draft-ietf-udlr-lltunnel-06.txt   rfc3077.txt 
Network Working Group E. Duros Network Working Group E. Duros
Internet-Draft UDcast Request for Comments: 3077 UDcast
January 2000 W. Dabbous Category: Standards Track W. Dabbous
Expires July 2001 INRIA Sophia-Antipolis INRIA Sophia-Antipolis
H. Izumiyama H. Izumiyama
N. Fujii N. Fujii
WIDE WIDE
Y. Zhang Y. Zhang
HRL HRL
March 2001
A Link-Layer Tunneling Mechanism for Unidirectional Links A Link-Layer Tunneling Mechanism for Unidirectional Links
<draft-ietf-udlr-lltunnel-06.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document specifies an Internet standards track protocol for the
all provisions of Section 10 of RFC2026. Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
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RFC editor as a Proposed Standard for the Internet Community.
Abstract Abstract
This document describes a mechanism to emulate full bidirectional This document describes a mechanism to emulate full bidirectional
connectivity between all nodes that are directly connected by a connectivity between all nodes that are directly connected by a
unidirectional link. The "receiver" uses a link-layer tunneling unidirectional link. The "receiver" uses a link-layer tunneling
mechanism to forward datagrams to "feeds" over a separate mechanism to forward datagrams to "feeds" over a separate
bidirectional IP network. As it is implemented at the link-layer, bidirectional IP (Internet Protocol) network. As it is implemented
protocols in addition to IP may also be supported by this mechanism. at the link-layer, protocols in addition to IP may also be supported
by this mechanism.
1. Introduction 1. Introduction
Internet routing and upper layer protocols assume that links are Internet routing and upper layer protocols assume that links are
bidirectional, i.e., directly connected hosts can communicate with bidirectional, i.e., directly connected hosts can communicate with
each other over the same link. each other over the same link.
This document describes a link-layer tunneling mechanism that allows This document describes a link-layer tunneling mechanism that allows
a set of nodes (feeds and receivers, see Section 2 for terminology) a set of nodes (feeds and receivers, see Section 2 for terminology)
which are directly connected by a unidirectional link to send which are directly connected by a unidirectional link to send
datagrams as if they were all connected by a bidirectional link. We datagrams as if they were all connected by a bidirectional link. We
present a generic topology in section 3 with a tunneling mechanism present a generic topology in section 3 with a tunneling mechanism
that supports multiple feeds and receivers. Note, this mechanism is that supports multiple feeds and receivers. Note, this mechanism is
not designed for topologies where a pair of nodes are connected by 2 not designed for topologies where a pair of nodes are connected by 2
unidirectional links in opposite direction. unidirectional links in opposite direction.
The tunneling mechanism requires that all nodes have an additional The tunneling mechanism requires that all nodes have an additional
interface to an IP interconnected infrastructure. interface to an IP interconnected infrastructure.
The tunneling mechanism is implemented at the link-layer of the The tunneling mechanism is implemented at the link-layer of the
interface of every node connected to the unidirectional link. The aim interface of every node connected to the unidirectional link. The
is to hide from higher layers, i.e. the network layer and above, the aim is to hide from higher layers, i.e., the network layer and above,
unidirectional nature of the link. The tunneling mechanism also the unidirectional nature of the link. The tunneling mechanism also
includes an automatic tunnel configuration protocol that allows nodes includes an automatic tunnel configuration protocol that allows nodes
to come up/down at any time. to come up/down at any time.
Generic Routing Encapsulation [rfc2784] is suggested as the tunneling Generic Routing Encapsulation [RFC2784] is suggested as the tunneling
mechanism as it provides a means for carrying IP, ARP datagrams, and mechanism as it provides a means for carrying IP, ARP datagrams, and
any other layer-3 protocol between nodes. any other layer-3 protocol between nodes.
The tunneling mechanism described in this document was discussed and The tunneling mechanism described in this document was discussed and
agreed upon by the UDLR working group. agreed upon by the UDLR working group.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [rfc2119]. document, are to be interpreted as described in [RFC2119].
2. Terminology 2. Terminology
Unidirectional link (UDL): A one way transmission link, e.g. a Unidirectional link (UDL): A one way transmission link, e.g., a
broadcast satellite link. broadcast satellite link.
Receiver: A router or a host that has receive-only connectivity to a Receiver: A router or a host that has receive-only connectivity to a
UDL. UDL.
Send-only feed: A router that has send-only connectivity to a UDL. Send-only feed: A router that has send-only connectivity to a UDL.
Receive capable feed: A router that has send-and-receive connectivity Receive capable feed: A router that has send-and-receive connectivity
to a UDL. to a UDL.
Feed: A send-only or a receive capable feed. Feed: A send-only or a receive capable feed.
Node: A receiver or a feed. Node: A receiver or a feed.
Bidirectional interface: a typical communication interface that can Bidirectional interface: a typical communication interface that can
send or receive packets, such as an Ethernet card, a modem, etc. send or receive packets, such as an Ethernet card, a modem, etc.
3. Topology 3. Topology
Feeds and receivers are connected via a unidirectional link. Send- Feeds and receivers are connected via a unidirectional link. Send-
only feeds can only send data over this unidirectional link, and only feeds can only send data over this unidirectional link, and
receivers can only receive data from it. Receive capable feeds have receivers can only receive data from it. Receive capable feeds have
both send and receive capabilities. both send and receive capabilities.
This mechanism has been designed to work with any topology with any This mechanism has been designed to work with any topology with any
number of receivers and one or more feeds. However, it is expected number of receivers and one or more feeds. However, it is expected
that the number of feeds will be small. In particular, the special that the number of feeds will be small. In particular, the special
case of a single send-only feed and multiple receivers is among the case of a single send-only feed and multiple receivers is among the
topologies supported. topologies supported.
A receiver has several interfaces, a receive-only interface and one A receiver has several interfaces, a receive-only interface and one
or more additional bidirectional communication interfaces. or more additional bidirectional communication interfaces.
A feed has several interfaces, a send-only or a send-and-receive A feed has several interfaces, a send-only or a send-and-receive
capable interface connected to the unidirectional link and one or capable interface connected to the unidirectional link and one or
more additional bidirectional communication interfaces. A feed MUST more additional bidirectional communication interfaces. A feed MUST
be a router. be a router.
Tunnels are constructed between the bidirectional interfaces of Tunnels are constructed between the bidirectional interfaces of
nodes, so these interfaces must be interconnected by an IP nodes, so these interfaces must be interconnected by an IP
infrastructure. In this document we assume that that infrastructure infrastructure. In this document we assume that that infrastructure
is the Internet. is the Internet.
Figure 1 depicts a generic topology with several feeds and several Figure 1 depicts a generic topology with several feeds and several
receivers. receivers.
Unidirectional Link Unidirectional Link
---->---------->------------------->------ ---->---------->------------------->------
| | | | | | | |
|f1u |f2u |r2u |r1u |f1u |f2u |r2u |r1u
-------- -------- -------- -------- ---------- -------- -------- -------- -------- ----------
|Feed 1| |Feed 2| |Recv 2| |Recv 1|---|subnet A| |Feed 1| |Feed 2| |Recv 2| |Recv 1|---|subnet A|
-------- -------- -------- -------- ---------- -------- -------- -------- -------- ----------
|f1b |f2b |r2b |r1b | |f1b |f2b |r2b |r1b |
| | | | | | | | | |
---------------------------------------------------- ----------------------------------------------------
| Internet | | Internet |
---------------------------------------------------- ----------------------------------------------------
Figure 1: Generic topology Figure 1: Generic topology
f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2) f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)
send-only interface. send-only interface.
f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2) f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
bidirectional interface connected to the Internet. bidirectional interface connected to the Internet.
r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver
2) receive-only interface. 2) receive-only interface.
r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver
2) bidirectional interface connected to the Internet. 2) bidirectional interface connected to the Internet.
Subnet A is a local area network connected to recv1. Subnet A is a local area network connected to recv1.
Note that nodes have IP addresses on their unidirectional and their Note that nodes have IP addresses on their unidirectional and their
bidirectional interfaces. The addresses on the unidirectional bidirectional interfaces. The addresses on the unidirectional
interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
network. In general the addresses on the bidirectional interfaces network. In general the addresses on the bidirectional interfaces
(f1b, f2b, r1b, r2b) will be drawn from different IP networks, and (f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
the Internet will route between them. the Internet will route between them.
