SIP

1
SIP

• Session Initiation Protocol
• Comes from IETF
• SIP long-term vision
• All telephone calls and video conference calls
  take place over the Internet
• People are identified by names or e-mail
  addresses, rather than by phone numbers.
• You can reach the callee, no matter where the
  callee roams, no matter what IP device the callee
  is currently using.

2
SIP Services

• Setting up a call
• Provides mechanisms for caller to let callee know
  she wants to establish a call
• Provides mechanisms so that caller and callee can
  agree on media type and encoding.
• Provides mechanisms to end call.

• Determine current IP address of callee.
• Maps mnemonic identifier to current IP address
• Call management
• Add new media streams during call
• Change encoding during call
• Invite others
• Transfer and hold calls

3
Setting up a call to a known IP address

• Alices SIP invite message indicates her port
  number IP address. Indicates encoding that
  Alice prefers to receive (PCM ulaw)
• Bobs 200 OK message indicates his port number,
  IP address preferred encoding (GSM)
• SIP messages can be sent over TCP or UDP here
  sent over RTP/UDP.
• Default SIP port number is 5060.

4
Setting up a call (more)

• Codec negotiation
• Suppose Bob doesnt have PCM ulaw encoder.
• Bob will instead reply with 606 Not Acceptable
  Reply and list encoders he can use.
• Alice can then send a new INVITE message,
  advertising an appropriate encoder.

• Rejecting the call
• Bob can reject with replies busy, gone,
  payment required, forbidden.
• Media can be sent over RTP or some other protocol.

5
Example of SIP message

• INVITE sipbob_at_domain.com SIP/2.0
• Via SIP/2.0/UDP 167.180.112.24
• From sipalice_at_hereway.com
• To sipbob_at_domain.com
• Call-ID a2e3a_at_pigeon.hereway.com
• Content-Type application/sdp
• Content-Length 885
• cIN IP4 167.180.112.24
• maudio 38060 RTP/AVP 0
• Notes
• HTTP message syntax
• sdp session description protocol
• Call-ID is unique for every call.

• Here we dont know
• Bobs IP address.
• Intermediate SIPservers will be necessary.

• Alice sends and receives SIP messages using
  the SIP default port number 506.
• Alice specifies in Viaheader that SIP client
  sends and receives SIP messages over UDP

6
Name translation and user locataion

• Caller wants to call callee, but only has
  callees name or e-mail address.
• Need to get IP address of callees current host
• user moves around
• DHCP protocol
• user has different IP devices (PC, PDA, car
  device)

• Result can be based on
• time of day (work, home)
• caller (dont want boss to call you at home)
• status of callee (calls sent to voicemail when
  callee is already talking to someone)
• Service provided by SIP servers
• SIP registrar server
• SIP proxy server

7
SIP Registrar

• When Bob starts SIP client, client sends SIP
  REGISTER message to Bobs registrar server
• (similar function needed by Instant Messaging)

Register Message

• REGISTER sipdomain.com SIP/2.0
• Via SIP/2.0/UDP 193.64.210.89
• From sipbob_at_domain.com
• To sipbob_at_domain.com
• Expires 3600

8
SIP Proxy

• Alice sends invite message to her proxy server
• contains address sipbob_at_domain.com
• Proxy responsible for routing SIP messages to
  callee
• possibly through multiple proxies.
• Callee sends response back through the same set
  of proxies.
• Proxy returns SIP response message to Alice
• contains Bobs IP address
• Note proxy is analogous to local DNS server

9
Example
Caller jim_at_umass.edu with places a call to
keith_at_upenn.edu (1) Jim sends INVITEmessage to
umass SIPproxy. (2) Proxy forwardsrequest to
upenn registrar server. (3) upenn server
returnsredirect response,indicating that it
should try keith_at_eurecom.fr
(4) umass proxy sends INVITE to eurecom
registrar. (5) eurecom regristrar forwards INVITE
to 197.87.54.21, which is running keiths SIP
client. (6-8) SIP response sent back (9) media
sent directly between clients. Note also a SIP
ack message, which is not shown.
10
Comparison with H.323

• H.323 is another signaling protocol for
  real-time, interactive
• H.323 is a complete, vertically integrated suite
  of protocols for multimedia conferencing
  signaling, registration, admission control,
  transport and codecs.
• SIP is a single component. Works with RTP, but
  does not mandate it. Can be combined with other
  protocols and services.

• H.323 comes from the ITU (telephony).
• SIP comes from IETF Borrows much of its concepts
  from HTTP. SIP has a Web flavor, whereas H.323
  has a telephony flavor.
• SIP uses the KISS principle Keep it simple
  stupid.

