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Internet Engineering Task Force (IETF) N. Zong
Request for Comments: 7263 X. Jiang
Category: Standards Track R. Even
ISSN: 2070-1721 Huawei Technologies
Y. Zhang
CoolPad / China Mobile
June 2014
An Extension to the REsource LOcation And Discovery (RELOAD) Protocol
to Support Direct Response Routing
Abstract
This document defines an optional extension to the REsource LOcation
And Discovery (RELOAD) protocol to support the direct response
routing mode. RELOAD recommends symmetric recursive routing for
routing messages. The new optional extension provides a shorter
route for responses, thereby reducing overhead on intermediate peers.
This document also describes potential cases where this extension can
be used.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7263.
Zong, et al. Standards Track [Page 1]
RFC 7263 P2PSIP DRR June 2014
Copyright Notice
Copyright (c) 2014 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.
Zong, et al. Standards Track [Page 2]
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Table of Contents
1. Introduction ....................................................4
2. Terminology .....................................................4
3. Overview ........................................................5
3.1. SRR and DRR ................................................5
3.1.1. Symmetric Recursive Routing (SRR) ...................6
3.1.2. Direct Response Routing (DRR) .......................6
3.2. Scenarios Where DRR Can Be Used ............................7
3.2.1. Managed or Closed P2P Systems .......................7
3.2.2. Wireless Scenarios ..................................8
4. Relationship between SRR and DRR ................................8
4.1. How DRR Works ..............................................8
4.2. How SRR and DRR Work Together ..............................8
5. DRR Extensions to RELOAD ........................................9
5.1. Basic Requirements .........................................9
5.2. Modification to RELOAD Message Structure ...................9
5.2.1. State-Keeping Flag ..................................9
5.2.2. Extensive Routing Mode .............................10
5.3. Creating a Request ........................................11
5.3.1. Creating a Request for DRR .........................11
5.4. Request and Response Processing ...........................11
5.4.1. Destination Peer: Receiving a Request and
Sending a Response .................................11
5.4.2. Sending Peer: Receiving a Response .................12
6. Overlay Configuration Extension ................................12
7. Security Considerations ........................................12
8. IANA Considerations ............................................13
8.1. A New RELOAD Forwarding Option ............................13
8.2. A New IETF XML Registry ...................................13
9. Acknowledgments ................................................13
10. References ....................................................13
10.1. Normative References .....................................13
10.2. Informative References ...................................14
Appendix A. Optional Methods to Investigate Peer Connectivity .....15
A.1. Getting Addresses to Be Used as Candidates for DRR .........15
A.2. Public Reachability Test ...................................16
Appendix B. Comparison of Cost of SRR and DRR .....................17
B.1. Closed or Managed Networks .................................17
B.2. Open Networks ..............................................19
Zong, et al. Standards Track [Page 3]
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1. Introduction
The REsource LOcation And Discovery (RELOAD) protocol [RFC6940]
recommends symmetric recursive routing (SRR) for routing messages and
describes the extensions that would be required to support additional
routing algorithms. In addition to SRR, two other routing options --
direct response routing (DRR) and relay peer routing (RPR) -- are
also discussed in Appendix A of [RFC6940]. As we show in Section 3,
DRR is advantageous over SRR in some scenarios in that DRR can reduce
load (CPU and link bandwidth) on intermediate peers. For example, in
a closed network where every peer is in the same address realm, DRR
performs better than SRR. In other scenarios, using a combination of
DRR and SRR together is more likely to provide benefits than if SRR
is used alone.
Note that in this document we focus on the DRR mode and its
extensions to RELOAD to produce a standalone solution. Please refer
to [RFC7264] for details on the RPR mode.
We first discuss the problem statement in Section 3. How to combine
DRR and SRR is presented in Section 4. An extension to RELOAD to
support DRR is defined in Section 5. Some optional methods to check
peer connectivity are introduced in Appendix A. In Appendix B, we
give a comparison of the cost of SRR and DRR in both managed and open
networks.
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].
