CPS-356- Computer Networks
Class 8: IP Forwarding+ Routing
Theophilus Benson
Based partly on lecture notes by Rodrigo Fonseca, David Mazières, Phil Levis, John Jannotti
Admini-strivia
• Midterm 1:
– Day after UNC game: New proposed dates:
• 02/24/2015
• HW #1: Going Up Tomorrow on Website (due
in a week 02/12/2015)
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Debugging the network: ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing
– Distance Vector Protocol
• Loop Detection + Avoidance
Format of IP Addresses
Classed Addresses
• Pros
– Very simple to use and
implement
– Allows for hierarchical routing
– Use first 3 bits to determine
addresses class (A, B, C)
– Based on class you know what
bits to ignore
• Cons
– Wasteful allocation
– Statically specify network and
host portion of address
CIDR Addresses
• Pros
– Efficient allocation of
resources
– dynamically specify network
and host portion of address
• Cons
– More complex to implement
in hardware
Format of IP Addresses
Classed Addresses
(Static partitioning of Network/host
portions)
• Class A (8-bit prefix), B (16bit), C (24-bit)
CIDR
(Dynamic partitioning of
Network/hosts portions)
128.23.16.12/31
Specifies the prefix size: the number of
bits in the network portion (NetMask)
10000000.00010111.00010000.00001100
11111111.11111111.11111111.11111110
128.23.92.12
10000000
Prefix size = 31 bits
Host size = 1 bit
32-31=1
Only 2^1 hosts in the network
Other CIDR Examples
128.23.16.12/24
128.23.16.12/32
10000000.00010111.00010000.00001100
10000000.00010111.00010000.00001100
11111111.11111111.11111111.00000000
11111111.11111111.11111111.11111111
Prefix size = 24 bits
Host size = 8 bits
32 – 24 = 8
Only 2^8 hosts in the network
Prefix size = 32 bits
Host size = 0 bit
32-32=0
Only 2^0 hosts in the network
Where Does IP-Address Fit Into a
packet?
Src Port
Dst Port
Seq Number
Ack Number
Window
Offset Reserved
V
TOS
Identification
V
Data (Payload)
Total Length
MMM
Frag
Protocol Hdr checksum
TTL
Source IP Address
Destination IP Address
Options
Padding
Destination MAC Address
Source MAC Address
Length
Type
IP
Ethernet
IP v4 packet format
• Forward based on destination
address
Hdr
len
vers
TOS
Identification
TTL
Protocol
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
IP v4 packet format
Hdr
len
vers
TOS
Identification
TTL
Protocol
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
• Forward based on destination
address
• TTL = Time to Live
• Prevents forwarding loops
• Decremented at each hop
IP v4 packet format
DF
Hdr
len
vers
TOS
Identification
TTL
Protocol
MF
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
• Forward based on destination
address
• TTL = Time to Live
• Prevents forwarding loops
• Decremented at each hop
• Cut large packets into smaller
ones
• E.g. from Ethernet to ATM
• From 1500B to 64B
• MF: more fragments
• DF: don’t fragment (return
an error to the sender)
IP v4 packet format
DF
Hdr
len
vers
TOS
Identification
TTL
Protocol
MF
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
• Forward based on destination
address
• TTL = Time to Live
• Prevents forwarding loops
• Decremented at each hop
• Cut large packets into smaller
ones
• E.g. from Ethernet to ATM
• From 1500B to 64B
• MF: more fragments
• DF: don’t fragment (return
an error to the sender)
• Version = IPv4 or IPv6
IP v4 packet format
Hdr
len
vers
TOS
Identification
TTL
Protocol
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
• Forward based on destination
address
• TTL = Time to Live
• Prevents forwarding loops
• Decremented at each hop
• Cut large packets into smaller
ones
• E.g. from Ethernet to ATM
• From 1500B to 64B
• MF: more fragments
• DF: don’t fragment (return
an error to the sender)
• Version = IPv4 or IPv6
• Protocol = TCP/UDP?
IP v4 packet format
Hdr
len
vers
TOS
Identification
TTL
Protocol
Total Length
Fragment Offset
Hdr Checksum
Source IP Address
Destination IP Address
Options
Padding
• Forward based on destination
address
• TTL = Time to Live
• Prevents forwarding loops
• Decremented at each hop
• Cut large packets into smaller
ones
• E.g. from Ethernet to ATM
• From 1500B to 64B
• MF: more fragments
• DF: don’t fragment (return
an error to the sender)
• Version = IPv4 or IPv6
• Protocol = TCP/UDP?
Header length == size of the header, which can vary because you can have an
arbitrary number of options
Total length == length of header + payload
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Network Error Messages (Debugging): ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing: RIP
Why Do you need to Fragment
Packets?
