SRH Mechanics & Packet Walk¶

Understanding how the Segment Routing Header (SRH) works at the packet level is essential for troubleshooting and designing SRv6 networks. This page walks through the mechanics step by step.

SRH Format (RFC 8754)¶

The SRH is an IPv6 Routing Header with Routing Type = 4.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |
|               |               |     (= 4)     |     (SL)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Last Entry   |    Flags      |              Tag              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|            Segment List[0] (128-bit IPv6 address)             |
|                        (Last SID)                             |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|            Segment List[1] (128-bit IPv6 address)             |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|            Segment List[n] (128-bit IPv6 address)             |
|                       (First SID)                             |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                  Optional TLV objects                         |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Key Fields¶

Field	Size	Description
Next Header	8 bits	Type of the next header (e.g., 41 = IPv6, 4 = IPv4)
Hdr Ext Len	8 bits	Length of SRH in 8-octet units, not including first 8 octets
Routing Type	8 bits	Always 4 for SRH
Segments Left (SL)	8 bits	Index into the segment list pointing to the current active segment
Last Entry	8 bits	Index of the last element in the segment list (0-based)
Flags	8 bits	Currently unused (reserved)
Tag	16 bits	Packet grouping / marking
Segment List	128 bits each	Ordered list of SIDs in reverse order

Reverse order!

The segment list is encoded in reverse order: Segment List[0] is the last SID to be processed, and Segment List[n] is the first. The Segments Left pointer starts at n and decrements toward 0.

Complete Packet Walk¶

Let's trace a packet through a 3-node SRv6 network:

Source (A) → Node B → Node C → Destination (D)

SRv6 SIDs:
  B::100  = End behavior on Node B
  C::200  = End.DT4 behavior on Node C (decap to IPv4)

Step 1: Ingress (Source A)¶

Source A receives an IPv4 packet destined to 10.0.0.2 and needs to steer it through B then C.

Original packet:
  [IPv4: 10.0.0.1 → 10.0.0.2][Payload]

After SRv6 encapsulation:
  ┌──────────────────────────────────────────┐
  │ Outer IPv6 Header                         │
  │   Source:      A::1                       │
  │   Destination: B::100  ← Active SID      │
  │   Next Header: 43 (Routing)               │
  ├──────────────────────────────────────────┤
  │ SRH                                       │
  │   Segments Left: 1                        │
  │   Last Entry:    1                        │
  │   Segment List[0]: C::200  (last SID)     │
  │   Segment List[1]: B::100  (first SID)    │
  ├──────────────────────────────────────────┤
  │ Inner IPv4 Header                         │
  │   10.0.0.1 → 10.0.0.2                    │
  ├──────────────────────────────────────────┤
  │ Payload                                   │
  └──────────────────────────────────────────┘

DA = Active SID

The IPv6 Destination Address always equals the currently active SID (Segment List[Segments Left]).

Step 2: Transit (Node B — End Behavior)¶

Node B receives the packet. The DA matches its local SID B::100 with End behavior:

Decrement Segments Left: 1 → 0
Copy Segment List[0] to IPv6 DA: DA = C::200
Forward based on new DA

After Node B processing:
  ┌──────────────────────────────────────────┐
  │ Outer IPv6 Header                         │
  │   Source:      A::1                       │
  │   Destination: C::200  ← Updated!        │
  ├──────────────────────────────────────────┤
  │ SRH                                       │
  │   Segments Left: 0     ← Decremented!    │
  │   Last Entry:    1                        │
  │   Segment List[0]: C::200                 │
  │   Segment List[1]: B::100                 │
  ├──────────────────────────────────────────┤
  │ Inner IPv4: 10.0.0.1 → 10.0.0.2          │
  ├──────────────────────────────────────────┤
  │ Payload                                   │
  └──────────────────────────────────────────┘

Step 3: Egress (Node C — End.DT4 Behavior)¶

Node C receives the packet. The DA matches its local SID C::200 with End.DT4 behavior:

SL = 0, so this is the last segment
Decapsulate: remove outer IPv6 + SRH
Lookup the inner IPv4 DA in the specified VRF table
Forward the original IPv4 packet

After Node C processing:
  [IPv4: 10.0.0.1 → 10.0.0.2][Payload]  ← Original packet restored!

