Network 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-sugimoto-multihome-shim-api-00
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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 also for
accessing to information about failure detection and path
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exploration.
This document is based on an assumption that a multhomed host is
equipped with a 'shim' layer which essentially maintains mappings
between identifiers and locators at the IP layer. SHIM6 and HIP are
examples of this shim layer. Hence, the API can be commonly used by
SHIM6 and HIP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Socket Options for Multihomed Shim Layer . . . . . . . . . . . 9
6.1. SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 12
6.2. SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 12
6.3. SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . . 13
6.4. SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . . 13
6.5. SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 13
6.6. SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 14
6.7. SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 14
6.8. SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 15
6.9. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 15
6.10. SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 15
6.11. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 16
6.12. SHIM_FEEDBACK_POSITIVE . . . . . . . . . . . . . . . . . . 16
6.13. SHIM_FEEDBACK_NEGATIVE . . . . . . . . . . . . . . . . . . 16
6.14. SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 17
6.15. SHIM_IF_RECV . . . . . . . . . . . . . . . . . . . . . . . 17
6.16. SHIM_IF_SEND . . . . . . . . . . . . . . . . . . . . . . . 17
6.17. Error Handling . . . . . . . . . . . . . . . . . . . . . . 17
7. Access to Locator Information . . . . . . . . . . . . . . . . 18
7.1. Get Locator Information from Incoming Packet . . . . . . . 19
7.2. Specify Locator Information for Outgoing Packet . . . . . 20
8. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Placeholder for Locator Information . . . . . . . . . . . 20
8.1.1. addrinfo structure . . . . . . . . . . . . . . . . . . 20
8.1.2. sockaddr_storage structure . . . . . . . . . . . . . . 21
9. Implications for Existing Socket API Extensions . . . . . . . 22
10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Issues with a Context Shared by Applications . . . . . . . 23
10.2. Issues with Shim Unaware Application . . . . . . . . . . . 24
10.2.1. Initial Contact with Multiple Locator Pairs . . . . . 24
10.2.2. Naming at Socket Layer . . . . . . . . . . . . . . . . 25
10.3. Additional Requirements from Application . . . . . . . . . 25
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10.4. Issues of Header Conversion among Different Address
Family . . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.5. Handling of Unknown Locator Provided by Application . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
12. Security Considerations . . . . . . . . . . . . . . . . . . . 26
13. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 27
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
15.1. Normative References . . . . . . . . . . . . . . . . . . . 27
15.2. Informative References . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Introduction
This document specifies a socket API for the multihoming shim layer.
The API aims to enable interactions between application and the
multihoming shim layer for advanced locator management and for
accessing to information about failure detection and path
exploration.
This document is based on an assumption that a multhomed host is
equipped with a 'shim' layer which essentially maintains mapping
between identifiers and locators at the IP layer. SHIM6 and HIP are
examples of the shim. Hence, the API can be commonly used by SHIM6
and HIP.
We suggest that the ID/Locator adaptation is done only once inside
the network stack. In other words, if there exist multiple shim
sublayers at the IP layer, any one of them should be exclusively
applied for a given flow.
We try to make this document be in line with Posix standard [POSIX]
as much as possible. And the API 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).
2. Target
The primary target reader of this document is application programmers
who develop application software which may run on top of a multihomed
environment. In particular, the API should be beneficial for
application development of the software which takes advantage of
multihomed environment aiming to achieve better failover.
Secondly, this document should be of interest for the developers of a
given protocol stack for the shim layer (e.g. SHIM6 and HIP). This
is because this document specifies what kinds of information exchange
should be possible between the applications and the shim layer.
3. Terminology
This document does not intend to give new definitions for technical
terms that are relevant to multihomed environments but tries to
inherit definitions provided in the existing documents as listed
below:
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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]
For clarification, we provide definition for the terms that are
frequently used in this document:
o Endpoint Identifier (EID) - An identifier used by the application
to specify an endpoint of the communication. As addressed in
[I-D.ietf-shim6-app-refer], application may handle and EID in
various ways in different types of communication models such as
long-lived connections, callbacks, and referrals.
