SHIM6 Working Group M. Komu
Internet-Draft HIIT
Intended status: Informational M. Bagnulo
Expires: April 29, 2010 UC3M
K. Slavov
S. Sugimoto, Ed.
Ericsson
October 26, 2009
Socket Application Program Interface (API) for Multihoming Shim
draft-ietf-shim6-multihome-shim-api-10
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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 "shim") inside the IP
layer that maintains mappings between identifiers and locators.
Examples of the shim are SHIM6 and HIP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Socket Options for Multihoming Shim Sub-layer . . . . . . . . 9
5.1. SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 13
5.2. SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 14
5.3. SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . 15
5.4. SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . 15
5.5. SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 16
5.6. SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . 17
5.7. SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 18
5.8. SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . 19
5.9. SHIM_LOC_LOCAL_SEND . . . . . . . . . . . . . . . . . . . 19
5.10. SHIM_LOC_PEER_SEND . . . . . . . . . . . . . . . . . . . 21
5.11. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . 21
5.12. SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 23
5.13. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . 23
5.14. SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 24
5.15. Applicability . . . . . . . . . . . . . . . . . . . . . . 25
5.16. Error Handling . . . . . . . . . . . . . . . . . . . . . 25
6. Ancillary Data for Multihoming Shim Sub-layer . . . . . . . . 26
6.1. Get Locator from Incoming Packet . . . . . . . . . . . . 27
6.2. Set Locator for Outgoing Packet . . . . . . . . . . . . . 27
6.3. Notification from Application to Multihoming Shim
Sub-layer . . . . . . . . . . . . . . . . . . . . . . . . 27
6.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 28
7. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. Placeholder for Locator Information . . . . . . . . . . . 28
7.1.1. Handling Locator behind NAT . . . . . . . . . . . . . 29
7.2. Path Exploration Parameter . . . . . . . . . . . . . . . 30
7.3. Feedback Information . . . . . . . . . . . . . . . . . . 31
8. System Requirements . . . . . . . . . . . . . . . . . . . . . 32
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9. Implications for Existing Socket API Extensions . . . . . . . 32
10. Resolving Conflicts with Preference Values . . . . . . . . . . 33
10.1. Implicit Forking . . . . . . . . . . . . . . . . . . . . 33
11. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.1. Naming at Socket Layer . . . . . . . . . . . . . . . . . 34
11.2. Additional Requirements from Applications . . . . . . . . 34
11.3. Issues of Header Conversion among Different Address
Family . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.4. Handling of Unknown Locator Provided by Application . . . 35
12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.1. Changes from version 00 to version 01 . . . . . . . . . . 35
12.2. Changes from version 01 to version 02 . . . . . . . . . . 35
12.3. Changes from version 02 to version 03 . . . . . . . . . . 36
12.4. Changes from version 03 to version 04 . . . . . . . . . . 36
12.5. Changes from version 04 to version 05 . . . . . . . . . . 36
12.6. Changes from version 05 to version 06 . . . . . . . . . . 36
12.7. Changes from version 06 to version 07 . . . . . . . . . . 36
12.8. Changes from version 07 to version 08 . . . . . . . . . . 36
12.9. Changes from version 08 to version 09 . . . . . . . . . . 36
12.10. Changes from version 09 to version 10 . . . . . . . . . . 36
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
14. Security Considerations . . . . . . . . . . . . . . . . . . . 37
15. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 37
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
17.1. Normative References . . . . . . . . . . . . . . . . . . 38
17.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Context Forking . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
HIP and SHIM6 have a commonality in their protocol design in the
sense that the semantic roles of an IP address, i.e., an identifier
and a locator, are distinguished. Separation of identifier and
locator is done by introducing a "shim" inside the IP layer which
maintains mapping of the identifier and associated locators. This
design principle is called "identifier/locator separation" and the
shim is referred to as a "shim sub-layer" in this document.
The shim sub-layer provides a nice property to present a stable
communication endpoints (i.e., identifiers) to the upper layer
protocols. An on-going session can be maintained even when the
locator associated with the identifier is changed, for instance, upon
a re-homing event under a multihomed environment. Therefore, upper
layer protocols, especially connection-oriented applications are no
more annoyed by the locator change thanks to the identifier/locator
separation mechanism.
While the identifier/locator separation removes negative impact of
locator change, it does not necessarily mean that applications are
always ignorant about locators. We rather think that applications
may want to have a control of locators in some cases. For instance,
an application may want to use a specific locator to send IP packets.
Such a control of locators is referred to as "locator management" in
this document. Besides, applications may want to turn on or off the
identifier/locator separation mechanism. This document defines API
that provides locator management and additional control of shim sub-
layer for applications.
This document recommends that the switching of identifier and locator
is done only once inside the TCP/IP stack of an endhost. That is, if
multiple shim sub-layers exist at the IP layer, any one of them
should be applied exclusively for a given flow.