Step 4: Delivery¶

The original IPv4 packet is delivered to destination 10.0.0.2 as if SRv6 was never involved.

Packet Walk Sequence Diagram¶

sequenceDiagram
    participant A as Source A
    participant B as Node B (End)
    participant C as Node C (End.DT4)
    participant D as Destination

    A->>A: Receive IPv4 packet for 10.0.0.2
    A->>A: Encap: IPv6(DA=B::100) + SRH[C::200, B::100] + IPv4
    A->>B: Forward to B::100 (SL=1)

    B->>B: Match B::100 → End behavior
    B->>B: SL: 1→0, DA: B::100→C::200
    B->>C: Forward to C::200 (SL=0)

    C->>C: Match C::200 → End.DT4 behavior
    C->>C: Decapsulate, lookup IPv4 in VRF
    C->>D: Forward original IPv4 packet

PSP, USP, and USD Flavors¶

SRv6 endpoints can support different processing flavors that optimize how the SRH is handled:

Flavor	Full Name	Description
PSP	Penultimate Segment Pop	The penultimate node (SL=1) removes the SRH before forwarding to the last segment. Reduces overhead on the egress node.
USP	Ultimate Segment Pop	The ultimate (last) node removes the SRH itself.
USD	Ultimate Segment Decap	The ultimate node decapsulates the entire outer IPv6 header + SRH. Used with End.DT4/DT6.

PSP is the most common

PSP is widely deployed because it offloads SRH removal from the egress PE, improving efficiency. Most modern implementations advertise End with PSP or End with PSP-USP-USD.

SRH with TLVs¶

The SRH can carry optional TLV (Type-Length-Value) objects after the segment list:

TLV	Description
HMAC TLV	Cryptographic authentication of the SRH
Padding TLV	Alignment padding
NSH Carrier TLV	Carries NSH metadata for service chaining

Reduced Encapsulation¶

H.Encaps.Red (Reduced Encapsulation, RFC 8986) optimizes the SRH by omitting the first SID of the path from the segment list, since the headend already places it in the IPv6 DA. This saves 16 bytes per packet, and with a single-SID path it removes the SRH entirely.

Path: SID-1 → SID-2 → SID-3   (segment list is stored in reverse order)

H.Encaps:      SRH [SID-3, SID-2, SID-1], SL=2, DA = SID-1
H.Encaps.Red:  SRH [SID-3, SID-2],        SL=2, DA = SID-1  ← SID-1 only in the DA

Verification Commands¶

LinuxCisco IOS-XRJuniper

# Capture SRv6 packets (Routing Header type 43)
tcpdump -i eth0 -vv 'ip6 proto 43'

# Show local SID table
ip -6 route show | grep seg6local

# Decode with tshark
tshark -i eth0 -O ipv6 -Y 'ipv6.routing.type == 4'

show segment-routing srv6 sid
show cef ipv6 <SID> detail
debug segment-routing srv6 dataplane

show route table inet6.0 protocol srv6
show spring-traffic-engineering srv6-sid

References¶

RFC 8754 - IPv6 Segment Routing Header (SRH) - Full specification of the SRH format, fields, and packet processing rules
RFC 8986 - SRv6 Network Programming - Defines SRv6 endpoint behaviors including End, End.X, End.DT4, PSP, USP, and USD flavors
SRv6 Linux Kernel Implementation - Configuration guide for seg6 and seg6local lightweight tunnels in the Linux kernel
Cisco IOS-XR: Configure Segment Routing over IPv6 (SRv6) - Cisco IOS-XR SRv6 configuration guide for NCS 5500 routers