ch 4. the network layer myungchul kim [email protected]
TRANSCRIPT
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– Datagram– Routers do not run application- and transport- layer protocols– Forwarding vs routing– Forwarding: router-local action of transfering a packet from an in
put link to the appropriate out link– Routing: network-wide process determining the end-to-end paths
that packets take from source to destination– Thr routing algorithm determines the values that are inserted into
the routers’ forwarding table.– Packet switch
Link-layer switch Router
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Network service model
– Defines the characteristics of end-to-end transport of packets between sending and receiving end systems.
Guaranteed delivery Guaranteed delivery with bounded delay In-order packet delivery Guranteed minimal bandwidth Guaranteed maximum jitter Security service
– Best-effort service: no service at all– ATM service model
Constant bit rate (CBR): as if a dedicated fixed-bandwidth transmission link
Available bit rate (ABR): cells cannot be reordered and a min cell transmission rate is guaranteed
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Virtual circuit and datagram networks
Network layer vs transport layer– Host-to-host services, process-to-process services– Network layer: host-to-host connectionless service (datagram ne
tworks), host-to-host connection service (virtual-circuit networks)– End systems for transport layer vs routers and end systems for n
etwork layers
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Virtual circuit networks– ATM and frame relay– Virtual circuit
A path VC numbers Entries in the forwarding table Page 345
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Forwarding table
12 22 32
1 23
VC number
interfacenumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …
Forwarding table innorthwest router:
Routers maintain connection state information!
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– VC setup -> Data transfer -> VC tear down by ATM’s Q.2931 signaling protocol
– Connection set up at the transport layer?
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Datagram networks– Prefix match at the page 348– Longest prefix matching rule– Forwarding tables can be modifed at any time -> packets go diffe
rent paths and arrive out of order
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Forwarding table
Destination Address Range Link Interface
11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111
otherwise 3
4 billion possible entries
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What’s inside a router
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Input ports– A shadow copy of the forwarding table is typically stored at ech i
nput port and updated by the routing processor– Increase lookup speeds: content addressable memories (CAM)
allows a 32-bit IP address to be presented to the CAM, which returns the content of the forwarding table entry for that address in essentially constant time.
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Switching fabric
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Output ports
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Where does queueing occur– Packet queue can form at both the input ports and the output por
ts– Packet loss – Packet scheduler at the output port must choose one packet am
ong those queued for transmission First-come-first-served Weighted fair queueing For quality-of-service guarantees
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IP: forwarding and addressing in the Internet
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IP datagram fragmentation– Maximum transmission unit: a hard limit on the length of an IP da
tagram– Jolt2 attack: none of fragments has an offset of zero or overlappi
ng IP fragments
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IPv4 addressing– 32 bits long (4 bytes)– Dotted-decimal notation– Globally unique– subnet
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– Classless interdomain routing (CIDR)– a.b.c.d/x network portion of th IP address = prefix– Classfule addressing: C(/24) = 254 hosts, B(/16) = 65,634 hosts,
broadcast = 255.255.255.255
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Obtaining a block of addresses– Internet Corporation for Assigned Name and Numbers (ICANN)
Allocate IP addresses to regional Internet registries Manage the DNS root servers
Obtaining a host address: the Dynamic Host Configuration Protocol (DHCP)
– DHCP server discovery– DHCP server offer(s)– DHCP request– DHCP ACK– A TCP connection maintanence problem for a mobile node
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Network address translation (NAT)– Private addresses have meaning within that network– The NAT router behaves to the outside world as a single device
with a single IP address.
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– Arguments on Network address translation (NAT) Prot numbers for addressing processes not for addressing ho
sts Routers are supposed to process packets only up to layer 3 Violates the end-to-end arguments IPv6
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Internet Control Message Protocol (ICMP)– Error reporting– Ping program– Source quench message– Tracerout
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IPv6– IPv5 (ST-2 similar to RSVP)– Datagram format
Expanded addressing capabilities: unicast, multicast, anycast address
A streamlined 40-byte header Flow labeling and priority
– IPv4 vs IPv6 Fragmentation/reassembly Header checksum Options
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– Transition from IPv4 to IPv6 A flag day Dual-stack approach Tunneling
– The US Office of Management and Budget (OMB): to IPv6 by June 2008
– Europe’s Third Generation Partnership Program (3GPP) 2007.
– Difficult to change network-layer protocols
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IP security– IPsec– Virtual Private Networks (VPN)
Cryptographic agreement on algorithms and keys Encryption of IP datagram payload Data integrity Origin authentication
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Routing algoritms
– Default router: the first-hop router– The least cost path
– Global routing algorithm: link-state (LS) algorithms– Decentralized routing algorithm: distance-vector (DV) algorithms
– Static routing algorithms vs dynamic routing
– Load-sensitive algorithms vs load-insensitive
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Hierarchical routing– Autonomous systems (ASs)– Gateway routers– Within an AS, all routers run the same intra-AS routing protocol.– The ASs run the same inter-AS routing protocol.
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Routing in the Internet
RIP (routing information protocol)– DV protocol– Hop count as a cost metric (max 15)– Routing updates every 30 seconds
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OSPF(open shortest path first)– LS protocol– Link’s state updates every 30 minutes– Advantages:
Security: MD5 Multiple same-cost paths Integrated support for unicast and multicast routing Support for hierarchy within a single routing domain
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BGP (Border Gateway Protocol)– Obtain subnet reachablility information from neighboring ASs– Propagate the reachablility information to all routers interanl to th
e AS– Determine “good” routes to subnets based on the reachability inf
ormation on AS policy.
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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 route from B via X to C– .. so X will not advertise to B a route to C
A
B
C
W X
Y
legend:
customer network:
provider network
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BGP routing policy (2)
A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C?
– No way! 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!
A
B
C
W X
Y
legend:
customer network:
provider network
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Broadcast and Multicast Routing
Broadcast routing algorithms– N-way unicast– Uncontrolled flooding -> broadcast storm
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– Controlled flooding Sequence-number-controlled flooding Reverse path forwarding (RPF)
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– Spanning-tree broadcast
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Multicast– To a subset of network nodes– Class D multicast IP address for multicast group– Internet Group Management Protocol and mulcast routing protoc
ols
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– Multicast routing algorithms Multicast routing using a group-shared tree Multicast routing using a source-based tree with pruning
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– Multicast routing in the Internet Distance Vector Multicast Routing Protocol (DVMRP) Protocol-Independent Multicast (PIM) routing protocol