Network Working Group S. Deering
Request for Comments: 1112 Stanford University
Obsoletes: RFCs 988, 1054 August 1989
Host Extensions for IP Multicasting
1. STATUS OF THIS MEMO
This memo specifies the extensions required of a host implementation
of the Internet Protocol (IP) to support multicasting. It is the
recommended standard for IP multicasting in the Internet.
Distribution of this memo is unlimited.
2. INTRODUCTION
IP multicasting is the transmission of an IP datagram to a "host
group", a set of zero or more hosts identified by a single IP
destination address. A multicast datagram is delivered to all
members of its destination host group with the same "best-efforts"
reliability as regular unicast IP datagrams, i.e., the datagram is
not guaranteed to arrive intact at all members of the destination
group or in the same order relative to other datagrams.
The membership of a host group is dynamic; that is, hosts may join
and leave groups at any time. There is no restriction on the
location or number of members in a host group. A host may be a
member of more than one group at a time. A host need not be a member
of a group to send datagrams to it.
A host group may be permanent or transient. A permanent group has a
well-known, administratively assigned IP address. It is the address,
not the membership of the group, that is permanent; at any time a
permanent group may have any number of members, even zero. Those IP
multicast addresses that are not reserved for permanent groups are
available for dynamic assignment to transient groups which exist only
as long as they have members.
Internetwork forwarding of IP multicast datagrams is handled by
"multicast routers" which may be co-resident with, or separate from,
internet gateways. A host transmits an IP multicast datagram as a
local network multicast which reaches all immediately-neighboring
members of the destination host group. If the datagram has an IP
time-to-live greater than 1, the multicast router(s) attached to the
local network take responsibility for forwarding it towards all other
networks that have members of the destination group. On those other
member networks that are reachable within the IP time-to-live, an
attached multicast router completes delivery by transmitting the
datagram as a local multicast.
This memo specifies the extensions required of a host IP
implementation to support IP multicasting, where a "host" is any
internet host or gateway other than those acting as multicast
routers. The algorithms and protocols used within and between
multicast routers are transparent to hosts and will be specified in
separate documents. This memo also does not specify how local
network multicasting is accomplished for all types of network,
although it does specify the required service interface to an
arbitrary local network and gives an Ethernet specification as an
example. Specifications for other types of network will be the
subject of future memos.
3. LEVELS OF CONFORMANCE
There are three levels of conformance to this specification:
Level 0: no support for IP multicasting.
There is, at this time, no requirement that all IP implementations
support IP multicasting. Level 0 hosts will, in general, be
unaffected by multicast activity. The only exception arises on some
types of local network, where the presence of level 1 or 2 hosts may
cause misdelivery of multicast IP datagrams to level 0 hosts. Such
datagrams can easily be identified by the presence of a class D IP
address in their destination address field; they should be quietly
discarded by hosts that do not support IP multicasting. Class D
addresses are described in section 4 of this memo.
Level 1: support for sending but not receiving multicast IP
datagrams.
Level 1 allows a host to partake of some multicast-based services,
such as resource location or status reporting, but it does not allow
a host to join any host groups. An IP implementation may be upgraded
from level 0 to level 1 very easily and with little new code. Only
sections 4, 5, and 6 of this memo are applicable to level 1
implementations.
Level 2: full support for IP multicasting.
Level 2 allows a host to join and leave host groups, as well as send
IP datagrams to host groups. It requires implementation of the
Internet Group Management Protocol (IGMP) and extension of the IP and
local network service interfaces within the host. All of the
following sections of this memo are applicable to level 2
implementations.
4. HOST GROUP ADDRESSES
Host groups are identified by class D IP addresses, i.e., those with
"1110" as their high-order four bits. Class E IP addresses, i.e.,
those with "1111" as their high-order four bits, are reserved for
future addressing modes.
In Internet standard "dotted decimal" notation, host group addresses
range from 224.0.0.0 to 239.255.255.255. The address 224.0.0.0 is
guaranteed not to be assigned to any group, and 224.0.0.1 is assigned
to the permanent group of all IP hosts (including gateways). This is
used to address all multicast hosts on the directly connected
network. There is no multicast address (or any other IP address) for
all hosts on the total Internet. The addresses of other well-known,
permanent groups are to be published in "Assigned Numbers".
