draft-ietf-udlr-lltunnel-02.txt   draft-ietf-udlr-lltunnel-03.txt 
Network Working Group E. Duros Network Working Group E. Duros
Internet-Draft W. Dabbous Internet-Draft W. Dabbous
June 1999 INRIA Sophia-Antipolis Feb 2000 INRIA Sophia-Antipolis
H. Izumiyama H. Izumiyama
N. Fujii N. Fujii
WIDE WIDE
Y. Zhang Y. Zhang
HRL HRL
A Link Layer Tunneling Mechanism for Unidirectional Links A Link Layer Tunneling Mechanism for Unidirectional Links
<draft-ietf-udlr-lltunnel-02.txt> <draft-ietf-udlr-lltunnel-03.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 38 skipping to change at page 1, line 39
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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A version of this draft document is intended for submission to the A version of this draft document is intended for submission to the
RFC editor as a Proposed Standard for the Internet Community. RFC editor as a Proposed Standard for the Internet Community.
Note to the RFC editor
Please replace all references to rfcXXXX with references to the new
GRE specification when it is published as Proposed Standard. It
currently exists as <draft-meyer-gre-update-03.txt>. The entry in the
"References" section may also need updating at this time.
Abstract Abstract
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. The "receiver" uses a link layer tunneling unidirectional link. The "receiver" uses a link layer tunneling
mechanism to forward datagrams to "feeds" over a bidirectional mechanism to forward datagrams to "feeds" over a separate
network. As it is implemented at link layer, other protocols than IP bidirectional IP network. As it is implemented at the link layer,
may also use this tunneling mechanism. 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
nodes which are directly connected by a unidirectional link (feeds nodes which are directly connected by a unidirectional link (feeds
and receivers, see section 2 for terminology) to send datagrams as if and receivers, see Section 2 for terminology) to send datagrams as if
they were connected to a bidirectional link. We present a generic they were connected to a bidirectional link. We present a generic
topology with a tunneling mechanism that supports multiple feeds and topology with a tunneling mechanism that supports multiple feeds and
receivers. receivers.
The tunneling mechanism requires that all nodes have an additional
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 connected to the unidirectional link on every feed and interface of every node connected to the unidirectional link. The aim
every receiver. The aim is to hide from higher layers, i.e. network is to hide from higher layers, i.e. the network layer and above, the
layer and above, the unidirectional feature of the link. The unidirectional nature of the link. The tunneling mechanism also
tunneling mechanism also includes an automatic tunnel configuration includes an automatic tunnel configuration protocol that allows nodes
protocol that allows feeds and receivers to come up/down at any time. to come up/down at any time.
The tunneling mechanism proposes to use Generic Routing Encapsulation Generic Routing Encapsulation [rfcXXXX] is suggested as the tunneling
[rfc1701] and provides a means for carrying IP, ARP datagrams and any mechanism as it provides a means for carrying IP, ARP datagrams, and
layer-3 protocol from receivers to feeds. 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.
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 that has receive-only connectivity to an UDL. Receiver: A router that has receive-only connectivity to a UDL.
Send-only feed: A router that has send-only connectivity to an 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 an 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.
3. Topology 3. Topology
In general, feeds and receivers are connected via a unidirectional Feeds and receivers are connected via a unidirectional link. Send-
link. Send-only feeds can only send data over this unidirectional only feeds can only send data over this unidirectional link, and
link, and receivers can only receive data from it. Receive capable receivers can only receive data from it. Receive capable feeds have
feeds have both send and receive capabilities. In this document, we both send and receive capabilities.
consider the case of several feeds (send-only and/or receive capable)
and many receivers. This mechanism has been designed to work with any topology with any
number of receivers and one or more feeds. However, it is expected
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
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. A receiver or more additional bidirectional communication interfaces. A receiver
is required to be a router. MUST be a router.
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.
In the entire document we assume that feeds and receivers are Tunnels are constructed between the bidirectional interfaces of
connected to the Internet via one of their bidirectional interfaces. nodes, so these interfaces must be interconnected by an IP
A receiver and a feed can then communicate with each other using this infrastructure. In this document we assume that that infrastructure
specific bidirectional interface (Ethernet interface, PPP connection, is the Internet.
etc.).
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
-------- -------- -------- -------- ---------- -------- -------- -------- -------- ----------
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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
bidirectional interfaces. The addresses on the unidirectional
interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
network. In general the addresses on the bidirectional interfaces
(f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
the Internet will route between them.
4. Problems related to unidirectional links 4. Problems related to unidirectional links
Most protocols in the Internet assume that links are bidirectional. Receive-only interfaces are "dumb" and send-only interfaces are
In particular, routing protocols used by directly connected routers "deaf". Thus a datagram passed to the link layer driver of a
no longer behave properly in the presence of a unidirectional link. receive-only interface is simply discarded. The link layer of a
Indeed, receivers cannot send requests/responses or routing messages send-only interface never receives anything.
to feeds through their receive-only interface.
