1. Computer Networking TCP/IP Part 2
Dr Sandra I. Woolley

2. Contents TCP/IP Part 2 Transport Layer Protocols: UDP and TCP
Internet Routing Protocols:
Interior Gateway Protocol (IGP)
Exterior Gateway Protocol (EGP)
Open Shortest Path First (OSPF)
Border Gateway Protocol
DHCP

3. Transport Layer Protocols: UDP and TCP

4. UDP – User Datagram Protocol
Best effort datagram service
Simple transmitter and receiver
Connectionless: no handshaking and no connection state
Low header overhead
No flow control, no error control, no congestion control
UDP datagrams can be lost or out-of-order
Applications
multimedia (e.g. RTP)
network services (e.g. DNS, SNMP)

5. UDP Datagram Source and destination port numbers
0-255
Well-known ports
256-1,023
Less well-known ports
1,024-65,536
Ephemeral client ports
Source and destination port numbers
Client ports are ephemeral
Server ports are well-known
Max number is 65,535
UDP length
Total number of bytes in datagram (including header)
8 bytes ≤ length ≤ 65,535
UDP Checksum
Optionally detects errors in UDP datagram (all 0's if not used)
Source Port Destination Port
UDP Length UDP Checksum
Data

6. TCP – Transmission Control Protocol
Reliable byte-stream service
More complex transmitter and receiver
Connection-oriented: full-duplex unicast connection between client and server processes
Connection setup, connection state, connection release
Higher header overhead
Error control, flow control, and congestion control
Higher delay than UDP
Most applications use TCP
HTTP, SMTP, FTP, POP3, …

7. TCP Segment Format
Source port Destination port
Sequence number
Acknowledgment number
Checksum Urgent pointer
Options Padding U R G A C K P S H T Y N F I
Header
length
Reserved
Window size
Data
Each TCP segment has header of 20 or more bytes + 0 or more bytes of data

8. TCP Header Port Numbers
A socket address identifies a connection endpoint
IP address + port
A connection is specified by a socket address pair.
Well-known ports
FTP 20
DNS 53
HTTP 80
Sequence Number
Byte count
First byte in segment.
Initial sequence number is selected during connection setup.

9. Initial Sequence Number
Select initial sequence numbers (ISN) to avoid overlap with segments from prior connections that may circulate in the network and arrive at a later time.

10. TCP Connection Establishment
“Three-way Handshake”
Initial sequence numbers (ISN) chosen to protect against segments from prior connections
Host A
Host B
SYN, Seq_no = x
SYN, Seq_no = y, ACK, Ack_no = x+1
Seq_no = x+1, ACK, Ack_no = y+1

11. If Host Always Uses the Same ISN
Host B
SYN, Seq_no = n, ACK, Ack_no = n+1
Seq_no = n+1, ACK, Ack_no = n+1
Delayed segment with
Seq_no = n+2
will be accepted

12. Maximum Segment Size Maximum Segment Size (MSS)
largest block of data that TCP sends to other end.
Each end can announce its MSS during connection establishment.
Default is 576 bytes including 20 bytes for IP header and 20 bytes for TCP header.
Ethernet implies MSS of 1,460 bytes.
IEEE implies 1,452

13. TCP Connection Closing
“Graceful Close”
Acknowledge FIN statements
FIN, seq = 5086
Ack = 5087
Data, seq. = 303, Ack=5087
Delivers data bytes
FIN, seq. =453, Ack = 5087
Ack = 454
Host A
Host B
Ack = 453

14. Internet Routing Protocols

15. Routing and Forwarding
How to determine the routing table entries
carried out by routing daemon
Forwarding
Look up routing table and forward packet from input to output port
carried out by IP layer
Routers exchange information using routing protocols to develop the routing tables.

16. Autonomous Systems
Global Internet viewed as collection of autonomous systems.
Autonomous system (AS) is a set of routers or networks administered by a single organization.
Same routing protocol need not be run within the AS.
But, to the outside world, an AS should present a consistent picture of what ASs are reachable through it.
Stub AS: has only a single connection to the outside world.
Multihomed AS: has multiple connections to the outside world, but refuses to carry transit traffic
Transit AS: has multiple connections to the outside world, and can carry transit and local traffic.

