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IETF RFC 6316
Last modified on Friday, July 22nd, 2011
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Internet Engineering Task Force (IETF) M. Komu
Request for Comments: 6316 Aalto University
Category: Informational M. Bagnulo
ISSN: 2070-1721 UC3M
K. Slavov
S. Sugimoto, Ed.
Ericsson
July 2011
Sockets Application Program Interface (API) for Multihoming Shim
 Abstract
This document specifies sockets API extensions for the multihoming
shim layer. The API aims to enable interactions between applications
and the multihoming shim layer for advanced locator management, and
access to information about failure detection and path exploration.
This document is based on an assumption that a multihomed host is
equipped with a conceptual sub-layer (hereafter called "shim sub-
layer") inside the IP layer that maintains mappings between
identifiers and locators. Examples of the shim are Shim6 and the
Host Identity Protocol (HIP).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/RFC 6316.
Komu, et al. Informational PAGE 1 
RFC 6316 Multihoming Shim API July 2011
Copyright Notice
Copyright (c) 2011 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
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publication of this document. Please review these documents
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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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
2. Requirements Language ...........................................4
3. Terminology and Background ......................................4
4. System Overview .................................................7
5. Requirements ....................................................8
6. Socket Options for Multihoming Shim Sub-Layer ..................10
6.1. SHIM_ASSOCIATED ...........................................14
6.2. SHIM_DONTSHIM .............................................15
6.3. SHIM_HOT_STANDBY ..........................................16
6.4. SHIM_LOC_LOCAL_PREF .......................................17
6.5. SHIM_LOC_PEER_PREF ........................................18
6.6. SHIM_LOC_LOCAL_RECV .......................................19
6.7. SHIM_LOC_PEER_RECV ........................................20
6.8. SHIM_LOC_LOCAL_SEND .......................................20
6.9. SHIM_LOC_PEER_SEND ........................................22
6.10. SHIM_LOCLIST_LOCAL .......................................23
6.11. SHIM_LOCLIST_PEER ........................................25
6.12. SHIM_APP_TIMEOUT .........................................26
6.13. SHIM_PATHEXPLORE .........................................27
Komu, et al. Informational PAGE 2
RFC 6316 Multihoming Shim API July 2011
6.14. SHIM_DEFERRED_CONTEXT_SETUP ..............................28
6.15. Applicability ............................................28
6.16. Error Handling ...........................................29
7. Ancillary Data for Multihoming Shim Sub-Layer ..................29
7.1. Get Locator from Incoming Packet ..........................30
7.2. Set Locator for Outgoing Packet ...........................30
7.3. Notification from Application to Multihoming Shim
Sub-Layer .................................................31
7.4. Applicability .............................................31
8. Data Structures ................................................32
8.1. Data Structure for Locator Information ....................32
8.1.1. Handling Locator behind NAT ........................33
8.2. Path Exploration Parameter ................................34
8.3. Feedback Information ......................................35
9. System Requirements ............................................36
10. Relation to Existing Sockets API Extensions ...................36
11. Operational Considerations ....................................37
11.1. Conflict Resolution ......................................37
11.2. Incompatibility between IPv4 and IPv6 ....................38
12. IANA Considerations ...........................................38
13. Protocol Constant .............................................38
14. Security Considerations .......................................38
14.1. Treatment of Unknown Locator .............................39
14.1.1. Treatment of Unknown Source Locator ...............39
14.1.2. Treatment of Unknown Destination Locator ..........39
15. Acknowledgments ...............................................40
16. References ....................................................40
16.1. Normative References .....................................40
16.2. Informative References ...................................41
Appendix A. Context Forking .......................................42
1. Introduction
This document defines sockets API extensions by which upper-layer
protocols may be informed about and control the way in which a
multihoming shim sub-layer in the IP layer manages the dynamic choice
of locators. Initially, the multihoming shim sub-layer refers to
Shim6 and/or HIP, but it is defined generically.
The role of the multihoming shim sub-layer (hereafter called "shim
sub-layer" in this document) is to avoid impacts to upper-layer
protocols that may be caused when the endhost changes its attachment
point to the Internet -- for instance, in the case of a rehoming
event under the multihomed environment. There is, however, a need
for an API in the cases where 1) the upper-layer protocol is
particularly sensitive to impacts, or 2) the upper-layer protocol
wants to benefit from better knowledge of what is going on
underneath.
Komu, et al. Informational PAGE 3
RFC 6316 Multihoming Shim API July 2011
There are various kinds of technologies that aim to solve the same
issue (the multihoming issue). Note that there will be conflict when
more than one shim sub-layer is active at the same time. The
assumption made in this document is that there is only a single shim
sub-layer (HIP or Shim6) activated on the system.
The target readers of this document are application programmers who
develop application software that may benefit greatly from multihomed
environments. In addition, this document aims to provide necessary
information for developers of shim protocols to implement APIs for
enabling advanced locator management.
2. Requirements Language
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].
3. Terminology and Background
This section provides terminology used in this document. Basically,
most of the terms used in this document are taken from the following
documents:
o Shim6 Protocol Specification [RFC 5533]
o HIP Architecture [RFC 4423]
o Reachability Protocol (REAP) [RFC 5534]
In this document, the term "IP" refers to both IPv4 and IPv6, unless
the protocol version is specifically mentioned. The following are
definitions of terms frequently used in this document:
o Endpoint Identifier (EID) -- The identifier used by the
application to specify the endpoint of a given communication.
Applications may handle EIDs in various ways, such as long-lived
connections, callbacks, and referrals [SHIM6-APP-REFER].
* In the case of Shim6, an identifier called a ULID (Upper-Layer
Identifier) serves as an EID. A ULID is chosen from locators
available on the host.
* In the case of HIP, an identifier called a Host Identifier
serves as an EID. A Host Identifier is derived from the public
key of a given host. For the sake of backward compatibility
with the sockets API, the Host Identifier is represented in the
form of a hash of a public key.
Komu, et al. Informational PAGE 4
RFC 6316 Multihoming Shim API July 2011
* Note that the EID appears in the standard sockets API as an
address, and does not appear in the extensions defined in this
document, which only concern locators.
o Locator - The IP address actually used to deliver IP packets.
Locators are present in the source and destination fields of the
IP header of a packet on the wire. A locator as discussed in this
document could be either an IPv4 address or an IPv6 address. Note
that HIP can handle both IPv4 and IPv6 locators, whereas Shim6 can
handle only IPv6 locators. For the HIP case, a locator can be a
private IPv4 address when the host is behind a NAT. Section 8.1.1
gives a detailed description about the handling of a locator
behind a NAT.
* List of locators - A list of locators associated with an EID.
There are two lists of locators stored in a given context. One
is associated with the local EID, and the other is associated
with the remote EID. As defined in [RFC 5533], the list of
locators associated with an EID 'A' is denoted as Ls(A).
* Preferred locator - The (source/destination) locator currently
used to send packets within a given context.
* Unknown locator - Any locator that does not appear in the
locator list of the shim context associated with the socket.
When there is no shim context associated with the socket, any
source and/or destination locator requested by the application
is considered to be an unknown locator.
* Valid locator - A valid locator means that the locator is
considered to be valid in the security sense. More
specifically, the validity indicates whether the locator is
part of a Hash-Based Address (HBA) set [RFC 5535].