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. Recv 1 network layer delivers the packet to the driver from Feed 1. The network layer of Recv 1 delivers the packet to the
of the receive-only interface, which obviously cannot send it to the driver of the receive-only interface, which obviously cannot send it
feed. to the feed.
More generally, a datagram of any protocol that passes from the Most protocols in the Internet assume that links are bidirectional.
network layer to the driver of a receive-only interface is simply In particular, routing protocols used by directly connected routers
discarded. no longer behave properly in the presence of a unidirectional link.
5. Emulating a broadcast bidirectional network 5. Emulating a broadcast bidirectional network
Some unidirectional links (e.g., satellite links) allow by nature The simplest solution is to emulate a broadcast capable link layer
communication from a feed to a set of receivers: a feed can send network. This will allow the immediate deployment of existing higher
packets to a particular receiver and it can send broadcast packets. level protocols without change. Though other network structures, such
However, any other communication is simply impossible: a receiver as NBMA, could also be emulated, a broadcast network is more
cannot send packets to a receiver or a feed, a feed cannot send a generally useful. Though a layer 3 network could be emulated, a link
packet to a send-only feed. layer network allows the immediate use of any other network layer
protocols, and most particularly allows the immediate use of ARP.
A solution to this problem based on a link layer (LL) tunneling A link layer (LL) tunneling mechanism which emulates bidirectional
mechanism which emulates a bidirectional connectivity in the presence connectivity in the presence of a unidirectional link will be
of a unidirectional link will be described in the next section. We described in the next Section. We first consider the various
first consider the various communication scenarios which characterize communication scenarios which characterize a broadcast network in
a broadcast network in order to define what functionalities the link order to define what functionalities the link layer tunneling
layer tunneling mechanism has to perform to emulate a bidirectional mechanism has to perform in order to emulate a bidirectional
broadcast link. broadcast link.
Here follows the scenarios which would be feasible on a broadcast Here we enumerate the scenarios which would be feasible on a
network, i.e if feeds and receivers were connected by a bidirectional broadcast network, i.e. if feeds and receivers were connected by a
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 feed and a receiver). 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
unidirectional 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 Scenario 5: A feed can send a broadcast/multicast packet on the link
unidirectional link to all nodes (point-to-multipoint). to all nodes (point-to-multipoint).
Scenario 6: A feed can send a packet to receiver or a receive capable Scenario 6: A feed can send a packet to a receiver or a receive
feed. This is the default communication over a unidirectional link. capable feed.
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 tunneling
mechanism should therefore provide the functionalities to permit mechanism should therefore provide the functionality to support
scenarios 1 to 5 to happen. Note that the scenarios above allow a scenarios 1 to 5.
node to send a packet to any destination IP address on the Internet.
The next hop address at the receiver will be in this case the address
of another router (a feed or a receiver) which will relay the packet.
6. Link layer tunneling mechanism Note that regular IP forwarding over such an emulated network (i.e.
using the emulated network as a transit network) works correctly; the
next hop address at the receiver will be the unidirectional link
address of another router (a feed or a receiver) which will then
relay the packet.
This section describes a link layer tunneling mechanism allowing 6. Link layer tunneling mechanism
bidirectional communication between feeds and receivers in the
presence of a unidirectional link. This mechanism has been designed
to work with any topology regardless of the number of feeds and
receivers. In particular, the special case of a single send-only feed
and multiple receivers is among the topologies supported.
This link layer tunneling mechanism operates underneath the network This link layer tunneling mechanism operates underneath the network
layer. Its aim is to emulate a bidirectional connectivity. It is layer. Its aim is to emulate bidirectional link layer connectivity.
transparent to the network layer: the link appears and behaves with This is transparent to the network layer: the link appears and
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
/-----***************----\ /-->---***************--\ /-----***************----\ /-->---***************--\
| | | | | | | |
| | | | | | | |
skipping to change at page 6, line 43 skipping to change at page 7, line 42
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 LL Tunneling Mechanism Figure 2: Scenario 1 using the LL Tunneling Mechanism
6.1. Tunneling mechanism on the receiver 6.1. Tunneling mechanism on the receiver
A datagram is delivered from the network layer to the link layer of On the receiver, a datagram is delivered to the link layer of the
the unidirectional interface (see Figure 2). It is then encapsulated unidirectional interface for transmission (see Figure 2). It is then
behind a MAC header corresponding to the unidirectional link. This encapsulated behind a MAC header corresponding to the unidirectional
packet cannot be sent over the link and is then processed by the link. This packet cannot be sent directly over the link, so it is
tunneling mechanism. then processed by the tunneling mechanism.