4. Problems related to unidirectional links 4. Problems related to unidirectional links
Receive-only interfaces are "dumb" and send-only interfaces are Receive-only interfaces are "dumb" and send-only interfaces are
"deaf". Thus a datagram passed to the link-layer driver of a receive- "deaf". Thus a datagram passed to the link-layer driver of a
only interface is simply discarded. The link-layer of a send-only receive-only interface is simply discarded. The link-layer of a
interface never receives anything. send-only interface never receives anything.
The network layer has no knowledge of the underlying transmission The network layer has no knowledge of the underlying transmission
technology except that it considers its access as bidirectional. technology except that it considers its access as bidirectional.
Basically, for outgoing datagrams, the network layer selects the Basically, for outgoing datagrams, the network layer selects the
correct first hop on the connected network according to a routing correct first hop on the connected network according to a routing
table and passes the packet(s) to the appropriate link-layer driver. table and passes the packet(s) to the appropriate link-layer driver.
Referring to Figure 1, Recv 1 and Feed 1 belong to the same network. Referring to Figure 1, Recv 1 and Feed 1 belong to the same network.
However, if Recv 1 initiates a 'ping f1u', it cannot get a response However, if Recv 1 initiates a 'ping f1u', it cannot get a response
from Feed 1. The network layer of Recv 1 delivers the packet to the from Feed 1. The network layer of Recv 1 delivers the packet to the
driver of the receive-only interface, which obviously cannot send it driver of the receive-only interface, which obviously cannot send it
to the feed. to the feed.
Many protocols in the Internet assume that links are bidirectional. Many protocols in the Internet assume that links are bidirectional.
In particular, routing protocols used by directly connected routers In particular, routing protocols used by directly connected routers
no longer behave properly in the presence of a unidirectional link. no longer behave properly in the presence of a unidirectional link.
5. Emulating a broadcast bidirectional network 5. Emulating a broadcast bidirectional network
The simplest solution is to emulate a broadcast capable link-layer The simplest solution is to emulate a broadcast capable link-layer
network. This will allow the immediate deployment of existing higher network. This will allow the immediate deployment of existing higher
level protocols without change. Though other network structures, such level protocols without change. Though other network structures,
as NBMA, could also be emulated, a broadcast network is more such as NBMA, could also be emulated, a broadcast network is more
generally useful. Though a layer 3 network could be emulated, a link- generally useful. Though a layer 3 network could be emulated, a
layer network allows the immediate use of any other network layer link-layer network allows the immediate use of any other network
protocols, and most particularly allows the immediate use of ARP. layer protocols, and most particularly allows the immediate use of
ARP.
A link-layer tunneling mechanism which emulates bidirectional A link-layer tunneling mechanism which emulates bidirectional
connectivity in the presence of a unidirectional link will be connectivity in the presence of a unidirectional link will be
described in the next Section. We first consider the various described in the next Section. We first consider the various
communication scenarios which characterize a broadcast network in communication scenarios which characterize a broadcast network in
order to define what functionalities the link-layer tunneling order to define what functionalities the link-layer tunneling
mechanism has to perform in order to emulate a bidirectional mechanism has to perform in order to emulate a bidirectional
broadcast link. broadcast link.
Here we enumerate the scenarios which would be feasible on a Here we enumerate the scenarios which would be feasible on a
broadcast network, i.e. if feeds and receivers were connected by a broadcast network, i.e., if feeds and receivers were connected by a
bidirectional broadcast link: bidirectional broadcast link:
Scenario 1: A receiver can send a packet to a feed (point-to-point Scenario 1: A receiver can send a packet to a feed (point-to-point
communication between a receiver and a feed). communication between a receiver and a feed).
Scenario 2: A receiver can send a broadcast/multicast packet on the Scenario 2: A receiver can send a broadcast/multicast packet on the
link to all nodes (point-to-multipoint). link to all nodes (point-to-multipoint).
Scenario 3: A receiver can send a packet to another receiver (point- Scenario 3: A receiver can send a packet to another receiver (point-
to-point communication between two receivers). to-point communication between two receivers).
Scenario 4: A feed can send a packet to a send-only feed (point-to- Scenario 4: A feed can send a packet to a send-only feed (point-to-
point communication between two feeds). point communication between two feeds).
Scenario 5: A feed can send a broadcast/multicast packet on the link Scenario 5: A feed can send a broadcast/multicast packet on the link
to all nodes (point-to-multipoint). to all nodes (point-to-multipoint).
Scenario 6: A feed can send a packet to a receiver or a receive Scenario 6: A feed can send a packet to a receiver or a receive
capable feed (point-to-point). capable feed (point-to-point).
These scenarios are possible on a broadcast network. Scenario 6 is These scenarios are possible on a broadcast network. Scenario 6 is
already feasible on the unidirectional link. The link-layer tunneling already feasible on the unidirectional link. The link-layer
mechanism should therefore provide the functionality to support tunneling mechanism should therefore provide the functionality to
scenarios 1 to 5. support scenarios 1 to 5.
Note that regular IP forwarding over such an emulated network (i.e. Note that regular IP forwarding over such an emulated network (i.e.,
using the emulated network as a transit network) works correctly; the using the emulated network as a transit network) works correctly; the
next hop address at the receiver will be the unidirectional link next hop address at the receiver will be the unidirectional link
address of another router (a feed or a receiver) which will then address of another router (a feed or a receiver) which will then
relay the packet. relay the packet.
6. Link-layer tunneling mechanism 6. Link-layer tunneling mechanism
This link-layer tunneling mechanism operates underneath the network This link-layer tunneling mechanism operates underneath the network
layer. Its aim is to emulate bidirectional link-layer connectivity. layer. Its aim is to emulate bidirectional link-layer connectivity.
This is transparent to the network layer: the link appears and This is transparent to the network layer: the link appears and
behaves to the network layer as if it was bidirectional. behaves to the network layer as if it was bidirectional.
Figure 2 depicts a layered representation of the link-layer tunneling Figure 2 depicts a layered representation of the link-layer tunneling
mechanism in the case of Scenario 1. mechanism in the case of Scenario 1.
Send-only Feed Receiver Send-only Feed Receiver
decapsulation encapsulation decapsulation encapsulation
/-----***************----\ /-->---***************--\ /-----***************----\ /-->---***************--\
| | | | | | | |
| | | | | | | |
--|---------------------- | | ---------------------|--- --|---------------------- | | ---------------------|---
| | f1b | f1u | | | | x r1u | r1b | | | | f1b | f1u | | | | x r1u | r1b | |
| | | ^ | | IP | | | | v | | | | ^ | | IP | | | | v |
| ^ | | | v | | | | | | | ^ | | | v | | | | | |
| | | | | | | | v | | | | | | | | | | | v | | |
|-|---------|-------|---| | | |----|------|--------|--| |-|---------|-------|---| | | |----|------|--------|--|
| | | | | | ^ | | | | | | | | | | | ^ | | | | |
| | | | | | LL | | | | | | | | | | | | LL | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | |
| | | O------/ \<------O | | | | | | O------/ \<------O | | |
|-|---------|-----------| |-----------|--------|--| |-|---------|-----------| |-----------|--------|--|
| | | | | | | | | | | | | | | |
| | | | PHY | | | | | | | | PHY | | | |
| | | | | | v | | | | | | | v |
| | | | | | | | | | | | | | | | | | | |
--|-----------|---------- ----------|----------|--- --|-----------|---------- ----------|----------|---
| Bidir | Send-Only Recv-Only | Bidir | | Bidir | Send-Only Recv-Only | Bidir |
^ Interf | Interf UDL Interf | Interf | ^ Interf | Interf UDL Interf | Interf |
| \------------>------->------------/ | | \------------>------->------------/ |
\----------------------<------------------------<--------/ \----------------------<------------------------<--------/
Bidirectional network Bidirectional network
x : IP layer at the receiver generates a datagram to be forwarded x : IP layer at the receiver generates a datagram to be forwarded
on the receive-only interface. on the receive-only interface.
O : Entry point where the link-layer tunneling mechanism is O : Entry point where the link-layer tunneling mechanism is
triggered. triggered.
Figure 2: Scenario 1 using the link-layer Tunneling Mechanism Figure 2: Scenario 1 using the link-layer Tunneling Mechanism
6.1. Tunneling mechanism on the receiver 6.1. Tunneling mechanism on the receiver
On the receiver, a datagram is delivered to the link-layer of the On the receiver, a datagram is delivered to the link-layer of the
unidirectional interface for transmission (see Figure 2). It is then unidirectional interface for transmission (see Figure 2). It is then
encapsulated within a MAC header corresponding to the unidirectional encapsulated within a MAC header corresponding to the unidirectional
link. This packet cannot be sent directly over the link, so it is link. This packet cannot be sent directly over the link, so it is
then processed by the tunneling mechanism. then processed by the tunneling mechanism.