* Verified locator - A verified locator means that the locator is
considered to be reachable according to the result of a REAP
return routability check. Note that the verification applies
only to the peer's locator.
o Shim - The conceptual sub-layer inside the IP layer. This sub-
layer maintains mappings between EIDs and locators. An EID can be
associated with more than one locator at a time when the host is
multihomed. The term "shim" does not refer to a specific protocol
but refers to the conceptual sub-layer inside the IP layer.
Komu, et al. Informational PAGE 5
RFC 6316 Multihoming Shim API July 2011
o Identifier/locator adaptation - The adaptation performed at the
shim sub-layer. This adaptation may end up re-writing the source
and/or destination addresses of an IP packet. In the outbound
packet processing, the EID pair is converted to the associated
locator pair. In the inbound packet processing, the locator pair
is converted to the EID pair.
o Context - The state information shared by a given pair of peers.
Context stores a binding between the EID and associated locators.
Contexts are maintained by the shim sub-layer. Deferred context
setup is a scenario where a context is established after the
communication starts. Deferred context setup is possible if the
ULID is routable, such as in the case of Shim6.
o Reachability detection - The procedure to check reachability
between a given locator pair.
o Path - The sequence of routers that an IP packet goes through to
reach the destination.
o Path exploration - The procedure to explore available paths for a
given set of locator pairs.
o Outage - The incident that prevents IP packets flowing from the
source locator to the destination locator. When there is an
outage, it means that there is no reachability between a given
locator pair. The outage may be caused by various reasons, such
as a shortage of network resources, congestion, and human error
(faulty operation).
o Working address pair - Considered to be "working" if the packet
can safely travel from the source to the destination, where the
packet contains the first address from the pair as the source
address and the second address from the pair as the destination
address. If reachability is confirmed in both directions, the
address pair is considered to be working bi-directionally.
o Reachability Protocol (REAP) - The protocol for detecting failure
and exploring reachability in a multihomed environment. REAP is
defined in [RFC 5534].
Komu, et al. Informational PAGE 6
RFC 6316 Multihoming Shim API July 2011
In this document, syntax and semantics of the API are given in the
same way as in the Portable Operating System Interface (POSIX)
standard [POSIX]. The API specifies how to use ancillary data (aka
cmsg) to access the locator information with recvmsg() and/or
sendmsg() I/O calls. The API is described in C language, and data
types are defined in the POSIX format; intN_t means a signed integer
of exactly N bits (e.g., int16_t), and uintN_t means an unsigned
integer of exactly N bits (e.g., uint32_t).
The distinction between "connected" sockets and "unconnected" sockets
is important when discussing the applicability of the sockets API
defined in this document. A connected socket is bound to a given
peer, whereas an unconnected socket is not bound to any specific
peers. A TCP socket becomes a connected socket when the TCP
connection establishment is completed. UDP sockets are unconnected,
unless the application uses the connect() system call.
4. System Overview
Figure 1 illustrates the system overview. The shim sub-layer and
REAP component exist inside the IP layer. Applications use the
sockets API defined in this document to interface with the shim
sub-layer and the transport layer for locator management, failure
detection, and path exploration.
It is also possible that the shim sub-layer interacts with the
transport layer; however, such an interaction is outside the scope of
this document.
Komu, et al. Informational PAGE 7
RFC 6316 Multihoming Shim API July 2011
+------------------------+
| Application |
+------------------------+
^ ^
~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~
| v
+-----------|------------------------------+
| | Transport Layer |
+-----------|------------------------------+
^ |
+-------------|-----|-------------------------------------+
| v v |
| +-----------------------------+ +----------+ | IP
| | Shim |<----->| REAP | | Layer
| +-----------------------------+ +----------+ |
| ^ ^ |
+-----------------------|----------------------|----------+
v v
+------------------------------------------+
| Link Layer |
+------------------------------------------+
Figure 1: System Overview
5. Requirements
The following is a list of requirements from applications:
o Turn on/off shim. An application should be able to request to
turn on or turn off the multihoming support by the shim layer:
* Apply shim. The application should be able to explicitly
request that the shim sub-layer apply multihoming support.
* Don't apply shim. The application should be able to request
that the shim sub-layer not apply the multihoming support but
apply normal IP processing at the IP layer.
* Note that this function is also required by other types of
multihoming mechanisms, such as the Stream Control Transmission
Protocol (SCTP) and multipath TCP, to avoid potential conflict
with the shim sub-layer.
Komu, et al. Informational PAGE 8
RFC 6316 Multihoming Shim API July 2011
o Locator management.
* It should be possible to set a preferred source and/or
destination locator within a given context.
* It should be possible to get a preferred source and/or
destination locator within a given context.
* It should be possible to set a list of source and/or
destination locators within a given context: Ls(local) and
Ls(remote).
* It should be possible to get a list of source and/or
destination locators within a given context: Ls(local) and
Ls(remote).
o Notification from applications and upper-layer protocols to the
shim sub-layer about the status of the communication. The
notification occurs in an event-based manner. Applications and/or
upper-layer protocols may provide positive feedback or negative
feedback to the shim sub-layer. Note that these types of feedback
are mentioned in [RFC 5534]:
* Applications and/or upper-layer protocols (e.g., TCP) may
provide positive feedback to the shim sub-layer informing that
the communication is going well.
* Applications and/or upper-layer protocols (e.g., TCP) may
provide negative feedback to the shim sub-layer informing that
the communication status is not satisfactory. TCP may detect a
problem when it does not receive any expected ACK message from
the peer. The REAP module may be triggered by the negative
feedback and invoke the path exploration procedure.
o Feedback from applications to the shim sub-layer. Applications
should be able to inform the shim sub-layer of the timeout values
for detecting failures, sending keepalives, and starting the
exploration procedure. In particular, applications should be able
to suppress keepalives.
o Hot-standby. Applications may request the shim sub-layer for a
hot-standby capability. This means that alternative paths are
known to be working in advance of a failure detection. In such a
case, it is possible for the shim sub-layer to immediately replace
the current locator pair with an alternative locator pair.
Komu, et al. Informational PAGE 9
RFC 6316 Multihoming Shim API July 2011
o Eagerness for locator exploration. An application should be able
to inform the shim sub-layer of how aggressively it wants the REAP
mechanism to perform a path exploration (e.g., by specifying the
number of concurrent attempts of discovery of working locator
pairs) when an outage occurs on the path between the locator pair
in use.
o Providing locator information to applications. An application
should be able to obtain information about the locator pair that
was actually used to send or receive packets.
* For inbound traffic, the application may be interested in the
locator pair that was actually used to receive the packet.
* For outbound traffic, the application may be interested in the
locator pair that was actually used to transmit the packet.
In this way, applications may have additional control of the
locator management. For example, an application becomes capable
of verifying if its preference for a locator is actually applied
to the flow or not.
o Applications should be able to know if the shim sub-layer supports
deferred context setup or not.
o An application should be able to know if the communication is now
being served by the shim sub-layer or not.
o An application should be able to use a common interface to access
an IPv4 locator and an IPv6 locator.
6. Socket Options for Multihoming Shim Sub-Layer
In this section, socket options that are specific to the shim
sub-layer are defined.
Table 1 shows a list of the socket options that are specific to the
shim sub-layer. All of these socket options are defined at the level
SOL_SHIM. When an application uses one of the socket options by
getsockopt() or setsockopt(), the second argument MUST be set to
SOL_SHIM.