The packet is encapsulated behind an IP header whose destination is The packet is encapsulated behind an IP header whose destination is
the IP address of a feed bidirectional interface (f1b or f2b), also the IP address of a feed bidirectional interface (f1b or f2b). This
called the tunnel end-point. The mechanism for a receiver to learn destination address is also called the tunnel end-point. The
these addresses and to choose the feed is explained in Section 6.3. mechanism for a receiver to learn these addresses and to choose the
The type of encapsulation is described in Section 7. feed is explained in Section 7. The type of encapsulation is
described in Section 8.
The new datagram is passed to the network layer that forwards it
according to its destination address. The destination address of the
encapsulated datagram is a feed bidirectional interface which is
reachable via the Internet. The datagram is then forwarded via the
receiver bidirectional interface (r1b).
We have to distinguish several cases as the datagram is to be In all cases the packet is encapsulated, but the tunnel end-point (an
tunneled according to the destination MAC address. If the destination IP address) depends on the encapsulated packet's destination MAC
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 encapsulated,
the destination address of the new datagram is the feed tunnel the destination address of the encapsulating datagram is the
end-point (f1b referring to Figure 2). 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 is
encapsulated, the destination address of the new datagram is the encapsulated, the destination address of the encapsulating
default feed tunnel end-point. See Section 6.3.4 for further datagram is the default feed tunnel end-point. See Section 7.4
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,
and sent to the tunnel end-point of the default feed. the destination address of the encapsulating datagram is the
default feed tunnel end-point.
At this point, the network layer passes a datagram to the link layer The encapsulated datagram is passed to the network layer which
of the receive-only interface, it is encapsulated and sent to a feed forwards it according to its destination address. The destination
via its bidirectional interface. address is a feed bidirectional interface which is reachable via the
Internet. In this case, the encapsulated datagram is forwarded via
the receiver bidirectional interface (r1b).
6.2. Tunneling mechanism on the feed 6.2. Tunneling mechanism on the feed
A feed processes packets in two different ways: packets must be A feed processes unidirectional link related packets in two different
forwarded over the unidirectional link (e.g. packets generated by a ways:
local application or a packet routed by the IP layer, see section - packets generated by a local application or packets routed as
6.2.1) and encapsulated packets received from one of the receivers or usual by the IP layer may have to be forwarded over the
the feeds that need particular processing (section 6.2.2). unidirectional link (Section 6.2.1)
- encapsulated packets received from another receiver or feed need
tunnel processing (Section 6.2.2).
A feed cannot send directly over the unidirectional link a packet to A feed cannot directly send a packet to a send-only feed over the
a send-only feed. In order to emulate this type of communication, a unidirectional link (Scenario 4). In order to emulate this type of
feed MUST maintain a list of send-only feed tunnel end-points. This communication, feeds have to tunnel packets to send-only feeds. A
is configured manually at the feed by the local administrator. In feed MUST maintain a list of all other feed tunnel end-points. This
fact, as stated in Section 3, the number of feeds is expected to be list MUST indicate which are send-only feed tunnel end-points. This
relatively small, therefore a manual configuration can be envisaged. is configured manually at the feed by the local administrator, as
How to use this list is detailed in the next section. described in Section 7.
6.2.1. Forwarding packets over the unidirectional link 6.2.1. Forwarding packets over the unidirectional link
The way a packet is forwarded depends on its destination MAC address, When a datagram is delivered to the link layer of the unidirectional
if it is: interface of a feed for transmission, its treatment depends on the
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 the 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 is
encapsulated and sent to the send-only feed tunnel end-point. encapsulated and sent to the send-only feed tunnel end-point.
The type of encapsulation is described in Section 7. 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 with a link layer header sent over the unidirectional link. Concurrently, a copy of this
corresponding to the broadcast/multicast addressing scheme. packet is encapsulated and sent to every feed of the list of
Currently, a copy of this packet is encapsulated and sent to send-only feed tunnel end-points. Thus the broadcast/multicast
every feed of the list of send-only feed tunnel end-points. will reach all receivers and all send-only feeds.
6.2.2. Receiving encapsulated packets 6.2.2. Receiving encapsulated packets
Feeds listen to incoming encapsulated datagrams on their tunnel end- Feeds listen for incoming encapsulated datagrams on their tunnel end-
point. An encapsulated packet which is received from the points. Encapsulated packets will have been received on a
bidirectional interface, traverses the IP stack and enters a bidirectional interface, and traversed their way up the IP stack.
decapsulation process (See Figure 2). Note that the original datagram They will then enter a decapsulation process (See Figure 2).
was encapsulated and therefore its payload (especially MAC and IP
header) has not been modified by intermediate routers. It is
decapsulated and further actions depend on the destination MAC
address which can be:
1) the MAC address of the feed interface connected to the Decapsulation reveals the original link layer packet. Note that this
unidirectional link, this corresponds to Scenarios 1 and 4. The has not been modified in any way by intermediate routers; in
packet is passed to the link layer of the interface connected to particular, the original MAC header will be intact.
the unidirectional link which then delivers it to the network
layer. As a result, the datagram is processed as if it was
coming from the unidirectional link. At this point, Scenarios 1
and 4 are now feasible, a receiver or a feed can send a packet
to a feed.
