1 ICS 156: Lecture 2 (part 2) Data link layer protocols Address resolution protocol Notes on lab 2
Dec 14, 2015
2
TCP/IP Protocol Stack
ApplicationLayer
TransportLayer
NetworkLayer
(Data) LinkLayer
• The TCP/IP protocol stack runs on top of multiple data link layers.
• Two data link layer technologies
•Broadcast
•Point-to-Point
Logical LinkControl (LLC)
Media AccessControl (MAC)
Sublayer inLocal AreaNetworks
3
Data Link Layer
• The main tasks of the data link layer are:• Transfer data from the network layer of one machine to
the network layer of another machine• Convert the raw bit stream of the physical layer into
groups of bits (“frames”)
NetworkLayer
Data LinkLayer
PhysicalLayer
NetworkLayer
Data LinkLayer
PhysicalLayer
4
Two types of networks at the data link layer
– Broadcast Networks: All stations share a single communication channel
– Point-to-Point Networks: Pairs of hosts (or routers) are directly connected
• Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point
Broadcast Network Point-to-Point Network
5
Local Area Networks
• Local area networks (LANs) connect computers within a building or a enterprise network
• Almost all LANs are broadcast networks• Typical topologies of LANs are bus or ring or star• We will work with Ethernet LANs. Ethernet has a bus or star
topology.
•Bus LAN •Ring LAN
6
MAC and LLC
• In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel
• The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol
• The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer
• The higher portion of the data link layer is often called Logical Link Control (LLC)
Logical LinkControl
Medium AccessControlD
ata
Link
Laye
r
to Physical Layer
to Network Layer
7
IEEE 802 Standards
•IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers
80
2.3
80
2.4
80
2.5
80
2.1
1
802.2
802.1
IEEE 802 standard
MediumAccessControl
PhysicalLayer
Logical LinkControl
IEEEReference
Model
PhysicalLayer
Data LinkLayer
HigherLayer
Higher layer issues
LLC
CS
MA
/CS
Token
bus
Token
ring
Wireless
lan
8
Ethernet
• Speed: 10Mbps -10 Gbps• Standard: 802.3, Ethernet II (DIX)
• Most popular physical layers for Ethernet:
• 10Base5 Thick Ethernet: 10 Mbps coax cable• 10Base2 Thin Ethernet: 10 Mbps coax cable• 10Base-T 10 Mbps Twisted Pair • 100Base-TX 100 Mbps over Category 5 twisted pair• 100Base-FX 100 Mbps over Fiber Optics• 1000Base-FX 1Gbps over Fiber Optics• 10000Base-FX 1Gbps over Fiber Optics (for wide area
links)
10
• Starting with 10Base-T, stations are connected to a hub in a star configuration
Star Topology
Hub
11
Ethernet Hubs vs. Ethernet Switches
• An Ethernet switch is a packet switch for Ethernet frames • Buffering of frames prevents collisions. • Each port is isolated and builds its own collision domain
• An Ethernet Hub does not perform buffering:• Collisions occur if two frames arrive at the same time.
HighS
peedB
ackplane
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
OutputBuffers
InputBuffers
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Hub Switch
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Ethernet and IEEE 802.3: Any Difference?
• There are two types of Ethernet frames in use, with subtle differences:
• “Ethernet” (Ethernet II, DIX (Digital-Intel-Xerox)• An industry standards from 1982 that is based on the first
implementation of CSMA/CD by Xerox.• Predominant version of CSMA/CD in the US.
• 802.3:• IEEE’s version of CSMA/CD from 1985.• Interoperates with 802.2 (LLC) as higher layer.
• Difference for our purposes: Ethernet and 802.3 use different methods to encapsulate an IP datagram.
