Steve Bellovin
Internet Draft AT&T Labs Research
Document: draft-ietf-itrace-04.txt Marcus Leech
Tom Taylor
Nortel Networks
Expires: August 2003 February 2003
ICMP Traceback Messages
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Abstract
It is often useful to learn the path that packets take through the
Internet, especially when dealing with certain denial-of-service
attacks. We propose a new ICMP message, emitted randomly by routers
along the path and sent randomly to the destination (to provide
useful information to the attacked party) or to the origin (to
provide information to decipher reflector attacks).
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Table of Contents
Status of this Memo............................................1
Abstract.......................................................1
Table of Contents..............................................2
1. Introduction................................................3
1.1 Requirements Keywords...................................3
1.2 Definitions.............................................3
2. Message Definition..........................................4
2.1 Conventions For Presentation............................4
2.2 Overall Message Format..................................4
2.3 Forward and Backward Link Elements......................5
2.3.1 Back Link (TYPE=0x01)..............................6
2.3.2 Forward link (TYPE=0x02)...........................7
2.3.3 Interface Identifier (TYPE=0x81)...................7
2.3.4 IPV4 address pair (TYPE=0x82)......................7
2.3.5 IPV6 address pair (TYPE=0x83)......................8
2.3.6 MAC address pair (TYPE=0x84).......................8
2.3.7 Operator-defined link identifier (TYPE=0x85).......8
2.4 Timestamp (TYPE=0x03)...................................9
2.5 Traced packet (TYPE=0x04)...............................9
2.6 Probability (TYPE=0x05).................................9
2.7 RouterId (TYPE=0x06)...................................10
2.8 Authentication data....................................10
2.8.1 HMAC Authentication data (TYPE=0x07)..............10
2.8.2 Key Disclosure List (TYPE=0x08)...................11
2.8.3 Key Disclosure (TYPE=0x86)........................11
2.8.4 Disclosure Signature (TYPE=0x87)..................13
3. Procedures.................................................13
3.1 Generation Of Traceback Messages.......................13
3.1.1 Implementation Requirements -- Message Generation.14
3.1.2 Implementation Requirements -- Message Reception..14
3.2 Configuration..........................................15
3.3 Processing Of Received Messages........................15
4. Related Work...............................................15
5. Security Considerations....................................16
6. IANA Considerations........................................16
7. Acknowledgements...........................................16
8. References.................................................16
9. Author Information.........................................17
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1. Introduction
It is often useful to learn the path that packets take through the
Internet. This is especially important for dealing with certain
denial-of-service attacks, where the source IP is forged. There are
other uses as well, including path characterization and detection of
asymmetric routes. There are existing tools, such as traceroute,
but these generally provide the forward path, not the reverse.
We propose an ICMP [RFC792] Traceback message to help solve this
problem. When forwarding packets, routers can, with a low
probability, generate a Traceback message that is sent along to the
destination or back to the source. With enough Traceback messages
from enough routers along the path, the traffic source and path of
forged packets can be determined.
1.1 Requirements Keywords
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119].
1.2 Definitions
Element: a component of the proposed message which is explicitly
identified by a type code, and is encoded using a Type-Length-Value
(TLV) format. Some elements will contain other elements as
described below.
Field: a component of the proposed message which is identified
through its relative position within the header or within a
particular element.
Generator: the router which itself generates the ICMP Traceback
message or on behalf of which this message is generated by some
other entity.
Link: a logical connection between the Generator and another entity,
along which the traced packet has passed.
Peer: the entity at the other end of the link, which either sent the
traced packet to the Generator or received it from the Generator.
Traced Packet: the packet which is the subject of an ICMP TRACEBACK
message.
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2. Message Definition
2.1 Conventions For Presentation
As indicated below, aside from the initial octet, the elements of
the ICMP TRACEBACK message are concatenated without any padding to
create word boundary alignment. The fields within each element are
similarly concatenated without intervening padding. The diagrams
presenting the individual elements therefore show the length and
relative order of the fields making them up, but do NOT indicate
alignment on any specific boundary. Each field beyond the initial
type code and length is shown beginning on a separate line, although
in fact fields are contiguous in the actual message.
2.2 Overall Message Format
The proposed message is carried in an ICMP packet, with ICMP TYPE of
TRACEBACK. (The numeric values for this field will be assigned by
IANA. For IPv6, the TRACEBACK should be classified as
Informational.) The CODE field MUST always be set to 0 (no code),
and MUST be ignored by the receiver.
