Internet DRAFT - draft-ietf-p2psip-drr
draft-ietf-p2psip-drr
P2PSIP N. Zong, Ed.
Internet-Draft X. Jiang
Intended status: Standards Track R. Even
Expires: April 24, 2014 Huawei Technologies
Y. Zhang
CoolPad
October 21, 2013
An Extension to REsource LOcation And Discovery (RELOAD) Protocol to
Support Direct Response Routing
draft-ietf-p2psip-drr-11
Abstract
This document proposes an optional extension to REsource LOcation And
Discovery (RELOAD) protocol to support direct response routing mode.
RELOAD recommends symmetric recursive routing for routing messages.
The new optional extension provides a shorter route for responses
reducing the overhead on intermediate peers and describes the
potential cases where this extension can be used.
Status of This Memo
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This Internet-Draft will expire on April 24, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. SRR and DRR . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Symmetric Recursive Routing (SRR) . . . . . . . . . . 4
3.1.2. Direct Response Routing (DRR) . . . . . . . . . . . . 5
3.2. Scenarios where DRR can be used . . . . . . . . . . . . . 6
3.2.1. Managed or closed P2P systems . . . . . . . . . . . . 6
3.2.2. Wireless scenarios . . . . . . . . . . . . . . . . . 6
4. Relationship between SRR and DRR . . . . . . . . . . . . . . 6
4.1. How DRR works . . . . . . . . . . . . . . . . . . . . . . 7
4.2. How SRR and DRR work together . . . . . . . . . . . . . . 7
5. Comparison on cost of SRR and DRR . . . . . . . . . . . . . . 8
5.1. Closed or managed networks . . . . . . . . . . . . . . . 8
5.2. Open networks . . . . . . . . . . . . . . . . . . . . . . 9
6. DRR extensions to RELOAD . . . . . . . . . . . . . . . . . . 9
6.1. Basic requirements . . . . . . . . . . . . . . . . . . . 9
6.2. Modification to RELOAD message structure . . . . . . . . 10
6.2.1. State-keeping flag . . . . . . . . . . . . . . . . . 10
6.2.2. Extensive routing mode . . . . . . . . . . . . . . . 10
6.3. Creating a request . . . . . . . . . . . . . . . . . . . 11
6.3.1. Creating a request for DRR . . . . . . . . . . . . . 11
6.4. Request and response processing . . . . . . . . . . . . . 12
6.4.1. Destination peer: receiving a request and sending a
response . . . . . . . . . . . . . . . . . . . . . . 12
6.4.2. Sending peer: receiving a response . . . . . . . . . 12
7. Overlay configuration extension . . . . . . . . . . . . . . . 12
8. Security considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13
9.1. A new RELOAD forwarding option . . . . . . . . . . . . . 13
9.2. A new IETF XML registry . . . . . . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1. Normative references . . . . . . . . . . . . . . . . . . 14
11.2. Informative references . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Appendix A. Optional methods to investigate peer connectivity . 14
A.1. Getting addresses to be used as candidates for DRR . . . 15
A.2. Public reachability test . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
REsource LOcation And Discovery (RELOAD) protocol [I-D.ietf-p2psip-
base] recommends symmetric recursive routing (SRR) for routing
messages and describes the extensions that would be required to
support additional routing algorithms. Other than SRR, two other
routing options: direct response routing (DRR) and relay peer routing
(RPR) are also discussed in Appendix A of [I-D.ietf-p2psip-base]. As
we show in section 3, DRR is advantageous over SRR in some scenarios
by reducing 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 bring benefits
than if SRR is used alone.
Note that in this document, we focus on DRR routing mode and its
extensions to RELOAD to produce a standalone solution. Please refer
to RPR draft [I-D.ietf-p2psip-rpr] for RPR routing mode.
We first discuss the problem statement in Section 3, then how to
combine DRR and SRR is presented in Section 4. In Section 5, we give
comparison on the cost of SRR and DRR in both managed and open
networks. An extension to RELOAD to support DRR is proposed in
Section 6. Some optional methods to check peer connectivity are
introduced in Appendix A.
