1. Lecture 11:
LDP, RSVP,
RSVP-TE

2. Routing in MPLS Hop-by-hop routing:
IP router learns about the topology of its domain by exchanging information with the other IP routers. It then calculates the next hop IP router for each destination using the shortest path algorithm. This next hop is stored in its FIB. MPLS uses the same next hop information in order to set up an LSP.
Explicit routing:
LSP that follows an explicit route through a network which might not necessarily correspond to the hop-by-hop path.
An explicit route might be set up to satisfy a QoS criterion, such as minimizing the total end-to-end delay and maximizing throughput.
Also, explicit routing can be used to provide load-balancing, by forcing some of the traffic to follow different paths through a network, so that the utilization of the network links is as even as possible.
Finally, explicit routing can be used to set up MPLS-based tunnels and virtual private networks (VPN).

3. Label Distribution Protocols
LDP:
A new signaling protocol. It is used to distribute label bindings for an LSP associated with a FEC.
RSVP:
Extending of an existing IP control protocol, so that it can carry label bindings.

4. LDP (Label Distribution Protocol)
LDP creates LSP trees rooted at each egress router. The ingress point is at every router running LDP.
Two adjacent routers running LDP and become neighbors, they establish an LDP session to exchange label information in order to control the establishment of new LSPs.

5. LDP (IETF RFC 5036 )
LDP is used to establish and maintain label bindings for an LSP associated with a FEC.
Two LSRs that use LDP to exchange label bindings are known as LDP peers.
LDP provides several LDP messages:
Discovery: to announce and maintain the presence of an LSR in the network.
Session: in order for two LDP peers to exchange information, they have to first establish an LDP session. The session messages are used to establish, maintain, and terminate LDP sessions between LDP peers.
Advertisement: to create, change, and delete label bindings to FECs.
Notification: to provide advisory information and to signal error information.
LDP runs on top of TCP for reliability, with the exception of the LDP discovery messages that run over UDP.

6. LDP messages
Discovery messages (UDP/TCP).
Session messages (TCP).
Advertisement messages (TCP).
Notification messages (TCP).

7. LDP LDP works in downstream-unsolicited way
label distribution always done from downstream to upstream
downstream-unsolicited: new route => send new label
downstream-on-demand: upstream LSR asks for label

8. Discovery messages
Using discovery messages, the LSRs announce their presence in the network.
Discovery messages are literally sending “Hello messages” periodically, therefore those message broadcasts over UDP to LDP (port 646).

9. Session messages
When an LSR learns about new LSR, it establishes a TCP connection (port 646) and creates an LDP session on top of the TCP connection.
The session messages use to establish, maintain, and terminate the sessions between two LDP peers.
When the initialization procedure completes successfully, the two routers became LDP peers, and can exchange advertisement messages.

10. Advertisement messages
Advertisement messages create, change, and delete label mappings for Forwarding Equivalence Classes (FECs).
Requesting a label or advertising a label mapping to a peer is a decision made by the local router. In general, the router requests a label mapping from a neighboring router when it needs one and advertises a label mapping to a neighboring router when it wants the neighbor to use a label.

11. Sending request/release message
When do we sent Label request message:
The LSR routing table doesn’t contain a mapping for a new LDP peer as next-hop.
The LSR routing table doesn’t contain a mapping for next-hop with a new FEC that jest received.
The LSR routing table doesn’t contain a mapping for next-hop with a FEC that change.
When do we sent Label release message:
There is no longer needs to map the LSP across the network.

12. LDP Label mapping message:
An LSR uses the message to advertise a mapping (i.e., a binding) of a label to a FEC to its LDP peers.
A FEC element could be either a prefix or a full IP address of a destination host
The FEC element identifies a set of packets that can be mapped to the corresponding LSP.
The message contains the label associated with the FEC.

13. Notification messages
Notification messages provide advisory information and signal error information. LDP sends notification messages to report errors and other events of interest. There are two kinds of LDP notification messages:
Fatal errors notification - leads to termination of the LDP session and deleting all the labels which learned through that session.
Advisory notifications - carrying information about the LDP session or the status of some previous message received from the peer.

14. … Discovery messages Session messages Advertisement messages
Notification messages

15. RSVP (Resource Reservation Protocol)
RSVP is 4-Layer protocol designed to reserve resources across a network for an integrated services Internet.
RSVP used by routers to request or deliver specific levels of quality of service (QoS) for data streams or flows and it’s operation will result in resources reservations along a path.

16. Flowspec RSVP reserves resources for a flow.
For each flow, RSVP identifies the particular QoS it’s required. This QoS specific information is called a flowspec. RSVP passes the flowspec from one LSR to other along the path. Each LSR analyze the flowspec to accept and reserve the resources.
A flowspec contains:
Service class.
Reservation spec - defines the QoS.
Traffic spec - describes the data flow.