Komu, et al. Informational PAGE 10
RFC 6316 Multihoming Shim API July 2011
The first column of Table 1 gives the name of the option. The second
column indicates whether the value for the socket option can be read
by getsockopt(), and the third column indicates whether the value for
the socket option can be written by setsockopt(). The fourth column
provides a brief description of the socket option. The fifth column
shows the type of data structure specified along with the socket
option. By default, the data structure type is an integer.
+-----------------------------+-----+-----+-----------------+-------+
| optname | get | set | description | dtype |
+-----------------------------+-----+-----+-----------------+-------+
| SHIM_ASSOCIATED | o | | Get the | int |
| | | | parameter that | |
| | | | indicates | |
| | | | whether the | |
| | | | socket is | |
| | | | associated (1) | |
| | | | with any shim | |
| | | | context or not | |
| | | | (0). | |
| SHIM_DONTSHIM | o | o | Get or set the | int |
| | | | parameter that | |
| | | | indicates | |
| | | | whether or not | |
| | | | to employ | |
| | | | multihoming | |
| | | | support by the | |
| | | | shim sub-layer. | |
| SHIM_HOT_STANDBY | o | o | Get or set the | int |
| | | | parameter to | |
| | | | request the | |
| | | | shim sub-layer | |
| | | | to prepare a | |
| | | | hot-standby | |
| | | | connection. | |
| SHIM_LOC_LOCAL_PREF | o | o | Set the | Note |
| | | | preference | 1 |
| | | | value for a | |
| | | | source locator | |
| | | | for outbound | |
| | | | traffic. Get | |
| | | | the preferred | |
| | | | locator for the | |
| | | | source locator | |
| | | | for outbound | |
| | | | traffic. | |
Komu, et al. Informational PAGE 11
RFC 6316 Multihoming Shim API July 2011
| SHIM_LOC_PEER_PREF | o | o | Set the | Note |
| | | | preference | 1 |
| | | | value for a | |
| | | | destination | |
| | | | locator for | |
| | | | outbound | |
| | | | traffic. Get | |
| | | | the preferred | |
| | | | locator for the | |
| | | | destination | |
| | | | locator for | |
| | | | outbound | |
| | | | traffic. | |
| SHIM_LOC_LOCAL_RECV | o | o | Request the | int |
| | | | shim sub-layer | |
| | | | to store the | |
| | | | destination | |
| | | | locator of the | |
| | | | received IP | |
| | | | packet in an | |
| | | | ancillary data | |
| | | | object. | |
| SHIM_LOC_PEER_RECV | o | o | Request the | int |
| | | | shim sub-layer | |
| | | | to store the | |
| | | | source locator | |
| | | | of the received | |
| | | | IP packet in an | |
| | | | ancillary data | |
| | | | object. | |
| SHIM_LOC_LOCAL_SEND | o | o | Get or set the | Note |
| | | | source locator | 1 |
| | | | of outgoing IP | |
| | | | packets. | |
| SHIM_LOC_PEER_SEND | o | o | Get or set the | Note |
| | | | destination | 1 |
| | | | locator of | |
| | | | outgoing IP | |
| | | | packets. | |
| SHIM_LOCLIST_LOCAL | o | o | Get or set the | Note |
| | | | list of | 2 |
| | | | locators | |
| | | | associated with | |
| | | | the local EID. | |
Komu, et al. Informational PAGE 12
RFC 6316 Multihoming Shim API July 2011
| SHIM_LOCLIST_PEER | o | o | Get or set the | Note |
| | | | list of | 2 |
| | | | locators | |
| | | | associated with | |
| | | | the peer's EID. | |
| SHIM_APP_TIMEOUT | o | o | Get or set the | int |
| | | | Send Timeout | |
| | | | value of REAP. | |
| SHIM_PATHEXPLORE | o | o | Get or set | Note |
| | | | parameters for | 3 |
| | | | path | |
| | | | exploration and | |
| | | | failure | |
| | | | detection. | |
| SHIM_CONTEXT_DEFERRED_SETUP | o | | Get the | int |
| | | | parameter that | |
| | | | indicates | |
| | | | whether | |
| | | | deferred | |
| | | | context setup | |
| | | | is supported or | |
| | | | not. | |
+-----------------------------+-----+-----+-----------------+-------+
Table 1: Socket Options for Multihoming Shim Sub-Layer
Note 1: Pointer to a shim_locator as defined in Section 8.
Note 2: Pointer to an array of shim_locator data.
Note 3: Pointer to a shim_pathexplore as defined in Section 8.
Figure 2 illustrates how the shim-specific socket options fit into
the system model of sockets API. The figure shows that the shim sub-
layer and the additional protocol components (IPv4 and IPv6) below
the shim sub-layer are new to the system model. As previously
mentioned, all the shim-specific socket options are defined at the
SOL_SHIM level. This design choice brings the following advantages:
1. The existing sockets APIs continue to work at the layer above the
shim sub-layer. That is, those legacy APIs handle IP addresses
as identifiers.
2. With newly defined socket options for the shim sub-layer, the
application obtains additional control of locator management.
Komu, et al. Informational PAGE 13
RFC 6316 Multihoming Shim API July 2011
3. The shim-specific socket options can be kept independent from
address family (IPPROTO_IP or IPPROTO_IPV6) and transport
protocol (IPPROTO_TCP or IPPROTO_UDP) settings.
s1 s2 s3 s4
| | | |
+----------------|--|-------|--|----------------+
| +-------+ +-------+ |
| IPPROTO_TCP | TCP | | UDP | |
| +-------+ +-------+ |
| | \ / | |
| | ----- | |
| | / \ | |
| +------+ +------+ |
| IPPROTO_IP | IPv4 | | IPv6 | IPPROTO_IPV6 |
| +------+ +------+ |
| \ / SOL_SOCKET
| +--------\-------/--------+ |
| SOL_SHIM | shim | |
| +--------/-------\--------+ |
| / \ |
| +------+ +------+ |
| | IPv4 | | IPv6 | |
| +------+ +------+ |
| | | |
+------------------|----------|-----------------+
| |
IPv4 IPv6
Datagram Datagram
Figure 2: System Model of Sockets API with Shim Sub-Layer
6.1. SHIM_ASSOCIATED
The SHIM_ASSOCIATED option is used to check whether or not the socket
is associated with any shim context.
This option is meaningful when the locator information of the
received IP packet does not tell whether or not the identifier/
locator adaptation is performed. Note that the EID pair and the
locator pair may be identical in some cases.
Note that the socket option is read-only, and the option value can be
read by getsockopt(). The result (0/1/2) is set in the option value
(the fourth argument of getsockopt()).
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
Komu, et al. Informational PAGE 14
RFC 6316 Multihoming Shim API July 2011
The data type of the option value is an integer. The option value
indicates the presence of shim context. A return value of 1 means
that the socket is associated with a shim context at the shim
sub-layer. A return value of 0 indicates that there is no shim
context associated with the socket. A return value of 2 means that
it is not known whether or not the socket is associated with a shim
context, and this MUST be returned only when the socket is
unconnected. In other words, the returned value MUST be 0 or 1 when
the socket is connected.
For example, the option can be used by the application as follows:
int optval;
int optlen = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_ASSOCIATED, &optval, &optlen);
6.2. SHIM_DONTSHIM
The SHIM_DONTSHIM option is used to request that the shim layer not
provide the multihoming support for the communication established
over the socket.
The data type of the option value is an integer, and it takes 0 or 1.
