NettGain [FLASH] and Venturi [FOURELLE] are examples of such products. VSAT network PEP implementations generally share the following characteristics: - They focus on improving TCP performance; - They use an asymmetric distributed implementation; - They use a split connection approach with local acknowledgments and local retransmissions; - They support some form of compression to reduce the amount of bandwidth required (with emphasis on saving inroute bandwidth). The key differentiators between VSAT network PEP implementations are: - The maximum throughput they attempt to support (mainly a function of the amount of buffer space they use);
- The protocol used over the satellite link. Some implementations use a modified version of TCP while others use a proprietary protocol running on top of UDP; - The type of compression used. Third party VSAT network PEP implementations generally focus on application (e.g., HTTP) specific compression algorithms while PEP implementations integrated into the VSAT network generally focus on link specific compression. PEP implementations integrated into a VSAT product are generally transparent to the end systems. Third party PEP implementations used with VSAT networks usually require configuration changes in the remote site end systems to route TCP packets to the remote site proxies but do not require changes to the hub site end systems. In some cases, the PEP implementation is actually integrated transparently into the end system node itself, using a "bump in the stack" approach. In all cases, the use of a PEP is non-transparent to the user, i.e., the user is aware when a PEP implementation is being used to boost performance. 5.1.3 VSAT Network PEP Motivation VSAT networks, since the early stages of their deployment, have supported the use of local termination of a protocol (e.g., SDLC and X.25) on each side of the satellite link to hide the satellite link from the applications using the protocol. Therefore, when LAN capabilities were added to VSAT networks, VSAT customers expected and, in fact, demanded, the use of similar techniques for improving the performance of IP based traffic, in particular TCP traffic. As indicated in Section 5.1, VSAT networks are primarily used to implement intranets with Internet connectivity limited to and closely controlled at the hub site of the VSAT network. Therefore, VSAT customers are not as affected (or at least perceive that they are not as affected) by the Internet related implications of using PEPs as are other technologies. Instead, what is more important to VSAT customers is the optimization of the network. And, VSAT customers, in general, prefer that the optimization of the network be done by the network itself rather than by implementing changes (such as enabling the TCP scaled window option) to their own equipment. VSAT customers prefer to optimize their end system configuration for local communications related to their local mission critical functions and let the VSAT network hide the presence of the satellite link as much as possible. VSAT network vendors have also been able to use PEP functionality to provide value added "services" to their customers such as extending the useful of life of older equipment which includes older, "non-modern" TCP stacks.
Of course, as the line between intranets and the Internet continues to fade, the implications of using PEPs start to become more significant for VSAT networks. For example, twelve years ago security was not a major concern because the equipment cost related to being able to intercept VSAT traffic was relatively high. Now, as technology has advanced, the cost is much less prohibitive. Therefore, because the use of PEP functionality in VSAT networks prevents the use of IPsec, customers must rely on the use of higher layer security mechanisms such as TLS or on proprietary security mechanisms implemented in the VSAT networks themselves (since currently many applications are incapable of making (or simply don't make) use of the standardized higher layer security mechanisms). This, in turn, affects the cost of the VSAT network as well as affects the ability of the customers to make use of Internet based capabilities. 5.2 W-WAN Environments In mobile wireless WAN (W-WAN) environments the wireless link is typically used as the last-hop link to the end user. W-WANs include such networks as GSM [GSM], GPRS [GPRS],[BW97], CDPD [CDPD], IS-95 [CDMA], RichoNet, and PHS. Many of these networks, but not all, have been designed to provide mobile telephone voice service in the first place but include data services as well or they evolve from a mobile telephone network. 5.2.1 W-WAN Network Characteristics W-WAN links typically exhibit some combination of the following link characteristics: - low bandwidth (with some links the available bandwidth might be as low as a few hundred bits/sec) - high latency (minimum round-trip delay close to one second is not exceptional) - high BER resulting in frame or packet losses, or long variable delays due to local link-layer error recovery - some W-WAN links have a lot of internal buffer space which tend to accumulate data, thus resulting in increased round-trip delay due to long (and variable) queuing delays - on some W-WAN links the users may share common channels for their data packet delivery which, in turn, may cause unexpected delays to the packet delivery of a user due to simultaneous use of the same channel resources by the other users