Traceback Message
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message body |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .....-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The body of any ICMP TRACEBACK message consists of a series of
individual elements that are self-identifying, using a TYPE-LENGTH-
VALUE scheme as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE | LENGTH | VALUE... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This structure is recursive, in that for certain element types the
VALUE field will contain one or more components which are also in
TYPE-LENGTH-VALUE (TLV) format. Top-level elements may appear in
any order, and a receiver MUST be capable of processing them in any
order. Elements contained within the VALUE field of a parent
element may also appear in any order within that field and present a
similar requirement to the receiver. Elements are placed
consecutively within the message body without intervening padding;
hence elements in general are not aligned to word boundaries.
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The TYPE field is a single octet, with values in the range 0x01 to
0x7f for top-level elements and 0x81 to 0xff for sub-elements. The
top-level type codes assigned in this document are as follows:
Type Element Name
0x01 Back Link
0x02 Forward Link
0x03 Timestamp
0x04 Traced Packet Contents
0x05 Probability
0x06 RouterId
0x07 HMAC Authentication Data
0x08 Key Disclosure List
The sub-element type codess assigned in this document are as
follows:
Type Element Name Notes
0x81 Interface Name 1
0x82 IPv4 Address Pair 1,2
0x83 IPv6 Address Pair 1,2
0x84 MAC Address Pair 1,3
0x85 Operator-Defined Link Identifier 1,3
0x86 Key Disclosure 4
0x87 Disclosure Signature 4
Note 1: this item is a sub-element within Back or Forward Link
elements.
Note 2: at least one of these elements MUST be present within a
Link element.
Note 3: either the MAC Address Pair or the Operator-Defined
Link Identifier element but not both MUST be present within a
Link element.
Note 4: the Key Disclosure List MUST contain one or more Key
Disclosure elements and exactly one Disclosure Signature
element.
LENGTH is always set to the length of the VALUE field in octets, and
always occupies two octets, even when the length of the VALUE field
is less than 256 octets.
2.3 Forward and Backward Link Elements
An ICMP TRACEBACK message MUST contain one Forward Link element or
one Back Link element; it MAY contain one instance of each. A Link
element specifies a link along which the traced packet travelled to
or from the Generator. The purpose of the Forward and Back Link
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elements is to permit easy construction of a chain of Traceback
messages. They are further designed for examination by network
operations personnel, and thus contain human-useful information such
as interface names.
The Value field of a link element consists of three components:
* the interface name at the Generator only. (It is assumed
that the Generator does not know its neighbors' interface
names.) This is encoded in an Interface Name element.
* the source and destination IP addresses of the Generator and
its peer. These are encoded in an IPv4 or IPv6 Address
Pair element.
* the link-level association string. The association string
is an opaque blob which is used to tie together Traceback
messages emitted by adjacent routers. Thus all Link
elements referring to the same link MUST use the same value
for the association string, regardless of which entity
generates them.
On LANs, the association string is constructed by
concatenating the source and destination MAC addresses of
the two interfaces to the link, and is encoded in a MAC
Address Pair element. If there are no such addresses (say,
for a point-to-point link), a suitable string MUST be
provisioned in both routers; this is encoded in an Operator-
Defined Link Identifier element.
The fields of the Address Pair elements are always arranged in
"forward order" from the point of view of the traced packet. That
is, the "destination" field is always the address of the entity
closer to the ultimate recipient of the traceback packet. Thus, in
Back Link elements, the generator's own address is placed in the
destination field of the IP and MAC Address Pair subelements; in
Forward Link elements, the generator's address is placed in the
source field.
2.3.1 Back Link (TYPE=0x01)
The Back Link element provides identifying information, from the
perspective of the Generator, about the link that the traced packet
arrived from. The VALUE field of this element consists of three TLV
subelements, one each for the Interface Identifier, the IP Address
Pair, and the association string. Element lengths shown include the
type and length fields. Elements may appear in a different order
from that shown.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x01 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| INTERFACE IDENTIFIER (variable length) .
+ IPV4 or IPV6 ADDRESS PAIR (11 or 35 octets) +
. MAC ADDRESS PAIR (15 octets) or OPERATOR-DEFINED LINK .
+ IDENTIFIER (variable length) +
. ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.2 Forward link (TYPE=0x02)
The Forward Link element provides identifying information, from the
perspective of the Generator, about the link that the traced packet
was forwarded on. Its structure is the same as that of the Back
Link element.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x02 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| INTERFACE IDENTIFIER (variable length) .