An option value of 0 means that the shim sub-layer is employed if
available. An option value of 1 means that the application does not
want the shim sub-layer to provide the multihoming support for the
communication established over the socket.
The default value is set to 0, which means that the shim sub-layer
performs identifier/locator adaptation if available.
Any attempt to disable the multihoming shim support MUST be made by
the application before the socket is connected. If an application
makes such an attempt for a connected socket, error code EOPNOTSUPP
MUST be returned.
For example, an application can request that the system not apply the
multihoming support as follows:
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval));
Komu, et al. Informational PAGE 15
RFC 6316 Multihoming Shim API July 2011
For example, the application can check the option value as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len);
6.3. SHIM_HOT_STANDBY
The SHIM_HOT_STANDBY option is used to control whether or not the
shim sub-layer employs a hot-standby connection for the socket. A
hot-standby connection is an alternative working locator pair to the
current locator pair. This option is effective only when there is a
shim context associated with the socket.
The data type of the option value is an integer.
The option value can be set by setsockopt().
The option value can be read by getsockopt().
By default, the value is set to 0, meaning that hot-standby
connection is disabled.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
For example, an application can request establishment of a hot-
standby connection by using the socket option as follows:
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval,
sizeof(optval));
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For example, an application can get the option value by using the
socket option as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len);
6.4. SHIM_LOC_LOCAL_PREF
The SHIM_LOC_LOCAL_PREF option is used to set the preference value
for a source locator for outbound traffic, or to get the preference
value of the source locator for outbound traffic that has the highest
preference value.
This option is effective only when there is a shim context associated
with the socket.
By default, the option value is set to NULL, meaning that the option
is disabled.
The preference of a locator is defined by a combination of priority
and weight as per DNS SRV [RFC 2782]. Note that the Shim6 base
protocol defines the preference of a locator in the same way.
The data type of the option value is a pointer to the shim_locator
information data structure as defined in Section 8.1.
When an application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when the validation of the
specified locator fails.
An application can set the preference value for a source locator for
outbound traffic by setsockopt() with the socket option. Note that
lc_ifidx and lc_flags (as defined in Section 8.1) have no effect in a
set operation. Below is an example of such a set operation.
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struct shim_locator lc;
struct in6_addr ip6;
/* ...set the locator (ip6)... */
memset(&lc, 0, sizeof(shim_locator));
lc.lc_family = AF_INET6; /* IPv6 */
lc.lc_ifidx = 0;
lc.lc_flags = 0;
lc.lc_prio = 1;
lc.lc_weight = 10;
memcpy(&lc.lc_addr, &ip6, sizeof(in6_addr));
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc,
sizeof(optval));
An application can get the source locator for outbound traffic that
has the highest preference value by using the socket option. Below
is an example of such a get operation.
struct shim_locator lc;
int len;
len = sizeof(lc);
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len);
6.5. SHIM_LOC_PEER_PREF
The SHIM_LOC_PEER_PREF option is used to set the preference value for
a destination locator for outbound traffic, or to get the preference
value of the destination locator for outbound traffic that has the
highest preference value.
This option is effective only when there is a shim context associated
with the socket.
By default, the option value is set to NULL, meaning that the option
is disabled.
As defined earlier, the preference of a locator is defined by a
combination of priority and weight as per DNS SRV [RFC 2782]. When
there is more than one candidate destination locator, the shim
sub-layer makes a selection based on the priority and weight
specified for each locator.
The data type of the option value is a pointer to the shim_locator
information data structure as defined in Section 8.1.
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When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when the validation of the
requested locator fails.
Error code EUNREACHABLELOCATOR is returned when the requested locator
is determined to be unreachable according to a reachability check.
The usage of the option is the same as that of SHIM_LOC_LOCAL_PREF.
6.6. SHIM_LOC_LOCAL_RECV
The SHIM_LOC_LOCAL_RECV option can be used to request that the shim
sub-layer store the destination locator of the received IP packet in
an ancillary data object that can be accessed by recvmsg(). This
option is effective only when there is a shim context associated with
the socket.
The data type of the option value is an integer. The option value
MUST be binary (0 or 1). By default, the option value is set to 0,
meaning that the option is disabled.
An application can set the option value by setsockopt().
An application can get the option value by getsockopt().
See Section 7 for the procedure to access locator information stored
in the ancillary data objects.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
For example, an application can request the shim sub-layer to store a
destination locator by using the socket option as follows:
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval,
sizeof(optval));
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For example, an application can get the option value as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, &len);
6.7. SHIM_LOC_PEER_RECV
The SHIM_LOC_PEER_RECV option is used to request that the shim
sub-layer store the source locator of the received IP packet in an
ancillary data object that can be accessed by recvmsg(). This option
is effective only when there is a shim context associated with the
socket.
The data type of the option value is an integer. The option value
MUST be binary (0 or 1). By default, the option value is set to 0,
meaning that the option is disabled.
The option value can be set by setsockopt().
The option value can be read by getsockopt().
See Section 7 for the procedure to access locator information stored
in the ancillary data objects.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
The usage of the option is the same as that of the
SHIM_LOC_LOCAL_RECV option.
6.8. SHIM_LOC_LOCAL_SEND
The SHIM_LOC_LOCAL_SEND option is used to request that the shim
sub-layer use a specific locator as the source locator for the IP
packets to be sent from the socket. This option is effective only
when there is a shim context associated with the socket.
The data type of the option value is a pointer to the shim_locator
data structure.
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An application can set the local locator by setsockopt(), providing a
locator that is stored in a shim_locator data structure. When a
zero-filled locator is specified, the pre-existing setting of the
local locator is inactivated.
An application can get the local locator by getsockopt().
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when an invalid locator is
specified.
For example, an application can request the shim sub-layer to use a
specific local locator by using the socket option as follows:
struct shim_locator locator;
struct in6_addr ia6;
/* an IPv6 address preferred for the source locator is copied
to the parameter ia6 */
memset(&locator, 0, sizeof(locator));
/* fill shim_locator data structure */
locator.lc_family = AF_INET6;
locator.lc_ifidx = 0;
locator.lc_flags = 0;
locator.lc_prio = 0;
locator.lc_weight = 0;
memcpy(&locator.lc_addr, &ia6, sizeof(ia6));
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
sizeof(locator));
For example, an application can get the designated local locator by
using the socket option as follows:
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struct shim_locator locator;
memset(&locator, 0, sizeof(locator));
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
sizeof(locator));
/* check locator */
6.9. SHIM_LOC_PEER_SEND
The SHIM_LOC_PEER_SEND option is used to request that the shim
sub-layer use a specific locator for the destination locator of IP
packets to be sent from the socket. This option is effective only
when there is a shim context associated with the socket.
The data type of the option value is a pointer to the shim_locator
data structure.
An application can set the remote locator by setsockopt(), providing
a locator that is stored in a shim_locator data structure. When a
zero-filled locator is specified, the pre-existing setting of the
remote locator is inactivated.
An application can get the specified remote locator by getsockopt().
The difference between the SHIM_LOC_PEER_SEND option and the
SHIM_LOC_PEER_PREF option is that the former guarantees the use of a
requested locator when applicable, whereas the latter does not.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when the validation of the
requested locator fails.
Error code EUNVERIFIEDLOCATOR is returned when reachability for the
requested locator has not been verified yet.
Error code EUNREACHABLELOCATOR is returned when the requested locator
is determined to be unreachable according to a reachability check.
The usage of the option is the same as that of the
SHIM_LOC_LOCAL_SEND option.
