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A Framework of Multipath Transport System Based on Application-Level Relay (MPTS-AR)
draft-leiwm-tsvwg-mpts-ar-05

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Lei Weimin , Wei Zhang , Shaowei Liu
Last updated 2016-01-19
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draft-leiwm-tsvwg-mpts-ar-05
Network Working Group                                             W. Lei
Internet-Draft                                                  W. Zhang
Intended Status: Experimental                                     S. Liu
Expires: July 19, 2016                           Northeastern University
                                                        January 19, 2016

          A Framework of Multipath Transport System Based on 
                   Application-Level Relay (MPTS-AR)
                      draft-leiwm-tsvwg-mpts-ar-05

Abstract

   Multipath transport is an important way to improve the efficiency of
   data delivery. This document defines a multipath transport system
   framework in which application-level relays are deployed to provide
   the conditions to enable multiple paths between source and
   destination. In the proposed framework, endpoints are allowed to use
   multiple paths, including the default IP path and relay paths, to
   transport data in a single session. A relay path may via one or more
   application-level relays which provide application-level relay
   services for endpoints. This framework defines three kinds of logical
   entities including user agent, relay server and relay controller.
   Relay server provides relay service for user agents based on a local
   path-table. Relay controller manages relay servers and relay paths.
   User agent maintains multiple end-to-end paths which include a
   default path and multiple relay paths. The framework also defines a
   relay service control protocol named OpenPath protocol in control
   plane to manage relay servers and relay paths, and a profile of
   multipath transport protocol suite in data plane to facilitate
   multipath data transport. The multipath transport system framework
   can support various applications including applications requiring
   timely delivery of real-time data such as streaming media, and
   applications requiring ordered reliable delivery of stream of data
   such as file transfer.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF). Note that other groups may also distribute working
   documents as Internet-Drafts. The list of current Internet-Drafts is
   at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
 

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   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 19, 2016.

Copyright Notice
   
   Copyright (c) 2016 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 (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 to this document. Code Components
   extracted from this document must include Simplified BSD License text
   as described in section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1  Deployment and organization of relay controller and relay
          server  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2  Relay path service provided by relay controller . . . . . . 10
     4.3  End-to-end transmission paths managed by user agent . . . . 11
     4.4  Relay service control protocol  . . . . . . . . . . . . . . 12
     4.5  Multipath transport protocol suite and profile  . . . . . . 13
   5.  Usage Scenarios  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1  Usage Scenario in SIP system  . . . . . . . . . . . . . . . 17
   6.  User Agent Behavior  . . . . . . . . . . . . . . . . . . . . . 19
     6.1  Multipath session management  . . . . . . . . . . . . . . . 19
     6.2  Path management . . . . . . . . . . . . . . . . . . . . . . 20
     6.3  Flow partitioning and scheduling  . . . . . . . . . . . . . 21
     6.4  Subflow packaging . . . . . . . . . . . . . . . . . . . . . 22
     6.5  Subflow and flow recombination  . . . . . . . . . . . . . . 22
     6.6  Subflow reporting . . . . . . . . . . . . . . . . . . . . . 23
   7.  Relay Server Behavior  . . . . . . . . . . . . . . . . . . . . 23
     7.1  Connection Management and Registrations . . . . . . . . . . 23
     7.2  Path-Table Management . . . . . . . . . . . . . . . . . . . 24
     7.3  Path Validity Management  . . . . . . . . . . . . . . . . . 27
 

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     7.4  Relay Service Management  . . . . . . . . . . . . . . . . . 27
     7.5  MPTP Packet Processing  . . . . . . . . . . . . . . . . . . 28
     7.6  Topology Discovery and Performance Measurement  . . . . . . 28
   8.  Relay Controller Behavior  . . . . . . . . . . . . . . . . . . 29
     8.1  Relay Server Management . . . . . . . . . . . . . . . . . . 29
     8.2  Topology Maintenance  . . . . . . . . . . . . . . . . . . . 29
     8.3  Relay Path Allocation . . . . . . . . . . . . . . . . . . . 30
   9.  OpenPath Protocol  . . . . . . . . . . . . . . . . . . . . . . 31
     9.1  Protocol Overview . . . . . . . . . . . . . . . . . . . . . 31
       9.1.1  Relay-to-Controller . . . . . . . . . . . . . . . . . . 31
       9.1.2  Controller-to-Relay . . . . . . . . . . . . . . . . . . 32
       9.1.3  User agent-to-Controller  . . . . . . . . . . . . . . . 33
       9.1.4 Symmetric  . . . . . . . . . . . . . . . . . . . . . . . 34
     9.2  Common Structures . . . . . . . . . . . . . . . . . . . . . 34
       9.2.1  OpenPath Common Header  . . . . . . . . . . . . . . . . 34
       9.2.2  Common Body of OpenPath Failure Responses . . . . . . . 36
       9.2.3  Transport Address Structure . . . . . . . . . . . . . . 37
     9.3  Message Format of OpenPath Request and Success Response . . 37
       9.3.1  HELLO . . . . . . . . . . . . . . . . . . . . . . . . . 37
       9.3.2  START/STOP/BYE  . . . . . . . . . . . . . . . . . . . . 38
       9.3.3  ECHO  . . . . . . . . . . . . . . . . . . . . . . . . . 38
       9.3.4  NOTIFY/DELETE_PATH  . . . . . . . . . . . . . . . . . . 40
       9.3.5  ADD_PATH/UPDATE_PATH  . . . . . . . . . . . . . . . . . 41
       9.3.6  ALLOCATE_PATH . . . . . . . . . . . . . . . . . . . . . 42
       9.3.7  RELEASE_PATH  . . . . . . . . . . . . . . . . . . . . . 45
       9.3.8  FEATURES  . . . . . . . . . . . . . . . . . . . . . . . 46
       9.3.9  STATISTICS  . . . . . . . . . . . . . . . . . . . . . . 46
   10.  MPTP Profile  . . . . . . . . . . . . . . . . . . . . . . . . 46
     10.1  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 46
     10.2  MPTP Fixed Header Fields . . . . . . . . . . . . . . . . . 47
       10.2.1  Fixed Header Fields of MPTP Data Packet  . . . . . . . 47
       10.2.2  Fixed Header Fields of MPTP Control Packet . . . . . . 49
   11.  SDP Considerations  . . . . . . . . . . . . . . . . . . . . . 51
     11.1  Signaling MPTP Capability in SDP . . . . . . . . . . . . . 51
     11.2  Relay Path Advertisement in SDP  . . . . . . . . . . . . . 51
   12.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
     12.1  SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 52
   13.  Security Considerations . . . . . . . . . . . . . . . . . . . 53
   14.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 53
     14.1  Normative References . . . . . . . . . . . . . . . . . . . 53
     14.2  Informative References . . . . . . . . . . . . . . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55

 

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1.  Introduction

   For end-to-end multimedia session, multipath transport has more
   benefits than single path transmission. Multiple disjoint (or
   partially disjoint) paths could provide greater transmission
   bandwidth, and redundancy among multiple disjoint paths increases
   transmission reliability by protecting multiple paths from failure of
   one, so multipath transport can promote quality of transmission
   service. Moreover, from the perspective of whole network transmission
   efficacy, multipath transport can achieve load balance and increase
   the efficiency of the network resource usage.

   In order to achieve multipath transport, the following two problems
   need to be considered: 1) how to build multiple paths between
   communication endpoints, and 2) given multipath paths, how to
   implement multipath transport between communication endpoints.

   The current underlying IP routing protocol can only build a single
   transport path between endpoints. This implies that although the
   Internet routing infrastructure is highly redundant, current
   underlying routing protocols fail to fully utilize the network
   redundancy. And it is nearly impossible to update protocol stack of
   existing network devices to support multipath routing due to
   tremendous cost. Now the main usage scenario of multipath transport
   is based on multi-homed host, which requires at least one of
   communicating endpoints is multi-homed. The multi-homed host
   multipath usage scenario relies on the end host equipped with several
   access networks, which is hard to be satisfied in practical
   applications.

   An alternative approach for establishing multiple paths between
   source and destination is to provide support mechanisms in the
   application layer while retaining the underlying network
   infrastructure and end devices. This scheme of multipath transport is
   a kind of overlay network technologies actually. Overlay networks
   have emerged as an effective way to support new applications, as well
   as protocols without any changes in the underlying network layer.
   Multiple disjoint (partially disjoint) transmission paths, which pass
   one or more application-level overlay nodes, can be established
   between end hosts. This method is compatible with existing protocol
   stack, and gets rid of the restrictions of physical network
   conditions. Therefore, it is relatively easier to establish multipath
   transport scenario.

   This document defines a framework of multipath transport system based
   on application-level relay (MPTS-AR). A large amount of application-
   level overlay nodes are deployed to provide relay service to the
   communicating endpoints. The upper application programs are provided
 

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   with opportunities to autonomously select one or multiple paths,
   including the default IP path and relay paths, to transport data in a
   session. A relay path may go through one or multiple overlay nodes
   which provide relay services for endpoints. This framework defines
   three kinds of logical entities including user agent, relay server
   and relay controller, and describes their function blocks and
   behaviors. The framework also defines a relay service control
   protocol named OpenPath protocol in control plane to manage relay
   paths, and a profile of multipath transport protocol suite in data
   plane to facilitate multipath data transport.  

   Relay controller and relay servers constitute a relay service system,
   which provides relay service to user agents. Relay controller is
   responsible for managing relay servers, allocating relay paths, QoS
   condition evaluation and so on. It also provides a unified access
   interface of relay service to user agents or out-of-band signaling
   entities. Main function of relay server is to forward the
   application-level data flow, by receiving media stream from the
   address and port of last hop, and then forwarding to the address and
   port of next hop. A relay path may pass one or more relay servers.

   OpenPath, relay service control protocol, mainly includes two kinds
   of messages: the first is exchanged between relay controller and
   relay server which is used to manage relay servers and relay paths.
   Any application software or special server, which implements data
   relay services and supports OpenPath protocol as described in this
   document, can dynamically register to a relay controller and provide
   relay service for user agents in the region of this relay controller.
   The second is exchanged between relay controller and user agent or
   out-of-band signaling entity which is used to manage relay path
   including relay path inquiry, establishment, teardown, etc. 

   Transport requirements of various applications may be quite diverse.
   These applications are sensitive to different routing metrics such as
   latency, loss, throughput and so on. In order to support a variety of
   applications, the proposed MPTS-AR needs to work with a suite of
   multipath transport protocol (MPTP) which consists of multiple
   application-specific MPTPs. Each application-specific MPTP is aimed
   to meet the transmission requirements of a specific category of upper
   applications. In order to extend easily a new application-specific
   MPTP for an emerging application and simplify implementation of user
   agents, this document gives a common profile for all application-
   specific MPTPs. MPTP profile provides application-level multipath
   routing mechanism which is common in all application-specific MPTPs.
   All application-specific MPTPs MUST follow the common rules defined
   by MPTP profile. This document gives the definition of MPTP profile.
   Application-specific MPTPs are defined in companion documents.

 

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   Main principles for designing this framework include:

   1) Establishment of multipath transport scenarios depends on the
   relay service provided by relay service system. Relay server does not
   care end-to-end transmission characteristics of data flows forwarded
   by it. Therefore, a variety of upper applications can use multipath
   transport services provided by the relay service system.

   2) Management and service access interfaces of relay service are
   standardized so that any organizations and individuals can provide
   specialized relay services.