The packet is encapsulated within an IP header whose destination is The packet is encapsulated within an IP header whose destination is
the IP address of a feed bidirectional interface (f1b or f2b). This the IP address of a feed bidirectional interface (f1b or f2b). This
destination address is also called the tunnel end-point. The destination address is also called the tunnel end-point. The
mechanism for a receiver to learn these addresses and to choose the mechanism for a receiver to learn these addresses and to choose the
feed is explained in Section 7. The type of encapsulation is feed is explained in Section 7. The type of encapsulation is
described in Section 8. described in Section 8.
In all cases the packet is encapsulated, but the tunnel end-point (an In all cases the packet is encapsulated, but the tunnel end-point (an
IP address) depends on the encapsulated packet's destination MAC IP address) depends on the encapsulated packet's destination MAC
address. If the destination MAC address is: address. If the destination MAC address is:
1) the MAC address of a feed interface connected to the 1) the MAC address of a feed interface connected to the
unidirectional link (Scenario 1). The datagram is encapsulated, unidirectional link (Scenario 1). The datagram is
the destination address of the encapsulating datagram is the encapsulated, the destination address of the encapsulating
feed tunnel end-point (f1b referring to Figure 2). datagram is the feed tunnel end-point (f1b referring to Figure
2).
2) a MAC broadcast/multicast address (Scenario 2). The datagram is 2) a MAC broadcast/multicast address (Scenario 2). The datagram
encapsulated, the destination address of the encapsulating is encapsulated, the destination address of the encapsulating
datagram is the default feed tunnel end-point. See Section 7.4 datagram is the default feed tunnel end-point. See Section 7.4
for further details on the default feed. for further details on the default feed.
3) a MAC address that belongs to the unidirectional network but is 3) a MAC address that belongs to the unidirectional network but is
not a feed address (Scenario 3). The datagram is encapsulated, not a feed address (Scenario 3). The datagram is encapsulated,
the destination address of the encapsulating datagram is the the destination address of the encapsulating datagram is the
default feed tunnel end-point. default feed tunnel end-point.
The encapsulated datagram is passed to the network layer which The encapsulated datagram is passed to the network layer which
forwards it according to its destination address. The destination forwards it according to its destination address. The destination
address is a feed bidirectional interface which is reachable via the address is a feed bidirectional interface which is reachable via the
Internet. In this case, the encapsulated datagram is forwarded via Internet. In this case, the encapsulated datagram is forwarded via
the receiver bidirectional interface (r1b). the receiver bidirectional interface (r1b).
6.2. Tunneling mechanism on the feed 6.2. Tunneling mechanism on the feed
A feed processes unidirectional link related packets in two different A feed processes unidirectional link related packets in two different
ways: ways:
- packets generated by a local application or packets routed as
usual by the IP layer may have to be forwarded over the - packets generated by a local application or packets routed as
unidirectional link (Section 6.2.1) usual by the IP layer may have to be forwarded over the
- encapsulated packets received from another receiver or feed need unidirectional link (Section 6.2.1)
tunnel processing (Section 6.2.2).
- encapsulated packets received from another receiver or feed need
tunnel processing (Section 6.2.2).
A feed cannot directly send a packet to a send-only feed over the A feed cannot directly send a packet to a send-only feed over the
unidirectional link (Scenario 4). In order to emulate this type of unidirectional link (Scenario 4). In order to emulate this type of
communication, feeds have to tunnel packets to send-only feeds. A communication, feeds have to tunnel packets to send-only feeds. A
feed MUST maintain a list of all other feed tunnel end-points. This feed MUST maintain a list of all other feed tunnel end-points. This
list MUST indicate which are send-only feed tunnel end-points. This list MUST indicate which are send-only feed tunnel end-points. This
is configured manually at the feed by the local administrator, as is configured manually at the feed by the local administrator, as
described in Section 7. described in Section 7.
6.2.1. Forwarding packets over the unidirectional link 6.2.1. Forwarding packets over the unidirectional link
When a datagram is delivered to the link-layer of the unidirectional When a datagram is delivered to the link-layer of the unidirectional
interface of a feed for transmission, its treatment depends on the interface of a feed for transmission, its treatment depends on the
packet's destination MAC address. If the destination MAC address is: packet's destination MAC address. If the destination MAC address is:
1) the MAC address of a receiver or a receive capable feed 1) the MAC address of a receiver or a receive capable feed
(Scenario 6). The packet is sent over the unidirectional link. (Scenario 6). The packet is sent over the unidirectional link.
This is classical "forwarding". This is classical "forwarding".
2) the MAC address of a send-only feed (Scenario 4). The packet is 2) the MAC address of a send-only feed (Scenario 4). The packet
encapsulated and sent to the send-only feed tunnel end-point. is encapsulated and sent to the send-only feed tunnel end-
The type of encapsulation is described in Section 8. point. The type of encapsulation is described in Section 8.
3) a broadcast/multicast destination (Scenario 5). The packet is 3) a broadcast/multicast destination (Scenario 5). The packet is
sent over the unidirectional link. Concurrently, a copy of this sent over the unidirectional link. Concurrently, a copy of
packet is encapsulated and sent to every feed of the list of this packet is encapsulated and sent to every feed of the list
send-only feed tunnel end-points. Thus the broadcast/multicast of send-only feed tunnel end-points. Thus the
will reach all receivers and all send-only feeds. broadcast/multicast will reach all receivers and all send-only
feeds.
6.2.2. Receiving encapsulated packets 6.2.2. Receiving encapsulated packets
Feeds listen for incoming encapsulated datagrams on their tunnel end- Feeds listen for incoming encapsulated datagrams on their tunnel
points. Encapsulated packets will have been received on a end-points. Encapsulated packets will have been received on a
bidirectional interface, and traversed their way up the IP stack. bidirectional interface, and traversed their way up the IP stack.
They will then enter a decapsulation process (See Figure 2). They will then enter a decapsulation process (See Figure 2).
Decapsulation reveals the original link-layer packet. Note that this Decapsulation reveals the original link-layer packet. Note that this
has not been modified in any way by intermediate routers; in has not been modified in any way by intermediate routers; in
particular, the original MAC header will be intact. particular, the original MAC header will be intact.
Further actions depend on the destination MAC address of the link- Further actions depend on the destination MAC address of the link-
layer packet, which can be: layer packet, which can be:
1) the MAC address of the feed interface connected to the 1) the MAC address of the feed interface connected to the
unidirectional link, i.e. own MAC address (Scenarios 1 and 4). unidirectional link, i.e., own MAC address (Scenarios 1 and 4).
The packet is passed to the link-layer of the interface The packet is passed to the link-layer of the interface
connected to the unidirectional link which can then deliver it connected to the unidirectional link which can then deliver it
up to higher layers. As a result, the datagram is processed as up to higher layers. As a result, the datagram is processed as
if it was coming from the unidirectional link, and being if it was coming from the unidirectional link, and being
delivered locally. Scenarios 1 and 4 are now feasible, a delivered locally. Scenarios 1 and 4 are now feasible, a
receiver or a feed can send a packet to a feed. receiver or a feed can send a packet to a feed.
2) a receiver address (Scenario 3). The packet is passed to the 2) a receiver address (Scenario 3). The packet is passed to the
link-layer of the interface connected to the unidirectional link-layer of the interface connected to the unidirectional
link. It is directly sent over the unidirectional link, to the link. It is directly sent over the unidirectional link, to the
indicated receiver. Note, the packet must not be delivered indicated receiver. Note, the packet must not be delivered
locally. Scenario 3 is now feasible, a receiver can send a locally. Scenario 3 is now feasible, a receiver can send a
packet to another receiver. packet to another receiver.
3) a broadcast/multicast address, this corresponds to Scenarios 2 3) a broadcast/multicast address, this corresponds to Scenarios 2
and 5. We have to distinguish two cases, either (i) the and 5. We have to distinguish two cases, either (i) the
encapsulated packet was sent from a receiver or (ii) from a feed encapsulated packet was sent from a receiver or (ii) from a
(encapsulated broadcast/multicast packet sent to a send-only feed (encapsulated broadcast/multicast packet sent to a send-
feed). These cases are distinguished by examining the source only feed). These cases are distinguished by examining the
address of the encapsulating packet and comparing it with the source address of the encapsulating packet and comparing it
configured list of feed IP addresses. The action then taken is: with the configured list of feed IP addresses. The action then
taken is:
i) the feed was designated as a default feed by a receiver to i) the feed was designated as a default feed by a receiver to
forward the broadcast/multicast packet. The feed is then in forward the broadcast/multicast packet. The feed is then in
charge of sending the multicast packet to all nodes. Delivery charge of sending the multicast packet to all nodes.
to all nodes is accomplished by executing all 3 of the Delivery to all nodes is accomplished by executing all 3 of
following actions: the following actions:
- The packet is encapsulated and sent to the list of send-
only feed tunnel end-points. - The packet is encapsulated and sent to the list of send-
- Also, the packet is passed to the link-layer of the only feed tunnel end-points.
interface which forwards it directly over the - Also, the packet is passed to the link-layer of the
unidirectional link (all receivers and receive capable interface which forwards it directly over the
feeds receive it). unidirectional link (all receivers and receive capable
- Also, the link-layer delivers it locally to higher layers. feeds receive it).