- unexpected link disconnections (or intermittent link outages) may occur frequently and the period of disconnection may last a very long time - (re)setting the link-connection up may take a long time (several tens of seconds or even minutes) - the W-WAN network typically takes care of terminal mobility: the connection point to the Internet is retained while the user moves with the mobile host - the use of most W-WAN links is expensive. Many of the service providers apply time-based charging. 5.2.2 W-WAN PEP Implementations Performance Enhancing Proxies implemented for W-WAN environments generally focus on improving the interactive response time but at the same time aim at improving throughput, mainly by reducing the transfer volume over the inherently slow link in various ways. To achieve this, typically enhancements are applied at almost all protocol layers. 22.214.171.124 Mowgli System The Mowgli system [KRA94] is one of the early approaches to address the challenges induced by the problematic characteristics of low bandwidth W-WAN links. The indirect approach used in Mowgli is not limited to a single layer as in many other split connection approaches, but it involves all protocol layers. The basic architecture is based on split TCP (UDP is also supported) together with full support for application layer proxies with a distributed PEP approach. An application layer proxy pair may be added between a client and server, the agent (local proxy) on a mobile host and the proxy on an intermediate node that provides the mobile host with the connection to the wireline Internet. Such a pair may be either explicit or fully transparent to the applications, but it is, at all times, under end-user control thus allowing the user to select the traffic that traverses through the PEP implementation and choose end-to-end IP for other traffic. In order to allow running legacy applications unmodified and without recompilation, the socket layer implementation on the mobile host is slightly modified to connect the applications, which are configured to traverse through the PEP, to a local agent while retaining the original TCP/IP socket semantics. Two types of application layer agent-proxy pairs can be configured for mobile host application use.
A generic pair can be used with any application and it simply provides split transport service with some optional generic enhancements like compression. An application-specific pair can be retailed for any application or a group of applications that are able to take leverage on the same kind of enhancements. A good example of enhancements achieved with an application-specific proxy pair is the Mowgli WWW system that improves significantly the user perceived response time of Web browsing mainly by reducing the transfer volume and the number of round trips over the wireless link [LAKLR95], [LHKR96]. Mowgli provides also an option to replace the TCP/IP core protocols on the last-hop link with a custom protocol that is tuned for low- bandwidth W-WAN links [KRLKA97]. This protocol was designed to provide the same transport service with similar semantics as regular TCP and UDP provide, but use a different protocol implementation that can freely apply any appropriate protocol mechanisms without being constrained by the current TCP/IP packet format or protocol operation. As this protocol is required to operate over a single logical link only, it could partially combine the protocol control information and protocol operation of the link, network, and transport layers. In addition, the protocol can operate on top of various link services, for example on top of different raw link services, on top of PPP, on top of IP, or even on top of a single TCP connection using it as a link service and implementing "TCP multiplexing" over it. In all other cases, except when the protocol is configured to operate on top of raw (wireless) link service, IP may co-exist with the custom protocol allowing simultaneous end-to- end IP delivery for the traffic not traversing through the PEP implementation. Furthermore, the custom protocol can be run in different operation modes which turn on or off certain protocol functions depending on the underlying link service. For example, if the underlying link service provides reliable data delivery, the checksum and the window-based error recovery can be turned off, thus reducing the protocol overhead; only a very simple recovery mechanism is needed to allow recovery from an unexpected link disconnection. Therefore, the protocol design was able to use extremely efficient header encoding (only 1-3 bytes per packet in a typical case), reduce the number of round trips significantly, and various features that are useful with low-bandwidth W-WAN links were easy to add. Such features include suspending the protocol operation over the periods of link disconnection or link outage together with fast start once the link becomes operational again, priority-based multiplexing of user data over the W-WAN link thus offering link capacity to interactive
applications in a timely manner even in presence of bandwidth- intensive background transfers, and link-level flow control to prevent data from accumulating into the W-WAN link internal buffers. If desired, regular TCP/IP transport, possibly with corresponding protocol modifications in TCP (and UDP) that would tune it more suitable for W-WAN links, can be employed on the last-hop link. 126.96.36.199 Wireless Application Protocol (WAP) The Mowgli system was designed to support mobile hosts that are attached to the Internet over constrained links, but did not address the specific challenges with low-end mobile devices. Many mobile wireless devices are power, memory, and processing constrained, and the communication links to these devices have lower bandwidth and less stable connections. These limitations led designers to develop the Wireless Application Protocol (WAP) that specifies an application framework and network protocols intended to work across differing narrowband wireless network technologies bringing Internet content and advanced data services to low-end digital cellular phones and other mobile wireless terminals, such as pagers and PDAs. The WAP model consists of a WAP client (mobile terminal), a WAP proxy, and an origin server. It requires a WAP proxy between the WAP client and the server on the Internet. WAP uses a layered, scalable architecture [WAPARCH], specifying the following five protocol layers to be used between the terminal and the proxy: Application Layer (WAE) [WAPWAE], Session Layer (WSP) [WAPWSP], Transaction Layer (WTP) [WAPWTP], Security Layer (WTLS) [WAPWTLS], and Transport Layer (WDP) [WAPWDP]. Standard Internet protocols are used between the proxy and the origin server. If the origin server includes WAP proxy functionality, it is called a WAP Server. In a typical scenario, a WAP client sends an encoded WAP request to a WAP proxy. The WAP proxy translates the WAP request into a WWW (HTTP) request, performing the required protocol conversions, and submits this request to a standard web server on the Internet. After the web server responds to the WAP proxy, the response is encoded into a more compact binary format to decrease the size of the data over the air. This encoded response is forwarded to the WAP client [WAPPROXY]. WAP operates over a variety of bearer datagram services. When communicating over these bearer services, the WAP transport layer (WDP) is always used between the WAP client and WAP proxy and it provides port addressed datagram service to the higher WAP layers. If the bearer service supports IP (e.g., GSM-CSD, GSM-GPRS, IS-136, CDPD), UDP is used as the datagram protocol. However, if the bearer
service does not support IP (e.g., GSM-SMS, GSM-USSD, GSM Cell Broadcast, CDMS-SMS, TETRA-SDS), WDP implements the required datagram protocol as an adaptation layer between the bearer network and the protocol stack. The use of the other layers depends on the port number. WAP has registered a set of well-known ports with IANA. The port number selected by the application for communication between a WAP client and proxy defines the other layers to be used at each end. The security layer, WTLS, provides privacy, data integrity and authentication. Its functionality is similar to TLS 1.0 [RFC2246] extended with datagram support, optimized handshake and dynamic key refreshing. If the origin server includes WAP proxy functionality, it might be used to facilitate the end-to-end security solutions, otherwise it provides security between the mobile terminal and the proxy. The transaction layer, WTP, is message based without connection establishment and tear down. It supports three types of transaction classes: an unconfirmed request (unidirectional), a reliable (confirmed) request (unidirectional), and a reliable (confirmed) request-reply transaction. Data is carried in the first packet and 3-way handshake is eliminated to reduce latencies. In addition acknowledgments, retransmission, and flow control are provided. It allows more than one outstanding transaction at a time. It handles the bearer dependence of a transfer, e.g., selects timeout values and packet sizes according to the bearer. Unfortunately, WTP uses fixed retransmission timers and does not include congestion control, which is a potential problem area as the use of WAP increases [RFC3002]. The session layer, WSP, supports binary encoded HTTP 1.1 with some extensions such as long living session with suspend/resume facility and state handling, header caching, and push facility. On top of the architecture is the application environment (WAE). 5.2.3 W-WAN PEP Motivation As indicated in Section 5.2.1, W-WAN networks typically offer very low bandwidth connections with high latency and relatively frequent periods of link disconnection and they usually are expensive to use. Therefore, the transfer volume and extra round-trips, such as those associated with TCP connection setup and teardown, must be reduced and the slow W-WAN link should be efficiently shielded from excess traffic and global (wired) Internet congestion to make Internet access usable and economical. Furthermore, interactive traffic must be transmitted in a timely manner even if there are other simultaneous bandwidth intensive (background) transfers and during the periods with connectivity the link must be kept fully utilized
due to expensive use. In addition, the (long) periods of link disconnection must not abort active (bulk data) transfers, if an end-user so desires. As (all) applications cannot be made mobility/W-WAN aware in short time frame or maybe ever, support for mobile W-WAN use should be implemented in a way which allows most applications, at least those running on fixed Internet hosts, to continue their operation unmodified. 5.3 W-LAN Environments Wireless LANs (W-LAN) are typically organized in a cellular topology where an access point with a W-LAN transceiver controls a single cell. A cell is defined in terms of the coverage area of the base station. The access points are directly connected to the wired network. The access point in each of the cells is responsible for forwarding packets to and from the hosts located in the cell. Often the hosts with W-LAN transceivers are mobile. When such a mobile host moves from one cell to another cell, the responsibility for forwarding packets between the wired network and the mobile host must be transferred to the access point of the new cell. This is known as a handoff. Many W-LAN systems also support an operation mode enabling ad-hoc networking. In this mode access points are not necessarily needed, but hosts with W-LAN transceiver can communicate directly with the other hosts within the transceiver's transmission range. 5.3.1 W-LAN Network Characteristics Current wireless LANs typically provide link bandwidth from 1 Mbps to 11 Mbps. In the future, wide deployment of higher bandwidths up to 54 Mbps or even higher can be expected. The round-trip delay with wireless LANs is on the order of a few milliseconds or tens of milliseconds. Examples of W-LANs include IEEE 802.11, HomeRF, and Hiperlan. Wireless personal area networks (WPAN) such as Bluethooth can use the same PEP techniques. Wireless LANs are error-prone due to bit errors, collisions and link outages. In addition, consecutive packet losses may also occur during handoffs. Most W-LAN MAC protocols perform low level retransmissions. This feature shields upper layers from most losses. However, unavoidable losses, retransmission latency and link outages still affect upper layers. TCP performance over W-LANs or a network path involving a W-LAN link is likely to suffer from these effects.
