Mehrdad Nourani

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Data Network Security

• Mehrdad Nourani

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Session 17

• IP Security

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• Review of
• TCP/IP Model and Features

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Why TCP/IP Is Preferred?

• Originated in ARPA protocol
• Simplified protocol stack
• Funded by the US government in early years
• Supported in Berkeley Unix (a free OS)
• Higher speed, lower price and more availability

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Some of Protocols in TCP/IP Suite
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Who Standardizes TCP/IP?

• The Internet Society
• Has both organizational and individual members
• Gets technical advice from the Internet
  Architecture Board (IAB )
• The IAB standardizes the protocols used on the
  Internet
• Specification documents are called Requests for
  Comments (RFCs).
• IAB oversees the Internet Engineering Task Force
  (IETF )
• Goal Standardization as a part of implementation
• Working documents are called Internet Drafts
• Divided into working groups focused on specific
  standards

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Who Standardizes TCP/IP? (cont.)

• Overall responsibility for names and IP
  addresses ICANN
• IP addresses American Registry for Internet
  Numbers (ARIN)
• Domain names Network Solutions, Inc. and many
  other registrars
• Numerical parameters Internet Assigned Numbers
  Authority (IANA)

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Internetworking (When One is not Enough)

• Why not have a single physical network for the
  entire planet?
• Requires centralized coordination
• Difficult to integrate heterogeneous networks
• Growth by scaling difficult (impossible?)
• Alternative Interconnected networks that look
  like a single network

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Internetworking Layer by Layer

• Layer 1 internetworking Goal is to connect two
  similar physical networks so that they function
  as one
• Typical internetworking device Repeating hub
• Layer 2 internetworking Connect two (possibly
  dissimilar) physical networks so that traffic
  flows from one to the other only if necessary
• Typical internetworking devices Bridge, Layer 2
  switch
• Layer 3 internetworking Goal is to connect
  diverse networks so that layers above the network
  layer see only a single large network
• Typical internetworking devices Router , Layer3
  switch
• Layer 4 internetworking Filter applications and
  network addresses to limit access
• Typical internetworking device Firewall

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Internetworking in the OSI Model
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Internetworking Modes

• There are two basic modes of internetworking at a
  particular protocol layer
• 1. Protocol Translation
• The Protocol Data Units (PDUs) of network A are
  replaced with network B PDUs
• Example A bridge (Layer 2 internetworking
  device) between an Ethernet LAN and a token-ring
  LAN removes the Ethernet framing and encapsulates
  the contents of each Ethernet frame in token-ring
  frames
• Problems arise when networks A and B offer
  dissimilar services

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Internetworking Modes (cont.)

• 2. Protocol Encapsulation
• At an edge node between two dissimilar networks,
  A and B, the network-layer protocol data units
  (PDUs) of network A are encapsulated in PDUs of
  network B
• Encapsulation nearly always works, so it is the
  IETFs usual approach for implementing IP in a
  non-IP network
• Example IP over ATM
• Disadvantage Large overhead

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The Internet Protocol (IP)

• The vision A virtual network, This is as
  important as virtual memory and networked (i.e.,
  virtual) file systems
• Modern memory and file systems present the same
  user interface, regardless of
• The physical location of the data
• The technology used to access the data
• Design goal for a virtual network
• Make hosts on other physical networks look and
  feel as if they were on the same physical network
  as your computer
• The worlds most important data network protocol
• If your network speaks IP, you can talk to
  networks anywhere

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Main Features of IP

• IP (Internet Protocol)
• Layer 3 (network)
• End-to-end encapsulation thus, hardware details
  are hidden
• Datagrams do not have to be explicitly routed
• Routing is performed hop-by-hop for each
  datagram, not end-to-end over a path set up in
  advance
• Transparent, connectionless, unreliable datagram
  transport
• No flow control

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Main Features of TCP

• TCP (the Transmission Control Protocol )
• Layer 4 (transport) thus, Hides network details
• Transparent, connection-oriented, reliable stream
  transport
• Flow and congestion control
• Sequence numbers and sliding windows
• Insensitive to details of routing

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TCP and UDP

• TCP (Transmission Control Protocol)
• connection-oriented
• Reliable packet delivery in sequence
• UDP (User Datagram Protocol )
• connectionless (datagram)
• Unreliable packet delivery
• Packets may arrive out of sequence or duplicated