Appendix II contains some background discussion of several issues
related to host group addresses.
5. MODEL OF A HOST IP IMPLEMENTATION
The multicast extensions to a host IP implementation are specified in
terms of the layered model illustrated below. In this model, ICMP
and (for level 2 hosts) IGMP are considered to be implemented within
the IP module, and the mapping of IP addresses to local network
addresses is considered to be the responsibility of local network
modules. This model is for eXPository purposes only, and should not
be construed as constraining an actual implementation.
Upper-Layer Protocol Modules
__________________________________________________________
--------------------- IP Service Interface -----------------------
__________________________________________________________
ICMP IGMP
IP ____________________________
Module
__________________________________________________________
---------------- Local Network Service Interface -----------------
__________________________________________________________
Local IP-to-local address mapping
Network (e.g., ARP)
Modules _____________________________
(e.g., Ethernet)
To provide level 1 multicasting, a host IP implementation must
support the transmission of multicast IP datagrams. To provide level
2 multicasting, a host must also support the reception of multicast
IP datagrams. Each of these two new services is described in a
separate section, below. For each service, extensions are specified
for the IP service interface, the IP module, the local network
service interface, and an Ethernet local network module. Extensions
to local network modules other than Ethernet are mentioned briefly,
but are not specified in detail.
6. SENDING MULTICAST IP DATAGRAMS
6.1. Extensions to the IP Service Interface
Multicast IP datagrams are sent using the same "Send IP" operation
used to send unicast IP datagrams; an upper-layer protocol module
merely specifies an IP host group address, rather than an individual
IP address, as the destination. However, a number of extensions may
be necessary or desirable.
First, the service interface should provide a way for the upper-layer
protocol to specify the IP time-to-live of an outgoing multicast
datagram, if such a capability does not already exist. If the
upper-layer protocol chooses not to specify a time-to-live, it should
default to 1 for all multicast IP datagrams, so that an explicit
choice is required to multicast beyond a single network.
Second, for hosts that may be attached to more than one network, the
service interface should provide a way for the upper-layer protocol
to identify which network interface is be used for the multicast
transmission. Only one interface is used for the initial
transmission; multicast routers are responsible for forwarding to any
other networks, if necessary. If the upper-layer protocol chooses
not to identify an outgoing interface, a default interface should be
used, preferably under the control of system management.
Third (level 2 implementations only), for the case in which the host
is itself a member of a group to which a datagram is being sent, the
service interface should provide a way for the upper-layer protocol
to inhibit local delivery of the datagram; by default, a copy of the
datagram is looped back. This is a performance optimization for
upper-layer protocols that restrict the membership of a group to one
process per host (such as a routing protocol), or that handle
loopback of group communication at a higher layer (such as a
multicast transport protocol).
6.2. Extensions to the IP Module
To support the sending of multicast IP datagrams, the IP module must
be extended to recognize IP host group addresses when routing
outgoing datagrams. Most IP implementations include the following
logic:
if IP-destination is on the same local network,
send datagram locally to IP-destination
else
send datagram locally to GatewayTo( IP-destination )
To allow multicast transmissions, the routing logic must be changed
to:
if IP-destination is on the same local network
or IP-destination is a host group,
send datagram locally to IP-destination
else
send datagram locally to GatewayTo( IP-destination )
If the sending host is itself a member of the destination group on
the outgoing interface, a copy of the outgoing datagram must be
looped-back for local delivery, unless inhibited by the sender.
(Level 2 implementations only.)
The IP source address of the outgoing datagram must be one of the
individual addresses corresponding to the outgoing interface.
A host group address must never be placed in the source address field
or anywhere in a source route or record route option of an outgoing
IP datagram.
6.3. Extensions to the Local Network Service Interface
No change to the local network service interface is required to
support the sending of multicast IP datagrams. The IP module merely
specifies an IP host group destination, rather than an individual IP
destination, when it invokes the existing "Send Local" operation.