+ IPV4 or IPV6 ADDRESS PAIR (11 or 35 octets) +
. MAC ADDRESS PAIR (15 octets) or OPERATOR-DEFINED LINK .
+ IDENTIFIER (variable length) +
. ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.3 Interface Identifier (TYPE=0x81)
This element contains the name of the interface to the link at the
generating router. The length is variable. The VALUE field
typically contains a human-readable character string.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x81 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| INTERFACE NAME (variable length) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.4 IPV4 address pair (TYPE=0x82)
This element contains two 4-octet IPV4 addresses of the ends of the
corresponding link; hence the LENGTH field is always 0x0008. As
noted above, the addresses MUST always be presented in the order of
their traversal by the traced packet.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x82 | LENGTH=0x0008 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UPSTREAM ADDRESS (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DOWNSTREAM ADDRESS (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.5 IPV6 address pair (TYPE=0x83)
This element contains two 16-octet IPV6 addresses of the ends of the
corresponding link; hence the LENGTH field is always 0x0020. As
noted above, the addresses MUST always be presented in the order of
their traversal by the traced packet.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x83 | LENGTH=0x0020 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UPSTREAM ADDRESS (16 octets) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DOWNSTREAM ADDRESS (16 octets) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.6 MAC address pair (TYPE=0x84)
This element contains two 6-octet IEEE MAC addresses of the ends of
the corresponding link; hence the LENGTH field is always 0x000C. As
noted above, the addresses MUST always be presented in the order of
their traversal by the traced packet.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x84 | LENGTH=0x000C |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UPSTREAM ADDRESS (6 octets) .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. DOWNSTREAM ADDRESS (6 octets) .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.7 Operator-defined link identifier (TYPE=0x85)
The value of this element is an opaque field of varying length. If
the peer also emits ICMP TRACEBACK messages for the same link, it
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MUST use the same value. Further definition will emerge in a later
document.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x85 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LINK IDENTIFIER (variable length) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.4 Timestamp (TYPE=0x03)
This element contains the time, in NTP timestamp format (eight
octets) [RFC1305], at which the ICMP Traceback packet was generated.
This element MUST be present at the top level within the TRACEBACK
message. The timestamp MUST be consistent with (i.e. lie between)
the starting and ending time of validity of the applicable hash key
as reported in Key Disclosures in subsequent ICMP Traceback packets.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x03 | LENGTH=0x0008 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| INTEGER PART (4 octets) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. FRACTION PART (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.5 Traced packet (TYPE=0x04)
This element provides the contents of the traced packet, as much as
can reasonably fit, subject to link and router resource constraints.
This element MUST be present at the top level within the TRACEBACK
message, and MUST contain at least the IP header and as much of the
remainder of the traced packet as will fit without making the ICMP
Traceback packet exceed the minimum path MTU (576 octets for IPv4,
as specified in [RFC2460] for IPv6). [ISSUE: should this wording
stand or should we go with the more restrictive value?]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x04 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Complete Packet Header (>=20 octets) .
. Packet body (>= 8 octets) .
. ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.6 Probability (TYPE=0x05)
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This element contains the inverse of the probability used to select
the traced packet. It appears as an unsigned integer, of one, two,
or four octets. This element SHOULD be present at the top level
within the TRACEBACK message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x05 | LENGTH=0x0001/2/4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VALUE (1, 2,.or 4 octets) . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.7 RouterId (TYPE=0x06)
This element contains opaque identifying information, useful to the
organization that operates the router emitting the ITRACE message.
This element MUST be present at the top level within the TRACEBACK
message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x06 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ROUTER IDENTIFIER (variable length) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.8 Authentication data
An attacker may try to generate fake Traceback messages, primarily
to conceal the source of the real attack traffic, but also to act as
another form of attack. We thus need authentication techniques that
are robust but quite cheap to verify.
The ideal form of authentication would be a digital signature. It
is unlikely, though, that routers will be able to afford such
signatures on all Traceback packets. Thus, although we leave hooks
for such a variant, we do not further define it at this time.
What is provided instead is a hash code (the HMAC Authentication
Data element), supported by signed disclosure of the keys most
recently used (the Key Disclosure and Public Key Information
elements). The current key MUST NOT be included in this disclosure.