We use terminology and definitions from the base RELOAD specification
[RFC6940] extensively in this document. We also use terms defined in
the NAT behavior discovery document [RFC5780]. Other terms used in
this document are defined inline when used and are also defined below
for reference.
Publicly Reachable: A peer is publicly reachable if it can receive
unsolicited messages from any other peer in the same overlay.
Note: "Publicly" does not mean that the peers must be on the
public Internet, because the RELOAD protocol may be used in a
closed network.
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Direct Response Routing (DRR): "DRR" refers to a routing mode in
which responses to Peer-to-Peer SIP (P2PSIP) requests are returned
to the sending peer directly from the destination peer based on
the sending peer's own local transport address(es). For
simplicity, the abbreviation "DRR" is used in the rest of this
document.
Symmetric Recursive Routing (SRR): "SRR" refers to a routing mode
in which responses follow the reverse path of the request to get
to the sending peer. For simplicity, the abbreviation "SRR" is
used in the rest of this document.
Relay Peer Routing (RPR): "RPR" refers to a routing mode in which
responses to P2PSIP requests are sent by the destination peer to
the transport address of a relay peer that will forward the
responses towards the sending peer. For simplicity, the
abbreviation "RPR" is used in the rest of this document.
3. Overview
RELOAD is expected to work under a great number of application
scenarios. The situations where RELOAD is to be deployed differ
greatly. For instance, some deployments are global, such as a
Skype-like system intended to provide public service, while others
run in small-scale closed networks. SRR works in any situation, but
DRR may work better in some specific scenarios.
3.1. SRR and DRR
RELOAD is a simple request-response protocol. After sending a
request, a peer waits for a response from a destination peer. There
are several ways for the destination peer to send a response back to
the source peer. In this section, we will provide detailed
information on two routing modes: SRR and DRR.
Some assumptions are made in the illustrations that follow:
1) Peer A sends a request destined to a peer who is the responsible
peer for a Resource-ID k.
2) Peer X is the root peer responsible for Resource-ID k.
3) The intermediate peers for the path from A to X are peers B, C,
and D.
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3.1.1. Symmetric Recursive Routing (SRR)
For SRR, when the request sent by peer A is received by an
intermediate peer B, C, or D, each intermediate peer will insert
information on the peer from whom they got the request in the
Via List, as described in RELOAD [RFC6940]. As a result, the
destination peer X will know the exact path that the request has
traversed. Peer X will then send back the response in the reverse
path by constructing a Destination List based on the Via List in the
request. Figure 1 illustrates SRR.
A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
| | | |<-----------|
| | | Response | |
| | |<-----------| |
| | Response | | |
| |<-----------| | |
| Response | | | |
|<-----------| | | |
| | | | |
Figure 1: SRR Mode
SRR works in any situation, especially when there are NATs or
firewalls. A downside of this solution is that the message takes
several hops to return to the peer, increasing the bandwidth usage
and CPU/battery load of multiple peers.
3.1.2. Direct Response Routing (DRR)
In DRR, peer X receives the request sent by peer A through
intermediate peers B, C, and D, as in SRR. However, peer X sends the
response back directly to peer A based on peer A's local transport
address. In this case, the response is not routed through
intermediate peers. Figure 2 illustrates DRR. Using a shorter route
means less overhead on intermediate peers, especially in the case of
wireless networks where the CPU and uplink bandwidth are limited.
For example, in the absence of NATs, or if the NAT implements
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endpoint-independent filtering, this is the optimal routing
technique. Note that establishing a secure connection requires
multiple round trips. Please refer to Appendix B for a cost
comparison between SRR and DRR.
A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
|<-----------+------------+------------+------------|
| | | | |
Figure 2: DRR Mode
3.2. Scenarios Where DRR Can Be Used
This section lists several scenarios where using DRR would work and
identifies when the increased efficiency would be advantageous.
