See this page as a slide show

CT320: Routing

Thanks to:

• Dr. Indrajit Ray, CSU
• Dr. James Walden, NKU
• Russ Wakefield, CSU

for the contents of these slides.

The Internet Network layer

Remember the layers?

• Application Layer
• Transport Layer
• Network Layer
• Data Link Layer

The Internet Network layer

Host, router network layer functions:

• IP protocol
  • addressing conventions
  • datagram format
  • packet handling conventions
• Routing protocols
  • Path selection
  • RIP, OSPF, BGP
• ICMP protocol
  • error reporting
  • router “signaling”

IP Fragmentation & Reassembly

• Like a big text message
• Network links have MTU (max.transfer size) largest possible link-level frame.
  • different link types, different MTUs
• Large IP datagram divided (“fragmented”) within net
  • one datagram becomes several datagrams
  • “reassembled” only at final destination
  • IP header bits used to identify, order related fragments
• Fragmentation:
  • in: one large datagram
  • out: several smaller datagrams

IP Fragmentation and Reassembly Example

• 4000 byte datagram
• MTU = 1500 bytes (1480 bytes of data)

Original:

length=4000

ID

fragflag=0

offset=0

…

Fragments:

length=1500	ID	fragflag=1	offset=0	…
length=1500	ID	fragflag=1	offset=185	…
length=1040	ID	fragflag=1	offset=370	…

(185 = 1480 / 8)

Hierarchical Addressing: Route Aggregation

How it’s not done:

┌─────────┐       ┌────────┐    ┌────────┐       ┌─────────┐
│ 1.2.3.4 │·······│ ISP #1 │    │ ISP #2 │·······│ 3.1.4.1 │
└─────────┘   :   └────────┘    └────────┘   :   └─────────┘
┌─────────┐   :      :                :      :   ┌─────────┐
│ 2.4.6.8 │···:      :  ┌──────────┐  :      :···│ 2.7.1.8 │
└─────────┘   :      ···│ Internet │···      :   └─────────┘
┌─────────┐   :         └──────────┘         :   ┌─────────┐
│ 9.8.7.6 │····               :              ····│ 2.2.3.6 │
└─────────┘                   :                  └─────────┘

ISP #1: “Internet, forward 1.2.3.4, 2.4.6.8, and 9.8.7.6 to me.”
ISP #2: “Internet, forward 3.1.4.1, 2.7.1.8, and 2.2.3.6 to me.”

Imagine if the ISPs had thousands of customers!

Hierarchical Addressing: Route Aggregation

How it’s done:

┌─────────┐       ┌────────┐    ┌────────┐       ┌─────────┐
│ 1.2.3.4 │·······│ ISP #1 │    │ ISP #2 │·······│ 3.1.4.1 │
└─────────┘   :   └────────┘    └────────┘   :   └─────────┘
┌─────────┐   :      :                :      :   ┌─────────┐
│ 1.2.3.8 │···:      :  ┌──────────┐  :      :···│ 3.1.4.8 │
└─────────┘   :      ···│ Internet │···      :   └─────────┘
┌─────────┐   :         └──────────┘         :   ┌─────────┐
│ 1.2.3.6 │····               :              ····│ 3.1.4.6 │
└─────────┘                   :                  └─────────┘

ISP #1: “Internet, forward 1.2.3/24 to me.”
ISP #2: “Internet, forward 3.1.4/24 to me.”

This scales nicely.

Hierarchical Addressing: More Specific Routes

┌─────────┐     ┌────────┐  ┌────────┐···································
│ 1.2.3.4 │·····│ ISP #1 │  │ ISP #2 │     ┌─────────┐     ┌─────────┐  :
└─────────┘  :  └────────┘  └────────┘·····│ 3.1.4.1 │  ···│ School  │···
┌─────────┐  :         :       :        :  └─────────┘  :  └─────────┘
│ 1.2.3.8 │··:        ┌──────────┐      :  ┌─────────┐  :  ┌─────────┐
└─────────┘  :        │ Internet │      :··│ 3.1.4.8 │  :··│ 9.8.7.6 │
┌─────────┐  :        └──────────┘      :  └─────────┘  :  └─────────┘
│ 1.2.3.6 │···              :           :  ┌─────────┐  :  ┌─────────┐
└─────────┘                 :           ···│ 3.1.4.6 │  ···│ 9.8.1.2 │
                                           └─────────┘     └─────────┘

ISP #1: “Internet, forward 1.2.3/24 to me.”
ISP #2: “Internet, forward 3.1.4/24 and 9.8/16 to me.”

