SHIM6 Working Group M. Komu
Internet-Draft HIIT
Expires: August 5, 2006 M. Bagnulo
UC3M
K. Slavov
S. Sugimoto, Ed.
Ericsson
February 2006
Socket Application Program Interface (API) for Multihoming Shim
draft-ietf-shim6-multihome-shim-api-01
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies a socket API for the multihoming shim layer.
The API aims to enable interactions between the applications and the
multihoming shim layer for advanced locator management and access to
information about failure detection and path exploration.
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This document is based on an assumption that a multhomed host is
equipped with a conceptual sublayer (here after "shim") inside the IP
layer that maintains mappings between identifiers and locators.
Examples of the shim are SHIM6 and HIP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Socket Options for Multihoming Shim Layer . . . . . . . . . . 9
5.1. SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 12
5.2. SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 13
5.3. SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . . 13
5.4. SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . . 14
5.5. SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 15
5.6. SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 16
5.7. SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 17
5.8. SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 18
5.9. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 18
5.10. SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 19
5.11. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 19
5.12. SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 20
5.13. Error Handling . . . . . . . . . . . . . . . . . . . . . . 21
6. Ancillary Data for Multihoming Shim . . . . . . . . . . . . . 21
6.1. Get Locator Information from Incoming Packet . . . . . . . 23
6.2. Specify Locator Information for Outgoing Packet . . . . . 23
6.3. Notification from Application to Multihoming Shim . . . . 23
6.3.1. SHIM_FEEDBACK_POSITIVE . . . . . . . . . . . . . . . . 24
6.3.2. SHIM_FEEDBACK_NEGATIVE . . . . . . . . . . . . . . . . 24
7. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 24
7.1. Placeholder for Locator Information . . . . . . . . . . . 24
7.2. Path Exploration Parameter . . . . . . . . . . . . . . . . 25
8. Implications for Existing Socket API Extensions . . . . . . . 26
9. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Issues with a Context Shared by Applications . . . . . . . 27
9.2. Issues with Shim Unaware Application . . . . . . . . . . . 27
9.2.1. Initial Contact with Multiple Locator Pairs . . . . . 27
9.2.2. Naming at Socket Layer . . . . . . . . . . . . . . . . 29
9.3. Additional Requirements from Application . . . . . . . . . 29
9.4. Issues of Header Conversion among Different Address
Family . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.5. Handling of Unknown Locator Provided by Application . . . 30
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
14.1. Normative References . . . . . . . . . . . . . . . . . . . 31
14.2. Informative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Introduction
HIP and SHIM6 have a commonality in their protocol design; separation
of identifier and locator (hereafter identifier/locator separation).
Both protocols aim to solve problems that are specific to multihoming
environment in a host centric approach. In these protocols, a sub-
layer within the IP layer maintains mappings of identifiers and
locators.
The shim layer is useful in a sense that the IP layer can maintain
the mapping of an identifier to corresponding locators. Under a
multihomed environment, typically, a host has more than one IP
address at a time. During a given transaction, a host may be
required to switch the IP address used for the communication to
another IP address to preserve the communication. A care is needed
to not disrupt the upper layer protocols by the address update. The
shim layer can make this locator update transparent to the upper
layer protocols.
In a system which is based on identifier/locator separation, upper
layer protocols are expected to deal with identifiers for
establishing and handling the communications. If an application
wants to have a multihoming support by the shim layer, the IP
addresses specified as source and destination addresses must be
identifiers. However, this does not necessarily mean that
applications are prohibited to choose specific locators in its
communication. It may be useful for applications, in some situation,
to specify a preferred locator for the flow.
This document recommends that the identifier/locator adaptation is
done only once inside the network stack of a host. That is, if
multiple shim sublayers exist at the IP layer, any one of them should
be applied exclusively for a given flow.
As this document specifies socket API, it is written so that the
contents are in line with Posix standard [POSIX] as much as possible.
