Lecture 5 TCP/IP Network Layer (3)

Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking

Lecture 5

TCP/IP Network Layer (3)


Outline (Network Layer)

• Principles behind network layer services:– Virtual circuit and datagram

networks– Routing algorithms

• Link State• Distance Vector• Hierarchical• Routing Tables

• What’s inside a router• Internet Protocol

– Datagram format– IPv4 addressing– CIDR– IPv6

• Routing in the Internet– RIP– OSPF– BGP

• Broadcast and multicast routing• Miscellaneous protocols and

programs


Routing Protocols in the Internet• Internet consists of inter connected Autonomous Systems (ASs).

• ASs typically consists of many networks.

• Datagrams are routed within each AS using Intra-AS routing protocols, also known as Interior Gateway Protocols (IGP).

• Datagrams among ASs are routed using inter-AS routing protocoal, also known as Exterior Gateway Protocols (EGP).

• Most common Intra-AS routing protocols:

– RIP: Routing Information Protocol– OSPF: Open Shortest Path First

• Most common Inter-AS routing protocols:– BGP: Border Gateway Protocol


RIP ( Routing Information Protocol)• Distance vector algorithm• Included in BSD-UNIX Distribution in 1982• Distance metric: # of hops, each hop has cost of 1, (max

= 15 hops)• Distance vectors: exchanged among neighbors every 30

sec via Response Message (also called advertisement)• Each advertisement: list of up to 25 destination nets within

AS

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2


RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1…. …. ....

w x y

z

A

C

D B

Routing table in D


RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....Routing table in D

w x y

z

A

C

D B

Dest Next hops w - - x - - z C 4 …. … ...

Advertisementfrom A to D


RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declared dead– routes via neighbor invalidated– new advertisements sent to neighbors– neighbors in turn send out new advertisements

(if tables changed)– link failure info quickly propagates to entire net– poison reverse used to prevent count to infinity

problem (infinite distance = 16 hops)


RIP Table processing• RIP routing tables

managed by application-level process. (e.g. in UNIX route-d (daemon))

• advertisements sent in UDP packets, periodically repeated

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable


RIP Message Format


RIP Message Format• Command—Indicates whether the packet is a request or a

response. The request asks that a router send all or part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables.

• Version Number—Specifies the RIP version used. This field can signal different potentially incompatible versions.

• Zero—Not used.• Address-Family Identifier (AFI)—Specifies the address family used.

RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address being specified. The AFI for IP is 2.

• Address—Specifies the IP address for the entry.• Metric—Indicates how many internetwork hops (routers) have been

traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.

• Up to 25 occurrences of the AFI, address, and metric fields are permitted in a single IP RIP packet.


RIP Table example (netstat command)C:\Documents and Settings\Administrator>netstat -rRoute Table===================================================================

========Interface List0x1 ........................... MS TCP Loopback interface0x2 ...00 12 79 db e6 94 ...... Broadcom NetXtreme Gigabit Ethernet - Packet Scheduler

Miniport==============================================================================================================================Active Routes:Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 172.16.20.1 172.16.10.29 30 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 169.254.0.0 255.255.0.0 172.16.10.29 172.16.10.29 30 172.16.0.0 255.255.0.0 172.16.10.29 172.16.10.29 30 224.0.0.0 240.0.0.0 172.16.10.29 172.16.10.29 30Default Gateway: 172.16.20.1===============================================================Persistent Routes: None


RIP Message Format (v2)


RIP Message Format (v2)• Command—Indicates whether the packet is a request or a response. The request

asks that a router send all or a part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing-table entries. Multiple RIP packets are used to convey information from large routing tables.

• Version—Specifies the RIP version used. In a RIP packet implementing any of the RIP 2 fields or using authentication, this value is set to 2.

• Routing Domain: An identifier of the routing daemon, to which this packet belongs.• Address-Family Identifier (AFI)—Specifies the address family used. RIP is designed

to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address specified. The address-family identifier for IP is 2. If the AFI for the first entry in the message is 0xFFFF, the remainder of the entry contains authentication information. Currently, the only authentication type is simple password.

• Route Tag—Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).

• IP Address—Specifies the IP address for the entry.• Subnet Mask—Contains the subnet mask for the entry. If this field is zero, no subnet

mask has been specified for the entry.• Next Hop—Indicates the IP address of the next hop to which packets for the entry

should be forwarded.• Metric—Indicates how many internetwork hops (routers) have been traversed in the

trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.










programs


OSPF (Open Shortest Path First)

• “open”: publicly available• Uses Link State algorithm

– LS packet dissemination– Topology map at each node– Route computation using Dijkstra’s algorithm

• OSPF advertisement carries one entry per neighbor router

• Advertisements disseminated to entire AS (via flooding)– Carried in OSPF messages directly over IP (rather

than TCP or UDP


OSPF “advanced” features (not in RIP)

• Security: all OSPF messages authenticated (to prevent malicious intrusion)

• Multiple same-cost paths allowed (only one path in RIP)

• For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)

• Integrated uni- and multicast support: – Multicast OSPF (MOSPF) uses same

topology data base as OSPF• Hierarchical OSPF in large domains.


Hierarchical OSPF


Hierarchical OSPF• Two-level hierarchy: local area, backbone.

– Link-state advertisements only in area – each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.

• Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

• Backbone routers: run OSPF routing limited to backbone.

• Boundary routers: connect to other AS’s.










programs


Internet inter-AS routing: BGP• BGP (Border Gateway Protocol): the de facto standard• Path Vector protocol:

– similar to Distance Vector protocol– each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to

destination– E.g., Gateway X may send its path to destination. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

• BGP routes to networks (ASs), not individual hosts


BGP

• Suppose: gateway X sends its path, Path (X,Z) = X,Y1,Y2,Y3,Z, to peer gateway W

• W may or may not select path offered by X– cost, policy (don’t route via competitors AS), loop

prevention reasons.

• If W selects path advertised by X, then:– Path (W,Z) = W, Path (X,Z)= W,X,Y1,Y2,Y3,Z

• Note: X can control incoming traffic by controlling it route advertisements to peers:– e.g., don’t want to route traffic to Z -> don’t advertise

any routes to Z


BGP Routing Policy

• A,B,C are provider networks• X,W,Y are customer (of provider networks)• X is dual-homed: attached to two networks

– X does not want to have a route from B to C via X– .. so X will not advertise to B a route to C


BGP Routing Policy

• A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW?

– No. B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

– B wants to force C to route to w via A– B wants to route only to/from its customers!


BGP• Routers’ responsibility

– Receiving and filtering route advertisements from directly attached neighbor(s).

– Route selection.• To route to destination X, which path (of several advertised)

will be taken?– Sending route advertisements to neighbors.

• BGP Messages:– BGP messages exchanged using TCP. – OPEN: opens TCP connection to peer and

authenticates sender– UPDATE: advertises new path (or withdraws old)– KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request– NOTIFICATION: reports errors in previous message;

also used to close connection


Why different Intra- and Inter-AS routing ?

Policy: • Inter-AS: admin wants control over how its traffic

routed, who routes through its net. • Intra-AS: single admin, so no policy decisions

needed

Scale:• hierarchical routing saves table size, reduced

update trafficPerformance: • Intra-AS: can focus on performance• Inter-AS: policy may dominate over performance


Reading Material• Chapter 4 – text3 (Kurose)

• Chapter 5 – text2 (Tanenbaum)

• Chapter 10 – text1 (Stevens)

Lecture 5 TCP/IP Network Layer (3)

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Transcript of Lecture 5 TCP/IP Network Layer (3)