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TCP/UDP Standard

• TCP
• RFC 793, RFC 1122
• Outgoing data is logically a stream of octets
  from user
• Stream broken into blocks of data, or segments
• TCP accumulates octets from user until segment is
  large enough, or data marked with PUSH flag
• Data marked with URGENT flag causes user to be
  signaled
• Similarly, incoming data is a stream of octets
  presented to user
• Data marked with PUSH flag triggers delivery of
  data to user, otherwise TCP decides when to
  deliver data
• UDP
• RFC 768
• Connectionless, unreliable, Less overhead
• Simply adds port addressing to IP
• Checksum is optional

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Fragmentation and Reassembly

• Networks may have different maximum packet size
• Router may need to fragment datagrams before
  sending to next network
• Fragments may need further fragmenting in later
  networks
• Reassembly done only at final destination since
  fragments may take different routes

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Encapsulation in TCP/IP
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A Decoded Ethernet Frame
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Role of IP

• IP provides functionality for interconnecting end
  systems across multiple networks.
• IP should be implemented in each end systems and
  routers in between
• Higher level data at a source are encapsulated in
  an IP data unit (PDU) for transmission
• E.g. in TCP/IP, the source IP layer attaches a
  header that specifies destination global address.
  If destination is in another subnetwork, in the
  router the IP hands its data to LLC (Logical Link
  Control) and later to MAC (Medium Access Control)
  layer that can be forwarded to the next router.

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Configuration of TCP/IP
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IP Headers
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IPv6 Packet with Extension Headers
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Routing What Makes Internet Possible

• Routers are specialized computers that forward
  datagrams
• Each network connected to the router communicates
  through a dedicated physical or logical network
  interface
• Many types exist
• Store and forward (e.g., general-purpose
  computer)
• Routing switch (ASIC forwarding engines, switch
  fabric)

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What Makes Routing Successful

• 1. The Robustness
• Principle (quoted from RFC 1123)
• At every layer of the protocols, there is a
  general rule whose application can lead to
  enormous benefits in robustness and
  interoperability
• Be liberal in what you accept, and
  conservative in what you send

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What Makes Routing Successful

• 2. Scalability
• A system that is designed to support growth to an
  arbitrarily large size without degradation of the
  services that it offers is called scalable
• For a network, size number of hosts or users
• The Internet Protocol (v4) has allowed the
  construction of a global, heterogeneous network
  of moderately large size by distributing the
  control
• The challenge for future protocols, systems and
  applications is to provide for scalability of the
  global Internet to a much larger size than it has
  at present

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• TCP/IP Operation

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Operation of TCP/IP
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Operation of TCP/IP (cont.)
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Operation of TCP/IP At Sender Side
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Operation of TCP/IP At Router
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Operation of TCP/IP At Receiver
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Packet Switch (Datagram Approach)

• Data transmitted in short blocks, or packets
• Packet length lt 1000 octets
• Each packet contains user data plus control info
  (routing)
• Store and forward
• Advantages
• flexibility, resource sharing, robust, responsive
• Disadvantages
• Time delays in distributed network, overhead
  penalties
• Need for routing and congestion control

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Packet Switch (Virtual Circuit Approach)

• Frame relay and ATM are variants of
  packet-switching
• Datagram
• Each packet sent independently of the others
• No call setup
• More reliable (can route around failed nodes or
  congestion)
• Virtual circuit
• Fixed route established before any packets sent
• No need for routing decision for each packet at
  each node

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• IP Security Overview

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IP Security

• Internet community has considered some
  application specific security mechanisms, e.g.
• Electronic Mail (S/MIME, PGP)
• Client-Server communication (Kerberos)
• Web Access (Secure Socket Layer SSL/HTTPS)
• however there are security concerns that cut
  across protocol layers, e.g.
• IP spoofing intruders create packets with false
  IP address and exploit applications that use
  authentication based on IP
• Packet sniffing attackers read transmitted
  information including logon information and
  database contents
• would like security implemented by the network
  for all applications

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IPSec

• general IP Security mechanisms
• provides
• authentication
• confidentiality
• key management
• applicable to use over LANs, across public
  private WANs, for the Internet. Examples
• Secure branch office connectivity over the
  Internet
• Secure remote access over the Internet
• Establishing Extranet/Intranet connectivity with
  partners
• Enhancing electronic commerce security

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IPSec Principal Features

• encrypts and/or authenticate all traffic at the
  IP level. Thus all distributed applications will
  benefit, such as
• logon, client-server, email, file transfer, web
  access
• is implemented in a firewall or router and
  provides strong security that can be applied to
  all traffic crossing the perimeter.
• when implemented in a firewall is resistant to
  bypass if all traffic from the outside must use
  IP.
• In routing applications, IPSec ensures that a
  router advertisement comes from a legitimate
  router and a routing update (or a redirect
  message) is not forged.