The API specified in this document defines how to use ancillary data
(aka cmsg) to access locator information with recvmsg() and/or
sendmsg() I/O calls. Definition of API is presented in C language
and data types follow 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 environment. In addition, this document should be of
interest for the developers of a given shim protocol, as the shim
layer should provide the interface to the application.
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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[I-D.ietf-shim6-proto]
o HIP Architecture[I-D.ietf-hip-arch]
o Reachability Protocol (REAP)[I-D.ietf-shim6-failure-detection]
In this document, the term "IP" refers to both IPv4 and IPv6, unless
the protocol version is specifically mentioned. The followings are
definitions of the terms that are frequently used in this document:
o Endpoint Identifier (EID) - An identifier used by the application
to specify the endpoint of a given communication. Applications
may handle EID 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 an ULID serves as an
EID. An ULID is chosen from locators available on the host.
* In the case of HIP, an identifier which specifies communication
endpoints is derived from the public key of the host, which is
called a Host Identifier. For the sake of backward
compatibility of the socket API, the Host Identifier is
represented in a form of hash of public key.
o Locator - An IP address actually used to deliver IP packets.
Locators should be 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 [I-D.ietf-shim6-proto], the
list of locators associated with an EID 'A' can be denoted as
Ls(A).
* Preferred Locator - The (source/destination) locator currently
used to send packets within a given context. As defined in
[I-D.ietf-shim6-proto], the preferred locator of a host 'A' is
denoted as Lp(A).
o Shim - A conceptual (sub-)layer inside the IP Layer which
maintains mappings of EIDs and locators. An EID can be associated
with more than one locators at a time when the host is multihomed.
The term 'shim' does not refer to a specific protocol but refers
to the conceptual sublayer inside the IP layer.
o identifier/locator adaptation - An adaptation performed at the
shim layer between EIDs and locators within a given context. The
adaptation may end up re-writing the source and destination
addresses of the IP packet. In the outbound packet processing,
the EID pair is converted to the associated locator pair, while
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the locator pair is converted to the EID pair in the inbound
packet processing.
o Context - State information shared by a given pair of peers, which
stores a binding between the EIDs and associated locators. The
context is maintained at the shim layer.
o Reachability Detection - A procedure to check reachability between
a given locator pair.
o Path - A sequence of routers that an IP packet goes through to
reach the destination.
o Path Exploration - A procedure to explore available paths for a
given set of locator pairs.
o Outage - An 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 can be caused by various reasons, such
as shortage of network resources, congestions, and human error
(faulty operation).
o Working Address Pair - An address pair is said to be working if
the packet containing the first address from the pair as source
address and the second address from the pair as destination
address can safely travel from the source to the destination. If
the reachability is confirmed in both directions, the address
pairs is said to be bi-directional. Otherwise, it's
unidirectional.
o Reachability Protocol (REAP) - A protocol for detecting failure
and exploring reachability in a multihomed environment. REAP is
defined in [I-D.ietf-shim6-failure-detection].
3. System Overview
Figure 1 illustrates the system overview. The shim layer and REAP
component exist inside the IP layer. Applications can use the socket
API defined in this document to interface the shim layer and
transport layer for locator management and failure detection and path
exploration.
It is also possible that the shim layer interacts with transport
layers, but the interactions are 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 the list of requirements from the application
perspective:
o Locator management. The shim layer selects a pair of locators for
sending IP packets within a given context. The selection is made
by taking miscellaneous conditions into account such as
reachability of the path, application's preference, and
characteristics of path. From the application's perspective:
* It should be possible to obtain the lists of locators of a
given context: Ls(local) and Ls(remote).
* It should be possible to obtain the preferred locators of a
given context: Lp(local) and Lp(remote).
o Notification from the application to the shim layer about the
status of the communication. Note that the notification is made
in an event based manner. There are mainly two aspects of the
feedback that application or upper layer protocol may provide for
the shim layer, positive and negative feedbacks [NOTE: These
feedbacks are mentioned in [I-D.ietf-shim6-failure-detection]]:
* Positive feedback could be given by the application or upper
layer protocol (e.g. TCP) to the shim layer informing that the
communication is going well.