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Improving QOS in IP Networks

• Thus far making the best of best effort
• Future next generation Internet with QoS
  guarantees
• RSVP signaling for resource reservations
• Differentiated Services differential guarantees
• Integrated Services firm guarantees
• simple model for sharing and congestion
  studies

12
Principles for QOS Guarantees

• Example 1MbpsI P phone, FTP share 1.5 Mbps
  link.
• bursts of FTP can congest router, cause audio
  loss
• want to give priority to audio over FTP

Principle 1
packet marking needed for router to distinguish
between different classes and new router policy
to treat packets accordingly
13
Principles for QOS Guarantees (more)

• what if applications misbehave (audio sends
  higher than declared rate)
• policing force source adherence to bandwidth
  allocations
• marking and policing at network edge
• similar to ATM UNI (User Network Interface)

Principle 2
provide protection (isolation) for one class from
others
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Principles for QOS Guarantees (more)

• Allocating fixed (non-sharable) bandwidth to
  flow inefficient use of bandwidth if flows
  doesnt use its allocation

Principle 3
While providing isolation, it is desirable to use
resources as efficiently as possible
15
Principles for QOS Guarantees (more)

• Basic fact of life can not support traffic
  demands beyond link capacity

Principle 4
Call Admission flow declares its needs, network
may block call (e.g., busy signal) if it cannot
meet needs
16
Summary of QoS Principles
Lets next look at mechanisms for achieving this
.
17
Scheduling And Policing Mechanisms

• scheduling choose next packet to send on link
  allocate link capacity and output queue buffers
  to each connection (or connections aggregated
  into classes)
• FIFO (first in first out) scheduling send in
  order of arrival to queue
• discard policy if packet arrives to full queue
  who to discard?
• Tail drop drop arriving packet
• priority drop/remove on priority basis
• random drop/remove randomly

18
Need for a Scheduling Discipline

• Why do we need a non-trivial scheduling
  discipline?
• Per-connection delay, bandwidth, and loss are
  determined by the scheduling discipline
• The NE can allocate different mean delays to
  different connections by its choice of service
  order
• it can allocate different bandwidths to
  connections by serving at least a certain number
  of packets from a particular connection in a
  given time interval
• Finally, it can allocate different loss rates to
  connections by giving them more or fewer buffers

19
FIFO Scheduling

• Disadvantage with strict FIFO scheduling is that
  the scheduler cannot differentiate among
  connections -- it cannot explicitly allocate some
  connections lower mean delays than others
• A more sophisticated scheduling discipline can
  achieve this objective (but at a cost)
• The conservation law
• the sum of the mean queueing delays received by
  the set of multiplexed connections, weighted by
  their fair share of the links load, is
  independent of the scheduling discipline

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Requirements

• A scheduling discipline must satisfy four
  requirements
• Ease of implementation -- pick a packet every few
  microsecs a scheduler that takes O(1) and not
  O(N) time
• Fairness and Protection (for best-effort
  connections) -- FIFO does not offer any
  protection because a misbehaving connection can
  increase the mean delay of all other connections.
  Round-robin scheduling?
• Performance bounds -- deterministic or
  statistical common performance parameters
  bandwidth, delay (worst-case, average),
  delay-jitter, loss
• Ease and efficiency of admission control -- to
  decide given the current set of connections and
  the descriptor for a new connection, whether it
  is possible to meet the new connections
  performance bounds without jeopardizing the
  performance of existing connections

21
Max-Min Fair Share

• Fair Resource allocation to best-effort
  connections?
• Fair share allocates a user with a small demand
  what it wants, and evenly distributes unused
  resources to the big users.
• Maximize the minimum share of a source whose
  demand is not fully satisfied.
• Resources are allocated in order of increasing
  demand
• no source gets a resource share larger than its
  demand
• sources with unsatisfied demand s get an equal
  share of resource

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Schedulable Region
23
Designing a scheduling discipline

• Four principal degrees of freedom
• the number of priority levels
• whether each level is work-conserving or
  non-work-conserving
• the degree of aggregation of connections within a
  level
• service order within a level
• Each feature comes at some cost
• for a small LAN switch -- a single priority FCFS
  scheduler or at most 2-priority scheduler may be
  sufficient
• for a heavily loaded wide-area public switch with
  possibly noncooperative users, a more
  sophisticated scheduling discipline may be
  required.

24
Priority Scheduling

• transmit highest priority queued packet
• multiple classes, with different priorities
• class may depend on marking or other header info,
  e.g. IP source/dest, port numbers, etc..