   3) The MPTP profile defines common rules of multipath transport based
   on application-level relay for various upper applications. To support
   a specific category of upper applications, a corresponding
   application-specific MPTP should be defined in an additional
   document.

2.  Terminology

   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 RFC 2119 [RFC2119].

3.  Definitions

   1) Subflow: flow of data packets along a specific path, i.e., a
   subset of the packets belonging to a data flow. The combination of
   all subflows forms the complete original data flow. Typically, a
   subflow should map to a unique path.

   2) Session: an association between two participants transmitting
   data. An endpoint may be involved in multiple sessions at the same
   time. For example, in a multimedia session, each medium is typically
   carried in a separate real-time transport protocol (RTP) [3] session
   which is a type of 'session' defined here. Another typical example of
   session is to transfer one or multiple files between two endpoints.
   This framework allows the variations defined here for different
   applications.

   3) Multipath Session: a special type of session in which the original
   data flow is split into multiple subflows and each subflow is
   forwarded along a unique path.

   4) Path: sequence of logical links between a source and a
   destination. We define it by a set of addresses. All available paths
   for a session include a default path and one or more relay paths.
 

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   5) Default path: a path between a sender and a receiver, which is
   same as the path negotiated and established by a normal session. In
   Session Initiation Protocol (SIP) [4] and Session Description
   Protocol (SDP) [5] case, the default path is determined by the c= and
   m= lines in SDP during session setup. If either the source or the
   destination is not behind a symmetric NAT, the default path may be
   the direct network path between the source and the destination.
   Otherwise, it may traverse a third-party node, such as a TURN server
   or a media server which is responsible for relaying packets. 

   6) Relay path: a path via one or multiple relay servers between a
   source and a destination. A relay path is defined by a sequence of
   (S, R1, ..., Rm, D), where Ri denotes the address of the i-th relay
   server and m denotes the number of relay servers in this path.

   7) Candidate path: a path that is either a default path or a relay
   path.

   8) Active path: a path that carries media data.

   9) User Agent: a logical entity that can act as either a sender or a
   receiver. A sender is responsible for managing all candidate paths
   and multipath session, partitioning and scheduling subflows, and
   packaging MPTP packets. A receiver is responsible for recombinating
   subflow packets and sending back QoS feedback to the sender for each
   subflow. The behavior of a user agent is further defined in Section
   6.

   10) Relay server: an intermediary entity that primarily plays the
   role of forwarding subflow packets according to a Path-Table. Relay
   server receives subflow packets from its upstream entity of the
   subflow that the received packets belong to, obtains the next-hop
   transport address based on the matching results in the Path-Table,
   and forwards the packets to the corresponding next-hop transport
   address. The behavior of a relay server is further defined in Section
   7.

   11) Path-Table: a table consisting of a set of path entries, which
   may be added, updated or deleted by a remote relay controller via
   OpenPath protocol. Each relay server maintains its own path-table.

   12) Relay Controller: a logical entity that manages all relay servers
   in its region and allocates relay paths for each session. The
   behavior of a relay controller is further defined in Section 8.

   13) Overlay Link: the connection between two relay servers. It is
   usually composed of one or more physical links.

 

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4.  Overview

   In the multipath transport system based on application-level relay,
   relay server provides application-level data forwarding service, 
   which can used to establish multiple relay paths between 
   communication endpoints to achieve multipath transport. With 
   appropriate multipath transport control mechanisms and protocols, not
   only can it achieve higher quality of end-to-end transmission 
   service, but it balance network load and enhance efficiency of the 
   network resource usage.

   Figure 1 illustrates the structure of the proposed multipath 
   transport system framework and the relationship among user agent, 
   relay server and relay controller. Relay controller and relay server
   constitute the relay service system which provides relay service to 
   user agent. User agent maintains multiple end-to-end relay paths, 
   and dispatches data flow along multipath paths following MPTP 
   protocol suite.

   (1)Out-of-band Signaling +-----------------+ (1)Out-of-band Signaling
      +-------------------->|   Out-of-band   |<-------------------+  
      |                     | Signaling Server|                    |
      |                     +-----------------+                    |
      |                              ^                             |
      |                              |(1)OpenPath                  |
      |               ...............|.................            |
      |               .              V                .            |
      |or (2)OpenPath .     +------------------+      .            |
      |  +----------------->| Relay Controller |      .            |   
      |  |            .     +------------------+      .            |  
      |  |            .              ^                .            |
      |  |            .              |                .            |
      V  V            .              |                .            V
   +------------+     .              |OpenPath        .  +------------+
   | User Agent |     .              |                .  | User Agent |
   | (Sender)   |     .              |                .  | (Receiver) |
   +------------+     .              V                .  +------------+
        ||            .          +-----------------+  .          ^ ^
        ||            .          |  Relay server   |  .          | |
        ||            .       +-----------------+  |  .          | |
        |+------------------->|  Relay server   |--+  .          | |
        |  MPTP       .    +-----------------+  |----------------+ |
        +----------------->|  Relay server   |--+     .   MPTP     |
           MPTP       .    |                 |---------------------+
                      .    +-----------------+        .   MPTP
                      .          relay service system .
                      .................................

 

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   Figure 1. The structure of the multipath transport system framework 
                   based on application-level relay (MPTS-AR)

4.1  Deployment and organization of relay controller and relay server

   The relay controller is the central component of the relay service
   system. It provides service access and management interfaces of relay
   service to user agents and relay servers. The main functions of relay
   controller are to manage relay servers, evaluate the QoS conditions
   of relay paths, and provide relay path service to user agents (or
   provide relay path service to user agent indirectly through out-of-
   band signaling server). Relay controller that is usually deployed in
   the core of the network by the overlay service provider usually
   possess high-capacity computing, storage, and bandwidth resources. 

   As the number of user agents and relay servers increases, the
   calculating and storing workload of relay controller also increases,
   which would cause the relay controller become a bottleneck within the
   system and reduce the scalability of MPTS-AR system. 

   When there are a large number of user agents or relay servers,
   multiple relay controllers can be deployed and each of them provides
   management service for part of user agents and relay servers. Relay
   controller cluster may be organized in a variety of ways, such as
   flat mode and hierarchical mode. In flat mode, all relay controllers
   are equivalent. Each relay controller is in charge of managing part
   of relay servers distributed across the network and providing relay
   path service for part of user agents distributed across the network.
   They work independently without sharing any information between them.
   User agents and relay controllers can get the address information of
   relay controllers through a variety of ways such as by querying DNS
   SRV records. In hierarchical mode, relay controllers are divided into
   two types: master relay controllers and slave relay controllers. The
   master relay controller has the global view of the system. It divides
   relay servers into several fields each of which is managed by one
   slave relay controller. When the source and the destination of a data
   flow are located into different fields, the allocation process of
   relay paths is completed coordinately by a master relay controller
   and several slave relay controller. 

   Relay servers provide data relay service to the communicating agents.
   Considering the efficiency of the forwarding service, MPTS-AR adopts
   UDP as the underlying transport protocol. Relay servers may be
   deployed in a variety of ways. In one way, a number of Internet
   service providers (ISP) can actively deploy proprietary overlay nodes
   with high network bandwidth and computing performance in their
   domains and offer them to overlay service providers (OSP). ISPs may
   also provide additional support for the operation of the overlay
 

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   network to the OSP and charge OSP for the resources and support
   provided. The OSP can lease a number of overlay nodes from multiple
   ISPs and deploy multipath transport system on top of them.The OSP
   offers multipath transport service to interested users and charges
   the users for the services. In this way, overlay networks can be
   enhanced with available routing information to reduce path quality
   measurement costs and to provide best routes. In another way, the
   participating nodes that access the same application may also self-
   organize to form a dynamic overlay network among which the nodes with
   higher performance can provide relay service to others. The
   organization of relay servers is a kind of typical overlay network
   technology. Overlay network model of relay servers that is closely
   related to the allocation of relay paths is outside the scope of this
   document. This document does not recommend any specific overlay
   network model and only rules that all relay servers need to register
   to a relay controller and provide data relay service to user agents.

4.2  Relay path service provided by relay controller

   All relay servers register to a relay controller in their region. 
   Relay controller is responsible for managing the relay servers and 
   providing direct or indirect relay path services to the user agent by
   OpenPath protocol including establishing, inquiring, and removing of 
   relay paths.

   Considering the execution efficiency, the relay path is designed to 
   be unidirectional. A bidirectional data flow, such as in a 
   conversational use-case, is regarded as two independent 
   unidirectional flows in opposite directions. Relay paths 
   are assigned for each unidirectional flow.

   User agent sender, who is responsible for sending out data flow, and 
   relay servers need to know the relay path information so they can 
   correctly forward subflow data along a particular relay path. An 
   alternative approach, similar to source routing, is that the user 
   agent sender can store the entire route in MPTP packet headers. Each
   intermediate node will simply examine the headers of a received 
   packet and forward it to the next node as indicated in the source 
   route. The advantage of the method is to simplify the implementation 
   of relay servers. They need not store any path information and can 
   perform routing of MPTP packets only based on MPTP packet headers. 
   But this method introduces traffic overhead considerably, especially 
   when the payload traffic is relatively small. 

   In practice, the user agent sender and relay servers need not to know
   the complete information of the associated path. They just need to 
   know the next-hop transport address for each path associated with
   them. A pair of transport address comprises one network address plus 
 

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   one port. When receiving OpenPath path allocation request messages 
   from user agents directly or indirectly (for example via signaling 
   server such as SIP or RTSP), relay controller generates a set of 
   disjoint relay paths based on information carried by the OpenPath 
   path allocation request message including the media types, quality of
   experience (QoE) expectations or requirements, the number of relay 
   paths expected, and so on. Relay controller associates a unique path 
   identifier to each relay path. On the one hand, relay controller 
   instructs the corresponding relay servers to assign the appropriate 
   resources for incoming data forwarding. On the other hand, relay 
   controller sends OpenPath response message back to the service 
   requester. Usually, the response message includes the path 
   identifier, the relay service address of first-hop relay server and 
   instant QoS of each path. This document neither defines a uniform 
   relay path generation algorithm, nor recommends QoS evaluation method
   of relay path. These methods can be designed by the overlay service 
   provider.

   Relay controller needs to maintain the mapping between the user agent
   and the allocated relay paths. When receiving OpenPath path removal
   request messages, relay controller instructs the associated relay
   servers to release resources and generates a response message back to
   the requester.

4.3  End-to-end transmission paths managed by user agent

   End-to-end transmission paths between user agents includes two types:
   1) default path (DP), which does not go through any relay server; 2)
   relay path (RP), which goes through one or more relay servers.

   In order to establish multiple end-to-end transmission paths, user
   agent sender, needs to collect candidate paths before transmitting
   data. As shown in figure 1, this document provides two alternative
   ways to be used for collecting candidate paths by the user agent
   sender. In the first way, the user agent sender obtains candidate
   paths from the out-of-band signaling used for establishing an
   association between the user agent sender and user agent receiver.
   More specifically, user agents use a kind of out-of-band signaling
   (e.g., SIP, Real-Time Streaming Protocol (RTSP) [6]) to negotiate and
   establish a session with the remote peer before transmitting data.
   The signaling server, which out-of-band signaling passes through
   during session setup, is extended to support the access interfaces of
   relay path service provided by OpenPath protocol. The signaling
   server requests the relay controller to allocate relay paths for the
   session to be established on behalf of the user agent sender. If
   successful, the signaling server inserts the information of the
   allocated relay paths into corresponding signaling messages to inform
   the participating user agents. In the second way, the user agent
 

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   sender obtains candidate paths through direct interaction with the
   relay controller using OpenPath Protocol. The advantage of the first
   way is to avoid a large number of connections of the relay controller
   with user agents, and improve the security of the relay controller
   through limiting communication only with the trusted signaling
   server.