As this document specifies sockets API extensions, it is written so
that the syntax and semantics are in line with the Posix standard
[POSIX] as much as possible. The API specified in this document
defines how to use ancillary data (aka cmsg) to access the locator
information with recvmsg() and/or sendmsg() I/O calls. The
definition of API is presented in C language and data types follow
the Posix format; intN_t means a singed 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 target readers of this document are application programmers who
develop application software which may benefit greatly from
multihomed environments. In addition, this document aims to provide
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necessary information for developers of multihoming shim protocols to
implement API for enabling advanced locator management.
2. Terminology
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[RFC5533]
o HIP Architecture[RFC4423]
o Reachability Protocol (REAP)[RFC5534]
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[I-D.ietf-shim6-app-refer].
* In the case of SHIM6, an identifier called a ULID 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 a
form of hash of public key.
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.
* 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 [RFC5533], 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. As defined in
[RFC5533], the preferred locator of a host 'A' is denoted as
Lp(A).
o Shim - The conceptual sub-layer inside the IP layer which
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.
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o Identifier/locator adaptation - The adaptation performed at the
shim sub-layer which 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,
which stores a binding between the EID and associated locators.
Contexts are maintained by the shim sub-layer.
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 to flow 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 shortage of network resources, congestion, and human error
(faulty operation).
o Working address pair - The address pair is 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 [RFC5534].
3. 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 may also be possible that the shim sub-layer interacts with the
transport layer, however, such an interaction is outside the scope of
this document.
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+------------------------+
| Application |
+------------------------+
^ ^
~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~
| v
+-----------|------------------------------+
| | Transport Layer |
+-----------|------------------------------+
^ |
+-------------|-----|-------------------------------------+
| v v |
| +-----------------------------+ +----------+ | IP
| | Shim |<----->| REAP | | Layer
| +-----------------------------+ +----------+ |
| ^ ^ |
+-----------------------|----------------------|----------+
v v
+------------------------------------------+
| Link Layer |
+------------------------------------------+
Figure 1: System overview
4. Requirements
The following is a list of requirements from applications:
o Locator management.
* It should be possible to set preferred source and/or
destination locator within a given context: Lp(local) and/or
Lp(remote).
* It should be possible to get preferred source and/or
destination locator within a given context: Lp(local) and/or
Lp(remote).
* 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 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 feedbacks or negative feedbacks to the shim sub-
layer. Note that these feedbacks are mentioned in [RFC5534]]:
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* Applications and/or upper layer protocols (e.g., TCP) may
provide positive feedbacks to the shim sub-layer informing that
the communication is going well.
* Applications and/or upper layer protocols (e.g., TCP) may
provide negative feedbacks 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. Besides, a receipt of an ICMP error message could be
a clue for the application to detect problems. The REAP module
may be triggered by these negative feedbacks 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 the
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 host to immediately replace the
current locator pair with an alternative locator pair.
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 which
was actually used to send or receive the packet.
* For inbound traffic, the application may be interested in the
locator pair which was actually used to receive the packet.
* For outbound traffic, the application may be interested in the
locator pair which was actually used to transmit the packet.
In this way, applications may have additional control on the
locator management. For example, an application becomes able to
verify if its preference for locator is actually applied to the
flow or not.
o Applications should be able to specify if they want to defer the
context setup, or if they want context establishment to be started
immediately in the case where there is no available context. A
deferred context setup means that the initiation of communication
should not be blocked to wait for completion of the context
establishment.
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:
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* Apply shim. The application should be able to explicitly
request the shim sub-layer to apply multihoming support.
* Don't apply shim. The application should be able to request
the shim sub-layer not to apply the multihoming support but to
apply normal IP processing at the IP layer.
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.
5. 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
multihoming shim sub-layer. An application may use these socket
options for a given socket either by the getsockopt() system call or
by the setsockopt() system call. All of these socket options are
defined at level SOL_SHIM.
The first column of Table 1 gives the name of the option. The second
and third columns indicate whether the option can be handled by the
getsockopt() system call and/or by the setsockopt() system call. 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.