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* Negative feedback could be given by the application or upper
layer protocol (e.g. TCP) to the shim layer informing that the
communication status is not satisfactory. TCP could detect a
problem when it does not receive expected ACK from the peer.
ICMP error messages delivered to the upper layer protocol could
be a clue for application to detect potential problems. REAP
module may be triggered by these negative feedbacks and invoke
procedure of path exploration.
o Feedback from application to shim layer. The application should
be able to inform the shim layer of the timeout values for
detecting failures, for sending keepalives, for starting the
exploration procedure. In particular, the application should be
able to suppress the keepalives.
o Hot-standby. The application may request the shim layer for hot-
standby capabilities. In this case, alternative paths are known
to be working before a failure is detected. Hence it is possible
for the host to immediately replace the current locator pair with
an alternative locator pair. Hot-standby may allow applications
to achieve better failover.
o Eagerness of locator exploration. The application should be able
to inform the shim layer how aggressive it wants REAP mechanism to
perform path exploration (e.g. specifying the number of concurrent
attempts of discovering working locator pair) when an outage
occurs on the path between the currently selected locator pair.
o Providing locator information to application. The 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, the application may have additional control on the
locator management. For example, the application can verify if
its preference of locator is actually applied to the flow or not.
o The application should be able to specify if it wants to defer the
context setup or if it wants context establishment to be started
immediately in case there is no available context. With deferred
context setup, there should be no additional delay imposed by
context establishment in initiation of communication.
o Turn on/off shim. The application should be able to request to
turn on/off the multihoming support by the shim layer:
* Apply shim. The application should be able to explicitly
request the shim layer to apply multihoming support.
* Don't apply shim. The application should be able to request
the shim layer not to apply the multihoming support but to
apply normal IP processing at the IP layer.
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o The application should be able to know if the communication is now
served by the shim layer or not.
o The application should be able to access locator information
regardless of its address family. In other words, no matter
whether the target locator is IPv4 or IPv6, the application should
be able to use common interface to access the locator information.
5. Socket Options for Multihoming Shim Layer
In this section, the socket options for the interface between the
application and the multihomed shim layer are defined. These options
can be used either by getsockopt() or setsockopt() system call for an
open socket. Table 1 provides a list of the socket options. Note
that all socket options are defined at level SOL_SHIM.
The first column of the table gives the name of the option. The
second and third columns indicate whether the option is for
getsockopt() and/or setsockopt(), respectively. The fourth column
provides a brief description of the socket option. The fifth column
shows the type of data structure specified with the socket option,
which can store an argument for setsockopt() and result for
getsockopt(). By default, the data structure type is an integer.