* In case of SHIM6, the EID is called ULID. The ULID is chosen
from one of the locators available on the host.
* In case of HIP, the EID is essentially a public key of the
host. In order to preserve backward compatibility with legacy
applications, a hash of public key called Host Identity Tag
(HIT) is used by the applications as a handle for the EID.
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 that appears on wire. Normally, a
locator is assigned to the network interface of the host. And the
IP packet destined to a given locator is delivered to the
correspondent network interface by the routing system.
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 locator at a time when the host is multihomed.
It should be noted that the term 'shim' does not refer to a
specific protocol but refers to the generic concept of a layer
that enables the mapping between identifiers and locators. SHIM6
and HIP are examples of the shim.
o Context - A state information shared by the peers, which
essentially stores a binding between the EIDs and associated
locators. The context is maintained at the shim layer of the
host.
o 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).
o Preferred Locator - The (source/destination) locator currently
used to send packets. As defined in [I-D.ietf-shim6-proto], the
preferred locator of a host which EID is 'A' can be denoted as
Lp(A).
o Reachability Detection - A procedure to detect reachability
between a given locator pair.
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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 meaning that the reachability among a given
locator pair is lost. The outage could be caused by any kind of
problems inside the routing infrastructure and/or problems of the
network interfaces of the end hosts.
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 REAP - A protocol for detecting failure and exploring reachability
in a multihomed environment. REAP is defined in[I-D.ietf-shim6-
failure-detection].
o Endpoint Descriptor (ED) - The representation of endpoints is
hidden from the applications. ED is a "handle" or "pointer" to
the actual EID.
4. System Overview
+------------------------+
| Application |
+------------------------+
^ ^
~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~
| v
+-----------|------------------------------+
| | Transport Layer |
+-----------|------------------------------+
^ |
+-------------|-----|-------------------------------------+
| v v |
| +-----------------------------+ +----------+ | IP
| | Shim |<----->| REAP | | Layer
| +-----------------------------+ +----------+ |
| ^ ^ |
+-----------------------|----------------------|----------+
v v
+------------------------------------------+
| Link Layer |
+------------------------------------------+
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Figure 1: System overview
Figure 1 illustrates the system overview. The application can use
the socket API to interact with the shim layer and the transport
layer for better control of locator management, failure detection and
path exploration.
Inside the IP layer, there is the shim which closely interacts with
REAP component. There could be interactions between the shim and the
transport layer, however they are outside of scope of this document.
The scope of this document is an interface from the application to
the shim layer, which is enabled via the socket interface.
5. Requirements
The list of requirements from the application perspective is the
following. These requirements are mainly identified during the
discussions on SHIM6 WG mailing list. Some requirements are derived
from Reachability Protocol document[I-D.ietf-shim6-failure-
detection].
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 addressed in section 4.3 and section 5.2 of REAP
specification]:
* 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.
* 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 receives 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
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procedure of path exploration.
o Feedback from application to shim layer. The application should
be able to inform the shim layer about 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
the 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.
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 the
target locator is IPv4 or IPv6, the application should be able to
use common interface to access the locator information.