• Different networks have different MTUs.
– Router may need to fragment packets to allow
them to cross different mediums
DukeNet
(Ethernet)
MTU=1500
Le Theo Net
(ATM)
MTU=64
ATT
(Ethernet)
MTU=1500
Implication of Fragmentation
• If a fragment is lost, must retransmit the
whole packet!!! Why?
• Fragmentation delays reassembly of packet
until all fragments are received
• Some people avoid fragmentation!!!!
What do Fragmented Packets look
like?
• Use ‘identification’, ‘fragment offset’ and ‘MF’ bit
in IP header
– Set the ‘MF’ bit
– Use the same ‘Id’ for all fragments
– Offset present position in original packet
Start of header
1
0
Rest of header
213
Start of header
0
0
Rest of header
213
1400 Bytes
512 bytes
Start of header
1 64
Rest of header
213
512 bytes
Start of header
0 128
Rest of header
213
376 bytes
Internet Control Message Protocol
(ICMP)
•
•
•
•
•
•
•
•
Echo (ping)
Redirect
Destination unreachable (protocol, port, or host)
TTL exceeded
Checksum failed
Reassembly failed
Can’t fragment
Many ICMP messages include part of packet that
triggered them
• See http://www.iana.org/assignments/icmpparameters
ICMP message format
Example: Time Exceeded
• Code usually 0 (TTL exceeded in transit)
• Discussion: traceroute
Example: Can’t Fragment
• Sent if DF=1 and packet length > MTU
• What can you use this for?
• Path MTU Discovery
– Can do binary search on packet sizes
– But better: base algorithm on most common
MTUs
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Debugging the network: ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing: RIP
How do you Make a Packet
Src Port
Dst Port
Seq Number
Ack Number
Window
Offset Reserved
V
TOS
Identification
V
TTL
Data (Payload)
Total Length
MMM
Frag
Protocol Hdr checksum
Source IP Address
IP
Destination IP Address
Options
Padding
Destination MAC Address
Source MAC Address
Length
Type
DNS gives this
to you
???????
Comes with
your hardware
Ethernet
Obtaining Host IP Addresses - DHCP
• Address must be assigned to each host by his
network.
– Manually: Tedious and error-prone:
– Automatically: Dynamic Host Configuration Protocol
• Client: DHCP Discover to 255.255.255.255 (broadcast)
• Server(s): DHCP Offer to 255.255.255.255 (why
broadcast?)
• Client: choose offer, DHCP Request (broadcast, why?)
• Server: DHCP ACK (again broadcast)
• Result: IP-address, gateway, netmask, DNS server
How do you Make a Packet
Src Port
Dst Port
Seq Number
Ack Number
Window
Offset Reserved
V
TOS
Identification
V
TTL
Data (Payload)
Total Length
MMM
Frag
Protocol Hdr checksum
Source IP Address
IP
DHCP
Destination IP Address
Options
Padding
Destination MAC Address
Source MAC Address
Length
Type
DNS gives this
to you
???????
Comes with
your hardware
Ethernet
What is the Destination Address?
• If dest. is in your network (e.g. Alice to Bob)
– Then use the Destination’s Ethernet address.
• If dest. is not in your network (e.g Alice to Google)
– Then use the gateway router’s Ethernet address.
– The destination may use a different protocol
Ethernet
ATM
ATM
Ethernet
DukeNet
(Ethernet)
Alice
Le Theo Net
(ATM)
Ethernet
Google
Bob
How do you find this destination
address?
Ethernet
• Check local ARP table
– If found use it. (DONE!)
– Start sending packets!
Ethernet
DukeNet
(Ethernet)
Alice
Ethernet
Bob
ATM
How do you find this destination
address?
• Check local ARP table
– If found use it. (DONE!)
• Compare my IP with dest IP
Alice: 128.23.16.12/30
Bob: 128.23.16.14
Google: 128.16.16.16
– In same network?
• Then ARP request for Dest IP
– In different Networks?
• Then ARP request for Router IP
DukeNet:
128.23.16.12/30  4 addresses
128.23.16.12– 128.23.16.16
Alice->Bob: same network
Alice->Google: diff network
How do you find this destination
address?
Ethernet
Alice: 128.23.16.12/30
Bob: 128.23.16.14
Google: 128.16.16.16
Ethernet
DukeNet
(Ethernet)
Alice
Ethernet
Bob
DukeNet:
128.23.16.12/30  4 addresses
128.23.16.12– 128.23.16.16
Alice->Bob: same network
Alice->Google: diff network
ATM
How Ethernet
ARP ATM
works.
Ethernet
DukeNet
(Ethernet)
Alice
I am:
128.23.16.12
Who is IP:
128.23.16.14
Ethernet
Bob
How Ethernet
ARP ATM
works.