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+-----------------------------+-----+-----+-----------------+-------+
| optname | get | set | description | dtype |
+-----------------------------+-----+-----+-----------------+-------+
| SHIM_ASSOCIATED | o | | Get the | int |
| | | | parameter which | |
| | | | indicates | |
| | | | whether if the | |
| | | | socket is | |
| | | | associated with | |
| | | | any shim | |
| | | | context or not. | |
| SHIM_DONTSHIM | o | o | Get or set the | int |
| | | | parameter which | |
| | | | indicates | |
| | | | whether to | |
| | | | employ the | |
| | | | multihoming | |
| | | | support by the | |
| | | | shim sub-layer | |
| | | | or not. | |
| 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 | Get or set the | *1 |
| | | | preferred | |
| | | | locator on the | |
| | | | local side for | |
| | | | the context | |
| | | | associated with | |
| | | | the socket. | |
| SHIM_LOC_PEER_PREF | o | o | Get or set the | *1 |
| | | | preferred | |
| | | | locator on the | |
| | | | remote side for | |
| | | | the context | |
| | | | associated with | |
| | | | the socket. | |
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| SHIM_LOC_LOCAL_RECV | o | o | Get or set the | int |
| | | | parameter which | |
| | | | is used to | |
| | | | request the | |
| | | | shim sub-layer | |
| | | | to store the | |
| | | | destination | |
| | | | locator of the | |
| | | | received IP | |
| | | | packet. | |
| SHIM_LOC_PEER_RECV | o | o | Get or set the | int |
| | | | parameter which | |
| | | | is used to | |
| | | | request the | |
| | | | shim sub-layer | |
| | | | to store the | |
| | | | source locator | |
| | | | of the received | |
| | | | IP packet. | |
| SHIM_LOC_LOCAL_SEND | o | o | Get or set the | *2 |
| | | | source locator | |
| | | | of outgoing IP | |
| | | | packets. | |
| SHIM_LOC_PEER_SEND | o | o | Get or set the | *2 |
| | | | destination | |
| | | | locator of | |
| | | | outgoing IP | |
| | | | packets. | |
| SHIM_LOCLIST_LOCAL | o | o | Get or set the | *3 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the local EID. | |
| SHIM_LOCLIST_PEER | o | o | Get or set the | *3 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the peer's EID. | |
| SHIM_APP_TIMEOUT | o | o | Get or set the | int |
| | | | timeout value | |
| | | | for detecting | |
| | | | failure. | |
| SHIM_PATHEXPLORE | o | o | Get or set | *4 |
| | | | parameters for | |
| | | | path | |
| | | | exploration and | |
| | | | failure | |
| | | | detection. | |
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| SHIM_CONTEXT_DEFERRED_SETUP | o | o | Get or set the | int |
| | | | parameter which | |
| | | | indicates | |
| | | | whether | |
| | | | deferred | |
| | | | context setup | |
| | | | is supported or | |
| | | | not. | |
+-----------------------------+-----+-----+-----------------+-------+
Table 1: Socket options for multihoming shim sub-layer
*1: Pointer to a shim_locator which is defined in Section 7.
*2: Pointer to shim_locator data structure.
*3: Pointer to an array of shim_locator.
*4: Pointer to a shim_pathexplore which is defined in Section 7.
Figure 2 illustrates how the shim specific socket options fit into
the system model of socket 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 API continue to work at the layer above the
shim sub-layer. That is, those legacy API handle IP addresses as
identifiers.
2. With newly defined socket options for the shim sub-layer, the
application obtains additional control of locator management.
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).
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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
5.1. SHIM_ASSOCIATED
The SHIM_ASSOCIATED option is used to check whether the socket is
associated with any shim context or not.
This option is meaningful when the locator information of the
received IP packet does not tell whether the identifier/locator
adaptation is performed or not. Note that the EID pair and the
locator pair may be identical in some cases.
This option can be specified by getsockopt(). Thus, the option is
read-only and the result (0 or 1) is set in the option value (the
fourth argument of getsockopt()).
The data type of the option value is an integer. The option value
indicates the presence of shim context. A returned value 1 means
that the socket is associated with a shim context at the shim sub-
layer. A return value 0 indicates that there is no shim context
associated with the socket.
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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);
5.2. SHIM_DONTSHIM
The SHIM_DONTSHIM option is used to request the shim layer not to
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 0 means that the multihoming shim sub-layer is
employed if available. An option value 1 means that the application
does not want the multihoming shim sub-layer to provide the
multihoming support for the communication established over the
socket.
Default value is set as 0, which means that the multihoming 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, an error code
EOPNOTSUPP MUST be returned.
For example, an application can request the system not to apply the
multihoming support as follows:
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval));
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);
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5.3. SHIM_HOT_STANDBY
The SHIM_HOT_STANDBY option is used to control the shim sub-layer
whether to employ a hot-standby connection for the socket or not. 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.
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));
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);
5.4. SHIM_PATHEXPLORE
The application may use this socket option to specify 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.
The option is effective only when there is a shim context associated
with the socket.
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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 7.
By default, the option value is set to NULL, meaning that the option
is disabled.
An error ENOENT will be returned when there is no context associated
with the socket.
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);
5.5. SHIM_LOC_LOCAL_PREF
The SHIM_LOC_LOCAL_PREF option is used to get or set preferred
locator on local side within a given context. Hence 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 a locator
information data structure which is defined in Section 7.
By default, the option value is set to NULL, meaning that the option
is disabled.
The preferred locator can be set by setsockopt(). The shim sub-layer
shall verify requested locator before it updating the preferred
locator.
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An application can get the preferred locator by getsockopt().
An error ENOENT will be returned when there is no context associated
with the socket.
An error EINVALIDLOCATOR will be returned when the validation of the
specified locator failed.
For example, an application can set the preferred locator by using
the socket option as follows. Note that some members of the
shim_locator (lc_ifidx and lc_flags) are ignored in the set
operation.
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_preference = 255;
memcpy(lc.lc_addr, &ip6, sizeof(in6_addr));
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc,
sizeof(optval));
For example, an application can get the preferred locator by using
the socket option as follows.
struct shim_locator lc;
int len;
len = sizeof(lc);
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len);
5.6. SHIM_LOC_PEER_PREF
The SHIM_LOC_PEER_PREF option is used to get or set preferred locator
on peer side within a given context. Hence 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 locator
information data structure which is defined in Section 7.
