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Versions: 00 01 draft-ietf-savi-fcfs
Network Working Group E. Nordmark
Internet-Draft Sun
Intended status: Standards Track M. Bagnulo
Expires: July 26, 2009 UC3M
January 22, 2009
First-Come First-Serve Source-Address Validation Implementation
draft-bagnulo-savi-fcfs-01
Status of this Memo
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Abstract
This memo describes FCFS SAVI a mechanism to provide source address
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validation for IPv4 and IPv6 networks using the First-Come First-
Serve approach. The proposed mechanism is intended to complement
ingress filtering techniques to provide a higher granularity on the
control of the source addresses used.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design considerations . . . . . . . . . . . . . . . . . . . . 3
2.1. Scope of FCFS SAVI . . . . . . . . . . . . . . . . . . . . 3
2.2. Constraints for FCFS SAVI . . . . . . . . . . . . . . . . 3
2.3. Address ownership proof . . . . . . . . . . . . . . . . . 4
2.4. Special cases . . . . . . . . . . . . . . . . . . . . . . 5
3. FCFS SAVI specification . . . . . . . . . . . . . . . . . . . 5
3.1. FCFS SAVI Data structures . . . . . . . . . . . . . . . . 5
3.2. FCFS SAVI algorithm . . . . . . . . . . . . . . . . . . . 6
3.2.1. Processing of data packets . . . . . . . . . . . . . . 6
3.2.2. Processing of control packets . . . . . . . . . . . . 7
3.3. IPv4 Neighbor Unreachability Detection Procedure . . . . . 9
3.3.1. ARP-based Neighbor Unreachability Detection
procedure . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2. ICMP-based Neighbor Unreachability Detection
procedure . . . . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
6. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
This memo describes FCFS SAVI, a mechanism to provide source address
validation for IPv4 and IPv6 networks using the First-Come First-
Serve approach. The proposed mechanism is intended to complement
ingress filtering techniques to provide a higher granularity on the
control of the source addresses used.
2. Design considerations
2.1. Scope of FCFS SAVI
The application scenario for FCFS SAVI is limited to the local-link.
This means that the goal of FCFS SAVI is verify that the source
address of the packets generated by the hosts attached to the local
link have not been spoofed. FCFS SAVI can be used in IPv4 and in
IPv6 networks.
In any link there usually are hosts and routers attached. Hosts
generate packets with their own address as the source address. This
is the so-called local traffic. while routers send packets containing
a source address other than their own, since they are forwarding
packets generated by other hosts (usually located in a different
link). This what the so-called transit traffic.
The applicability of FCFS SAVI is limited to the local traffic i.e.
to verify if the traffic generated by the hosts attached to the local
link contains a valid source address. The verification of the source
address of the transit traffic is out of the scope of FCFS SAVI.
Other techniques, like ingress filtering [RFC2827], are recommended
to validate transit traffic. In that sense, FCFS SAVI complements
ingress filtering, since it relies on ingress filtering to validate
transit traffic but is provides validation of local traffic, which is
not provided by ingress filtering. Hence, the security level is
increased by using these two techniques.
2.2. Constraints for FCFS SAVI
FCFS SAVI is designed to be susceptible of deployment in existing
networks requiring a minimum set of changes. For that reason, FCFS
SAVI does not require any changes in the hosts which source address
is to be verified. Any verification must solely rely in the usage of
already available protocols. This means that FCFS SAVI cannot define
a new protocol nor to define any new message on existing protocols
nor to require that a host uses an existent protocol message in a
different way. In other words, the requirement is no host changes.
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FCFS SAVI validation is performed by the FSFC SAVI function. Such
function can be placed in different type of devices, including a
router or a layer-2 bridge. The basic idea is that the FCFS SAVI
function is located in the points of the topology that can enforce
the correct usage of source address by dropping the non-compliant
packets.
2.3. Address ownership proof
The main function performed by FCFS SAVI is to verify that the source
address used in data packets actually belongs to the originator of
the packet. Since FCFS SAVI scope is limited to the local-link, the
originator of the packet is attached to the local-link. In order to
to define any source address validation solution, we need to define
some address ownership proof concept i.e. what it means to be able to
proof that a given host owns a given address in the sense that the
host is entitled to send packet with that source address.
Since no hast changes are acceptable, we need to find the means to
proof address ownership without requiring a new protocol. In FCFS
SAVI the address ownership proof is based in the First-Come first
Serve approach. This means that the first host that uses a given
source address is the owner of the address until further notice.