3.2.1. Managed or Closed P2P Systems
The properties that make P2P technology attractive, such as the lack
of need for centralized servers, self-organization, etc., are
attractive for managed systems as well as unmanaged systems. Many of
these systems are deployed on private networks where peers are in the
same address realm and/or can directly route to each other. In such
a scenario, the network administrator can indicate preference for DRR
in the peer's configuration file. Peers in such a system would
always try DRR first, but peers MUST also support SRR in case DRR
fails. During the process of establishing a direct connection with
the sending peer, if the responding peer receives a request with SRR
as the preferred routing mode (or it fails to establish the direct
connection), the responding peer SHOULD NOT use DRR but instead
switch to SRR. The simple policy is to try DRR and, if this fails,
switch to SRR for all connections. In a finer-grained policy, a peer
would keep a list of unreachable peers based on trying DRR and then
would use only SRR for those peers. The advantage of using DRR is
network stability, since it puts less overhead on the intermediate
peers that will not route the responses. The intermediate peers will
need to route fewer messages and will save CPU resources as well as
link bandwidth usage.
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3.2.2. Wireless Scenarios
In some mobile deployments, using DRR may help reduce radio battery
usage and bandwidth by the intermediate peers. The service provider
may recommend using DRR based on his knowledge of the topology.
4. Relationship between SRR and DRR
4.1. How DRR Works
DRR is very simple. The only requirement is for the source peers to
provide their potential (publicly reachable) transport address to the
destination peers, so that the destination peer knows where to send
the response. Responses are sent directly to the requesting peer.
4.2. How SRR and DRR Work Together
DRR is not intended to replace SRR. It is better to use these two
modes together to adapt to each peer's specific situation. In this
section, we give some informative suggestions for how to transition
between the routing modes in RELOAD.
According to [RFC6940], SRR MUST be supported. An overlay MAY be
configured to use alternative routing algorithms, and alternative
routing algorithms MAY be selected on a per-message basis. That is,
a node in an overlay that supports SRR and some other routing
algorithm -- for example, DRR -- might use SRR some of the time and
DRR some of the time. A node joining the overlay should get the
preferred routing mode from the configuration file. If an overlay
runs within a private network and all peers in the system can reach
each other directly, peers MAY send most of the transactions with
DRR. However, DRR SHOULD NOT be used in the open Internet or if the
administrator does not feel he has enough information about the
overlay network topology. A new overlay configuration element
specifying the usage of DRR is defined in Section 6.
Alternatively, a peer can collect statistical data on the success of
the different routing modes based on previous transactions and keep a
list of non-reachable addresses. Based on this data, the peer will
have a clearer view of the success rate of different routing modes.
In addition to data on the success rate, the peer can also get data
of finer granularity -- for example, the number of retransmissions
the peer needs to achieve a desirable success rate.
A typical strategy for the peer is as follows. A peer chooses to
start with DRR based on the configuration. Based on the success rate
as indicated by statistics on lost messages or by responses that used
DRR, the peer can either continue to offer DRR first or switch to
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SRR. Note that a peer should use the DRR success statistics to
decide whether to continue using DRR or fall back to SRR. Making
such a decision per specific connection is not recommended; this
should be an application decision.
5. DRR Extensions to RELOAD
Adding support for DRR requires extensions to the current RELOAD
protocol. In this section, we define the required extensions,
including extensions to message structure and message processing.
5.1. Basic Requirements
All peers MUST be able to process requests for routing in SRR and MAY
support DRR routing requests.
5.2. Modification to RELOAD Message Structure
RELOAD provides an extensible framework to accommodate future
extensions. In this section, we define a ForwardingOption structure
to support DRR mode. Additionally, we present a state-keeping flag
to inform intermediate peers if they are allowed to not maintain
state for a transaction.
5.2.1. State-Keeping Flag
RELOAD allows intermediate peers to maintain state in order to
implement SRR -- for example, for implementing hop-by-hop
retransmission. If DRR is used, the response will not follow the
reverse path, and the state in the intermediate peers will not be
cleared until such state expires. In order to address this issue, we
define a new flag, state-keeping flag, in the ForwardingOption
structure to indicate whether the state-keeping is required in the
intermediate peers.
Flag: 0x08 IGNORE-STATE-KEEPING
If IGNORE-STATE-KEEPING is set, any peer receiving this message but
who is not the destination of the message SHOULD forward the message
with the full Via List and SHOULD NOT maintain any internal state.