2) a receiver address, this corresponds to Scenario 3. The packet Further actions depend on the destination MAC address of the link
is passed to the link layer of the interface connected to the layer packet, which can be:
unidirectional link. It is directly sent over the unidirectional
link with the proper link layer encapsulation to the receiver
(note, the packet must not be delivered to the network layer).
Scenario 5 is now feasible, a receiver can send a packet to
another receiver.
3) a broadcast/multicast address, this corresponds to Scenario 2. 1) the MAC address of the feed interface connected to the
unidirectional link, i.e. own MAC address (Scenarios 1 and 4).
The packet is passed to the link layer of the interface
connected to the unidirectional link which can then deliver it
up to higher layers. As a result, the datagram is processed as
if it was coming from the unidirectional link, and being
delivered locally. Scenarios 1 and 4 are now feasible, a
receiver or a feed can send a packet to a feed.
We have to distinguish two cases, either (i) the encapsulated 2) a receiver address (Scenario 3). The packet is passed to the
packet was sent from a receiver or (ii) from a feed link layer of the interface connected to the unidirectional
link. It is directly sent over the unidirectional link, to the
indicated receiver. Note, the packet must not be delivered
locally. Scenario 3 is now feasible, a receiver can send a
packet to another receiver.
3) a broadcast/multicast address, this corresponds to Scenarios 2
and 5. We have to distinguish two cases, either (i) the
encapsulated packet was sent from a receiver or (ii) from a feed
(encapsulated broadcast/multicast packet sent to a send-only (encapsulated broadcast/multicast packet sent to a send-only
feed): feed). These cases are distinguished by examining the source
address of the encapsulating packet and comparing it with the
configured list of feed IP addresses. The action then taken is:
i) the feed was designed 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. An charge of sending the multicast packet to all nodes. Delivery
encapsulation process at the feed encapsulates the packet and to all nodes is accomplished by executing all 3 of the
sends a copy to the list of send-only feed tunnel end-points. following actions:
The packet is also passed to the link layer of the interface - The packet is encapsulated and sent to the list of send-
which forwards it directly over the unidirectional link (all only feed tunnel end-points.
receivers and receive capable feeds receive it). The link - Also, the packet is passed to the link layer of the
layer also delivers it locally to the network layer. Caution: interface which forwards it directly over the
a receiver which sends an encapsulated broadcast/multicast unidirectional link (all receivers and receive capable
packet to a default feed will receive its own packet via the feeds receive it).
unidirectional link. Correct filtering as described in - Also, the link layer delivers it locally to higher layers.
[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 delivers
it to the network layer. 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.
6.3. 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 configuration of the send-only feeds list is performed manually The number of feeds is expected to be relatively small (Section 3),
at the feed. The administrator sets up tunnels to all send-only so at every feed the list of all feeds is configured manually. This
feeds. A tunnel end-point is an IP address of a send-only feed. list should note which are send-only feeds, and which are receive
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
a send-only feed.
For scalability reasons this cannot be done at the receivers. Tunnels For scalability reasons, manual configuration cannot be done at the
must be configured and maintained dynamically in order to cope with receivers. Tunnels must be configured and maintained dynamically by
the following events: receivers, both for scalability, and in order to cope with the
following events:
1) when a new feed comes up, every receiver must create a tunnel to 1) New feed detection.
enable a bidirectional communication with it. Static tunneling When a new feed comes up, every receiver must create a tunnel to
configuration is not appropriate, as new feeds can be connected enable bidirectional communication with it.
to the unidirectional link at any time.
2) when the unidirectional link is down, receivers must disable 2) Loss of unidirectional link detection.
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. Indeed there are protocols that consider a link as receivers. Protocols that consider a link as operational if they
operational if they receive datagrams from it (e.g., the RIP receive datagrams from it (e.g. the RIP protocol [rfc2453])
protocol [rfc2453]). require this behavior for correct operation.
3) when a feed is down, receivers must disable their corresponding 3) Loss of feed detection.
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 tunneled
which would overload Internet. For instance, there is no need which might overload the Internet. For instance, there is no
for receivers to forward a broadcast message through a tunnel need for receivers to forward a broadcast message through a
whose end-point is down. tunnel whose end-point is down.
