Add acn
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John Gatward
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docs/lectures/acn/14_bgp.md
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docs/lectures/acn/14_bgp.md
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# Border Gateway Protocol
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#### Routing Protocols
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The main job is to distribute the data to build forwarding tables
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- These are **intra-domain routing protocols**
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- Or **Interior gateway protocols**
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- When the source and destination are **inside** the **same network**
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It is important to distinguish between local and global protocols
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- Interior vs Exterior Gateway Protocol (IGP vs EGP)
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##### BGPv4
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The internet inter-domain routing protocol
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- Derives from GGP & EGP
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- Deals in IP prefixes and **autonomous systems**
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- autonomous systems are purely administrative
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- Purpose is to enable *policy* to be applied
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- Only prefixes matter in the data-plane
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- Internet policy domains
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- Logical construct only
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- No meaning outside BGP
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- Do not map simply onto ISPs or networks
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- Currently ~493,000 prefixes & ~46,000 ASs
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- Because we have less ASs, the routing is easily -> less complex
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- Reduces complexity
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- Speeds up performance
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BGP uses TCP as transport
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- `OPEN`, `UPDATE`, `KEEPALIVE`, `NOTIFICATION`
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Sessions between peers have:
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- Simple capability negotiation
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- Manage simultaneous `OPEN` connections
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- Lose everything on session failure
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#### Sessions and Routing Information Base (RIBs)
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A BGP peer typically has many sessions
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- Logically, for each peer, it receives the information about routing from peers, this is sorted into `Adj-RIB-in` table.
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- After processing, it produces a `Adj-RIB-out` table which it sends to other peers
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- Advertisements received and to be sent
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- Generates a local RIB table from `Adj-RIB-in`
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- Routes to use and potentially distribute
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- Resolved into per-port forwarding tables
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- Generate `Adj-RIB-out` from `Loc-RIB` and policy
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#### Update messages
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- Incremental - indicate *changes* to state
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- These updates could be:
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- Withdrawn routes
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- Path attributes, common to all advertised routes
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- Advertised routes, known as NLRI
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- There are ~27 path attributes
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- Only ~12 are in common use
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- Communicate information about prefixes
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- Used to apply policy in BGP *decision process*
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##### Path Attributes
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Mandatory - every AS has to inform the other ASs about these 3 attributes:
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- Next hop
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- AS Path
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- Origin
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Discretionary
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- Local preferences
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- Allows prioritisation of ASs
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Optional & transitive
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- Aggregator
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- Community
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- Extended Communities
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Optional & non-transitive
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- Multi-exit discriminator
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- Originator ID
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### Path Vectors
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**Distance Vector** - prefer lowest cost path (not always)
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**Path Vector**
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- How do we know if an AS has seen this advert before
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- Store the list of ASs in the packet
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- This is called the `AS_PATH`
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- This way loops can be broken
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- If our ASN appears in a received `AS_PATH`, drop the advert
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##### Decision Process
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Drop prefix if:
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- `NEXT_HOP` is unreachable via local routing table
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- Local AS appears in `AS_PATH` (packet in a loop)
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Then (commonly) apply following preference:
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1. Higher `weight` (local to this router)
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2. Highest `LOCAL_PREF`
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3. Shortest `AS_PATH`
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4. Lowest `ORIGIN`
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5. Lowest `MED`
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6. `EGP` to `IGP` (hot potato)
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7. Shortest internal path
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8. Prefer oldest route
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- Oldest routes are often most stable
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9. Lowest interface IP address
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### Consistency
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Learn external routes on `EBGP` sessions
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- `EBGP` defined as peers having different ASNs
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- Must ensure every router knows all external routes
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- Redistribute external routes inside the network
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- Via `IGP` - only in small networks
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- via `IBGP` - gives full control over route distribution
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###### Scaling
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Can distribute `IBGP` routes on `IBGP` sessions
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- Have to maintain $N\cdot \frac{(N-1)}{2}$ `IBGP` sessions
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- Each carrying up to 490k routes x2 tables
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- Two standard solutions
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1. **Route Reflectors**
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- Super nodes re-advertising `IBGP` routes
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- Allows for hierarchy
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2. **AS Confederations**
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- split AS up into mini-ASs
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###### Failures
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- Handling link failures
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- Bind to loopback
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- If it cant talk to other nodes, will only support communication internally
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- Flap damping
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- A warning message saying don't send traffic to me
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- This can make things worse if this message is delayed
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- Process failures
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- Out of memory error due to too many routes
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##### Network Inter-connection
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- Networks interconnect via `EBGP` sessions
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- POPs - points of presence or IX internet exchanges
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- Multi-homing
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- This is all logical
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- http://0x0.st/ooCs.png
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