2.8.1 HMAC Authentication data (TYPE=0x07)
This element MUST be present. It contains four subfields:
* algorithm, two octets. This identifies the hash algorithm
used. It must be one of the hash algorithm codepoint values
defined in http://www.iana.org/assignments/ipsec-registry.
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* keyid: eight octet key identifier
* MAC data: variable
The MAC data field covers the entire IP datagram, including
header information. Where header information is mutable
during transport, such information is set to zero (0x00) for
purposes of calculating the HMAC. Mutable fields for IPv4
and IPv6 are identified in [RFC2402] section 3.3.3.1.
This field is as long as is appropriate for the given MAC
algorithm.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x07 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC ALG (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ KEY IDENTIFIER (8 octets) +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC DATA (algorithm dependent) .
+ +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.8.2 Key Disclosure List (TYPE=0x08)
A packet SHOULD contain a list of the keys most recently used to
create HMAC authentication values for ICMP Traceback packets. The
key currently in use MUST NOT be included in this disclosure. The
disclosure need not appear if there are no keys to be disclosed
according to the criteria [TBD].
[EDITOR's NOTE: it has been suggested that we need to state an upper
limit for the number of keys to be disclosed. The Editor suggests
that the basic principle governing the number of keys that should be
disclosed is that there be a reasonable probability (e.g. 80%) that
a host receiving an ITrace packet will also receive a key disclosure
for that packet. This makes the number of keys a function of the
rate of generation of ITrace packets and the rate at which keys are
changed. Further analysis may give more concrete results.]
The key disclosure is provided in the Key Disclosure List element.
This element MUST contain at least one Key Disclosure sub-element,
and MUST also contain a Public Key Information sub-element pointing
to the keys used to sign the Key Disclosures. In accordance with
the general rule for construction of the TRACEBACK message, the sub-
elements may be presented in any order and the receiver MUST be able
to process them regardless of the order in which they are presented.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x08 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ KEY DISCLOSURE(s) and PUBLIC KEY INFORMATION +
. ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.8.3 Key Disclosure (TYPE=0x86)
The primary content of the Key Disclosure element consists of a key
used to authenticate previous TRACEBACK messages and the starting
and ending times between which that key was used. The algorithm is
assumed to be the same as that used to authenticate the current
message (and shown in the HMAC Authentication Data element). The
element MUST also contain a digital signature covering the Key
Disclosure element.
The structure of the Key Disclosure element is as follows:
* keyid for the key being disclosed: eight octets
* validity: two NTP timestamps giving validity period (start,
end)
* key length: one octet
* key material: variable [key length] octets
Keying material for the chosen HMAC function MUST conform to
the requirements for keys outlined in [RFC2104].
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x86 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ KEY IDENTIFIER (8 octets) +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ START TIME (8 octets) +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ END TIME (8 octets) +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| KEY LEN (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| KEY MATERIAL (KEY LEN octets) .
+ ... +
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. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.8.4 Disclosure Signature (TYPE=0x87)
The Disclosure Signature sub-element contains a digital signature
for the the Key Disclosure List. The signature covers the entire
Key Disclosure List element, including the Disclosure Signature
element, but excluding the signature length and actual signature
within that element. This element also contains a URL, suitable for
retrieving an X.509 certificate that contains the public key used to
sign key-disclosure elements. See [HTTP-CERTSTOR] for a possible
implementation.
The URL is present because digital signatures are useless without
some way of authenticating the public key of the signer. The ideal
form of authentication would be a certificate-based scheme rooted in
the address registries. That is, the registries are the
authoritative source of information on who owns which addresses;
they are thus the only party that can easily issue such
certificates.
Until such a PKI is in existence, we suggest that each ISP publish
its own root public key. Current registry-based databases can be
used to verify the owner of an address block; this information can
in turn be used to locate the appropriate root key.
The Disclosure Signature element has the following structure:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=0x87 | LENGTH (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIGNATURE LENGTH (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIGNATURE (SIG LEN octets) .
+ ... +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
+ URL (variable) +
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3. Procedures
3.1 Generation Of Traceback Packets
A router implementing this scheme SHOULD generate and emit an ICMP
Traceback packet with probability of about 1/20,000, although local
site policy MAY adjust this to better suit local link utilization
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metrics. It MUST then randomly select with equal probability to
send this packet to the origin or the destination of the sampled
packet.
