BGP and ICMP

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BGP and ICMP
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Exterior Gateway Protocol (EGP)

• Like RIP, but no metrics. Just if reachable. Rtr
  inside a domain collects reachability information
  and informs the rtr on the boarder of the domain.
  Boarder rtr informs the internal rtr about
  reachability outside the domain.
• Rtrs establish com with pkts nbr acquisition
  request, nbr acquisition reply or refusal, nbr
  cease request, nbr cease ack.
• Theoretically only legal topology (but tree would
  work)

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Exterior Gateway Protocol (EGP)
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Border Gateway Protocol (BGP)

• Different autonomous systems exchange routing
  information using BGP.
• BGP messages are exchanged between AS border
  routers.
• The protocol for exchanging messages between
  border routers of the same AS is Internal BGP
  (IBGP), and the protocol for exchanging messages
  between border routers of different AS is
  external BGP (EBGP).

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Border Gateway Protocol (BGP)

• BGP neighbors are configured. They exchange the
  control information through TCP connections.
• BGP is a distance vector protocol that is path
  based, i.e. for each network a sequence of AS to
  that network is advertised (path) to avoid loops.

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BGP
ASBR
ASBE
ASBR
IBGP
ASBR
ASBR
ASBR
ASBR
EBGP
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BGP Packets

• Between any two BGP routers a reliable TCP
  connection is established.
• Open establish com between rtrs of different AS
• Update carries routing info
• Notification last message before a connection is
  closed. It reports an error
• Keepalive to inform about presence of nbr.

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BGP Packet Header

• Keepalive packet equals the BGP packet header

Packet type
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BGP Packet Open
Max time during which rtr does not receive packet
before concluding a problem.
IP address of a rtr sender
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BGP Packet Update
Variable
2B
Variable
Variable
2B
Unfeasible Routes Lngth
Withdrawn Routes
Total Path Attribute Lngth
Path Attributes
Network Layer Reachability
Charasteristics of a path being advertised
List of prefixes being advertised
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Kinds of Path Attributes

• Well known vs. optional. Well known must be
  recognized and understood.
• Mandatory vs. discretionary. A mandatory must
  appear in the description of a route.
• Partial has been passed along if a router does
  not recognize it.
• Transitive vs. nontransitive. A transitive
  attribute is passed along by the router that does
  not recognize it and marked as partial.
  Non-transitive is not passed.

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Path Attributes

• Origin. Values IGP, EGP, Incomplete. It is
  well-known mandatory attribute.
• AS Path. Sequence of AS numbers which are along
  the path to the destination. Well-known and
  mandatory.
• Next Hop. The rtr to which a packet should be
  next forwarded toward the destination. Well-known
  and mandatory.
• Unreachable. Well known and discretionary.

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Path Attributes

• Multi Exit Disc. Degree of preference that is
  received from AS nbr. Optional nontransitive.
• Local Preference. Degree of preference given to
  different routes within AS. Well-known
  discretionary.
• Atomic Aggregate. Denotes if some address is a
  result of the aggregation.Well-known
  discretionary.
• Aggregator. AS number and the router IP address
  that did the aggregation. Optional and transitive.

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BGP Packet Notification
1B
Variable
1B
Error Subcode
Error Data
Error Code
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Error Codes

• Message Header Error Indicates a problem with a
  message header, such as unacceptable message
  length, unacceptable marker field value, or
  unacceptable message type.
• Open Message Error Indicates a problem with an
  open message, such as unsupported version number,
  unacceptable autonomous system number or IP
  address, or unsupported authentication code.
• Update Message Error Indicates a problem with an
  update message, such as a malformed attribute
  list, attribute list error, or invalid next-hop
  attribute.
• Hold Time Expired Indicates that the hold-time
  has expired, after which time a BGP node will be
  considered nonfunctional.
• Finite State Machine Error Indicates an
  unexpected event.
• Cease Closes a BGP connection at the request of
  a BGP device in the absence of any fatal errors.

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IBGP

• In order to avoid loop in AS, ASBR are connected
  in a full mesh.
• Alternatively, in each region there is a
  designated router which is a Route Reflector
  (RR). RRs are connected in a full mesh, and they
  receive updates from each rtr of the region and
  forward those to other routers.
• Or, AS is divided into confederations in which
  ASBRs are connected into full meshes. Each
  confederation has its own ID, and the path of
  confederations are added into the BGP packets.

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Policy Based Routing in BGP

• Filters for receiving or advertising or
  forwarding info for certain prefixes can be
  implemented.
• Examples Only prefixes that originate in certain
  AS can be accepted, or ads through certain AS
  cannot be accepted, or would not be forwarded
  etc.
• Policy based routing may cause instabilities in
  the network.

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Internet Control Message Protocol
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Internet Control Message Protocol

• Commands Ping and Traceroute are based on ICMP
  messages.
• Ping determines if certain IP address is
  reachable, and what is RTT to it.
• Traceroute determines all routers on the way
  between two IP addresses.