Ethernet
DukeNet
(Ethernet)
Alice
I am:
128.23.16.12
Who is IP:
128.23.16.14
Now I know who:
128.23.16.12 is!
Now I know who:
128.23.16.12 is!
Ethernet
Bob
How Ethernet
ARP ATM
works.
Ethernet
DukeNet
(Ethernet)
Alice
Ethernet
Bob
I am:128.23.16.14
MacAdd: 02………..
Now I know
who:
128.23.16.14 is!
Now I know who:
128.23.16.14 is!
ARP Ethernet frame format
• Why include source hardware address?
How do you Make a Packet
Src Port
Dst Port
Seq Number
Ack Number
Window
Offset Reserved
V
TOS
Identification
V
TTL
Data (Payload)
Total Length
MMM
Frag
Protocol Hdr checksum
Source IP Address
IP
DHCP
Destination IP Address
Options
Padding
Destination MAC Address
Source MAC Address
Length
Type
DNS gives this
to you
ARP
Comes with
your hardware
Ethernet
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Debugging the network: ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing
– Distance Vector Protocol
• Loop Detection + Avoidance
Routing
• Routing is the process of updating forwarding
tables
– Routers exchange messages about routers or
networks they can reach
– Goal: find optimal route for every destination
– … or maybe a good route, or any route
(depending on scale)
• Challenges
– Dynamic topology
– Decentralized
– Scale
Scaling Issues
• Every router must be able to forward based on
any destination IP address
– Given address, it needs to know next hop
– Naïve: one entry per address
– There would be 108 entries!
• Solutions
– Hierarchy (many examples)
– Address aggregation
• Address allocation is very important (should mirror
topology)
– Default routes
IP Connectivity
• For each destination address, must either:
– Have prefix mapped to next hop in forwarding
table
– Know “smarter router” – default for unknown
prefixes
• Route using longest prefix match, default is
prefix 0.0.0.0/0
• Core routers know everything – no default
• Manage using notion of Autonomous System
(AS)
Internet structure, 1990
• Several independent organizations
• Hierarchical structure with single
backbone
Internet structure, today
• Multiple backbones, more arbitrary
structure
Autonomous Systems
• Correspond to an administrative domain
– AS’s reflect organization of the Internet
– E.g., DukeNet, large company, etc.
– Identified by a 16-bit number
• AS are also called ISP
– ISP = Internet Service Providers
Lnk 1
DukeNet
B
Lnk2
A
ATT
Le Theo Net
C
D
• AS’s choose their own local routing algorithm
• How should A,B,C,D do routing?
• AS’s want to set policies about non-local routing
• Should DukeNet use Link 1 or 2 to ATT?
• AS’s need not reveal internal topology of their network
• That Duke Net has 4 routers
Inter and Intra-domain routing
• Routing organized in two levels
• Intra-domain routing
– Complete knowledge, strive for optimal paths
– Scale to ~100 networks
– Today
• Inter-domain routing
– Aggregated knowledge, scale to Internet
– Dominated by policy
• E.g., route through X, unless X is unavailable, then route
through Y. Never route traffic from X to Y.
– Policies reflect business agreements, can get complex
– Next lecture
Lnk 1
DukeNet
B
Lnk2
A
ATT
Le Theo Net
C
D
Intradomain:
Routing inside DukeNET
Interdomain:
Routing across DukeNet, ATT, TheoNet
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Debugging the network: ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing
– Distance Vector Protocol
• Loop Detection + Avoidance
Network as a graph
• Nodes are routers
• Assign cost to each edge
– Can be based on latency, b/w, queue length, …
• Problem: find lowest-cost path between
nodes
– Each node individually computes routes
Basic Algorithms
• Two classes of intra-domain routing algorithms
• Distance Vector (Bellman-Ford SP Algorithm)
– Requires only local state
– Harder to debug
– Can suffer from loops
• Link State (Djikstra-Prim SP Algorithm)
– Each node has global view of the network
– Simpler to debug
– Requires global state
Distance Vector
• Local routing algorithm
• Each node maintains a set of triples
– <Destination, Cost, NextHop>
• Exchange updates with neighbors
– Periodically (seconds to minutes)
– Whenever table changes (triggered update)
• Each update is a list of pairs
– <Destination, Cost>
• Update local table if receive a “better” route
– Smaller cost
• Refresh existing routes, delete if time out
DV Example
B only exchanges
information
with A and C
Distance Vector
• Local routing algorithm
• Each node maintains a set of triples