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 the terminology and definitions from the RELOAD base draft
[I-D.ietf-p2psip-base] extensively in this document. We also use
terms defined in NAT behavior discovery [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): refers to a routing mode in which
responses to 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 instead in the rest of the document.
Symmetric Recursive Routing (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
instead in the rest of the 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
closed networks of small scale. 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 following illustrations.
1) Peer A sends a request destined to a peer who is the responsible
peer for Resource-ID k;
2) Peer X is the root peer being responsible for resource k;
3) The intermediate peers for the path from A to X are peer B, C, D.
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. As a result, the destination peer X
will know the exact path which 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.
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A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
| | | |<-----------|
| | | Response | |
| | |<-----------| |
| | Response | | |
| |<-----------| | |
| Response | | | |
|<-----------| | | |
| | | | |
Figure 1. SRR routing 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 peer 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 is limited. For
example, in the absence of NATs, or if the NAT implements endpoint-
independent filtering, this is the optimal routing technique. Note
that establishing a secure connection requires multiple round trips.
Please refer to Section 5 for cost comparison between SRR and DRR.
A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
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| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
|<-----------+------------+------------+------------|
| | | | |
Figure 2. DRR routing 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. If during the process of establishing a direct connection
with the sending peer, 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 switch
to SRR. The simple policy is to try DRR and if fails switch to SRR
for all connections. A finer grained policy is when a peer keeps a
list of unreachable peers based on trying DRR and use only SRR for
these peers. The advantage in using DRR is on the network stability
since it puts less overhead on the intermediate peers that will not
route the responses. The intermediate peers will need to route less
messages and save CPU resources as well as the link bandwidth usage.
3.2.2. Wireless scenarios
In some mobile deployments, using DRR may help with reducing radio
battery usage and bandwidth by the intermediate peers. The service
provider may recommend using DRR based on his/her knowledge of the
topology.
4. Relationship between SRR and DRR
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4.1. How DRR works
DRR is very simple. The only requirement is for the source peers to
provide their potential (publically 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 on how to transition
between the routing modes in RELOAD.
According to base draft [I-D.ietf-p2psip-base], 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. I.e., a node in an overlay which 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 from the configuration file the preferred routing mode. 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. On the contrary, using DRR SHOULD be discouraged in the
open Internet or if the administrator does not feel he have enough
information about the overlay network topology. A new overlay
configuration element specifying the usage of DRR is defined in
Section 7.
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 about the success rate of different routing
modes. Other than the success rate, the peer can also get data of
finer granularity, for example, the number of retransmission 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
seen from the lost message statistics or responses that used DRR, the
peer can either continue to offer DRR first or switch to SRR. Note
that a peer should use the DRR success statistic to decide if to
continue using DRR or fall back to SRR. It is not recommended to
make such decision per specific connection but this is an application
decision.
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5. Comparison on cost of SRR and DRR
The major advantages in using DRR are in going through less
intermediate peers on the response. By doing that it reduces the
load on those peers' resources like processing and communication
bandwidth.
5.1. Closed or managed networks
As described in Section 3, many P2P systems run in a closed or
managed environment (e.g., carrier networks) so that network
administrators would know that they could safely use DRR.
SRR brings out more routing hops than DRR. Assuming that there are N
peers in the P2P system and Chord is applied for routing, the number
of hops for a response in SRR and DRR are listed in the following
table. Establishing a secure connection between the sending peer and
the responding peer with (D)TLS requires multiple messages. Note
that establishing (D)TLS secure connections for P2P overlay is not
optimal in some cases, e.g., direct response routing where (D)TLS is
heavy for temporary connections. Therefore, in the following table,
we show the cases of: 1) no (D)TLS in DRR; 2) still using DTLS in DRR
as sub-optimal. As the worst-cost case, 7 messages are used during
the DTLS handshaking [DTLS]. (TLS Handshake is a two round-trip
negotiation protocol while DTLS handshake is a three round-trip
negotiation protocol.)