More precisely, whenever a source address is used for the first time,
a state is created in the device that is performing the FCFS SAVI
function binding the source address to the layer-2 information that
the FCFS SAVI box has available (e.g. the MAC address in a LAN, or
the port in a switched LAN). Following data packets containing that
IP source address must use the same layer-2 information in order to
be compliant.
There are however additional considerations to be taken into account.
For instance, consider the case of a host that moves from one segment
of a LAN to another segment of the same subnetwork and it keeps the
same IP address. In this case, the host is still the owner of the IP
address, but the associated layer-2 information has changed. In
order to cope with this case, FCFS SAVI performs an active check to
verify if the host is still reachable using the previous layer-2
information. In order to do that FCFS SAVI uses ARP protocol in IPv4
and ND in IPv6. If the host is no longer reachable at the previously
recorded layer-2 information, FCFS SAVI assumes that the new location
is valid and creates a new binding using the new LAyer-2 information.
In case the host is still reachable using the previously recorded
information, the packets coming from the new layer-2 information are
dropped (see some caveats described in the following section).
Note that this only applies to local traffic. Transit traffic
generated by a router would be verified using alternative techniques,
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such as ingress filtering. ARP or ND checks would not be fulfilled
by the transit traffic, since the router is not the owner of the
source address contained in the packets.
Layer-2 considerations:TBD
2.4. Special cases
The following special cases that need to be considered
o Hosts with multiple physical interfaces, potentially connected to
different networks.
o Anycast i.e. multiple hosts using the same source address to send
packets.
o Proxy ARP/ND i.e. host sending packets on behalf of other, in a
layer-3 transparent manner.
3. FCFS SAVI specification
3.1. FCFS SAVI Data structures
FCFS SAVI function relies on state information binding the source
address used in data packets to the layer-2 information that
contained the first packet that used that source IP address. Such
information is stored in FCFS SAVI Data Base (DB). The FCFS SAVI DB
will contain a set of entries about the currently used IP source
addresses. So each entry will contain the following information:
o IP source address
o Layer-2 information, such as Layer-2 address, port through which
the packet was received, etc
o Lifetime
o Status:either tentative or valid
o Creation time: the value of the local clock when the entry was
firstly created
In addition to this, FCFS SAVI need to know what are the prefixes
that are directly connected, so it maintains a data structure called
the the FCFS SAVI prefix list, which contains:
o Prefix
o Interface where prefix is directly connected
Finally, FCFS SAVI keep a list of the routers that are directly
connected, since the FCFS SAVI checks will not directly apply to
them. In the FCFS SAVI Router List, the following information is
stored:
o Router IP address (of the directly connected interface)
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o Router Layer-2 information such as layer-2 address or port which
the router is connected to
3.2. FCFS SAVI algorithm
3.2.1. Processing of data packets
The FCFS SAVI function is located in a forwarding device, such as a
router or a layer-2 bridge. Upon the reception of a data packet, the
packet will be passed to the FCFS SAVI function which will perform
the processing detailed in this section. The outcome of such
processing can be that the packet is discarded or that is forwarded
as usual.
After a data packet is received, the FCFS SAVI function checks
whether the received data packet is local traffic or transit traffic.
It does so by verifying if the source address of the packet belongs
to one of the directly connected prefixes available in the receiving
interface. It does so by searching the FCFS SAVI Prefix List.
o If the IP source address belongs to one of the local prefixes of
the receiving interface, the data packet is local traffic and the
FCFS SAVI algorithm is executed as described next.
o If the IP source address does not belong to one of the local
prefixes of the receiving interface, this means that the dat
packet is transit traffic. The FCFS SAVI SHOULD verify if the
layer-2 information of the packet corresponds to one of the
routers available in the receiving interface, by using the
information available in the FCFS SAVI router list. If the packet
comes from one of the know routers for that interface, then the
packet is passed so additional checks such as ingress filtering
can be performed. If the packet does not comes from one of the
known routers, then the packet SHOULD be discarded. The FCFS SAVI
function MAY send an ICMP Destination Unreachable Error back to
the source address of the data packet. (In ICMPv4, code 0 (net
unreachable) should be used and in ICMPv6, code 5 (Source address
failed ingress/egress policy) should be used) (Note; we could skip
this verification altogether and simply pass it to the ingress
filters, but it think this could be useful, especially if used
along with SeND)
After checking that the data packet is local traffic, the FCFS SAVI
function will verify the source address used in the packet. In order
to do so, it searches the FCFS SAVI DB using the IP source address as
a key.