By default, the option value is set to NULL, meaning that the option
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is disabled.
The preferred locator can be set by setsockopt(). The shim sub-layer
shall verify requested locator before it updating the preferred
locator.
An application can get the preferred locator by getsockopt().
An error ENOENT will be returned when there is no context associated
with the socket.
An error EINVALIDLOCATOR will be returned when the validation of the
requested locator fails.
The usage of the option is same as that of SHIM_LOC_LOCAL_PREF. Note
that some members of the shim_locator (lc_ifidx and lc_flags) are
ignored in the set operation.
5.7. SHIM_LOC_LOCAL_RECV
The SHIM_LOC_LOCAL_RECV option can be used to request the shim sub-
layer to store the destination locator of the received IP packet in
an ancillary data object which can be accessed by recvmsg(). Hence
this option is effective only when there is a shim context associated
with the socket.
The data type of the option value is integer. The option value
should 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 6 for the procedure to access locator information stored
in the ancillary data objects.
An error ENOENT will be returned when there is no context associated
with the socket.
For example, an application can request the shim sub-layer to store
destination locator by using the socket option as follows.
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval,
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sizeof(optval));
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);
5.8. SHIM_LOC_PEER_RECV
The SHIM_LOC_PEER_RECV option is used to request the shim sub-layer
to store the source locator of the received IP packet in an ancillary
data object which can be accessed by recvmsg(). Hence this option is
effective only when there is a shim context associated with the
socket.
The data type of the option value is integer. The option value
should 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 6 for the procedure to access locator information stored
in the ancillary data objects.
An error ENOENT will be returned when there is no context associated
with the socket.
The usage of the option is same as that of SHIM_LOC_LOCAL_RECV
option.
5.9. SHIM_LOC_LOCAL_SEND
The SHIM_LOC_LOCAL_SEND option is used to request the shim sub-layer
to use a specific locator as the source locator for the IP packets to
be sent from the socket. Hence this option is effective only when
there is a shim context associated with the socket.
The data type of option value is pointer to shim_locator data
structure.
An application can set the local locator by setsockopt() providing a
valid locator which is stored in a shim_locator data structure. When
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a zero-filled locator is specified, pre-existing setting of local
locator is inactivated.
An application can get the local locator by getsockopt().
An error ENOENT will be returned when there is no context associated
with the socket.
An error EINVALIDLOCATOR will be returned when 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 = 1;
locator.lc_flags = 0;
locator.lc_preference = 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 preferred local locator by
using the socket option as follows.
struct shim_locator locator;
memset(&locator, 0, sizeof(locator));
getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
sizeof(locator));
/* check locator */
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5.10. SHIM_LOC_PEER_SEND
The SHIM_LOC_PEER_SEND option is used to request the shim sub-layer
to use a specific locator for the destination locator of IP packets
to be sent from the socket. Hence 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 shim_locator data
structure.
An application can set the remote locator by setsockopt() providing a
valid locator which is stored in a shim_locator data structure. When
a zero-filled locator is specified, pre-existing setting of remote
locator is inactivated.
An application can get the specified remote locator by getsockopt().
An error ENOENT will be returned when there is no context associated
with the socket.
An error EINVALIDLOCATOR when invalid locator is specified.
The usage of the option is as the same as that of SHIM_LOC_LOCAL_SEND
option.
5.11. 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. Hence 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 7 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 by 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 by optval argument should contain an array of locator list.
An error ENOENT will be returned when there is no context associated
with the socket.
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An error EINVALIDLOCATOR will be returned when the validation of the
specified locator failed.
For example, an application can set a list of locators to be
associated with the local EID by using the socket option as follows:
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_preference = 1;
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_preference = 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.
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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 */
...
5.12. 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. Hence 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 7 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 by 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 by optval argument should contain an array of locator list.
An error ENOENT will be returned when there is no context associated
with the socket.
An error EINVALIDLOCATOR will be returned when the validation of the
specified locator failed.
The usage of the option is same as that of SHIM_LOCLIST_LOCAL.
5.13. SHIM_APP_TIMEOUT
The SHIM_APP_TIMEOUT option is used to get or set the timeout value
for application to detect failure. Hence 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
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to 0, meaning that the option is disabled. When the option is
disabled, the REAP mechanism follows its default value of Send
Timeout value as specified in [RFC5534]
If the timeout value specified is longer than the Send Timeout
configured in the REAP component, the REAP Keepalive message should
be suppressed.
An error ENOENT will be returned when there is no context associated
with the socket.
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);
5.14. SHIM_DEFERRED_CONTEXT_SETUP
The SHIM_DEFERRED_CONTEXT_SETUP option is used to specify whether to
enable deferred context setup or not. 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.
The data type for the option value is an integer. The option value
should be binary (0 or 1). By default, the value is set to 1,
meaning that the context setup is deferred. In order to disable the
option, the application should call setsockopt() with option value
set to 0.
For example, an application can disable the deferred context setup by
using the socket option as follows:
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int optval;
optval = 0;
setsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
&optval, sizeof(optval));
For example, an application can get the option value as follows.