17. Filterspec
filterspec defines the set of packets that shall be affected by a flowspec. A filterspec selects a subset of packets processed by a node according to it’s sender IP / address / port.
The RSVP reservation styles:
Fixed filter - reserves resources for a specific flow.
Shared explicit - reserves resources for several flows.
Wildcard filter - reserves resources for a general type of flow.

18. A RSVP reservation request consists of a flowspec and a filterspec and the pair is called a flowdescriptor.
The effects at the node of each spec are that while the flowspec sets the parameters of the packet scheduler at a node, the filterspec sets the parameters at the packet classifier.

19. RSVP messages
A LER that needs to send a data flow with specific QoS will transmit a RSVP path message that will travel along the network based on IP routing tables. Other LER who is interested to receive that flow, send a corresponding resv message which travel the path backwards to the sender.
Each LSR along the path that receives the resv message will:
Make a reservation based on the request parameters. If they can’t support it, they send a reject message.
Forward the request upstream.
Store the nature of the flow, and also police it.
RSVP is soft-state, so if nothing is heard for a certain timeout period, the reservation will be cancelled.

20. RSVP - Downstream-on-demand
eth
subnet 1 2
eth
subnet 1
Massage to:
Label request
LER-
upstream A
Label mapping B C
Label request
Label mapping
MPLS domain
Label request
LER-
downstream D E
Label mapping F
eth
subnet 1 2
eth
subnet 1

21. RSVP-TE
An extension of the RSVP for traffic engineering - an ability to establish sets of LSP tunnels which can be useful during reroute operations or in spreading a traffic trunk over multiple paths.
RSVP-TE allows the reservation of resources across the network and can indicate nodes the bandwidth, jitter, maximum burst of a packet streams they want to receive

22. RSVP-Traffic Engineering
is used in MPLS to set up LSPs using either the next hop information in the routing table or an explicit route.
RSVP-TE uses downstream-on-demand label allocation to set up an LSP.
RSVP-TE enables the reservation of resources along the LSP. For example, bandwidth can be allocated to an LSP using standard RSVP reservations.

23. RSVP - LDP compare

24. Reliability
RSVP-TE
Unreliable
Does not use TCP to exchange messages.
LDP
Reliable
Use TCP to exchange messages.
No need to explicitly configure LDP peers, because each LSR actively discovers all other LSRs to which it is connected to.

25. Downstream on-demand / unsolicited
RSVP-TE
Downstream-on-demand
MPLS devices do not signal a FEC-to-label binding until requested to do.
LDP
Downstream-unsolicited
MPLS devices don’t wait for a request from other device before signaling FEC-to-label bindings.
LSR which learns a route sends a binding to all LDPs peer, both upstream and downstream.

26. Soft/Hard-state protocol
RSVP-TE
Soft-state protocol
The session information is temporarily saved on each LSR and must be refreshed periodically. In case that the session haven’t been refreshed during timeout period - the session detriment.
LDP
Hard-state protocol
After the LSP is established, it is remain in place until it has been explicitly torn down.

27. Traffic-engineering
RSVP-TE
Used primarily for MPLS applications that require traffic engineering (TE) or quality of service (QoS) capabilities
Support best-effort LSPs.
LDP
Have no traffic-engineering capability and support only best-effort LSPs.

28. IP Routers – internal view
Lecture 12:
IP Routers – internal view

33. Unlike standard computer memory (eg
Unlike standard computer memory (eg. RAM) in which the user supplies a memory
address and the RAM returns the data word stored at that address, a CAM is designed such that the user supplies a data word and the CAM searches its entire memory to see if that data word is stored anywhere in it.
If the data word is found, the CAM returns a list of one or more storage addresses where the word was found
Thus, a CAM is the hardware embodiment of what in software terms would be called
an associative array.

36. Longest Prefix Match Harder than Exact Match
destination address of arriving packet does not carry information to determine length of longest matching prefix
need to search space of all prefix lengths; as well as space of prefixes of given length

37. LPM in IPv4: exact match Use 32 exact match algorithms Network Address
against prefixes
of length 1
Network Address
Exact match
against prefixes
of length 2
Priority
Encode
and pick
Port
Exact match
against prefixes
of length 32

38. IP Address Lookup
routing tables contain (prefix, next hop) pairs
address in packet compared to stored prefixes, starting at left
prefix that matches largest number of address bits is desired match
packet forwarded to specified next hop
routing table
next hop
prefix
10* 7
01* 5
110* 3
1011* 5
0001*
0101 1* 7
0001 0* 1 * 2 * 3 * 5 * 6 * 4 * 8 * 10 * 9
Problem - large router may have 100,000 prefixes in its list
address:

39. Address Lookup Using Tries
next-hop-ptr (if prefix)
left-ptr
right-ptr
Trie node P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 A 1 B
prefixes “spelled” out by following path from root
to find best prefix, spell out address in tree
last green node marks longest matching prefix
Lookup 10111
adding prefix easy 1 C
add P5=1110* I P5 D P2 1 1 F E P1 G P3 1 H P4