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6. Socket Options for Multihomed 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() and/or setsockopt() system calls
for an opened 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 indicates whether if 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 data structure specified with the socket option, which can
store an argument for setsockopt() and result for getsockopt(). By
default, the data structure 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 | Request the | int |
| | | | shim layer to | |
| | | | prepare a | |
| | | | hot-standby | |
| | | | connection (in | |
| | | | addition to the | |
| | | | current path). | |
| SHIM_LOC_LOCAL_PREF | o | o | Get or set the | *1 |
| | | | preferred | |
| | | | locator on the | |
| | | | local side for | |
| | | | the context | |
| | | | associated with | |
| | | | the socket. | |
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| 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 | Request for the | int |
| | | | destination | |
| | | | locator of the | |
| | | | received IP | |
| | | | packet. | |
| SHIM_LOC_PEER_RECV | | o | Request for the | int |
| | | | source locator | |
| | | | of the received | |
| | | | IP packet. | |
| SHIM_LOCLIST_LOCAL | o | o | Get or set a | *1 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the local EID. | |
| SHIM_LOCLIST_PEER | o | o | Get or set a | *1 |
| | | | list of | |
| | | | locators | |
| | | | associated with | |
| | | | the peer's EID. | |
| SHIM_APP_TIMEOUT | | o | Inform the shim | int |
| | | | layer about a | |
| | | | timeout value | |
| | | | for detecting | |
| | | | failure. | |
| SHIM_FEEDBACK_POSITIVE | | o | Provide a | int |
| | | | positive | |
| | | | feedback to the | |
| | | | shim layer. | |
| SHIM_FEEDBACK_NEGATIVE | | o | Provide a | *2 |
| | | | negative | |
| | | | feedback to the | |
| | | | shim layer. | |
| SHIM_PATHEXPLORE | | o | Specify how | *3 |
| | | | path | |
| | | | exploration | |
| | | | should be | |
| | | | performed in | |
| | | | case of | |
| | | | failure. | |
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| SHIM_CONTEXT_DEFERRED_SETUP | | o | Specify if the | int |
| | | | context setup | |
| | | | can be deferred | |
| | | | or not. | |
| SHIM_IF_RECV | | o | Request for a | int |
| | | | receiving | |
| | | | interface. | |
| SHIM_IF_SEND | | o | Request for an | int |
| | | | outgoing | |
| | | | interface. | |
+-----------------------------+-----+-----+-----------------+-------+
Table 1: Shim specific socket options for getsockopt() and
setsockopt()
*1: Pointer to the buffer which stores arrays of locator information.
The buffer is actually the chained list of addrinfo structure.
*2: TBD.
*3: TBD.
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) nor any transport protocol
(SOCK_STREAM or SOCK_DGRAM or SOCK_RAW).
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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
6.1. SHIM_ASSOCIATED
This option can be specified by getsockopt() to check if the socket
is associated with a shim context or not. Thus, the option is read-
only and the result (0 or 1) is set in optval. A returned value 1
means that the socket is associated with a given shim context at the
shim layer, while a return value 0 indicates that there is no context
associated with the socket.
This option is particularly meaningful in a case where locator
information of the received IP packet is not enough for identifying
if the ID/Locator adaptation is performed or not. Note that the EID
pair and locator pair maybe identical in some case.
ISSUE: Should we limit this option only for 'connected' socket ?
6.2. SHIM_DONTSHIM
This option can be specified either by getsockopt() or setsockopt().
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The application can specify the option by setsockopt() taking the
argument optval with value 1 to request the shim layer not to apply
any multihoming support for the communication. The application can
also obtain the current setting by specifying the the socket option
in getsockopt(). The result should be binary (0 or 1).
By default, the value is set to 0, meaning that the shim layer will
try to apply ID/Locator adaptation for the communication over a given
socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.3. SHIM_HOT_STANDBY
This option can be specified by setsockopt().
By setting 1 in the optval for the setsockopt(), the application can
request the shim layer to use a hot-standby connection. The hot-
standby connection is another working locator pair than the current
locator pair.
By default, the value is set to 0, meaning that hot-standby
connection is disabled.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.4. SHIM_PATHEXPLORE
This option can be specified either by setsockopt() or getsockopt().
The value specified by the option indicates how aggressive the
application wants path exploration to be performed in case of
failure. Therefore, this option is effective only when there is
associated shim context for the socket.
The information to be provided by this socket option should contain:
suggested number of attempts for path exploration, frequency of the
path exploration, and so on. Need further discussions.