   As mentioned above, relay path is designed to be unidirectional. A
   bidirectional data flow is regarded as two independent unidirectional
   flows in opposite directions. Two participating nodes of the
   bidirectional session are the user agent sender of the corresponding
   unidirectional flows respectively. Both user agent senders needs to
   collect candidate paths for corresponding unidirectional flow.

   After collecting multiple end-to-end transmission paths, user agent
   sender needs to evaluate the quality of the part of a relay path from
   the user agent sender itself to the first-hop relay server (the first
   relay server on a relay path) for each relay path. Combined with path
   QoS information carried by OpenPath path allocation response message,
   user agent sender then calculates end-to-end transmission qualities
   of all relay paths, which are used as the basis of subsequent path
   selection and load distribution.

   During the lifecycle of multipath transport session, user agent
   sender usually needs to dynamically evaluate QoS condition of all
   paths including default path and relay paths using MPTP and
   corresponding measurement mechanism. Dynamic path quality information
   can be used to optimize load distribution process and achieve a
   better transmission service quality.

   User agents should establish a multipath session before using
   multiple paths to transport data flow. It can be set up in many
   possible ways e.g., during session establishment, or at anytime
   during the session. To reduce session startup time, the user agent
   sender can start transporting data via the default path and then
   perform path connectivity checks for relay paths. If there are one or
   multiple available relay paths, the use agent sender updates the
   single-path session to a multipath session.

4.4  Relay service control protocol

   Different from multipath transport scenario based on multi-homed  
   hosts, in the multipath transport scenario based on application-level
   relay, the generation and normal operations of relay paths depend on 
   the relay service system which includes relay controller and relay 
   server. 

   Relay controller provides control functions of relay service, and 
 

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   relay sever provides executive functions of relay service. Relay 
   server needs to report the status to the relay controller, and relay 
   controller needs to control the behaviors of relay servers. In 
   addition, in order to provide relay path service to user agents or 
   out-of-band signaling server, relay controller needs to provide relay
   service access interfaces. Therefore, this document defines a relay 
   service control protocol named OpenPath, which provides interfaces 
   among relay controller, relay server and user agents or out-of-band 
   signaling server.

   The definition of OpenPath protocol will promote the progress of
   standardization of the application-level multipath relay service. As
   long as OpenPath protocol and MPTP are followed, third-party relay
   servers implemented and deployed by any organization and individual
   can interact with relay controller and provide relay service to user
   agents. This document defines OpenPath protocol in detail.

4.5  Multipath transport protocol suite and profile

   Existence of multiple end-to-end transmission paths between
   participating nodes is only the basis of enabling multipath
   transmission. User agents need to use a kind of multipath transport
   protocol to exchange data via multiple transmission path in a
   session. The design of this multipath transport protocol needs to
   focus on considering the following factors: 1) express multipath
   transport control mechanism fully, such as subflow partition and
   recombination mechanism, multipath routing, etc. 2) meet various
   transport requirements of different overlay applications, such as
   real-time transport requirement, reliable transport requirement, etc.
   3) meet transport requirements of relay server. As relay server only
   supports efficient UDP forwarding, multipath transport protocol is
   required to be a UDP-based application-layer protocol. 4) minimize
   the overhead of multipath transport control. The ratio of transport
   control messages should be as small as possible. Some existing
   multipath transmission control protocols, including MPTCP [7], SCTP
   [8], can not meet the aforementioned requirements. In addition,
   multiple multipath transport protocols need to be designed to meet
   different transport requirement of overlay applications.

   In this document, multipath transport protocol (MPTP) is designed to
   be a protocol suite which consists of one MPTP profile and multiple
   application-specific MPTPs. The MPTP profile is aimed to provide
   application-level multipath routing mechanism and each application-
   specific MPTP is aimed to meet the transmission requirements of a
   specific category of upper applications. All application-specific
   MPTPs MUST follow MPTP profile defined in this document. Relay
   servers need to support MPTP profile, and user agents need to support
   MPTP profile and one or multiple application-specific MPTPs.
 

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   MPTP is a UDP-based application-layer transport protocol. The 
   protocol stack architecture of MPTP is shown in figure 2. 

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Applications                         |
   |(VoIP, video conference, streaming, file transfer,...) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     MPTP                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     UDP                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     IP                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 2. The protocol stack architecture of MPTP

   In network protocol stack, the transport layer provides end-to-end 
   communication services for applications. Reliable transport 
   protocols, such as TCP and SCTP, are not particularly suitable for 
   real-time applications such as streaming media and real-time 
   multiplayer games. And they are complex and can be a problem for 
   relay servers which provide relay service for huge numbers of user 
   agents. In contrast, UDP only provides checksums for data integrity, 
   and multiplexing for different applications. It delegates other 
   functions to the above application programs. Therefore it typically 
   gives higher throughput and shorter latency. Based on these 
   considerations, MPTP uses UDP as underlying transport protocol. Each
   UDP datagram carries one MPTP packet. On the one hand, this design 
   decision simplifies the behaviors and enhances service capabilities 
   of relay servers. In other words, relay server works in a stateless 
   manner and provides the UDP-based relay service. On the other hand, 
   all upper applications, that need either timely delivery or reliable
   delivery, can use the transport service provided by MPTP. The 
   delivery requirements of upper application need to be meet by MPTP 
   profile and application-specific MPTPs.

   In addition to carrying payload data passed from upper application 
   programs through multiple paths, MPTP also need to provide path 
   control functions including keeping path alive and monitoring the 
   quality of data delivery on each path.

5.  Usage Scenarios

   The multipath transport system framework based on application-level 
   relay can provide many application scenarios including point-to-
   point, many-to-one and one-to-many communications. These 
   applications could be delay sensitive or reliability sensitive, such
   as real-time communication, parallel streaming media, file transfer 
 

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   or file sharing.

   Figure 3 illustrates a point-to-point multipath session. There are 
   three paths between source and destination, including the default 
   path, one relay path via one relay server and another relay path via
   two relay servers. User agent sender can choose a data partitioning 
   method according to its particular requirements. Then, each flow is 
   assigned to a path. The user agent receiver reassembles the received
   flows using a resequencing buffer to retrieve the reconstructed flow
   which is delivered to the above application.

                         Relay path(A, Relay-1, B)
                              +-----------+  
        +-------------------->|  Relay-1  |-----------------------+  
        |                     +-----------+                       |
        |                                                         V
   +--------+               Default path(A,B)                 +--------+
   | User A |<----------------------------------------------->| User B |
   +--------+                                                 +--------+
        |                                                         ^
        |             Relay path(A, Relay-2, Relay-3, B)          |
        |     +-----------+                     +-----------+     |
        +---->|  Relay-2  |-------------------->|  Relay-3  |-----+  
              +-----------+                     +-----------+

               Figure 3. A point-to-point multipath session

   A wide range of applications require data transmission from 
   geographically distributed sources to one destination. For instance, 
   large volume data of high definition movies are stored at multiple 
   geographically distributed locations. The audiences need to retrieve 
   and integrate data from several locations. This usage scenario can be
   completed by a many-to-one multipath session, which is depicted in 
   Figure 4. In the figure, the user agent receiver streams different 
   portions of a video from three servers concurrently. The path between
   the server and the user agent receiver may go through one or more 
   relay servers.

 

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   +---+                               
   | A |----------------------------------------------------+
   +---+                                                    |
                                                            |
                                                            V
   +---+                  +-----------+                   +---+
   | B |------------------|  Relay-1  |------------------>| D | 
   +---+                  +-----------+                   +---+
                                                            ^
                                                            |
   +---+                  +-----------+                     |
   | C |------------------|  Relay-2  |---------------------+
   +---+                  +-----------+ 

          Figure 4. A many-to-one multipath session

   Many video applications are typically characterized by a wide range 
   of connection qualities and receiving devices which are with 
   different capabilities ranging from cell phones with small screens 
   and restricted processing power to high-end PC with high-definition 
   display. These systems are usually adopting layered coding. Layered 
   coding enables the encoding of a high-quality video bit stream 
   containing one or more subset bit streams that can themselves be 
   decoded independently. This usage scenario can be completed by a 
   one-to-many multipath session, which is depicted in figure 5. A video
   source is encoded into multiple streams, each of which is transported
   by a source tree for video multicast. 

 

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                                                          +---+
                               +------------------------->| A |
                               |                     +--->+---+
                               |                     |
                          +-----------+              |    
     +------------------->|  Relay-1  |--------------|------+
     |                    +-----------+              |      |
     |                         |                     |      | 
     |                         |                     |      V 
   +---+                       +------------------+  |    +---+
   | S |                                          |  |    | B | 
   +---+                       +------------------|--+    +---+
     |                         |                  |         ^
     |                         |                  |         |
     |                    +-----------+           |         |
     +------------------->|  Relay-2  |-----------|---------+
                          +-----------+           |
                               |                  |
                               |                  +------>+---+ 
                               +------------------------->| C |
                                                          +---+

             Figure 5. A one-to-many multipath session

5.1  Usage Scenario in SIP system

   Figure 6 depicts a kind of usage scenario in which SIP is used as a 
   separate signaling to advertise relay paths information. 

 

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             (1)INVITE     +-----------------+       (3)INVITE
     +-------------------->|                 |----------------------+  
     |   +-----------------|     SIP Server  |<-----------------+   |
     |   |   (6)200OK      +-----------------+       (4)200OK   |   |
     |   |                          ^                           |   |
     |   |                          |(2)(5)                     |   |
     |   |                          V                           |   |
     |   |                 +-----------------+                  |   |
     |   |                 | Relay Controller|                  |   |   
     |   |                 +-----------------+                  |   |  
     |   |                   ^     ^       ^                    |   |
     |   |           (2)(5)  |     |(2)(5) | (2)(5)             |   | 
     |   V           +-------+     V       +-------+            |   V
   +--------+ P-1/P-3|       +------------+        |  P-1/P-3 +--------+
   | User A |<-------|------>|  Relay-1   |<-------|--------->| User B |
   +--------+        |       +------------+        |          +--------+
        ^            |                             |               ^
        |            V                             V               |
        |     +-----------+                     +-----------+      |
        +---->|  Relay-2  |<------------------->|  Relay-3  |<-----+
    P-2/P-4   +-----------+                     +-----------+   P-2/P-4

             Figure 6. Usage scenario for multipath transport 
                     system framework in SIP System

   (1) User A sends an INVITE to the SIP server that serves her domain. 

   (2) SIP server extracts the current addressable locations of the 
   caller and the callee, and the media information including media 
   types and listed media codecs. Then SIP server sends an Openpath path
   allocation request, ALLOCATE_PATH, carrying the above information to 
   the relay controller and requests it to assign the appropriate relay 
   paths for the media flow from user B to user A. In this example, the 
   relay controller selects two relay paths for the media flow from user
   B to user A. They are (B, Relay-1, A) and (B, Relay-3, Relay-2, A), 
   named P-1 and P-2 respectively. And each relay path is associated 
   with a globally unique path identifier, named PID-1 and PID-2 
   respectively. The relay controller sends each of the three selected 
   relay server an ADD_PATH request message of OpenPath protocol. 
   ADD_PATH request includes the corresponding path identifier and 
   next-hop transport address of the receiving relay server. For 
   Relay-1, the path identifier is PID-1 and the next-hop transport 
   address is the current addressable location of user A. For Relay-2, 
   the path identifier is PID-2 and the next-hop transport address is 
   the current addressable location of user A. For Relay-3, the path 
   identifier is PID-2 and the next-hop transport address is Relay-2's
   IP address and relay service port.