+-----------------------------+-----+-----+-----------------+-------+
| optname | get | set | description | dtype |
+-----------------------------+-----+-----+-----------------+-------+
| SHIM_ASSOCIATED | o | | Check if the | int |
| | | | socket is | |
| | | | associated with | |
| | | | any shim | |
| | | | context or not. | |
| SHIM_DONTSHIM | o | o | Request the | int |
| | | | shim layer not | |
| | | | to apply any | |
| | | | multihoming | |
| | | | support for the | |
| | | | communication. | |
| SHIM_HOT_STANDBY | o | o | Request the | int |
| | | | shim layer to | |
| | | | prepare a | |
| | | | hot-standby | |
| | | | connection (in | |
| | | | addition to the | |
| | | | current path). | |
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| 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. | |
| SHIM_LOC_LOCAL_RECV | o | o | Request for the | int |
| | | | destination | |
| | | | locator of the | |
| | | | received IP | |
| | | | packet. | |
| SHIM_LOC_PEER_RECV | o | o | Request for the | int |
| | | | source locator | |
| | | | of the received | |
| | | | IP packet. | |
| SHIM_LOCLIST_LOCAL | o | o | Get or set a | *2 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the local EID. | |
| SHIM_LOCLIST_PEER | o | o | Get or set a | *2 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the peer's EID. | |
| SHIM_APP_TIMEOUT | o | o | Inform the shim | int |
| | | | layer of a | |
| | | | timeout value | |
| | | | for detecting | |
| | | | failure. | |
| SHIM_PATHEXPLORE | o | o | Specify | *3 |
| | | | behavior of | |
| | | | path | |
| | | | exploration and | |
| | | | failure | |
| | | | detection. | |
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| SHIM_CONTEXT_DEFERRED_SETUP | o | o | Specify if the | int |
| | | | context setup | |
| | | | can be deferred | |
| | | | or not. | |
+-----------------------------+-----+-----+-----------------+-------+
Table 1: Shim specific socket options for getsockopt() and
setsockopt()
*1: Pointer to a shim_locator which is defined in Section 7.
*2: Pointer to an array of shim_locator.
*3: 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. In the figure, it can be seen that
the shim layer and the additional protocol components (IPv4 and IPv6)
below the shim layer are new to the system model. As previously
mentioned, all the shim specific socket options are defined at
SOL_SHIM level. This design choice brings the following advantages:
1. It is assured that the existing socket API continue to work at
the layer above the shim layer. That is, those legacy API deal
with 'identifier' aspect of the IP addresses.
2. With newly defined socket options for the shim layer, the
application obtains additional control on locator management.
3. The shim specific socket options are not specific to any address
family (IPPROTO_IP or IPPROTO_IPV6) or any 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 socket API with shim layer
5.1. SHIM_ASSOCIATED
The SHIM_ASSOCIATED option can be used to check whether the socket is
associated with any shim context or not.
This option is particularly meaningful in a case where 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 locator pair may be identical in some case.
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()).
Data type of the option value is integer. The option value indicates
presence of shim context. A returned value 1 means that the socket
is associated with a certain shim context at the shim layer, while a
return value 0 indicates that there is no 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 can be used to request the shim layer to not
apply the multihoming support for the communication established over
the socket.
Data type of the option value is integer. The option value indicates
whether the multihoming shim support is deprecated or not. The
option value is binary (0 or 1). By default, the value is set to 0,
meaning that the shim layer applies identifier/locator adaptation for
the communication. In order to disable the socket option, the
application should call setsockopt() with optval set as 0.
For example, the option can be disabled by the application as
follows.
int optval;
optval = 0;
setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval));
For example, the option value can be checked by the application as
follows.
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len);
5.3. SHIM_HOT_STANDBY
The SHIM_HOT_STANDBY option can be used to check if the shim layer
uses hot-standby connection or not for the communication established
over the socket. Hot-standby connection is another working locator
pair than the current locator pair. Hence this option is effective
only when there is a shim context associated with the socket.
Data type of the option value is integer.
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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, the option can be activated by the application as
follows.
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval,
sizeof(optval));
For example, the option value can be checked by the application as
follows.
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len);
5.4. SHIM_PATHEXPLORE
This option can be used to specify behavior of path exploration to be
carried out. Path exploration is a procedure to find an alternative
locator pair when the host finds any problem with current locator
pair. A message used for finding an alternative locator pair is
called a Probe message and it is sent per locator pair. Default
value is defined for Initial Probe Timeout (0.5 seconds) and Initial
Probe (4 times) in the REAP specification.
The option is effective only when there is a shim context associated
with the socket.
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 as NULL, meaning that the option
is disabled.
An error ENOENT will be returned when there is no context associated
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with the socket.
For example, the parameters for the path exploration can be set 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, the parameters for the path exploration can be read 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 can be used to read 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.
Data type of the option value is a pointer to the a specific data
structure which stores the locator information. The data structure
is defined in Section 7.