Note that these tunnels have no existence at the IP layer and are not The DTCP protocol provides a means for receivers to dynamically
considered as tunnel interfaces. The DTCP protocol provides a means discover the presence of feeds and to maintain a list of operational
for receivers to discover dynamically the presence of feeds and to tunnel end-points. Feeds periodically announce their tunnel end-point
maintain a list of operational tunnel end-points. It is based on feed addresses over the unidirectional link. Receivers listen to these
periodical announcements over the unidirectional link that contain announcements and maintain a list of tunnel end-points.
tunnel end-point addresses. Receivers listen to these announcements
and maintain a list of tunnel end-points.
6.3.1. 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 2. Fields contain binary The packet format is shown in Figure 3. Fields contain binary
integers, in normal Internet order with the most significant byte 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 6.3.5: written in uppercase and are defined in Section 7.5:
Vers (4 bits): DTCP version number. MUST be DTCP_VERSION. Vers (4 bit unsigned integer): DTCP version number. MUST be
DTCP_VERSION.
Com (4 bits): 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 same layer-3 protocol. The recommended value is messages for the IP protocol in "IP Vers". Must be > 0. The
HELLO_INTERVAL. This field MUST not be changed while sending. recommended value is HELLO_INTERVAL. If this value is increased,
the feed MUST continue to send HELLO messages at the old rate for
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 bits): IP protocol version of the feed bidirectional IP IP Vers (4 bit unsigned integer): IP protocol version of the feed
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
the feed; receivers MUST use this form of tunnel encapsulation when
tunneling to the feed.
47 = GRE [rfcXXXX] (recommended)
Other values may be used, but their interpretation is not specified
here.
Tunnel Type (8 bits): tunneling protocol supported by feed, Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed
corresponds to the type of encapsulation used by receivers to addresses which are enumerated in the HELLO message
encapsulate packets which are tunneled:
47 = GRE [rfc1701] (recommended)
x = any other tunneling supporting the UDL MAC packets.
Nb of FBIP (8 bits): Number of bidirectional IP feed 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 bit field. The feed bidirectional IP Feed BDL IP addr (32 or 128 bits). The bidirectional IP address feed
address is the IP address of a feed bidirectional interface (tunnel is the IP address of a feed bidirectional interface (tunnel end-
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 tunnel
end-point. end-point.
6.3.2. 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. HELLO_PORT with a TTL of 1.
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 6.3.1. 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 to: 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 the
unidirectional link. unidirectional link.
6.3.3. 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. The list of tunnel end-points is the entries the tunnel end-points for those feeds.
(FUIP,FBIP1,...,FBIPn) and is initially empty.
For each active feed, and each IP protocol supported, at least the
following information will be kept:
FUIP - feed unidirectional link IP address
FUMAC - MAC address corresponding to the above IP
address
(FBIP1,...,FBIPn) - list of tunnel end-points
tunnel type - tunnel type supported by this feed
Sequence - "Sequence" value from the last HELLO received
from this feed
timer - used to timeout this entry
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
specified here.
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 After successfully checking the version number further action depends
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, the entry (FUIP,FBIP1,...,FBIPn) is added to the If it does not, a new entry, for feed FUIP, is created and added
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. The addresses to read is deduced from the 'Nb of FBID' field. These
tunnel type is also read and recorded for this FUIP. A timer set tunnel end-points (FBIP1,...,FBIPn) can then be added to the new
to HELLO_LEAVE is associated with this entry. entry. The tunnel type and Seq values are also taken from the
HELLO packet and recorded in the new entry. A timer set to
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 it is contained in the previous HELLO packet sent by this feed. If they
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 reset. 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 which recv 2) have a list of active feeds containing two entries: Feed 1
are (f1u,(f1b)) and (f2u,(f2b)). 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).
- LEAVE: - LEAVE:
The process checks if there is an entry with the value FUIP that The process checks if there is an entry for FUIP in the list of
belongs to the list of active feeds. If it does, the entry active feeds. If there is, the timer is disabled and the entry is
(FUIP,FBIP1,...,FBIPn) is deleted from the list and the deleted from the list. The LEAVE message provides a means of
corresponding timer is disabled. The LEAVE message provides a quickly updating the list of active feeds.
means of quickly updating the list of active feeds.
A timeout occurs for 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 are
received from the feeds. All the timeouts associated with received from any of the feeds, and they will each timeout
entries (FUIP,FBIP1,...,FBIPn) expire at HELLO_LEAVE after the independently. The timeout of each entry depends on its
last JOIN message from the unidirectional link. individual HELLO_LEAVE value, and when the last JOIN message was
sent by that feed, before the unidirectional link went down.
In both cases, the associated (FUIP,FBIP1,...,FBIPn) entries are In either case, bidirectional connectivity can no longer be ensured
removed from the list of active feeds. As either the feed does not between the receiver and the feed (FUIP): either the feed is no
route datagrams over the unidirectional link or the link is down, longer routing datagrams over the unidirectional link, or the link is
bidirectional connectivity can no longer be ensured between the down. Thus the associated entry is removed from the list of active
receiver and the feed (FUIP). 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 the receivers with the list of usable The HELLO protocol provides receivers with a list of feeds, and a
tunnel end-points (FBIP1,..., FBIPn) per feed. In the following list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed. In
section, we describe how to integrate the HELLO protocol into the the following Section, we describe how to integrate the HELLO
tunneling mechanism described in Sections 6.1 and 6.2. protocol into the tunneling mechanism described in Sections 6.1 and
6.2.