6.4. Extensions to an Ethernet Local Network Module
The Ethernet directly supports the sending of local multicast packets
by allowing multicast addresses in the destination field of Ethernet
packets. All that is needed to support the sending of multicast IP
datagrams is a procedure for mapping IP host group addresses to
Ethernet multicast addresses.
An IP host group address is mapped to an Ethernet multicast address
by placing the low-order 23-bits of the IP address into the low-order
23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
Because there are 28 significant bits in an IP host group address,
more than one host group address may map to the same Ethernet
multicast address.
6.5. Extensions to Local Network Modules other than Ethernet
Other networks that directly support multicasting, such as rings or
buses conforming to the IEEE 802.2 standard, may be handled the same
way as Ethernet for the purpose of sending multicast IP datagrams.
For a network that supports broadcast but not multicast, such as the
Experimental Ethernet, all IP host group addresses may be mapped to a
single local broadcast address (at the cost of increased overhead on
all local hosts). For a point-to-point link joining two hosts (or a
host and a multicast router), multicasts should be transmitted
exactly like unicasts. For a store-and-forward network like the
ARPANET or a public X.25 network, all IP host group addresses might
be mapped to the well-known local address of an IP multicast router;
a router on such a network would take responsibility for completing
multicast delivery within the network as well as among networks.
7. RECEIVING MULTICAST IP DATAGRAMS
7.1. Extensions to the IP Service Interface
Incoming multicast IP datagrams are received by upper-layer protocol
modules using the same "Receive IP" operation as normal, unicast
datagrams. Selection of a destination upper-layer protocol is based
on the protocol field in the IP header, regardless of the destination
IP address. However, before any datagrams destined to a particular
group can be received, an upper-layer protocol must ask the IP module
to join that group. Thus, the IP service interface must be extended
to provide two new operations:
JoinHostGroup ( group-address, interface )
LeaveHostGroup ( group-address, interface )
The JoinHostGroup operation requests that this host become a member
of the host group identified by "group-address" on the given network
interface. The LeaveGroup operation requests that this host give up
its membership in the host group identified by "group-address" on the
given network interface. The interface argument may be omitted on
hosts that support only one interface. For hosts that may be
attached to more than one network, the upper-layer protocol may
choose to leave the interface unspecified, in which case the request
will apply to the default interface for sending multicast datagrams
(see section 6.1).
It is permissible to join the same group on more than one interface,
in which case duplicate multicast datagrams may be received. It is
also permissible for more than one upper-layer protocol to request
membership in the same group.
Both operations should return immediately (i.e., they are non-
blocking operations), indicating success or failure. Either
operation may fail due to an invalid group address or interface
identifier. JoinHostGroup may fail due to lack of local resources.
LeaveHostGroup may fail because the host does not belong to the given
group on the given interface. LeaveHostGroup may succeed, but the
membership persist, if more than one upper-layer protocol has
requested membership in the same group.
7.2. Extensions to the IP Module
To support the reception of multicast IP datagrams, the IP module
must be extended to maintain a list of host group memberships
associated with each network interface. An incoming datagram
destined to one of those groups is processed exactly the same way as
datagrams destined to one of the host's individual addresses.
Incoming datagrams destined to groups to which the host does not
belong are discarded without generating any error report or log
entry. On hosts with more than one network interface, if a datagram
arrives via one interface, destined for a group to which the host
belongs only on a different interface, the datagram is quietly
discarded. (These cases should occur only as a result of inadequate
multicast address filtering in a local network module.)
An incoming datagram is not rejected for having an IP time-to-live of
1 (i.e., the time-to-live should not automatically be decremented on
arriving datagrams that are not being forwarded). An incoming
datagram with an IP host group address in its source address field is
quietly discarded. An ICMP error message (Destination Unreachable,
Time Exceeded, Parameter Problem, Source Quench, or Redirect) is
never generated in response to a datagram destined to an IP host
group.
The list of host group memberships is updated in response to
JoinHostGroup and LeaveHostGroup requests from upper-layer protocols.
Each membership should have an associated reference count or similar
mechanism to handle multiple requests to join and leave the same
group. On the first request to join and the last request to leave a
group on a given interface, the local network module for that
interface is notified, so that it may update its multicast reception
filter (see section 7.3).