Caution: a receiver which sends an encapsulated - Also, the link-layer delivers it locally to higher
broadcast/multicast packet to a default feed will receive its layers.
own packet via the unidirectional link. Correct filtering as
described in [rfc1112] must be applied. Caution: a receiver which sends an encapsulated
broadcast/multicast packet to a default feed will receive
its own packet via the unidirectional link. Correct
filtering as described in [RFC1112] must be applied.
ii) the feed receives the packet and keeps it for local ii) the feed receives the packet and keeps it for local
delivery. The packet is passed to the link-layer of the delivery. The packet is passed to the link-layer of the
interface connected to the unidirectional link which delivers interface connected to the unidirectional link which
it to higher layers. delivers it to higher layers.
Scenario 2 is now feasible, a receiver can send a Scenario 2 is now feasible, a receiver can send a
broadcast/multicast packet over the unidirectional link and it broadcast/multicast packet over the unidirectional link and it
will be heard by all nodes. will be heard by all nodes.
7. Dynamic Tunnel Configuration Protocol (DTCP) 7. Dynamic Tunnel Configuration Protocol (DTCP)
Receivers and feeds have to know the feed tunnel end-points in order Receivers and feeds have to know the feed tunnel end-points in order
to forward encapsulated datagrams (e.g. Scenarios 1 and 4). to forward encapsulated datagrams (e.g., Scenarios 1 and 4).
The number of feeds is expected to be relatively small (Section 3), The number of feeds is expected to be relatively small (Section 3),
so at every feed the list of all feeds is configured manually. This so at every feed the list of all feeds is configured manually. This
list should note which are send-only feeds, and which are receive list should note which are send-only feeds, and which are receive
capable feeds. The administrator sets up tunnels to all send-only capable feeds. The administrator sets up tunnels to all send-only
feeds. A tunnel end-point is an IP address of a bidirectional link on feeds. A tunnel end-point is an IP address of a bidirectional link
a send-only feed. on a send-only feed.
For scalability reasons, manual configuration cannot be done at the For scalability reasons, manual configuration cannot be done at the
receivers. Tunnels must be configured and maintained dynamically by receivers. Tunnels must be configured and maintained dynamically by
receivers, both for scalability, and in order to cope with the receivers, both for scalability, and in order to cope with the
following events: following events:
1) New feed detection. 1) New feed detection.
When a new feed comes up, every receiver must create a tunnel to When a new feed comes up, every receiver must create a tunnel
enable bidirectional communication with it. to enable bidirectional communication with it.
2) Loss of unidirectional link detection. 2) Loss of unidirectional link detection.
When the unidirectional link is down, receivers must disable When the unidirectional link is down, receivers must disable
their tunnels. The tunneling mechanism emulates bidirectional their tunnels. The tunneling mechanism emulates bidirectional
connectivity between nodes. Therefore, if the unidirectional connectivity between nodes. Therefore, if the unidirectional
link is down, a feed should not receive datagrams from the link is down, a feed should not receive datagrams from the
receivers. Protocols that consider a link as operational if they receivers. Protocols that consider a link as operational if
receive datagrams from it (e.g. the RIP protocol [rfc2453]) they receive datagrams from it (e.g., the RIP protocol
require this behavior for correct operation. [RFC2453]) require this behavior for correct operation.
3) Loss of feed detection. 3) Loss of feed detection.
When a feed is down, receivers must disable their corresponding When a feed is down, receivers must disable their corresponding
tunnel. This prevents unnecessary datagrams from being tunneled tunnel. This prevents unnecessary datagrams from being
which might overload the Internet. For instance, there is no tunneled which might overload the Internet. For instance,
need for receivers to forward a broadcast message through a there is no need for receivers to forward a broadcast message
tunnel whose end-point is down. through a tunnel whose end-point is down.
The DTCP protocol provides a means for receivers to dynamically The DTCP protocol provides a means for receivers to dynamically
discover the presence of feeds and to maintain a list of operational discover the presence of feeds and to maintain a list of operational
tunnel end-points. Feeds periodically announce their tunnel end-point tunnel end-points. Feeds periodically announce their tunnel end-
addresses over the unidirectional link. Receivers listen to these point addresses over the unidirectional link. Receivers listen to
announcements and maintain a list of tunnel end-points. these announcements and maintain a list of tunnel end-points.
7.1. The HELLO message 7.1. The HELLO message
The DTCP protocol is a 'unidirectional protocol', messages are only The DTCP protocol is a 'unidirectional protocol', messages are only
sent from feeds to receivers. sent from feeds to receivers.
The packet format is shown in Figure 3. Fields contain binary The packet format is shown in Figure 3. Fields contain binary
integers, in normal Internet order with the most significant bit integers, in normal Internet order with the most significant bit
first. Each tick mark represents one bit. first. Each tick mark represents one bit.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Com | Interval | Sequence | | Vers | Com | Interval | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| res |F|IP Vers| Tunnel Type | Nb of FBIP | reserved | | res |F|IP Vers| Tunnel Type | Nb of FBIP | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIP1) (32/128 bits) | | Feed BDL IP addr (FBIP1) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... | | ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIPn) (32/128 bits) | | Feed BDL IP addr (FBIPn) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Packet Format Figure 3: Packet Format
Every datagram contains the following fields, note that constants are Every datagram contains the following fields, note that constants are
written in uppercase and are defined in Section 7.5: written in uppercase and are defined in Section 7.5:
Vers (4 bit unsigned integer): DTCP version number. MUST be Vers (4 bit unsigned integer): DTCP version number. MUST be
DTCP_VERSION. DTCP_VERSION.
Com (4 bit unsigned integer): Command field, possible values are Com (4 bit unsigned integer): Command field, possible values are
1 - JOIN A message announcing that the feed sending this message 1 - JOIN A message announcing that the feed sending this message
is up and running. is up and running.
2 - LEAVE A message announcing that the feed sending this message 2 - LEAVE A message announcing that the feed sending this message
is being shut down. is being shut down.
Interval (8 bit unsigned integer): Interval in seconds between HELLO Interval (8 bit unsigned integer): Interval in seconds between HELLO
messages for the IP protocol in "IP Vers". Must be > 0. The messages for the IP protocol in "IP Vers". Must be > 0. The
recommended value is HELLO_INTERVAL. If this value is increased, recommended value is HELLO_INTERVAL. If this value is increased,
the feed MUST continue to send HELLO messages at the old rate for the feed MUST continue to send HELLO messages at the old rate for
at least the old HELLO_LEAVE period. at least the old HELLO_LEAVE period.
Sequence (16 bit unsigned integer): Random value initialized at boot Sequence (16 bit unsigned integer): Random value initialized at boot
time and incremented by 1 every time a value of the HELLO message time and incremented by 1 every time a value of the HELLO message
is modified. is modified.
res (3 bits): Reserved/unused field, MUST be zero. res (3 bits): Reserved/unused field, MUST be zero.
F (1 bit): bit indicating the type of feed: F (1 bit): bit indicating the type of feed:
0 = Send-only feed 0 = Send-only feed
1 = Receive-capable feed 1 = Receive-capable feed
IP Vers (4 bit unsigned integer): IP protocol version of the feed IP Vers (4 bit unsigned integer): IP protocol version of the feed
bidirectional IP addresses (FBIP): bidirectional IP addresses (FBIP):
4 = IP version 4 4 = IP version 4
6 = IP version 6 6 = IP version 6
Tunnel Type (8 bit unsigned integer): tunneling protocol supported by Tunnel Type (8 bit unsigned integer): tunneling protocol supported by
the feed. This value is the IP protocol number defined in [rfc1700] the feed. This value is the IP protocol number defined in
[iana/protocol-numbers] and their legitimate descendents. Receivers [RFC1700] [iana/protocol-numbers] and their legitimate
MUST use this form of tunnel encapsulation when tunneling to the descendents. Receivers MUST use this form of tunnel encapsulation
feed. when tunneling to the feed.
