Chapter 5 Gateway and Routing Protocols

• gateway device that performs routing functions, usually as a standalone device, that also can perform protocol translation from one network to another.
• bridge network device that connects two or more networks that use the same protocol [DataLink layer]
• router network node that forwards datagrams around the network [Network layer]
• repeater device that extends the distance of the network [physical layer]
• brouter - performs functions of both router and bridge
• packet-switching - all transfers are based on a self-contained packet of data; others are message switching and line switching

• core gateways (stub gateways) - administered by Internet Network Operations Center (INOC) [requires the implementation of the Gateway-to-Gateway Protocol (GGP)]
• non-core gateways (nonrouting)
• autonomous systems - the structure of the network it is attached to is not visible to the rest of the internetwork (gateway-separated)
• interior neighbors - two gateways connected to the same LAN [requires Interior Gateway Protocols (IGPs) - Routing Information Protocol (RIP) or HELLO Protocol]
• messages between two exterior gateways uses Exterior Gateway Protocol (EGP)
• RIP, HELLO and EGP all rely on 30 sec. updates of the routing tables; GGP has a routing table of all other core gateways

	#	@gated.egp
	#	sample EGP configuration file

		traceoption general kernal icmp egp protocol;
		autonomoussystem 519;
		rip no;
		egp yes {
			group ASin 519 {
				neighbor 128.212.64.1;
			};
		};
		static {
			default gateway 128.212.64.1 pref 100;
		};
		propogate proto egp as 519 {
			proto rip gateway 128.212.64.1 {
				announce 128.212 metric 2;
			};
			proto direct {
				announce 128.212 metric 2;
			};
		};
		propogate proto rip {
			proto default {
				announce 0.0.0.0 metric 1;
			};
			proto rip {
				noannounce all;
			};
		};
	

1. fixed table - mao of network which must be modified every time there is a physical change on the network
2. dynamic table - evaluates traffic load and messages from other nodes to refine the table (most frequently used on the Internet)
3. fixed central routing table - loaded from a central repository

• Fewest-Hops Routing
  A table of distances (hops) is kept
• Type of Service (TOS) or QOS Routing - uses speed and reliability of connections; requires dynamic updating of tables (TTL is important for dynamic tables)
• Updating Gateway Routing Informatin

• GGP is a vector-distance protocol (needs address and # of hops)
• sends an echo message to another gateway [must receive 3 out of 4 replies to maintain connection]
• each message between gateways has a sequence # to monitor errors
  

  GGP message format

  Type(8 bits)	Unused (8 bits)
  Sequence # (16 bits)
  Update	# Distances
  Distance 1	# Networks at 1
  First Network at Distance 1(24 bits)
  Second Network at Distance 1(24 bits)
  etc...
  Last Network at Distance 1(24 bits)
  Distance 2	# Networks at 2
  First Network at Distance 2(24 bits)
  Second Network at Distance 2(24 bits)
  etc...
  Last Network at Distance 2(24 bits)
  etc...

• Neighbors and EGP Confirms the routing

  EGP Commands

  Command Name	Command Description	Response Name	Response Description
  Request	Requests that a neighbor become a gateway	Confirm/Refuse	Agre or refuse a request
  Cease	Request the termination of a neighbor	Cease-Ack	Agree to terminiation
  Hello	Request confirmation of routing to a neighbor(neighbor reachability)	IHU
  Poll	Requests that the neighbor provide network information(network reachability)	Update	Provides network information

• EGP Messages
  1. Neighbor Acquisition Messages

     EGP Neighbor Acquisition format

     Vers	Type	Code	Status
     Checksum		System #
     Sequence #		Hello Interval
     Poll Inteval

  2. Neighbor Reachability Messages
  3. Poll Messages
  4. Update Messages
  5. Error Messages
• EGP to GGP Messages (autonomous (non-core) gateways send reachability information to other gateways via a core gateway
• EGP State Variables and Timers

  EGP States

  State	Description
  0	Idle - gateway not involved in any activity
  1	Acquisition - gateway can transmit/receive messages but not act as a full messaging gateway
  2	Down - gateway is not operational
  3	Up - gateway is processing and responding to EGP messages
  4	Cease - gateway ceases all updating operations, but still sends/receives CEASE messages

  EGP commands and their responses

  Command	Response
  Request	Confirm
  Refuse	none
  Error	none
  Cease	Cease ACK
  Hello	IHU
  Poll	Update

  EGP parameters and timers

  Name	Description
  M	Hello polling mode
  P1	minimum interval acceptable between successive received Hello commands (default - 30 sec)
  R	Receive sequence #
  T1	Interval between Hello command retransmissions

• The Routing Protocol (RIP) - gateways broadcast routing tables to other gateways on the network at regular intervals; [uses a vector-distance system] {uses UDP and port # 520}

  RIP message format

  Command Value

  Version #

  Reserved

  Family protocol

  Network Address

  Network Address

  Network Address

  Metric(distance in hops)

• The HELLO Protocol (often used with TCP/IP)

  HELLO message format

  Checksum

  Date

  Time

  Timestamp (used to calculate round-trip delays)

  Offset (pointer)

  # of hosts

  Delay for Host 1

  Offset for Host 1 (difference between sending and receiving clock)

• The Open Shortest Path First (OSPF) Protocol (developed by IETF) - determines "optimum path" [uses the destination address and TOS in an IP datagram header to determine a route]
  determines shortest route using topology & cost metrics (route speed, traffic, reliability, security, etc.)
  • Type 1 route - uses same calculations for external routes as internal
  • Type 2 route - uses OSPF algorithm only to calculate route to the gateway of the destination system
  • internal router - all connections belong to the same area
  • border router - has connections outside an area
  • backbone router - has an interface to the backbone
  • boundary router - is a gateway that has a connection to another autonomous system

  gateways send messages to each other through advertisements send through HELLO update messages
  

  • Router Link advertisement - provides info on a local router's(gateway) connection in an area
  • Network Links advertisement - provides a list of routers that are connected to a network
  • Summary Links advertisement - contains info about routes outside the area (sent by border routers)
  • Autonomous System Extended Links advertisement - contains info on routes in external autonomous systems (used by boundary routers)

  OSPF maintains several tables for determining routes:

  1. protocol data table
  2. area data table / backbone data table
  3. interface data table
  4. routing data table
  1. OSPF Packets (IP is used with multicast addresses (224.0.0.5) and (224.0.0.6)

     OSPF message header format

     Version(8 bits)

     Type(8 bits)

     Packet Length(16 bits)

     Router ID(32 bits)

     Area ID(32 bits)

     Checksum(16 bits)

     Authenitication Type(16 bits)

     Authentication(64 bits)

  2. HELLO Packets

     OSPF HELLO packet format

     OSPF Header (192 bits)

     Network Mask(32 bits)

     Hello Interval(16 bits)

     Options(8 bits)

     Router Priority(8 bits)

     Dead Interval(32 bits)

     Designated Router(32 bits)

     Backup Router(32 bits)

     Neighbor 1(32 bits)

     etc.

  3. Link State Request and Update Packets