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6.10. SHIM_LOCLIST_LOCAL
The SHIM_LOCLIST_LOCAL option is used to get or set the locator list
associated with the local EID of the shim context associated with the
socket. This option is effective only when there is a shim context
associated with the socket.
The data type of the option value is a pointer to the buffer in which
a locator list is stored. See Section 8 for the data structure for
storing the locator information. By default, the option value is set
to NULL, meaning that the option is disabled.
An application can get the locator list by getsockopt(). Note that
the size of the buffer pointed to by the optval argument SHOULD be
large enough to store an array of locator information. The number of
the locator information is not known beforehand.
The local locator list can be set by setsockopt(). The buffer
pointed to by the optval argument MUST contain an array of locator
structures.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when the validation of any of
the specified locators failed.
Error code ETOOMANYLOCATORS is returned when the number of locators
specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
the buffer provided by the application is not large enough to store
the locator list provided by the shim sub-layer.
For example, an application can set a list of locators to be
associated with the local EID by using the socket option as follows.
Note that an IPv4 locator can be handled by HIP and not by Shim6.
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struct shim_locator locators[SHIM_MAX_LOCATORS];
struct sockaddr_in *sin;
struct sockaddr_in6 *sin6;
memset(locators, 0, sizeof(locators));
...
/* obtain local IP addresses from local interfaces */
...
/* first locator (an IPv6 address) */
locators[0].lc_family = AF_INET6;
locators[0].lc_ifidx = 0;
locators[0].lc_flags = 0;
locators[0].lc_prio = 1;
locators[0].lc_weight = 0;
memcpy(&locators[0].lc_addr, &sa6->sin6_addr,
sizeof(sa6->sin6_addr));
...
/* second locator (an IPv4 address) */
locators[1].lc_family = AF_INET;
locators[1].lc_ifidx = 0;
locators[1].lc_flags = 0;
locators[1].lc_prio = 0;
locators[1].lc_weight = 0;
memcpy(&locators[1].lc_addr, &sa->sin_addr,
sizeof(sa->sin_addr));
setsockopt(fd, SOL_SHIM, SHIM_LOCLIST_LOCAL, locators,
sizeof(locators));
For example, an application can get a list of locators that are
associated with the local EID by using the socket option as follows:
struct shim_locator locators[SHIM_MAX_LOCATORS];
memset(locators, 0, sizeof(locators));
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, locators,
sizeof(locators));
/* parse locators */
...
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6.11. SHIM_LOCLIST_PEER
The SHIM_LOCLIST_PEER option is used to get or set the locator list
associated with the peer EID of the shim context associated with the
socket. This option is effective only when there is a shim context
associated with the socket.
The data type of the option value is a pointer to the buffer where a
locator list is stored. See Section 8 for the data structure for
storing the locator information. By default, the option value is set
to NULL, meaning that the option is disabled.
An application can get the locator list by getsockopt(). Note that
the size of the buffer pointed to by the optval argument SHOULD be
large enough to store an array of locator information. The number of
the locator information is not known beforehand.
An application can set the locator list by setsockopt(). The buffer
pointed to by the optval argument MUST contain an array of locator
list items.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when the validation of any of
the specified locators failed.
Error code EUNVERIFIEDLOCATOR is returned when reachability for the
requested locator has not been verified yet.
Error code EUNREACHABLELOCATOR is returned when the requested locator
is determined to be unreachable according to a reachability check.
Error code ETOOMANYLOCATORS is returned when the number of locators
specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
the buffer provided by the application is not large enough to store
the locator list provided by the shim sub-layer.
The usage of the option is the same as that of SHIM_LOCLIST_LOCAL.
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6.12. SHIM_APP_TIMEOUT
The SHIM_APP_TIMEOUT option is used to get or set the Send Timeout
value of REAP [RFC 5534]. This option is effective only when there is
a shim context associated with the socket.
The data type of the option value is an integer. The value indicates
the period of timeout in seconds to send a REAP Keepalive message
since the last outbound traffic. By default, the option value is set
to 0, meaning that the option is disabled. When the option is
disabled, the REAP mechanism follows its default Send Timeout value
as specified in [RFC 5534].
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
When there is no REAP instance on the system, error code EOPNOTSUPP
is returned to the application.
For example, an application can set the timeout value by using the
socket option as follows:
int optval;
optval = 15; /* 15 seconds */
setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval,
sizeof(optval));
For example, an application can get the timeout value by using the
socket option as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len);
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6.13. SHIM_PATHEXPLORE
The application MAY use this socket option to get or set parameters
concerning path exploration. Path exploration is a procedure to find
an alternative locator pair to the current locator pair. As the REAP
specification defines, a peer may send Probe messages to find an
alternative locator pair.
This option is effective only when there is a shim context associated
with the socket.
The data type of the option value is a pointer to the buffer where a
set of information for path exploration is stored. The data
structure is defined in Section 8.
By default, the option value is set to NULL, meaning that the option
is disabled.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
For example, an application can set parameters for path exploration
by using the socket option as follows:
struct shim6_pathexplore pe;
pe.pe_probenum = 4; /* times */
pe.pe_keepaliveto = 10; /* seconds */
pe.pe_initprobeto = 500; /* milliseconds */
pe.pe_reserved = 0;
setsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, sizeof(pe));
For example, an application can get parameters for path exploration
by using the socket option as follows:
struct shim6_pathexplore pe;
int len;
len = sizeof(pe);
getsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, &len);
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6.14. SHIM_DEFERRED_CONTEXT_SETUP
The SHIM_DEFERRED_CONTEXT_SETUP option is used to check whether or
not deferred context setup is possible. Deferred context setup means
that the context is established in parallel with the data
communication. Note that Shim6 supports deferred context setup and
HIP does not, because EIDs in HIP (i.e., Host Identifiers) are non-
routable.
Note that the socket option is read-only, and the option value can be
read by getsockopt().
The data type for the option value is an integer. The option value
MUST be binary (0 or 1). The option value of 1 means that the shim
sub-layer supports deferred context setup.
When the application specifies the socket option to an unconnected
socket, error code EOPNOTSUPP is returned to the application.
For example, an application can check whether deferred context setup
is possible or not as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
&optval, &len);
6.15. Applicability
All the socket options defined in this section except for the
SHIM_DONTSHIM option are applicable to applications that use
connected sockets.
All the socket options defined in this section except for the
SHIM_ASSOCIATED, SHIM_DONTSHIM, and SHIM_CONTEXT_DEFERRED_SETUP
options are effective only when there is a shim context associated
with the socket.
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6.16. Error Handling
If successful, getsockopt() and setsockopt() return 0; otherwise, the
functions return -1 and set errno to indicate an error.
The following are new error values defined for some shim-specific
socket options indicating that the getsockopt() or setsockopt()
finished incompletely:
EINVALIDLOCATOR
This indicates that the locator is not part of the HBA set
[RFC 5535] within the shim context associated with the socket.
EUNVERIFIEDLOCATOR
This indicates that the reachability of the locator has not been
confirmed. This error is applicable to only the peer's locator.
EUNREACHABLELOCATOR
This indicates that the locator is not reachable according to the
result of the reachability check. This error is applicable to
only the peer's locator.
7. Ancillary Data for Multihoming Shim Sub-Layer
This section provides definitions of ancillary data to be used for
locator management and notification from/to the shim sub-layer to/
from the application.