6.3.4. 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 done by the receiver administrator The choice of the default feed is made independently at each
according to a local policy. This policy is out of scope of in this receiver. The choice is a matter of local policy, and this policy is
document. However, as an example, the default feed may be the feed out of scope for this document. However, as an example, the default
that has the lowest round trip time to the receiver. feed may be the feed that has the lowest round trip time to the
receiver.
When a receiver sends a packet to a feed it chooses the tunnel end- When a receiver sends a packet to a feed, it must choose a tunnel
point within the FBIP list. The 'preferred FBIP' is generally FBIP1 end-point from within the FBIP list. The 'preferred FBIP' is
(see 6.3.1). However, for some reasons, a receiver may have a better generally FBIP1 (Section 7.1). For various reasons, a receiver may
connectivity to another FBIPi of this feed. In that case, it is decide to use a different FBIP, say FBIPi instead of FBIP1, as the
possible to use FBIPi instead of FBIP1 as tunnel end-point. This tunnel end-point. For example, the receiver may have better
decision is taken by the receiver administrator. connectivity to FBIPi. This decision is taken by the receiver
administrator.
Several cases are enumerated in Section 6.1 to determine where to Here we show how the list of active feeds is involved when a receiver
forward a MAC packet according to its destination address. If the tunnels a link layer packet. Section 6.1 listed the following cases,
destination address 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 with unidirectional link: This is TRUE if the address matches a FUMAC
the MAC address of an FUIP in the list of active feeds. The address in the list of active feeds. The packet is tunneled to
datagram is encapsulated and sent the preferred FBIP of the the preferred FBIP of the matching feed.
feed. The encapsulation type depends on the tunnel type required
by the feed FUIP.
2) the broadcast address of the unidirectional link or a multicast 2) the broadcast address of the unidirectional link or a multicast
address: a copy of the datagram is encapsulated and sent to the address:
preferred FBIP of the default feed. The encapsulation type This is determined by the MAC address format rules, and the list
depends on the tunnel type required by the default feed. 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 3) an address that belongs to the unidirectional network but is not
a feed address (i.e., it does not match a MAC address of any a feed address:
FUIP in the list of active feeds): the datagram is encapsulated This is TRUE if the address is neither broadcast nor multicast,
and sent to the preferred FBIP of the default feed. The nor found in the list of active feeds. The packet is tunneled to
encapsulation type depends on the tunnel type required by the the preferred FBIP of the default feed.
default feed.
6.3.5. Constant definitions In all cases, the encapsulation type depends on the tunnel type
required by the feed which is selected.
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 224.0.1.124. "DTCP announcement" multicast group is 224.0.1.124.
HELLO_PORT is 652. It is a reserved system port, no other traffic HELLO_PORT is 652. It is a reserved system port, no other traffic
must be allowed. must be allowed.
HELLO_LEAVE is 3*HELLO_INTERVAL, i.e. 15 seconds. HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e. 15
seconds if the default HELLO_INTERVAL was advertised.
7. Tunnel encapsulation format 8. Tunnel encapsulation format
The tunneling mechanism operates at the link layer level and emulates The tunneling mechanism operates at the link layer and emulates
bidirectional connectivity between 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. The
hardware (or the driver) of the receiver can then filter the incoming hardware (or the driver) of the receiver can then filter the incoming
packets sent over the unidirectional links without any assumption of packets sent over the unidirectional links without any assumption
the encapsulated data type. 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 over the unidirectional link via its tunnels. broadcast MAC packets via its tunnels. Link layer packets are
The encapsulation process should encapsulate link layer level encapsulated. As a result, after decapsulating an incoming packet,
packets. As a result, after decapsulating an incoming packet, the the feed can perform link layer filtering as if the data came
feed can perform link layer filtering as if data was directly coming directly from the unidirectional link (See Figure 2).
from the unidirectional link (See Figure 2).
The Generic Routing Encapsulation (GRE) [rfc1701] suits our Generic Routing Encapsulation (GRE) [rfcXXXX] suits our requirements
requirements because it specifies a protocol for encapsulating because it specifies a protocol for encapsulating arbitrary packets,
arbitrary packets within IP as the delivery protocol. Alternatively, and allows use of IP as the delivery protocol.
we can also encapsulate directly a MAC level packet within an IP
datagram.