 

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   (3) SIP server inserts the path identifiers and the next-hop 
   transport addresses of user B into SDP body of INVITE and forward the
   modified INVITE to user B. The next-hop transport addresses of user B
   for P-1 and P-2 are separately the rely address of Relay-1 and 
   Relay-3.

   (4) User B answers the call and sends back a 200 OK response. 

   (5) In the same way, SIP server obtains the allocated relay paths 
   for the media flow from user A to user B from the relay controller. 
   In this example, the relay controller assigns the same relay paths 
   with opposite direction for the media flow from user A to user B 
   based on symmetry principle. They are (A, Relay-1, B) and (A, 
   Relay-2, Relay-3, B), named P-3 and P-4 respectively, associated with
   the path identifiers of PID-3 and PID-4. The relay controller sends 
   an ADD_PATH request message to each of the three selected relay 
   servers. For Relay-1, the path identifier is PID-3 and the next-hop 
   transport address is the current addressable location of user B. For 
   Relay-2, the path identifier is PID-4 and the next-hop transport 
   address is Relay-3's IP address and relay service port. For Relay-3, 
   the path identifier is PID-4 and the next-hop transport address is 
   the current addressable location of user B.

   (6) SIP server inserts the path identifiers and the next-hop 
   transport addresses of user A into SDP body of 200 OK response and 
   forwards it to user A. The next-hop transport addresses of user A for
   P-3 and P-4 are separately the rely address of Relay-1 and Relay-2.

   Through the signaling process above, user A and user B separately 
   obtain three candidate paths including the default path and two relay
   paths as the sending peer of the corresponding media flow. After 
   connectivity check, user A and user B can take advantage of multiple 
   paths to transport the media flow.

6.  User Agent Behavior

   Given multiple paths, user agent needs to provide essential support 
   for multipath transport, including session and path management, flow
   partitioning and scheduling, subflow recombination, QoS feedback for
   each subflow.

6.1  Multipath session management

   In general, a session needs to be established between two 
   participants before transmitting data. At the same way, a multipath 
   session needs to be established before transporting data on multiple 
   paths. The multipath session setup uses a way of out-of-band
   (e.g., SDP in SIP or RTSP). The capability exchange and relay path 
 

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   advertisement should be done during the signaling process. A 
   multipath session may be setup from the beginning, or may be upgraded
   from a normal single path session.

6.2  Path management

   The user agent sender obtains the default path from the out-of-band 
   signaling for the session setup. In SIP/SDP case, the default path is
   determined by the c= and m= lines in SDP. It may be the direct 
   network path between the sender and the receiver, or may traverse a 
   third-party node, such as a TURN server or a dedicated relay server. 
   For the latter, IP address and port of the third-party node is in the
   c= and m= lines in SDP. The path identifier of the default path is 
   set to zero.

   As stated in Section 5, the user agent sender can use two alternative
   ways to collect candidate relay paths. One way is that the user agent
   sender obtains candidate relay paths from the out-of-band signaling 
   for the session setup. The signaling server, which out-of-band 
   signaling passes through during session setup, requests the relay 
   controller to allocate relay paths for the session to be established 
   using OpenPath protocol. And then the signaling server inserts the 
   information of the allocated relay paths into corresponding signaling
   messages to inform the participating user agents. Another way is that
   the user agent sender obtains candidate paths through direct 
   interaction with the relay controller using OpenPath Protocol.

   There may be one or multiple data flows in a single session. If there
   are multiple flows in a session, user agent should determine which 
   flows to be multipath transmitted. For example, there may be one 
   audio media flow and one video media flow in an oncoming multimedia
   session. User agent may select single transmission for audio media
   flow and multipath transimission for video media flow according to 
   the bandwidth requirements. If multiple flows in a session need to be
   multiple transmitted, user agent sender or signaling server can 
   request controller server to allocate relay paths for all flows at 
   once.

   The user agent sender needs to perform path probes to determine if 
   the path is available and if so, obtains the transmission quality of 
   the path at the same time. After obtaining a new candidate path, the 
   user agent sender first performs path probe process, if the path is 
   available, marks it available and puts it into the available path 
   list ordered based on a decreasing priority level; if the path is not
   available, marks it unavailable and puts it into the unavailable path
   list. The user agent sender will periodically perform path probes for
   all the paths in available path list to actively detect path failure 
   and perceive the dynamic changes to the path. If the probe process 
 

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   for a particular path fails, the path will be marked unavailable and 
   removed from the available path list to the unavailable path list. 
   The user agent sender should also perform path probes for the paths 
   in unavailable path list in a longer cycle. If the probe process for 
   a particular path successes, the path will be marked available and 
   removed from the unavailable path list to the available path list.

   If no data is received on a relay path within a given time, relay 
   servers will withdraw the corresponding resources allocated for this 
   path. Therefore, all the relay paths should be kept alive actively by
   the user agent sender. To meet this requirement, the user agent 
   sender SHOULD send MPTP keep-alive packets periodically for both 
   active paths and non-active paths.

   Using the information in the subflow MPTP reports, a user agent 
   sender can monitor delivery quality of active paths. If failure 
   (e.g., errors, unreachability, and congestion) happens in an active 
   path, the user agent sender may perform flow repartitioning and 
   spread the payload across other active paths, or may select a new 
   path from the available path list to replace the failure path.

6.3  Flow partitioning and scheduling

   This document does not limit the usage of multiple paths. User agent
   sender may concurrently use multiple paths to obtain higher 
   throughput, or may send all traffic on a specified path while all 
   other available paths are used only rarely to enhance resilience 
   (e.g., for retransmission, for error-repair, or for redundancy 
   packets). User agent sender MUST only use the default path and the 
   relay paths in available path list as the active path. How to use 
   multiple paths, concurrently, redundantly, or mixedly, is related to
   the transmission requirements of the flow. How to select multiple 
   paths among all available paths and how many paths to use 
   concurrently or redundantly are outside the scope of this document.

   If multiple paths are used concurrently, the original data stream 
   should be divided into several substreams. Flow partitioning methods
   are outside the scope of this document. Application-specific MPTP 
   documents may introduce some flow partitioning methods for specific 
   applications. A simple flow partitioning scheme is to assign packets 
   to multiple subflows using the round-robin algorithm. Specifically, 
   the original data stream is first divided into blocks of equal-sized
   temporal or spacial length. Then, a subflow is assembled by picking 
   blocks periodically from the original blocks in an increasing order. 
   This method is simple and can be applied to all of the upper 
   applications. However, the data are treated equally and not 
   distinguished based on their different importance in terms of the 
   whole data flow. And great correlations exist among subflows which 
 

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   are sent along paths with different transport capacity.

   User agent sender should associate a subflow with an active path
   based on a scheduling strategy. A scheduling policy should jointly 
   consider various factors including the estimated path bandwidth 
   information, the path reception statistics (e.g. RTT, loss-rate, 
   delay etc.), the relative importance of subflow data and so on. Due 
   to the changes in path characteristics, user agent sender should be 
   able to change its scheduling strategy during an ongoing session to 
   fully explore the potential of multipath transport. 

6.4  Subflow packaging

   In a multipath session, user agent sender formats data flow into MPTP
   data packets following MPTP profile defined in this document and the 
   corresponding application-specific MPTP defined in one application-
   specific MPTP documents. Then the user agent sender dispatches MPTP 
   data packets along corresponding relay paths.   

   The common header of MPTP packets includes three key fields: path
   identifier, subflow-specific sequence number (SSSN) and flow sequence
   number (FSN). The path identifier of a relay path, which is globally
   unique, is generated by the relay controller. The path identifier of
   the default path is fixed to zero. The path identifier is used to
   identify a specific path by the user agent sender and relays.
   Meanwhile, the path identifier is also used to identify a subflow by
   the user agent receiver.

   Subflow-specific sequence number is the sequence number of a MPTP 
   data packet in a specific subflow. It is used to help calculate the 
   quality of data delivery such as fractional losses, jitter, RTT, etc,
   for each path. The initial subflow sequence number is randomly 
   generated when the subflow is first established in the multipath 
   session.

   Flow sequence number is the sequence number of a MPTP data packet in
   an original flow. It is used to recombine the original flow. The 
   initial flow sequence number is randomly generated when the multipath
   session is first established.

6.5  Subflow and flow recombination

   The user agent receiver recombines the original data flow according 
   to MPTP data packets received from multiple paths. The user agent 
   receiver first restores the order of each subflow using path 
   identifier and subflow sequence number in MPTP data packet headers. 
   The user agent receiver then restores the order of the original flow 
   using flow sequence number. Concrete implementation work for subflow 
 

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   and flow recombination is different according to specific 
   applications. This work SHOULD be defined in detail in application-
   specific MPTP documents.

6.6  Subflow reporting

   In order to monitor the quality of path delivery and to meet specific
   requirements of some kind of applications, user agents SHOULD 
   generate MPTP reports for per subflow to provide subflow information.
   The user agent sender generates MPTP Subflow Sender Reports (SSR) for
   each unique subflow and sends them along the same path as the MPTP 
   data packets in this subflow. As the relay path is unidirectional and
   the default path is bidirectional, the user agent receiver generates
   MPTP Subflow Receiver Reports (SRR) for each unique subflow and sends
   them along the default path.

   Although MPTP SSRs and SRRs are not sent along the same path, 
   they still can be used to measure the quality of path delivery. For 
   example, the calculated round-trip propagations to the user agent 
   receiver along different paths using MPTP SSRs and SRRs still 
   can correctly represent relative size of transmission delays of 
   different paths.

   When user agent generates MPTP report packets is outside the scope of
   this document. It SHOULD be defined in detail in application-specific
   MPTP documents. In general, timely MPTP reports are necessary for 
   multipath transport environments. MPTP reporting interval should be 
   frequent enough for user agents to quickly adapt to path fluctuation 
   and transmission errors. Meanwhile the traffic overhead introduced by
   MPTP reporting SHOULD also be taken into account when designing an 
   application-specific MPTP for some specific applications. 

   In addition, what MPTP report packets contain and what to do when 
   receiving an MPTP report packet are related to the requirements of 
   specific application programs. They are outside the scope of this 
   document and SHOULD also be defined in detail in application-specific
   MPTP documents.

7.  Relay Server Behavior

   Relay server performs MPTP packets lookups and forwarding based on a 
   local Path-Table. Relays are managed by the relay controller over 
   connections using a protocol referred to as OpenPath protocol. 

7.1  Connection Management and Registrations

   Relay server communicates with the relay controller over a connection
   which may be encrypted using TLS or run directly over TCP. Relay 
 

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   server initiates a standard TLS or TCP connection to the relay 
   controller. Relay server can get the IP address of the relay 
   controller in a number of ways, such as manual configuration, DNS 
   domain name queries and so on.

   When a connection is established, relay server completes the 
   registration process by exchanging HELLO messages. The version field
   in HELLO request is set to the highest OpenPath protocol version 
   supported by the relay server. Relay controller needs to check the 
   included OpenPath protocol version in HELLO request. If the version 
   is not supported, relay controller responds to the HELLO with a 
   failure response.