The data type for the argument optval is TBD.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.5. SHIM_LOC_LOCAL_PREF
This option can be specified either by setsockopt() or getsockopt().
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When specified by setsockopt(), the preferred locator on local side
is explicitly given to the shim layer. The shim layer shall
accordingly update the preferred locator of the context associated
with the socket.
When specified by getsockopt(), the preferred locator on local side
is returned by the shim layer.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.6. SHIM_LOC_PEER_PREF
This option can be specified either by setsockopt() or getsockopt().
When specified by setsockopt(), the preferred locator on remote side
is explicitly given to the shim layer. The shim layer shall
accordingly update the preferred locator of the context associated
with the socket.
When specified by getsockopt(), the preferred locator on remote side
is returned by the shim layer.
An error ENOSHIMCONTEXT 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.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.7. SHIM_LOC_LOCAL_RECV
This option can be specified by setsockopt().
When specified by setsockopt(), the shim layer stores the destination
locator of the received IP packet in an ancillary data object which
can be accessed by recvmsg(). The argument optval value should be
set to 1.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
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effective until it is deactivated (sticky option).
6.8. SHIM_LOC_PEER_RECV
This option can be specified by setsockopt().
When specified by setsockopt(), the shim layer stores the source
locator of the received IP packet in an ancillary data object which
can be accessed by recvmsg(). The argument optval value should be
set to 1.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.9. SHIM_LOCLIST_LOCAL
This option can be specified either by getsockopt() or setsockopt().
When specified by setsockopt(), the application provides a list of
locators which is associated with the local EID to the shim layer.
Accordingly, the shim layer shall update the list of locators
Ls(local). The argument optval should contain a pointer to the
buffer in which a list of locators are stored. See Section 8 for
detail.
When specified by getsockopt(), the application obtains a list of
locators which is associated with the local EID.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.10. SHIM_LOCLIST_PEER
This option can be specified either by getsockopt() or setsockopt().
When specified by setsockopt(), the application provides a list of
locators which is associated with the remote EID to the shim layer.
Accordingly, the shim layer shall update the list of locators
Ls(remote). The argument optval should contain a pointer to the
buffer in which a list of locators are stored. See Section
Section 8.1for detail.
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When specified by getsockopt(), the application obtains a list of
locators which is associated with the remote EID.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.11. SHIM_APP_TIMEOUT
This option can be specified by setsockopt().
The application can inform the shim layer about the timeout value for
detecting failure. The argument optval should contain the timeout
value in seconds. Accordingly, the shim layer shall update the
strategy for reachability test. Especially, this is efficient in a
case where the informed timeout value is shorter than the interval of
keepalive. In such case, keepalives to be performed by REAP may be
suppressed.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.12. SHIM_FEEDBACK_POSITIVE
This option can be specified by setsockopt().
The application can simply inform the shim layer that its
communication is going well. The argument optval should be set to 1.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
6.13. SHIM_FEEDBACK_NEGATIVE
This option can be specified by setsockopt().
The application can inform the shim layer that its communication is
not going well. The argument optval should be TBD.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
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6.14. SHIM_DEFERRED_CONTEXT_SETUP
This option can be specified by setsockopt().
When specified by the setsockopt(), optval should be set to 1 if the
context setup can be deferred. Otherwise, the context setup is
invoked immediately when there is no shim context setup for the flow.
By default, the value is set to 1.
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 establishment should be made prior
to communication.
6.15. SHIM_IF_RECV
This option can be specified by setsockopt().
The application can request the shim layer to provide information
about interface from which the packet was received. Once the socket
option is successfully set, the interface information can be obtained
by recvmsg() from the ancillary data. The argument optval should be
set to 1.
An error ENOSHIMCONTEXT will be returned when there is no context
associated with the socket.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.16. SHIM_IF_SEND
This option can be specified by setsockopt().