As TCP wrongly interprets these packet losses to be network congestion, the TCP sender reduces its congestion window and is often forced to timeout in order to recover from the consecutive losses. The result is often unacceptably poor end-to-end performance. 5.3.2 W-LAN PEP Implementations: Snoop Berkeley's Snoop protocol [SNOOP] is a TCP-specific approach in which a TCP-aware module, a Snoop agent, is deployed at the W-LAN base station that acts as the last-hop router to the mobile host. Snoop aims at retaining the TCP end-to-end semantics. The Snoop agent monitors every packet that passes through the base station in either direction and maintains soft state for each TCP connection. The Snoop agent is an asymmetric PEP implementation as it operates differently on TCP data and ACK channels as well as on the uplink (from the mobile host) and downlink (to the mobile host) TCP segments. For a data transfer to a mobile host, the Snoop agent caches unacknowledged TCP data segments which it forwards to the TCP receiver and monitors the corresponding ACKs. It does two things: 1. Retransmits any lost data segments locally by using local timers and TCP duplicate ACKs to identify packet loss, instead of waiting for the TCP sender to do so end-to-end. 2. Suppresses the duplicate ACKs on their way from the mobile host back to the sender, thus avoiding fast retransmit and congestion avoidance at the latter. Suppressing the duplicate ACKs is required to avoid unnecessary fast retransmits by the TCP sender as the Snoop agent retransmits a packet locally. Consider a system that employs the Snoop agent and a TCP sender S that sends packets to receiver R via a base station BS. Assume that S sends packets A, B, C, D, E (in that order) which are forwarded by BS to the wireless receiver R. Assume the first transmission of packet B is lost due to errors on the wireless link. In this case, R receives packets A, C, D, E and B (in that order). Receipt of packets C, D and E trigger duplicate ACKs. When S receives three duplicate ACKs, it triggers fast retransmit (which results in a retransmission, as well as reduction of the congestion window). The Snoop agent also retransmits B locally, when it receives three duplicate ACKs. The fast retransmit at S occurs despite the local retransmit on the wireless link, degrading throughput. Snoop deals with this problem by dropping TCP duplicate ACKs appropriately at BS.