By default, the option value is set as NULL, meaning that the option
is disabled.
The preferred locator can be set by setsockopt(). Verification of
the locator shall be done by the shim layer before updating the
preferred locator.
The preferred locator can be read 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
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specified locator failed.
For example, a preferred locator can be set as follows. It should be
noted that some members of the shim_locator (lc_ifidx and lc_flags)
are ignored in the write operation.
struct shim_locator lc;
struct in6_addr ip6;
/* ...set the locator (ip6)... */
bzero(&lc, 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, the preferred locator of the context can be read by
application 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 can be used to read 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.
Data type of the option value is a pointer to the a specific data
structure which stores the locator information. The data structure
is defined in Section 7.
By default, the option value is set as NULL, meaning that the option
is disabled.
The preferred locator can be set by setsockopt(). Necessary
verification of the locator shall be done by the shim layer before
updating the preferred locator.
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The preferred locator can be read 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, a preferred locator can be set as follows. It should be
noted that some members of the shim_locator (lc_ifidx and lc_flags)
are ignored in the write operation.
The usage of the option is same as that of SHIM_LOC_LOCAL_PREF.
5.7. SHIM_LOC_LOCAL_RECV
The SHIM_LOC_LOCAL_RECV option can be used to request the shim 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.
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 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, the option can be activated by the application as
follows:
int optval;
optval = 1;
setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval,
sizeof(optval));
For example, the option value can be checked by the application as
follows:
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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 can be used to request the shim 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.
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 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_LOCLIST_LOCAL
The SHIM_LOCLIST_LOCAL option can be used to read 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.
Data type of option value is pointer to the buffer where a locator
list is stored. See section Section 7 for the data structure for
storing the locator information. By default, the option value is set
as NULL, meaning that the option is disabled.
The locator list can be read 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.
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The 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.
An error EINVALIDLOCATOR will be returned when the validation of the
specified locator failed.
Example is TBD.
5.10. SHIM_LOCLIST_PEER
The SHIM_LOCLIST_LOCAL option can be used to read 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.
Data type of option value is pointer to the buffer where a locator
list is stored. See section Section 7 for the data structure for
storing the locator information. By default, the option value is set
as NULL, meaning that the option is disabled.
The locator list can be read 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 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.
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.
5.11. SHIM_APP_TIMEOUT
The SHIM_APP_TIMEOUT option indicates timeout value for application
to detect failure. Hence this option is effective only when there is
a shim context associated with the socket.
Data type of the option value is 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 as 0,
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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 [I-D.ietf-shim6-failure-detection]
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, a specific timeout value can be configured by the
application as follows:
int optval;
optval = 15; /* 15 seconds */
setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval,
sizeof(optval));
For example, the option value namely the period of timeout can be
checked by the application as follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len);
5.12. SHIM_DEFERRED_CONTEXT_SETUP
The SHIM_DEFERRED_CONTEXT_SETUP option indicates how initiation of
context setup is made in terms of timing (before or after) the
initial communication flow. Deferred context means that the
establishment of context does not put additional delay for an initial
transaction.
Data type for the option value is integer. The option value should
binary (0 or 1). By default, the value is set as 1, meaning that the
context setup is deferred. In order to disable the option, the
application should call setsockopt() with option value set as 0.
However, it should be noted that in some case, deferred context setup
is not possible; given EID is non-routable address and there is no
way to transmit any IP packet unless there is a context providing the
locators. In such case, context should be established prior to the
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communication.
For example, the option can be disabled by the application as
follows:
int optval;
optval = 0;
setsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
&optval, sizeof(optval));
For example, the option value can be checked by the application as
follows:
int optval;
int len;
len = sizeof(optval);
getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
&optval, &len);
5.13. Error Handling
If successful, getsockopt() and setsockopt() return 0; otherwise, the
functions return -1 and set errno to indicate error.
