draft-ietf-udlr-lltunnel-04.txt   draft-ietf-udlr-lltunnel-05.txt 
Network Working Group E. Duros Network Working Group E. Duros
Internet-Draft W. Dabbous Internet-Draft UDcast
April 2000 INRIA Sophia-Antipolis November 2000 W. Dabbous
Exprires April 2001 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-04.txt> <draft-ietf-udlr-lltunnel-05.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.
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http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
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.
Abstract Abstract
This document describes a mechanism to emulate bidirectional This document describes a mechanism to emulate full bidirectional
connectivity between 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 network. As it is implemented at the link-layer,
protocols in addition to IP may also be supported by this 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 a set of nodes (feeds and receivers, see Section 2 for terminology)
and receivers, see Section 2 for terminology) to send datagrams as if which are directly connected by a unidirectional link to send
they were connected to a bidirectional link. We present a generic datagrams as if they were all connected by a bidirectional link. We
topology with a tunneling mechanism that supports multiple feeds and present a generic topology in section 3 with a tunneling mechanism
receivers. that supports multiple feeds and receivers. Note, this mechanism is
not designed for topologies where a pair of nodes are connected by 2
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 aim
is to hide from higher layers, i.e. the network layer and above, the is to hide from higher layers, i.e. the network layer and above, the
unidirectional nature of the link. The tunneling mechanism also 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.
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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
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
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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 "deaf". Thus a datagram passed to the link-layer driver of a
receive-only interface is simply discarded. The link layer of a receive-only interface is simply discarded. The link-layer of a
send-only 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, such
as NBMA, could also be emulated, a broadcast network is more 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 (LL) 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).
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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. 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 tunneling
mechanism should therefore provide the functionality to support mechanism should therefore provide the functionality to support
scenarios 1 to 5. 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 | |
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| | | | | | | | | | | | | | | | | | | |
--|-----------|---------- ----------|----------|--- --|-----------|---------- ----------|----------|---
| 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 LL 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 behind 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 behind 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:
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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 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 8. The type of encapsulation is described in Section 8.
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send-only feed tunnel end-points. Thus the broadcast/multicast send-only feed tunnel end-points. Thus the broadcast/multicast
will reach all receivers and all send-only feeds. 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 end-
points. Encapsulated packets will have been received on a 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 feed
(encapsulated broadcast/multicast packet sent to a send-only (encapsulated broadcast/multicast packet sent to a send-only
feed). These cases are distinguished by examining the source feed). These cases are distinguished by examining the source
address of the encapsulating packet and comparing it with the address of the encapsulating packet and comparing it with the
configured list of feed IP addresses. The action then taken is: 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. Delivery
to all nodes is accomplished by executing all 3 of the to all nodes is accomplished by executing all 3 of the
following actions: following actions:
- The packet is encapsulated and sent to the list of send- - The packet is encapsulated and sent to the list of send-
only feed tunnel end-points. only feed tunnel end-points.
- Also, the packet is passed to the link layer of the - Also, the packet is passed to the link-layer of the
interface which forwards it directly over the interface which forwards it directly over the
unidirectional link (all receivers and receive capable unidirectional link (all receivers and receive capable
feeds receive it). feeds receive it).
- Also, the link layer delivers it locally to higher layers. - Also, the link-layer delivers it locally to higher layers.
Caution: a receiver which sends an encapsulated Caution: a receiver which sends an encapsulated
broadcast/multicast packet to a default feed will receive broadcast/multicast packet to a default feed will receive its
its own packet via the unidirectional link. Correct own packet via the unidirectional link. Correct filtering as
filtering as described in [rfc1112] must be applied. 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 higher layers. 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
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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; receivers MUST use this form of tunnel encapsulation when the feed. This value is the IP protocol number defined in [rfc1700]
tunneling to the feed. [iana/protocol-numbers] and their legitimate descendents. Receivers
MUST use this form of tunnel encapsulation when tunneling to the
feed.
47 = GRE [rfc2784] (recommended) 47 = GRE [rfc2784] (recommended)
Other values may be used, but their interpretation is not specified Other protocol types allowing link-layer encapsulation are
here. 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
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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 FUMAC
address in the list of active feeds. The packet is tunneled to address in the list of active feeds. The packet is tunneled to
the preferred FBIP of the matching feed. 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 This is determined by the MAC address format rules, and the list
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"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*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. 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 packets sent over the unidirectional links without any assumption
about 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 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.