For a data transfer from a mobile host, the Snoop agent detects the packet losses on the wireless link by monitoring the data segments it forwards. It then employs either Negative Acknowledgements (NAK) locally or Explicit Loss Notifications (ELN) to inform the mobile sender that the packet loss was not related to congestion, thus allowing the sender to retransmit without triggering normal congestion control procedures. To implement this, changes at the mobile host are required. When a Snoop agent uses NAKs to inform the TCP sender of the packet losses on the wireless link, one possibility to implement them is using the Selective Acknowledgment (SACK) option of TCP [RFC2018]. This requires enabling SACK processing at the mobile host. The Snoop agent sends a TCP SACK, when it detects a hole in the transmission sequence from the mobile host or when it has not received any new packets from the mobile host for a certain time period. This approach relies on the advisory nature of the SACKs: the mobile sender is advised to retransmit the missing segments indicated by SACK, but it must not assume successful end-to-end delivery of the segments acknowledged with SACK as these segments might get lost later in the path to the receiver. Instead, the sender must wait for a cumulative ACK to arrive. When the ELN mechanism is used to inform the mobile sender of the packet losses, Snoop uses one of the 'unreserved' bits in the TCP header for ELN [SNOOPELN]. The Snoop agent keeps track of the holes that correspond to segments lost over the wireless link. When a (duplicate) ACK corresponding to a hole in the sequence space arrives from the TCP receiver, the Snoop agent sets the ELN bit on the ACK to indicate that the loss is unrelated to congestion and then forwards the ACK to the TCP sender. When the sender receives a certain number of (duplicate) ACKs with ELN (a configurable variable at the mobile host, e.g., two), it retransmit the missing segment without performing any congestion control measures. The ELN mechanism using one of the six bits reserved for future use in the TCP header is dangerous as it exercises checks that might not be correctly implemented in TCP stacks, and may expose bugs. A scheme such as Snoop is needed only if the possibility of a fast retransmit due to wireless errors is non-negligible. In particular, if the wireless link uses link-layer recovery for lost data, then this scheme is not beneficial. Also, if the TCP window tends to stay smaller than four segments, for example, due to congestion related losses on the wired network, the probability that the Snoop agent will have an opportunity to locally retransmit a lost packet is small. This is because at least three duplicate ACKs are needed to trigger the local retransmission, but due to small window the Snoop
agent may not be able to forward three new packets after the lost packet and thus induce the required three duplicate ACKs. Conversely, when the TCP window is large enough, Snoop can provide significant performance improvement (compared with standard TCP). In order to alleviate the problem with small TCP windows, Snoop proposes a solution in which a TCP sender is allowed to transmit a new data segment for each duplicate ACK it receives as long as the number of duplicate ACKs is less than the threshold for TCP fast retransmission (three duplicate ACKs). If the new segment reaches the receiver, it will generate another duplicate ACK which, in turn, allows the sender to transmit yet another data segment. This continues until enough duplicate ACKs have accumulated to trigger TCP fast retransmission. This proposal is the same as the "Limited Transfer" proposal [RFC3042] that has recently been forwarded to the standards track. However, to be able to benefit from this solution, it needs to be deployed on TCP senders and therefore it is not ready for use in a short time frame. Snoop requires the intermediate node (base station) to examine and operate on the traffic between the mobile host and the other end host on the wired Internet. Hence, Snoop does not work if the IP traffic is encrypted. Possible solutions involve: - making the Snoop agent a party to the security association between the client and the server; - IPsec tunneling mode, terminated at the Snooping base station. However, these techniques require that users trust base stations. Snoop also requires that both the data and the corresponding ACKs traverse the same base station. Furthermore, the Snoop agent may duplicate efforts by the link layer as it retransmits the TCP data segments "at the transport layer" across the wireless link. (Snoop has been described by its designers as a TCP-aware link layer. This is the right approach: the link and network layers can be much more aware of each other than strict layering suggests.) 5.3.3 W-LAN PEP Motivation Wireless LANs suffer from an error prone wireless channel. Errors can typically be considered bursty and channel conditions may change rapidly from mobility and environmental changes. Packets are dropped from bit errors or during handovers. Periods of link outage can also be experienced. Although the typical MAC performs retransmissions, dropped packets, outages and retransmission latency still can have serious performance implications for IP performance, especially TCP.
PEPs can be used to alleviate problems caused by packet losses, protect TCP from link outages, and to add priority multiplexing. Techniques such as Snoop are integrally implemented in access points, while priority and compression schemes are distributed across the W- LAN. 6. Security Considerations The use of Performance Enhancing Proxies introduces several issues which impact security. First, (as described in detail in Section 4.1.1,) using PEPs and using IPsec is generally mutually exclusive. Unless the PEP is also both capable and trusted to be the endpoint of an IPsec tunnel (and the use of an IPsec tunnel is deemed good enough security for the applicable threat model), a user or network administrator must choose between improved performance and network layer security. In some cases, transport (or higher) layer security can be used in conjunction with a PEP to mitigate the impact of not having network layer security. But, support by applications for the use of transport (or higher) layer security is far from ubiquitous. Additionally, the PEP itself needs to be protected from attack. First, even when IPsec tunnels are used with the PEP, the PEP represents a point in the network where traffic is exposed. And, the placement of a PEP in the network makes it an ideal platform from which to launch a denial of service or man in the middle attack. (Also, taking the PEP out of action is a potential denial of service attack itself.) Therefore, the PEP must be protected (e.g., by a firewall) or must protect itself from improper access by an attacker just like any other device which resides in a network. 7. IANA Considerations This document is an informational overview document and, as such, does not introduce new nor modify existing name or number spaces managed by IANA. 8. Acknowledgements This document grew out of the Internet-Draft "TCP Performance Enhancing Proxy Terminology", RFC 2757 "Long Thin Networks", and work done in the IETF TCPSAT working group. The authors are indebted to the active members of the PILC working group. In particular, Joe Touch and Mark Allman gave us invaluable feedback on various aspects of the document and Magdolna Gerendai provided us with essential help on the WAP example.