13
Ethernet II, DIX Encapsulation (RFC 894)
802.3 MAC
destinationaddress
6
sourceaddress
6
type
2
data
46-1500
CRC
4
0800
2
IP datagram
38-1492
0806
2
ARP request/reply
28
PAD
10
0835
2
RARP request/reply
28
PAD
10
14
IEEE 802.2/802.3 Encapsulation (RFC 1042)
802.3 MAC
destinationaddress
6
sourceaddress
6
length
2
DSAPAA
1
SSAPAA
1
cntl03
1
org code0
3
type
2
data
38-1492
CRC
4
802.2 LLC 802.2 SNAP
- destination address, source address:MAC addresses are 48 bit
- lengt h : frame length in number of bytes- DSAP, SSAP : always set to 0xaa- Ctrl: set t o 3- org code: set to 0- type field identifies the content of the
data field- CRC: cylic redundancy check
0800
2
IP datagram
38-1492
0806
2
ARP request/reply
28
PAD
10
0835
2
RARP request/reply
28
PAD
10
15
Dial-Up Access
AccessRouter
Modems
Point-to-Point (serial) links
• Many data link connections are point-to-point serial links:– Dial-in or DSL access connects hosts to
access routers– Routers are connected by
high-speed point-to-point links
• Here, IP hosts and routers are connected by a serial cable
• Data link layer protocols for point-to-point links are simple:– Main role is encapsulation of IP
datagrams– No media access control needed
Point-to-Point Links
Router
Router
Router Router
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Data Link Protocols for Point-to-Point links
• SLIP (Serial Line IP) • First protocol for sending IP datagrams over dial-up links (from
1988)• Encapsulation, not much else
• PPP (Point-to-Point Protocol):• Successor to SLIP (1992), with added functionality• Used for dial-in and for high-speed routers
• HDLC (High-level Data Link Control) :• Widely used and influential standard (1979)• Default protocol for serial links on Cisco routers• Actually, PPP is based on a variant of HDLC
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PPP - IP encapsulation
• The frame format of PPP is similar to HDLC and the 802.2 LLC frame format:
• PPP assumes a duplex circuit• Note: PPP does not use addresses• Usual maximum frame size is 1500
7E
flag
1
FF
addr
1
03
ctrl
1 2
protocol
<= 1500
data
2
CRC
7E
flag
1
0021 IP datagram
C021 link control data
8021 network control data
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Additional PPP functionality
• In addition to encapsulation, PPP supports:– multiple network layer protocols (protocol multiplexing)– Link configuration– Link quality testing– Error detection– Option negotiation– Address notification – Authentication
• The above functions are supported by helper protocols: – LCP – PAP, CHAP– NCP
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PPP Support protocols
• Link management: The link control protocol (LCP) is responsible for establishing, configuring, and negotiating a data-link connection. LCP also monitors the link quality and is used to terminate the link.
• Authentication: Authentication is optional. PPP supports two authentication protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
• Network protocol configuration: PPP has network control protocols (NCPs) for numerous network layer protocols. The IP control protocol (IPCP) negotiates IP address assignments and other parameters when IP is used as network layer.
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NetworkLayer
Link Layer
IP
ARP NetworkAccess RARP
Media
ICMP IGMP
TransportLayer
TCP UDP
Overview
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ARP and RARP
• Note: – The Internet is based on IP addresses – Data link protocols (Ethernet, FDDI, ATM) may have
different (MAC) addresses• The ARP and RARP protocols perform the translation
between IP addresses and MAC layer addresses• We will discuss ARP for broadcast LANs, particularly Ethernet
LANs
RARP
Ethernet MACaddress(48 bit)
ARPIP address(32 bit)
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Processing of IP packets by network device drivers
loopbackDriver
IP Input
Put on IPinput queue
ARPdemultiplex
Ethernet Frame
Ethernet
IP destination of packet= local IP address ?
IP destination = multicastor broadcast ?
IP Output
Put on IPinput queue
No: get MACaddress withARP
ARPPacket
IP datagram
No
Yes
YesEthernet
Driver
24
Address Translation with ARP
ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of 128.143.137.1?”
Argon128.143.137.144
00:a0:24:71:e4:44
Router137128.143.137.1
00:e0:f9:23:a8:20
ARP Request:What is the MAC addressof 128.143.71.1?