Other encapsulations are possible, such as directly encapsulating a
MAC level packet within an IP datagram.
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
8.2. Encapsulation of UDL MAC level packets
An alternative is to encapsulate the MAC level packet within IP. The
protocol field in the IP datagram is then set to the MAC type of the
unidirectional link. Figure 5 presents the entire encapsulated
packet.
----------------------------------------
| IP delivery header |
| destination addr = FBIP |
| IP proto = MAC type of the UDL |
----------------------------------------
| Payload packet |
| MAC packet |
----------------------------------------
Figure 5: 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 a
receiver and a feed has varying delays because packets go through the receiver and a feed has varying delays because packets go through the
Internet. Furthermore, a typical example of a unidirectional link is Internet. Furthermore, a typical example of a unidirectional 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, 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" an
IP address to a MAC address, they just need to perform the simple 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.
skipping to change at page 19, line 17 skipping to change at page 19, line 46
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. Security Considerations 10. Security Considerations
Security in a network using the link layer tunneling mechanism should Many unidirectional link technologies are characterised by the ease
with which the link contents can be received. If sensitive or
valuable information is being sent, then link-layer security
mechanisms are an appropriate measure. For the UDLR protocol itself,
the feed tunnel end-point addresses, sent in HELLO messages, may be
considered sensitive. In such cases link-layer security mechanisms
may be used.
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 uses GRE[rfc2784], it is as the link-layer tunneling mechanism requires the use of tunnels, it
expected that GRE authentication mechanism combined with a specific introduces a potential for unauthorised access to the service. In
link layer security mechanism on the back-channel will help to particular, ARP and IP spoofing are potential threats because nodes
enhance security in a unidirectional link environment. may not be authorised to tunnel packets. This can be countered by
authenticating all tunnels. The authenticating mechanism is not
specified in this document, it can take place either in the delivery
IP protocol (e.g. AH[rfc2402]) or in an authentication protocol
integrated with the tunneling mechanism.
In order to prevent unauthorised users from providing fake routing At a higher level, receivers may not be authorised to provide routing
information, routing protocols running on top of the link layer information even though they are connected to the unidirectional
tunneling mechanism MUST use authentication mechanisms when link. In order to prevent unauthorised receivers from providing fake
routing information, routing protocols running on top of the link-
layer tunneling mechanism MUST use authentication mechanisms when
available. available.
11. Acknowledgments 11. 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.
We would like to thank Patrick Cipiere (INRIA) for his valuable input We would like to thank Patrick Cipiere (UDcast) for his valuable
concerning the design of the encapsulation mechanism. input 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).
A. Conformance and interoperability A. Conformance and interoperability
skipping to change at page 20, line 16 skipping to change at page 21, line 11
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. A
usage profile for a particular environment will require the 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."
References 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
[rfc2119] 'Key words for use in RFCs to Indicate Requirement Levels', [rfc1700] 'ASSIGNED NUMBERS', J. Reynolds, J. Postel, ISI, October
[rfc2401] 'Security Architecture for the Internet Protocol', S. Kent, 1994
BBN Corp, R. Atkinson, @Home Network
[rfc2119] 'Key words for use in RFCs to Indicate Requirement Levels',
[rfc2402] 'IP Authentication Header', S. Kent, BBN Corp, R. Atkinson, [rfc2402] 'IP Authentication Header', S. Kent, BBN Corp, R. Atkinson,
@Home Network @Home Network
[rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998 [rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998
[rfc2780] 'IANA Allocation Guidelines For Values In the Internet
Protocol and Related Headers', S. Bradner, Harvard
[rfc2784] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li, [rfc2784] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li,
Procket Networks, S. Hanks, Enron Communications, D. Meyer, Procket Networks, S. Hanks, Enron Communications, D. Meyer,
Author's address Author's address
Emmanuel Duros Emmanuel Duros
INRIA Sophia Antipolis UDcast
2004, Route des Lucioles BP 93 Les Taissounieres - HB3
06902 Sophia Antipolis 1681, route des Dolines
06560 Sophia-Antipolis
France France
Phone : +33 4 92 38 79 42 Phone : +33 4 93 00 16 60
Fax : +33 4 92 38 79 78 Fax : +33 4 93 00 16 61
Email : Emmanuel.Duros@inria.fr 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
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