47 = GRE [rfc2784] (recommended) 47 = GRE [RFC2784] (recommended)
Other protocol types allowing link-layer encapsulation are Other protocol types allowing link-layer encapsulation are
permitted. Obtaining new values is documented in [rfc2780]. permitted. Obtaining new values is documented in [RFC2780].
Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed
addresses which are enumerated in the HELLO message addresses which are enumerated in the HELLO message
reserved (8 bits): Reserved/unused field, MUST be zero. reserved (8 bits): Reserved/unused field, MUST be zero.
Feed BDL IP addr (32 or 128 bits). The bidirectional IP address feed Feed BDL IP addr (32 or 128 bits). The bidirectional IP address feed
is the IP address of a feed bidirectional interface (tunnel end- is the IP address of a feed bidirectional interface (tunnel end-
point) reachable via the Internet. A feed has 'Nb of FBIP' IP point) reachable via the Internet. A feed has 'Nb of FBIP' IP
addresses which are operational tunnel end-points. They are addresses which are operational tunnel end-points. They are
enumerated in preferred order. FBIP1 being the most suitable tunnel enumerated in preferred order. FBIP1 being the most suitable
end-point. tunnel end-point.
7.2. DTCP on the feed: sending HELLO packets 7.2. DTCP on the feed: sending HELLO packets
The DTCP protocol runs on top of UDP. Packets are sent to the "DTCP The DTCP protocol runs on top of UDP. Packets are sent to the "DTCP
announcement" multicast address over the unidirectional link on port announcement" multicast address over the unidirectional link on port
HELLO_PORT with a TTL of 1. Due to existing deployements a feed HELLO_PORT with a TTL of 1. Due to existing deployments a feed
SHOULD also support the use of the old DTCP announcement address, see SHOULD also support the use of the old DTCP announcement address, as
Appendix B. described in Appendix B.
The source address of the HELLO packet is set to the IP address of The source address of the HELLO packet is set to the IP address of
the feed interface connected to the unidirectional link. In the rest the feed interface connected to the unidirectional link. In the rest
of the document, this value is called FUIP (Feed Unidirectional IP of the document, this value is called FUIP (Feed Unidirectional IP
address). address).
The process in charge of sending HELLO packets fills every field of The process in charge of sending HELLO packets fills every field of
the datagram according to the description given in Section 7.1. the datagram according to the description given in Section 7.1.
As long as a feed is up and running, it periodically announces its As long as a feed is up and running, it periodically announces its
presence to receivers. It MUST send HELLO packets containing a JOIN presence to receivers. It MUST send HELLO packets containing a JOIN
command every HELLO_INTERVAL over the unidirectional link. command every HELLO_INTERVAL over the unidirectional link.
Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends HELLO Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends HELLO
messages with the FBIP1 field set to f1b (resp. f2b). messages with the FBIP1 field set to f1b (resp. f2b).
When a feed is about to be shut down, or when routing over the When a feed is about to be shut down, or when routing over the
unidirectional link is about to be intentionally interrupted, it is unidirectional link is about to be intentionally interrupted, it is
recommended that feeds: recommended that feeds:
1) stop sending HELLO messages containing a JOIN command. 1) stop sending HELLO messages containing a JOIN command.
2) send a HELLO message containing a LEAVE command to inform 2) send a HELLO message containing a LEAVE command to inform
receivers that the feed is no longer performing routing over the receivers that the feed is no longer performing routing over
unidirectional link. the unidirectional link.
7.3. DTCP on the receiver: receiving HELLO packets 7.3. DTCP on the receiver: receiving HELLO packets
Based on the reception of HELLO messages, receivers discover the Based on the reception of HELLO messages, receivers discover the
presence of feeds, maintain a list of active feeds, and keep track of presence of feeds, maintain a list of active feeds, and keep track of
the tunnel end-points for those feeds. the tunnel end-points for those feeds.
For each active feed, and each IP protocol supported, at least the For each active feed, and each IP protocol supported, at least the
following information will be kept: following information will be kept:
FUIP - feed unidirectional link IP address
FUMAC - MAC address corresponding to the above IP FUIP - feed unidirectional link IP address
address FUMAC - MAC address corresponding to the above IP
(FBIP1,...,FBIPn) - list of tunnel end-points address
tunnel type - tunnel type supported by this feed (FBIP1,...,FBIPn) - list of tunnel end-points
Sequence - "Sequence" value from the last HELLO received tunnel type - tunnel type supported by this feed
from this feed Sequence - "Sequence" value from the last HELLO received
timer - used to timeout this entry from this feed
timer - used to timeout this entry
The FUMAC value for an active feed is needed for the operation of The FUMAC value for an active feed is needed for the operation of
this protocol. However, the method of discovery of this value is not this protocol. However, the method of discovery of this value is not
specified here. specified here.
Initially, the list of active feeds is empty. Initially, the list of active feeds is empty.
When a receiver is started, it MUST run a process which joins the When a receiver is started, it MUST run a process which joins the
"DTCP announcement" multicast group and listens to incoming packets "DTCP announcement" multicast group and listens to incoming packets
on the HELLO_PORT port from the unidirectional link. on the HELLO_PORT port from the unidirectional link.
Upon the reception of a HELLO message, the process checks the version Upon the reception of a HELLO message, the process checks the version
number of the protocol. If it is different from HELLO_VERSION, the number of the protocol. If it is different from HELLO_VERSION, the
packet is discarded and the process waits for the next incoming packet is discarded and the process waits for the next incoming
packet. packet.
After successfully checking the version number further action depends After successfully checking the version number further action depends
on the type of command: on the type of command:
- JOIN: - JOIN:
The process verifies if the address FUIP already belongs to the The process verifies if the address FUIP already belongs to the
list of active feeds. list of active feeds.
If it does not, a new entry, for feed FUIP, is created and added If it does not, a new entry, for feed FUIP, is created and added
to the list of active feeds. The number of feed bidirectional IP to the list of active feeds. The number of feed bidirectional IP
addresses to read is deduced from the 'Nb of FBID' field. These addresses to read is deduced from the 'Nb of FBID' field. These
tunnel end-points (FBIP1,...,FBIPn) can then be added to the new tunnel end-points (FBIP1,...,FBIPn) can then be added to the new
entry. The tunnel Type and Sequence values are also taken from the entry. The tunnel Type and Sequence values are also taken from
HELLO packet and recorded in the new entry. A timer set to the HELLO packet and recorded in the new entry. A timer set to
HELLO_LEAVE is associated with this entry. HELLO_LEAVE is associated with this entry.
If it does, the sequence number is compared to the sequence number If it does, the sequence number is compared to the sequence number
contained in the previous HELLO packet sent by this feed. If they contained in the previous HELLO packet sent by this feed. If they
are equal, the timer associated with this entry is reset to are equal, the timer associated with this entry is reset to
HELLO_LEAVE. Otherwise all the information corresponding to FUIP HELLO_LEAVE. Otherwise all the information corresponding to FUIP
is set to the values from the HELLO packet. is set to the values from the HELLO packet.
Referring to Figure 1 in Section 3, both receivers (recv 1 and Referring to Figure 1 in Section 3, both receivers (recv 1 and
recv 2) have a list of active feeds containing two entries: Feed 1 recv 2) have a list of active feeds containing two entries: Feed 1
with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed
2 with a FUIP of f2u and a list of tunnel end-points (f2b). 2 with a FUIP of f2u and a list of tunnel end-points (f2b).
- LEAVE: - LEAVE:
The process checks if there is an entry for FUIP in the list of The process checks if there is an entry for FUIP in the list of
active feeds. If there is, the timer is disabled and the entry is active feeds. If there is, the timer is disabled and the entry is
deleted from the list. The LEAVE message provides a means of deleted from the list. The LEAVE message provides a means of
quickly updating the list of active feeds. quickly updating the list of active feeds.
A timeout occurs for either of two reasons: A timeout occurs for either of two reasons:
1) a feed went down without sending a LEAVE message. As JOIN 1) a feed went down without sending a LEAVE message. As JOIN
messages are no longer sent from this feed, a timeout occurs at messages are no longer sent from this feed, a timeout occurs at
HELLO_LEAVE after the last JOIN message. HELLO_LEAVE after the last JOIN message.
2) the unidirectional link is down. Thus no more JOIN messages are 2) the unidirectional link is down. Thus no more JOIN messages
received from any of the feeds, and they will each timeout are received from any of the feeds, and they will each timeout
independently. The timeout of each entry depends on its independently. The timeout of each entry depends on its
individual HELLO_LEAVE value, and when the last JOIN message was individual HELLO_LEAVE value, and when the last JOIN message
sent by that feed, before the unidirectional link went down. was sent by that feed, before the unidirectional link went
down.