When the application performs locator management by sendmsg() or
recvmsg(), a member of the msghdr structure (given in Figure 3)
called msg_control holds a pointer to the buffer in which one or more
shim-specific ancillary data objects may be stored. An ancillary
data object can store a single locator. It should be possible to
process the shim-specific ancillary data object by the existing
macros defined in the POSIX standard and [RFC 3542].
struct msghdr {
caddr_t msg_name; /* optional address */
u_int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
u_int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_control; /* ancillary data, see below */
u_int msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Figure 3: msghdr Structure
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In the case of an unconnected socket, msg_name stores the socket
address of the peer. Note that the address is not a locator of the
peer but the identifier of the peer. SHIM_LOC_PEER_RECV can be used
to get the locator of the peer node.
Table 2 is a list of the shim-specific ancillary data that can be
used for locator management by recvmsg() or sendmsg(). In any case,
the value of cmsg_level MUST be set to SOL_SHIM.
+---------------------+-----------+-----------+-----------------+
| cmsg_type | sendmsg() | recvmsg() | cmsg_data[] |
+---------------------+-----------+-----------+-----------------+
| SHIM_LOC_LOCAL_RECV | | o | Note 1 |
| SHIM_LOC_PEER_RECV | | o | Note 1 |
| SHIM_LOC_LOCAL_SEND | o | | Note 1 |
| SHIM_LOC_PEER_SEND | o | | Note 1 |
| SHIM_FEEDBACK | o | | shim_feedback{} |
+---------------------+-----------+-----------+-----------------+
Table 2: Shim-Specific Ancillary Data
Note 1: cmsg_data[] within msg_control includes a single
sockaddr_in{} or sockaddr_in6{} and padding if necessary
7.1. Get Locator from Incoming Packet
An application can get locator information from the received IP
packet by specifying the shim-specific socket options for the socket.
When SHIM_LOC_LOCAL_RECV and/or SHIM_LOC_PEER_RECV socket options are
set, the application can retrieve a local and/or remote locator from
the ancillary data.
When there is no shim context associated with the socket, the shim
sub-layer MUST return zero-filled locator information to the
application.
7.2. Set Locator for Outgoing Packet
An application can specify the locators to be used for transmitting
an IP packet by sendmsg(). When the ancillary data of cmsg_type
SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the
application can explicitly specify the source and/or the destination
locators to be used for the communication over the socket. If the
specified locator pair is verified, the shim sub-layer overrides the
locator(s) of the outgoing IP packet. Note that the effect is
limited to the datagram transmitted by the sendmsg().
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When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
Error code EINVALIDLOCATOR is returned when validation of the
specified locator fails.
Error code EUNVERIFIEDLOCATOR is returned when reachability for the
requested locator has not been verified yet. The application is
recommended to use another destination locator until the reachability
check for the requested locator is done.
Error code EUNREACHABLELOCATOR is returned when the requested locator
is determined to be unreachable according to a reachability check.
The application is recommended to use another destination locator
when receiving the error.
7.3. Notification from Application to Multihoming Shim Sub-Layer
An application MAY provide feedback to the shim sub-layer about the
communication status. Such feedback is useful for the shim sub-layer
to monitor the reachability status of the currently used locator pair
in a given shim context.
The notification can be made by sendmsg() specifying a new ancillary
data called SHIM_FEEDBACK. The ancillary data can be handled by
specifying the SHIM_FEEDBACK option in cmsg_type.
When there is no shim context associated with the socket, error code
ENOENT is returned to the application.
See Section 8.3 for details of the data structure to be used.
It is outside the scope of this document to describe how the shim
sub-layer would react when feedback is provided by an application.
7.4. Applicability
All the ancillary data for the shim sub-layer is applicable to
connected sockets.
Care is needed when the SHIM_LOC_*_RECV socket option is used for
stream-oriented sockets (e.g., TCP sockets) because there is no one-
to-one mapping between a single send or receive operation and the
data (e.g., a TCP segment) being received. In other words, there is
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no guarantee that the locator(s) set in the SHIM_LOC_*_RECV ancillary
data is identical to the locator(s) that appears in the IP packets
received. The shim sub-layer SHOULD provide the latest locator
information to the application in response to the SHIM_LOC_*_RECV
socket option.
8. Data Structures
This section gives data structures for the shim sub-layer. These
data structures are either used as a parameter for setsockopt() or
getsockopt() (as mentioned in Section 6), or as a parameter for
ancillary data to be processed by sendmsg() or recvmsg() (as
mentioned in Section 7).
8.1. Data Structure for Locator Information
As defined in Section 6, the SHIM_LOC_*_PREF, SHIM_LOC_*_SEND, and
SHIM_LOCLIST_* socket options need to handle one or more locator
information points. Locator information includes not only the
locator itself but also additional information about the locator that
is useful for locator management. A new data structure is defined to
serve as a placeholder for the locator information.
Figure 4 illustrates the data structure called shim_locator, which
stores locator information.
struct shim_locator {
uint8_t lc_family; /* address family */
uint8_t lc_proto; /* protocol */
uint16_t lc_port; /* port number */
uint16_t lc_prio; /* preference value */
uint16_t lc_weight; /* weight */
uint32_t lc_ifidx; /* interface index */
struct in6_addr lc_addr; /* address */
uint16_t lc_flags; /* flags */
};
Figure 4: Shim Locator Structure
lc_family
Address family of the locator (e.g., AF_INET, AF_INET6). It is
required that the parameter contains a non-zero value indicating
the exact address family of the locator.
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lc_proto
Internet Protocol number for the protocol that is used to handle a
locator behind a NAT. The value MUST be set to zero when there is
no NAT involved. When the locator is behind a NAT, the value MUST
be set to IPPROTO_UDP.
lc_port
Port number that is used for handling a locator behind a NAT.
lc_prio
Priority of the locator. The range is 0-65535. The lowest
priority value means the highest priority.
lc_weight
Weight value indicates a relative weight for locators with the
same priority value. The range is 0-65535. A locator with higher
weight value is prioritized over the other locators with lower
weight values.
lc_ifidx
Interface index of the network interface to which the locator is
assigned. This field is applicable only to local locators, and
has no effect in set operations.
lc_addr
Contains the locator. In the case of IPv4, the locator MUST be
formatted in the IPv4-mapped IPv6 address as defined in [RFC 4291].
The locator MUST be stored in network byte order.
lc_flags
Each bit of the flags represents a specific characteristic of the
locator. The Hash-Based Address (HBA) is defined as 0x01. The
Cryptographically Generated Address (CGA) is defined as 0x02.
This field has no effect in set operations.
8.1.1. Handling Locator behind NAT
Note that the locator information MAY contain a locator behind a
Network Address Translator (NAT). Such a situation may arise when
the host is behind the NAT and uses a local address as a source
locator to communicate with the peer. Note that a NAT traversal
mechanism for HIP is defined, which allows a HIP host to tunnel
control and data traffic over UDP [RFC 5770]. Note also that the
locator behind a NAT is not necessarily an IPv4 address and can be an
IPv6 address. Below is an example where the application sets a UDP
encapsulation interface as a source locator when sending IP packets.