The followings are errno codes newly 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 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 return routability (check if the peer is actually
willing to receive data with the specified locator) and the other
is verifications based on given crypto identifier mechanisms
[RFC3972], [I-D.ietf-shim6-hba].
6. Ancillary Data for Multihoming Shim
In this section, definition and usage of the ancillary data which is
specific to multihoming shim are provided.
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As defined in Posix standard, sendmsg() and recvmsg() take msghdr
structure as its argument and they can additionally handle control
information along with data. Figure 18 shows the msghdr structure
which is defined in <sys/socket.h>. msg_control member 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 18: msghdr structure
The buffer pointed from the msg_control member of the msghdr
structure may contain a 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.
+------------------------+-----------+-----------+-------------+
| 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_POSITIVE | o | | TBD |
| SHIM_FEEDBACK_NEGATICE | o | | TBD |
+------------------------+-----------+-----------+-------------+
Table 2: Shim specific ancillary data
*1: cmsg_data[] should include padding (if necessary) and a single
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sockaddr_in{}/sockaddr_in6{}.
It should be noted that the above ancillary data can only be handled
in UDP and raw sockets, not in TCP sockets. As explained in
[RFC3542], there is no one-to-one mapping of send/receive operations
and the TCP segments being transmitted/received. In case of TCP,
application may use setsockopt() or getsockopt() to access or specify
some of locator information provided by the shim layer.
6.1. Get Locator Information from Incoming Packet
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. Specify Locator Information for Outgoing Packet
Application can specify the locators to be used for transmitting an
IP packet by sendmsg(). When ancillary data of cmsg_type
SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the
application can explicitly specify source and/or destination locators
to be used for the communication over the socket.
In addition, the application can specify the outgoing interface by
SHIM_IF_SEND ancillary data. The ancillary data should contain the
interface identifier of the physical interface over which the
application expects the packet to be transmitted.
Note that the effect is limited to the datagram transmitted by the
sendmsg().
If the specified locator pair seems to be valid, the shim layer
overrides the locator of the IP packet as requested.
An error EINVALIDLOCATOR will be returned when validation of the
specified locator failed.
6.3. Notification from Application to Multihoming Shim
Application may provide feedback to the shim layer in accordance with
its communication status. The notification can be made by specifying
shim specific ancillary data in sendmsg() call. Note that this
notification is dynamic rather than static.
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6.3.1. SHIM_FEEDBACK_POSITIVE
The application can simply inform the shim layer that its
communication is going well.
Data type is TBD.
An error ENOENT will be returned when there is no context associated
with the socket.
6.3.2. SHIM_FEEDBACK_NEGATIVE
The application can inform the shim layer that its communication is
not going well.
Data type is TBD.
An error ENOENT will be returned when there is no context associated
with the socket.
7. Data Structures
In this section, data structures specifically defined for the
multihoming shim layer are introduced. Those data structures are
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 19 illustrates the data structure called shim_locator which
stores a locator information.
struct shim_locator {
uint8_t lc_family; /* address family */
uint8_t lc_ifidx; /* interface index */
uint8_t lc_flags; /* flags */
uint8_t lc_preference; /* preference value */
uint8_t lc_addr[16]; /* locator */
};
Figure 19: shim locator structure
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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_ifidx
Interface index of the network interface to which the locator is
assigned. This field should be valid only in read (getsockopt())
operation.
lc_flags
Each bit of the flags represents a specific characteristics of the
locator. HBA is defined as 0x01. CGA is defined as 0x02. The
other bits are TBD.
lc_preference
Indicates preference of the locator. The preference is
represented by integer.
lc_addr
Contains the locator. For the cases where a locator whose size is
smaller than 16 bytes, encoding rule should be provided for each
locator of a given address family. For instance, in case of
AF_INET (IPv4), the last 4 bytes of lc_addr should contain the
IPv4 address.
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 20 illustrates the data structure. The
data structure can be used by 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 20: path explore structure
pe_probenum
Indicates the number of initial probe messages to be sent.