In either case, bidirectional connectivity can no longer be ensured In either case, bidirectional connectivity can no longer be ensured
between the receiver and the feed (FUIP): either the feed is no between the receiver and the feed (FUIP): either the feed is no
longer routing datagrams over the unidirectional link, or the link is longer routing datagrams over the unidirectional link, or the link is
down. Thus the associated entry is removed from the list of active down. Thus the associated entry is removed from the list of active
feeds, whatever the cause. As a result, the list only contains feeds, whatever the cause. As a result, the list only contains
operational tunnel end-points. operational tunnel end-points.
The HELLO protocol provides receivers with a list of feeds, and a The HELLO protocol provides receivers with a list of feeds, and a
list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed. In list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed.
the following Section, we describe how to integrate the HELLO In the following Section, we describe how to integrate the HELLO
protocol into the tunneling mechanism described in Sections 6.1 and protocol into the tunneling mechanism described in Sections 6.1 and
6.2. 6.2.
7.4. Tunneling mechanism using the list of active feeds 7.4. Tunneling mechanism using the list of active feeds
This Section explains how the tunneling mechanism uses the list of This Section explains how the tunneling mechanism uses the list of
active feeds to handle datagrams which are to be tunneled. Referring active feeds to handle datagrams which are to be tunneled. Referring
to Section 6.1, it shows how feed tunnel end-points are selected. to Section 6.1, it shows how feed tunnel end-points are selected.
The choice of the default feed is made independently at each The choice of the default feed is made independently at each
receiver. The choice is a matter of local policy, and this policy is receiver. The choice is a matter of local policy, and this policy is
out of scope for this document. However, as an example, the default out of scope for this document. However, as an example, the default
feed may be the feed that has the lowest round trip time to the feed may be the feed that has the lowest round trip time to the
receiver. receiver.
When a receiver sends a packet to a feed, it must choose a tunnel When a receiver sends a packet to a feed, it must choose a tunnel
end-point from within the FBIP list. The 'preferred FBIP' is end-point from within the FBIP list. The 'preferred FBIP' is
generally FBIP1 (Section 7.1). For various reasons, a receiver may generally FBIP1 (Section 7.1). For various reasons, a receiver may
decide to use a different FBIP, say FBIPi instead of FBIP1, as the decide to use a different FBIP, say FBIPi instead of FBIP1, as the
tunnel end-point. For example, the receiver may have better tunnel end-point. For example, the receiver may have better
connectivity to FBIPi. This decision is taken by the receiver connectivity to FBIPi. This decision is taken by the receiver
administrator. administrator.
Here we show how the list of active feeds is involved when a receiver Here we show how the list of active feeds is involved when a receiver
tunnels a link-layer packet. Section 6.1 listed the following cases, tunnels a link-layer packet. Section 6.1 listed the following cases,
depending on whether the MAC destination address of the packet is: depending on whether the MAC destination address of the packet is:
1) the MAC address of a feed interface connected to the 1) the MAC address of a feed interface connected to the
unidirectional link: This is TRUE if the address matches a FUMAC unidirectional link: This is TRUE if the address matches a
address in the list of active feeds. The packet is tunneled to FUMAC address in the list of active feeds. The packet is
the preferred FBIP of the matching feed. tunneled to the preferred FBIP of the matching feed.
2) the broadcast address of the unidirectional link or a multicast 2) the broadcast address of the unidirectional link or a multicast
address: address:
This is determined by the MAC address format rules, and the list
of active feeds is not involved. The packet is tunneled to the
preferred FBIP of the default feed.
3) an address that belongs to the unidirectional network but is not This is determined by the MAC address format rules, and the
a feed address: list of active feeds is not involved. The packet is tunneled
This is TRUE if the address is neither broadcast nor multicast, to the preferred FBIP of the default feed.
nor found in the list of active feeds. The packet is tunneled to
the preferred FBIP of the default feed. 3) an address that belongs to the unidirectional network but is
not a feed address:
This is TRUE if the address is neither broadcast nor multicast,
nor found in the list of active feeds. The packet is tunneled
to the preferred FBIP of the default feed.
In all cases, the encapsulation type depends on the tunnel type In all cases, the encapsulation type depends on the tunnel type
required by the feed which is selected. required by the feed which is selected.
7.5. Constant definitions 7.5. Constant definitions
DTCP_VERSION is 1. DTCP_VERSION is 1.
HELLO_INTERVAL is 5 seconds. HELLO_INTERVAL is 5 seconds.
"DTCP announcement" multicast group is 224.0.0.36, assigned by IANA. "DTCP announcement" multicast group is 224.0.0.36, assigned by IANA.
HELLO_PORT is 652. It is a reserved system port assigned by IANA, no HELLO_PORT is 652. It is a reserved system port assigned by IANA, no
other traffic must be allowed. other traffic must be allowed.
HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e. 15 HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e., 15
seconds if the default HELLO_INTERVAL was advertised. seconds if the default HELLO_INTERVAL was advertised.
8. Tunnel encapsulation format 8. Tunnel encapsulation format
The tunneling mechanism operates at the link-layer and emulates The tunneling mechanism operates at the link-layer and emulates
bidirectional connectivity amongst receivers and feeds. We assume bidirectional connectivity amongst receivers and feeds. We assume
that hardware connected to the unidirectional link supports broadcast that hardware connected to the unidirectional link supports broadcast
and unicast MAC addressing. That is, a feed can send a packet to a and unicast MAC addressing. That is, a feed can send a packet to a
particular receiver using a unicast MAC destination address or to a particular receiver using a unicast MAC destination address or to a
set of receivers using a broadcast/multicast destination address. The set of receivers using a broadcast/multicast destination address.
hardware (or the driver) of the receiver can then filter the incoming The hardware (or the driver) of the receiver can then filter the
packets sent over the unidirectional links without any assumption incoming packets sent over the unidirectional links without any
about the encapsulated data type. assumption about the encapsulated data type.
In a similar way, a receiver should be capable of sending unicast and In a similar way, a receiver should be capable of sending unicast and
broadcast MAC packets via its tunnels. Link-layer packets are broadcast MAC packets via its tunnels. Link-layer packets are
encapsulated. As a result, after decapsulating an incoming packet, encapsulated. As a result, after decapsulating an incoming packet,
the feed can perform link-layer filtering as if the data came the feed can perform link-layer filtering as if the data came
directly from the unidirectional link (See Figure 2). directly from the unidirectional link (See Figure 2).
Generic Routing Encapsulation (GRE) [rfc2784] suits our requirements Generic Routing Encapsulation (GRE) [RFC2784] suits our requirements
because it specifies a protocol for encapsulating arbitrary packets, because it specifies a protocol for encapsulating arbitrary packets,
and allows use of IP as the delivery protocol. and allows use of IP as the delivery protocol.
The feed's local administrator decides what encapsulation it will The feed's local administrator decides what encapsulation it will
demand that receivers use, and sets the tunnel type field in the demand that receivers use, and sets the tunnel type field in the
HELLO message appropriately. The value 47 (decimal) indicates GRE. HELLO message appropriately. The value 47 (decimal) indicates GRE.
Other values can be used, but their interpretation must be agreed Other values can be used, but their interpretation must be agreed
upon between feeds and receivers. Such usage is not defined here. upon between feeds and receivers. Such usage is not defined here.
8.1. Generic Routing Encapsulation on the receiver 8.1. Generic Routing Encapsulation on the receiver
A GRE packet is composed of a header in which a type field specifies A GRE packet is composed of a header in which a type field specifies
the encapsulated protocol (ARP, IP, IPX, etc.). See [rfc2784] for the encapsulated protocol (ARP, IP, IPX, etc.). See [RFC2784] for
details about the encapsulation. In our case, only support for the details about the encapsulation. In our case, only support for the
MAC addressing scheme of the unidirectional link MUST be implemented. MAC addressing scheme of the unidirectional link MUST be implemented.
A packet tunneled with a GRE encapsulation has the following format: A packet tunneled with a GRE encapsulation has the following format:
the delivery header is an IP header whose destination is the tunnel the delivery header is an IP header whose destination is the tunnel
end-point (FBIP), followed by a GRE header specifying the link-layer end-point (FBIP), followed by a GRE header specifying the link-layer
type of the unidirectional link. Figure 4 presents the entire type of the unidirectional link. Figure 4 presents the entire
encapsulated packet. encapsulated packet.