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IPSec Uses
Individual security (when needed)
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Benefits of IPSec

• in a firewall/router provides strong security to
  all traffic crossing the perimeter
• Typically, IPSec
• encrypt/compress data going into WAN
• decrypt/decompress traffic coming from WAN
• is resistant to bypass (affects all traffic with
  no exception)
• is below transport layer (TCP, UDP), hence
  transparent to applications (e.g. servers and
  workstations in LAN)
• can be transparent to end users
• can provide security for individual users if
  desired
• can be also used in routing applications to make
  sure that a new router is authorized in the
  neighborhood.

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IP Security Architecture

• specification has become quite complex
• defined in numerous RFCs
• including RFC 2401/2402/2406/2408
• many others, grouped by category
• mandatory in IPv6, optional in IPv4
• Security features are implemented as extension
  headers that follow the main IP header

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IPSec Services

• Access control
• Connectionless integrity
• Data origin authentication
• Rejection of replayed packets
• a form of partial sequence integrity
• Confidentiality (encryption)
• Limited traffic flow confidentiality

Encapsulating Security Payload Protocol
Authentication Header Protocol
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Security Associations (SA)

• SA is a one-way relationship between sender
  receiver that affords security services to the
  traffic carried on it
• defined by 3 parameters
• Security Parameters Index (SPI) (carried in
  SA/ESP headers)
• IP Destination Address (endpoint of the SA)
• Security Protocol Identifier (says if SA is an AH
  or ESP)
• The IP destination address is in IPv4/IPv6 header
  and SPI in the enclosed extension header (AH or
  ESP)
• SA has a number of other parameters (see book)
• sequence number, sequence counter overflow,
  Anti-replay window, AH info, ESP info, lifetime,
  protocol mode, etc.
• Through these parameters, authentication and
  privacy are specified independent of specific
  key-management mechanism.
• have a database of Security Associations (see
  details of Security Policy Database (SPD) in the
  textbook).

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Transport and Tunnel Modes

• Transport Mode
• Used for end-to-end communications (e.g. two
  workstations or a client and a server)
• ESP encrypts and optionally authenticate the IP
  payload but not the IP header
• AH authenticate the IP payload and selected
  portions of the IP header
• Tunnel Mode
• Used when one or both ends of communication is a
  security gateway such as a firewall or a router
  that implements IPSec.
• The entire original (inner) packet travels
  through tunnel and no router along the way is
  able to examine the inner IP header
• After AH and ESP fields are added to IP packet,
  the entire packet plus security fields is treated
  as the payload of new outer IP packet with a
  new outer IP header.

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Transport and Tunnel Modes (cont.)
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Transport Tunnel Modes

• Uses Transport mode, e.g.
• Workstation and server share a protected secret
  key

• Uses Tunnel mode, e.g.
• to access the entire internal network or
• because the requested server does not support the
  authentication feature.

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Authentication Header (AH)

• provides support for data integrity
  authentication of IP packets
• end system/router can authenticate
  user/application
• prevents address spoofing attacks by tracking
  sequence numbers (spoofing is the creation of
  TCP/IP packets using somebody else's IP address.
  Then, the responses may be directed to the
  attacker).
• The authentication Data field holds a value
  referred to as Integrity Check Value (ICV) which
  is based on use of a MAC
• HMAC-MD5-96 or HMAC-SHA-1-96
• parties must share a secret key

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Authentication Header
(identifies a security association)
(A counter value up to 232 for one SA to provide
anti-replay function)
(also called ICV-96 bits)
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Scope of AH Authentication

• Transport Mode AH
• In IPv4, AH is inserted after the original IP
  header and before the IP payload.
• In IPv6, AH is viewed as an end-to-end payload,
  i.e. it is not examined or processed by
  intermediate routers
• In both IPv4 and IPv6, authentication covers the
  entire packet, excluding mutable fields that are
  set to zero for MAC calculation

(dest options extension header can be before or
after AH)
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Scope of AH Authentication (cont.)