9. References [BBKT97] P. Bhagwat, P. Bhattacharya, A. Krishma, S.K. Tripathi, "Using channel state dependent packet scheduling to improve TCP throughput over wireless LANs," ACM Wireless Networks, March 1997, pp. 91 - 102. Available at: http://www.acm.org/pubs /articles/journals/wireless/1997-3-1/p91-bhagwat/p91- bhagwat.pdf [BPK97] H. Balakrishnan, V.N. Padmanabhan, R.H. Katz, "The Effects of Asymmetry on TCP Performance," Proc. ACM/IEEE Mobicom, Budapest, Hungary, September 1997. [BW97] G. Brasche, B. Walke, "Concepts, Services, and Protocols of the New GSM Phase 2+ general Packet Radio Service," IEEE Communications Magazine, Vol. 35, No. 8, August 1997. [CDMA] Electronic Industry Alliance (EIA)/Telecommunications Industry Association (TIA), IS-95: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, 1993. [CDPD] Wireless Data Forum, CDPD System Specification, Release 1.1, 1995. [CTC+97] H. Chang, C. Tait, N. Cohen, M. Shapiro, S. Mastrianni, R. Floyd, B. Housel, D. Lindquist, "Web Browsing in a Wireless Environment: Disconnected and Asynchronous Operation in ARTour Web Express," Proc. MobiCom'97, Budapest, Hungary, September 1997. [FMSBMR98] D.C. Feldmeier, A.J. McAuley, J.M. Smith, D.S. Bakin, W.S. Marcus, T.M. Raleigh, "Protocol Boosters," IEEE Journal on Selected Areas of Communication, Vol. 16, No. 3, April 1998. [FLASH] Flash Networks Ltd., performance boosting products technology vendor based in Holmdel, New Jersey. Website at http://www.flashnetworks.com. [FOURELLE] Fourelle Systems, performance boosting products technology vendor based in Santa Clara, California. Website at http://www.fourelle.com. [GPRS] ETSI, "General Packet Radio Service (GPRS): Service Description, Stage 2," GSM03.60, v.6.1.1, August 1998.
[GSM] M. Rahnema, "Overview of the GSM system and protocol architecture," IEEE Communications Magazine, Vol. 31, No. 4, pp. 92-100, April 1993. [HNS] Hughes Network Systems, Inc., VSAT technology vendor based in Germantown, Maryland. Website at http://www.hns.com. [I-TCP] A. Bakre, B.R. Badrinath, "I-TCP: Indirect TCP for Mobile Hosts," Proc. 15th International Conference on Distributed Computing Systems (ICDCS), May 1995. [KRA94] M. Kojo, K. Raatikainen, T. Alanko, "Connecting Mobile Workstations to the Internet over a Digital Cellular Telephone Network," Proc. Workshop on Mobile and Wireless Information Systems (MOBIDATA), Rutgers University, NJ, November 1994. Revised version published in Mobile Computing, pp. 253-270, Kluwer, 1996. [KRLKA97] M. Kojo, K. Raatikainen, M. Liljeberg, J. Kiiskinen, T. Alanko, "An Efficient Transport Service for Slow Wireless Telephone Links," IEEE Journal on Selected Areas of Communication, Vol. 15, No. 7, September 1997. [LAKLR95] M. Liljeberg, T. Alanko, M. Kojo, H. Laamanen, K. Raatikainen, "Optimizing World-Wide Web for Weakly- Connected Mobile Workstations: An Indirect Approach," Proc. of the 2nd Int. Workshop on Services in Distributed and Networked Environments, Whistler, Canada, pp. 132- 