– <Destination, Cost, NextHop>
• Exchange updates with neighbors
– Periodically (seconds to minutes)
– Whenever table changes (triggered update)
• Each update is a list of pairs
– <Destination, Cost>
• Update local table if receive a “better” route
– Smaller cost
• Refresh existing routes, delete if time out
DV Example
B only exchanges
information
with A and C
D, 1
A, 1
B’s routing table
@ time = 0
Destination
Cost
Next Hop
A
1
A
C
1
C
D
infinity
--
E
infinity
--
F
infinity
--
G
infinity
--
DV Example
B only exchanges
information
with A and C
D, 1
A, 1
B’s routing table
@ time = 0
Destination
Cost
Next Hop
A
1
A
C
1
C
D
2
C
E
infinity
--
F
infinity
--
G
infinity
--
Distance Vector
• Local routing algorithm
• Each node maintains a set of triples
– <Destination, Cost, NextHop>
• Exchange updates with neighbors
– Periodically (seconds to minutes)
– Whenever table changes (triggered update)
• Each update is a list of pairs
– <Destination, Cost>
• Update local table if receive a “better” route
– Smaller cost
• Refresh existing routes, delete if time out
Calculating the best path
• Bellman-Ford equation
• Let:
– Db(d) denote the current best distance from b to d
– C(b,c) denote the cost of a link from a to b
• Then Db(d) = mind(Db(d) , c(b,c) + Dc(d))
• Routing messages
contain D
C’s update
Destination
Cost
1
• D is any additiveD,metric
Next Hop
1
A delay 1
A
– e.g, number ofA,hops,
queue length,
C
– log can convert multiplicative metric
into an1 additiveC
infinity
-one (e.g., probability of failure) D
E
Db(d) = mind(infinity, 1 + 1)
F
infinite
--
infinite
--
Db(A) = minA(1, 1 + 1) G
infinite
--
Calculating the best path
• Bellman-Ford equation
• Let:
– Db(d) denote the current best distance from b to d
– C(b,c) denote the cost of a link from a to b
• Then Db(d) = mind(Db(d) , c(b,c) + Dc(d))
• Routing messages contain D
• D is any additive metric
– e.g, number of hops, queue length, delay
– asdf
DV Example
B’s routing table
Destination
Cost
Next Hop
A
1
A
C
1
C
D
2
C
E
2
A
F
2
A
G
3
A
Adapting to Failures
G, 3, D
G, 2, D
G, 3,C
2, F
∞,-
G, 1, G
G, 4,
3, A
1, A
GG, ∞,
4,
•
•
•
•
F-G fails
F sets distance to G to infinity, propagates
A sets distance to G to infinity
A receives periodic update from C with 2-hop path to
G
• A sets distance to G to 3 and propagates
• F sets distance to G to 4, through A
Count-to-Infinity
•
•
•
•
•
•
•
Link from A to E fails
A advertises distance of infinity to E
B and C advertise a distance of 2 to E
B decides it can reach E in 3 hops through C
A decides it can reach E in 4 hops through B
C decides it can reach E in 5 hops through A, …
When does this stop?
Good news travels fast
B
1
4
1
A
C
10
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Bad news travels slowly
A
4
A
C
1
C
B
B
4 B
C
5 B
4
1
A
A
5 B
B
1 B
C
10
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
A
6
C
A
6
C
C
1
C
12
B
11
C
C
10
C
B
4
1
A
A
5 B
B
1 B
C
10
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
A
7
C
C
1
C
12
B
11
C
C
10
C
B
4
1
A
A
6 B
B
1 B
C
10
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
A
11
C
C
1
C
12
B
11
C
C
10
C
B
4
1
A
A
10 C
B
1
B
C
10
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
How to avoid loops
• IP TTL field prevents a packet from living
forever
– Does not repair a loop
• Simple approach: consider a small cost n (e.g.,
16) to be infinity
– After n rounds decide node is unavailable
– But rounds can be long, this takes time
• Problem: distance vector based only on local
information
Bad news travels slowly
A
11
C
C
1
C
12
B
11
C
C
10
C
B
4
1
A
A
10 C
B
1
C
10
• Why did it take a while to converge?
B
Better loop avoidance
• Split Horizon
– When sending updates to node A, don’t include
routes you learned from A
– Prevents B and C from sending cost 2 to A
• Split Horizon with Poison Reverse
– Rather than not advertising routes learned from A,
explicitly include cost of ∞.
– Faster to break out of loops, but increases
advertisement sizes
Warning
• Split horizon/split horizon with poison reverse
only help between two nodes
– Can still get loop with three nodes involved
– Might need to delay advertising routes after
changes, but affects convergence time
Today’s Lecture
• Forwarding
– IP-Address/IP-Packet Format
– Fragmentation
– Network Error Messages (Debugging): ICMP
– Getting IP-Address: ARP + DHCP
• Routing
– Intra-Domain Routing: RIP
• Next class:
– Intra-Domain Routing: OSPF, OSPF v RIP