Some requirements are imposed on the IP header of the Traceback
packet. In particular, the source address SHOULD be that associated
with the interface on which the packet arrived. If that interface
has multiple addresses, the address chosen SHOULD, if possible, be
the one by which this router is known to the previous hop. If the
interface has no IP address, the "primary" IP address associated
with the router MAY be used. ("Primary" is discussed below.)
The initial TTL field MUST be set to 255. If the Traceback packet
follows the same path as the data packets, this provides an
unambiguous indication of the distance from this router to the
destination. More importantly, by comparing the distances with the
link elements, a chain can be constructed and partially verified
even without examining the authentication fields.
The TOS field SHOULD be copied from the TOS field of the traced
packet.
3.1.1 Implementation Requirements -- Message Generation
The probability of Traceback generation SHOULD be adjustable by the
operator of the router. A default value of about 1/20000 is
suggested. If the average maximum diameter of the Internet is 20
hops, that translates to a net increase in traffic at the origina
and destination of about .1%; this should not be an undue burden on
the recipient. The probability SHOULD NOT be greater than 1/1000.
Packet selection SHOULD be based on a pseudo-random number, rather
than a simple counter. This will help block attempts to time attack
bursts. There does not appear to be any requirement for
cryptographically strong pseudo-random numbers.
A suggested scheme involves examination of the low-order bits of a
linear congruential pseudo-random number generator (LCPRNG). If
they are all set to 1, the packet should be emitted. This permits
easy selection of probabilities 1/8191, 1/16383, etc. N.B. While
the low-order bits of LCPRNGs are not very random, that does not
matter here. As long as the period of the generator is maximal, all
values, including all 1s in the low-order bits, will occur with the
proper probability.
Although this document describes a router-based implementation of
Traceback messages, most of the functionality can be implemented via
outboard devices. For example, suitable laptop computers can be
used to monitor LANs, and emit the traceback messages as
appropriate, on behalf of all of the routers on that LAN.
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3.1.2 Implementation Requirements -- Message Reception
Hosts SHOULD be designed so that the operator can enable and disable
the collection and storage of ICMP TRACEBACK messages as required.
Hosts SHOULD also be designed so that the operator can limit the
rate at which the host accepts ICMP TRACEBACK messages. Messages
exceeding this rate would be silently dropped. If such
functionality is implemented, the host SHOULD provide a counter
displaying how many messages have been dropped.
3.2 Configuration
The association string used in the Forward and Back Link elements
can be built up from the MAC addresses of the link endpoints. If
there are no such addresses (say, for a point-to-point link), a
suitable string MUST be provisioned in both routers, to be used as
the Operator-Defined Link Identifier.
3.3 Processing Of Received Messages
To circumvent attacks in the course of which false ICMP TRACEBACK
messages are emitted, these messages SHOULD be validated before use.
Malformed messages SHOULD be silently discarded. Some further
validation can be done before the HMAC keying information is
disclosed. In particular, when messages appearing to relate to
adjacent segments of a chain have been identified, recipients SHOULD
use the TTL field differences in conjunction with the link elements
to verify the chain.
Because HMAC key disclosure is done only after the end of the period
of validity for the key, authentication of a given set of ICMP
TRACEBACK messages requires that further messages be collected and
examined beyond the period of interest, until the required key
appears. The processing entity SHOULD then verify the signature on
the key before applying the key itself to validation of the message.
4. Related Work
Another scheme proposed for packet Traceback is by Savage et al
[SWKA00]. It relies on a very clever encoding of the path in the IP
header's ID field. That is, in-flight packets may have their ID
field changed to provide information about the path. The recipient
can decode this information.
There are a number of advantages of this compared to ICMP Traceback.
No extra traffic is generated. More importantly, the trace
information is bound to the packets, and hence doesn't follow a
different path and isn't differentially blocked by firewalls or
policy routing mechanisms. However, there are disadvantages as
well. For one thing, the ID field cannot be changed if
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ICMP Traceback Messages February 2003
fragmentation is necessary (though they propose some schemes to
ameliorate this). Moreover, AH [RFC2402] provides cryptographic
protection for the ID field; if it is modified, the packet will be
discarded by the receiving system. And IPv6 has no ID field at all.
A number of other packet-marking schemes have been proposed.
A different approach is hash-based traceback, by Snoeren et al
[SPSSJTK01]. In this scheme, routers along the path are queried
about whether or not they have seen a certain packet; a very compact
representation is used to store recent history. The problem is that
queries must be done very soon after the attack, unless the routers
have some way of offloading historical data to bulk storage.