139, June 1995. [LHKR96] M. Liljeberg, H. Helin, M. Kojo, K. Raatikainen, "Mowgli WWW Software: Improved Usability of WWW in Mobile WAN Environments," Proc. IEEE Global Internet 1996 Conference, London, UK, November 1996. [M-TCP] K. Brown, S. Singh, "M-TCP: TCP for Mobile Cellular Networks," ACM Computer Communications Review Volume 27(5), 1997. Available at ftp://ftp.ece.orst.edu/pub/singh/papers/mtcp.ps.gz. [Pax99] V. Paxson, "End-to-End Internet Packet Dynamics," IEEE/ACM Transactions on Networking, Vol. 7, No. 3, 1999, pp. 277-292. [PILCWEB] http://pilc.grc.nasa.gov.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1122] Braden, R., "Requirements for Internet Hosts -- Communications Layers", STD 3, RFC 1122, October 1989. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC1958] Carpenter, B., "Architectural Principles of the Internet", RFC 1958, June 1996. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC2151] Kessler, G. and S. Shepard, "A Primer On Internet and TCP/IP Tools and Utilities", FYI 30, RFC 2151, June 1997. [RFC2246] Dierk, T. and E. Allen, "TLS Protocol Version 1," RFC 2246, January 1999. [RFC2393] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload Compression Protocol (IPcomp)", RFC 2393, December 1998. [RFC2401] Kent, S., and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC2488] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488, January 1999. [RFC2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header Compression", RFC 2507, February 1999.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [RFC2509] Engan, M., Casner, S. and C. Bormann, "IP Header Compression over PPP", RFC 2509, February 1999. [RFC2663] Srisuresh, P. and Y. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J., Henderson, T., Heidemann, J., Kruse, H., Ostermann, S., Scott, K., Semke, J., Touch, J. and D. Tran, "Ongoing TCP Research Related to Satellites", RFC 2760, February 2000. [RFC3002] Mitzel, D., "Overview of 2000 IAB Wireless Internetworking Workshop", RFC 3002, December 2000. [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001. [SHEL00] Z. Shelby, T. Saarinen, P. Mahonen, D. Melpignano, A. Marshall, L. Munoz, "Wireless IPv6 Networks - WINE," IST Mobile Summit, Ireland, October 2000. [SNOOP] H. Balakrishnan, S. Seshan, E. Amir, R. Katz, "Improving TCP/IP Performance over Wireless Networks," Proc. 1st ACM Conference on Mobile Communications and Networking (Mobicom), Berkeley, California, November 1995. [SNOOPELN] H. Balakrishnan, R. Katz, "Explicit Loss Notification and Wireless Web Performance," Proc. IEEE Globecom 1998, Internet Mini-Conference, Sydney, Australia, November 1998. [SPACENET] Spacenet, VSAT technology vendor based in Mclean, Virginia. Website at http://www.spacenet.com. [SRC84] J.H. Saltzer, D.P. Reed, D.D. Clark, "End-To-End Arguments in System Design," ACM TOCS, Vol. 2, No. 4, pp. 277-288, November 1984. [WAPARCH] Wireless Application Protocol Architecture Specification, April 1998, http://www.wapforum.org.