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 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 the resources such 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.
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3) In the cases when N < 256, DRR uses more messages than SRR (but
still uses fewer hops than SRR). So the consideration on whether
using DRR or SRR depends on other factors like using less resources
(bandwidth and processing) from the intermediate peers. Section 4
provides use cases where DRR has better chance to work or where the
intermediary resources considerations are important.
5.2. Open networks
In open networks (e.g., 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 in Section 5.1, the number
of hops, 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), 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 becomes better as the
scale of the network increases.
6. DRR extensions to RELOAD
Adding support for DRR requires extensions to the current RELOAD
protocol. In this section, we define the extensions required to the
protocol, including extensions to message structure and to message
processing.
6.1. Basic requirements
All peers MUST be able to process requests for routing in SRR, and
MAY support DRR routing requests.
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6.2. 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.
6.2.1. 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
propose a new flag, state-keeping flag, in the message header 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 and
which is not the destination of the message SHOULD forward the
message with the full via_list and SHOULD NOT maintain any internal
state.
6.2.2. Extensive routing mode
This draft introduces a new forwarding option for an extensive
routing mode. This option conforms to the description in section
6.3.2.3 of [I-D.ietf-p2psip-base].
We first define a new type to define the new option,
extensive_routing_mode:
The option value is illustrated as below, defining the
ExtensiveRoutingModeOption structure:
enum {(0),DRR(1),(255)} RouteMode;
struct {
RouteMode routemode;
OverlayLinkType transport;
IpAddressPort ipaddressport;
Destination destinations<1..2^8-1>;
} ExtensiveRoutingModeOption;
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The above structure reuses OverlayLinkType, Destination and
IpAddressPort structure defined in section 6.5.1.1, 6.3.2.2 and
6.3.1.1 of [I-D.ietf-p2psip-base].
RouteMode: refers to which type of routing mode is indicated to the
destination peer.
OverlayLinkType: refers to the transport type which is used to
deliver responses from the destination peer to the sending peer.
IpAddressPort: refers to the transport address that the destination
peer use to send the response to. 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.
6.3. Creating a request
6.3.1. 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 ForwardingOptions 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 ForwardingOptions 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.
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6.4. 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 [I-D.ietf-p2psip-base] for RELOAD
base procedures.
6.4.1. 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 can not
understand extensive_routing_mode option in the request, it MUST
attempt to use SRR to return an "Error_Unknown_Extension" response
(defined in Section 6.3.3.1 and Section 14.9 of [I-D.ietf-p2psip-
base]) to the sending peer.
If the routing mode is DRR, the peer MUST construct the Destination
list for the response with only one entry, using the sending peer's
Node-ID from the option in the request as the value.
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 Section 6.3.3.1 and
Section 14.9 of [I-D.ietf-p2psip-base]) 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.
6.4.2. Sending peer: receiving a response
Upon receiving a response, the peer follows the rules in [I-D.ietf-
p2psip-base]. The peer SHOULD note if DRR worked in order to decide
if to offer DRR again. If the peer does not receive a response until
the timeout it SHOULD resend the request using SRR.
7. Overlay configuration extension
This document extends the RELOAD overlay configuration (see
Section 11.1 of [I-D.ietf-p2psip-base]) by adding one new element,
"route-mode", inside each "configuration" element.
The Compact Relax NG Grammar for this element is:
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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.
Mode can be DRR or RPR and if specified in the configuration should
be the preferred routing mode used by the application.
8. Security considerations
The normative security recommendations of Section 13 of base draft
[I-D.ietf-p2psip-base] are applicable to this document. As a routing
alternative, the security part of DRR conforms to Section 13.6 of the
base draft which describes routing security. For example, the DRR
routing option provides the information about the route back to the
source. According to Section 13.6 of the base draft the enter DRR
routing message MUST be digitally signed and sent over by protected
channel to protect the DRR routing information.