The IP module must also be extended to implement the IGMP protocol,
specified in Appendix I. IGMP is used to keep neighboring multicast
routers informed of the host group memberships present on a
particular local network. To support IGMP, every level 2 host must
join the "all-hosts" group (address 224.0.0.1) on each network
interface at initialization time and must remain a member for as long
as the host is active.
(Datagrams addressed to the all-hosts group are recognized as a
special case by the multicast routers and are never forwarded beyond
a single network, regardless of their time-to-live. Thus, the all-
hosts address may not be used as an internet-wide broadcast address.
For the purpose of IGMP, membership in the all-hosts group is really
necessary only while the host belongs to at least one other group.
However, it is specified that the host shall remain a member of the
all-hosts group at all times because (1) it is simpler, (2) the
frequency of reception of unnecessary IGMP queries should be low
enough that overhead is negligible, and (3) the all-hosts address may
serve other routing-oriented purposes, such as advertising the
presence of gateways or resolving local addresses.)
7.3. Extensions to the Local Network Service Interface
Incoming local network multicast packets are delivered to the IP
module using the same "Receive Local" operation as local network
unicast packets. To allow the IP module to tell the local network
module which multicast packets to accept, the local network service
interface is extended to provide two new operations:
JoinLocalGroup ( group-address )
LeaveLocalGroup ( group-address )
where "group-address" is an IP host group address. The
JoinLocalGroup operation requests the local network module to accept
and deliver up subsequently arriving packets destined to the given IP
host group address. The LeaveLocalGroup operation requests the local
network module to stop delivering up packets destined to the given IP
host group address. The local network module is expected to map the
IP host group addresses to local network addresses as required to
update its multicast reception filter. Any local network module is
free to ignore LeaveLocalGroup requests, and may deliver up packets
destined to more addresses than just those specified in
JoinLocalGroup requests, if it is unable to filter incoming packets
adequately.
The local network module must not deliver up any multicast packets
that were transmitted from that module; loopback of multicasts is
handled at the IP layer or higher.
7.4. Extensions to an Ethernet Local Network Module
To support the reception of multicast IP datagrams, an Ethernet
module must be able to receive packets addressed to the Ethernet
multicast addresses that correspond to the host's IP host group
addresses. It is highly desirable to take advantage of any address
filtering capabilities that the Ethernet hardware interface may have,
so that the host receives only those packets that are destined to it.
Unfortunately, many current Ethernet interfaces have a small limit on
the number of addresses that the hardware can be configured to
recognize. Nevertheless, an implementation must be capable of
listening on an arbitrary number of Ethernet multicast addresses,
which may mean "opening up" the address filter to accept all
multicast packets during those periods when the number of addresses
exceeds the limit of the filter.
For interfaces with inadequate hardware address filtering, it may be
desirable (for performance reasons) to perform Ethernet address
filtering within the software of the Ethernet module. This is not
mandatory, however, because the IP module performs its own filtering
based on IP destination addresses.
7.5. Extensions to Local Network Modules other than Ethernet
Other multicast networks, such as IEEE 802.2 networks, can be handled
the same way as Ethernet for the purpose of receiving multicast IP
datagrams. For pure broadcast networks, such as the Experimental
Ethernet, all incoming broadcast packets can be accepted and passed
to the IP module for IP-level filtering. On point-to-point or
store-and-forward networks, multicast IP datagrams will arrive as
local network unicasts, so no change to the local network module
should be necessary.
APPENDIX I. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)
The Internet Group Management Protocol (IGMP) is used by IP hosts to
report their host group memberships to any immediately-neighboring
multicast routers. IGMP is an asymmetric protocol and is specified
here from the point of view of a host, rather than a multicast
router. (IGMP may also be used, symmetrically or asymmetrically,
between multicast routers. Such use is not specified here.)
Like ICMP, IGMP is a integral part of IP. It is required to be
implemented by all hosts conforming to level 2 of the IP multicasting
specification. IGMP messages are encapsulated in IP datagrams, with
an IP protocol number of 2. All IGMP messages of concern to hosts
have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Type Unused Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Group Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version
This memo specifies version 1 of IGMP. Version 0 is specified
in RFC-988 and is now obsolete.