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
&optval, &len);
5.15. Applicability
All the socket options for the multihoming shim sub-layer are
applicable only to connected sockets. The reason behind this
restriction is that it is necessary for the multihoming shim layer to
identify a target multihoming shim context when an application gives
preference value(s) by a socket option for the multihoming shim sub-
layer. Multihoming shim contexts are, by definition, identified by a
pair of EIDs. Therefore, it is possible for the multihoming shim
sub-layer to identify the target context only when the source and
destination IP addresses of the application session are known. When
any socket options for the multihoming shim sub-layer is set for an
unconnected socket, EINVAL error code MUST be returned.
5.16. 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 at least one of the necessary validations
inside the shim sub-layer for the specified locator has failed.
In case of SHIM6, there are two kinds of verifications required
for security reasons prior to sending an IP packet to the peer's
new locator; one is the return routability (check if the peer is
actually willing to receive data with the specified locator) and
the other one is the verification based on crypto identifier
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mechanisms [RFC3972], [RFC5535].
6. Ancillary Data for Multihoming Shim Sub-layer
In this section, the definition and the usage of the ancillary data
specific to multihoming shim sub-layer are provided.
As defined in the Posix standard, sendmsg() and recvmsg() input a
msghdr structure as their arguments. These system calls can handle
control information along with data. Figure 3 shows the msghdr
structure which is defined in <sys/socket.h>. The member msg_control
holds a pointer to the buffer where the shim specific ancillary data
objects can be stored in addition to other ancillary data objects.
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
The buffer pointed by the member msg_control of the msghdr structure
may contain locator information which is a single locator and it
should be possible to process them with the existing macros defined
in Posix and [RFC3542]. Each cmsghdr{} should be followed by data
which stores a single locator.
In case of non-connected socket, msg_name member stores the socket
address of the peer which should be considered as an identifier
rather than a locator. The locator of the peer node should be
retrieved by SHIM_LOC_PEER_RECV as specified below.
Table 2 is a list of the shim specific ancillary data which can be
used for recvmsg() or sendmsg(). In any case, SOL_SHIM must be set
as cmsg_level.
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+---------------------+-----------+-----------+-----------------+
| cmsg_type | sendmsg() | recvmsg() | cmsg_data[] |
+---------------------+-----------+-----------+-----------------+
| SHIM_LOC_LOCAL_RECV | | o | *1 |
| SHIM_LOC_PEER_RECV | | o | *1 |
| SHIM_LOC_LOCAL_SEND | o | | *1 |
| SHIM_LOC_PEER_SEND | o | | *1 |
| SHIM_FEEDBACK | o | | shim_feedback{} |
+---------------------+-----------+-----------+-----------------+
Table 2: Shim specific ancillary data
*1: cmsg_data[] should include padding (if necessary) and a single
sockaddr_in{}/sockaddr_in6{}.
6.1. 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 local and/or remote locator from
the ancillary data.
6.2. 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.
Note that the effect is limited to the datagram transmitted by the
sendmsg().
If the specified locator pair is verified, the shim sub-layer
overrides the locators of the IP packet.
An error EINVALIDLOCATOR will be returned when validation of the
specified locator failed.
6.3. Notification from Application to Multihoming Shim Sub-layer
An application may provide feedbacks to the shim sub-layer about the
communication status. Such feedbacks are particularly useful for the
shim sub-layer in the absence of REAP mechanism to monitor the
reachability status of the currently used locator pair in a given
shim context.
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The notification can be made by sendmsg() specifying a new ancillary
data called SHIM_FEEDBACK. The ancillary data can be handled by
specifying SHIM_FEEDBACK option in cmsg_type.
An error ENOENT will be returned when there is no context associated
with the socket.
See Section 7.3 for details of the data structure to be used.
It is outside the scope of this document how the shim sub-layer would
react when a feedback is provided by an application.
6.4. Applicability
It is important to note that the ancillary data specified in this
section are applicable only to datagram-oriented sockets (e.g., UDP
sockets or raw sockets) and that they are not applicable to stream-
oriented sockets (e.g., TCP sockets). The reason behind this
restriction is that there is no one-to-one mapping between a single
send or receive operation and a TCP segment being transmitted or
received.
Due to the above restriction and the restriction addressed in
Section 5.15, SHIM_LOC_LOCAL_RECV or SHIM_LOC_PEER_RECV socket
options are, in practice, applicable only to connected UDP sockets.
7. Data Structures
In this section, data structures specifically defined for the
multihoming shim sub-layer are introduced. These data structures are
either used as a parameter for setsockopt()/getsockopt() (as
mentioned in Section 5) or as a parameter for ancillary data to be
processed by sendmsg()/recvmsg() (as mentioned in Section 6).
7.1. Placeholder for Locator Information
As defined in Section 5, the SHIM_LOC_LOCAL_PREF, SHIM_LOC_PEER_PREF,
SHIM_LOCLIST_LOCAL, and SHIM_LOCLIST_PEER socket options need to
handle one or more locator information. Locator information includes
not only the locator itself but also additional information about the
locator which 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 a locator information.