It is the feed's local administrator who decides what encapsulation Other encapsulations are possible, such as directly encapsulating a
is used by a receiver to send a packet via a tunnel to this feed. The MAC level packet within an IP datagram.
tunnel type field in the HELLO message is either set to GRE or to any
other encapsulation supporting UDL MAC level packets.
7.1. Generic Routing Encapsulation on the receiver The feed's local administrator decides what encapsulation it will
demand that receivers use, and sets the tunnel type field in the
HELLO message appropriately. The value 47 (decimal) indicates GRE.
Other values can be used, but their interpretation must be agreed
upon between feeds and receivers. Such usage is not defined here.
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 the encapsulated protocol (ARP, IP, IPX, etc.). See [rfcXXXX] for
[rfc1701][rfc1702] for details about the encapsulation. In our case, details about the encapsulation. In our case, only support for the
only the support of the MAC addressing scheme of the unidirectional MAC addressing scheme of the unidirectional link MUST be implemented.
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) |
---------------------------------------- ----------------------------------------
skipping to change at page 16, line 16 skipping to change at page 18, line 13
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 level 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
7.2. Encapsulation of UDL MAC level packets 8.2. Encapsulation of UDL MAC level packets
An alternative is to encapsulate the MAC level packet within IP. The An alternative is to encapsulate the MAC level packet within IP. The
protocol field in the IP datagram is then set to the MAC level type protocol field in the IP datagram is then set to the MAC type of the
of the unidirectional link. Figure 5 presents the entire encapsulated unidirectional link. Figure 5 presents the entire encapsulated
packet. packet.
---------------------------------------- ----------------------------------------
| IP delivery header | | IP delivery header |
| destination addr = FBIP | | destination addr = FBIP |
| IP proto = MAC level of the UDL | | IP proto = MAC type of the UDL |
---------------------------------------- ----------------------------------------
| Payload packet | | Payload packet |
| MAC packet | | MAC packet |
---------------------------------------- ----------------------------------------
Figure 5: Encapsulated packet Figure 5: Encapsulated packet
8. Issues 9. Issues
8.1. Hardware address resolution 9.1. Hardware address resolution
Regardless of unidirectional or bidirectional links, if a feed sends Regardless of whether the link is unidirectional or bidirectional, if
a packet over a broadcast type network it requires the data link a feed sends a packet over a non-point-to-point type network, it
address of the destination. ARP [rfc826] is used on an Ethernet requires the data link address of the destination. ARP [rfc826] is
network for that 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 back-channel between delays between every (feed, receiver) pair. The backchannel between a
a receiver and a feed has varying delays because packets go through receiver and a feed has varying delays because packets go through the
the Internet. Furthermore, a typical example of a unidirectional Internet. Furthermore, a typical example of a unidirectional link is
link is a GEO satellite link whose delay is about 250 milliseconds. a GEO satellite link whose delay is about 250 milliseconds.
Because of long round trip delays, current address resolution such as Because of long round trip delays, reactive address resolution
[rfc826] may not be efficient. E.g., a feed has to forward packets at methods such as ARP [rfc826] may not work well. For example, a feed
high data rates to a receiver whose hardware address is unknown. The may have to forward packets at high data rates to a receiver whose
stream of packets is passed to the link layer driver of the feed hardware address is unknown. The stream of packets is passed to the
send-only interface. When the first packet arrives, the link layer link layer driver of the feed send-only interface. When the first
realizes it does not have the corresponding hardware address of the packet arrives, the link layer realizes it does not have the
next hop, and sends an ARP request. While the link layer is waiting corresponding hardware address of the next hop, and sends an ARP
for the response (at least 250 ms for GEO satellite), IP packets are request. While the link layer is waiting for the response (at least
buffered by the feed. If it runs out of space before the ARP response 250 ms for the GEO satellite case), IP packets are buffered by the
arrives, IP packets will be dropped. feed. If it runs out of space before the ARP response arrives, IP
packets will be dropped.
The inefficiency of address resolution protocols is not addressed in This problem of address resolution protocols is not addressed in this
this document. An ad-hoc solution is proposed when the MAC address is document. An ad-hoc solution is possible when the MAC address is
configurable (which is the case in some satellite receiver cards). It configurable, which is possible in some satellite receiver cards. A
consists in mapping the IP address on a MAC address. In this case, no simple transformation (maybe null) of the IP address can then be used
address resolution protocol is required. as the MAC address. In this case, senders do not need to "resolve" an
IP address to a MAC address, they just need to perform the simple
transformation.
8.2. Routing protocols 9.2. Routing protocols
The link layer tunneling mechanism hides from the network layer and The link layer tunneling mechanism hides from the network and higher
above 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 in unidirectional link. Communication is bidirectional, but asymmetric
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 have to run routing protocols. They will work fine and receivers must run routing protocols. These protocols will work
because feeds and receivers consider themselves as directly fine because the tunneling mechanism results in bidirectional
connected, and can exchange routing messages over the unidirectional connectivity between all feeds and receivers. Thus routing messages
link. can be exchanged as on any bidirectional network.