• Tunnel Mode AH
• Entire IP packet is authenticated
• AH is inserted between the original IP header and
  a new outer IP header
• The inner header has source / destination
  addresses
• The outer header has address of firewall or other
  security gateways
• Entire inner IP packet, including the entire IP
  header, is protected by AH

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Encapsulating Security Payload (ESP)

• provides message content confidentiality
  limited traffic flow confidentiality
• can optionally provide the same authentication
  services as AH
• supports range of ciphers, modes, padding
• including DES, 3-key triple-DES, RC5, 3-key
  triple-IDEA, CAST, etc.
• CBC most common
• pad to meet block-size, for traffic flow

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Encapsulating Security Payload
(To prevent replay attack)

• - Transport mode transport level segment
• Tunnel mode IP packet

(also called ICV computed over ESP packet minus
Authentication Data field)
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Transport vs. Tunnel Mode ESP

• transport mode is used to encrypt optionally
  authenticate IP data
• data protected but header left in clear
• attacker can do traffic analysis
• good for ESP host to host traffic
• tunnel mode encrypts entire IP packet
• add new header for next hop
• good for virtual private networks (VPNs), gateway
  to gateway security

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Transport vs. Tunnel Mode ESP (cont.)

• Transport Mode
• Encryption (and optionally authentication) is
  provided directly between two hosts.
• Tunnel Mode
• E.g. Four private networks are interconnected
  across the Internet. Hosts use internet to
  communicate among themselves only. The tunnel and
  security gateway do not allow hosts to interact
  with other Internet-based hosts.

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Scope of ESP Encryption Authentication

• Transport Mode ESP
• In IPv4, ESP header is inserted after the
  original IP header and before the transport layer
  header (e.g. TCP, UDP, ICMP).
• In IPv6, ESP is viewed as an end-to-end payload,
  i.e. it is not examined or processed by
  intermediate routers
• In both IPv4 and IPv6, authentication covers the
  cipher plus the ESP header
• The destination node examines and processes the
  IP and extensions headers. Then based on SPI in
  ESP header, decrypts the remainder of packet to
  recover transport-layer segment.

ESP
Used if authentication is selected
ESP Trailerpadding, pad length, next header
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Scope of ESP Encryption Authentication (cont.)

• Tunnel Mode ESP
• Entire IP packet is authenticated
• ESP is inserted between the original IP header
  and a new outer IP header
• The new header provides information for routers
  for routing but not for traffic analysis
• Packet plus ESP trailer is encrypted
• The destination firewall examines and processes
  the outer IP header plus any extension headers.
  Then based on SPI in the ESP header decrypts the
  packet and then send it to the internal network.

ESP
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Combining Security Associations

• An individual SA can implement either the AH or
  ESP but not both
• to implement both need to combine SAs
• form a security bundle
• have 4 cases that must be supported by compliant
  IPSec hosts (e.g. workstations and servers) or
  security gateways (e.g. firewall, router).

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Combining Security Associations
Security is provided between any two system with
IPSec (sharing secret key)
Cases (1) and (2) are combined
Security is provided only between gateways
(routers, firewalls, etc.)
Case (1) plus support for a remote host to reach
firewall and server behind it
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Key Management

• handles key generation distribution
• typically need 2 pairs of keys
• transmit for AH transmit for ESP
• receive for AH receive for ESP
• manual key management
• System admin manually configures every system
• automated key management
• automated system for on demand creation of keys
  for SAs in large systems
• Default automated key management protocol for
  IPSec is referred to as ISAKMP/Oakley elements

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Oakley

• a key exchange protocol
• based on Diffie-Hellman (DH) key exchange
• adds features to address weaknesses
• It employs a mechanism known as Cookies to solve
  clogging attack (to solve pseudorandom numbers
  problem)
• It enables two parties to negotiate a group (to
  set global parameters for Diffie-Hellman key
  exchange)
• It uses nonces to ensures against replay attacks
• It enables DH key exchange with authentication
  (to solve the man-in-the-middle attack)
• can use arithmetic in prime fields or elliptic
  curve fields
• See book for examples.

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ISAKMP

• ISAKMP stands for Internet Security Association
  and Key Management Protocol
• defines procedures and packet formats to
  establish, negotiate, modify, delete security
  associations (SAs)
• The payload format, defined by ISAKMP, provides
  framework for key management independent of
• key exchange protocol
• encryption algorithm
• authentication method

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ISAKMP
(A pseudorandom number)
(unique ID for this message)
(header plus payload in octets)
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ISAKMP Payload Types
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ISAKMP Message Exchange Types
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ISAKMP Message Exchange Types (cont.)
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Summary

• have considered
• IPSec security framework
• AH
• ESP
• key management Oakley/ISAKMP