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struct shim_locator {
uint8_t lc_family; /* address family */
uint8_t lc_proto; /* protocol */
uint16_t lc_port; /* port number */
uint16_t lc_flags; /* flags */
uint16_t lc_pref; /* preference value */
uint32_t lc_ifidx; /* interface index */
struct in6_addr lc_addr; /* address */
};
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 non-zero value indicating the
exact address family of the locator.
lc_proto
Internet Protocol number for the protocol which is used to handle
locator behind NAT. Typically, this value is set as UDP (17) when
the locator is a UDP encapsulation interface.
lc_port
Port number which is used for handling locator behind NAT.
lc_flags
Each bit of the flags represents a specific characteristics of the
locator. Hash Based Address (HBA) is defined as 0x01.
Cryptographically Generated Address (CGA) is defined as 0x02.
lc_pref
Preference of the locator. The preference is represented by an
integer.
lc_ifidx
Interface index of the network interface to which the locator is
assigned. This field should be valid only in a read
(getsockopt()) operation.
lc_addr
Contains the locator. In the case where a locator whose size is
smaller than 16 bytes, an encoding rule should be provided for
each locator of a given address family. For instance, in case of
AF_INET (IPv4), the locator should be in the format of an IPv4-
mapped IPv6 address as defined in [RFC4291].
7.1.1. 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 HIP host to tunnel control
and data traffic over UDP[I-D.ietf-hip-nat-traversal]. Note also
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that the locator behind NAT is not necessarily an IPv4 address but it
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.
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_flags = 0;
locator.lc_pref = 0;
locator.lc_ifidx = 3;
memcpy(&locator.lc_addr, &ia6, sizeof(ia6));
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
sizeof(locator));
Figure 5: Handling locator behind NAT
7.2. Path Exploration Parameter
As defined in Section 5, SHIM_PATHEXPLORE allows 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 {
uint8_t pe_probenum; /* # of initial probe */
uint8_t pe_keepaliveto; /* Keepalive Timeout */
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.
Default value of this parameter should follow what is specified in
[RFC5534].
pe_keepaliveto
Indicates timeout value 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, path exploration
procedure will be carried out by sending a REAP Probe message.
Default value of this parameter should follow what is specified in
[RFC5534].
pe_initprobeto
Indicates retransmission timer of 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. Default value of this parameter should
follow what is specified in [RFC5534].
pe_reserved
A reserved field for future extension. By default, the field
should be initialized to zero.
7.3. Feedback Information
As mentioned in Section 6.3, 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
direction
Indicates direction of reachability between a locator pair in
question. A value 0 indicates outbound and a value 1 indicates
inbound direction.
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.
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* 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.
reserved
Reserved field. Must be ignored by the receiver.
8. System Requirements
As discussed in Section 5, all the socket options for multihoming
shim sub-layer are applicable only to connected sockets. To break
this down into system requirements, the operating system (kernel)
should be able to establish and maintain an association between a
socket instance and one or more multihoming shim context. It is,
however, outside the scope of this document how the operating system
would establish and maintain associations between sockets and
multihoming shim contexts. An association can be established on
creation of a multihoming shim context, or at any stage. On creation
of a shim context, the multihoming shim sub-layer on the initiator
side should be aware of the triggering packet and it should be
possible to figure out the originating socket. It is more difficult
to establish an association on the responder side.
9. Implications for Existing Socket API Extensions
Some of the socket options defined in this document are overlapping
with existing sockets API and care should be taken for the usage not
to confuse with the overlapping features.
The socket options for requesting specific locators to be used for a
given transaction (SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF) are
semantically similar to the existing sockets API (IPV6_PKTINFO). The
socket options for obtaining the locator information from the
received IP packet (SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV) are
semantically similar to the existing sockets API (IP_RECVDSTADDR and
IPV6_PKTINFO).
In IPv4, application can obtain the destination IP address of the
received IP packet (IP_RECVDSTADDR). If the shim sub-layer performs
identifier/locator adaptation for the received packet, the
destination EID should be stored in the ancillary data
(IP_RECVDSTADDR).
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In IPv6, [RFC3542] defines that IPV6_PKTINFO can be used to specify
source IPv6 address and the outgoing interface for outgoing packets,
and retrieve destination IPv6 address and receiving interface for
incoming packets. This information is stored in ancillary data being
IPV6_PKTINFO specified as cmsg_type. Existing sockets API should
continue to work above the shim sub-layer, that is, the IP addresses
handled in IPV6_PKTINFO should be EIDs, not the locators.
Baseline is that the above existing sockets API (IP_RECVDSTADDR and
IPV6_PKTINFO) is assumed to work above the multihoming shim sub-
layer. In other words, the IP addresses those socket options deal
with are EIDs rather than locators.
10. Resolving Conflicts with Preference Values
Since the multihoming shim API allows application to specify
preference value for the context which is associated with the socket
instance, there may be a conflict with preference values specified by
different applications. For instance, application A and B may
establish communication with the same EID pair while both
applications have different preference in their choice of local
locator.
SHIM6 supports a notion of context forking in which a context is
split when there is a conflict with preference values specified by
multiple applications. Thus, context forking can simply resolve the
conflicting situation which may be caused by the use of socket
options for multihoming shim sub-layer.