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Address Translation with ARP
ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address
Argon128.143.137.144
00:a0:24:71:e4:44
Router137128.143.137.1
00:e0:f9:23:a8:20
ARP Reply:The MAC address of 128.143.71.1is 00:e0:f9:23:a8:20
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ARP Packet Format
Destinationaddress
6
ARP Request or ARP Reply
28
Sourceaddress
6 2
CRC
4
Type0x8060
Padding
10
Ethernet II header
Hardware type (2 bytes)
Hardware addresslength (1 byte)
Protocol addresslength (1 byte)
Operation code (2 bytes)
Target hardware address*
Protocol type (2 bytes)
Source hardware address*
Source protocol address*
Target protocol address*
* Note: The length of the address fields is determined by the corresponding address length fields
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Example
• ARP Request from Argon:
Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: 00:00:00:00:00:00Target protocol address: 128.143.137.1
• ARP Reply from Router137:
Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144
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ARP Cache
• Since sending an ARP request/reply for each IP datagram is inefficient, hosts maintain a cache (ARP Cache) of current entries. The entries expire after a time interval.
• Contents of the ARP Cache:(128.143.71.37) at 00:10:4B:C5:D1:15 [ether] on eth0
(128.143.71.36) at 00:B0:D0:E1:17:D5 [ether] on eth0
(128.143.71.35) at 00:B0:D0:DE:70:E6 [ether] on eth0
(128.143.136.90) at 00:05:3C:06:27:35 [ether] on eth1
(128.143.71.34) at 00:B0:D0:E1:17:DB [ether] on eth0
(128.143.71.33) at 00:B0:D0:E1:17:DF [ether] on eth0
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Proxy ARP
• Proxy ARP: Host or router responds to ARP Request that arrives from one of its connected networks for a host that is on another of its connected networks.
128.143.137.1/1600:e0:f9:23:a8:20
128.143.71.1/24
128.143.0.0/16Subnet
128.143.71.0/24Subnet
Router137
ARP Request:What is the MAC addressof 128.143.71.21?
128.143.137.144/16128.143.171.21/2400:20:af:03:98:28
Argon Neon
ARP Reply:The MAC address of128.143.71.21 is00:e0:f9:23:a8:20
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Things to know about ARP
• What happens if an ARP Request is made for a non-existing host? Several ARP requests are made with increasing time
intervals between requests. Eventually, ARP gives up.
• On some systems (including Linux) a host periodically sends ARP Requests for all addresses listed in the ARP cache. This refreshes the ARP cache content, but also introduces traffic.
• Gratuitous ARP Requests: A host sends an ARP request for its own IP address:– Useful for detecting if an IP address has already been
assigned.
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Vulnerabilities of ARP
1. Since ARP does not authenticate requests or replies, ARP Requests and Replies can be forged
2. ARP is stateless: ARP Replies can be sent without a corresponding ARP Request
3. According to the ARP protocol specification, a node receiving an ARP packet (Request or Reply) must update its local ARP cache with the information in the source fields, if the receiving node already has an entry for the IP address of the source in its ARP cache. (This applies for ARP Request packets and for ARP Reply packets)
Typical exploitation of these vulnerabilities:• A forged ARP Request or Reply can be used to update the ARP cache of
a remote system with a forged entry (ARP Poisoning)• This can be used to redirect IP traffic to other hosts
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What is a single-segment network?
• A single-segment network consists of interfaces connected by a single physical link, either a point-to-point link or a broadcast link.
• Interfaces on the same single-segment network have the same network prefix.
128.195.1.100
128.195.1.200
128.195.1.300
128.195.1.1
128.195.2.100
128.195.2.200
128.195.3.100 128.195.3.200
128.195.2.1
128.195.3.1
128.195.1.0/24128.195.2.0/24
128.195.3.0/24
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How to identify a single segment IP network
• Detach interfaces from routers or hosts• Each isolated island is a single segment IP network• Each interface on the same single segment IP network must have
the same network address prefix
128.195.1.100
128.195.1.200
128.195.1.300
128.195.1.1 128.195.2.1
128.195.3.1
128.195.2.100
128.195.2.200
128.195.3.100 128.195.3.200
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Protocol specification vs implementation
• According to the ARP protocol specification, a node receiving an ARP packet (Request or Reply) must update its local ARP cache with the information in the source fields, if the receiving node already has an entry for the IP address of the source in its ARP cache. (This applies for ARP Request packets and for ARP Reply packets)
• Implementation may differ from the specification
• What you observe in the lab may not be universally true.