The application can specify outgoing interface of the outbound
traffic over the socket. Application should specify the requested
interface identifier in the argument optval. Alternatively, this
option can also be specified in ancillary data in along with
sendmsg() call.
Once the socket option is specified by setsockopt(), it remains
effective until it is deactivated (sticky option).
6.17. Error Handling
If successful, getsockopt() and setsockopt() return 0; otherwise, the
functions return -1 and set errno to indicate error.
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Following are errno codes newly defined for some shim specific socket
options indicating that the getsockopt() or setsockopt() finished
incompletely:
ENOSHIIMCONTEXT
There is no shim context associated with the socket.
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 prior
sending an IP packet to the peer's new address; 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].
7. Access to Locator Information
In this section, the way how to access locator information with some
I/O calls is presented. As defined in Posix, sendmsg() and recvmsg()
take msghdr structure as its argument and they can additionally
handle control information in along with data. Figure 3 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 are stored.
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
ISSUE: Should we introduce a new flag for msg_flags (e.g.
MSG_SHIMMED) ? Following the practice, it seems reasonable to do so,
but not sure how much it is useful.
The buffer pointed from the msg_control member of the msghdr
structure should contain 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 a data which stores
a single locator.
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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_IF_RECV | | o | int |
| SHIM_IF_SEND | o | | int |
+---------------------+-----------+-----------+-------------+
Table 2: Shim specific ancillary data
*1: cmsg_data[] should include padding (if necessary) and a single
sockaddr_storage{} a protocol independent placeholder for socket
addresses.
ISSUE: Is the design choice (to use sockaddr_storage{}) reasonable ?
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.
7.1. 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.
In addition, the application can get the receiving interface from the
ancillary data marked with SHIM_IF_RECV. The ancillary data should
contain an interface identifier of the physical interface which was
actually used to receive the packet.
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7.2. 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 an
interface identifier of the physical interface over which the
application expect the packet to be transmitted.
Note that the effect is limited to the datagram transmitted by the
sendmsg(). If the specified locator pair seem 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.
ISSUE: Is there any other type of error that we should specifically
handle ?
8. Data Structures
Some of the socket options defined in this document requires specific
data structures for exchanging information. Those data structures
are illustrated in this section.
8.1. Placeholder for Locator Information
Some of the socket options defined in this document handle locator
information. Locator information could be a single locator or an
array of locators. An important requirement is that the locator
information should be handled in a protocol independent manner. In
other words, an interface to the locator information should not be
dependent to any address family.
8.1.1. addrinfo structure
addrinfo structure in along with getaddrinfo() function are defined
in Posix, which is useful for programming applications in protocol
independent manner. As mentioned earlier, protocol independency is
required for the locator management at the shim layer, thus we
propose to use addrinfo structure as a placeholder for locators.
A chain of addrinfo structures can be used to represent a list of
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locators. Note that addrinfo structure itself does not contain the
locator data but it holds a pointer to sockaddr structure where the
actual data of a given locator is stored. Figure 4 illustrates the
addrinfo structure defined in <netdb.h>.
struct addrinfo {
int ai_flags; /* input flags */
int ai_family; /* protocol family for socket */
int ai_socktype; /* socket type */
int ai_protocol; /* protocol for socket */
socklen_t ai_addrlen; /* length of socket-address */
struct sockaddr *ai_addr; /* socket-address for socket */
char *ai_canonname; /* canonical name for
service location */
struct addrinfo *ai_next; /* pointer to next in list */
};
Figure 4: addrinfo structure
8.1.2. sockaddr_storage structure
[RFC3493] specifies a protocol independent placeholder for socket
address, called sockaddr_storage structure as shown in Figure 5. By
definition, the structure can store socket address of any protocol
(IPv4 or IPv6) and is simply suitable for a placeholder for the
locator information. In this document, we suggest to use
sockaddr_storage structure to store the locator information to be
specified in the ancillary data. In those cases, the locator
information is a single locator.