10.1. Implicit Forking
Socket options defined in Section 5 may cause conflicting situation
when the target context is shared by multiple applications. In such
case, socket handler should inform the shim sub-layer that context
forking is required. In SHIM6, when a context is forked, an unique
identifier called Forked Instance Identifier (FII) is assigned to the
newly forked context. The forked context is then exclusively
associated with the socket through which non-default preference value
was specified. The forked context is maintained by the multihoming
shim sub-layer during the lifetime of associated socket instance.
When the socket is closed, the multihoming shim sub-layer SHOULD
delete associated context. In this way, garbage collection can be
carried out to cleanup unused forked contexts. Upon garbage
collection, every forked context SHOULD be checked if there is no
socket (process) associated with the context. If there is none, the
forked context should be deleted. When a forked context is torn
down, SHIM6 should notify the peer about the deletion of forked
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context.
As opposed to socket options, context forking MUST NOT be triggered
by any use of ancillary data that is specific to multihoming shim
sub-layer as defined in Section 6.
11. Discussion
In this section, open issues are introduced.
11.1. Naming at Socket Layer
The getsockname() and getpeername() system calls are used to obtain
the 'name' of an endpoint which is actually a pair of IP address and
port number assigned to a given socket. getsockname() is used when an
application wants to obtain the local IP address and port number
assigned for a given socket instance. getpeername() is used when an
application obtains the remote IP address and port number.
The above is based on a traditional system model of the sockets API
where an IP address is expected to play both the role of identifier
and the role of locator.
In a system model where a shim sub-layer exists inside the IP layer,
both getsockname() and getpeername() deal with identifiers, namely
EIDs. In this sense, the shim sub-layer serves to (1) hide locators
and (2) provide access to the identifier for the application over the
legacy socket APIs.
11.2. Additional Requirements from Applications
At the moment, it is not certain if following requirements are common
in all the multihomed environments (SHIM6 and HIP). These are mainly
identified during discussions made on SHIM6 WG mailing list.
o The application should be able to set preferences for the
locators, local and remote ones, and also to the preferences of
the local locators that will be passed to the peer.
11.3. Issues of Header Conversion among Different Address Family
The shim sub-layer performs identifier/locator adaptation.
Therefore, in some cases, the whole IP header can be replaced with
new IP header of a different address family (e.g. conversion from
IPv4 to IPv6 or vice versa). Hence, there is an issue how to make
the conversion with minimum impact. Note that this issue is common
in other protocol conversion such as SIIT[RFC2765].
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As addressed in SIIT specification, some of the features (IPv6
routing headers, hop-by-hop extension headers, or destination
headers) from IPv6 are not convertible to IPv4. In addition, notion
of source routing is not exactly the same in IPv4 and IPv6. Hence,
there is a certain limitation in protocol conversion between IPv4 and
IPv6.
The question is how should the shim sub-layer behave when it faces
with limitation problem of protocol conversion. Should we introduce
new error something like ENOSUITABLELOCATOR ?
11.4. Handling of Unknown Locator Provided by Application
There might be a case where application provides the shim layer new
locator with the SHIM_LOC_*_PREF socket options or SHIM_LOC_*_SEND
ancillary data. Then there is a question how should the shim sub-
layer treat the new locator informed by the application.
In principle, locator information are exchanged by the shim protocol.
However, there might be a case where application acquires information
about the locator and prefers to use it for its communication.
12. Changes
12.1. Changes from version 00 to version 01
The followings are changes from version 00 to version 01:
o Define shim_locator{} data type which is a placeholder for
locator.
o Define shim_pathexplore{} data type in which a set of REAP
parameters are stored.
o Remove descriptions about "stickiness" of socket options.
o Deprecate SHIM_IF_RECV and SHIM_IF_SEND socket options.
o Give default value and how to disable given socket option.
12.2. Changes from version 01 to version 02
The followings are changes from version 01 to version 02:
o Add section describing context forking.
o Rephrase conclusion section.
o Separate normative references from informative references.
o Remove texts from discussion section that are not relevant to the
contents of the document.
o Add section describing change history (this section).
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12.3. Changes from version 02 to version 03
The followings are changes from version 02 to version 03:
o Add an Appendix section describing the issue of context forking.
12.4. Changes from version 03 to version 04
The followings are changes from version 03 to version 04:
o Updated reference.
o Correct typo and grammatical errors.
12.5. Changes from version 04 to version 05
The followings are changes from version 04 to version 05:
o Added definition of SHIM_FEEDBACK ancillary data.
o Added an example of code using the SHIM_LOCLIST_LOCAL
o Added SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options.
12.6. Changes from version 05 to version 06
The followings are changes from version 04 to version 05:
o Updated references.
12.7. Changes from version 06 to version 07
The followings are changes from version 06 to version 07:
o Resolved editorial issues.
12.8. Changes from version 07 to version 08
No changes are made except for updates of the references.
12.9. Changes from version 08 to version 09
The followings are changes from version 08 to version 09:
o Updated texts for Section 1 and Section 5 according to the
comments provided by Samu Varjonen.
o Made it clear that downgrading the multihoming shim support (i.e.,
specifying value 1 with the SHIM_DONTSHIM socket option) is only
allowed before the socket is connected.
o Updated locator information (shim_locator{}) so that it can
contain a locator behind NAT.