Default value of this parameter should follow what is specified in
[I-D.ietf-shim6-failure-detection].
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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
[I-D.ietf-shim6-failure-detection].
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 [I-D.ietf-shim6-failure-detection].
pe_reserved
A reserved field for future extension. By default, the field
should be initialized with zero.
8. Implications for Existing Socket API Extensions
Some of the socket options defined in this document have some
overlapping with existing socket API and care should be made for the
usage not to confuse the 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 socket 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 socket 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 layer performs
identifier/locator adaptation for the received packet, the
destination EID should be stored in the ancillary data
(IP_RECVDSTADDR).
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 socket API should
continue to work above the shim layer, that is, the IP addresses
handled in IPV6_PKTINFO should be EIDs, not the locators.
Baseline is that the above existing socket API (IP_RECVDSTADDR and
IPV6_PKTINFO) is assumed to work above the multihoming shim layer.
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In other words, the IP addresses those socket options deal with are
EIDs rather than locators.
9. Discussion
In this section, open discussion issues are noted.
9.1. Issues with a Context Shared by Applications
A context is by definition, system-wide. This means that a context
could be shared by applications whose communications are using the
same EID pair.
When a context is shared by applications, there may be some problems
when the shim layer needs to handle feedbacks from the multiple
applications. As mentioned in Section Section 6, an application may
provide the shim layer feedback about timeout values from its own
settings. This implies that there is potentially a race condition at
the shim layer.
First of all, the socket options must be used with a proper
privilege. Feedback from the application which is run under a root
privilege must always override the feedback provided by application
which is run under normal user privilege.
For other cases, one could rely on a kind of heuristics of the
configuration. For instance, prioritizing feedback with higher
demand (e.g. timeout value 300 seconds are more demanding then
timeout value 600 seconds) may make sense in some cases. However, it
is still an open issue what kind of timer value could be handled in
this way.
Further discussions are needed how the shim layer can accommodate
feedbacks from multiple applications within a same context.
9.2. Issues with Shim Unaware Application
In multihomed environment where either of the peers or both of the
peers have multiple locators, there are some issues with shim unaware
application which uses legacy socket API.
9.2.1. Initial Contact with Multiple Locator Pairs
In a connection oriented communication, the connect() system call is
used to make the initial contact to the peer, which typically
requires IP address and port number to specify the endpoint. Hence,
name-to-address resolution should be performed prior to connect().
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The application needs to resolve the FQDN of the peer to an IP
address by any available name-to-address conversion method.
In typical case, the application receives information from the
resolver. If the application ends up with receiving multiple IP
addresses to reach the peer, it should iterate through each
destination address one-by-one. It should be noted that the host may
also have multiple source addresses.
The different resulting address pairs may have different reachability
status so, in order to find a working address pair, it may be
required to explore all the available address pairs (as opposed to
explore all available destination addresses).
In normal case, the application issues a connect() by specifying the
resolved IP address of the peer. If the connect() fails, it iterates
through the available IP addresses one by one sequentially until
working pair is found. Another approach is to initiate concurrent
connect() with every locator of the peer. connect() can also be
called in a sequence which would probably require more time to find
the working pair.
There is a case where involvement of the shim layer is expected for
handling initial contact. In such case, behavior of the shim layer
will depend on presence of the required context. This case occurs
when there exists a context for the EID specified in connect(), the
initial contact can be made in accordance with the context
information. Otherwise, the shim layer should invoke context
establishment with the peer EID specified in the argument for
connect().
Additional efforts would be required in a case where the peer cannot
be reachable through the EID (for example, EID is non-routable or
non-IP reachable) but it can be reached through alternative locator.
In particular, the shim layer should somehow discover the alternate
locator for the EID to establish context. [I-D.nordmark-shim6-esd]
addresses the possible approach to perform reverse DNS lookup from
EID to FQDN, then perform forward lookup again to find the full-set
of locators and EID.