The tunneling mechanism allows one to forward all the IP traffic, and The tunneling mechanism allows any IP traffic, not just routing
not only routing protocol messages from receivers to feeds. Receivers protocol messages, to be forwarded between receivers and feeds.
can route datagrams on the Internet using the most suitable feed or Receivers can route datagrams on the Internet using the most suitable
receiver as a next hop. feed or receiver as a next hop. Administrators may want to set the
metrics used by their routing protocols in order to reflect in
routing tables the asymmetric characteristics of the link, and thus
direct traffic over appropriate paths.
Feeds and receivers can run multicast routing daemon and therefore Feeds and receivers can run multicast routing daemons 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.
8.3. 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 a more general issue such as the related to this protocol but to more general issues such as the
maximum number of nodes which can be connected to a link. This is out maximum number of nodes which can be connected to any link. This is
of scope of this document. out of scope of this document.
9. Security Considerations 10. Security Considerations
Receivers send packets through tunnels to feeds. Because of Confidentiality and integrity concerns may arise from the lower layer
scalability reasons, there is no specific mechanism in this document technologies employed, e.g. if the unidirectional link is a satellite
to ensure that a receiver is allowed to set a tunnel to a feed. Any link and the backchannel is the public internet. Since this protocol
malicious individual that gains access to the unidirectional link can aims to support a link layer, link layer confidentiality and
get full connectivity to feeds. Therefore it is highly recommended on integrity mechanisms may be appropriate. In the case of the
the feed site to have some firewall capabilities. backchannel only, IPSEC [rfc2401] may provide appropriate services.
10. Acknowledgments Theft of service and denial of service attacks become possible to
systems which can discover the feed tunnel end-point addresses and
can direct packets to them. It may be appropriate for feeds to
authenticate tunnel sources, i.e. receivers. Feeds can validate the
IP source addresses of tunneled packets, but this can be easily
spoofed. MAC layer filtering may also be possible. Adequate
protection can be ensured using IPSEC [rfc2401] AH [rfc2402] to
provide strong authentication of tunnel sources. For reasons of
scalability, no particular mechanism is specified in this protocol.
11. Acknowledgments
We would like to thank Tim Gleeson (Cisco Japan) for his valuable
editing and technical input during the finalization phase of the
document.
We would like to thank Patrick Cipiere (INRIA) for his valuable input We would like to thank Patrick Cipiere (INRIA) for his valuable input
concerning the design of the encapsulation mechanism. concerning the design of the encapsulation mechanism.
We would like also to thank for their participation: Akihiro Tosaka We would like also to thank for their participation: Akihiro Tosaka
(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).
11. References A. Conformance and interoperability
This document describes a mechanism to emulate bidirectional
connectivity between nodes that are directly connected by a
unidirectional link. Applicability over a variety of equipment and
environments is ensured by allowing a choice of several key system
parameters.
Thus in order to ensure interoperability of equipment it is not
enough to simply claim conformance with the mechanism defined here. A
usage profile for a particular environment will require the
definition of several parameters:
- the MAC format used
- the tunneling mechanism to be used (GRE is recommended)
- the "tunnel type" indication if GRE is not used
For example, a system might claim to implement "the link layer
tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
GRE encapsulation for the tunnels."
References
[rfc826] 'An Ethernet Address Resolution Protocol', David C. Plummer, [rfc826] 'An Ethernet Address Resolution Protocol', David C. Plummer,
November 1982. November 1982.
[rfc1112] 'Host Extensions for IP Multicasting', S. Deering, Stanford [rfc1112] 'Host Extensions for IP Multicasting', S. Deering, Stanford
University, August 1989 University, August 1989
[rfc1702] 'Generic Routing Encapsulation over IPv4 networks', S. [rfc2401] 'Security Architecture for the Internet Protocol', S. Kent,
Hanks, NetSmiths, Ltd., T. Li, D. Farinacci, P. Traina, BBN Corp, R. Atkinson, @Home Network
Cisco System, October 1994.
[rfc1701] 'Generic Routing Encapsulation (GRE)', S. Hanks, NetSmiths, [rfc2402] 'IP Authentication Header', S. Kent, BBN Corp, R. Atkinson,
Ltd., T. Li, D. Farinacci, P. Traina, Cisco System, October @Home Network
1994.
[rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998 [rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998
[rfcXXXX] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li,
S. Hanks, D. Meyer, P. Traina.
Author's address Author's address
Emmanuel Duros Emmanuel Duros
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 79 42 Phone : +33 4 92 38 79 42
Fax : +33 4 92 38 79 78 Fax : +33 4 92 38 79 78
Email : Emmanuel.Duros@inria.fr Email : Emmanuel.Duros@inria.fr
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