12.10. Changes from version 09 to version 10
The followings are changes from version 09 to version 10:
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o Addressed applicability of socket options and ancillary data for
the multihoming shim sub-layer.
o Addressed system requirements.
o Removed unnecessary description about deprecated socket option
(SHIM_IF_RECV).
13. IANA Considerations
This document contains no IANA consideration.
14. Security Considerations
This document does not specify any security mechanism for the shim
sub-layer. Fundamentally, the shim sub-layer has a potential to
impose security threats, as it changes the source and/or destination
IP addresses of the IP packet being sent or received. Therefore, the
basic assumption is that the security mechanism defined in each
protocol of the shim sub-layer is strictly applied.
15. Conclusion
In this document, the Application Program Interface (API) for
multihoming shim sub-layer is specified. The sockets API allows
applications to have additional control of the locator management and
interface to the REAP mechanism inside the multihoming shim sub-
layer.
Socket options for multihoming shim sub-layer can be used by
getsockopt() and/or setsockopt() system calls. Besides, applications
can use some ancillary data that are specific to multihoming shim
sub-layer to get locator from received packet or to set locator for
outgoing packet.
From an architectural point of view, the sockets API provides extends
the existing sockets API framework in the face of ID/Locator
separation. With regard to API that relate to IP address management,
it is assured that existing sockets API continue to work above the
shim sub-layer dealing with identifiers, while multihoming shim API
deals with locators.
16. Acknowledgments
Authors would like to thank Jari Arkko who participated in the
discussion that lead to the first version of this document, and
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Tatuya Jinmei who thoroughly reviewed the early version of this draft
and provided detailed comments on sockets API related issues. Thomas
Henderson provided valuable comments especially from HIP
perspectives.
Authors sincerely thank to the following people for their help to
improve this document: Samu Varjonen and Dmitriy Kuptsov.
17. References
17.1. Normative References
[POSIX] "IEEE Std. 1003.1-2001 Standard for Information Technology
-- Portable Operating System Interface (POSIX). Open group
Technical Standard: Base Specifications, Issue 6,
http://www.opengroup.org/austin", December 2001.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, May 2006.
[RFC5533] Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
protocol", RFC 5533, June 2009.
[RFC5534] Arkko, J. and I. Beijnum, "Failure Detection and Locator
Pair Exploration Protocol for IPv6 Multihoming", RFC 5534,
June 2009.
17.2. Informative References
[I-D.ietf-hip-nat-traversal]
Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
Keranen, "Basic HIP Extensions for Traversal of Network
Address Translators", Internet
Draft draft-ietf-hip-nat-traversal-09, October 2009.
[I-D.ietf-shim6-app-refer]
Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress),
July 2005.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
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[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5535] Bagnulo, M., "Hash Based Addresses (HBA)", RFC 5535,
June 2009.
Appendix A. Context Forking
In this section, an issue concerning context forking and its relation
to the multihoming shim API are discussed.
SHIM6 supports a notion of context forking. A peer may decide to
fork a context for certain reason (e.g. upper layer protocol prefers
to use different locator pair than the one defined in available
context). The procedure of forking context is done similar 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 initiator.
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
implementation specific issue. However, on the responder side, there
is a question on how the outbound packet can be multiplexed by the
shim sub-layer. Since there are more than one SHIM6 contexts that
match with the ULID pair of the packet flow. There is a need to
differentiate packet flows not only by the ULID pairs but some other
information and associate a given packet flow with 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 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
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forking occurs on the initiator side, a context forking needs to
occur also on the responder side.
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
To overcome the problem mentioned above, there are some solutions.
One viable approach is to let the system implicitly maintain an
association between the socket and the associated context by keeping
the record of inbound packet processing. That is, the system stores
the information about the context on which the inbound packet flow
was demultiplexed. The information comprises the ULID pair and FII
of the context and is stored in the socket instance. Later, the
system can use the information to identify the associated context in
outbound packet processing. This approach should be feasible as far
as there is bi-directional user traffic.
Another viable approach is to extend SHIM6 protocol by adding
capability of exchanging additional information to identify the
packet flow from others which needs to be handled by a newly forked
context. The information exchange can be done during the context
establishment. The initiator appends 5 tuple of the packet flow to
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be handled by the newly forked context. Note that the additional
information provided by the 5 tuple are source and destination port
numbers and upper layer protocol. The information is later used by
the shim sub-layer to multiplex the outbound packet flow on the
responder side.
The socket options for multihoming shim can be used by the
application to trigger the context forking in implicit manner. The
peer becomes an initiator in the establishment of the forked context.
Once the forked context is established between the peers, application
on each end can influence the preference on context by utilizing the
multihoming shim API.
Authors' Addresses
Miika Komu
Helsinki Institute for Information Technology
Tammasaarenkatu 3
Helsinki
Finland
Phone: +358503841531
Fax: +35896949768
Email: miika@iki.fi
URI: http://www.hiit.fi/
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
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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@sfc.wide.ad.jp
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