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5.2.2. Extensive Routing Mode
This document introduces a new forwarding option for an extensive
routing mode. This option conforms to the description in
Section 6.3.2.3 of [RFC6940].
We first define a new type to define the new option,
extensive_routing_mode:
The option value that defines the ExtensiveRoutingModeOption
structure is illustrated below:
enum {(0),DRR(1),(255)} RouteMode;
struct {
RouteMode routemode;
OverlayLinkType transport;
IpAddressPort ipaddressport;
Destination destinations<1..2^8-1>;
} ExtensiveRoutingModeOption;
The above structure reuses the OverlayLinkType, Destination, and
IpAddressPort structures as defined in Sections 6.5.1.1, 6.3.2.2, and
6.3.1.1 of [RFC6940], respectively.
RouteMode: refers to which type of routing mode is indicated to the
destination peer.
OverlayLinkType: refers to the transport type that is used to deliver
responses from the destination peer to the sending peer.
IpAddressPort: refers to the transport address that the destination
peer will use for sending responses. This will be a sending peer
address for DRR.
Destination: refers to the sending peer itself. If the routing mode
is DRR, then the destination only contains the sending peer's
Node-ID.
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5.3. Creating a Request
5.3.1. Creating a Request for DRR
When using DRR for a transaction, the sending peer MUST set the
IGNORE-STATE-KEEPING flag in the ForwardingHeader. Additionally, the
peer MUST construct and include a ForwardingOption structure in the
ForwardingHeader. When constructing the ForwardingOption structure,
the fields MUST be set as follows:
1) The type MUST be set to extensive_routing_mode.
2) The ExtensiveRoutingModeOption structure MUST be used for the
option field within the ForwardingOption structure. The fields
MUST be defined as follows:
2.1) routemode set to 0x01 (DRR).
2.2) transport set as appropriate for the sender.
2.3) ipaddressport set to the peer's associated transport
address.
2.4) The destination structure MUST contain one value, defined
as type "node" and set with the sending peer's own values.
5.4. Request and Response Processing
This section gives normative text for message processing after DRR is
introduced. Here, we only describe the additional procedures for
supporting DRR. Please refer to [RFC6940] for RELOAD base
procedures.
5.4.1. Destination Peer: Receiving a Request and Sending a Response
When the destination peer receives a request, it will check the
options in the forwarding header. If the destination peer cannot
understand the extensive_routing_mode option in the request, it MUST
attempt to use SRR to return an "Error_Unknown_Extension" response
(defined in Sections 6.3.3.1 and 14.9 of [RFC6940]) to the sending
peer.
If the routing mode is DRR, the destination peer MUST construct the
Destination List for the response with only one entry, using the
requesting peer's Node-ID from the Via List in the request as the
value.
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In the event that the routing mode is set to DRR and there is not
exactly one destination, the destination peer MUST try to return an
"Error_Unknown_Extension" response (defined in Sections 6.3.3.1 and
14.9 of [RFC6940]) to the sending peer using SRR.
After the peer constructs the Destination List for the response, it
sends the response to the transport address, which is indicated in
the ipaddressport field in the option using the specific transport
mode in the ForwardingOption. If the destination peer receives a
retransmit with SRR preference on the message it is trying to respond
to now, the responding peer SHOULD abort the DRR response and
use SRR.
5.4.2. Sending Peer: Receiving a Response
Upon receiving a response, the peer follows the rules in [RFC6940].
The peer SHOULD note if DRR worked, in order to decide whether to
offer DRR again. If the peer does not receive a response until the
timeout, it SHOULD resend the request using SRR.
6. Overlay Configuration Extension
This document extends the RELOAD overlay configuration (see
Section 11.1 of [RFC6940]) by adding one new element, "route-mode",
inside each "configuration" element.
The Compact Regular Language for XML Next Generation (RELAX NG)
grammar for this element is:
namespace route-mode = "urn:ietf:params:xml:ns:p2p:route-mode"
parameter &= element route-mode:mode { xsd:string }?