Komu, et al. Informational PAGE 33
RFC 6316 Multihoming Shim API July 2011
struct shim_locator locator;
struct in6_addr ia6;
/* copy the private IPv4 address to the ia6 as an IPv4-mapped
IPv6 address */
memset(&locator, 0, sizeof(locator));
/* fill shim_locator data structure */
locator.lc_family = AF_INET;
locator.lc_proto = IPPROTO_UDP;
locator.lc_port = 50500;
locator.lc_ifidx = 0;
locator.lc_flags = 0;
locator.lc_prio = 0;
locator.lc_weight = 0;
memcpy(&locator.lc_addr, &ia6, sizeof(ia6));
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
sizeof(locator));
Figure 5: Handling Locator behind NAT
8.2. Path Exploration Parameter
As defined in Section 6, SHIM_PATHEXPLORE allows an application to
set or read the parameters for path exploration and failure
detection. A new data structure called shim_pathexplore is defined
to store the necessary parameters. Figure 6 illustrates the data
structure. The data structure can be passed to getsockopt() or
setsockopt() as an argument.
struct shim_pathexplore {
uint16_t pe_probenum; /* # of initial probes */
uint16_t pe_keepaliveto; /* Keepalive Timeout */
uint16_t pe_keepaliveint; /* Keepalive Interval */
uint16_t pe_initprobeto; /* Initial Probe Timeout */
uint32_t pe_reserved; /* reserved */
};
Figure 6: Path Explore Structure
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pe_probenum
Indicates the number of initial Probe messages to be sent. The
value MUST be set as per [RFC 5534].
pe_keepaliveto
Indicates the timeout value in seconds for detecting a failure
when the host does not receive any packets for a certain period of
time while there is outbound traffic. When the timer expires, the
path exploration procedure will be carried out by sending a REAP
Probe message. The value MUST be set as per [RFC 5534].
pe_keepaliveint
Indicates the interval of REAP Keepalive messages in seconds to be
sent by the host when there is no outbound traffic to the peer
host. The value MUST be set as per [RFC 5534].
pe_initprobeto
Indicates the retransmission timer of the REAP Probe message in
milliseconds. Note that this timer is applied before exponential
back-off is started. A REAP Probe message for the same locator
pair may be retransmitted. The value MUST be set as per
[RFC 5534].
pe_reserved
A reserved field for future extension. By default, the field MUST
be initialized to zero.
8.3. Feedback Information
As mentioned in Section 7.3, applications can inform the shim
sub-layer about the status of unicast reachability of the locator
pair currently in use. The feedback information can be handled by
using ancillary data called SHIM_FEEDBACK. A new data structure
named shim_feedback is illustrated in Figure 7.
struct shim_feedback {
uint8_t fb_direction; /* direction of traffic */
uint8_t fb_indicator; /* indicator (1-3) */
uint16_t fb_reserved; /* reserved */
};
Figure 7: Feedback Information Structure
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fb_direction
Indicates the direction of reachability between the locator pair
in question. A value of 0 indicates outbound direction, and a
value of 1 indicates inbound direction.
fb_indicator
A value indicating the degree of satisfaction of a unidirectional
reachability for a given locator pair.
* 0: Default value. Whenever this value is specified, the
feedback information MUST NOT be processed by the shim
sub-layer.
* 1: Unable to connect. There is no unidirectional reachability
between the locator pair in question.
* 2: Unsatisfactory. The application is not satisfied with the
unidirectional reachability between the locator pair in
question.
* 3: Satisfactory. There is satisfactory unidirectional
reachability between the locator pair in question.
fb_reserved
Reserved field. MUST be ignored by the receiver.
9. System Requirements
As addressed in Section 6, most of the socket options and ancillary
data defined in this document are applicable to connected sockets.
It is assumed that the kernel is capable of maintaining the
association between a connected socket and a shim context. This
requirement is considered to be reasonable because a pair of source
and destination IP addresses is bound to a connected socket.
10. Relation to Existing Sockets API Extensions
This section explains the relation between the sockets API defined in
this document and the existing sockets API extensions.
As mentioned in Section 6, the basic assumption is that the existing
sockets API continues to work above the shim sub-layer. This means
that the existing sockets API deals with identifiers, and the sockets
API defined in this document deals with locators.
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RFC 6316 Multihoming Shim API July 2011
SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options are
semantically similar to the IPV6_PKTINFO sockets API in the sense
that both provide a means for an application to set the source IP
address of outbound IP packets.
SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV socket options are
semantically similar to the IP_RECVDSTADDR and IPV6_PKTINFO sockets
APIs in the sense that both provide a means for an application to get
the source and/or destination IP address of inbound IP packets.
getsockname() and getpeername() enable an application to get the
"name" of the communication endpoints, which is represented by a pair
of IP addresses and port numbers assigned to the socket.
getsockname() gives the IP address and port number assigned to the
socket on the local side, and getpeername() gives the IP address and
port number of the peer side.
11. Operational Considerations
This section gives operational considerations of the sockets API
defined in this document.
11.1. Conflict Resolution
There can be a conflicting situation when different applications
specify different preferences for the same shim context. For
instance, suppose that applications A and B establish communication
with the same EID pair while both applications have different
preferences in their choice of local locator. The notion of context
forking in Shim6 can resolve the conflicting situation.
It is possible that socket options defined in Section 6 cause a
conflicting situation when the target context is shared by multiple
applications. In such a case, the socket handler should inform the
shim sub-layer that context forking is required. In Shim6, when a
context is forked, a unique identifier called the Forked Instance
Identifier (FII) is assigned to the newly forked context. The forked
context is then exclusively associated with the socket through which
a non-default preference value was specified. The forked context is
maintained by the shim sub-layer during the lifetime of the
associated socket instance. When the socket is closed, the shim
sub-layer SHOULD delete the associated context.
When the application specifies SHIM_LOC_*_SEND specifying a different
source or destination locator that does not have the highest priority
and weight specified by the SHIM_LOC_*_PREF, the shim sub-layer
SHOULD supersede the request made by SHIM_LOC_*_SEND over the
preference specified by SHIM_LOC_*_PREF.
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RFC 6316 Multihoming Shim API July 2011
When the peer provides preferences of the locators (e.g., a Shim6
peer sends a locator with a Locator Preferences Option) that conflict
with preferences specified by the applications either by
SHIM_LOC_PEER_SEND or SHIM_LOC_PEER_PREF, the shim sub-layer SHOULD
supersede the preferences made by the applications over the
preferences specified by the peer.
11.2. Incompatibility between IPv4 and IPv6
The shim sub-layer performs identifier/locator adaptation.
Therefore, in some cases, the whole IP header can be replaced with a
new IP header of a different address family (e.g., conversion from
IPv4 to IPv6 or vice versa). Hence, there is an issue regarding how
to make the conversion with minimum impact. Note that this issue is
common in other protocol conversion techniques [RFC 2765] [RFC 6145].
As studied in the previous works on protocol conversion [RFC 2765],
[RFC 6145] some of the features (IPv6 routing headers, hop-by-hop
extension headers, and destination headers) from IPv6 are not
convertible to IPv4. In addition, the notion of source routing is
not exactly the same in IPv4 and IPv6. This means that an error may
occur during the conversion of the identifier and locator. It is
outside the scope of this document to describe how the shim sub-layer
should behave in such erroneous cases.
12. IANA Considerations
There are no IANA considerations for the socket options (SHIM_*), the
ancillary data, and the socket level (SOL_SHIM) that are defined in
this document. All the numbers concerned are not under the control
of the IETF or IANA, but they are platform-specific.
13. Protocol Constant
This section defines a protocol constant.
SHIM_MAX_LOCATORS The maximum number of locators to be included in a
locator list. The value is set to 32.