Hierarchical Addressing

• Without hierarchical addressing, top-level routers would have to know the location of every host in the world.
• With hierarchical addressing, top-level routers only have to know where big hunks of hosts are.
• It’s a lot easier to treat all of CSU as one entity, as opposed to having sixty-five thousand entries for all the possible CSU hosts.
• Now, imagine how many hosts Comcast has. Or HP. Or the DoD.

IP Addressing: ICANN

• How does an ISP get block of addresses?
• ICANN: Internet Corporation for Assigned Names and Numbers
  • allocates addresses
  • manages DNS
  • assigns domain names, resolves disputes

Addressing at CSU

• All of CSU is 129.82.0.0/16
• That’s more than sixty-five thousand possible hosts.
• Does the CSU main router have to have sixty-five thousand entries?
• This is the same problem as we have at the top level of the internet.
• So, use the same solution!

Hierarchy at CSU

• Hierarchy saves the day again!
• Groups within CSU get blocks within CSU’s 129.82.0.0/16:
  • 129.82.28/22 Morgan Library
  • 129.82.40/22 College of Business
  • 129.82.44/22 Computer Science
  • 129.82.48/22 Atmospheric Science
  • 129.82.76/22 Chemistry
  • 129.82.220/22 Lory Student Center
  • 129.82.224/21 Engineering (more than CS!?)
    • wait—how is /21 more than /22?
  • and many more
• ICANN neither knows nor cares about this.

Hierarchy at CSU

┌───────────────┐     ┌──────────┐  ┌────────┐     ┌───────────────┐
│ 129.82.40.123 │·····│ Business │  │   CS   │·····│ 129.82.44.12  │
└───────────────┘  :  │ router   │  │ router │  :  └───────────────┘
┌───────────────┐  :  └──────────┘  └────────┘  :  ┌───────────────┐
│ 129.82.43.4   │··:    :                  :    :··│ 129.82.45.85  │
└───────────────┘  :    :  ┌────────────┐  :    :  └───────────────┘
┌───────────────┐  :    ···│ CSU router │···    :  ┌───────────────┐
│ 129.82.42.66  │···       └────────────┘       ···│ 129.82.47.234 │
└───────────────┘                :                 └───────────────┘

• The CSU router doesn’t need 65000-some entries, just one per subnet: one for Business, one for CS, one for Engineering, etc.
• When the Business School adds another computer, the CSU router doesn’t have to be updated, since the new computer is within 129.82.76/22.
• It’s like if your aunt & uncle have another kid—send the birthday cards to the same address as the previous kids.

IP datagram format, redux

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Version				IHL				Type of service								Total length
Identification																Flags				Fragment offset
Time to live								Protocol								Header checksum
Source IP address
Destination IP address
Options (optional)
IP data payload (many bytes)

ICMP: Internet Control Message Protocol

• used by hosts & routers to communicate network-level information
  • error reporting: unreachable host, network, port, protocol
  • echo request/reply (used by ping)
• network-layer “above” IP:
  • ICMP msgs carried in IP datagrams
• ICMP message: type, code plus first 8 bytes of IP datagram causing error

Ping

• Create an IP packet with type=8, code=0.
• Send it to the destination
• Destination will create an IP packet with type=0, code=0, and send it back.
  • If it feels like it.

% ping google.com
PING google.com (216.58.217.14) 56(84) bytes of data.
64 bytes from den03s09-in-f14.1e100.net (216.58.217.14): icmp_seq=1 ttl=58 time=2.89 ms
64 bytes from den03s09-in-f14.1e100.net (216.58.217.14): icmp_seq=2 ttl=58 time=2.90 ms
^C
--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 2.895/2.897/2.900/0.053 ms
% ping hp.com
PING hp.com (15.216.241.18) 56(84) bytes of data.

Traceroute and ICMP

• Source sends series of UDP packets to dest
  • First has TTL=1
  • Second has TTL=2, etc.
  • Unlikely port number
• When nth datagram eventually arrives to nth router:
  • Router discards datagram
  • And sends to source an ICMP message packet (type 11, code 0)
  • Message includes name of source router & IP address
• When a packet actually gets to the destination:
  • UDP segment arrives at destination host
  • Destination returns ICMP “port unreachable” (type 3, code 3)
  • When source gets this ICMP, stops.