17. Inter and Intra Domain Routing
Interior Gateway Protocol (IGP): routing within AS
RIP, OSPF
Exterior Gateway Protocol (EGP): routing between AS’s
BGPv4
Border Gateways perform IGP & EGP routing
IGP R
EGP
IGP R R R R R
AS A
AS C R R
IGP
AS B

18. Open Shortest Path First (OSPF)
RFC 2328 (v2)
Enables each router to learn complete network topology.
Each router monitors the link state to each neighbour and floods the link-state information to other routers.
Each router builds an identical link-state database.
Allows router to build shortest path tree with itself as root.
OSPF typically converges fast when there is a failure in the network.
To improve scalability, AS may be partitioned into areas
Area is identified by 32-bit Area ID
Router in area only knows complete topology inside area and limits the flooding of link-state information to area.
Area border routers summarize info from other areas.

19. OSPF Protocol
OSPF packets transmitted directly on IP datagrams; Protocol ID 89
Five OSPF packet types:
Hello
Database description
Link state request; Link state update; Link state ack
OSPF Stages
Discover neighbours by sending Hello packets (every 10 sec) and designated router elected in multi-access networks.
Adjacencies are established and link state databases are synchronized.
Link state information is propagated (and updated every 30 mins) & routing tables are calculated.

20. Exterior Gateway Protocols
The Internet is woven into a coherent whole by Exterior Gateway Protocols (EGPs) that operate between AS’s.
EGP enables two AS’s to exchange routing information about:
The networks that are contained within each AS.
The AS’s that can be reached through each AS.
EGP path selection is guided by policy rather than path optimality
Trust, peering arrangements, etc.

21. EGP Example AS1 AS2 AS3 R1 R2 R3 R4 N1
EGP routers within an AS, e.g. R3 and R2, are kept consistent.
Suppose AS2 is willing to handle transit packets from AS1 to N1.
R2 advertises to AS1 the reachability of N1 through AS2.
R1 applies its policy to decide whether to send to N1 via AS2.
N1 reachable through AS2

22. Border Gateway Protocol v4
BGP (RFC 1771) is an EGP “routing” protocol to exchange network reachability information among BGP routers (also called BGP speakers.)
Network reachability information contains sequence of ASs that packets traverse to reach a destination network.
Information exchanged between BGP speakers allows a router to construct a graph of AS connectivity
Routing loops can be pruned
Routing policy at AS level can be applied
AS1
AS2
AS3
AS4
AS5
AS6
AS7

23. BGP Speaker & AS Relationship
BGP speaker: a router running BGP
Peers or neighbours: two speakers exchanging information on a connection.
BGP peers use TCP (port 179) to exchange messages.
Initially, BGP peers exchange entire BGP routing table
Incremental updates sent subsequently
Reduces bandwidth usage and processing overhead
Keepalive messages sent periodically (30 seconds)
Internal BGP (iBPG) between BGP routers in same AS.
External BGP (eBGP) connections across AS borders.

24. iBGP & eBGP R
iBGP
eBGP
eBGP to exchange reachability information in different AS’s
eBGP peers directly connected
iBGP to ensure net reachability information is consistent among the BGP speakers in the same AS

25. Path Selection Each BGP speaker
Evaluates paths to a destination from an AS border router
Selects the best path that complies with policies
Advertises that route to all BGP neighbours
BGP assigns a preference order to each path and selects path with highest value; BGP does not keep a cost metric to any path.
When multiple paths to a destination exist, BGP maintains all of the paths, but only advertises the one with highest preference value.

26. BGP Policy Examples of policy: Never use AS X.
Never use AS X to get to a destination in AS Y.
Never use AS X and AS Y in the same path.
Import policies to accept, deny, or set preferences on route advertisements from neighbours.
Export policies to determine which routes should be advertised to which neighbours.
A route is advertised only if AS is willing to carry traffic on that route.

27. Additional Notes

28. Additional Notes
Dynamic Host Configuration Protocol – protocol for assigning IP addresses.
Queueing (bookwork basics – what is Little’s formula?)
Quality-of-Service.

29. Thank You