This namespace is added into the <mandatory-extension> element in the
overlay configuration file. The defined routing modes include DRR
and RPR.
The mode can be DRR or RPR and, if specified in the configuration,
should be the preferred routing mode used by the application.
7. Security Considerations
The normative security recommendations of Section 13 of [RFC6940] are
applicable to this document. As a routing alternative, the security
part of DRR conforms to Section 13.6 of [RFC6940], which describes
routing security. For example, the DRR routing option provides
information about the route back to the source. According to
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RFC 7263 P2PSIP DRR June 2014
Section 13.6 of [RFC6940], the entire DRR routing message MUST be
digitally signed and sent over via a protected channel to protect the
DRR routing information.
8. IANA Considerations
8.1. A New RELOAD Forwarding Option
A new RELOAD Forwarding Option type has been added to the "RELOAD
Forwarding Option" registry defined in [RFC6940].
Code: 2
Forwarding Option: extensive_routing_mode
8.2. A New IETF XML Registry
IANA has registered the following URN in the "XML Namespaces" class
of the "IETF XML Registry" in accordance with [RFC3688].
URI: urn:ietf:params:xml:ns:p2p:route-mode
Registrant Contact: The IESG
XML: This specification
9. Acknowledgments
David Bryan helped extensively with this document and helped provide
some of the text, analysis, and ideas contained here. The authors
would like to thank Ted Hardie, Narayanan Vidya, Dondeti Lakshminath,
Bruce Lowekamp, Stephane Bryant, Marc Petit-Huguenin, and Carlos
Jesus Bernardos Cano for their constructive comments.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC6940] Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
Base Protocol", RFC 6940, January 2014.
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10.2. Informative References
[Chord] Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
Scalable Peer-to-Peer Lookup Protocol for Internet
Applications", IEEE/ACM Transactions on Networking
Volume 11, Issue 1, 17-32, February 2003.
[DTLS] Modadugu, N. and E. Rescorla, "The Design and
Implementation of Datagram TLS", Proc. 11th Network and
Distributed System Security Symposium (NDSS),
February 2004.
[IGD2] UPnP Forum, "WANIPConnection:2 Service", September 2010,
<http://upnp.org/specs/gw/
UPnP-gw-WANIPConnection-v2-Service.pdf>.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
Using Session Traversal Utilities for NAT (STUN)",
RFC 5780, May 2010.
[RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol
(NAT-PMP)", RFC 6886, April 2013.
[RFC7264] Zong, N., Jiang, X., Even, R., and Y. Zhang, "An Extension
to the REsource LOcation And Discovery (RELOAD) Protocol
to Support Relay Peer Routing", RFC 7264, June 2014.
[wikiChord]
Wikipedia, "Chord (peer-to-peer)", 2013,
<http://en.wikipedia.org/w/
index.php?title=Chord_%28peer-to-peer%29&oldid=549516287>.
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Appendix A. Optional Methods to Investigate Peer Connectivity
This section is for informational purposes only and provides some
mechanisms that can be used when the configuration information does
not specify if DRR can be used. It summarizes some methods that can
be used by a peer to determine its own network location compared with
NAT. These methods may help a peer to decide which routing mode it
may wish to try. Note that there is no foolproof way to determine
whether a peer is publicly reachable, other than via out-of-band
mechanisms. This document addresses UNilateral Self-Address Fixing
(UNSAF) [RFC3424] considerations by specifying a fallback plan to SRR
[RFC6940]. SRR is not an UNSAF mechanism. This document does not
define any new UNSAF mechanisms.
For DRR to function correctly, a peer may attempt to determine
whether it is publicly reachable. If it is not, the peer should fall
back to SRR. If the peer believes it is publicly reachable, DRR may
be attempted. NATs and firewalls are two major contributors to
preventing DRR from functioning properly. There are a number of
techniques by which a peer can get its reflexive address on the
public side of the NAT. After obtaining the reflexive address, a
peer can perform further tests to learn whether the reflexive address
is publicly reachable. If the address appears to be publicly
reachable, the peer to which the address belongs can use DRR for
responses.