Type
There are two types of IGMP message of concern to hosts:
1 = Host Membership Query
2 = Host Membership Report
Unused
Unused field, zeroed when sent, ignored when received.
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the 8-octet IGMP message. For computing
the checksum, the checksum field is zeroed.
Group Address
In a Host Membership Query message, the group address field
is zeroed when sent, ignored when received.
In a Host Membership Report message, the group address field
holds the IP host group address of the group being reported.
Informal Protocol Description
Multicast routers send Host Membership Query messages (hereinafter
called Queries) to discover which host groups have members on their
attached local networks. Queries are addressed to the all-hosts
group (address 224.0.0.1), and carry an IP time-to-live of 1.
Hosts respond to a Query by generating Host Membership Reports
(hereinafter called Reports), reporting each host group to which they
belong on the network interface from which the Query was received.
In order to avoid an "implosion" of concurrent Reports and to reduce
the total number of Reports transmitted, two techniques are used:
1. When a host receives a Query, rather than sending Reports
immediately, it starts a report delay timer for each of its
group memberships on the network interface of the incoming
Query. Each timer is set to a different, randomly-chosen
value between zero and D seconds. When a timer expires, a
Report is generated for the corresponding host group. Thus,
Reports are spread out over a D second interval instead of
all occurring at once.
2. A Report is sent with an IP destination address equal to the
host group address being reported, and with an IP
time-to-live of 1, so that other members of the same group on
the same network can overhear the Report. If a host hears a
Report for a group to which it belongs on that network, the
host stops its own timer for that group and does not generate
a Report for that group. Thus, in the normal case, only one
Report will be generated for each group present on the
network, by the member host whose delay timer expires first.
Note that the multicast routers receive all IP multicast
datagrams, and therefore need not be addressed explicitly.
Further note that the routers need not know which hosts
belong to a group, only that at least one host belongs to a
group on a particular network.
There are two exceptions to the behavior described above. First, if
a report delay timer is already running for a group membership when a
Query is received, that timer is not reset to a new random value, but
rather allowed to continue running with its current value. Second, a
report delay timer is never set for a host's membership in the all-
hosts group (224.0.0.1), and that membership is never reported.
If a host uses a pseudo-random number generator to compute the
reporting delays, one of the host's own individual IP address should
be used as part of the seed for the generator, to reduce the chance
of multiple hosts generating the same sequence of delays.
A host should confirm that a received Report has the same IP host
group address in its IP destination field and its IGMP group address
field, to ensure that the host's own Report is not cancelled by an
erroneous received Report. A host should quietly discard any IGMP
message of type other than Host Membership Query or Host Membership
Report.
Multicast routers send Queries periodically to refresh their
knowledge of memberships present on a particular network. If no
Reports are received for a particular group after some number of
Queries, the routers assume that that group has no local members and
that they need not forward remotely-originated multicasts for that
group onto the local network. Queries are normally sent infrequently
(no more than once a minute) so as to keep the IGMP overhead on hosts
and networks very low. However, when a multicast router starts up,
it may issue several closely-spaced Queries in order to build up its
knowledge of local memberships quickly.
When a host joins a new group, it should immediately transmit a
Report for that group, rather than waiting for a Query, in case it is
the first member of that group on the network. To cover the
possibility of the initial Report being lost or damaged, it is
recommended that it be repeated once or twice after short delays. (A
simple way to accomplish this is to act as if a Query had been
received for that group only, setting the group's random report delay
timer. The state transition diagram below illustrates this
approach.)
Note that, on a network with no multicast routers present, the only
IGMP traffic is the one or more Reports sent whenever a host joins a
new group.
State Transition Diagram
IGMP behavior is more formally specified by the state transition
diagram below. A host may be in one of three possible states, with
respect to any single IP host group on any single network interface:
- Non-Member state, when the host does not belong to the group
on the interface. This is the initial state for all
memberships on all network interfaces; it requires no storage
in the host.
- Delaying Member state, when the host belongs to the group on
the interface and has a report delay timer running for that
membership.
- Idle Member state, when the host belongs to the group on the
interface and does not have a report delay timer running for
that membership.
