week14 lec2
DESCRIPTION
Computer NetworksTRANSCRIPT
Chapter 4Network Layer
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July
2007.
Path Vector RoutingLoop Prevention
If a router receives a message, checks to see if its autonomous system is in the path list to the destination.
If it is, looping is involved and the message is ignored.
Policy RoutingIf one of the path in the path vector
is against the policy it can ignore that path and does not update its routing table.
Inter-AS Routing in Internet: BGP BGP (Border Gateway Protocol)
BGP provides each AS a means to:1. Obtain subnet reachability information from
neighboring ASs.2. Propagate reachability information to all AS-
internal routers.3. Determine “good” routes to subnets based
on reachability information and policy. Allows subnet to advertise its existence
to rest of Internet: “I am here”
BGP Basics Pairs of routers (BGP peers) exchange routing info over
TCP connections using port 179. For each TCP connection, the two routers at the end of
the connection are called BGP Peers The TCP connection along with all the BGP messages
sent over the connection is called a BGP Session Internal (IBGP) neighbours
A pair of BGP speakers within the same AS External (EBGP) neighbours
Two BGP speakers from two different AS
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
BGP Basics BGP allows each AS to learn which
destinations are reachable via its neighboring ASs.
Destinations are not hosts but instead are CIDRized prefixes. E.g. AS1 has four subnets attached to it
It will aggregate the prefixes of these four and will advertise the single prefix
AS1 and AS2 send reachability information trough their gateway routers 1b and 2a.
When a gateway router receives e-BGP learned prefixes, the gateway router uses iBGP sessions to distribute the prefixes to other routers in the AS.
Distributing Reachability info 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 When router learns of new prefix, it creates
entry for prefix in its forwarding table.
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
Path Attributes & BGP Routes
In BGP an AS is recognized by its globally unique Autonomous System Number (ASN).Assigned by ICANN Regional
Registries.Advertised prefix includes BGP
attributes. prefix + attributes = “route”
Two important attributes:AS-PATH and NEXT-HOP
Path Attributes & BGP Routes
AS-PATHContains ASs through which prefix
advertisement has passed.When a prefix is passed into an AS, the
As adds its ASN to the AS-PATH attribute.E.g suppose that the prefix
138.16.64.0/24 is first advertised from AS2 to AS1.
If AS1 then advertises the prefix to AS3, AS-PATH would be AS2 AS1.
Detect and prevent looping advertisements
Routing Loop Prevention
AS1
AS2
AS3
AS4
Net1
[Net1, (AS4)]
[ Net1, (AS3,AS4) ]
[ Net1, (AS1,AS2,AS3,AS4)]
[ Net1, (AS2,AS3,AS4)]
AS3 won’t forward this further
NEXT-HOP The route advertised from 3a to 1c contains advertised
prefix say x and NEXT-HOP attribute.NEXT-HOP is the IP address of the router 3a interface that
leads to 1c. Router 1d learns about this route from iBGP. Remember that intra AS protocol finds the least cost
path to all the subnet attached to the routers in AS1 Router 1d makes entry into the table (l,x) where l is the
interface that begins the least cost path to the gateway router 1c.
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
AS 6431
135.207.0.0/16Next Hop = 12.125.133.90
AS 7018
AS 12654
12.125.133.90
135.207.0.0/16Next Hop = 12.127.0.121
12.127.0.121
NEXT-HOP
BGP Route Selection Router may learn about more than one route
to the some prefix. Router must select one route.
BGP sequentially invokes the following elimination rules until one route remains
1. The route with highest local preference value. This is a policy decision.
2. Shortest AS-PATH. 3. Hot Potato Routing.
Additional criteria (SEE RFC 4271)
BGP speakers receive a lot of path-vector advertisements from its EBGP neighbours
Advertisement processing involves 3 stepsImport policiesPath selection
Which route should I choose? Export policies
Which route should I export to my EBGP neighbors?
BGP Route Selection
CompanyAS2
ISP1AS3
ISP2 AS4 Net1
[Net1, (AS2)]
[ Net1, (AS1,AS4,AS2)]
If AS1 forwards this to AS3, then ISP1 may use AS1 to reach Net1 if it chooses.However, AS1 may not want that.
Company AS1
[Net1, (AS4,AS2)]
Net2
[Net1, (AS4,AS2)]
Policy Routing
CompanyAS2
ISP1AS3
ISP2 AS4
Net1[Net2, (AS4,AS1)]
[ Net2, (AS1)]
If AS1 doesn’t want to become a transitnetwork, it does not export routes learntfrom EBGP. It exports only its own routes.
Company AS1
[Net1, (AS4,AS2)]
Net2[Net2, (AS3,AS1)]
AS1 won’t export this
Policy Routing
Why different Intra- and Inter-AS routing ?
Policy: Inter-AS: admin wants control over how its traffic
routed, who routes through its network. Intra-AS: single admin, so no policy decisions
needed
Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance
Chapter 5 Link Layer
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July
2007.
Link Layer: Introduction Layer-2 packet is a frame,
encapsulates datagram Link layer has responsibility
of transferring datagram from one node to adjacent node over a link
Datagram may be carried by different link-layer protocols on different links in the path. Services provided by link layer
protocols may be different May or may not provide
reliable data transfer over link
Link layer and Network Layer
Transportation Analogy
Travel Agent planning a trip for a tourist Trip from Princeton to Lausanne
Car: Princeton to JFK Plane: JFK to Geneva Train: Geneva to Lausanne
Tourist = Datagram Transportation Mode = Link layer protocol Travel Agent = Routing Algorithm
Link Layer Services Framing:
Encapsulate datagram into frame, adding header, trailer
The structure of the frame is specified by the Link Layer Protocol
Link Access Channel access if shared medium Medium Access Control (MAC) protocol specifies the
rules by which a frame is transmitted onto the link Reliable Delivery between Adjacent Nodes
Link Layer like Transport Layer provides reliability with ACKs and retransmissions.
Wireless links: high error rates Seldom used on low bit-error link (fiber, some twisted
pair)
Link Layer Services
Flow Control: Pacing between adjacent sending and receiving
nodes
Error Detection: Errors caused by signal attenuation, noise. Receiver detects presence of errors
Error correction: Receiver identifies and corrects bit error(s) without
resorting to retransmissions Transport layer provides reliable delivery between
processes on an end-to-end basis Link layer provides reliability between two nodes
connected by a single link.
Where is the link layer implemented? In each and every host Link layer implemented in
“adaptor” ( Network Interface Card NIC)
Attaches into host’s system buses
Network Adaptor Link Layer controller
Single special purpose chip Implements many of the
link layer services Mostly in hardware (Framing,
flow control, error detection etc)
Software Component Receiving the datagram,
addressing, activating the controller hardware
controller
physicaltransmission
cpu memory
host bus (e.g., PCI)
network adapter card
host
applicationtransportnetwork
link
linkphysical
Adaptors Communicating
Sending Side: Encapsulates datagram
in frame Adds error checking
bits, flow control, etc.
Receiving Side looks for errors, flow
control, etc extracts datagram,
passes to upper layer at receiving side
frame
controller controller
sending host receiving host
datagram datagram
datagram