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Lecture 27

CSE 123 Lecture Notes - Lecture 27: Source Routing, Hypertext Transfer Protocol, Tcp Tuning

Computer Science and Engineering
Course Code
CSE 123
Alex Snoeren

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Encapsulation via Headers
| Ethernet | IP | TCP | HTTP | Payload
Start End
Layers add overhead!
Know the fields of each header, how to fragment, etc.
Phy/(MAC) Link Layer
Signal encoding
Media access - “sharing” the channel
- Arbitrate which nodes can send frames at ANY point in time
- Not ALWAYS necessary
DNS Example
Go to local DNS server
Hit root DNS
Then hit the appropriate TLD
Go to authoritative DNS - get answer
NA(p)T Example (another example of naming)
Builds a table that maps a port number to a “temporary” one
Internet does NOT know about LAN addresses (these are not even routable)
In general, rewriting a set of IP addresses with another
- Multiplexiing port # to the LAN
Routing Challenges
How to choose “best path”?
How to scale?
Forwarding Options
Source routing
- COMPLETE path listed in packet
Virtual circuits
- Set up path “out-of-band” and store path identifier in routers
- Local path identifier in packet
Destination-based forwarding
- Router looks up address in forwarding table
Classless of Routing Protocols
Link-state Routing
- Every router tells about ITSELF
- Used in intra-AS
2 phases
1) Reliable “flooding” - send LSP packets so every router learned about every other router
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- Every router has a map (no need to coordinate about how to route!)
2) Path calculation (Dijkstra’s)
Alternative to Dijkstra’s:
Distance Vector Algorithm
ONLY tell our neighbors, but “gossip
EVERY router would compute the cost of how to get to the dest
- It then tells its neighbors
ALSO intra-AS
We don’t have to tell everyone EVERYTHING!
- Scales with sizeof(neighbors)
- Requires Bellman-Ford alg
Problem: ONLY tell neighbors how much it costs
Counting to Infinity
Nodes may depend on each other!
Mitigation Strategies
Hold downs
- As metric increases, DELAY propagation info
Loop avoidance
- FULL path info in route ad
Split Horizon
- NEVER advertise dest through its next hop
- Poison reverse: Send NEGATIVE info when advertising a destination through its next
- Deliberately send an “infinite” cost
Autonomous Systems
Distinct regions of administrative control
- Tier 1 - default-free (NO default entry in routing tables)
- Nationwide BACKBONE
Interaction between AS
- Cutthroat competitors!
- Have their own set of customers
- Have to cooperate with other ASes
Inter-domain Routing
Border routers
- Speak intra AND inter-domain protocols!
Path-vector Routing
Extension of DV routing
- Support flexible routing policies
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