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Performance-enhancing proxies (PEPs) are network agents designed to improve the end-to-end performance of some communication protocols. PEP standards are defined in RFC 3135 (PEPs intended to mitigate link-related degradations) and RFC 3449 (TCP performance implications of network path asymmetry).

Classification

Available PEP implementations use different methods to enhance performance.

Proxy type: A PEP can either 'split' a connection or 'snoop' into it. In the first case, the proxy pretends to be the opposite endpoint of the connection in each direction, literally splitting the connection into two. In the latter case, the proxy controls the transmissions of the TCP segments in both directions, by ack filtering and reconstruction in the existing connection (see protocol spoofing). This is based on the OSI level of implementation of the PEP.^[1]
Distribution: PEPs can be either integrated or distributed. Integrated PEP will run on a single box, while distributed PEP will require to be installed on both sides of the link that cause the performance degradation. This is quite common in commercial PEP devices, which act as a black box, using more or less open protocols to communicate between them in the place of TCP.
Symmetry: A PEP implementation may be symmetric or asymmetric. Symmetric PEPs use identical behavior in both directions; the actions taken by the PEP occur independent from which interface a packet is received. Asymmetric PEPs operate differently in each direction, which can cause, for example, only one link direction performance to be enhanced.

Types

There are a range of different types of PEPs. Each is used to solve a link related problem. Some common types include:

Split-TCP
Ack decimation
Snoop
D-proxy

Split TCP

スプリット TCP は通常、大きな往復遅延時間のTCP問題を解決するために使用されます。一般的なシステムでは、スプリットTCP PEPsを使用して改善 [[伝送制御プロトコル|[[サテライトリンク]上のTCPパフォーマンス。[エンドツーエンド接続]を複数の接続に分割し、異なるパラメータを使用して異なる足を渡してデータを転送することで、TCP機能を分割します。エンドシステムは、変更なしで標準 TCP を使用し、間に PEP の存在を知る必要はありません。スプリット TCP は、エンド・システムからの TCP 接続をインターセプトして終了します。これにより、エンド・システムは変更されずに実行でき、サテライト通信に対して設定が低すぎるエンド・システム上の TCP ウィンドウ・サイズに関するいくつかの問題を克服できます。

Ack filtering/decimation

Ack filtering or decimation is used on highly asymmetric links. In asymmetric links the upstream and downstream rates vary widely. A common example is satellite broadband where a downstream satellite link provides significantly greater bandwidths than the upstream dialup modem link. In this scenario, the speed at which the modem can return TCP acknowledgements can be a limiting factor. As TCP acknowledgements are cumulatively acknowledged some can be decimated or filtered to improve performance.

Snoop

The Snoop proxy^[2] is an example of an integrated proxy. It is designed to hide interference or collision-based packet loss over a wireless link. Snoop proxies detect losses by monitoring TCP transmissions for duplicate acknowledgements. When duplicate TCP acknowledgements, indicating a packet loss, are received by Snoop, they will be silently dropped and lost data packet will be retransmitted. The TCP sender should have no knowledge of the loss. This should prevent TCP senders from unnecessarily reducing the TCP window.

D-Proxy

D-Proxy^[3]^[4] is also designed to hide interference or collision based packet loss over a wireless link. D-Proxy is a new distributed TCP proxy, requiring a proxy on either side of the lossy link. Like Snoop, it uses TCP sequence numbers to detect lost packets. However, it has a proactive approach, monitoring the TCP sequence numbers on data packets rather than acknowledgements. When packet loss occurs, the TCP stream will be temporarily buffered until the missing packet can be recovered and re-sequenced.

References

^ [1]: PERFORMANCE ENHANCEMENT PROXY (PEP) : TCP in Wireless Network
^ Balakrishnan, Hari; Srinivasan Seshan; Randy H. Katz (December 1995). “Improving TCP/IP Performance over Wireless Networks”. ACM Wireless Networks 1 (4).
^ Murray, David; Terry Koziniec; Michael Dixon (2009). “Solving Ack Inefficiencies in 802.11 Networks”. IEEE International Conference on Internet Multimedia Systems Architecture and Applications.
^ Murray, David; Terry Koziniec; Michael Dixon (2010). “D-Proxy: Reliability in Wireless Networks”. 16th Asia-Pacific Conference on Communications (APCC).

External links

PEPsal : A GPL licensed, Linux-based, integrated splitting PEP implementation
PEP server MediaSputnik : PEP server MediaSputnik 2402 has been developed by MediaSputnik as I-PEP compatible server compliant with SatLabs Group (ESA) recommendations to support DVB-RCS standards and networks
RFC 3135 : The whole RFC (Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations)

[1] [1]: PERFORMANCE ENHANCEMENT PROXY (PEP) : TCP in Wireless Network

[2] Balakrishnan, Hari; Srinivasan Seshan; Randy H. Katz (December 1995). “Improving TCP/IP Performance over Wireless Networks”. ACM Wireless Networks 1 (4).

[3] Murray, David; Terry Koziniec; Michael Dixon (2009). “Solving Ack Inefficiencies in 802.11 Networks”. IEEE International Conference on Internet Multimedia Systems Architecture and Applications.

[4] Murray, David; Terry Koziniec; Michael Dixon (2010). “D-Proxy: Reliability in Wireless Networks”. 16th Asia-Pacific Conference on Communications (APCC).

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