Some conditions that are unique in P2PSIP architecture could be
leveraged to facilitate the tests. In a P2P overlay network, each
peer has only a partial view of the whole network and knows of a few
peers in the overlay. P2P routing algorithms can easily deliver a
request from a sending peer to a peer with whom the sending peer has
no direct connection. This makes it easy for a peer to ask other
peers to send unsolicited messages back to the requester.
In the following sections, we first introduce several ways for a peer
to get the addresses needed for further tests. Then, a test for
learning whether a peer may be publicly reachable is proposed.
A.1. Getting Addresses to Be Used as Candidates for DRR
In order to test whether a peer may be publicly reachable, the peer
should first get one or more addresses that will be used by other
peers to send him messages directly. This address is either a local
address of a peer or a translated address that is assigned by a NAT
to the peer.
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Session Traversal Utilities for NAT (STUN) is used to get a reflexive
address on the public side of a NAT with the help of STUN servers.
NAT behavior discovery using STUN is specified in [RFC5780]. Under
the RELOAD architecture, a few infrastructure servers can be
leveraged for discovering NAT behavior, such as enrollment servers,
diagnostic servers, bootstrap servers, etc.
The peer can use a STUN Binding request to one of the STUN servers to
trigger a STUN Binding response, which returns the reflexive address
from the server's perspective. If the reflexive transport address is
the same as the source address of the Binding request, the peer can
determine that there is likely no NAT between it and the chosen
infrastructure server. (Certainly, in some rare cases, the allocated
address happens to be the same as the source address. Further tests
will detect this case and rule it out in the end.) Usually, these
infrastructure servers are publicly reachable in the overlay, so the
peer can be considered publicly reachable. On the other hand, using
the techniques in [RFC5780], a peer can also decide whether it is
behind a NAT with endpoint-independent mapping behavior. If the peer
is behind a NAT with endpoint-independent mapping behavior, the
reflexive address should also be a candidate for further tests.
The Universal Plug and Play Internet Gateway Device (UPnP-IGD) [IGD2]
is a mechanism that a peer can use to get the assigned address from
its residential gateway, and after obtaining this address to
communicate it with other peers, the peer can receive unsolicited
messages from outside, even though it is behind a NAT. So, the
address obtained through the UPnP mechanism should also be used for
further tests.
Another way that a peer behind NAT can learn its assigned address by
NAT is via the NAT Port Mapping Protocol (NAT-PMP) [RFC6886]. As
with UPnP-IGD, the address obtained using this mechanism should also
be tested further.
The above techniques are not exhaustive. These techniques can be
used to get candidate transport addresses for further tests.
A.2. Public Reachability Test
Using the transport addresses obtained by the above techniques, a
peer can start a test to learn whether the candidate transport
address is publicly reachable. The basic idea of the test is that a
peer sends a request and expects another peer in the overlay to send
back a response. If the response is successfully received by the
sending peer and the peer giving the response has no direct
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connection with the sending peer, the sending peer can determine that
the address is probably publicly reachable and hence the peer may be
publicly reachable at the tested transport address.
In a P2P overlay, a request is routed through the overlay and finally
a destination peer will terminate the request and give the response.
In a large system, there is a high probability that the destination
peer has no direct connection with the sending peer. Every peer
maintains a connection table, particularly in the RELOAD
architecture, so it is easier for a peer to see whether it has direct
connection with another peer.
If a peer wants to test whether its transport address is publicly
reachable, it can send a request to the overlay. The routing for the
test message would be different from other kinds of requests because
it is not for storing or fetching something to or from the overlay,
or for locating a specific peer; instead, it is to get a peer who can
deliver to the sending peer an unsolicited response and who has no
direct connection with him. Each intermediate peer receiving the
request first checks to see whether it has a direct connection with
the sending peer. If there is a direct connection, the request is
routed to the next peer. If there is no direct connection, the
intermediate peer terminates the request and sends the response back
directly to the sending peer with the transport address under test.
After performing the test, if the peer determines that it may be
publicly reachable, it can try DRR in subsequent transactions.