There are five significant events that can cause IGMP state
transitions:
- "join group" occurs when the host decides to join the group on
the interface. It may occur only in the Non-Member state.
- "leave group" occurs when the host decides to leave the group
on the interface. It may occur only in the Delaying Member
and Idle Member states.
- "query received" occurs when the host receives a valid IGMP
Host Membership Query message. To be valid, the Query message
must be at least 8 octets long, have a correct IGMP
checksum and have an IP destination address of 224.0.0.1.
A single Query applies to all memberships on the
interface from which the Query is received. It is ignored for
memberships in the Non-Member or Delaying Member state.
- "report received" occurs when the host receives a valid IGMP
Host Membership Report message. To be valid, the Report
message must be at least 8 octets long, have a correct IGMP
checksum, and contain the same IP host group address in its IP
destination field and its IGMP group address field. A Report
applies only to the membership in the group identified by the
Report, on the interface from which the Report is received.
It is ignored for memberships in the Non-Member or Idle Member
state.
- "timer expired" occurs when the report delay timer for the
group on the interface expires. It may occur only in the
Delaying Member state.
All other events, such as receiving invalid IGMP messages, or IGMP
messages other than Query or Report, are ignored in all states.
There are three possible actions that may be taken in response to the
above events:
- "send report" for the group on the interface.
- "start timer" for the group on the interface, using a random
delay value between 0 and D seconds.
- "stop timer" for the group on the interface.
In the following diagram, each state transition arc is labelled with
the event that causes the transition, and, in parentheses, any
actions taken during the transition.
________________
---------> Non-Member <---------
________________
leave group join group leave group
(stop timer) (send report,
start timer)
________________ ________________
<---------
<-------------------
query received
Delaying Member (start timer) Idle Member
------------------->
report received
(stop timer)
_________________------------------->_________________
timer expired
(send report)
The all-hosts group (address 224.0.0.1) is handled as a special case.
The host starts in Idle Member state for that group on every
interface, never transitions to another state, and never sends a
report for that group.
Protocol Parameters
The maximum report delay, D, is 10 seconds.
APPENDIX II. HOST GROUP ADDRESS ISSUES
This appendix is not part of the IP multicasting specification, but
provides background discussion of several issues related to IP host
group addresses.
Group Address Binding
The binding of IP host group addresses to physical hosts may be
considered a generalization of the binding of IP unicast addresses.
An IP unicast address is statically bound to a single local network
interface on a single IP network. An IP host group address is
dynamically bound to a set of local network interfaces on a set of IP
networks.
It is important to understand that an IP host group address is NOT
bound to a set of IP unicast addresses. The multicast routers do not
need to maintain a list of individual members of each host group.
For example, a multicast router attached to an Ethernet need
associate only a single Ethernet multicast address with each host
group having local members, rather than a list of the members'
individual IP or Ethernet addresses.
Allocation of Transient Host Group Addresses
This memo does not specify how transient group address are allocated.
It is anticipated that different portions of the IP transient host
group address space will be allocated using different techniques.
For example, there may be a number of servers that can be contacted
to acquire a new transient group address. Some higher-level
protocols (such as VMTP, specified in RFC-1045) may generate higher-
level transient "process group" or "entity group" addresses which are
then algorithmically mapped to a subset of the IP transient host
group addresses, similarly to the way that IP host group addresses
are mapped to Ethernet multicast addresses. A portion of the IP
group address space may be set aside for random allocation by
applications that can tolerate occasional collisions with other
multicast users, perhaps generating new addresses until a suitably
"quiet" one is found.
In general, a host cannot assume that datagrams sent to any host
group address will reach only the intended hosts, or that datagrams
received as a member of a transient host group are intended for the
recipient. Misdelivery must be detected at a level above IP, using
higher-level identifiers or authentication tokens. Information
transmitted to a host group address should be encrypted or governed
by administrative routing controls if the sender is concerned about
unwanted listeners.
Author's Address
Steve Deering
Stanford University
Computer Science Department
Stanford, CA 94305-2140
Phone: (415) 723-9427
EMail: deering@PESCADERO.STANFORD.EDU