   The relay server identifier field is set to zero in the initial HELLO
   request. This indicates that it is the first time for this relay 
   server to register with the relay controller. The relay controller 
   needs to assign a unique identifier for this relay server and insert 
   the assigned relay server identifier in the corresponding HELLO 
   response message. Subsequent HELLO request messages may carry the 
   relay server identifier directly. 

   The HELLO request contains the IP address and port of the relay 
   server for the relay service. 

   If a failure response to the HELLO message is received, relay server 
   then terminates the connection. Otherwise, the connection proceeds 
   and the relay server may start relay service.

   During the lifetime of the connection, ECHO requests should be sent 
   periodically from either relay server or relay controller, and the 
   request receiver must return an ECHO reply. 

   After connection establishment, relay server may receive FEATURES 
   requests from relay controller. It must respond with a FEATURES reply
   that specifies its capabilities.

   During the lifetime of the connection, relay controller may 
   periodically collect statistics from a relay server by STATISTICS 
   requests. The relay server must respond with a STATISTICS reply that 
   specifies its current statistics.

7.2  Path-Table Management

   The Path-Table contains a set of path entries each of which 
   corresponds to an associated relay path. Each path entry consists of 
   match fields, result fields and counters (see table 1). 

   Match fields are used to match against MPTP packets. Match fields 
 

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   only consist of path identifier in this document. 

   Path Identifier: a 32 bit unsigned integer uniquely identifying the 
   associated relay path.

   Result fields include next-hop transport address, idle timeout and 
   hard timeout. Next-hop transport address is to determine where the 
   matched packets are forwarded. Idle timeout and hard timeout are used
   for relay to actively clear those expired path entries.

   Next-hop Transport Address Type: corresponds to the type of the 
   next-hop transport address. Namely:
      1: IPv4 address
      2: IPv6 address

   Next-hop Transport IP Address: the address to which the relay server 
   forwards the matched packets.

   Next-hop Transport Port: the port number to which the relay server 
   forwards the matched packets.

   Counters are maintained for each path entry and updated for matching 
   MPTP packets.

   Received packets: the amount of packets the path has matched.

   Received bytes: the amount of bytes the path has matched. 

   Duration: the amount of time the path has been installed in the relay
   server.

 

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                     Table 1: Fields in a path entry.   

   +-------------------------------------+---------------------------+
   | Fields                              | Bits                      |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   | Match fields                                                    |
   +-----------------------------------------------------------------+
   | Path Identifier                     | 32                        |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   | Result fields                                                   |
   +-----------------------------------------------------------------+
   | Next-hop transport address type     | 8                         |
   +-------------------------------------+---------------------------+
   |                                     | For an IPv6 address, this | 
   |Next-hop transport IP address        | is 128;for an IPv4 address|
   |                                     | , this is 32.             |
   +-------------------------------------+---------------------------+
   | Next-hop transport port             | 16                        |
   +-------------------------------------+---------------------------+
   | Idle timeout                        | 16                        |
   +-------------------------------------+---------------------------+
   | Hard timeout                        | 16                        |
   +-----------------------------------------------------------------+
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   | Counters                                                        |
   +-----------------------------------------------------------------+
   | Received packets                    | 64                        |
   +-------------------------------------+---------------------------+
   | Received bytes                      | 64                        |
   +-------------------------------------+---------------------------+
   | Duration (seconds)                  | 32                        |
   +-------------------------------------+---------------------------+
   | Duration (nanoseconds               | 32                        |
   +-----------------------------------------------------------------+

   Path-Table of relay server is managed by relay controller through 
   Path-Table modification messages. For ADD_PATH requests, relay server
   must first check if any path entry with the same path identifier has 
   existed in the Path-Table. If an overlap conflict exists between an 
   existing path entry and the ADD_PATH request, relay server must 
   refuse the addition and respond with a failure response. For valid 
   ADD_PATH requests, relay server must insert a new path entry in the 
   Path-Table and respond to the relay controller with a success 
   response. The counters of received packets and received bytes in the 
   new inserted entry are set to zero.

   For DELETE_PATH requests, if a matching entry exists in the Path-
   Table,it must be removed, and a success response is returned to the 
 

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   relay controller. For DELETE_PATH requests, if no path entry matches,
   no path entry modification occurs, and a failure response is returned
   to the relay controller.

   For UPDATE_PATH requests, if a matching entry exists in the 
   Path-Table, the result fields of this entry is updated with the value
   from the request, and counter fields are left unchanged. For 
   UPDATE_PATH requests, if no path entry matches, no path entry 
   modification occurs, and a failure response is returned to the relay 
   controller.

7.3  Path Validity Management

   Each path entry has an idle timeout and a hard timeout associated 
   with it. These two fields control how quickly a path entry expires. 
   When a path entry is inserted by an ADD_PATH request or modified by a
   UPDATE_PATH request, its idle timeout and hard timeout are set with 
   the values from the message.

   If either value is non-zero, relay server must note the arrival time 
   of MPTP packet on the associated path, as it may need to evict the 
   path entry later. A non-zero hard timeout field causes the path entry
   to be removed after the given number of seconds, regardless of how 
   many ackets it has matched. A non-zero idle timeout field causes the 
   path entry to be removed when it has matched no packets in the given 
   number of seconds. Using these two fields, relay server can actively 
   clear those expired path entries. In addition, relay controller may 
   actively remove path entries by sending DELETE_PATH messages.

   If a relay server actively removes a path entry, it must send a 
   NOTIFY message to relay controller. The NOTIFY message contains a 
   complete description of the path entry including the reason for 
   removal, the path entry duration and statistics at the time of 
   removal.

7.4  Relay Service Management

   After successful registration, relay server may send a START message 
   to relay controller to indicate that it has enough capacity to 
   provide relay service. 

   In the case that a relay server is overloaded, or under other some 
   situations, the relay server may send a STOP message to relay 
   controller to indicate that it will no longer receive new relay 
   service requests (i.e. ADD_PATH messages) for a while. During this 
   period, the relay server still provides relay service for those 
   existing relay paths. And ECHO messages still need to be sent 
   periodically between the relay server and the relay controller.
 

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   When a relay server recovers from an overloaded state, it may send a 
   START message to relay controller to indicate that it has additional 
   capacities to provide new relay services.

   When a relay server wants to stop relay service permanently, it 
   should actively send a BYE message to relay controller before 
   terminating the connection. In this way, relay controller can be 
   ready in time to do some remedial measures. For instance, relay 
   controller may assign new relay paths for the affected media flow.

7.5  MPTP Packet Processing

   The main function of a relay server is to provide relay service to 
   the associated subflows. All subflows associated with a relay server 
   share a common relay port of this relay server. 

   After receiving a MPTP packet, relay server extracts the path 
   identifier from the received MPTP packet and does matching in the 
   Path-Table. If the received MPTP packet does not match a path entry 
   in the Path-Table, relay server has nothing to do but to drop the 
   packet. If the received packet matches a path entry in the 
   Path-Table, relay server forwards it to the associated next-hop 
   transport address in the matched path entry. Meanwhile, relay server 
   updates the associated statistical counters in the matched path 
   entry.

7.6  Topology Discovery and Performance Measurement

   In this document, the term topology refers to the topology that
   connects all the relay servers in a MPTS-AR. The relay controller is
   responsible for managing the topology of relay overlay network that
   is composed of relay servers. The connection between relay servers
   can be based on the manual configuration or other mechanism. For
   instance, the relay controller can obtain relay network topology
   information with the help of the topology discovery process in relay
   servers. During the registration process, relay controller may select
   several registered relay servers randomly or according to certain
   strategy, and insert their information in the corresponding HELLO
   success response message. ECHO messages can be used to accomplish the
   same function.

   After obtaining other relay servers information by manual
   configuration or being provisioned by relay controller, relay server
   performs topology discovery process. Topology discovery process is
   mainly to decide whether there is an overlay link between two relay
   servers. An alternative way may be based on the following rules. If
   two relay servers are located in the same autonomous domain, there
   will be an overlay link connecting the two relay servers. If there is
 

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   an inter-domain link between two domains, there is an overlay link
   which connects the two relay servers from the two domains. Relay
   server should report the result of topology discovery process to
   relay controller via ECHO messages periodically. The global overlay
   topology is formed in the relay controller based on the distributed
   topology observation of relay servers.

   An overlay link is usually composed of multiple physical links.
   Besides overlay traffic, other nonoverlay traffic would be using the
   same physical links. In addition, relay functions provided by relay
   server work in application layer, that is, on top of transport layer.
   Thus, relay server cannot control or manage the IP-layer resource.
   Relay server can only rely on measurement mechanism to obtain the
   performance of overlay links associated to it. With performance
   measurement, relay server can track dynamically overlay link
   capacity. Overlay link capacity can be expressed in terms of the
   quality metrics such as loss rate, delay, available bandwidth and so
   on. The concrete measurement methods that can be adopted are outside
   the scope of this document. 

   Relay server should report dynamic capacity of overlay links
   associated with it to relay controller via ECHO messages
   periodically. These capacity information of overlay links will help
   the relay controller allocate superior relay paths for user agents.

8.  Relay Controller Behavior

   Relay controller is responsible for managing relay servers and 
   selecting one or multiple relay paths for a data flow. 

8.1  Relay Server Management

   Relay controller manages all relay servers in its region, and 
   maintains registration and status information of relay servers such 
   as relay capacity, availability and work load. For each relay server,
   relay controller collects its capabilities information by FEATURES 
   messages, and periodically collects its statistics information by 
   STATISTICS messages.

8.2  Topology Maintenance

   Relay controller manages the topology of a network of relay servers
   in its region. Topology information include if an overlay link exists
   between any two relay servers and if so, how about the quality of the
   overlay link.

   Relay controller can obtain topology information in several ways. An
   alternative way, as mentioned in section 7.6, is to obtain topology
 

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   information from relay servers in its region via ECHO messages. In
   another alternative way, relay controller obtains topology
   information of relay overlay network itself with underlying network
   information.

   Many popular overlay applications often generate a huge amount of
   cross-ISP traffic overloading links that are frequently subject to
   congestion [9]. Besides resulting in a poor experience for the user,
   such transits can be quite costly to the network operator. One of the
   important reasons is that overlay applications do not have reliable
   information of the underlying network topology when selecting overlay
   paths. In the IETF, Application-Layer Traffic Optimization (ALTO)
   working group has been working on standardization of the ALTO service
   [9, 10]. This working group advocates network operators to take the
   initiative to provide distributed applications with more reliable
   underlying network information, thus achieving performance
   optimization of both sides of underlying networks and distributed
   applications. In this scenario, relay controller, as an ALTO client,
   periodically obtains network maps and cost maps from one or more ALTO
   servers. With the reliable network information, relay controller is
   enable to organize relay servers into groups in accordance with the
   underlying network topology.

8.3  Relay Path Allocation

   During establishing a multipath session, the user agent sender or the
   signaling server which out-of-band signaling pass through during 
   session setup, requests relay controller to allocate relay paths by 
   ALLOCATE_PATH messages. The ALLOCATE_PATH request carries the related
   information of the data flow which is going to establish, such as 
   location information of participants, transport requirements of the 
   data flow and so on. Relay controller selects one or multiple relay 
   paths according to the information carried in ALLOCATE_PATH request 
   and the status information of the registered relay servers. For each 
   selected relay path, the relay controller sends an ADD_PATH request 
   to each relay server on the selected relay path. The ADD_PATH request
   carries the path identifier, next-hop transport address of the 
   receiving relay server, etc. A relay path is assigned successfully 
   only when each relay server on this relay path replies with an 
   ADD_PATH success response. After successful path allocation, relay 
   controller replies an ALLOCATE_PATH success response to inform the 
   user agent sender or the signaling server about the allocated relay 
   path information including the path identifier, user agent sender's 
   next-hop transport address, etc.