14. Security Considerations
This section gives security considerations of the API defined in this
document.
Komu, et al. Informational PAGE 38
RFC 6316 Multihoming Shim API July 2011
14.1. Treatment of Unknown Locator
When sending IP packets, there is a possibility that an application
will request the use of an unknown locator for the source and/or
destination locators. Note that the treatment of an unknown locator
can be a subject of security considerations, because the use of an
invalid source and/or destination locator may cause a redirection
attack.
14.1.1. Treatment of Unknown Source Locator
The shim sub-layer checks to determine if the requested locator is
available on any local interface. If not, the shim sub-layer MUST
reject the request and return an error message with the
EINVALIDLOCATOR code to the application. If the locator is confirmed
to be available, the shim sub-layer SHOULD initiate the procedure to
update the locator list.
Use of the following socket options and ancillary data requires
treatment of an unknown source locator:
o SHIM_LOC_LOCAL_SEND
o SHIM_LOC_LOCAL_PREF
o SHIM_LOCLIST_LOCAL
14.1.2. Treatment of Unknown Destination Locator
If the shim sub-layer turns out to be Shim6, the Shim6 layer MUST
reject the request for using an unknown destination locator.
If the shim sub-layer turns out to be HIP, the HIP layer MUST reject
the request for using an unknown destination locator. There is,
however, an exceptional case where the HIP layer SHOULD accept the
request, provided that the HIP association is in the UNASSOCIATED
state. Details of locator handling in HIP are described in
Section 4.6 of [RFC 6317].
Use of the following socket options and ancillary data requires
treatment of an unknown destination locator:
o SHIM_LOC_PEER_SEND
o SHIM_LOC_PEER_PREF
o SHIM_LOCLIST_PEER
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RFC 6316 Multihoming Shim API July 2011
15. Acknowledgments
The authors would like to thank Jari Arkko, who participated in the
discussion that led to the first version of this document, and Tatuya
Jinmei, who thoroughly reviewed the early draft version of this
document and provided detailed comments on sockets API-related
issues. Thomas Henderson provided valuable comments, especially from
the HIP perspective.
The authors sincerely thank the following people for their helpful
comments regarding the document: Samu Varjonen, Dmitriy Kuptsov,
Brian Carpenter, Michael Scharf, Sebastien Barre, and Roni Even.
16. References
16.1. Normative References
[POSIX] "IEEE Std. 1003.1-2008 Standard for Information
Technology -- Portable Operating System Interface
(POSIX). Open group Technical Standard: Base
Specifications, Issue 7", September 2008,
<http://www.opengroup.org/austin>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC 4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, May 2006.
[RFC 5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC 5534] Arkko, J. and I. van Beijnum, "Failure Detection and
Locator Pair Exploration Protocol for IPv6 Multihoming",
RFC 5534, June 2009.
Komu, et al. Informational PAGE 40
RFC 6316 Multihoming Shim API July 2011
16.2. Informative References
[RFC 2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC 2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
June 2009.
[RFC 5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and
A. Keranen, Ed., "Basic Host Identity Protocol (HIP)
Extensions for Traversal of Network Address Translators",
RFC 5770, April 2010.
[RFC 6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
[RFC 6317] Komu, M. and T. Henderson, "Basic Socket Interface
Extensions for the Host Identity Protocol (HIP)",
RFC 6317, July 2011.
[SHIM6-APP-REFER]
Nordmark, E., "Shim6 Application Referral Issues", Work
in Progress, July 2005.
Komu, et al. Informational PAGE 41
RFC 6316 Multihoming Shim API July 2011
Appendix A. Context Forking
In this section, an issue concerning context forking and its relation
to the multihoming shim API are discussed.
Shim6 supports the notion of context forking. A peer may decide to
fork a context for a certain reason (e.g., an upper-layer protocol
prefers to use a different locator pair than the one defined in an
available context). The procedure of context forking is done
similarly to the normal context establishment, performing the 4-way
message exchange. A peer who has decided to fork a context initiates
the context establishment. Hereafter, we call this peer the
"initiator". The peer of the initiator is called the "responder".
Once the forked context is established between the peers, on the
initiator side, it is possible to apply forked context to the packet
flow, since the system maintains an association between the forked
context and the socket owned by the application that has requested
the context forking. How this association is maintained is an
implementation-specific issue. However, on the responder side, there
is a question of how the outbound packet can be multiplexed by the
shim sub-layer, because there is more than one Shim6 context that
matches with the ULID pair of the packet flow. There is a need to
differentiate packet flows not only by the ULID pairs but by some
other information and associate a given packet flow with a specific
context.
Figure 8 gives an example of a scenario where two communicating peers
fork a context. Initially, there has been a single transaction
between the peers, by the application 1 (App1). Accordingly, another
transaction is started, by application 2 (App2). Both of the
transactions are made based on the same ULID pair. The first context
pair (Ctx1) is established for the transaction of App1. Given the
requests from App2, the shim sub-layer on Peer 1 decides to fork a
context. Accordingly, a forked context (Ctx2) is established between
the peers, which should be exclusively applied to the transaction of
App2. Ideally, multiplexing and demultiplexing of packet flows that
relate to App1 and App2 should be done as illustrated in Figure 8.
However, as mentioned earlier, the responder needs to multiplex
outbound flows of App1 and App2 somehow. Note that if a context
forking occurs on the initiator side, a context forking needs to also
occur on the responder side.
Komu, et al. Informational PAGE 42
RFC 6316 Multihoming Shim API July 2011
Peer 1 Peer 2
(initiator) (responder)
+----+ +----+ +----+ +----+
|App1| |App2| |App1| |App2|
+----+ +----+ +----+ +----+
|^ |^ ^| ^|
v| v| |v |v
-----S1-------------S2----- -----S1-------------S2-----
|| || || ||
|| || || ||
Ctx1 Ctx2 Ctx1 Ctx2
ULID:<A1,B1> ULID:<A1,B1> ULID:<B1,A1> ULID:<B1,A1>
Loc: <A1,B2> Loc: <A1,B3> Loc: <B2,A1> Loc: <B3,A1>
FII: 0 FII: 100 FII: 0 FII: 100
|^ |^ ^| ^|
|| || || ||
|| || || ||
\..............||....................../| ||
\.............||......................./ ||
|| ||
\|...................................../|
\....................................../
Figure 8: Context Forking
How to solve the issue described above is a topic for further study.
Komu, et al. Informational PAGE 43
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Authors' Addresses
Miika Komu
Aalto University
Espoo
Finland
Phone: +358505734395
Fax: +358947025014
EMail: miika@iki.fi
URI: http://cse.aalto.fi/research/groups/datacommunications/people/
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes 28911
SPAIN
Phone: +34 91 6248837
EMail: marcelo@it.uc3m.es
URI: http://it.uc3m.es/marcelo
Kristian Slavov
Ericsson Research Nomadiclab
Hirsalantie 11
Jorvas FI-02420
Finland
Phone: +358 9 299 3286
EMail: kristian.slavov@ericsson.com
Shinta Sugimoto (editor)
Nippon Ericsson K.K.
Koraku Mori Building
1-4-14, Koraku, Bunkyo-ku
Tokyo 112-0004
Japan
Phone: +81 3 3830 2241
EMail: shinta.sugimoto@ericsson.com
Komu, et al. Informational PAGE 44
RFC TOTAL SIZE: 93076 bytes
PUBLICATION DATE: Friday, July 22nd, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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