o If no valid entry is found, then a new entry is created, using the
information of the data packet, including all the related layer-2
information of where the packet was received from and the lifetime
of the entry is set to LIFETIME. The status is set to valid. The
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packet is forwarded as usual. (NOTE: AS defined FCFS SAVI treats
tentative entries as if they did not existed i.e. a data packet
preempts the DAD procedure, this probably requires more
discussion)
o If a valid entry is found and the layer-2 information of the
received data packet matches to the information contained in the
existing entry, then the lifetime is set of LIFETIME and the
packet is forwarded as usual.
o If a valid entry is found and the layer-2 information of the
received data packet does not match the information contained in
the existing matching entry, then the FCFS SAVI performs a
Neighbor Unreachability Detection procedure as described in
[RFC4861] for IPv6 and in Section 3.3 for IPv4. It uses the IP
source address and Layer-2 information available in the FCFS SAVI
DB entry.
* If the procedure determines that the neighbor is no longer
reachable using the information available in the FCFS SAVI DB
entry, then the entry information is modified to include the
new information about the data packet received (in particular
the new layer-2 information) and lifetime of the entry is
updated to LIFETIME. The packet is forwarded as usual.
* If the procedure determines that the neighbor is still
reachable using the information available in the FCFS SAVI DB,
then the data packet is discarded and the lifetime of the entry
is set to LIFETIME. The FCFS SAVI function MAY send an ICMP
Destination Unreachable Error back to the source address of the
data packet. (In ICMPv4, code 0 (net unreachable) should be
used and in ICMPv6, code 5 (Source address failed ingress/
egress policy) should be used)
3.2.2. Processing of control packets
Processing of IPv6 ND packets
The FCFS SAVI function will also create state based on control
packets. In particular, in IPv6, when a host configures an address,
it performs the Duplicate Address Detection (DAD) procedure, to
verify that the address is unique in the link. FCFS SAVI keeps track
of the DAD procedure and creates modify the FCFS SAVI DB state
accordingly.
Upon the reception of a Neighbor Solicitation message containing the
unspecified source address FCFS SAVI retrieves the address contained
in the Target Address filed of the NSOL message and performs the
following actions:
o If no valid entry is found in the FCFS SAVI DB for that address,
then it creates a new entry, includes the Target Address and the
link layer information contained in the NSOL message and sets the
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status to tentative. At that point FCFS SAVI will keep track of
the Neighbor Advertisement messages.
* If a NADV message containing the address in the NADV Target
Address field is received before DADTimeout then the entry is
deleted.
* If no NADV message for that Target Address is received in
DADTimeout, then the status of the entry is change to valid and
the lifetime of the entry is set to LIFETIME. In addition, if
the address contained in the newly created entry is a link
local address, FCFS SAVI MAY as well create entries for the
global addresses resulting from concatenating the Interface
Identifier of the link local address and the global prefixes
contained in the Prefix List for the Interface through which
the NSOL message was received.
o If a valid entry is found in the FCFS SAVI DB for that address, no
additional processing is performed. (Note: there is no point of
tracking the NADV at this point. Either the SAVI DB is updated
and there is no new information or it is not, which we will find
out when we receive a data packet. Moreover, tracking NADV
messages could enable an attacker to overwrite an existing entry.)
Processing of IPv4 ARP packets
IPv4 Address Conflict Detection (ACD) is defined in [RFC5227] and
provides the means to verify if there is an address conflict in IPv4.
The FCFS SAVI function will also create state based on IPv4 ACD
control packets. In IPv4, when a host configures an address, it
performs the Address Conflict Detection (ACD) procedure, to verify
that the address is unique in the link. FCFS SAVI keeps track of the
ACD procedure and creates modify the FCFS SAVI DB state accordingly.
Upon the reception of a ARP Probe (defined as an ARP Request message,
broadcast on the local link, with an all-zero 'sender IP address'),
FCFS SAVI retrieves the address contained in the 'target IP address'
field of the ARP Request message and performs the following actions:
o If no valid entry is found in the FCFS SAVI DB for that address,
then it creates a new entry, includes the 'target IP address' and
the link layer information contained in the ARP Request message
and sets the status to tentative. At that point FCFS SAVI will
keep track of ARP Announcement messages. ARP Announcement
messages are defined as an ARP Request message, broadcast on the
local link, with a non all-zero 'sender IP address')
* If an ARP Announcement message containing the tentative address
in both the 'sender IP address' and the 'target IP address' and
it contains the same link-layer information stored in the
tentative entry in the SAVI DB is received before 3*ACDTimeout.