   How relay controller selects superior candidate relay paths for an
   oncoming transmission session between user agents is critical. If the
   assigned relay path has poor performance, MPTS-AR will not reach the
 

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   potential of multipath transport, and also may get even worse
   performance than single-path transmission case of the default IP
   path. How to select superior relay paths is outside the scope of this
   document. An ideal approach for selecting relay paths should take
   into account both underlying network topology and transport
   requirements of upper applications.

9.  OpenPath Protocol

9.1  Protocol Overview

   OpenPath is based on a request/response transaction model. Each
   transaction consists of a request and a response. A response uses the
   same transaction id as is in the associated request to facilitate
   pairing. The transaction id is a 32-bit identifier generated by the
   request sender.

   OpenPath messages are guaranteed delivery over a connection-oriented
   channel. All integer fields are carried in network octet order, that
   is, most significant byte first. Octets designated as padding have
   the value zero.

   All OpenPath messages begin with an OpenPath common header. OpenPath
   messages MAY contain a message body. The structure and interpretation
   of a body depends on the message type.

   OpenPath message types fall into four classes: relay-to-controller,
   controller-to-relay, user agent-to-controller and symmetric.

   Relay-to-controller messages are initiated by relay server and used
   to manage the channel connection and update relay controller of
   changes to the relay server. Controller-to-relay messages are
   initiated by relay controller and used to manage the Path-Table or
   inspect the state of relay servers. User agent-to-controller messages
   are initiated by user agent or out-of-band signaling server, and used
   for relay path allocation. Symmetric messages are initiated by either
   relay controller or relay server and used to keep alive the channel
   connection and topology maintenance. 

9.1.1  Relay-to-Controller

   Relay-to-controller message is initiated by a relay server and 
   requires a response message from relay controller.

   HELLO: Relay server registers with relay controller by sending a 
   HELLO request. Relay controller must respond with a HELLO response 
   that indicates the outcome of registration. This is commonly 
   performed upon establishment of the OpenPath connection channel.
 

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   The relay server identifier field is set to zero in the initial HELLO
   request. Relay controller must assign a unique identifier for this 
   relay server and insert the assigned relay server identifier in the 
   corresponding HELLO response. Subsequent HELLO requests may carry the
   assigned relay server identifier directly.

   The HELLO request contains a message body. The body contains the IP 
   address and port of relay server for the relay service. 

   The HELLO success response may contains a message body. The body 
   contains the information of one or more other registered relay 
   servers. The information of a relay server here include the IP 
   address for relay service and the relay server identifier.

   START: Relay server starts relay service by sending a START request. 
   Relay controller must respond with a START response that indicates 
   the outcome of relay service startup.

   STOP: Relay server may stop relay service temporarily by sending a 
   STOP request. For instance, when a relay server is overloaded, it may
   want to refuse accepting any new relay service requests for a while. 
   Relay controller must respond with a STOP response that indicates the
   outcome of relay service pause. During relay service pause, this 
   relay server still provides relay service for those existing relay 
   paths. This relay server may restart relay service by sending a START
   request after a while.

   BYE: Relay server may terminate the relay service permanently by 
   sending a BYE request, such as before terminating the OpenPath 
   connection channel or exiting. Meanwhile, this relay server ceases 
   relay service for all existing relay paths. If this relay server 
   wants to start relay service again, it must first perform 
   registration with relay controller.

   NOTIFY: When a relay server actively removes a path entry, it may 
   notify relay controller by sending a NOTIFY request. For instance, in
   order to save the resource, relay server actively removes those path 
   entries which lack activity.

9.1.2  Controller-to-Relay

   Controller-to-relay message is initiated by a relay controller and 
   require a response message from relay server.

   FEATURES: Relay controller may request the capabilities of a relay 
   server by sending a FEATURES request. This relay server must respond 
   with a FEATURES response that specifies its capabilities. This is 
   commonly performed after successful registration of this relay 
 

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   server.

   STATISTICS: Relay controller may periodically collect statistics from
   relay servers by sending STATISTICS requests. Relay server must 
   respond with a STATISTICS response that specifies its current 
   statistics.

   ADD_PATH: When relay controller assigns relay paths for a data flow, 
   it must send an ADD_PATH request to each relay server on the assigned
   relay path. The relay server must respond with an ADD_PATH response 
   that specifies the outcome of adding a new path entry. A relay path 
   is assigned successfully only when each relay server replies with an 
   ADD_PATH success response.

   UPDATE_PATH: Relay controller may want to modify an existing path 
   entry in the Path-Table of a relay server by sending a UPDATE_PATH 
   request. For instance, a data flow may be "put on hold" and data 
   transmission may be ceased for a while. In this case, relay 
   controller may update the idle timeout and hard timeout of the 
   corresponding path entries of all the relay servers on the affected 
   relay paths to a longer time. Relay server must respond with an 
   UPDATE_PATH response that specifies the outcome of updating an 
   existing path entry.

   DELETE_PATH: Relay controller may delete an existing path entry in 
   the Path-Table of a relay server by sending a DELETE_PATH request, 
   such as when a data flow ends normally. Relay server must respond 
   with a DELETE_PATH response that specifies the outcome of deleting an
   existing path entry.

9.1.3  User agent-to-Controller

   User agent-to-controller message is initiated by a user agent or an 
   out-of-band signaling server, and requires a response message from 
   relay controller.

   ALLOCATE_PATH: During establishing or the process of a multipath 
   session, user agent sender may request relay controller to allocate 
   candidate relay paths for one or multiple media flows in a session
   by exchanging ALLOCATE_PATH messages directly with relay controller. 
   Or the signaling server requests relay controller to allocate 
   candidate relay paths for user agents, and inserts the information of
   the allocated relay paths into corresponding signaling messages to 
   inform user agents. Relay controller must respond with an 
   ALLOCATE_PATH response that specifies the outcome of path allocation 
   and the information of the assigned relay path if exists.

   RELEASE_PATH: Before tearing down a multipath session, user agent 
 

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   sender may request relay controller to release the relay paths 
   assigned for the multipath session by exchanging RELEASE_PATH 
   messages directly with relay controller. RELEASE_PATH request message
   carries the session identifier information. Or during the process of
   a multipath session, user agent may request relay controller to 
   release the relay paths assigned for one or multiple specified media
   flow. RELEASE_PATH request message carries the information of one or 
   more relay path identifiers. Alternatively, the signaling server 
   requests relay controller to release the assigned relay paths on 
   behalf of the user agent sender. Relay controller must respond with 
   an RELEASE_PATH response that specifies the outcome of deleting relay
   paths.

9.1.4 Symmetric

   Symmetric message is sent in either direction between relay server 
   and relay controller.

   ECHO: ECHO requests MUST be sent periodically from either relay 
   controller or relay server. ECHO messages can be used to measure the 
   latency of the connection between relay controller and relay server, 
   as well as verify the peer's liveness. The receiver of an ECHO 
   request must respond with an ECHO response.

   Relay server can insert the information of topology and overlay link 
   capacity in ECHO requests or ECHO responses.

9.2  Common Structures

   This section describes several common structures used by multiple 
   messages.

9.2.1  OpenPath Common Header

   All OpenPath messages begin with an OpenPath common header which is 
   defined below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Peer Identifier                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Transaction Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version (V): 6 bits
 

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   This field identifies the version of OpenPath protocol version being 
   used. The version defined by this document is one (1). 

   R: 1 bit
   If the R bit is set, the message is an OpenPath request; otherwise, 
   the message is an OpenPath response.

   S: 1 bit
   When the message is a request, this bit is reserved. When the message
   is a response, if the S bit is set, the message is a success 
   response; otherwise, the message is a failure response.

   Type: 8 bits
   This field identifies the type of the OpenPath messages. This 
   document defines 13 OpenPath message types:

   +----------------------------------------------------------------+
   |  Type Value  |   Type Name   |    Sending Direction            |
   +--------------+---------------+---------------------------------+ 
   |  1           |   HELLO       |    Relay -> Controller          |
   +--------------+---------------+---------------------------------+ 
   |  2           |   BYE         |    Relay -> Controller          |
   +--------------+---------------+---------------------------------+ 
   |  3           |   ECHO        |    Relay -> Controller  Or      | 
   |              |               |    Controller -> Relay          | 
   +--------------+---------------+---------------------------------+ 
   |  4           |   START       |    Relay -> Controller          | 
   +--------------+---------------+---------------------------------+ 
   |  5           |   STOP        |    Relay -> Controller          | 
   +--------------+---------------+---------------------------------+ 
   |  6           |   NOTIFY      |    Relay -> Controller          |
   +--------------+---------------+---------------------------------+
   |  7           |   FEATURES    |    Controller -> Relay          |
   +--------------+---------------+---------------------------------+ 
   |  8           |   STATISTICS  |    Controller -> Relay          | 
   +--------------+---------------+---------------------------------+ 
   |  9           |   ADD_PATH    |    Controller -> Relay          | 
   +--------------+---------------+---------------------------------+ 
   |  10          |   DETELE_PATH |    Controller -> Relay          | 
   +--------------+---------------+---------------------------------+ 
   |  11          |   UPDATE_PATH |    Controller -> Relay          | 
   +----------------------------------------------------------------+
   |  12          | ALLOCATE_PATH |    User Agent -> Controller     | 
   +--------------+---------------+---------------------------------+ 
   |  13          |  RELEASE_PATH |    User Agent -> Controller     | 
   +----------------------------------------------------------------+

   Length: 16 bits
 

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   The length field indicates the total length of the message in 32-bit 
   words including the header and any padding.

   Peer Identifier: 32 bits
   This field is a 32-bit identifier that corresponds to a globally 
   unique relay server, user agent or out-of-band signaling server. It 
   is generated by relay controller during registration process of a 
   relay server, user agent or out-of-band signaling server. This 
   identifier remains unchanged during the entire lifecycle of the 
   OpenPath connection with relay controller. For OpenPath messages 
   exchanged between relay controller and relay server, this field 
   corresponds to a relay server identifier. For OpenPath messages 
   exchanged between relay controller and user agent, this field 
   corresponds to a user agent identifier. For OpenPath messages 
   exchanged between relay controller and out-of-band signaling server, 
   this field corresponds to a signaling server identifier. For the last
   two cases, it is uniformly called user agent identifier.

   Transaction Identifier: 32 bits
   This field identifies the transaction id associated with this 
   message. It is randomly generated by the request sender and discarded
   when the associated response message is received. The transaction 
   identifier of a response message must always match the requests that 
   prompted them.

9.2.2  Common Body of OpenPath Failure Responses

   OpenPath failure responses of all message types MAY contain an 
   optional body after the OpenPath common header. If present, the body 
   conforms to a common structure defined below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Status code |     Rlength   |           reason              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             reason                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Status code: 8 bits
   This field is a numeric result code that indicates the outcome of a 
   request processing. 

   Rlength: 8 bits
   This field is the length of the reason phrase in 16-bit word length.

