1. CS 5565 Network Architecture and Protocols
Lecture 23
Godmar Back

2. Announcements Project 2B due in 2 parts: Extra Credit Opportunities:
Apr 29 and May 6
Extra Credit Opportunities:
Expand simulator (and your implementation) to introduce multiple link failures and link resurrection
Additional, requiring reading posted
Andersen et al [SIGCOMM’08]: Accountable Internet Protocol (AIP)
Casado et al [HotNets’08]: Rethinking Packet Forwarding Hardware
CS 5565 Spring 2009
7/4/2018

3. Error Detection & Correction
Data Link Layer
Error Detection & Correction

4. Error Detection EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
Error detection not 100% reliable!
protocol may miss some errors, but rarely
larger EDC field yields better detection and correction
CS 5565 Spring 2009
7/4/2018

5. Coding Theory Coding Theory is the study of codes and their properties
Error Detection Codes:
CRC, Parity, Checksums
Error Correction Codes: FEC – Forward Error Correction:
Hamming, Reed Solomon
General guidelines to evaluate effectiveness of codes
Minimum distance of codes
Burst-detecting capabilities
Probability that random string is accepted as error free
CS 5565 Spring 2009
7/4/2018

6. Parity Checking Two Dimensional Bit Parity: Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
Odd Parity: odd number of 1’s,
including parity bit
Parity works well if single random
bit flips – does not handle burst
errors well
2D Parity can be used to correct single bit error
CS 5565 Spring 2009
7/4/2018

7. Hamming Codes (1) Hamming Distance between codes: Suppose you encode
Number of bits you have to change to transform one code word to another
Suppose you encode
0 as 000
1 as 111
Source:
CS 5565 Spring 2009
7/4/2018

8. Hamming Codes (2) Correct 1-bit error Hamming Rule d+p+1 ≤ 2p
Detect 2-bit error
Hamming Rule
d+p+1 ≤ 2p
d – number of data bits
p – number of parity bits
(d+p, d) code
Example d = 7, p = 4
12 ≤ 2^4
(11, 7) code
CS 5565 Spring 2009
7/4/2018

9. Internet Checksum RFC 1071: used for UDP/TCP segments & IP header
treat segment contents as sequence of 16-bit integers
checksum: 1’s complement sum of segment contents
1’s complement sum: carry wraps around and is added, if any
somewhat higher probability to declare segments correct for real data with many zeros [Partridge 95]
CS 5565 Spring 2009
7/4/2018

10. Internet Checksum (2) u_short cksum(u_short *buf, int count) {
register u_long sum = 0;
while (count--) {
sum += *buf++;
if (sum & 0xFFFF0000) {
/* carry, so wrap around */
sum &= 0xFFFF;
sum++; }
return ~(sum & 0xFFFF);
CS 5565 Spring 2009
7/4/2018

11. Checksumming: Cyclic Redundancy Check
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
can detect all burst errors less than r+1 bits
widely used in practice (ATM, HDLC)
CS 5565 Spring 2009
7/4/2018

12. CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R
if we divide D.2r by G, want remainder R
D.2r G
R = remainder[ ]
CS 5565 Spring 2009
7/4/2018

13. CRC (cont’d)
Formulation often as polynomials over algebraic field {0, 1}, e.g.
G = x3 + 1
D = x5 + x3 + x2 + x
CRC-32 IEEE polynomial
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
detects all burst errors up to 32bits
detects all burst errors of odd length
CS 5565 Spring 2009
7/4/2018

14. Multiple Access Protocols & Link Layer Addressing

15. MAC Addresses and ARP 32-bit IP address:
network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address:
used to get datagram from one interface to another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
CS 5565 Spring 2009
7/4/2018

16. Each adapter on LAN has unique LAN address
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
CS 5565 Spring 2009
7/4/2018

17. ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
Each IP node (Host, Router) on LAN has ARP table
ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
CS 5565 Spring 2009
7/4/2018

18. ARP Protocol ARP is “plug-and-play”:
A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
Dest MAC address = FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
CS 5565 Spring 2009
7/4/2018

19. Interaction of ARP and Routing
Send datagram from A to B via R A R B
Two ARP tables in router R, one for each IP network (LAN)
CS 5565 Spring 2009
7/4/2018

20. Proxy ARP
Default Gateway
LAN
Point-To-Point
Link, e.g. dialup
with subnet
/30
Upstream
Internet
58-23-D7-FA-20-B0
uses /24
answers ARP requests for
See arp(8), “arp –s … pub”
Forwards IP datagrams along
uses as default gateway
CS 5565 Spring 2009
7/4/2018

21. DHCP Dynamic Host Configuration Protocol Uses MAC-level broadcast
Precursors: RARP – Reverse ARP, bootp
Uses MAC-level broadcast
DHCP Discovery (“what DHCP servers are out there and who can give me an IP address?”)
DHCP Offer (“you may use this address”)
May be more than one
DHCP Request (“I’d like to use this address”)
DHCP Ack (“please do”)
DHCP relay agents for larger networks.
CS 5565 Spring 2009
7/4/2018