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/*
* Desired design of maximum size and alignment
*/
#define _SS_MAXSIZE 128 /* Implementation specific max size */
#define _SS_ALIGNSIZE (sizeof (int64_t))
/* Implementation specific desired alignment */
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE - sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t) +
_SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
sa_family_t ss_family; /* address family */
/* Following fields are implementation specific */
char __ss_pad1[_SS_PAD1SIZE];
int64_t __ss_align;
char __ss_pad2[_SS_PAD2SIZE];
};
Figure 5: sockaddr_storage structure
9. Implications for Existing Socket API Extensions
As the socket options proposed in this document allow the application
to specify the locators for transmitting IP packet, there may be
conflict with some of the existing socket API. As stated in
Section 6, a basic assumption is that the legacy API should continue
to work above the shim layer.
In IPv4, application can obtain the destination IP address of the
received IP packet (IP_RECVDSTADDR) as well as the receiving
interface (IP_RECVIF). If the shim layer performs ID/Locator
adaptation for the received packet, the destination EID should be
stored in the ancillary data (IP_RECVDSTADDR). Accordingly, the
receiving interface should be aligned with the destination EID of the
packet. That is, the shim layer should set appropriate interface to
which the destination EID is assigned in the ancillary data object.
However, from the application perspective, knowing the receiving
interface which is associated with the destination EID may not be
useful, especially in the case where application is particularly
interested in the characteristics of the receiving interface. Hence,
we suggest application programmer to use SHIM_IF_RECV instead of
IP_RECVIF in such case.
In IPv6, [RFC3542] defines that IPV6_PKTINFO can be used to specify
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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. Now, similar to the case of
IPv4, the shim layer may affect the behavior of socket API which
deals with IPV6_PKFINFO. We again would like note that 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. Hence we recommend application programmers to use shim
specific socket options (SHIM_IF_RECV or SHIM_IF_SEND) if the
interest in the communicating interface comes from lower level (e.g.
characteristics of physical interface). For the same reason, in
order to handle locator information, we suggest to use shim specific
socket options defined in Section 7.
In summary, a care should be taken in potential conflict with
existing socket API which treats the IP address as a locator rather
than identifier. Basic assumption is that the existing socket API
works above the shim layer.
10. Discussion
In this section, open discussion issues are noted.
10.1. 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 X, 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
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this way.
Further discussions are needed how the shim layer can accommodate
feedbacks from multiple applications within a same context.
10.2. Issues with Shim Unaware Application
In multihomed environment where either or both of the peers have
multiple locators, there are some issues with shim unaware
application which uses legacy socket API.
10.2.1. 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().
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().
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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
legacy applications as described in [I-D.henderson-hip-applications].
10.2.2. 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.
10.3. 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.
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10.4. Issues of Header Conversion among Different Address Family
The shim layer performs ID/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
error something like ENOSUITABLELOCATOR ?
10.5. 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.
11. IANA Considerations
This document contains no IANA consideration.
12. 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.
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13. 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 the application that takes full
advantage of multihomed environment. From architectural perspective,
the socket API aims to enhance software development environment in
terms of support for separation of identifier and locator. That is,
with new API, application can handle identifier and locator
separately still being allowed to use legacy socket API.
Shim specific socket options can be used by getsockopt() and/or
setcokopt() system calls, which allows applications to get
information about locator management. Additionally, applications can
specify locator information for outgoing packet and get locator
information from incoming packet by using ancillary data.
14. Acknowledgments
Jari Arkko participated in the discussion that lead to the first
version of this document.
15. References
15.1. 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-03 (work in progress),
December 2005.
[I-D.ietf-shim6-proto]
Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
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protocol", draft-ietf-shim6-proto-03 (work in progress),
December 2005.
[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.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
15.2. 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-01 (work in progress), October 2005.
[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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