(default value of 3*2 secs) then the entry status is set to
valid and the lifetime of the entry is set to LIFETIME.
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* If no such message is received for that Target Address in
3*ACDTimeout (default value of 3*2 secs), then the entry is
deleted.
o If a valid entry is found in the FCFS SAVI DB for that address, no
additional processing is performed.
3.3. IPv4 Neighbor Unreachability Detection Procedure
As opposed to IPv6, there is no general Neighbor Unreachability
Detection procedure defined for IPv4. Since this is needed in order
to verify if the original node is still using the IP address it once
used, in this section, we define the procedure to perform such
verification. However, unlike IPv6 Neighbor discovery, the IPv4 ARP
protocol [RFC0826] cannot be assumed to be available in all link
layers. So, we will define a ARP based procedure to be used in
layers 2 that the ARP protocol is available and an ICMP based
[RFC0792] procedure for the cases where the ARP protocols is not
available. The ARP based procedure is used whenever it is possible
and when ARP is not available, the ICMP based procedure is used.
3.3.1. ARP-based Neighbor Unreachability Detection procedure
Consider two nodes, S and T, directly connected through a layer 2
where the ARP protocol is available. Node S has with IP address IPS
and layer 2 address MACS and Node T has IP address IPT and layer 2
address MACT.
Node S wants to perform the ARP based Neighbor Unreachability
Detection Procedure for node T. Node S has both IPT and MACT
available. So, node S generates an ARP REQUEST packet, containing
the following information:
Ethernet transmission layer:
Ethernet address of destination: MACT
Ethernet address of sender: MACS
Protocol type = ether_type$ADDRESS_RESOLUTION
Ethernet packet data:
(ar$hrd) Hardware address space (e.g., Ethernet, Packet Radio
Net.)
(ar$pro) Protocol address space:0x0800 Internet Protocol
Version 4 (IPv4)
(ar$hln) byte length of each hardware address
(ar$pln) byte length of each protocol address: 4
(ar$op) opcode (ares_op$REQUEST)
(ar$sha) Hardware address of sender of this packet: MACS
(ar$spa) Protocol address of sender of this packet: IPS
(ar$tha) Hardware address of target of this packet: MACT
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(ar$tpa) Protocol address of target: IPT
Upon the reception of the ARP REQUEST, if node T follows current ARP
specification [RFC0826], it will reply with an ARP REPLY packet with
the following information:
Ethernet transmission layer:
Ethernet address of destination: MACS
Ethernet address of sender: MACT
Protocol type = ether_type$ADDRESS_RESOLUTION
Ethernet packet data:
(ar$hrd) Hardware address space (e.g., Ethernet, Packet Radio
Net.)
(ar$pro) Protocol address space:0x0800 Internet Protocol
Version 4 (IPv4)
(ar$hln) byte length of each hardware address
(ar$pln) byte length of each protocol address: 4
(ar$op) opcode (ares_op$REPLY)
(ar$sha) Hardware address of sender of this packet: MACT
(ar$spa) Protocol address of sender of this packet: IPT
(ar$tha) Hardware address of target of this packet: MACS
(ar$tpa) Protocol address of target: IPS
If node S receives the ARP REPLY message, the Neighbor Unreachability
procedure was successful and the neighbor T is still reachable with
the available information. If node S does not receives the ARP REPLY
message after ARPTIMEOUT, then the Neighbor Unreachability procedure
has failed and the neighbor T is no longer reachable with the current
information.
3.3.2. ICMP-based Neighbor Unreachability Detection procedure
Consider two nodes, S and T, directly connected through a layer 2.
Node S has with IP address IPS and layer 2 address LLAS and Node T
has IP address IPT and layer 2 address LLAT.