   Reason: variable size
   This field is a short textual description of the status code.
 

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9.2.3  Transport Address Structure

   Relay server needs to advertise relay controller about its transport 
   address for relay service. Relay controller also needs to tell relay 
   server about the transport address of its next-hop node for each 
   associated path. A transport address is defined as follow.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |     Pad(0)    |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Address Type: 8 bits
   This field is the type of transport address. Namely:
     1: IPv4 address
     2: IPv6 address

   Port: 16 bits
   This field is the port number part of transport address.

   Address: 4 octets (IPv4), 16 octets (IPv6)
   The IP address part of transport address.

9.3  Message Format of OpenPath Request and Success Response

9.3.1  HELLO

   A HELLO request contains a body beyond an OpenPath common header. 

   The body contains the transport address of the relay server to 
   provide relay service.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Relay Server Identifier                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Transaction Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |     Pad(0)    |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 

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   A HELLO success response may contains a message body. The body 
   contains information of one or more other registered relay servers. 
   The information of a relay server here include the address for relay 
   service and the relay server identifier.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Other Relay Server Identifier                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |     Pad(0)    |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                IP Address of Other Relay Server               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An example of A HELLO success response is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |0|1|       1       |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Relay Server Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Transaction Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Identifier of Other Relay Server               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |     Pad(0)    |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                IP Address of Other Relay Server               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ......                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.3.2  START/STOP/BYE

   START/STOP/BYE requests and their success responses have no body; 
   that is, they only contain an OpenPath common header.

9.3.3  ECHO

   An ECHO request consists of an OpenPath common header and an optional
   body. If the body is absent, an ECHO transaction is used to simply 
   verify liveness between relay controller and relay server. If the 
   body is present, it may contain a timestamp field to check latency 
   between relay controller and relay server. Example:

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Relay Server Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Transaction Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             NTP timestamp, most significant word              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             NTP timestamp,least significant word              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   NTP timestamp: Using the timestamp format of the Network Time 
   Protocol (NTP), which is in seconds relative to 0h UTC on 1 January 
   1900 [2]. The full resolution NTP timestamp is a 64-bit unsigned 
   fixed-point number with the integer part in the first 32 bits and the
   fractional part in the last 32 bits.

   If the body of an ECHO request only contain a timestamp field, its 
   response has the same message format as the associated ECHO request.

   An ECHO message (request message or response message) generated by a 
   relay server may contain the information of one or more overlay link 
   capacities in its body.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Relay Server Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Transaction Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Identifier of Neighboring Relay Server            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Link Type          |         Packet Loss Rate      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Transport Delay                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Transport Bandwidth                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Identifier of Neighboring Relay Server: 
   the identifier of the relay server which is connected to this ECHO 
   message sender by a overlay link.
 

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   Link Type: 16 bits
   This field identifies the type of the overlay link between this ECHO
   message sender and the relay server which is identified by the 
   identifier of neighboring relay server field. This document defines 2
   overlay link types:

   +-----------------------------------------------------------------+
   |  Type Value  |   Description                                    |
   +--------------+--------------------------------------------------+
   |  1           |The two relay servers which are associated with   |
   |              |the overlay link are located in the same          |
   |              |autonomous domain.                                |
   +--------------+--------------------------------------------------+
   |  2           |The two relay servers which are associated with   |
   |              |the overlay link are from two different autonomous|
   |              |domains. And there is an interdomain link between |
   |              |these two domains.                                |
   +--------------+--------------------------------------------------+

   Packet Loss Rate: 16 bits
   This field indicates the estimated packet loss rate of the overlay
   link since the last ECHO message which contains the information
   report of the same overlay link. This field is expressed in units of
   per thousand.

   Transport Delay: 32 bits
   This field indicates the estimated unidirectional transport delay of
   the overlay link from this ECHO message sender to the relay server
   which is identified by the identifier of neighboring relay server
   field. This field is expressed in units of 1/65536 seconds.

   Transport Bandwidth: 32 bits
   This field indicates the estimated transport bandwidth of the overlay
   link. This field is expressed in units of kilo-bits per second
   (kpbs).

9.3.4  NOTIFY/DELETE_PATH

   NOTIFY/DELETE_PATH requests contain a body beyond an OpenPath common 
   header. The body only contains a path identifier field.

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Relay Server Identifier                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Transaction Identifier                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Path Identifier                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Path Identifier: 32 bits
   This field is a 32-bit identifier that corresponds to a globally 
   unique path. It is generated by relay controller when assigning relay
   paths for a data flow.

   NOTIFY/DELETE_PATH success responses only contain an OpenPath common 
   header.

9.3.5  ADD_PATH/UPDATE_PATH

   ADD_PATH/UPDATE_PATH requests contain a body beyond an OpenPath 
   common header. The body contains match fields and result fields of a 
   path entry. The match fields contain a single path identifier field. 
   The result fields contain next-hop transport address and idle/hard 
   timeout fields.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Relay Server Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Transaction Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |     Pad(0)    |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Idle timeout           |         Hard timeout          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   

   Idle timeout: 16 bits
   This field is the number of seconds after which the path will timeout
 

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   if no traffic.

   Hard timeout: 16 bits
   This field is the number of seconds after which the path must expire 
   regardless of whether or not packets go through the path.

   ADD_PATH/UPDATE_PATH success responses only contain an OpenPath 
   common header.

9.3.6  ALLOCATE_PATH

   In a multimedia session, there may be more than one media flow that
   need multipath transport, and a media flow could be unidirectional or
   bidirectional. Relay path requesters can request relay controller to
   allocate relay paths for all unidirectional media flows in a
   multimedia session at once.

   ALLOCATE_PATH requests contain a body beyond an OpenPath common
   header. The body contains information of the multimedia session,
   which is going to be established, and one or more data flows in this
   session. Information of a media flow include flow identifier, two
   communicating endpoint addresses, and the transport requirements.

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     User Agent Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Transaction Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sess.ID. Length|                                               |
   +-+-+-+-+-+-+-+-+         Session Identifier                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | P1.Addr. Type | DT|  Flow ID  |         P1's Port             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         P1's Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | P2.Addr. Type |  PUM  |  AMT  |         P2's Port             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         P2's Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Required Transport Bandwidth                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Maximum End-to-end Transport Delay              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Another data flow's information              |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sess.ID. Length: 8 bits

   The length field indicates the total length of the following Session
   Identifier in bytes.

   Session Identifier:  variable size

   This field indicates the unique identifier of the ongoing multipath
   session. If its length is not (4*k-1) bytes, one or more additional
   padding octets which are not part of the session identifier should be
   appended at the end. 

   P1 & P2: P1 and P2 represent two communicating endpoints of the
   ongoing multipath session.

   DT: 2 bits

   This field indicates the direction of the corresponding data flow. If
   the binary value of this field is '10', the data flow is
 

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   unidirectional, and P1 and P2 are source side and destination side of
   this flow respectively. If the binary value of this field is '01',
   the data flow is unidirectional, and P1 and P2 are destination side
   and source side of this flow respectively. If the binary value of
   this field is '11', the data flow is bidirectional, and will be
   regarded as two independent unidirectional flows in opposite
   directions, that is, their 'DT' are '10' and '01' respectively. 

   Flow ID: 6 bits

   This field indicates the identifier of the corresponding data flow.
   The identifer of a flow MUST be unique in the scope of the multipath
   session.

   Path Usage Method (PUM): 4 bits

   This field indicates how to use multiple paths, concurrently,
   redundantly, or by other ways. This document defines two usage
   methods of multiple paths: UM = 1, indicates concurrent-mode; UM = 2,
   indicates redundant-mode.

   Application-specific MPTP type (AMT): 4 bits

   This field identifies the type of application-specific MPTP that
   packet format of this data flow conforms to. This document defines
   two application-specific MPTP types: AMT = 1, indicates that this
   MPTP packet conforms to RTP-based multimedia application-specific
   MPTP; AMT = 2, indicates that this MPTP packet conforms to reliable
   application-specific MPTP.

   Required Transport Bandwidth: 32 bits

   This field indicates the required transport bandwidth of the data
   flow which is going to be established. If the data flow is
   bidirectional, this field indicates the required bandwidth of the
   corresponding unidirectional flow. This field is expressed in units
   of kilo-bits per second (kpbs).    

   Maximum End-to-end Transport Delay: 32 bits

   This field indicates the maximum end-to-end transport relay that
   could be tolerated by the data flow which is going to be established.
   This field is expressed in units of millisecond (ms).

   An ALLOCATE_PATH success response contains a body beyond an OpenPath
   common header. The body contains information of the multimedia
   session and one or more allocated relay path including the path
   identifier and user agent sender's next-hop transport address.
 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     User Agent Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Transaction Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sess.ID. Length|                                               |
   +-+-+-+-+-+-+-+-+         Session Identifier                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | DT|  Flow ID  |    Pad(0)     |    the number of relay paths  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Address Type |Path Group ID. |             Port              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    the next path's information                |
   |                            ......                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Path Group ID. : 8 bits

   The allocated relay paths are divided into groups, each of which
   could meet the required transport bandwidth of the data flow stated
   in the corresponding ALLOCATE_PATH request. This field indicates the
   identifier of the path group that this relay path belongs to.

9.3.7  RELEASE_PATH

   RELEASE_PATH requests contain a body beyond an OpenPath common
   header. The body contains the session identifier and optionally one
   or more path identifier fields. Each path identifier corresponds to
   one relay path which is being requested to be released.

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      V    |R|S|     Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     User Agent Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Transaction Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sess.ID. Length|                                               |
   +-+-+-+-+-+-+-+-+         Session Identifier                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ......                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   RELEASE_PATH success responses only contain an OpenPath common
   header.

9.3.8  FEATURES

   A FEATURES request only contains an OpenPath common header.

   A FEATURES success response contains a body beyond an OpenPath common
   header.

   (to be continued)

9.3.9  STATISTICS

   A STATISTICS request only contains an OpenPath common header.

   A STATISTICS success response contains a body beyond an OpenPath
   common header. 

   (to be continued)

10.  MPTP Profile

10.1  Overview

   As stated in Section 5, MPTP obtain a simple, unreliable datagram 
   service from the underlying UDP. MPTP SHOULD meet various delivery 
   requirements of upper applications. As stated in the introduction, in
   order to be extensible and suitable for various applications, MPTP is
   designed to be a protocol suite which consists of one MPTP profile 
 

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   and multiple application-specific MPTPs. The MPTP profile only 
   defines common rules of multipath transport based on 
   application-level relay for various upper applications. It is aimed 
   to provide application-level multipath routing mechanism. Other 
   transmission requirements of upper applications, such as timely or 
   reliable delivery, SHOULD be met in corresponding 
   application-specific MPTP documents. 

   In addition to carrying payload data passed from upper application 
   programs through multiple paths, MPTP also need to provide path 
   control functions including keeping path alive and monitoring the 
   quality of data delivery on each path. Therefore, MPTP packets are 
   divided into two types: MPTP data packets and MPTP control packets. 
   MPTP control packets include MPTP keep-alive packets and MPTP report 
   packets.

   User agent sender formats the payload data passed from upper 
   application programs into MPTP data packets which are sent along the 
   associated path.

   User agent sender SHOULD send MPTP keep-alive packets periodically 
   for each path including both active path and non-active path.