In HIP, resolving HITs to IP addresses using DNS is not feasible
because HITs do not contain any hierarchical information. To
mitigate this problem, there are a few alternatives. Firstly,
resolver library on end-host can be modified to provide HIT-to-IP
mappings for HIP software module. Secondly, a distributed hash table
(DHT) service can be used for storing and looking up the mappings
because it supports non-hierarchical identifiers, such as HITs
[I-D.ietf-hip-arch]. Thirdly, it is possible to use IP addresses in
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legacy applications as described in [I-D.henderson-hip-applications].
9.2.2. Naming at Socket Layer
getsockname() and getpeername() system calls are used to obtain the
'name' of 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 wants to obtain the remote IP address and port number.
The above is based on a traditional system model of the socket 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 layer exists inside the IP layer, both
getsockname() and getpeername() deal with identifiers, namely EIDs.
In this sense, the shim layer serves to (1) hide locators and (2)
provide access to the identifier for the application over the legacy
socket APIs.
9.3. Additional Requirements from Application
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 one and also to the preferences of the
local locators that will be passed to the peer.
9.4. Issues of Header Conversion among Different Address Family
The shim layer performs identifier/locator adaptation. Therefore, in
some case, 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].
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 certain limitation in protocol conversion between IPv4 and
IPv6.
The question is how should the shim layer behave when it is face with
limitation problem of protocol conversion. Should we introduce new
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error something like ENOSUITABLELOCATOR ?
9.5. 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 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.
10. IANA Considerations
This document contains no IANA consideration.
11. Security Considerations
This document does not specify any security mechanism for the shim
layer. Fundamentally, the shim 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 layer is strictly applied.
12. Conclusion
In this document, the Application Program Interface (API) for
multihomed shim layer is specified. The socket API allows
applications to have additional control on the locator management and
interface to the REAP mechanism inside the shim layer. The socket
API is expected to be useful for applications that may greatly
benefit from multihomed environment. From the architectural
perspective, the socket API enhances software development environment
in a sense that it allows separate treatment of identifier and
locator at the IP layer. The API is designed with a care not to
break the semantics of existing socket API and minimize the impact to
the legacy applications.
Multihoming shim socket options defined in this document can be used
by getsockopt() and/or setcokopt() system calls, which allow
applications to have control of locator management. Additionally,
applications can specify locator information for outgoing packet and
get locator information from incoming packet by using ancillary data
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objects that are specific to the multihoming shim layer.
13. Acknowledgments
Authors would like to thank Jari Arkko who participated in the
discussion that lead to the first version of this document, and
Tatuya Jinmei who thoroughly reviewed the early version of this draft
and provided detailed comments on socket API related issues.
14. References
14.1. Normative References
[I-D.henderson-hip-applications]
Henderson, T. and P. Nikander, "Using HIP with Legacy
Applications", draft-henderson-hip-applications-03 (work
in progress), May 2006.
[I-D.ietf-hip-arch]
Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", draft-ietf-hip-arch-03 (work in progress),
August 2005.
[I-D.ietf-shim6-failure-detection]
Arkko, J. and I. Beijnum, "Failure Detection and Locator
Pair Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-06 (work in progress),
September 2006.
[I-D.ietf-shim6-proto]
Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-05 (work in progress),
May 2006.
[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.
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14.2. Informative References
[I-D.ietf-shim6-app-refer]
Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress),
July 2005.
[I-D.ietf-shim6-hba]
Bagnulo, M., "Hash Based Addresses (HBA)",
draft-ietf-shim6-hba-02 (work in progress), October 2006.
[I-D.nordmark-shim6-esd]
Nordmark, E., "Extended Shim6 Design for ID/loc split and
Traffic Engineering", draft-nordmark-shim6-esd-00 (work in
progress), February 2006.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
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Authors' Addresses
Miika Komu
Helsinki Institue 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
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
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