Node S wants to perform the ICMP based Neighbor Unreachability
Detection Procedure for node T. Node S has both IPT and LLAT
available. So, node S generates an ICMP ECHO packet [RFC0792] ,
containing the following information:
Link Layer fields:
Source address: LLAS
Destination address: LLAT
IP header fields:
IP Source Address: IPS
IP Destination Address: IPT
ICMP fields
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Type: 8
Identifier: set to random number by S
Upon the reception of the ICMP ECHO message, if node T follows
current ICMP specification [RFC0792], it will reply with an ECHO
REPLY packet with the following information:
Link Layer fields:
Source address: LLAT
Destination address: LLAS
IP header fields:
IP Source Address: IPT
IP Destination Address: IPS
ICMP fields
Type: 0
Identifier: copied from the ECHO message received
If node S receives a ECHO REPLY message, it will verify that the
source IP address and the source link layer address match to the
original ones used in the ECHO message. Besides, it will check that
the identifier matches to the one contained in the original ECHO
message. If these checks are successful the Neighbor Unreachability
procedure was successful and the neighbor T is still reachable with
the available information. If node S does not receives the ECHO
REPLY message after ICMPTIMEOUT, then the Neighbor Unreachability
procedure has failed and the neighbor T is no longer reachable with
the current information.
4. Security Considerations
First of all, it should be noted that any SAVI solution will be as
strong as the lower layer anchor that it uses. In particular, if the
lower layer anchor is forgeable, then the resulting SAVI solution
will be weak. For example, if the lower layer anchor is a MAC
address that can be easily spoofed, then the resulting SAVI will not
be stronger than that. On the other hand, if we use switch ports as
lower layer anchors (and there is only one host connected to each
port) it is likely that the resulting SAVI solution will be
considerably more secure.
Denial of service attacks
There are two types of DoS attacks that can be envisaged in a SAVI
environment. On one hand, we can envision attacks against the SAVI
device resources. On the other hand, we can envision DoS attacks
against the hosts connected to the network where SAVI is running.
The attacks against the SAVI device basically consist on making the
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SAVI device to consume its resource until it runs out of them. For
instance, a possible attack would be to send packets with different
source addresses, making the SAVI device to create state for each of
the addresses and waste memory. At some point the SAVI device runs
out of memory and it needs to decide how to react in this situation.
The result is that some form of garbage collection is needed to prune
the entries. It is recommended that when the SAVI device runs out of
the memory allocated for the SAVI DB, it creates new entries by
deleting the entries which Creation Time is higher. This implies
that older entries are preserved and newer entries overwrite each
other. In an attack scenario where the attacker sends a batch of
data packets with different source address, each new source address
is likely to rewrite another source address created by the attack
itself. It should be noted that entries are also garbage collected
using the LIFETIME, which is updated using data packets. The result
is that in order for an attacker to actually fill the SAVI DB with
false source addresses, it needs to continuously send data packets
for all the different source addresses, in order for the entries to
grow old and compete with the legitimate entries. The result is that
the cost of the attack for the attacker is highly increased.
The other type of attack is when an attacker manages to create state
in the SAVI device that will result in blocking the data packets sent
by the legitimate owner of the address. In the IPv4 case, the
simplest way of doing this is for the attacker to claim the ownership
of all the addresses available in the prefix assigned to the
subnetwork. That is, if an attacker sends data packets with all the
source addresses of the on-link prefix, it will claim address
ownership for all the available addresses and SAVI will block packets
sent by any other host. This is a very severe attack. The proposed
solution for this attack is to limit the number of IP addresses bound
to a give lower layer anchor. In this way, any host, including the
attacker, can only claim the address ownership of a limited number of
addresses. Of course, this is only effective if the attacker cannot
spoof the lower layer anchor. For instance, in the case where the
MAC address is used as lower layer anchor, this measure is hardly
sufficient, since the attacker can spoof the source address and still
perform the attack. As a result, it is recommended that when the
lower layer anchors are spoofable, SAVI should not discard non-
compliant packet, but rather log them, to enable proper
administrative action. Enabling SAVI in that case could expose the
network to the aforementioned DoS attack. If the lower layer anchor
is not easily spoofable, the proposed mechanism provides considerable
protection, since it limits the impact of the attack. In IPv6 these
attacks are not an issue thanks to the 2^64 addresses available in
each link.
Compare with Threat analysis and identify residual threats: TBD
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5. Acknowledgments
This draft benefited from the input from: Christian Vogt, Fred Baker,
Guang Yao, Dong Zhang, Frank Xia and Lin Tao. In particular the usage
of ARP and ND packet to create SAVI DB state was suggested by Guang
Yao in response to an attack described by Fred Baker.
Marcelo Bagnulo is partly funded by Trilogy, a research project
supported by the European Commission under its Seventh Framework
Program.
6. Normative References
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
July 2008.
Authors' Addresses
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Menlo Park, CA 94025
USA
Phone: +1 650 786 2921
Email: Erik.Nordmark@Sun.COM
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Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
Nordmark & Bagnulo Expires July 26, 2009 [Page 14]
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