   To monitor the transport quality of a path, user agents generate MPTP
   report packets for each subflow. Due to the content of MPTP report 
   packets depends on the delivery requirements of upper application 
   programs, this document does not give concrete content and processing
   of the MPTP report packets, which should be described in 
   application-specific MPTP documents. Here, we only outline the 
   delivery methods of MPTP report packets. The user agent sender 
   generates MPTP subflow sender reports for each subflow and sends them
   along the same path as the subflow MPTP data packets. The user agent 
   receiver responds with MPTP subflow receiver reports which are sent 
   along the default path.

   According to the delivery requirements of upper applications, the 
   user agent receiver optionally generates MPTP flow recombination 
   reports, which should also be described in application-specific MPTP
   documents. 

10.2  MPTP Fixed Header Fields

10.2.1  Fixed Header Fields of MPTP Data Packet

   All MPTP data packets have a fixed twelve octet-length header, which
   is defined below:

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=1|T|P|  AMT  |  TOS  |  Rsvd |             SSSN              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Path Identifier                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Flow Sequence Number                        |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   |                        Payload Data                           |
   |                                                               |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields have the following meaning:

   version (V): 2 bits
   This field identifies the version of MPTP. The version defined by 
   this document is one (1). 

   MPTP packet type (T): 1 bit
   This field identifies the type of a MPTP packet. If the MPTP packet 
   type bit is set, this packet is a MPTP data packet; otherwise, this 
   packet is a MPTP control packet.

   padding (P): 1 bit
   If the padding bit is set, the packet contains one or more additional
   padding octets at the end which are not part of the payload. The last
   octet of the padding contains a count of how many padding octets 
   should be ignored, including itself. Padding may be needed by some 
   encryption algorithms with fixed block sizes.

   Application-specific MPTP type (AMT): 4 bits
   This field identifies the type of application-specific MPTP that this
   packet format conforms to. It is identical as the same named 
   attribute in ALLOCATE_PATH message in Section 9.3.6.

   type of service (TOS): 4 bits
   This field, similar to TOS field in internet packet header, is
   specified along the abstract parameters precedence, delay,
   throughput, and reliability. These abstract parameters embody the 
   delivery requirements of upper application programs. The values of 
   these abstract parameters should be specified in application-specific
   MPTP documents.

      Precedence: An independent measure of the importance of the 
             payload data.
 

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      Delay: Prompt delivery is important for the payload data with 
             this indication.

      Throughput: High data rate is important for the payload data with 
             this indication. 

      Reliability: A higher level of effort to ensure delivery is 
             important for the payload data with this indication.

   Rsvd: 4 bits
   These 4 bits are reserved, which can be used and described in
   application-specific MPTP documents.  

   Subflow-Specific Sequence Number (SSSN): 16 bits
   This field is the sequence of the MPTP data packet in the subflow. 
   Each subflow has its own strictly monotonically increasing sequence 
   number space.

   Path Identifier: 32 bits
   This field is the identifier of the path that the MPTP packet is 
   associated with. For a relay path, the path identifier is globally 
   unique; for the default path, the path identifier is fixed to zero.

   Flow Sequence Number (FSN): 32 bits
   This field is the sequence of the MPTP data packet in the original
   flow. The original flow has the unique strictly monotonically
   increasing sequence number space.

   Payload Data: variable size
   The content of payload data should be described in 
   application-specific MPTP documents.

10.2.2  Fixed Header Fields of MPTP Control Packet

   MPTP control packets except MPTP flow recombination report packets 
   have a fixed eight octet-length header, which is defined below. In 
   comparison, MPTP flow recombination report packets have no path 
   identifier field.

 

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=1|T|P|  AMT  |      CT       |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Path Identifier                          |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   |                        Control Data                           |
   |                                                               |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of V, T, P, AMT and Path Identifier have the same meaning 
   as those fields in MPTP data packet. Other fields have the following 
   meaning:

   MPTP control packet type (CT): 8 bits

   This field identifies the type of MPTP control packet. Namely:

   +-------------------------------------------------------------------+
   |MPTP control packet type (CT)    |Use                              |
   +---------------------------------+---------------------------------+
   |1                                |Subflow Sender Report, for       |
   |                                 |transmission statistics from the |
   |                                 |user agent sender                |
   +---------------------------------+---------------------------------+
   |2                                |Subflow Receiver Report, for     |
   |                                 |reception statistics from the    |
   |                                 |user agent receiver              |
   +---------------------------------+---------------------------------+
   |3                                |Keep Alive, for keeping a path   |
   |                                 |alive                            |
   +-------------------------------------------------------------------+
   |4                                |Flow Recombination Report, for   |
   |                                 |flow recombination statistics    |
   |                                 |from the user agent receiver     |
   +-------------------------------------------------------------------+

   Length: 16 bits
   This field is the length of the encapsulated control data after the 
   MPTP fixed header in byte length.

   Control Data: variable size
   For MPTP keep-alive packet, control data may be empty. 
   For MPTP report packet, the content of control data should be 
   described in application-specific MPTP documents.
 

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11.  SDP Considerations

   The advertisement of MPTP capability and relay paths MUST be 
   performed by out-of-band signaling, for example, as part of a SIP 
   offer/answer exchange using SDP. This section defines dedicated 
   extensions in SDP. SDP syntax and procedures are available that can 
   be re-used or easily adapted to provide the necessary capabilities.

11.1  Signaling MPTP Capability in SDP

   A communication participant, who follows the framework of multipath 
   transport system based on application-level relay, MUST use SDP to 
   indicate that it supports and wants to use this framework. The 
   mptp-relay attribute defined here will be used to indicate the 
   support for this framework. The mptp-relay attribute is a media level
   parameter. The syntax of the mptp-relay attribute is defined using 
   the following Augmented BNF, as defined in [RFC5234].

   mptp-relay-attrib = "a=" "mptp-relay" ":" mptp-name CRLF

   The mptp-name field indicates an application-specific MPTP. In an 
   application-specific MPTP document, the value of mptp-name field MUST
   be given for that application-specific MPTP.

   When SDP is used following the offer/answer mode [RFC3264], the 
   "mptp-relay" attribute indicates the desire to transport media flow 
   on multiple paths. If the offerer supports and wishes to use MPTP, 
   it MUST include a media-level "a=mptp-relay" attribution in the 
   initial SDP offer. If the initial SDP offer contains "a=mptp-relay" 
   attribution and if the answerer supports and wishes to use MPTP, it 
   MUST include this attribute in the SDP answer. 

   When SDP is used in a declarative manner, the "mptp-relay" attribute 
   indicates that the message sender can send or receive media data over
   multiple paths. The message receiver SHOULD be capable to stream 
   media to multiple paths or be prepared to receive media from multiple
   paths.

11.2  Relay Path Advertisement in SDP

   The information of candidate relay paths MUST be advertised to the 
   user agent sender in the out-of-band signaling. The relay-path 
   attribute is extended to advertise candidate relay paths in SDP. The 
   syntax of the relay-path attribute is defined using the following 
   Augmented BNF, as defined in [RFC5234]. The definitions of DIGIT, SP 
   and CRLF are according to RFC4234.

   relay-path-attrib = "a=" relay-path-label ":" counter SP path-id SP 
 

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   transport-address CRLF
   relay-path-label = "relay-path"
   counter = 1*DIGIT
   path-id = 8*8HEXDIG
   transport-address = addrtype ":" unicast-address ":" port
     ; addrtype from RFC4566, typically "IP4" | "IP6"
     ; port from RFC4566
   unicast-address = IP4-address / IP6-address
     ; IP4-address from RFC4566
     ; IP6-address from RFC4566

   The path identifier and the next-hop transport address of the user 
   agent sender for each candidate relay path is encapsulated in a 
   relay-path attribute. 

   The <counter> parameter indicates the priority of the associated 
   relay path and it is a monotonically increasing positive integer 
   starting at 1. Number 1 is the highest priority. The counter must be 
   reset for each media line. The relay-path attributes are ordered 
   based on a decreasing priority level.

   The <path-id> parameter indicates the path identifier of the 
   associated relay path.

   The <transport-address> is the next-hop transport address of the 
   user agent sender for associated candidate relay path. The <addrtype>
   is the address type. This document only defines IP4 and IP6 for IPv4 
   addresses and IPv6 addresses respectively.

   Example:

   a=relay-path:1 1a3b6c9d0e2f6g1qazxsw23edcvfr45t IP4:192.0.3.2:10000

12.  IANA Considerations

   The following contact information shall be used for all registrations
   in this document:

       Contact:   Weimin Lei
                  mailto:leiweimin@ise.neu.edu.cn
                  tel:+86-24-8368-3048

   Note to the RFC-Editor: When publishing this document as an RFC, 
   please replace "RFC XXXX" with the actual RFC number of this document
   and delete this sentence.

12.1  SDP Attributes

 

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   In the registry for SDP parameters, the attributes named "mptp-relay"
   and "relay-path" have been registered as follows:

   SDP Attribute ("att-field"):

     Attribute Name:      mptp-relay
     Long form:           Multipath transport system framework based on
                          application-level relay
     Type of name:        att-field
     Type of attribute:   Media level only
     Subject to charset:  No
     Purpose:             This attribute is used to indicate support for
                          multipath transport system framework based on
                          application-level relay.
     Reference:           See this document
     Values:              See this document.

   SDP Attribute ("att-field"):

     Attribute Name:      relay-path
     Long form:           Relay Path of multipath transport system 
                          framework based on application-level relay
     Type of name:        att-field
     Type of attribute:   Media level only
     Subject to charset:  No
     Purpose:             This attribute is used to describe the 
                          information of a relay path including the path
                          identifier and the next-hop transport address 
                          of the user agent sender.
     Reference:           See this document
     Values:              See this document.

13.  Security Considerations

   TBD

14.  References

14.1  Normative References

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

   [2]  Mills, D., "Network Time Protocol (Version 3) Specification, 
        Implementation and Analysis", RFC 1305, March 1992.

14.2  Informative References

 

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   [3]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, 
        "RTP: A Transport Protocol for Real-Time Applications", STD 64, 
        RFC 3550, July 2003.

   [4]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: 
        Session Initiation Protocol", RFC 3261, June 2002.

   [5]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 
        Description Protocol", RFC 4566, July 2006.

   [6]  Schulzrinne, H., Rao, A., Lanphier, R., "Real Time Streaming 
        Protocol (RTSP)", RFC2326, April 1998.

   [7]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar, 
        "Architectural Guidelines for Multipath TCP Development", 
        RFC6182, March 2011.

   [8]  Stewart, R., "Stream Control Transmission Protocol", RFC4960, 
        September 2007.

   [9]  Seedorf, J., and E. Burger, "Application-Layer Traffic 
        Optimization (ALTO) Problem Statement", RFC5693, October 2009.

   [10] Alimi, R., Penno, R., et al. "Application-Layer Traffic 
        Optimization (ALTO) Protocol", RFC7285, September 2014.

 

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Authors' Addresses

   Weimin Lei
   Northeastern University
   Institute of Communication and Information System
   College of Information Science and Engineering
   Shenyang, China, 110819
   P. R. China

   Phone: +86-24-8368-3048
   Email: leiweimin@ise.neu.edu.cn

   Wei Zhang
   Northeastern University
   Institute of Communication and Information System
   College of Information Science and Engineering
   Shenyang, China, 110819
   P. R. China

   Email: zhangwei1@ise.neu.edu.cn

   Shaowei Liu
   Northeastern University
   Institute of Communication and Information System
   College of Information Science and Engineering
   Shenyang, China, 110819
   P. R. China

   Email: liushaowei.ise.neu@gmail.com
          liu_nongfu@163.com

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