[SDS00] descibes a scheme for coupling IDS systems. A sensor that
detects an attack tells its neighbors; they in turn look for the
same signature, and notify their neighbors. The current prototype
only works within an administrative domain; work is currently under
way to produce an inter-domain version.
5. Security Considerations
It is quite clear that this scheme cannot cope with all conceivable
denial of service attacks. It is limited to those where a
significant amount of traffic is coming from a relatively small
number of sources. Furthermore, those sources must themselves be in
some sense evil or corrupted. An attack based on inducing innocent
and uncorrupted machines to send traffic to the victim would be
traceable only to these machines, and not to the real attackers.
A lengthy discussion of the possibility of flooding attacks using
fake ITrace packets to fill host buffers and render the tool useless
took place after the previous version of this document was issued.
(Thread " Problems with implementation - DoS attacks possible" on
the ITrace E-mail list, initiated 1/20/2003 by Tomasz Grabowski). A
major issue is how quickly hash keys should be rotated so disclosure
can take place. Further work is needed to resolve this.
6. IANA Considerations
TBD
7.Acknowledgements
The ICMP Traceback message is the product of an informal research
group; members include (in alphabetical order) Steven M. Bellovin,
Matt Blaze, Bill Cheswick, Cory Cohen, Jon David, Jim Duncan, Jim
Ellis, Paul Ferguson, John Ioannidis, Marcus Leech, Perry Metzger,
Robert Stone, Vern Paxson, Ed Vielmetti, Wietse Venema.
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ICMP Traceback Messages February 2003
8. References
[RFC792] : J. Postel, "Internet Control Message Protocol", RFC 792,
Internet Engineering Task Force, September 1981.
[RFC1305]: David L. Mills, "Network Time Protocol (Version 3):
Specification, Implementation and Analysis", RFC 1305, Internet
Engineering Task Force, March 1992.
[RFC2104]: H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, Internet Engineering Task
Force, February 1997.
[RFC2119]: S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Internet Engineering Task Force,
March 1997.
[RFC2402]: S. Kent and R. Atkinson, "IP Authentication Header", RFC
2402, Internet Engineering Task Force, November 1998.
[RFC2460]: Deering, S. and R. Hinden, "Internet Protocol, Version 6,
(IPv6) Specification", RFC 2460, December 1998.
[RFC2463]: Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[RFC3279]: L. Bassham, R. Housley, W. Polk, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and CRL Profile", Internet Engineering Task Force, April
2002.
[HTTP-CERTSTOR]: Peter Gutmann, "Internet X.509 Public Key
Infrastructure: Operational Protocols: Certificate Store Access via
HTTP", Work in progress (draft-ietf-pkix-certstore-http-04.txt),
February 2003.
[SWKA00] : Stefan Savage, David Wetherall, Anna Karlin and Tom
Anderson, "Practical Network Support for IP Traceback", Technical
Report UW-CSE-2000-02-01, Department of Computer Science and
Engineering, University of Washington,
http://www.cs.washington.edu/homes/savage/traceback.html.
[SDS00] : D. Schnackenberg, K. Djahandari, and D. Sterne,
"Infrastructure for Intrusion Detection and Response", Proceedings
of the DARPA Information Survivability Conference and Exposition
(DISCEX), Hilton Head Island, SC, January 25-27, 2000.
[SPSSJTK01]: A.C. Snoeren, C. Partridge, L.A. Sanchez, W.T. Strayer,
C.E. Jones, F. Tchakountio, and S.T. Kent, "Hash-Based IP
Traceback", BBN Technical Memorandum No. 1284,
http://www.ir.bbn.com/documents/techmemos/TM1284.ps, February 7,
2001.
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ICMP Traceback Messages February 2003
9. Author Information
Steven M. Bellovin,
AT&T Labs Research
Shannon Laboratory
180 Park Avenue
Florham Park, NJ 07974
USA
Phone: +1 973-360-8656
Email: smb@research.att.com
Marcus D. Leech
Nortel Networks
P.O. Box 3511, Station C
Ottawa, ON
Canada, K1Y 4H7
Phone: +1 613-763-9145
Email: mleech@nortelnetworks.com
Tom Taylor [Editor]
Nortel Networks
P.O. Box 3511, Station C
Ottawa, ON
Canada, K1Y 4H7
Phone: +1 613-736-0961
Email: taylor@nortelnetworks.com
Bellovin Standards Track - Expires August 2003 18
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