---------------------------------------- ----------------------------------------
| IP delivery header | | IP delivery header |
| destination addr = FBIP | | destination addr = FBIP |
| IP proto = GRE (47) | | IP proto = GRE (47) |
---------------------------------------- ----------------------------------------
| GRE Header | | GRE Header |
| type = MAC type of the UDL | | type = MAC type of the UDL |
---------------------------------------- ----------------------------------------
| Payload packet | | Payload packet |
| MAC packet | | MAC packet |
---------------------------------------- ----------------------------------------
Figure 4: Encapsulated packet Figure 4: Encapsulated packet
9. Issues 9. Issues
9.1. Hardware address resolution 9.1. Hardware address resolution
Regardless of whether the link is unidirectional or bidirectional, if Regardless of whether the link is unidirectional or bidirectional, if
a feed sends a packet over a non-point-to-point type network, it a feed sends a packet over a non-point-to-point type network, it
requires the data link address of the destination. ARP [rfc826] is requires the data link address of the destination. ARP [RFC826] is
used on Ethernet networks for this purpose. used on Ethernet networks for this purpose.
The link-layer mechanism emulates a bidirectional network in the The link-layer mechanism emulates a bidirectional network in the
presence of an unidirectional link. However, there are asymmetric presence of an unidirectional link. However, there are asymmetric
delays between every (feed, receiver) pair. The backchannel between a delays between every (feed, receiver) pair. The backchannel between
receiver and a feed has varying delays because packets go through the a receiver and a feed has varying delays because packets go through
Internet. Furthermore, a typical example of a unidirectional link is the Internet. Furthermore, a typical example of a unidirectional
a GEO satellite link whose delay is about 250 milliseconds. link is a GEO satellite link whose delay is about 250 milliseconds.
Because of long round trip delays, reactive address resolution Because of long round trip delays, reactive address resolution
methods such as ARP [rfc826] may not work well. For example, a feed methods such as ARP [RFC826] may not work well. For example, a feed
may have to forward packets at high data rates to a receiver whose may have to forward packets at high data rates to a receiver whose
hardware address is unknown. The stream of packets is passed to the hardware address is unknown. The stream of packets is passed to the
link-layer driver of the feed send-only interface. When the first link-layer driver of the feed send-only interface. When the first
packet arrives, the link-layer realizes it does not have the packet arrives, the link-layer realizes it does not have the
corresponding hardware address of the next hop, and sends an ARP corresponding hardware address of the next hop, and sends an ARP
request. While the link-layer is waiting for the response (at least request. While the link-layer is waiting for the response (at least
250 ms for the GEO satellite case), IP packets are buffered by the 250 ms for the GEO satellite case), IP packets are buffered by the
feed. If it runs out of space before the ARP response arrives, IP feed. If it runs out of space before the ARP response arrives, IP
packets will be dropped. packets will be dropped.
This problem of address resolution protocols is not addressed in this This problem of address resolution protocols is not addressed in this
document. An ad-hoc solution is possible when the MAC address is document. An ad-hoc solution is possible when the MAC address is
configurable, which is possible in some satellite receiver cards. A configurable, which is possible in some satellite receiver cards. A
simple transformation (maybe null) of the IP address can then be used simple transformation (maybe null) of the IP address can then be used
as the MAC address. In this case, senders do not need to "resolve" an as the MAC address. In this case, senders do not need to "resolve"
IP address to a MAC address, they just need to perform the simple an IP address to a MAC address, they just need to perform the simple
transformation. transformation.
9.2. Routing protocols 9.2. Routing protocols
The link-layer tunneling mechanism hides from the network and higher The link-layer tunneling mechanism hides from the network and higher
layers the fact that feeds and receivers are connected by a layers the fact that feeds and receivers are connected by a
unidirectional link. Communication is bidirectional, but asymmetric unidirectional link. Communication is bidirectional, but asymmetric
in bandwidths and delays. in bandwidths and delays.
In order to incorporate unidirectional links in the Internet, feeds In order to incorporate unidirectional links in the Internet, feeds
and receivers might have to run routing protocols in some topologies. and receivers might have to run routing protocols in some topologies.
These protocols will work fine because the tunneling mechanism These protocols will work fine because the tunneling mechanism
results in bidirectional connectivity between all feeds and results in bidirectional connectivity between all feeds and
receivers. Thus routing messages can be exchanged as on any receivers. Thus routing messages can be exchanged as on any
bidirectional network. bidirectional network.
The tunneling mechanism allows any IP traffic, not just routing The tunneling mechanism allows any IP traffic, not just routing
protocol messages, to be forwarded between receivers and feeds. protocol messages, to be forwarded between receivers and feeds.
Receivers can route datagrams on the Internet using the most suitable Receivers can route datagrams on the Internet using the most suitable
feed or receiver as a next hop. Administrators may want to set the feed or receiver as a next hop. Administrators may want to set the
metrics used by their routing protocols in order to reflect in metrics used by their routing protocols in order to reflect in
routing tables the asymmetric characteristics of the link, and thus routing tables the asymmetric characteristics of the link, and thus
direct traffic over appropriate paths. direct traffic over appropriate paths.
Feeds and receivers may implement multicast routing and therefore Feeds and receivers may implement multicast routing and therefore
dynamic multicast routing can be performed over the unidirectional dynamic multicast routing can be performed over the unidirectional
link. However issues related to multicast routing (e.g. protocol link. However issues related to multicast routing (e.g., protocol
configuration) are not addressed in this document. configuration) are not addressed in this document.
9.3. Scalability 9.3. Scalability
The DTCP protocol does not generate a lot of traffic whatever the The DTCP protocol does not generate a lot of traffic whatever the
number of nodes. The problem with a large number of nodes is not number of nodes. The problem with a large number of nodes is not
related to this protocol but to more general issues such as the related to this protocol but to more general issues such as the
maximum number of nodes which can be connected to any link. This is maximum number of nodes which can be connected to any link. This is
out of scope of this document. out of scope of this document.
10. IANA Considerations 10. IANA Considerations
IANA has reserved the address 224.0.0.36 for the "DTCP announcement" IANA has reserved the address 224.0.0.36 for the "DTCP announcement"
multicast address as defined in Section 7. multicast address as defined in Section 7.
IANA has reserved the udp port 652 for the HELLO_PORT as defined in IANA has reserved the udp port 652 for the HELLO_PORT as defined in
Section 7. Section 7.
11. Security Considerations 11. Security Considerations
Many unidirectional link technologies are characterised by the ease Many unidirectional link technologies are characterised by the ease
with which the link contents can be received. If sensitive or with which the link contents can be received. If sensitive or
valuable information is being sent, then link-layer security valuable information is being sent, then link-layer security
mechanisms are an appropriate measure. For the UDLR protocol itself, mechanisms are an appropriate measure. For the UDLR protocol itself,
the feed tunnel end-point addresses, sent in HELLO messages, may be the feed tunnel end-point addresses, sent in HELLO messages, may be
considered sensitive. In such cases link-layer security mechanisms considered sensitive. In such cases link-layer security mechanisms
may be used. may be used.
Security in a network using the link-layer tunneling mechanism should Security in a network using the link-layer tunneling mechanism should
be relatively similar to security in a normal IPv4 network. However, be relatively similar to security in a normal IPv4 network. However,
as the link-layer tunneling mechanism requires the use of tunnels, it as the link-layer tunneling mechanism requires the use of tunnels, it
introduces a potential for unauthorised access to the service. In introduces a potential for unauthorised access to the service. In
particular, ARP and IP spoofing are potential threats because nodes particular, ARP and IP spoofing are potential threats because nodes
may not be authorised to tunnel packets. This can be countered by may not be authorised to tunnel packets. This can be countered by
authenticating all tunnels. The authenticating mechanism is not authenticating all tunnels. The authenticating mechanism is not
specified in this document, it can take place either in the delivery specified in this document, it can take place either in the delivery
IP protocol (e.g. AH[rfc2402]) or in an authentication protocol IP protocol (e.g., AH[RFC2402]) or in an authentication protocol
integrated with the tunneling mechanism. integrated with the tunneling mechanism.
At a higher level, receivers may not be authorised to provide routing At a higher level, receivers may not be authorised to provide routing
information even though they are connected to the unidirectional information even though they are connected to the unidirectional
link. In order to prevent unauthorised receivers from providing fake link. In order to prevent unauthorised receivers from providing fake
routing information, routing protocols running on top of the link- routing information, routing protocols running on top of the link-
layer tunneling mechanism MUST use authentication mechanisms when layer tunneling mechanism MUST use authentication mechanisms when
available. available.
