lecture 10 overview
DESCRIPTION
Lecture 10 Overview. Border Gateway Protocol(BGP). De facto standard for Internet inter-AS routing allows subnet to advertise its existence to rest of Internet: “ I am here ” BGP provides each AS a means to: Obtain subnet reachability information from neighboring ASs - PowerPoint PPT PresentationTRANSCRIPT
Lecture 10 Overview
Border Gateway Protocol(BGP)• De facto standard for Internet inter-AS routing• allows subnet to advertise its existence to rest
of Internet: “I am here”• BGP provides each AS a means to:– Obtain subnet reachability information from
neighboring ASs– Propagate reachability information to all AS-internal
routers– Determine “good” routes to subnets based on
reachability information and policyCPE 401/601 Lecture 10 : Border Gateway Protocol 2
BGP basics• Pairs of routers (BGP peers) exchange routing info
over semi-permanent TCP connections: BGP sessions– BGP sessions need not correspond to physical links
• when AS2 advertises a prefix to AS1:– AS2 promises it will forward datagrams towards that prefix– AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c2a
AS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
CPE 401/601 Lecture 10 : Border Gateway Protocol 3
Distributing reachability info
3b
1d
3a
1c2a
AS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
• using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1– 1c can then use iBGP do distribute new prefix info to all
routers in AS1– 1b can then re-advertise new reachability info to AS2 over
1b-to-2a eBGP session
CPE 401/601 Lecture 10 : Border Gateway Protocol 4
Path attributes & BGP routes• advertised prefix includes BGP attributes – prefix + attributes = “route”
• two important attributes– AS-PATH: contains ASs through which prefix
advertisement has passed: e.g, AS 67, AS 17 – NEXT-HOP: indicates specific internal-AS router to
next-hop AS• may be multiple links from current AS to next-hop-AS
• when gateway router receives advertisement, uses import policy to accept/decline
CPE 401/601 Lecture 10 : Border Gateway Protocol 5
BGP route selection
• router may learn about more than 1 route to some prefix– Router must select route
• elimination rules– local preference value attribute: policy decision– shortest AS-PATH – closest NEXT-HOP router: hot potato routing– additional criteria
CPE 401/601 Lecture 10 : Border Gateway Protocol 6
BGP messages
• BGP messages exchanged using TCP
• BGP messages:– 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 msg;
also used to close connectionCPE 401/601 Lecture 10 : Border Gateway Protocol 7
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
CPE 401/601 Lecture 10 : Border Gateway Protocol 8
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
CPE 401/601 Lecture 10 : Border Gateway Protocol 9
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, no need for policy decisions
• Scale:– hierarchical routing saves table size, reduced update
traffic
• Performance: – Intra-AS: can focus on performance– Inter-AS: policy may dominate over performance
CPE 401/601 Lecture 10 : Border Gateway Protocol 10
Lecture 11
Router Architectures
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollingerslides are modified from Dave Hollinger
Router Architecture Overview• Two key router functions: – run routing algorithms/protocol (RIP, OSPF, BGP)– forwarding datagrams from incoming to outgoing link
CPE 401/601 Lecture 11 : Router Architectures 12
Input Port Functions
• Goal: complete input port processing at ‘line speed’
• Decentralized switching: – given datagram dest., lookup output port using
forwarding table in input port memory
CPE 401/601 Lecture 11 : Router Architectures 13
Input Port Functions
Physical layer:bit-level reception
Data link layer:e.g., Ethernet
Network layer:Routing & forwarding
CPE 401/601 Lecture 11 : Router Architectures 14
Input Port Queuing• Queuing when datagrams arrive faster than
forwarding rate into switch fabric• queuing delay and loss due to input buffer overflow!• Head-of-the-Line (HOL) blocking: queued datagram
at front of queue prevents others in queue from moving forward
CPE 401/601 Lecture 11 : Router Architectures 15
Three types of switching fabrics
CPE 401/601 Lecture 11 : Router Architectures 16
Output Ports
• Buffering required when datagrams arrive from fabric faster than the transmission rate
• Scheduling discipline chooses among queued datagrams for transmission
CPE 401/601 Lecture 11 : Router Architectures 17
Output port queueing
• Queuing (delay) and loss due to output port buffer overflow!
CPE 401/601 Lecture 11 : Router Architectures 18
How much buffering?
• RFC 3439 rule of thumb: – buffering equal to typical RTT (say 250 msec)
times link capacity C• e.g., C = 10 Gps link:
– 2.5 Gbit buffer
• Recent recommendation: – with N flows, buffering equal to
RTT C.N
CPE 401/601 Lecture 11 : Router Architectures 19
Bridges
Bridge Software
• A bridge connects networks and forwards frames from one network to another.
21
BRIDGE
A B
C D
E F
G H
PORTSPORTS
CPE 401/601 Lecture 11 : Bridges
Selective Forwarding
• If A sends a frame to E– the frame must be forwarded by the bridge
• If A sends a frame to B– there is no reason to forward the frame
22
BRIDGE
A B
C D
E F
G H
CPE 401/601 Lecture 11 : Bridges
Bridge Database
• The bridge needs a database that contains information about which hosts are on which network.
• The realistic options are:– The system administrator can create and maintain
the database. – The bridge can acquire the database on the fly.
23
Hard to addnew computers
Some loss of efficiency
CPE 401/601 Lecture 11 : Bridges
Learning the host mapping
• The bridge forwards packets for which it does not know which network the destination is on
• Every time the bridge forwards a packet it can record the network on which the sender is located
• Each host mapping expires when it is unused for a “long” period of time.
24CPE 401/601 Lecture 11 : Bridges
“Learning” Bridge
• A host can be moved to another network.
• New hosts can be added at any time.
• Requires no setup information from humans.
25CPE 401/601 Lecture 11 : Bridges
What is the problem ?
26
Bridge 1 10
Bridge 2 10
A
C
B
D
F
E
CPE 401/601 Lecture 11 : Bridges
Possible Solutions
• Forget about smart bridges
• Tell customers to avoid topologies that include loops– design bridges so that they detect loops and
scream at the customer
• Design bridges that prune the network topology into something with no loops.
27CPE 401/601 Lecture 11 : Bridges
Spanning Tree Creation
• The bridges must communicate!– They send configuration bridge protocol data units
(BPDUs)– Multicast: special data link address
• Each bridge has a unique ID
• Use an algorithm to construct a spanning tree based on local messages
28CPE 401/601 Lecture 11 : Bridges
Spanning Tree
29
B
BB
B
B
B
B
BB
B
B
B
Prune
B bridge networkCPE 401/601 Lecture 11 : Bridges