9. IANA considerations
9.1. A new RELOAD forwarding option
A new RELOAD Forwarding Option type is added to the Forwarding Option
Registry defined in [I-D.ietf-p2psip-base].
Type: 0x02 - extensive_routing_mode
9.2. A new IETF XML registry
This section requests IANA to register 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
10. Acknowledgments
David Bryan has helped extensively with this document, and helped
provide some of the text, analysis, and ideas contained here. The
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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.
11. References
11.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[I-D.ietf-p2psip-base] Jennings, C., Lowekamp, B., Rescorla, E.,
Baset, S., and H. Schulzrinne, "REsource LOcation And Discovery
(RELOAD) Base Protocol", draft-ietf-p2psip-base-26 (work in
progress), February 2013.
[RFC3688] Mealling, M., "The IETF XML Registry ", BCP 81, RFC3688,
January 2004.
11.2. Informative references
[DTLS] Modadugu, N., Rescorla, E., "The Design and Implementation of
Datagram TLS", 11th Network and Distributed System Security Symposium
(NDSS), 2004.
[RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
Using STUN", RFC5780, May 2010.
[I-D.ietf-p2psip-rpr] Zong, N., Jiang, X., Even, R. and Zhang, Y.,
"An extension to RELOAD to support Relay Peer Routing", draft-ietf-
p2psip-rpr-11 (work in progress), October 2013.
[IGD2] UPnP Forum, "WANIPConnection:2 Service (http://upnp.org/specs/
gw/UPnP-gw-WANIPConnection-v2-Service.pdf)", September 2010.
[RFC6886] Cheshire, S., Krochmal M., and K. Sekar, "NAT Port Mapping
Protocol (NAT-PMP)", RFC6886, April 2013.
[RFC3424] Daigle, L., "IAB Considerations for UNilateral Self-Address
Fixing (UNSAF) Across Network Address Translation", RFC3424, November
2002.
12. References
Appendix A. Optional methods to investigate peer connectivity
This section is for informational purposes only for providing some
mechanisms that can be used when the configuration information does
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not specify if DRR can be used. It summarizes some methods which can
be used for 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
if a peer is publically reachable, other than via out-of-band
mechanisms. This document addresses the UNSAF [RFC3424] concerns by
specifying a fallback plan to SRR [p2psip-base-draft]. SRR is not an
UNSAF mechanism. The 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 peers should
fall back to SRR. If the peer believes it is publically reachable,
DRR may be attempted. NATs and firewalls are two major contributors
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 peers to which the address belongs can use DRR for
responses.
Some conditions are unique in P2PSIP architecture which could be
leveraged to facilitate the tests. In P2P overlay network, each peer
only has partial a 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 which 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 which is assigned by a NAT
to the peer.
STUN is used to get a reflexive address on the public side of a NAT
with the help of STUN servers. Discovery of NAT behavior using STUN
is specified in [RFC5780]. Under RELOAD architecture, a few
infrastructure servers can be leveraged for discovering NAT behavior,
such as enrollment servers, diagnostic servers, bootstrap servers,
etc.
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The peer can use a STUN Binding request to one of 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 likely is 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 severs are publicly reachable in the overlay, so the
peer can be considered publicly reachable. On the other hand, with
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.
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 use to learn its assigned
address by NAT is NAT-PMP [RFC6886]. Like in 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 for the test is for a
peer to send a request and expect another peer in the overlay to send
back a response. If the response is received by the sending peer
successfully and also the peer giving the response has no direct
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. Especially in
RELOAD architecture, every peer maintains a connection table. So it
is easier for a peer to check whether it has direct connection with
another peer.
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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/fetching something to/from the overlay or
locating a specific peer, instead it is to get a peer who can deliver
the sending peer an unsolicited response and which has no direct
connection with him. Each intermediate peer receiving the request
first checks whether it has a direct connections 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.
Authors' Addresses
Ning Zong (editor)
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
Email: hishigh@gmail.com
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