12. Acknowledgments 12. Acknowledgments
We would like to thank Tim Gleeson (Cisco Japan) for his valuable We would like to thank Tim Gleeson (Cisco Japan) for his valuable
editing and technical input during the finalization phase of the editing and technical input during the finalization phase of the
document. document.
skipping to change at page 21, line 9 skipping to change at page 21, line 9
(IMD), Akira Kato (Tokyo Univ.), Hitoshi Asaeda (IBM/ITS), Hiromi (IMD), Akira Kato (Tokyo Univ.), Hitoshi Asaeda (IBM/ITS), Hiromi
Komatsu (JSAT), Hiroyuki Kusumoto (Keio Univ.), Kazuhiro Hara (Sony), Komatsu (JSAT), Hiroyuki Kusumoto (Keio Univ.), Kazuhiro Hara (Sony),
Kenji Fujisawa (Sony), Mikiyo Nishida (Keio Univ.), Noritoshi Demizu Kenji Fujisawa (Sony), Mikiyo Nishida (Keio Univ.), Noritoshi Demizu
(Sony CSL), Jun Murai (Keio Univ.), Jun Takei (JSAT) and Harri (Sony CSL), Jun Murai (Keio Univ.), Jun Takei (JSAT) and Harri
Hakulinen (Nokia). Hakulinen (Nokia).
Appendix A: Conformance and interoperability Appendix A: Conformance and interoperability
This document describes a mechanism to emulate bidirectional This document describes a mechanism to emulate bidirectional
connectivity between nodes that are directly connected by a connectivity between nodes that are directly connected by a
unidirectional link. Applicability over a variety of equipment and unidirectional link. Applicability over a variety of equipment and
environments is ensured by allowing a choice of several key system environments is ensured by allowing a choice of several key system
parameters. parameters.
Thus in order to ensure interoperability of equipment it is not Thus in order to ensure interoperability of equipment it is not
enough to simply claim conformance with the mechanism defined here. A enough to simply claim conformance with the mechanism defined here.
usage profile for a particular environment will require the A usage profile for a particular environment will require the
definition of several parameters: definition of several parameters:
- the MAC format used - the MAC format used
- the tunneling mechanism to be used (GRE is recommended) - the tunneling mechanism to be used (GRE is recommended)
- the "tunnel type" indication if GRE is not used - the "tunnel type" indication if GRE is not used
For example, a system might claim to implement "the link-layer For example, a system might claim to implement "the link-layer
tunneling mechanism for unidirectional links, using IEEE 802 LLC, and tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
GRE encapsulation for the tunnels." GRE encapsulation for the tunnels."
Appendix B: DTCP announcement address transition plan Appendix B: DTCP announcement address transition plan
skipping to change at page 21, line 28 skipping to change at page 21, line 29
- the tunneling mechanism to be used (GRE is recommended) - the tunneling mechanism to be used (GRE is recommended)
- the "tunnel type" indication if GRE is not used - the "tunnel type" indication if GRE is not used
For example, a system might claim to implement "the link-layer For example, a system might claim to implement "the link-layer
tunneling mechanism for unidirectional links, using IEEE 802 LLC, and tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
GRE encapsulation for the tunnels." GRE encapsulation for the tunnels."
Appendix B: DTCP announcement address transition plan Appendix B: DTCP announcement address transition plan
Some older receivers listen for DTCP announcements on the 224.0.1.124 Some older receivers listen for DTCP announcements on the 224.0.1.124
multicast address (the "old DTCP announcement" address). In order to multicast address (the "old DTCP announcement" address). In order to
support such legacy receivers, feeds SHOULD be configurable to send support such legacy receivers, feeds SHOULD be configurable to send
all announcements simultaneously to both the "DTCP announcement" all announcements simultaneously to both the "DTCP announcement"
address, and the "old DTCP announcement" address. The default setting address, and the "old DTCP announcement" address. The default
is to send announcements to just the "DTCP announcement" address. setting is to send announcements to just the "DTCP announcement"
address.
In order to encourage the transition plan, the "old" feeds MUST be In order to encourage the transition plan, the "old" feeds MUST be
updated to send DTCP announcements as defined in this section. The updated to send DTCP announcements as defined in this section. The
number of "old" feeds originally deployed is relatively small and number of "old" feeds originally deployed is relatively small and
therefore the update should be fairly easy. "New" receivers only therefore the update should be fairly easy. "New" receivers only
support "new" feeds, i.e., they listen to DTCP announcements on the support "new" feeds, i.e., they listen to DTCP announcements on the
"DTCP announcement" address. "DTCP announcement" address.
References References
[rfc826] 'An Ethernet Address Resolution Protocol', David C. Plummer, [RFC826] Plummer, D., "An Ethernet Address Resolution Protocol", STD
November 1982. 37, RFC 826, November 1982.
[rfc1112] 'Host Extensions for IP Multicasting', S. Deering, Stanford [RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
University, August 1989 RFC 1112, August 1989
[rfc1700] 'ASSIGNED NUMBERS', J. Reynolds, J. Postel, ISI, October [RFC1700] Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2, RFC
1994 1700, October 1994.
[rfc2119] 'Key words for use in RFCs to Indicate Requirement Levels', [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
S. Bradner, Harvard University, March 1997 Requirement Levels", BCP 14, RFC 2119, March 1997.
[rfc2402] 'IP Authentication Header', S. Kent, BBN Corp, R. Atkinson, [RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
@Home Network 2402, November 1998.
[rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998 [RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
1998.
[rfc2780] 'IANA Allocation Guidelines For Values In the Internet [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Protocol and Related Headers', S. Bradner, Harvard Values In the Internet Protocol and Related Headers", BCP
University, V. Paxson, ACIRI, March 2000 37, RFC 2780, March 2000.
[rfc2784] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li, [RFC2784] Farinacci, D., Hanks, S., Meyer, D. and P. Traina, "Generic
Procket Networks, S. Hanks, Enron Communications, D. Meyer, Routing Encapsulation (GRE)", RFC 2784, March 2000.
Cisco Systems, P. Traina, Juniper Networks, March 2000
Author's address Authors' Addresses
Emmanuel Duros Emmanuel Duros
UDcast UDcast
1681, route des Dolines 1681, route des Dolines
Les Taissounieres - BP 355 Les Taissounieres - BP 355
06906 Sophia-Antipolis Cedex 06906 Sophia-Antipolis Cedex
France France
Phone : +33 4 93 00 16 60 Phone : +33 4 93 00 16 60
Fax : +33 4 93 00 16 61 Fax : +33 4 93 00 16 61
Email : Emmanuel.Duros@UDcast.com EMail : Emmanuel.Duros@UDcast.com
Walid Dabbous Walid Dabbous
INRIA Sophia Antipolis INRIA Sophia Antipolis
2004, Route des Lucioles BP 93 2004, Route des Lucioles BP 93
06902 Sophia Antipolis 06902 Sophia Antipolis
France France
Phone : +33 4 92 38 77 18 Phone : +33 4 92 38 77 18
Fax : +33 4 92 38 79 78 Fax : +33 4 92 38 79 78
Email : Walid.Dabbous@inria.fr EMail : Walid.Dabbous@inria.fr
Hidetaka Izumiyama Hidetaka Izumiyama
JSAT Corporation JSAT Corporation
Toranomon 17 Mori Bldg.5F Toranomon 17 Mori Bldg.5F
1-26-5 Toranomon, Minato-ku 1-26-5 Toranomon, Minato-ku
Tokyo 105 Tokyo 105
Japan Japan
Voice : +81-3-5511-7568
Phone : +81-3-5511-7568
Fax : +81-3-5512-7181 Fax : +81-3-5512-7181
Email : izu@jsat.net EMail : izu@jsat.net
Noboru Fujii Noboru Fujii
Sony Corporation Sony Corporation
2-10-14 Osaki, Shinagawa-ku 2-10-14 Osaki, Shinagawa-ku
Tokyo 141 Tokyo 141
Japan Japan
Voice : +81-3-3495-3092
Fax : +81-3-3495-3527
Email : fujii@dct.sony.co.jp
Phone : +81-3-3495-3092
Fax : +81-3-3495-3527
EMail : fujii@dct.sony.co.jp
Yongguang Zhang Yongguang Zhang
HRL HRL
RL-96, 3011 Malibu Canyon Road RL-96, 3011 Malibu Canyon Road
Malibu, CA 90265, Malibu, CA 90265,
USA USA
Phone : 310-317-5147 Phone : 310-317-5147
Fax : 310-317-5695 Fax : 310-317-5695
Email : ygz@hrl.com EMail : ygz@hrl.com
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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English.
The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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