Appendix B. Comparison of Cost of SRR and DRR
The major advantage of using DRR is that it reduces the number of
intermediate peers traversed by the response. This reduces the load,
such as processing and communication bandwidth, on those peers'
resources.
B.1. Closed or Managed Networks
As described in Section 3, many P2P systems run in a closed or
managed environment (e.g., carrier networks), so network
administrators would know that they could safely use DRR.
SRR uses more routing hops than DRR. Assuming that there are N peers
in the P2P system and Chord [Chord] [wikiChord] is applied for
routing, the number of hops for a response in SRR and in DRR are
listed in the following table. Establishing a secure connection
between the sending peer and the responding peer with Transport Layer
Security (TLS) or Datagram TLS (DTLS) requires multiple messages.
Note that establishing (D)TLS secure connections for a P2P overlay is
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not optimal in some cases, e.g., DRR where (D)TLS is heavy for
temporary connections. Therefore, in the following table we show the
cases of 1) no (D)TLS in DRR and 2) still using DTLS in DRR as
sub-optimal. As the worst-cost case, seven (7) messages are used
during DTLS handshaking [DTLS]. (The TLS handshake is a negotiation
protocol that requires two (2) round trips, while the DTLS handshake
is a negotiation protocol that requires three (3) round trips.)
Mode | Success | No. of Hops | No. of Msgs
------------------------------------------------
SRR | Yes | log(N) | log(N)
DRR | Yes | 1 | 1
DRR (DTLS) | Yes | 1 | 7+1
Table 1: Comparison of SRR and DRR in Closed Networks
From the above comparison, it is clear that:
1) In most cases when the number of peers (N) > 2 (2^1), DRR uses
fewer hops than SRR. Using a shorter route means less overhead
and resource usage on intermediate peers, which is an important
consideration for adopting DRR in the cases where such resources
as CPU and bandwidth are limited, e.g., the case of mobile,
wireless networks.
2) In the cases when N > 256 (2^8), DRR also uses fewer messages
than SRR.
3) In the cases when N < 256, DRR uses more messages than SRR (but
still uses fewer hops than SRR), so the consideration of whether
to use DRR or SRR depends on other factors such as using less
resources (bandwidth and processing) from the intermediate peers.
Section 4 provides use cases where DRR has a better chance of
working or where the considerations of intermediary resources are
important.
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B.2. Open Networks
In open networks (e.g., the Internet) where DRR is not guaranteed to
work, DRR can fall back to SRR if it fails after trial, as described
in Section 4. Based on the same settings as those listed in
Appendix B.1, the number of hops, as well as the number of messages
for a response in SRR and DRR, are listed in the following table:
Mode | Success | No. of Hops | No. of Msgs
----------------------------------------------------------------
SRR | Yes | log(N) | log(N)
DRR | Yes | 1 | 1
| Fail & fall back to SRR | 1+log(N) | 1+log(N)
DRR (DTLS) | Yes | 1 | 7+1
| Fail & fall back to SRR | 1+log(N) | 8+log(N)
Table 2: Comparison of SRR and DRR in Open Networks
From the above comparison, it can be observed that trying to first
use DRR could still provide an overall number of hops lower than
directly using SRR. Suppose that P peers are publicly reachable; the
number of hops in DRR and SRR is P*1+(N-P)*(1+logN) and N*logN,
respectively. The condition for fewer hops in DRR is
P*1+(N-P)*(1+logN) < N*logN, which is P/N > 1/logN. This means that
when the number of peers (N) grows, the required ratio of publicly
reachable peers P/N for fewer hops in DRR decreases. Therefore, the
chance of trying DRR with fewer hops than SRR improves as the scale
of the network increases.
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Authors' Addresses
Ning Zong
Huawei Technologies
EMail: zongning@huawei.com
Xingfeng Jiang
Huawei Technologies
EMail: jiang.x.f@huawei.com
Roni Even
Huawei Technologies
EMail: roni.even@mail01.huawei.com
Yunfei Zhang
CoolPad / China Mobile
EMail: hishigh@gmail.com
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