25
Priority Scheduling

• The scheduler serves a packet from priority level
  k only if there are no packets awaiting service
  in levels k1, k2, , n
• at least 3 levels of priority in an integrated
  services network?
• Starvation? Appropriate admission control and
  policing to restrict service rates from all but
  the lowest priority level
• Simple implementation

26
Round Robin Scheduling

• multiple classes
• cyclically scan class queues, serving one from
  each class (if available)
• provides protection against misbehaving sources
  (also guarantees a minimum bandwidth to every
  connection)

27
Weighted Fair Queueing

• generalized Round Robin (offers differential
  service to each connection/class)
• each class gets weighted amount of service in
  each cycle

28
Policing Mechanisms

• Goal limit traffic to not exceed declared
  parameters
• Three common-used criteria
• (Long term) Average Rate how many pkts can be
  sent per unit time (in the long run)
• crucial question what is the interval length
  100 packets per sec or 6000 packets per min have
  same average!
• Peak Rate e.g., 6000 pkts per min. (ppm) avg.
  1500 ppm peak rate
• (Max.) Burst Size max. number of pkts sent
  consecutively (with no intervening idle)

29
Traffic Regulators

• Leaky bucket controllers
• Token bucket controllers

30
Policing Mechanisms

• Token Bucket limit input to specified Burst Size
  and Average Rate.
• bucket can hold b tokens
• tokens generated at rate r token/sec unless
  bucket full
• over interval of length t number of packets
  admitted less than or equal to (r t b).

31
Policing Mechanisms (more)

• token bucket, WFQ combine to provide guaranteed
  upper bound on delay, i.e., QoS guarantee!

32
IETF Integrated Services

• architecture for providing QOS guarantees in IP
  networks for individual application sessions
• resource reservation routers maintain state info
  (a la VC) of allocated resources, QoS reqs
• admit/deny new call setup requests

Question can newly arriving flow be admitted
with performance guarantees while not violating
QoS guarantees made to already admitted flows?
33
Intserv QoS guarantee scenario

• Resource reservation
• call setup, signaling (RSVP)
• traffic, QoS declaration
• per-element admission control

request/ reply
34
RSVP
35
Call Admission

• Arriving session must
• declare its QOS requirement
• R-spec defines the QOS being requested
• characterize traffic it will send into network
• T-spec defines traffic characteristics
• signaling protocol needed to carry R-spec and
  T-spec to routers (where reservation is required)
• RSVP

36
Intserv QoS Service models rfc2211, rfc 2212

• Guaranteed service
• worst case traffic arrival leaky-bucket-policed
  source
• simple (mathematically provable) bound on delay
  Parekh 1992, Cruz 1988

• Controlled load service
• "a quality of service closely approximating the
  QoS that same flow would receive from an unloaded
  network element."

37
Chapter 6 outline

• 6.1 Multimedia Networking Applications
• 6.2 Streaming stored audio and video
• RTSP
• 6.3 Real-time, Interactivie Multimedia Internet
  Phone Case Study
• 6.4 Protocols for Real-Time Interactive
  Applications
• RTP,RTCP
• SIP

• 6.5 Beyond Best Effort
• 6.6 Scheduling and Policing Mechanisms
• 6.7 Integrated Services
• 6.8 RSVP
• 6.9 Differentiated Services

38
IETF Differentiated Services

• Concerns with Intserv
• Scalability signaling, maintaining per-flow
  router state difficult with large number of
  flows
• Flexible Service Models Intserv has only two
  classes. Also want qualitative service classes
• behaves like a wire
• relative service distinction Platinum, Gold,
  Silver
• Diffserv approach
• simple functions in network core, relatively
  complex functions at edge routers (or hosts)
• Dot define define service classes, provide
  functional components to build service classes

39
Diffserv Architecture
Edge router - per-flow traffic management -
marks packets as in-profile and out-profile
Core router - per class traffic management -
buffering and scheduling based on marking at
edge - preference given to in-profile packets -
Assured Forwarding
40
Edge-router Packet Marking

• profile pre-negotiated rate A, bucket size B
• packet marking at edge based on per-flow profile

User packets
Possible usage of marking

• class-based marking packets of different classes
  marked differently
• intra-class marking conforming portion of flow
  marked differently than non-conforming one

41
Classification and Conditioning

• Packet is marked in the Type of Service (TOS) in
  IPv4, and Traffic Class in IPv6
• 6 bits used for Differentiated Service Code Point
  (DSCP) and determine PHB that the packet will
  receive
• 2 bits are currently unused

42
Classification and Conditioning

• may be desirable to limit traffic injection rate
  of some class
• user declares traffic profile (eg, rate, burst
  size)
• traffic metered, shaped if non-conforming

43
Forwarding (PHB)

• PHB result in a different observable (measurable)
  forwarding performance behavior
• PHB does not specify what mechanisms to use to
  ensure required PHB performance behavior
• Examples
• Class A gets x of outgoing link bandwidth over
  time intervals of a specified length
• Class A packets leave first before packets from
  class B

44
Forwarding (PHB)

• PHBs being developed
• Expedited Forwarding pkt departure rate of a
  class equals or exceeds specified rate
• logical link with a minimum guaranteed rate
• Assured Forwarding 4 classes of traffic
• each guaranteed minimum amount of bandwidth
• each with three drop preference partitions

45
Multimedia Networking Summary

• multimedia applications and requirements
• making the best of todays best effort service
• scheduling and policing mechanisms
• next generation Internet Intserv, RSVP, Diffserv