[WAPPROXY] Wireless Application Protocol Push Proxy Gateway Service Specification, August 1999, http://www.wapforum.org. [WAPWAE] Wireless Application Protocol Wireless Application Environment Overview, March 2000, http://www.wapforum.org. [WAPWDP] Wireless Application Protocol Wireless Datagram Protocol Specification, February 2000, http://www.wapforum.org. [WAPWSP] Wireless Application Protocol Wireless Session Protocol Specification, May 2000, http://www.wapforum.org. [WAPWTLS] Wireless Application Protocol Wireless Transport Layer Security Specification, February 2000, http://www.wapforum.org. [WAPWTP] Wireless Application Protocol Wireless Transaction Protocol Specification, February 2000, http://www.wapforum.org. [Zhang00] Y. Zhang, B. Singh, "A Multi-Layer IPsec Protocol," Proc. proceedings of 9th USENIX Security Symposium, Denver, Colorado, August 2000. Available at http://www.wins.hrl.com/people/ygz/papers/usenix00.html. 10. Authors' Addresses Questions about this document may be directed to: John Border Hughes Network Systems 11717 Exploration Lane Germantown, Maryland 20876 Phone: +1-301-548-6819 Fax: +1-301-548-1196 EMail: firstname.lastname@example.org
Markku Kojo Department of Computer Science University of Helsinki P.O. Box 26 (Teollisuuskatu 23) FIN-00014 HELSINKI Finland Phone: +358-9-1914-4179 Fax: +358-9-1914-4441 EMail: email@example.com Jim Griner NASA Glenn Research Center MS: 54-5 21000 Brookpark Orad Cleveland, Ohio 44135-3191 Phone: +1-216-433-5787 Fax: +1-216-433-8705 EMail: firstname.lastname@example.org Gabriel Montenegro Sun Microsystems Laboratories, Europe 29, chemin du Vieux Chene 38240 Meylan, FRANCE Phone: +33 476 18 80 45 EMail: email@example.com Zach Shelby University of Oulu Center for Wireless Communications PO Box 4500 FIN-90014 Finland Phone: +358-40-779-6297 EMail: firstname.lastname@example.org
Appendix A - PEP Terminology Summary This appendix provides a summary of terminology frequently used during discussion of Performance Enhancing Proxies. (In some cases, these terms have different meanings from their non-PEP related usage.) ACK filtering Removing acknowledgments to prevent congestion of a low speed link, usually used with paths which include a highly asymmetric link. Sometimes also called ACK reduction. See Section 3.1.4. ACK spacing Delayed forwarding of acknowledgments in order to space them appropriately, for example, to help minimize the burstiness of TCP data. See Section 3.1.1. application layer PEP A Performance Enhancing Proxy operating above the transport layer. May be aimed at improving application or transport protocol performance (or both). Described in detail in Section 2.1.2. asymmetric link A link which has different rates for the forward channel (used for data segments) and the back (or return) channel (used for ACKs). available bandwidth The total capacity of a link available to carry information at any given time. May be lower than the raw bandwidth due to competing traffic. bandwidth utilization The actual amount of information delivered over a link in a given period, usually expressed as a percent of the raw bandwidth of the link. gateway Has several meanings with respect to PEPs, depending on context: - An access point to a particular link;
- A device capable of initiating and terminating connections on behalf of a user or end system (e.g., a firewall or proxy). Not necessarily, but could be, a router. in flight (data) Data sent but not yet acknowledged. More precisely, data sent for which the sender has not yet received the acknowledgement. link layer PEP A Performance Enhancing Proxy operating below the network layer. local acknowledgement The generation of acknowledgments by an entity in the path between two end systems in order to allow the sending system to transmit more data without waiting for end-to-end acknowledgments. Described (in the context of TCP) in Section 3.1.2. performance enhancing proxy An entity in the network acting on behalf of an end system or user (with or without the knowledge of the end system or user) in order to enhance protocol performance. Section 2 describes various types of performance enhancing proxies. Section 3 describes the mechanisms performance enhancing proxies use to improve performance. raw bandwidth The total capacity of an unloaded link available to carry information. Snoop A TCP-aware link layer developed for wireless packet radio and cellular networks. It works by caching segments at a wireless base station. If the base station sees duplicate acknowledgments for a segment that it has cached, it retransmits the missing segment while suppressing the duplicate acknowledgement stream being forwarded back to the sender until the wireless receiver starts to acknowledge new data. Described in detail in Section 5.3.2 and [SNOOP].
split connection A connection that has been terminated before reaching the intended destination end system in order to initiate another connection towards the end system. This allows the use of different connection characteristics for different parts of the path of the originally intended connection. See Section 2.4. TCP PEP A Performance Enhancing Proxy operating at the transport layer with TCP. Aimed at improving TCP performance. TCP splitting Using one or more split TCP connections to improve TCP performance. TCP spoofing Sometimes used as a synonym for TCP PEP. More accurately, TCP spoofing refers to using transparent (to the TCP stacks in the end systems) mechanisms to improve TCP performance. See Section 2.1.1. transparent In the context of a PEP, transparent refers to not requiring changes to be made to the end systems, transport endpoints and/or applications involved in a connection. See Section 2.5 for a more detailed explanation. transport layer PEP A Performance Enhancing Proxy operating at the transport layer. Described in detail in Section 2.1.1. tunneling In the context of PEPs, tunneling refers to the process of wrapping a packet for transmission over a particular link between two PEPs. See Section 3.2.
WAP The Wireless Application Protocol specifies an application framework and network protocols intended to work across differing narrow-band wireless network technologies. See Section 188.8.131.52.
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