40. Binary Tries For W-bit prefixes: O(W) lookup,
O(NW) storage, N number of entries (at the worst case for each entry has different prefix, disjoint path in a trie)
O(W) update
Advantages
Simplicity
Extensible to wider fields
Disadvantages
Worst case lookup slow
Wastage of storage space in chains

41. Leaf-pushed Binary Trie
Trie node A
left-ptr or next-hop
right-ptr or next-hop 1 B 1 C D P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 P2 P1 1 E P2 G P3 P4

42. PATRICIA
Patricia tree internal node
bit-position
left-ptr
right-ptr
PATRICIA
(Practical Algorithm To Retrieve Coded Information in Alphanumeric) A 2 1 B C P1 P2 3 1 E
Bitpos 12345 5 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 1 F G P3 P4

43. Bitpos 12345 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4
111*
Lookup: follow the key according to the bit position
if can not proceed and the node is “filled”: this is the output
if can not proceed and the node is not “filled”: lookup failed
if can proceed and arrived to a leaf: this is the output

44. Bitpos 12345 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 2 1
10*
111*
Lookup: follow the key according to the bit position
if can not proceed and the node is “filled”: this is the output
if can not proceed and the node is not “filled”: lookup failed
if can proceed and arrived to a leaf: this is the output

45. Bitpos 12345 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 2 1
111* 3
10* 1
1010*
Lookup: follow the key according to the bit position
if can not proceed and the node is “filled”: this is the output
if can not proceed and the node is not “filled”: lookup failed
if can proceed and arrived to a leaf: if the leaf corresponds to the prefix,
then this is the output

46. Bitpos 12345 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 2 1
111*
10* 3 1 5 1
1010*
10101
Lookup: follow the key according to the bit position
if can not proceed and the node is “filled”: this is the output
if can not proceed and the node is not “filled”: lookup failed
if can proceed and arrived to a leaf: this is the output

47. Example: lookup 10111 111* 10* 1010* 10101 P1 111* H1 P2 10* H2 P31
111* 3
10* 1 5 1
1010*
10101
Let’s lookup for ‘10111’ …
We can't proceed to check from the 5’th bit.
Backtrack to bit 3

48. (Example cont.) Inserting Operation
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4 P5
10111 H5 2 1
111* 3
10* 1 4 1 5
10111 1
10101
1010*

49. PATRICIA W-bit prefixes: O(W2) lookup O(N) storage
O(W) update complexity
Advantages
decreased storage
extensible to wider fields
Disadvantages
worst case lookup slow
backtracking makes implementation complex

50. Path-compressed Tree A C B D E Lookup 10111
1, , 2 A B C 1
10,P2,4 P4 P1 E D
1010,P3,5 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4
Lookup 10111
bit-position
left-ptr
right-ptr
variable-length bitstring
next-hop (if prefix present)
Path-compressed tree node structure

51. Path-compressed Tree
W-bit prefixes: O(W) lookup, O(N) storage and O(W) update complexity
Advantages
decreased storage
Disadvantages
worst case lookup slow

52. Multi-bit Tries Binary trie W Multi-ary trie W/k Depth = W Degree = 2
Stride = 1 bit
Depth = W/k
Degree = 2k
Stride = k bits
Multi-ary trie
W/k

53. Prefix Expansion with Multi-bit Tries
If stride = k bits, prefix lengths that are not a multiple of k need to be expanded
Prefix
Expanded prefixes 0*
00*, 01*
11*
E.g., k = 2:
Maximum number of expanded prefixes corresponding to one non-expanded prefix = 2k-1

54. 4-ary Trie (k=2) A four-ary trie node A B C Lookup 10111 P2 D E F P3
next-hop-ptr (if prefix) A
ptr00
ptr01
ptr10
ptr11 11 10 B C
Lookup 10111 P2 11 10 D E F 10 P3
P11
P12 10 11 H G
P41
P42 P1
111* H1 P2
10* H2 P3
1010* H3 P4
10101 H4

55. Prefix Expansion Increases Storage Consumption
replication of next-hop ptr
greater number of unused (null) pointers in a node
Time ~ W/k
Storage ~ NW/k * 2k-1

56. Generalization: Different Strides at Each Trie Level
split
split
24-8 split
split
In future, use cartoons esp. with questions like ‘optional exercise’…

57. Choice of Strides: Controlled Prefix Expansion
Given forwarding table and desired number of memory accesses in worst case (i.e., maximum tree depth, D)
A dynamic programming algorithm to compute optimal sequence of strides that minimizes storage requirements: runs in O(W2D) time
Advantages
Optimal storage under these constraints
Disadvantages
Updates lead to sub-optimality anyway
Hardware implementation difficult